B GIO DC V O TO TRNG I HC S PHM TP. H CH MINH

V Th Thy Dung

O T B UT I U U G

U G U G C O

T B G C G T C

U V T C VT

Thnh ph Ch inh -2012

B GIO DC V O TO TRNG I HC S PHM TP. H CH MINH

V Th Thy Dung

O T B UT I U U G

U G U G C O

T B G C G T C

Chuyn ngnh: Vt l nguyn t, ht nhn v nng lng cao

M s : 60 44 05

U V T C VT

gi hng dn khoa hc

TS. T G T G O

Thnh ph Ch inh-2012

- 1 -

LI CM N

Trong qu trnh hon thnh lun vn, ti nhn c rt nhiu s quan tm,

ng vin, gip ca qu thy c, gia nh v bn b. Xin cho php ti c by

t lng bit n chn thnh ca mnh n:

TS. Trng Th Hng Loan, ngi theo di sut qu trnh thc hin lun

vn ca ti. C l ngi ging dy, hng dn nhng bi hc u tin v phng

php m phng Monte Carlo v gi s dng chng trnh MCNP (Monte Carlo N

Particle) trong nghin cu ti ny. C cng l ngi truyn cho ti s say m

nghin cu khoa hc, c nhng gp qu bu cho ti trong qu trnh tin hnh

lun vn.

Cc thnh vin trong nhm MCNP ca B mn Vt l ht nhn : C Trng

Th Hng Loan, cc anh ch: ng Nguyn Phng, Trn i Khanh, L Thanh

Xun, Nguyn Th Cm Thu h tr, ng gp kin v lun bn cnh gip

ti trong qu trnh tin hnh lun vn.

ThS. Thi M Ph gip ti trong vic tin hnh o c thc nghim ti

bnh vin Nhi ng. Bc s Nguyn Anh Tun bnh vin Nhi ng I cho php

chng ti tin hnh o c thc nghim ti bnh vin.

Ngoi ra ti cng xin chn thnh cm n n cc k s ca hng Shimadzu l

nhng ngi cung cp cho ti ti liu v my X quang ca hng cng nh h tr

ti rt nhiu trong vic tm hiu v cu to ca my X quang.

Ti xin gi lng bit n n gia nh, bn b lun ng h ng vin ti ti

hon thnh kha hc.

Tp H Ch Minh, thng 9 nm 2012

V TH THY DUNG

- 2 -

MC LC

LI CM N ................................................................................................................ 1

MC LC ....................................................................................................................... 2

DANH MC CC K HIU V CC CH VIT TT .................................... 6

DANH MC HNH V V TH ........................................................................ 7

DANH MC CC BNG .......................................................................................... 10

M U......................................................................................................................... 11

CHNG 1. CU TO V NGUYN L MY PHT TIA X ..................... 16

1.1. Cu to my pht X quang thng thng ................................................... 16

1.1.1. Cu to ng pht tia X .......................................................................... 16

1.1.1.1. m cc ( Cathode) ......................................................................... 18

1.1.1.2. Dng cc (anode) ........................................................................ 20

1.1.1.3. V ng chn khng, du tn nhit, v kim loi, ca s ng pht tia X

..................................................................................................................... 32

1.1.2. B lc tia ............................................................................................... 35

1.1.3. H chun trc u n (Collimator) ..................................................... 36

1.2. Nguyn l ca qu trnh pht tia X .............................................................. 39

1.2.1. Nguyn l to tia X ............................................................................... 39

1.2.1.1. Bc x hm .................................................................................... 39

1.2.1.2. Bc x c trng ............................................................................ 43

1.2.2. Cc tnh cht ca tia X ......................................................................... 47

1.2.2.1. Bn cht ca tia X .......................................................................... 47

1.2.2.2.S hp th tia X .............................................................................. 49

1.2.2.3. S tn x tia X ................................................................................ 50

- 3 -

1.3. Nguyn l hot ng ca my pht tia X ................................................... 51

1.4. Cc yu t nh hng n cht lng liu ra tia X ..................................... 52

CHNG 2. AN TON BC X TRONG X QUANG CHN ON Y T56

2.1. Cc hiu ng sinh hc ca bc x ion ha .................................................. 56

2.1.1. C ch tc dng ca bc x ion ha ..................................................... 56

2.1.1.1.Giai on ha l ............................................................................... 56

2.1.1.2.Giai on sinh hc ........................................................................... 56

2.1.2. Nhng tn thng do bc x ion ha ................................................... 57

2.1.2.1. Tn thng mc phn t............................................................. 57

2.1.2.2.Tn thng mc t bo ................................................................ 57

2.1.2.3. Tn thng mc c th ............................................................... 58

2.1.2.4. Cc yu t nh hng n hiu ng sinh hc ca bc x ion ha 58

2.2. Cc tiu chun v an ton bc x ............................................................... 60

2.2.1. Lch s xy dng cc tiu chun an ton bc x trn th gii ............ 60

2.2.2. Cc khuyn co v an ton bc x ca ICRP ....................................... 60

2.2.3. Cc tiu chun v an ton bc x ca IAEA ....................................... 61

2.2.4. Gii hn liu ......................................................................................... 63

2.3. An ton bc x ti cc c quan y t theo tiu chun Vit Nam ................ 64

2.3.1. Cc quy ch an ton bc x c ban hnh Vit Nam ................. 64

2.3.2. Tiu chun Vit Nam TCVN 6561:1999 v an ton bc x ion ha ti

cc c s X quang y t .................................................................................... 64

2.3.2.1. Phm vi p dng ............................................................................. 65

2.3.2.2. Ni dung ......................................................................................... 65

- 4 -

CHNG 3. KHO ST PHN B SUT LIU XUNG QUANH PHNG

MY X QUANG CHN ON Y T BNG CHNG TRNH MCNP .... 71

3.1. Gii thiu chng trnh MCNP ................................................................... 71

3.1.1. Lch s ca chng trnh MCNP ............ Error! Bookmark not defined.

3.1.2. D liu ht nhn v phn ng ca MCNP ........................................... 73

3.1.3.Cu trc ca chng trnh MCNP ......................................................... 74

3.1.4. chnh xc ca kt qu v cc nhn t nh hng ............................ 76

3.2. Tally nh gi ............................................................................................. 77

3.2.1. Tally F4 ................................................................................................. 77

3.2.2. Tally Fmesh4 ........................................................................................ 78

3.2.3. Tally F2 ................................................................................................. 78

3.3. Kt qu kho st phn b liu xung quanh phng my X quang chn on y

t bng chng trnh MCNP .............................................................................. 78

3.3.1. M t phng X quang thng quy ti bnh vin Nhi ng I ................ 78

3.3.1.1. M t phng chp X quang thng quy bnh vin Nhi ng I .. 78

3.3.1.2. Thng s v cu trc hnh hc my X quang................................. 80

3.3.2. Kim tra hiu lc ca m hnh - chun ha kt qu m phng ............ 83

3.3.2.1. Kho st thc nghim sut liu chiu t chm tia X s cp theo

khong cch so vi tm pht ....................................................................... 83

3.3.2.2. M phng sut liu t chm tia X s cp theo khong cch so vi

tm pht v chun theo thc nghim .......................................................... 84

3.3.2.3. M phng ph tia X trong my X quang Nhi ng I ..................... 86

3.3.3. M phng phn b sut liu trong phng chp X quang bng tally Fmesh

vi cc ch chiu chp khc nhau ............................................................. 88

3.3.3.1. M phng phn b sut liu ng vi ch chp c kho st thc88

- 5 -

3.3.3.2. M phng phn b sut liu ng vi cc ch chp trn bng iu

khin ............................................................................................................ 89

3.3.3.3. M phng phn b sut liu trong phng chp khi gim kch thc

..................................................................................................................... 96

3.3.3.4. nh gi an ton bn ngoi phng chp X quang ......................... 97

KT LUN V KIN NGH ................................................................................... 103

TI LIU THAM KHO ......................................................................................... 105

- 6 -

DANH MC CC K HIU V CC CH VIT TT

ALARA As Low As Reasonably Achievable

ACTL The Activation Library

ENDF The Evaluated Nuclear Data File

ENDL The Evaluated Nuclear Data Library

IAEA International Atomic Energy Agency

ICRP International Commission on Radiological Protection

ICRU The International Commission on Radiation Units and Measurements

MCNP Monte Carlo N-Particle

NCRP National Council on Radiation Protection and Measurement

- 7 -

DANH MC HNH V V TH

Hnh 1.1. S khi h thng chp nh X quang thng thng .................................... 16

Hnh 1.2. Nhng b phn c bn ca mt ng pht tia X thng thng ....................... 17

Hnh 1.3. Cc b phn chnh ca ng tia X trong my chp X quang hin i ............ 18

Hnh 1.4. Cu trc cathode ca ng tia X si t Volfram nm trong chn hi t ..... 19

Hnh 1.5. Tc dng lm thay i hnh dng phn b chm electron ca chn ti t .... 20

Hnh 1.6. Cc thnh phn ca mt ng tia X c anode c nh gm bia Vonfram

gn vo mt khi ng .......................................................................................................... 21

Hnh 1.7. Vt hi t ca bng X quang c anode c nh ................................................ 22

Hnh 1.8. Hnh dng ca anode xoay .................................................................................. 23

Hnh 1.9. Cu to anode xoay ............................................................................................. 24

Hnh 1.10.a. Mt ct ca mt anode RTM ......................................................................... 25

Hnh 1.10.b. Mt ct ca mt anode RTM- than ch ...................................................... 25

Hnh 1.11. Vt tiu thc v vt tiu hiu dng ca anode ............................................... 26

Hnh 1.12. Vng tiu im thc v tiu im hiu dng .................................................. 27

Hnh 1.13. Phng php chp nh qua l ngm xc nh kch thc tiu im ..... 28

Hnh 1.14. Anode s dng hai vt tiu ln nh ................................................................ 29

Hnh 1.15. nh hng ca gc nghing anode ln kch thc vt tiu hiu dng .... ..30

Hnh 1.16. S phn b khng ng u chm tia X theo phng song song vi trc

Cathode - Anode ..................................................................................................................... 31

Hnh 1.17. nh hng ca hiu ng chn ln khong cch t phim ............................ 31

Hnh 1.18. Bu thy tinh cha anode quay ....................................................................... 32

Hnh 1.19. Mt ct ng tia X loi c anode quay ca hng Shimadzu ........................... 33

Hnh 1.20. B lc hp th cc photon nng lng thp v cho cc photon nng

lng cao i qua ..................................................................................................................... 35

- 8 -

Hnh 1.21. Ph tia X to ra in p nh 150 kVp i vi anode lm bng

Vonfram. .................................................................................................................................. 36

Hnh 1.22. Cu trc bn trong h chun trc u n ..................................................... 39

Hnh 1.23. Mt ct ngang b chun trc loi R20-J ca hng Shimadzu ..................... 38

Hnh 1.24. Bc x hm pht ra khi electron tng tc vi ht nhn bia ........................ 40

Hnh 1.25. Electron va chm trc din vi ht nhn lm pht ra bc x hm c nng

lng cc i ........................................................................................................................... 41

Hnh 1.26. S phn b nng lng bc x hm gi tr in p nh 90 kVp (trong

trng hp khng c b lc (ng t nt) v c b lc tia (lin nt).......................... 41

Hnh 1.27. Tng tc lm pht ra bc x tia X c trng ............................................... 44

Hnh 1.28. Cc dy ph ng vi cc chuyn di electron trong nguyn t .................. 46

Hnh 1.29. Cc vch bc x c trng ng vi s dch chuyn trn nn bc x hm

i vi Vonfram in p 90kVp ...................................................................................... 47

Hnh 1.30. Cng pht x tia X thay i mnh theo gi tr kVp, khi gi cng

mt gi tr dng qua ng v thi gian chiu khng i ..................................................... 53

Hnh 1.31. nh hng ca mA ln hiu sut pht tia X ................................................... 54

Hnh 3.1. Quang cnh phng chp X quang thng quy bnh vin Nhi ng I ........ 79

Hnh 3.2. M phng 3D phng X quang Nhi ng I bng chng trnh MCNP5 ....... 80

Hnh 3.3. M phng cc lp ch trn, ch tng, ch p ca, knh ch,v tr ng pht

tia X bng MCNP5 ................................................................................................................. 80

Hnh 3.4. My X quang s dng bnh vin Nhi ng I ................................................ 81

Hnh 3.5. Kch thc cu hnh u bng pht tia X tnh theo mm (inch) ..................... 81

Hnh 3.6. Cu trc collimator loi R-20J ............................................................................ 82

Hnh 3.7. M hnh ng pht tia X v h thng Collimator ca hng Shimadzu ............ 82

Hnh 3.8. V ng chn khng v cu trc bn trong collimator c ............................ 83

- 9 -

Hnh 3.9. Anode v h thng ca s Bakelite, cc lp trong collimator ....................... 83

Hnh 3.10. Hnh chp cc v tr t cc cell kho st liu v bng MCNP5 .................. 84

Hnh 3.11. th sut liu (theo khong cch ngun my o) .................................... 86

Hnh 3.12. M phng ph tia X ti gi tr in p 100kV ................................................ 87

Hnh 3.13. M phng phn b sut liu ti v tr bn bnh nhn ch chp 70

kVp, 200 mA, 100 ms ............................................................................................................ 89

Hnh 3.14. M phng phn b sut liu mt (xy) ca ch chp ngc ....................... 91

Hnh 3.15. M phng phn b sut liu mt (xy) ca ch chp chn ....................... 92

Hnh 3.16. M phng phn b sut liu mt (xy) ca ch chp tay .......................... 92

Hnh 3.17. M phng phn b sut liu mt (xy) ng vi chp bng 75kV ................. 93

Hnh 3.18. M phng phn b sut liu mt (xy) ca ch chp bng 90kV ............ 94

Hnh 3.19. M phng phn b sut liu mt (xy) ca ch chp nh lng ............. 93

Hnh 3.20. M phng phn b sut liu mt (x,y) ca ch chp s ........................... 93

Hnh 3.21. M phng phn b sut liu mt (x,y) ca ch chp u gi .................. 94

Hnh 3.22. M phng phn b sut liu mt (x,y) ca ch chp ngc AP ............... 94

Hnh 3.23. Phn b sut liu cho khu vc phng chp d kin thu hp kch thc ..... 96

Hnh 3.24. Phn b sut liu trong phng chp khi gim kch thc ............................. 97

Hnh 3.25. M phng s suy gim sut liu khi i qua khu vc tng .......................... 98

Hnh 3.26. M phng s suy gim sut liu khi i qua ca bnh nhn v tng ........ 99

Hnh 3.27. S suy gim chm tia khi i qua tng phng X quang ............................. 100

Hnh 3.28. M phng phn b liu mt (x,y) trong phng chp st trn nh .............. 100

Hnh 3.29. M phng phn b sut liu (x,y) trong khu vc trn c lt ch ................ 101

Hnh 3.30. S suy gim sut liu khi i qua tng ngn cch hnh lang bnh vin ... 102

- 10 -

DANH MC CC BNG

Bng 1.1. Tiu chun bn ca du my bin p ........................................................... 34

Bng 1.2. Nng lng lin kt ca electron lp K ng vi mt s vt liu anode ........ 46

Bng 1.3. Bc sng ca cc loi sng in t ................................................................. 47

Bng 2.1. Hiu ng sinh hc theo mc liu .................................................................. 59

Bng 2.2. Gii hn liu chiu khuyn co ca ICRP ........................................................ 64

Bng 2.3. Liu gii hn trong mt nm .............................................................................. 65

Bng 2.4. Liu khuyn co cho mt phim chp X quang quy c i vi bnh nhn

(TCVN 6561:1999) ................................................................................................................ 66

Bng 2.5. Liu khuyn co chp, chiu X quang qui c cho 1 ln chp 1 phim ........ 67

Bng 2.6. Kch thc tiu chun cho phng t my X quang cc loi theo (TCVN

6561:1999) .............................................................................................................................. 68

Bng 3.1. Kt qu o sut liu theo khong cch .............................................................. 84

Bng 3.2. Gi tr sut liu m phng ti cc khong cch kho st v sai s thng k

tng i tng ng ............................................................................................................... 85

Bng 3.3. Cc ch chp ca phng X quang chn on thng quy ti bnh vin

nhi ng I ................................................................................................................................. 87

Bng 3.4. So snh gi tr sut liu gia cc ch chp .................................................. 90

Bng 3.5. So snh kt qu m phng v thc nghim cc nh tia X c trng ca

Vonfram ................................................................................................................................... 99

- 11 -

M U

T khi Wilhelm Conrad Rntgen pht hin ra tia X c th chn on cu trc

xng, tia X c pht trin s dng cho chp hnh y t nh nhng u th vt

tri hn v tnh hiu qu, chnh xc, nhanh chng hn cc phng php chn on

bnh trc y. Vic s dng tia X c bit hu dng trong vic xc nh bnh l

v xng, nhng cng c th gip ch tm ra cc bnh v phn mm. My chp X

quang ngy nay c ph bin rng ri tt c cc bnh vin, phng khm a khoa,

phng khm t nhn p ng nhu cu khm cha bnh ca ngi dn. Cng vi

s pht trin ca khoa hc k thut trong cc lnh vc vt l, in t, c kh, ha

sinh, cng ngh thng tinthit b chn on X quang ngy cng c ci tin tr

thnh phng php chn on u vit khng th thiu trong y hc hin i.

Ngy nay my chp X quang c ci tin v pht trin s dng trong cc

lnh vc khm cha bnh chuyn bit, ngoi X quang thng quy (chp hu ht cc

b phn trn c th) cn c X quang rng, X quang v, X quang chp mch, X

quang o long xng, X quang c tng sng truyn hnhMy X quang

thng quy loi c c tn s thp (na sng, c sng), ngy nay thng s dng cc

my X quang cao tn dng phim hay cao tn k thut s. Trn th trng c rt

nhiu loi my X quang t nhiu hng sn xut trn th gii, a dng v chng loi,

s lng cung cp ra th trng cng tng nhanh trong nhng nm gn y.

Trc s pht trin ca lnh vc X quang th vn an ton bc x trong

chp chiu phim X quang chn on y t cng thu ht s quan tm ca cc nh

khoa hc ni ring, ton x hi ni chung. V bn cnh nhng u th vt tri so

vi nhng phng php chn on y t trc y cc thit b pht tia X chn

on v iu tr bnh cng n cha nhng nguy him nu khng c bin php bo

v thch ng. Tnh nguy him ca my X quang tuy thp hn so vi cc ngun

phng x v mc nh hng ch mang tnh cc b, nht thi, d dng qun l v

khc phc nu c s c v thit b, nhng mc nh hng cng khng phi nh

i vi cng ng v cc c s y t l ni tp trung ng ngi c bit l nhng

- 12 -

ngi ang suy gim sc khe. Nguy c liu chiu trn bnh nhn cao hn mc cn

thit, liu bc x lt ra ngoi cao nu mc che chn khng m bo. iu ny

a ti nhng hu qu ht sc tai hi, gy nh hng xu cho sc khe cho cuc

sng ca chnh bc s v k thut vin vn hnh thit b chp X quang, cho bnh

nhn phi chp chiu v cho c nhn vin, dn chng ni chung trong khu vc tc

dng ca chm tia X pht ra t my pht tia X.

Trn th gii, cng tc bo v an ton bc x trong lnh vc y t c nhiu

t chc quan tm v thng xuyn xy dng cc tiu chun an ton che chn i

vi thit b X quang trong chn on y t v tiu ch ALARA (As Low As

Reasonably Achievable) tr thnh tiu ch hng u trong vic thit k cc phng X

quang. Hng nm, t chc NCRP (National Council on Radiation Protection and

Measurement) v ICRU u a ra cc khuyn co mi nhm bo v an ton bc x

cho mi trng, nhn vin y t v bnh nhn iu tr.

C quan nng lng nguyn t quc t (IAEA - International Atomic

Energy Agency) thng xuyn t chc cc lp o to v hi tho lin quan n

thit k che chn thit b chn on y t sao cho nhn vin lm vic v bnh nhn

chu mt mc liu cng thp cng tt nhng vn m bo hiu qu chn on v li

ch v kinh t. Cc ti liu k thut (TECDOC 1040) v cc n phm v an ton

trong chn on v x tr (Safety Guide for Radiotherary) ca IAEA nhn mnh cc

mc ch che chn:

- Gim liu chiu ti a vi nhn vin y t, bnh nhn v dn chng mc

thp nht c th.

- Nghin cu ti u ha kch thc phng X quang v iu kin che chn

hp l m bo an ton v hiu qu kinh t.

nc ta, t php lnh nm 1996 cho n thng t 05/2006/BKHCN ngy

10/01/2006 v vic hng dn th tc khai bo cp giy ng k, cp giy php cho

cc hp ng lin quan n bc x, thng t mi nht-thng t 08/2010/TT-

BKHCN ngy 22/07/2010 ca BKHCN hng dn v vic khai bo, cp giy php

tin hnh cng vic bc x & CCNVBX, cc c s bc x y t ni chung hay cc

- 13 -

phng chp X quang ni ring u phi tun th theo cc tiu chun v kch thc

phng X quang v iu kin che chn m bo an ton bc x cho mi trng

xung quanh, nhn vin y t v bnh nhn trc khi c cp php s dng.

V thc trng s dng my X quang trong chn on y t ti thnh ph H

Ch Minh t nm 1996 n nay c gn khong 504 c s bc x thuc y t v

khong 900 cc thit b X quang chn on hnh nh nh CT Scanner, X quang

thng quy, X quang di ng, X quang chp nha...Thng k trung bnh cc nm

2009-2010 [13] cho thy c n 53% phng X quang tun th theo quy nh v

kch thc phng phi c cp giy php. c bit 100% phng my X quang

chp nha khng tun th kch thc phng theo quy nh. Mt trong nhiu nguyn

nhn ca s vic ny l do nhiu phng X quang c xy dng theo tiu chun c,

khng cn thch hp vi cc thit b chn on X quang hin i dng k thut s

vi liu lng bc x pht ra khng cao, cc quy nh v kch thc phng do nh

sn xut cung cp khng khp vi kch thc phng theo TCVN, ngoi ra tiu

chun kch thc cho mi loi X quang cng thay i nhiu t nm ny qua nm

kia m khng c nhng c s khoa hc r rng km theo. iu gy kh khn cho

c quan c thm quyn trong vic cp php hot ng cho cc c s X quang. Thc

t ny t ra nhu cu cn phi tnh ton li din tch cc phng X-quang cho ph

hp vi cc thit b mi sao cho va m bo an ton bc x rt a dng trong cc

c s bc x t nhn va gim c chi ph u t c s vt cht.

Nguyn l ca vic tnh ton che chn an ton cho X quang chn on c cho

trong bo co ca NCRP49. D NCRP49 duy tr d liu v tiu chun che chn

trong nhiu nm, nhiu tc gi ngh nhng m hnh che chn khc thch hp

c th tnh ton bng chng trnh trn my tnh, c chnh xc cao hn v gim

bt tnh cng nhc vn c ca NCRP49.

Phng php lun trong NCRP tnh ton mc liu chiu ca tia s cp, tia tn

x v bc x r r t ngun tia X ring l, n khng cho thng tin lin quan n s

kt hp ca tt c cc thng lng bc x khc nhau xuyn qua lp che chn ngoi

tr a vo xp x HVL (lp gi tr mt na). Archer v cng s [21] chng

- 14 -

minh vic a vo cng nhc ny dn ti mc liu chiu hng tun ca cc nhn

vin gim hn gi tr yu cu theo MPD (gii hn liu cho php cc i) thit

lp. ci tin phng php lun trong NCRP, B.R.Archer a ra phng php

xc nh b dy th cp chnh xc cn cho vic p ng cc tiu ch thit k. iu

ny nhm gim thiu che chn qu ln ca NCRP-49.

Do vic tnh ton liu chiu xung quanh phng chp X quang nh gi

mc an ton bc x t c nhng ci tin kp thi c s trang thit b k thut,

ra nhng tiu chun trong thit k xy dng phng chp X quang va m

bo tnh an ton v tnh kinh t l vn ht sc cn thit. y cng khng phi l

vn mi m v trc y c nhiu phng php tnh ton khc nhau, ring

trong lun vn ny chng ti s s dng phng php m phng Monte Carlo vi

chng trnh MCNP nghin cu vn ny. Vic tin hnh m phng s cho

mt kt lun chnh xc trc khi thit lp cc tiu chun che chn bc x cho phng

X quang sao cho va m bo an ton bc x, va tit kim kinh ph xy dng.

Trc y c mt s lun vn (ng gio vin hng dn) thc hin tnh

ton phn b sut liu phng chp X quang bng chng trnh MCNP. Tuy nhin

do thiu nhiu thng tin v cu trc my X quang nn kt qu c c cha

chnh xc. Lun vn ny k tha v nghin cu su, rng thnh qu c c t cc

lun vn trc, trong n lc tm kim thng tin tht chi tit n mc c th v

cu to ca my X quang bnh vin Nhi ng I nhm t c m hnh X quang

chnh xc hn.

Vi mc ch trn ni dung lun vn c phn b trong 3 chng nh sau:

Chng 1 Cu to v nguyn l my pht tia X: trnh by nhng vn v

cu to chi tit my pht tia X s dng trong X quang chn on y t, s to thnh

v tnh cht ca tia X.

Chng 2 An ton bc x trong X quang chn on y t: trnh by v cc

hiu ng sinh hc, cc tn thng do bc x ion ha. Cc gii hn liu chiu x,

cc tiu chun an ton bc x ca th gii, TCVN6561 ca Vit Nam v mt s

yu cu an ton bc x i vi X quang chn on.

- 15 -

Chng 3 Gii thiu v phng php Monte Carlo, chng trnh MCNP v

ng dng chng trnh vo vic m phng phng X quang ti bnh vin Nhi ng

1; kho st ph tia X, kho st phn b liu trong v ngoi phng X quang, nh gi

an ton che chn, tho lun v nhng kt qu thu c.

- 16 -

Chng 1

CU TO V NGUYN L MY PHT TIA X

1.1. Cu to my pht X quang thng thng

Mt h thng chp nh tia X thng dng bao gm cc khi chc nng sau:

Hnh 1.1. S khi h thng chp nh X quang thng thng

Bn iu khin l ni vn hnh c th thay i cc thng s ca tia X (in

th, cng , thi gian pht tia). My to in th cao l ni cung cp mt hiu

in th cao cho cc in cc ca ng tia X gia tc chm in t. ng tia X l

ni to ra tia X bng hin tng pht bc x hm. H thng ghi nh c th l mt

tm phim tia X hay mt h thng bin hnh nh tia X thnh hnh nh m mt thng

c th nhn thy trn mt mn hnh.

Di y tc gi tp trung vo vn phn tch cu to v nguyn l hot

ng ca ng pht tia X.

1.1.1. Cu to ng pht tia X

Chc nng: ng pht tia X l thit b chuyn i nng lng in thnh hai

dng khc l nng lng tia X v nhit. Trong nhit to ra l kt qu khng

mong mun, do ng pht tia X c thit k sao cho lng nhit to ra l t nht

v tiu tn cng nhanh cng tt.

Nhng ng pht tia X th h u tin c ch to vo khong cui th k 19

bi William Crookes v cc cng s c gi l ng pht m cc ngui. Dng in

t c gii phng khi m cc nhm ion ha lng khng kh t i bn trong bung

ng tia X

(X ray tube)

Bn iu khin

(console)

My to in th cao

(generator)

H thng ghi nh Bnh nhn

- 17 -

cha thy tinh gii phng ng in electron t do, ri sau gia tc chng bng

ngun cao th (c 100kV), bn ph vo bia dng cc Platin t nghing v pht ra

tia X.

Nm 1913, William Coolidge ci tin ng pht tia X bng cch dng m

cc si t thay th cho m cc ngui t c cng sut pht cao hn. S thay

i ny chnh l nn tng cho s pht trin ca th h ng pht tia X ngy nay.

V nguyn tc, mi ng tia X u c cu to t 2 bn cc: mt bn cc

m (cathode) l mt dy tc c nung nng bng dng in sn sinh ra cc

electron v mt bn cc dng l ni cc electron p vo pht sinh ra bc x

hm. ng tia X c ht chn khng electron khng b mt nng lng do va

chm vi cc phn t kh khi i t cathode n anode.

Hnh 1.2. Nhng b phn c bn ca mt ng pht tia X thng thng [26]

ng pht tia X hin i l thit b gm 2 thnh phn chnh l m cc

(cathode), dng cc (anode) v cc b phn ph: ng c quay dng cc ( oto

tato ), v ng, hp cha, du tn nhit, cng giao tip.

ng pht tia X hin i t s dng dng cc tnh nh trong cc th h ca

Crookes v Coolidge m dng dng cc quay bng cm ng in t. Nh vy, ng

pht tia X hin i l s kt hp gia m cc ngui, dng cc quay v cc kt cu

- 18 -

thch hp khc. Tuy nhin, th h ng pht tia X Coolidge vi dng cc tnh vn

c dng trong mt s k thut X- quang khc vi yu cu c th, nh X- quang

nha khoa, X- quang cm tay, tng sng truyn hnh di ng.

Hnh 1.3. Cc b phn chnh ca ng tia X trong my chp X quang hin i [26]

1.1.1.1. m cc ( Cathode)

m cc (Cathode) l mt b phn c chc nng c bn l gii phng chm

in t v hi t chng thnh mt chm tia xc nh nhm vo dng cc (anode).

V cu to: mt m cc in hnh bao gm hai thnh phn chnh l dy tc

(si filament) v chn hi t (focusing cup). Dy tc lm bng nhng si Volfram

c hnh l xo xon ( tng din tch bc x in t ) thng ng c gn chm

vo trong mt chn hi t nh hnh 1.4.

Vt liu lm tim n thng l hp kim Vonfram v Thorium v Vonfram l

mt kim loi nng, dn nhit tt v chu c nhit cao (nhit nng chy cao

33700C), Thorium thng c thm vo si t Vonfram tng cng hiu sut

pht x electron v tng tui th tim n.

- 19 -

Hnh 1.4. Cu trc cathode ca ng tia X si t Volfram nm trong chn hi t

[26]

Phn ln bng X quang hin nay u c 2 h thng chn hi t v 2 si tc

nhm m rng cho nhiu ng dng ring bit. Si tc ln c kch thc 1,2mm

dng chp nhng c quan ln, dy cn cng sut cao, si tc nh c kch thc

khong 0,6mm s dng cho nhng b phn nh s dng cng sut thp. Hai si tc

ny c nung nng bng mch in vi hiu in th 2 u l 10V, dng in qua

si tc bng X quang thay i t 3 n 7A.

Chm electron pht ra t cathode c gia tc n p vo anode l chm

phn k, nu khng c s can thip th chm tia ny s p vo anode trn din tch

rng, iu ny s lm m hnh nh X quang. Do cn thit s dng b phn b

sung, c chc nng hi t chm electron thng n anode. in cc b sung c hnh

dng c bit bao quanh cathode gi vai tr ny chnh l chn hi t cn c gi

l in cc Wehnelt ( hnh 1.4).

in cc ny c t vo cng gi tr in th vi tim n v in trng

m n sinh ra c tc dng lm thay i cc ng ng th ca in trng, gii

hn s m rng chm electron khin chm electron hi t li trn 1 im c bit

trn anode (focal spot), chnh iu ny lm tng nt ca hnh nh X quang.

- 20 -

Hnh 1.5. Tc dng lm thay i hnh dng phn b chm electron ca chn ti t

Nu in cc Wehnelt c thm vo mt in th m chnh lch in th

vi tim n th tc dng cng r rt, kch thc b rng vt tiu cng nh. Chn hi

t lm thay i hnh dng phn b chm electron khi n c cng gi tr in p vi

si tc (Hnh 1.5.a). Chn hi t c tch in th m c gi tr lch khong 100 V

so vi si tc c tc dng gim b rng chm electron mnh hn (Hnh 1.5.b) [26].

1.1.1.2. Dng cc (anode)

Anode l ni tia X c to ra, n l mt bia hng electron bng kim loi c

cu trc cng v c mt phn t cao ni vi in cc dng. Anode c 2 chc

nng: (1) chuyn nng lng ca electron thnh bc x tia X v (2) lm tiu tn

lng nhit to ra trong qu trnh pht tia X, do vt liu anode c chn p

ng 2 yu cu trn.

V cu to: cc ng tia X cng sut b chng hn dng trong chp nh rng

c anode l mt bia c nh, cn cc ng tia X c cng sut cao thng c anode

dng a, c th quay quanh mt trc.

Hnh 1.5.b Hnh 1.5.a

i

n t

h

V

(so

v

i ch

n h

i t

- 21 -

A. Anode c nh

Bao gm 1 gng phn x bng Volfram l ni chm electron bn vo to

ra tia X, c gn cht vo mt khi ng ln. Khi ng c vai tr nng bia

Volfram v tiu tn nhit cho bia Volfram v ng c kh nng dn nhit tt.

Hnh 1.6. Cc thnh phn ca mt ng tia X c anode c nh gm bia Vonfram

gn vo mt khi ng [26]

Anode thng c t nghing mt gc t 150- 200 so vi mt phng thng

ng, gc nghing ny quyt nh din tch hi t ca chm electron (l phn din

tch ca anode b chm electron bn vo).

Nhc im ca loi anode ny l din tch bia b bn ph nh nn gii hn

s tiu tn nhit v do gii hn dng cc i qua ng v hn ch hiu sut pht

tia.

Loi bng ny dng trong cc my X quang cng sut nh nh my X quang

nha, my X quang di ng, my chiu (fluoro).

- 22 -

Hnh 1.7. Vt hi t ca bng X quang c anode c nh

B. Anode quay

a. Cu to v vt liu ca a anode quay

Cu to

Khi s dng Anode c nh th chm electron ch p vo 1 v tr c nh

lm nhit v tr ny rt cao ng thi lm gim cng sut ca bng v lm gim

tui th ca anode, do ngi ta thng s dng anode quay thay i lin tc

im tip xc ca anode v chm electron, lm cho din tch vt hi t rt b so vi

din tch gii nhit. Chuyn ng quay lm cho v tr p ca electron ln anode

thay i lin tc, iu ny lm cho anode mn u nn khng nh hng n gc

pht tia. Chuyn ng quay cn c tc dng tiu tn lng nhit sinh ra trong qu

trnh pht tia X lm tng cng sut bng, tng tui th anode. l l do m anode

quay vn c s dng ph bin trong chp X quang d i hi nhng thit k v

k thut phc tp.

Cc anode quay c hnh dng a c g vt nghing mt gc c tc dng

hng tia X l ra pha bin ca ng pht. ng phn gic ca gc vt phi nm

trong vng l ra ca tia X, gc vt c thit k sao cho ph hp vi mc ch s

dng. Gc vt cng nh th phn gii khng gian cng ln nhng li lm gim

din tch hiu dng ca tiu im v din tch bao ph ca vng tia X pht x.

- 23 -

Hnh 1.8. Hnh dng ca anode xoay

Anode vi gc vt nh ( 70 90) thch hp vi cc thit b thu nhn c nh

nh my chup X quang ng mch, chp dy thn kinh Cc my X quang thng

quy thng dng yu cu vng chp ln th thng s dng anode c gc vt ln (

120 -15

0)

. Kch thc ca a anode nm trong khong t 5cm 12,5cm v kch

thc ny quyt nh kh nng chu nhit ca anode [26].

Vt liu

Khi chm electron p vo anode ch di 1 nng lng ca chng chuyn

ha thnh tia X, cn li hn 99 nng lng ca chm in t c chuyn ha

thnh nhit nn anode phi c kh nng chu nhit cao. Hu ht cc ng tia X u s

dng Volfram ( 4) lm vt liu anode v Volfram c s hiu nguyn t ln, kh

nng chu nhit cao do c im nng chy cao, kh nng bay hi rt thp. Trong

nhiu nm Volfram tinh khit thng c s dng lm vt liu anode.

Cng ngh anode quay ngy cng phc tp. Anode quay Vonfram th c

sn xut trn c s chun luyn kim bt. Hn ch ca cng ngh ny l s mi mn

b mt do s nn c nhit cao.

- 24 -

Hnh 1.9. Cu to anode xoay [26]

Hiu ng ny dn n s st gim nhanh chng cng tia X v gy nn

ci gi l hiu ng mt nghing. y, mt phn bc x trong nhng lp su hn

ca anode b hp th trc khi ri khi anode. Thm vo l s bin dng c hc

xy ra c th ph hy anode.

Trong nhng nm gn y hp kim ca Volfram v Reni c s dng lm

vt liu b mt anode. Vic s dng mt hp kim Vonfram Reni cho php s dng

ng pht tia X trong khong thi gian di. Cng ngh nng cao da trn nguyn liu

hn hp Reni -Vofram-Molyden (RTM). S ct ngang ca anode RTM biu din

trong hnh 1.10.a. RTM anode l loi anode c b mt l lp hp kim Reni-

Vonfram dy khong 1,3mm, tip theo l lp molybdenum dy 5-11mm (mt s

hng sn xut s dng Graphit thay cho Molybdenum). Hp kim Reni-Vonfram

thng c t l khi lng ty thuc vo mi hng sn xut thng l W-Re 3%, W-

Re 5%, W-Re 25%, W-Re 26% [28]. My X quang kho st trong lun vn ca

hng Shimadzu s dng loi anode RTM ny.

Mt gii php ti u iu khin kh nng tch tr nhit v bc x nhit c

th t c trong mt anode gii ni vi tm than ch (hnh 1.10.b). Vi anode

- 25 -

nhit, tm RTM c hn vo cht ti nhit than ch, ng c sn xut vi trng

ti ln hn 3.106 J hot ng ti cng sut nhit trung bnh 4 kW.

Hnh 1.10.a. Mt ct ca mt anode RTM

Hnh 1.10.b. Mt ct ca mt anode RTM- than ch [22]

Do Volfram vi nguyn t khi ln cho t l bc x hm cao hn so vi

Molybdenum v Rhodium nn trong vy trong X-quang v (Mammography) anode

thng lm bng bng cc nguyn t c nguyn t khi nh. Do yu cu ghi nhn

hnh nh cc m dy c ca tuyn v cn nhiu bc x tia X c trng. Hu ht

cc ng tia X s dng chp nh nh c b mt anode c ph Molypden l loi c

kh nng to ra tia X c nng lng thch hp cho ng dng ny. Mt s ng chp

nh nh c anode th 2 lm bng Rhodium ( 45), loi ny to ra bc x c nng

lng cao, kh nng m xuyn tt c th s dng ghi li cu trc dy c ca

tuyn v.

b. Vnh tiu im Vng tiu im thc Vng tiu im hiu dng

Khng phi ton b anode u tham gia vo vic to ra tia X m bc x ch

c to ra mt vng rt nh trn anode. Vng ny c gi l vt tiu ( focal

- 26 -

spot) c kch thc xc nh bi kch thc chm electron t cathode. T dn

n cc khi nim vnh tiu im, vng tiu im thc v vng tiu im hiu

dng.

Din tch vnh tiu im l khi nim m t phn din tch nghing trn bia

anode va chm vi chm electron. Vnh tiu im c bn knh trung bnh r, chu vi

trung bnh 2r v rng vnh r, din tch c tnh theo cng thc: S=2rr .

Nh vy, mt anode quay c din tch vnh tiu im ln hn khong 314 ln din

tch mt vng tiu im kch thc tng ng ca anode tnh.

Hnh 1.11. Vt tiu thc v vt tiu hiu dng ca anode [26]

Vng tiu im thc l vng hnh ch nht trn mt vt anode m chm

electron bn ph, kch thc vng ny ph thuc vo di ca si t v rng

ca hp iu tiu trn cathode. Trong hu ht cc ng tia X vt tiu ny c dng

hnh ch nht, kch thc thng nm trong khong t 0,1mm n 2 mm, kch

thc ca n nh hng n kh nng ti nhit ca anode. Vt tiu thc cng ln th

kh nng ti nhit cng tt v ngc li. Kch thc vt tiu thc quyt nh bi ba

yu t: kch thc v hnh dng si tc (filament), kch thc hnh dng ca chn

hi t, gc nghing anode.

- 27 -

Hnh 1.12. Vng tiu im thc v tiu im hiu dng

Vng tiu im thc ch mang ngha hnh hc, chng ta quan tm nhiu hn n

vng tiu im hiu dng.

rng vng tiu im hiu dng chnh bng rng ca chm in t v

rng vng tiu im thc. di vng tiu im hiu dng li ph thuc vo

gc vt dng cc v di vng tiu im thc, c tnh theo cng thc sau:

a = A sin (1.1)

b = B

Trong , A, B v a, b ln lt l di, rng ca vng tiu im thc v

vng tiu im hiu dng, l gc vt anode. Hnh chiu ca vt tiu thc ln mt

phng hng tia gi l vt tiu hiu dng. Kch thc vt tiu hiu dng l nhn t

quan trng quyt nh r nt ca phim, kch thc cng nh th cng r nt.

Trong thc nghim, vng tiu im hiu dng c th o c c bng

phng php chp nh qua l ngm.

Mt tm cn quang vi l ngm c ng knh c m c t vung gc

v nm gia ng pht tia X v phim hoc h thu nhn theo m hnh hnh 1.13, vi

d1, d2 ln lt l khong cch t l ngm n anode v t l ngm n phim, a,b v

a, b ln lt l kch thc vng tiu im hiu dng v kch thc nh trn phim.

o c cc kch thc d1, d2, a, b, ta tnh c a, b qua cng thc ng dng:

- 28 -

a a x (d1 / d2) (1.2)

b b x (d1 / d2) (1.3)

Hnh 1.13. Phng php chp nh qua l ngm xc nh kch thc tiu im

hiu dng

Kch thc vt tiu l mt trong nhng yu t cn xem xt khi la chn ng

pht tia X cho mt ng dng c th. Loi c kch thc nh s dng chp nhng

vng c kch thc nh, c kh nng hin th chi tit hnh nh vi lng bc x

tng i thp nh chp nh nh.

Vi cng kch thc vt tiu thc gc nghing anode cng nh th kch thc

vt tiu hiu dng cng nh dn n phn gii khng gian tt. Tuy nhin, gc

nghing nh li gii hn kch thc chm tia X hu dng nn trng bao ph chm

tia b thu hp u pht tia. Gc nghing ti u nht ph thuc vo trang thit b

ghi nhn hnh nh ca phng chp X quang. Mt s loi anode c thit k vi 2

gc nghing tng ng to ra 2 vt tiu hiu dng kch thc khc nhau.

Gc nghing anode cng nh hng n din tch bao ph ca chm bc x.

Khi gim gc nghing th din tch bao ph chm tia cng gim v d bia nghing

100 th c khu vc bao ph chm tia hp hn bia c gc nghing 200 tnh ti cng

mt khong cch tnh t ng pht.

- 29 -

Hnh 1.14. Anode s dng hai vt tiu ln nh [28]

Din tch bao ph chm tia thng khng gy nh hng n kt qu chp

nhng vi nhng bia anode c gc nghing nh th c th gy mt s vn v s

bao ph chm tia cn thit ln phim. V d vi anode c gc nghing 120 v khong

cch t phim l 40 inch th din tch bao ph c bn knh 8 inch, trong khi vi

anode nghing 100 cng khong cch t phim nh trn th din tch bao ph ch

c bn knh 7 inch. R rng l c 2 bia trn u khng th ph ht phim c kch

thc 14 x 17 ti khong cch 40 inch v phim ny i hi bn knh bao ph l

11inch.

- 30 -

Hnh 1.15. nh hng ca gc nghing anode ln kch thc vt tiu hiu dng [32]

Do vy xc nh s bao ph chm tia thch hp ta s dng cng thc tnh

bn knh che ph nh sau: tan ca gc nghing anode nhn vi khang cch t

phim (FFD) bng bn knh din tch khu vc che ph chm tia (RC):

tan *FFD = RC (1.4)

V d: nu gc nghing l 200 v FFD l 40 inch, bn knh khu vc che ph

chm tia s l 14 inch v RC = tan200 x 40 inch = 14 inch.

c. Hiu ng chn (Heel effect)

V anode c b tr nghing gc nn cng chm tia X pht ra dc theo

trc ng tia X s khc nhau, tc l c s phn b khng ng u dc theo hng

song song vi trc Cathode Anode. nh hng ny c gi l hiu ng chn

(Heel effect). S khc nhau ny l do bia anode hp th chnh photon m n pht ra

[26]. Quan st trong hnh 1.16 ta thy cc photon c gii phng ra t mt im

nm bn trong bia anode. T hnh v ta thy cc photon pht ra nm pha gn

cathode c ng i bn trong bia anode ngn hn cc photon nm pha cha

anode. Qung ng i trong bia di hn khin cc photon ny c khn nng b hp

th cao hn. l l do cng chm tia X pha gn cathode mnh hn cng

chm tia X nm pha anode.

- 31 -

Hnh 1.16. S phn b khng ng u chm tia X theo phng song song vi trc

Cathode- Anode

Hnh 1.17 cho thy t l phn trm cng bc x ng vi cc gc pht x

khc nhau gy ra do hiu ng gc nghing khi bia anode nghing gc 200. y

cn lu s nh hng ca khong cch t phim ln s thay i cng chm

tia. Nu phim X quang dng chp b phn nh s 1 l phim 1a th cng chm

tia ko di t 95 n 104%, khc bit nhau 9% v s khc bit ny l khng ng

k. Hay ni cch khc mt chm tia kh ng u. Nu phim X quang chp b

phn nh s 2 s dng phim 2a, cng chm tia s ko di t 31 n 95%,

khc bit 64 v iu ny l khng th chp nhn trong k thut chp X quang.

Nh vy ta c th thy khong cch t phim hay khu vc s dng chm tia cn

xem xt trong nh hng ca anode heel effect.

Hnh 1.17. nh hng ca hiu ng chn ln khong cch t phim

- 32 -

1.1.1.3. V ng chn khng, du tn nhit, v kim loi, ca s ng pht

tia X

a. V ng chn khng

V chn khng thng lm bng thy tinh bao bc cathode, anode, rotor to

thnh mt khng gian bn trong gn nh chn khng, p sut khong di 10-

5mmHg. Thy tinh lm v ng thng bng thy tinh loi borosilicate l loi knh

chng nhit c t nht 5% oxit boron (B2O3) thng dng lm knh ha hc a chu

nhit do c bn cao, h s n v nhit thp, im nng chy cao. Nhng c tnh

trn ca thy tinh borosilicat thch hp cho vic s dng lm v ng chn khng.

Hnh 1.18. Bu thy tinh cha anode quay [28]

B dy ca bu thy tinh nm trong khong t 1-2mm ty theo my ca mi

hng, mt s hng ch to loi thy tinh borisilicat c pha ch tr phn thy tinh

li ra nhm ngn chm tia X i ra theo cc hng khc.

Chc nng chnh ca v chn khng ny l h tr v cch in cho anode,

cathode ng thi duy tr mi trng chn khng bn trong ng pht tia X. S c

mt ca kh trong ng pht tia X s cho php dng in t truyn qua ng mt cch

t do hn l to thnh chm in t. iu ny s gy tr ngi trong vic to ra bc

x v c th gy h hi mch in. Mi trng chn khng cn c tc dng ngn

cn qu trnh oxy ha cc linh kin t bn trong c bit l dy tc.

- 33 -

Theo thi gian s dng s c s xm nhp ca cc phn t kh lm gim

chn khng ca ng, khi cn kim tra v ht chn khng trong ng. Dung tch v

ph thuc vo ng tia X s dng, v chn khng v nhng b phn cha trong n

c xem nh phn ring l t vo trong ng pht tia X, y l phn c thi gian

s dng hn ch v c th thay th ngay bn trong khoang cha (tube housing).

Phn ln cc ng tia X c v chn khng lm bng thy tinh, mc d c th s dng

cc loi vt liu khc nh kim loi, gm s.

Hnh 1.19. Mt ct ng tia X loi c anode quay ca hng Shimadzu

b. Du tn nhit

Vng khng gian gia v kim loi (tube housing) v bu thy tinh chn

khng c y mt loi du c bit xem nh vt liu cch ly bu thy tinh vi

ngun cao th v ti nhit t bu thy tinh ra bn ngoi, du ny cn c gi l

du bin p hay du tn nhit. Du bin p c hai nhim v chnh l:

- Chim ch khng kh trong cc thit b in p cao v lm nhim v cch

in, tng bn cch in ln rt nhiu.

- Lm mt, tng cng thot nhit do tn hao cng sut trong dy qun v

trong li thp my bin p sinh ra.

Du bin p l cht lng d chy nn nhit chp chy ca du bin p quy

nh khng c thp hn +1350C. Nhit ng c ca du khng c cao hn

- 34 -

-450C. Mt trong nhng c tnh quan trng c nhiu ngha thc tin l bn

ca du bin p. Tr s bn ca du bin p rt nhy cm vi m ca du.

bn in ca du cn gim nhiu hn nu nh trong du c nhng tp cht, n s

lm cu ni cho s phng in sm pht trin.

Mt tip xc gia lp du v lp v Ch c qut mt loi vt liu c bit

c nhim v gi cho lp Ch khng b oxi ha.

Bung cha du c b sung mt h co gin m bo p sut du khng

tng ln khi du gin n do nhit. Hp cha c tip mt vi t ngn in t

chuyn ng trong khng gian cha du. Bng 1.1. trnh by mt s tiu chun

bn ca du my bin p.

Bng 1.1. Tiu chun bn ca du my bin p

i vi thit b c in p lm

vic (kV)

in p phng in ca du khng nh hn

kV/2.5mm

Du mi Du c

6 25 20

35 30 25

110-220 40 35

330 50 45

Trong qu trnh s dng du bin p b gi ha, cc tnh cht ca du b

gim, mu ca du tr nn sm hn do khng kh lt vo gy hin tng oxy ha

du, s tip xc gia du v mt s kim loi, tc dng ca nh sng v cng t

trng cao. Do vy trong qu trnh bo dng my X quang cc k s s kim tra

cht lng du thay th, b sung lng du ph hp.

c. V kim loi

Hp cha (housing) ca ng pht tia X l mt khoang cha bng kim loi

ng dung dch du, ng vai tr trong vic gia tng s ng kn cng nh h tr

bo v ng chn khng khi mi trng bn ngoi. N c chc nng nh mt l

chn v hp th cc bc x pht ra tr bc x hu dng. N c b mt bn ngoi

- 35 -

tng i ln lm tiu tn hu ht nhit to ra trong ng. Lp ch bc ngoi khoang

cha nhm hp th tia l bt thng bao gm tia X chch hng t anode, tia X

sinh ra khi in t th cp va chm vi cc linh kin kim loi khc trong ng pht.

Bn ngoi lp ch l lp thp khng r hoc barit c b dy khc nhau ty theo thit

k ca tng hng.

d. Ca s ng

Chm bc x tia X pht ra t ng qua mt cng hoc ca s. Gc khi ca

chm bc x hnh nn thng l 400 500. Ca s thng c ch to bng mt

vt liu hp th bc x thp nh l cc kim loi nh c nguyn t s thp (chng

hn nh l beryllium). Mt s loi bng X quang cn s dng ca s lm bng nha

Bakelite. Ngay bn di ca s trong vng chm tia hiu dng l mt mn chn,

qu trnh m mn che chn cho php thay i c kch thc hiu dng ca chm

tia.

1.1.2. B lc tia

B lc tia X uc s dng nhm loi b nhng tia X nng lung thp v to

chm tia X c nng lng ng u hn. Nhng tia X c nng lng thp trong ph

tia X to ra rt nhiu liu cho bnh nhn nhng khng ng gp vo tn hiu thu

nhn. Bi vy vic lc b nhng tia X ny l cn thit, do trong cc my X

quang lun c b phn lc tia.

Hnh 1.20. B lc hp th cc photon nng lng thp v cho cc photon nng

lng cao i qua

- 36 -

C 2 b phn tham gia vo vic lc tia l b lc sn c (inherent filter) v b

lc b sung (additional filter). B lc sn c to nn do cc vt liu c sn trong

ng tia X nm trn li ra chm tia c chc nng lc tia gm v thy tinh, du cao

p, ca s bakelite. Mi hng c tiu chun b dy vt liu v ng khc nhau nn

b lc sn c c b dy tng ng 0,5mm-1mm Al.

B phn th hai l mt tm kp nhm ng t sau u pht tia X c b

dy cc tiu d ph thuc vo in th cc i s dng trong ng pht tia X (nhng

thng thng d 2,5mm i vi in p t 70kV tr ln). B lc ny loi b

nhng tia X c th xuyn qua b lc sn c nhng nng lng khng ln

ng gp vo vic to nh hiu qu trn phim. B lc c th loi b hiu qu nhng

tia X nng lng thp nhng ng thi cng i hi cng sut tia X cao hn. [11]

Hnh 1.21. Ph tia X to ra in p nh 150 kVp i vi anode lm bng

Vonfram.

Cc tia X nng lng thp (ng t nt) b hp th nh b lc tia

Mt dng ci tin khc l b lc cnh cung, b lc ny c kh nng lm gim

mnh nhng bc x vng bin m khng gy nh hng n vng gia. Do vy

b lc cnh cung c th hn ch cng ca chm bc x tn sc xut hin

vng bin ca vt th v lm gim liu chiu trn bnh nhn [11].

1.1.3. H chun trc u n (Collimator)

Chm tia X i ra t ng pht l chm tia phn k do khi n c th bnh

nhn mc ph ca chm tia s ln hn gi tr cn thit cho vic ghi nhn hnh

- 37 -

nh (FOV field of view). iu ny dn n 2 vn ny sinh: th nht l tng liu

chiu trn ngi bnh khng cn thit, th hai l lm tng nh hng photon tia X

tn x Compton ln phim.[26]

thay i kch thc hnh hc ca chm tia ngi ta s dng h chun trc

u n (Collimator) l b phn gm nhiu tm ch c th trt ln nhau dng

gii hn chm tia t gia vng pht tia v c th bnh nhn.

Hnh 1.22. Cu trc bn trong h chun trc u n [26]

H chun trc ca ng tia X c ni vi u ra ng tia X nh mt khp

xoay cho php thay i v tr ca n. Mt ngun sng kh kin c b tr bn trong

h chun trc xem nh mt vt tiu o, pht ra nh sng i n gng phn x t

nghing gc 450 ( hnh1.22) nh sng phn x t gng i ra khi b chun trc v

cho mt trng che ph trng vi trng che ph ca tia X. Nh quan st trng

chiu ca nh sng kh kin ny ta c th bit c kch thc trng chiu tia X

c nhng iu chnh thch hp. Trng chiu tia c xc nh nh cc tm ch

sp xp nh hnh 1.22.

Thng thng c 2 h thng chun trc u n. t gn u pht tia X hn

l h chun trc u n dng c nh, c tc dng lm gim hn lon ca tia X

c to ra v gii hn vng chiu hnh hc. H thng chun trc th hai t ngay

- 38 -

sau h th nht c tc dng lm gim liu chiu cho bnh nhn c gi l h chun

trc dng iu chnh c.

Ty theo tiu chun ca mi hng m cu trc b dy ca cc lp ch trong

b chun trc s c s khc nhau. My X quang kho st bnh vin nhi ng s

dng b chun trc loi R20-J ca hng Shimadzu c cu to nh hnh 1.23. gm 3

lp ch. [30]

Lp ch pha trc ( front leaf) gm 4 l ch di ng b dy 3mm, nm mt

di collimator ng vai tr chnh trong vic xc nh kch thc trng chiu. Lp

ch gia (middle leaf) gm 4 l ch di ng b dy 2mm ng vai tr loi b cc

bc x r r. Lp ch trong cng (inner leaf) gm 4 l ch c nh b dy 2mm c

vai tr lm gim bc x off focus (bc x to ra khi electron p vo anode ngoi

v tr tiu im). Gia lp ch gia v ch trong cng c mt tm Al (0,5mm) ng

vai tr l b lc b sung. Bn ngoi v collimator c nm iu khin thay i

khang cch gia cc l ch thuc lp ch trc v ch gia nhm thay i kch

thc trng chiu.

Hnh 1.23. Mt ct ngang b chun trc loi R20-J ca hng Shimadzu [30]

Trong mt s my X quang hin i c h chun trc ti u l h chun trc

c kh nng t ng gii hn kch thc trng chiu nm trong din tch n nhn

ca detector. B cm bin c hc nm trong gi casset cha phim s d tm kch

- 39 -

thc ca casset v t ng iu chnh cc l ch trong h chun trc sao cho trng

chiu tia trng khp vi kch thc ca casset cha phim.

Bn cnh h chun trc u n, mt h chun trc khc cng c s dng

t ngay sau bnh nhn c tc dng hp th nhng tia tn x trn bnh nhn, gim

nhiu v xo nh cho h thng.

Mt s b phn khc c b sung trong cu trc t hp ng pht nhm thc

hin mt s chc nng khc trong qu trnh pht tia X. Cm bin nhit dng

dng hot ng ca ng pht khi nhit du v bung cha vt ngng cho

php.

1.2. Nguyn l ca qu trnh pht tia X

1.2.1. Nguyn l to tia X

Khi cho mt chm electron c vn tc kh ln p vo anode lm bng kim

loi nng (Vonfram), tng tc ny lm pht ra bc x tia X.

C 2 dng tng tc to ra tia X, dng th nht l khi electron tng tc vi

lp v electron ca nguyn t kch thch s chuyn mc nng lng. Trng thi

kch thch c nng lng Ei pht ra cc tia X dng ph vch trong s chuyn mc Ei

Ek c bc sng ik c trng cho vt liu lm anode. Cc tia X ny v vy cn

c gi l cc tia X c trng. Dng th hai bc x tia X c ph nng lng lin

tc cn gi l bc x hm khi electron tng tc trc tip vi trng in t ca

ht nhn nguyn t.

1.2.1.1. Bc x hm

Khi cc electron mang nng lng i qua vt liu c nguyn t s (Z) ln b

lm lch hng di tc dng ca trng th Coulomb ca ht nhn mang in tch

dng. Theo l thuyt in ng lc hc, cc ht mang in khi tng tc hay gim

tc s pht ra bc x vi nng lng t l vi bnh phng ca gia tc. Cc electron

ny pht ra mt bc x c ph lin tc trn mt di rng vi s phn b cng

ph thuc vo nng lng ca electron ti. Bc x pht ra nh vy c gi l

Bremsstrahlung theo ting c c ngha l bc x hm hay bc x do lm chm cc

ht mang in.

- 40 -

Mt electron trc khi dng li c th pht ra nhiu photon, nng lng pht

ra ph thuc vo khong cch tng tc gia electron v ht nhn, nng lng

gim khi tng khong cch. Trong hnh 1.24 electron s 2 tng tc vi ht nhn

khong cch gn hn electron s 3 nn bc x hm pht ra i vi tng tc ca

electron s 2 ln hn i vi electron s 3. R rng l photon vi nng lng cc

i c to ra khi electron mt ht ng nng trong mt qu trnh tng tc duy

nht, trong hnh 1.24 l electron s 1. Trng hp ny xy ra khi electron tin ti

rt gn vi ht nhn va chm vi ht nhn v nh mt tt c ng nng ca mnh.

Photon pht ra c nng lng chnh bng ng nng ca electron, tc nu electron

c ng nng 100 keV th photon pht ra s c nng lng ng bng 100 keV [26].

Xc sut electron va chm trc din vi ht nhn th cc k him, v

thang o nguyn t, trong mt nguyn t vng khng gian trng chim phn ln,

tit din ht nhn th cc nh. Do tia X nng lng thp c to ra nhiu hn,

v s lng tia X nng lng cao gim gn nh tuyn tnh vi gi tr nng lng

ca electron ti.

Hnh 1.24. Bc x hm pht ra khi electron tng tc vi ht nhn bia

- 41 -

Hnh 1.25. Electron va chm trc din vi ht nhn lm pht ra bc x hm c nng

lng cc i

Ph bc x hm m t s phn b ca photon tia X nh l mt hm ca nng

lng. Ph bc x hm sau khi cha qua b phn lc tia (hnh 1.25.a) c dng dc

theo mi quan h gia s lng v nng lng tia X to ra, nng lng cc i

c quyt nh bi gi tr in p nh (kVp) t vo ng tia X. Khi c b lc tia

th cc tia X nng lng thp b hp th, do ph tia X lc ny c dng nh hnh

1.25b.

Hnh 1.26. S phn b nng lng bc x hm gi tr in p nh 90 kVp (trong

trng hp khng c b lc (ng t nt) v c b lc tia (lin nt)

Nhn t chnh nh hng n vic pht x tia X bao gm: s nguyn t ca

vt liu bia, ng nng ca chm electron ti (quy nh bi in th gia tc). T s

Photon c nng lng cc i

- 42 -

gn ng ca nng lng mt i cho vic pht bc x hm v nng lng mt i

trong cc va chm l:[26]

000.820

ZE

E

E K

L

R (1.5)

Trong : ER: Nng lng electron dng vo vic pht bc x,

EL: Nng lng electron mt i trong va chm,

EK : ng nng ca electron tnh theo keV,

Z: S hiu nguyn t ca vt liu anode.

V d: i vi chm electron nng lng 100 keV n p vo bia Vonfram

(Z=74), t s xp x ca nng lng bc x v nng lng mt khi va chm l:

%9,0000.820

74,100 , iu c ngha l c hn 99 nng lng ca electron chuyn

thnh nhit. i vi electron c nng lng 6 MeV, t l ny l 54%, tc lng

nhit khng cn thit to ra nh hn. Nh vy, khi nng lng ca electron tng ln

th hiu sut pht tia cng tng ln.

in t sau khi vt qua c hiu in th U s c nng lng eU. Khi va

chm vi nguyn t, mt phn nng lng c bc x ra di dng phtn, mt

phn nng lng truyn cho nguyn t, ch yu bin thnh nhit nng [5].

Nh vy: eU = hf + 4.T Q (1.6)

Trong :

hf l nng lng chuyn ha tia X

4.T Q l nhit nng khng mong mun

: h s nhit anode

Q: din tch anode

: hng s

T: nhit anode

Tn s cc i c xc nh t iu kin cho rng: mt electron p vo

cathode s pht ra mt lng t, v ton b ng nng ca electron dng to

thnh lng t. Lc ta c:

- 43 -

U.ef.h max

U.e

hcmin

Vi: Hng s Plank h =6,625.10-24 J.s

in tch electron e = 1,6.10-19C

Vn tc nh sng c =3.108m/s

Thay cc gi tr trn vo cng thc ta c: nm]V[U

5,1234min

1.2.1.2. Bc x c trng

Nu tng in p gia tc U ln na th n mt in p ti hn no th trn

phng ca ph lin tc s xut hin bc x c vch nh nhn. Tn s ca vch nh

nhn ch ph thuc vo in p gia tc U v hon ton c trng cho vt liu lm

anode. Do , bc x ny c gi l bc x c trng v nng lng ca n c

trng cho lp electron trong nguyn t. Lc cng c ngha l nng lng ca

in t gia tc bng hay ln hn mt gi tr xc nh c trng trong cht lm i

m cc.

C ch to ra ph tia X c trng, c th hnh dung nh sau: trong s cc

electron p vo anode b hm li, c nhng electron nng lng ln c th xuyn

su vo bn trong nguyn t, va chm vi electron lp trong cng ca nguyn t

lm cho electron ny bt ra khi nguyn t. Ch trng m electron va ri khi

nguyn t b cc electron lp cao hn xung chim ch. Qu trnh chim ch din

ra lin tip nhau km theo pht bc x photon .h .

Trng hp lp K b nh bt i 1 electron li mt ch trng lp tc t

cc lp bn ngai L,M,N electron s chuyn xung chim ch trng lp K s lm

xut hin cc vch ph K,K,K ... Tng t nh vy nu lp L b nh bt i 1

electron li mt ch trng, th t cc lp v ngoi M,N,O... electron chuyn

xung chim ch km theo pht x cc vch ph ,...L,L,L v tng t nh vy

khi electron lp ngai chuyn xung ch trng lp M cho cc vch ph

,...M,M,M

(1.7)

(1.8)

(1.9)

- 44 -

Hnh 1.27. Tng tc lm pht ra bc x tia X c trng [26]

Trn hnh 1.27 m t nguyn t Vonfram vi cc lp v K,L,M. Nng lng

lin kt ca electron trong nguyn t Vonfram cc lp K, L, M ln lt l 69,5

keV, 12 keV, 2 keV. Electron ti va chm vi mt electron lp K v lm bt

electron ra khi qu o, sau c 2 electron bay ra khi nguyn t. Khi c s th

ch cc electron s lm pht x ra 2 photon c nng lng l 57,5 keV (y l nng

lng chnh lch gia lp K v lp L).

Mt khc electron lp v ngoi M li chuyn xung th ch trng ca

electron lp L v lm pht ra photon c nng lng v 10 keV (phn nng lng

chnh lch gia lp L v M). Nh vy, nng lng tia X pht ra c trng cho mc

nng lng ca cc lp electron trong nguyn t Vonfram.

xc nh bc sng tia X c trng, ta vn dng biu thc nng lng

lin kt ca in t trong nguyn t phc tp (Z>1). V ch c nguyn t phc tp

mi cho ph tia X (tc l ch c cc nguyn t vi Z ln).[4]

- 45 -

2*

2l,n)Z(

n

RhcE

Vi e)aZ(eZ* gi l in tch trung bnh hiu dng, a l h s mn chn

tnh in tch m i vi ht nhn. H s a ny thay i ty theo tng lp in t.

i vi lp K th h s a =1, vy khi in t chuyn t lp L vo ch trng lp K s

cho vch ph K tng ng vi tn s:

22

22

KL )1Z.(RC4

3)1Z)(

2

1

1

1(C.R

h

EE

Trong : R l hng s Ritbe; C l vn tc nh sng; Z l nguyn t s ca

nguyn t ha hc ch to nn cathode.

Biu thc trn gi l nh lut Moseley, cng thc ny ng vi cc nguyn

t c nguyn t s Z>30 ( tc l nhng nguyn t t trung bnh n cc nguyn t

nng). Mi dy ph u c vch u v vch cui gi l vch ranh gii. Vch u l

M,L,K , vch cui l M,L,K ... (hnh 1.28).

Bc x c trng cn thit trong X quang chn on l bc x pht ra do s

dch chuyn electron t cc lp L,M,N... (thm ch electron bn ngoi nguyn t)

v lp y l trng lp K. Nu gi ch s biu th lp v k cn lp K, v l

lp v khng k cn v cc dch chuyn tng ng l K v K th tia X ng vi

K mang nng lng ln hn. i vi mi lp v, c nhng vch nng lng ri

rc ca cc lp v con l kt qu ca s tch nng lng tinh t ca tia X c trng.

i vi Vonfram, c 3 vch ni bt trn nn ph lin tc l 121 K,K,K nh trong

hnh 1.28.

(1.10)

(1.11)

- 46 -

Hnh 1.28. Cc dy ph ng vi cc chuyn di electron trong nguyn t [4]

Nhng vch bc x c trng khng pht ra bi s dch chuyn v lp K th

khng quan trng trong chp X quang chn on v chng gn nh b suy gim khi

i qua ca s ng pht tia v b lc. Bng 1.2 lit k cc gi tr nng lng lin kt

ca cc electron lp K tng ng vi nng lng bc x c trng ca lp K ng

vi mt s vt liu anode thng dng [26].

Bng 1.2. Nng lng lin kt ca electron lp K ng vi mt s vt liu

anode

Loi dch chuyn Vonfram Molybden Rodi

1K 59,32 17,48 20,22

2K 57,98 17,37 20,07

1K 67,24 19,61 22,72

Bc x tia X c trng ng vi lp K ch pht x khi electron ti anode c

nng lng ln hn nng lng lin kt ca electron lp K, tc in th gia tc

phi ln hn 69,5 kVp i vi bia Vonfram, ln hn 20 kVp i vi bia Molybden.

S lng tia X c trng so vi tia X bc x hm tng theo nng lng electron ti,

v d vi in p nh 80 kVp, c khong 5% tng s tia X pht ra l bc x c

- 47 -

trng, vi in p 100 kVp th t l ny tng ln 10 . Vic to ra tia X c trng

hu nh l kt qu ca tng tc gia electron-electron, tuy nhin tng tc gia

bc x hm v electron thng qua hiu ng quang in cng c th lm pht ra tia X

c trng.

Hnh 1.29. Cc vch bc x c trng ng vi s dch chuyn K v K trn nn

bc x hm i vi Vonfram in p 90kVp [26]

1.2.2. Cc tnh cht ca tia X

1.2.2.1. Bn cht ca tia X

Tia Rnghen c bn cht l sng in t nh nh sng nhng c bc sng

nh hn nhiu so vi bc sng nh sng. Ta c th so snh bc sng tia Rnghen

trong di sng in t sau y:

Bng 1.3. Bc sng ca cc loi sng in t

Bc x Bc sng

Tia Gamma 1mm

Tnh cht ca tia X:

- 48 -

- Tia X lm ion ha khng kh, do o mc ion ha ca khng kh c

th suy ra uc liu lng tia X. Ri vo cc vt, c bit l kim loi, tia X cng bt

c electron ra khi vt.

- Tnh truyn thng v m xuyn: l tnh cht ni bt ca tia X, n d dng

i qua cc vt khng trong sut vi nh sng thng thng nh g, giy, vi, cc

m mm nh tht, da. i vi cc m cng v kim loi th n i qua kh hn, kim

loi c nguyn t lng cng ln th tia X cng kh xuyn qua. Tia X c bc sng

cng ngn th kh nng m xuyn cng mnh, khi ta ni tia X cng cng. y

l c trng quan trng trong to hnh X quang.

- Tnh b hp thu: sau khi xuyn qua vt cht th cng chm tia X b

gim xung do mt phn nng lng b hp th. y l c s ca cc phng php

chn on X quang v liu php X quang.

-Tnh cht quang hc: ging nh nh sng, tia X cng c nhng hin tng

quang hc nh khc x, phn x, nhiu x v tn x. Nhng tnh cht ny to nn

nhng tia th cp trong c th khi n xuyn qua v gy nn gim tng phn

trn cc phim chp. chng li hin tng ny ngi ta c th dng loa khu tr,

ng nh chm tia, li lc....

- Tnh cht gy pht quang: di tc dng ca tia X mt s mui tr nn pht

quang nh clorua, Na, BA, Mg, Li,... v c cht tr nn sng nh Tungstat cadmi,

platino-cyanua Bari cc cht ny c dng ch to mn hunh quang dng khi

chiu X quang, tm tng quang.

- Tnh cht ho hc: tnh cht ho hc quan trng nht ca tia X l tc dng

ln mui bromua bc trn phim v giy nh lm cho n bin thnh bc khi chu tc

dng ca cc cht kh trong thuc hin hnh. Nh tnh cht ny m n cho php ghi

hnh X quang ca cc b phn trong c th ln phim v giy nh.

- Tc dng sinh hc: khi truyn qua c th tia X gy ra tc dng sinh l hy

dit t bo. Tc dng ny c s dng trong iu tr (iu tr ung th nng) ng

thi n cng gy nn nhng bin i c hi cho c th.

1.2.2.2. S hp th tia X

- 49 -

Do cc qu trnh tng tc quang in, Compton, to cp nn tia X khi i

xuyn qua vt cht nng lng, mt chm tia c th gim i v do cng

chm tia s b suy gim. S suy gim ny c tnh theo nh lut Beer. Nu I0 l

cng chm tia X ti, I l cng tia X ra khi lp vt cht, d l b dy lp vt

cht m tia X xuyn qua nh lut hp th Beer th hin nh sau:

d

0 e.II

Trong , l h s suy gim theo chiu di c trng cho bn cht hp th

tia X ca vt cht v mt vt cht. V qu trnh hp th ph thuc ngu nhin vo

xc sut xy ra cc hiu ng trn nn h s suy gim c xem nh l tng h s

suy gim ca tng hiu ng:

Vi ,, ln lt l h s suy gim ca hiu ng quang in, hiu ng

Compton v hiu ng to cp. Trong nhiu trng hp th hin r kh nng lm

gim cng bc x qua cc loi vt liu khc nhau ngi ta s dng h s suy

gim khi /m trong l mt vt cht. p dng cng thc Beer s lm

xut hin i lng mi .ddm (kg/m2)

gi l b dy khi ca bn vt cht. Khi

kho st mt cch nh tnh ta thy rng s hp th tia X trong vt cht ph thuc

vo mt s yu t sau:

- Th tch ca vt b chiu x: vt cng ln th tia X b hp thu cng nhiu.

B dy c th cng ln, s hp th tia X cng nhiu. Ngi ta nhn thy rng tia X

s suy gim 50 khi i qua lp vt liu no dy 1cm th s gim 25% na khi i

qua 1cm tip theo ca vt liu . V ch cn 12,5 khi i qua dy 3cm. Nh

vy sau khi i qua mt vt liu khng dy lm tia X s ch cn khng y 1 cng

ban u.

- Bc sng ca chm tia X: bc sng cng di tc l tia X cng mm th

s b hp th cng nhiu.

- Nguyn t s ca vt cht: s th t ca nguyn t vt cht cng cao th s

in t cha trong hnh cu nguyn t cng ln. ng knh nguyn t ln nn kh

(1.12)

(1.13)

- 50 -

nng va chm gia nhng in t v photon tia X ln nn qu trnh hp th cng d

xy ra. Ni cch khc s thp th tng theo trng lng nguyn t ca cht b chiu

x.

- Mt nguyn t: s nguyn t trong mt th tch nht nh ca vt cng

nhiu th s hp thu tia X cng tng. V d nc trng thi lng hp th tia X

nhiu hn trng thi hi.

1.2.2.3. S tn x tia X

S tn x ca tia Roentgen c quy lut khc vi s tn x ca nh sng trong

vng quang ph hc (l vng nh sng nhn thy hay tia t ngoi). Nh ta bit,

trong vng quang ph hc, di sng nh sng c , nh vy ln hn

nhiu kch thc ca nguyn t (c = cm) s tn x t l nghch vi ly

tha bc 4 ca bc sng.

Trong vng tia Roentgen, di bc sng ca n cng bc v gi tr vi

kch thc ca nguyn t. Nh vy quy lut v tn x vi tia Roentgen s phi

khc. C th, v s tn x c kho st nh kt qu ca nhng dao ng cng

bc ca nhng in t di tc dng ca trng in t ca sng Roentgen ti.

Thomson trc y a ra cng thc sau cho h s tn x nguyn t:

Vi : h s tn x nguyn t,

e: in tch ca electron ( Coulomb),

m: khi lng electron (kg),

C: tc nh sng (cm/s),

Z: s th t nguyn t.

C th kho st nh tit din hiu dng ca mt electron i vi s tn x

ca tia X. Khi t vo nhng gi tr bit:

(1.14)

- 51 -

Ta xc nh c tit din hiu dng ca nguyn t

4

25 2

2 4

86,57.10

3a

ecm

m C

Bn knh hiu dng ca tit din ny rt nh

cm

1.3. Nguyn l hot ng ca my pht tia X

Khi nung nng mt vt liu thch hp, mt s electron trong vt liu tr nn

linh ng v bt ra khi vt liu nh cc electron t do. Nhng electron t do ny

s bao quanh vt liu nh mt m my electron. l s pht x nhit cc

electron, c s dng to ra ngun electron t do trong ng tia X.

Khi mt in th chy qua cathode s sinh ra mt dng in t nng n n

di nhit pht x electron. Si t c nung nng vi mt dng in AC c

cng nm trong khong t 1 n 5A hiu in th trong khong t 4 -12V.

Dng in trong ng pht bc x tia X chy qua gia cathode v anode gn bng

0,1 dng t nng cathode.

Chm electron pht x nhit thot ra khi Cathode theo mi hng, do

cn hi t chm electron theo hng nhm thng vo v tr vt tiu trn anode. Do

cn c h thng hi t chm electron, h thng hi t thng c gi l chn

hi t c lp xung quanh tim n v c ni vi 1 cc ngun DC nhm to ra

mt in trng li cc electron. t c vt hi t nh, chn hi t s c

cp thm mt in th m, iu ny s to nn mt in trng c cc ng sc

cong hn, mc hi t s cao hn. Cc bng X quang thng c vt hi t

khong 0,6 1,6mm.

Khi t mt hiu in th vo gia anode v cathode, trong anode mang

in th dng th electron s chuyn ng v pha anode v to nn dng in chy

trong bng X quang c chiu t anode v cathode.

- 52 -

Nu chng ta duy tr nhit cathode mt gi tr no (do dng si t

quyt nh) th s lng electron bc x cathode s khng i. Khi tng dn in

th anode, s lng in t dch chuyn v anode s tng lm dng qua bng tng,

y l trng thi lm vic cha bo ha. trng thi ny dng in v in p ca

bng X quang cn ph thuc vo nhau.

Khi in p anode tng n mt gi tr m ti ton b s in t bc x

c ht ht v anode, lc ny bng X quang lm vic trng thi bo ha. trng

thi ny dng in v in p khng ph thuc vo nhau. y l trng thi lm vic

thc t ca bng X quang. Tuy vy, trong thc t khng phi lc no cng t c

trng thi ny.

to ra bc x cn thit dng cho chp nh bc x trong cng nghip th

in th dng gia tc cc electron phi nm trong khong t 30KV n 30MV.

Cc ng pht bc x tia X thng s dng in th gia tc cc electron ln n

420KV trong khi nhng gi tr in th gia tc cc electron ln hn c s

dng cho cc my gia tc tuyn tnh. Nng lng ca cc electron nhn c tng

ng vi di in th ny.

1.4. Cc yu t nh hng n cht lng liu ra tia X

nh gi cht lng liu ra ca tia X ngi ta thng s dng cc thut

ng: cht lng (quality) c trng cho kh nng m xuyn ca chm tia, s lng

(quantity) photon pht ra, liu chiu (exposure). Liu chiu l t s gia gi tr tuyt

i tng in tch dQ ca tt c cc ion cng du c to ra trong mt th tch

nguyn t khng kh (khi tt c cc electron v positron th cp do cc gamma to

ra b hm hon ton trong th tch khng kh ) v khi lng dm ca th tch

nguyn t khng kh [6]. Liu chiu chu s nh hng ca cht lng v s lng

chm tia pht ra. Hiu sut pht tia, liu chiu, cht lng v s lng chm tia

c quyt nh bi 6 yu t chnh: vt liu anode, in p, dng, thi gian chiu,

b lc, dng sng ca in p u vo.

- 53 -

Vt liu bia anode nh hng n hiu sut pht bc x hm theo cng thc:

000.820

ZE

E

E K

L

R ( trnh by trong phn 1.3.1.1). Chm electron ti tng tc nhiu

hn vi vt liu c nguyn t s Z ln hn nn hiu sut pht bc x tia X cao hn.

Mt khc, nng lng bc x c trng ph thuc vo vt liu bia (trnh by phn

1.3.1.2). Vy vt liu bia nh hng trc tip n s lng photon pht ra v cht

lng ca bc x c trng.

in p nh kVp quyt nh gi tr nng lng cc i ca bc x v nh

hng n cht lng ph bc x hm, vi gi tr kVp cng ln th nng lng bc

x cng ln. Thm vo hiu sut pht x tia X lin h trc tip vi gi tr kVp.

Trong ng pht bc x tia X s dng in th thp c 0,1 nng lng ca chm

electron c chuyn i thnh bc x tia X. Cc ng pht bc x tia X s dng

in th 100KV hiu sut pht bc x tia X tng ln c khong 1%. 2MV th

n c th t n 10% v 15MV c th ln hn 50 .

Liu chiu (exposure) gn nh t l vi bnh phng kVp:

Liu chiu ~ (kVp)2 [26]

V d, theo cng thc trn t l liu chiu ng vi in p 80 kVp so vi

chm tia ng vi in p 60 kVp ca cng mt ng tia X v cng thi gian chiu

tnh c (80/60)2 ~1, 8, c ngha liu chiu tng ln khong 78%.

Hnh 1.30. Cng pht x tia X thay i mnh theo gi tr kVp, khi gi cng

mt gi tr dng qua ng v thi gian chiu khng i

- 54 -

Ta cng thy rng gi tr nng lng ti c nhiu tia X nht ( im cao

nht trn ph) c gi tr cng ln khi kVp cng ln. Ngha l vi kVp ln, a phn

cc tia X trong chm l c nng lng cao. in p nh kVp l gi tr c bn quyt

nh gi tr nng lng hay kh nng m xuyn ca tia X. Gi tr kVp cng ln th

gi tr nng lng tia X t c cng ln, gi tr nng lng cao cho php tia X d

dng xuyn qua nhng b phn c th c mt m dy nn cht lng hnh nh

trn phim tt. Thm vo , vic s dng gi tr in p cao s to ra lng tia X

ln do gim c thi gian chiu. iu ny c li khi chp phim cho tr em hay

nhng i tng khng th kim sot s vn ng khi chp X quang.[26]

Dng qua ng (mA) bng s electron i t Cathode n Anode trong mt

n v thi gian. Vi cng gi tr kVp v b lc khng thay i th liu chiu ca

ng tia X t l vi gi tr mA. Nh bit, gi tr mA la chn cng ln th nhit

tim n cng cao nn s electron pht x ra cng nhiu. iu ny s lm tng s

lng hoc cng bc x tia X. Nh vy khi mA tng, din tch ph tng ln

nh hnh 1.30. Ta thy khi nh ph khng dch chuyn ngha l nng lng

chm tia khng thay i khi thay i gi tr mA.[15]

Hnh 1.31. nh hng ca mA ln hiu sut pht tia X

Vic tng thi gian chiu s dn ti vic tng lng tia X pht x ra, v d

nu tng thi gian chiu ln hai ln th lng tia X pht ra cng tng ln hai ln.

Trong mt s my hin i ngi vn hnh c th t lng mAs. i lng ny l

tch s gia cng dng in qua ng tia X (tnh bng mA) v thi gian n

(tnh bng giy). R rng khi mAs cng ln, th lng tia X i n phim cng nhiu,

ni chung hnh nh s r hn. Ty thuc vo b dy vng c th kho st m ngi

- 55 -

vn hnh c th chn mAs ph hp. Vic thay i thi gian chiu khng nh hng

ln nng lng ca chm tia X.

S thay i kVp phi cn bng vi s thay i mAs t c cng mt

gi tr liu chiu. v d trn, t s liu chiu ng vi gi tr in p 80 kVp v 60

kVp l 1,78. t c cng mt gi tr liu chiu nh nhau khi i qua c th

bnh nhn th mAs phi thay i theo ly tha bc 5 ca t s kVp:[26]

Tc nu liu chiu ng vi 60kVp, 40 mAs th ti gi tr 80kVp th mAs

tng ng phi l: mAs5,9mAs40.)80

60( 5

Hoc gi tr mAs cng c th c tnh theo cng thc:[26]

21

4

2

1 mAsmAs.)kVp

kVp( .

Vic la chn gia 2 cng thc trn ph thuc vo b dy v tnh suy gim

bc x khi i qua c th bnh nhn.

Qu trnh lc tia iu chnh s lng v cht lng tia X bng vic loi b

cc tia X nng lng thp ra khi ph. Vic lc tia lm tng gi tr nng lng trung

bnh bc x t lm tng cht lng tia X.

Dng sng ca in p nh hng n cht lng (kh nng m xuyn) ca

bc x. Vi cng gi tr kVp, in p mt pha cung cp hiu in th trung bnh

thp hn in p 3 pha. Do c cht lng v s lng tia X pht ra u b nh

hng.

21

5

2

1 mAsmAs.)kVp

kVp( (1.15)

(1.16)

- 56 -

Chng 2

AN TON BC X TRONG X QUANG CHN ON Y T

2.1. Cc hiu ng sinh hc ca bc x ion ha

2.1.1. C ch tc dng ca bc x ion ha

Cc qu trnh xy ra sau khi bc x i vo c th sng l mt chui lin tc,

bt u t nhng tng tc xy ra trong mt khong thi gian cc k ngn ngi, n

nhng qu trnh sinh hc c th m hng chc nm. Di tc dng ca bc x ion

ha, trong t chc sng tri qua hai giai on bin i l giai on ha l v giai

on sinh hc.

2.1.1.1. Giai on ha l

Giai on ny thng rt ngn, ch xy ra trong khong thi gian t 10-16

10-13

giy. Trong giai on ny cc phn t sinh hc chu tc dng trc tip hay gin

tip ca bc x ion ha.

Tc dng trc tip: bc x ion ha trc tip truyn nng lng v gy nn

qu trnh kch thch v ion ha cc phn t sinh hc dn n tn thng cc phn t

. Tc dng ny d dng quan st c trn thc nghim vi cc vt cht kh.

Tc dng gin tip: bc x ion ha tc dng ln phn t nc (chim 75% t

chc sng) gy hin tng x phn cc phn t nc ( H+ , OH-,....) cc hp cht c

kh nng ion ha cao (HO2, H2O2,...) nh ln cc phn t sinh hc gy tn thng

chng. Nhng tn thng trong giai on ny ch yu l cc tn thng ha sinh.

2.1.1.2. Giai on sinh hc

Giai on ny ko di t vi giy n vi chc nm sau chiu x. Nhng tn

thng ha sinh giai on u nu khng c hi phc s dn n nhng ri

lon v chuyn ha, tip n l nhng tn thng hnh thi v chc nng ca t bo.

Kt qu cui cng l nhng hiu ng sinh hc trn c th sng c biu hin mt

cch a dng v phong ph.

- 57 -

Chiu bc x ion ha

Tc dng trc tip Tc dng gin tip

H2O

Ion, gc t do phn t kch thch

Cc phn t sinh hc quan trng v cc c quan ca t bo

Ri lon chuyn ha cc chc nng t

bo Giai on sinh hc

(vi giy n vi chc nm)

Cc hiu ng sinh hc

2.1.2. Nhng tn thng do bc x ion ha

2.1.2.1. Tn thng mc phn t

Khi b chiu x, nng lng ca chm bc x lm ph v cc mi lin kt

ha hc hoc phn li cc phn t sinh hc. Tuy nhin, cc bc x ion ha thng

kh lm t ht cc mi lin kt ha hc m thng ch lm mt thuc tnh sinh hc

ca cc phn t sinh hc.

2.1.2.2.Tn thng mc t bo

S thay i c tnh ca t bo c th xy ra trong nhn v nguyn sinh cht ca

chng sau khi b chiu x. Trong nhiu trng hp ngi ta thy th tch t bo tng

ln do c s hnh thnh cc khong trng trong nhn v trong cht nguyn sinh sau

khi b chiu x. Nu b chiu x liu cao t bo c th b ph hy hon ton. Cc tn

thng phng x ln t bo c th khin:

- T bo cht do b tn thng nng nhn v cht nguyn sinh,

- T bo khng cht nhng khng th phn chia c,

O2

Giai

an

ha

l

(10-6

s)

- 58 -

- T bo khng phn chia c nhng nhim sc th tng ln gp i v tr

thnh t bo khng l,

- T bo vn c th phn chia nhng c ri lon trong c ch di truyn.

2.1.2.3. Tn thng mc c th

Tn thng gy ra bi bc x l h qu ca cc tn thng nhiu mc

lin tc din ra trong c th sng t tn thng phn t, t bo, m n tn thng

cc c quan v cc h thng ca c th. Hu qu ca nhng tn thng ny lm

pht sinh nhng triu chng lm sng, c th dn n t vong. Din tin ca tn

thng bc x lun i cng vi qu trnh phc hi tn thng. S phc hi ny

cng din ra mc t phn t, t bo, m n hi phc cc c quan v h thng

trong c th.

2.1.2.4. Cc yu t nh hng n hiu ng sinh hc ca bc x ion ha

[16]

a. Liu chiu

Liu chiu l yu t quan trng nht quyt nh tnh cht v cc tn thng

sau chiu x. Liu cng ln, tn thng cng nng v cng biu hin sm.

b. Sut liu chiu

Vi cng liu hp th nu thi gian chiu ko di s lm gim hiu ng sinh

hc ca bc x. Do vi nhng sut liu nh, tc pht trin nhng tn thng cn

bng vi tc hi phc ca c th. Nu tng sut liu ln th tc hi phc s

gim xung, mc tn thng tng ln, hiu ng sinh hc cng tng theo.

c. Din tch chiu

Mc tn thng sau khi chiu x cn ph thuc rt nhiu vo din tch b

chiu. Chiu mt phn hay chiu ton thn. Liu t vong khi chiu ton thn thp

hn nhiu so vi liu chiu cc b.

d. Cc nhn t khc

- Nhit

S thay i nhit sau khi chiu x nh hng r rt ln cc qu trnh tn

thng gy ra do bc x. Gim nhit s lm gim tc dng ca bc x ion ha.

- 59 -

Do khi h thp nhit , tc vn chuyn cc gc t do ti cc phn t sinh hc

gim, dn n gim s phn t sinh hc b tn thng do chiu x.

Bng 2.1. Hiu ng sinh hc theo mc liu

Liu Hiu ng

0,1 Gy

Khng c du hiu tn thng lm sng.

Tng sai lch nhim sc th c th pht

hin c

1 Gy Xut hin bnh nhim x trong s 5-7%

c th sau chiu x

2 -3 Gy

Rng lng, tc, c thy tinh th, gim

bch cu, xut hin ban trn da. Bnh

nhim x gp hu ht cc i tng b

chiu. T vong 10-30% s c th sau

chiu x

3- 5 Gy

Gim bch cu nghim trng.Ban, xut

huyt, nhim khun, rng lng,tc. T

vong 50% s c th sau chiu x

6 Gy

V sinh lu di c nam v n. T vong

hn 50 s c th b chiu ngay c khi

c iu tr tt nht

- Hm lng H2O

Nc ng vai tr quan trng gn lin vi vic to nn cc gc t do di

tc dng ca bc x ion ha, do hm lng nc ln lm tng tc dng ion ha

ca bc x.

- Hiu ng Oxy

nhy cm ca h sinh vt tng ln r rt khi tng p sut oxy ca mi

trng. Khi tng nng oxy, lng HO2, H2O2 to ra cng nhiu nn s phn t

sinh hc tn thng cng tng ln v ngc li.

- 60 -

2.2. Cc tiu chun v an ton bc x

2.2.1. Lch s xy dng cc tiu chun an ton bc x trn th gii

Vo cui th k 19 khi khm ph ra cht phng x ngi ta nhn thy cc

li ch v tc hi ca chng. Nm 1899 nh du thnh cng u tin trong vic s

dng tia X cha bnh ung th thng b trn mt mt ph n. i i vi vic

pht trin cc ng dng ca cht phng x trong y hc cng xut hin nhiu bo co

v cc hiu ng c hi ca bc x. iu i hi cc nh khoa hc phi xy dng

cc quy tc an ton bc x. Nm 1915 hi Roentgen Anh quc c thnh lp v

y ban X quang v Radium ca hi Roentgen Anh quc xut bn cc khuyn

co v an ton bc x nm 1921 v 192 . T thu ht cc t chc quc t quan

tm v tham gia thit lp cc tiu chun an ton bc x. Hai t chc quc t ng

vai tr quan trng nht trong vic khuyn co v ban hnh cc chun an ton bc x

quc t l y ban Quc t v An ton Bc x ICRP (International Commission on

Radiological Protection) (1928) v C quan Nng lng Nguyn t Quc t IAEA

(International Atomic Energy Agency).

2.2.2. Cc khuyn co v an ton bc x ca ICRP

Trong hi ngh phng x quc t ln th hai vo nm 1928, y ban Quc t

Bo v X quang v Radium c thnh lp. n nm 1950 y ban ny i tn

thnh y ban Quc t v An ton Bc x ICRP (International Commission on

Radiological Protection). y l t chc c cng nhn l t chc c uy tn nht

v cung cp cc khuyn co i vi cc vn an ton bc x.

T nhng nm 1930, ICRP khuyn co mi tip xc vi bc x vt qu

gii hn thng thng nn gi mc cng thp cng tt v a ra cc gii hn liu

nhng ngi lm vic trong iu kin bc x v dn chng ni chung khng b

chiu qu liu. C sau mt khong thi gian, khi tch ly c thm cc thng

tin cn thit v tc ng ca bc x ln con ngi, ICRP xem xt b sung, sa

i cc khuyn co c v a ra cc khuyn co mi. Khuyn co gn y nht do

ICRP a ra vo nm 1990.

- 61 -

Cc khuyn co ca ICRP mang tnh cht khi qut, v vy cc quc gia khc

nhau c th p dng vo lut l ca nc mnh. Nh c t chc ny m hu ht cc

quc gia trn th gii u s dng nhng nguyn tc trong lnh vc an ton phng

x nh nhau.

Cc khuyn co u tin ca ICRP da trn vic phng cc hiu ng bc

x c hi quan st c. Cc mc liu c khuyn co l 300mrem trong mt tun

( 3 mSv/tun) i vi cc m su hn 1cm gi l liu su v 600 mrem trong mt

tun ( 6 mSv/tun) i vi lp da su 0,007 cm gi l liu nng hay liu da. Nm

1959 liu 5 rem/ nm (50 mSv/nm) c ngh trong n phm ICRP 2 nhm

trnh hiu ng di truyn.

Nm 19 nhiu d liu thu c i vi cc nn nhn Nht Bn sng st

sau trn bom nguyn t cho thy khng nhn c cc hiu ng di truyn nn ICRP

cp nht cc khuyn co an ton bc x ca mnh. Khuyn co mi ICRP 26 tha

nhn ung th l hiu ng chnh cn trnh v cc c quan cng nh cc m khc

nhau trong c th c xc sut b ung th khc nhau. iu a n khi nim liu

hiu dng l liu tng ng tnh theo trng s m ca cc m i vi cc bnh

ung th. Do n phm ICRP 26 khuyn co liu hiu dng i vi chiu x ngh

nghip l 5 rem/nm (50 mSv/nm) v liu ny l tng liu chiu ngoi v liu

chiu trong.

Nm 1990 sau cc kt qu nghin cu ca cc nn nhn sng st sau trn

bom nguyn t Nht Bn cho thy sc xut gy ung th cao hn 4 ln so vi

khuyn co trc y. Do trong n phm ICRP 60 (1991) y ban khuyn co

gim gii hn liu hiu dng i vi chiu x ngh nghip xung thnh 20

mSv/nm c ly trung bnh trong 5 nm, trong liu gii hn cho mt nm n

l l 50 mSv. Khuyn co ny dng lm c s cho tiu chun v an ton bc x m

chng ta hin ang s dng.

2.2.3. Cc tiu chun v an ton bc x ca IAEA [6][34][35][38]

C quan Nng lng Nguyn t Quc t IAEA ( International Atomic

Energy Agency) l t chc c bit ca Lin hp quc thnh lp nm 1956 tr s ti

- 62 -

Viena, o c nhim v khuyn khch cc nc pht trin s nghip ng dng k

thut ht nhn v nng lng ht nhn v mc ch ha bnh. Do , IAEA ch

trng vic thit lp cc tiu chun an ton bc x v gip cc nc thc hin cc

tiu chun ny.

Hi ng thng c IAEA ln u tin thng qua cc bin php bo v v an

ton bc x vo thng 3 nm 1960 da trn cc khuyn co ca ICRP. Cc tiu

chun an ton u tin c Hi ng thng c IAEA duyt y nm 1962 v c

xut bn trong b sch v an ton Safety Series No.9. Bn hiu chnh c xut bn

nm 196 v bn hiu chnh ln 2 xut bn nm 1982.

Nm 1990 y ban hn hp gia cc T chc Quc t v An ton Bc x

IACRS (Inter- Agency Committee on Radiation Safety) c thnh lp trao i

kin v hp tc v cc vn lin quan n an ton bc x v ht nhn. Trong

khun kh ca h thng ny, cc t chc bo tr IAEA, FAO, ILO, OECD/NEA,

WHO v T chc Y t Pan American PAHO (Pan American Health Organization)

lp ra mt ban th k hn hp bin san tiu chun quc t c bn v bo v

i vi bc x ion ha v an ton i vi cc ngun bc x. Cc tiu chun c

th hin trong n phmTiu chun an ton quc t c bn v bo v bc x ion ha

v an ton i vi ngun bc x (International Basic Safety Standards for

Protection against Ionizing Radiation and the Safety of Radiation Sources) xut bn

trong b sch an ton-Safety Series No.115 nm 1996, gi tt l BSS (Basic Safety

Standards).Trong cc nguyn tc do nhm ny kin ngh i vi cc nh my in

ht nhn c nhiu nguyn tc thch hp vi cc c s v ngun bc x.

Cc tiu chun ny c hiu lc i vi cc t chc ng bo tr

IAEA,FAO,ILO, OECD/NEA,PAHO v WHO v khng bt buc cc quc gia coi

l lut nh i vi quc gia mnh v cng khng thay th cho cc iu khan ca

lut hay quy phm quc gia. Chng ch c xem l nhng iu hng dn thc t

i vi cc nh chc trch, cc t chc, cc ch c s, cc nhn vin, cc c quan

an ton bc x chuyn trch, c s x nghip v cc hi ng v an ton v y t.

- 63 -

2.2.4. Gii hn liu

Gii hn liu l gi tr liu cc i cho php i vi mt ngi b chiu x

trong mt khong thi gian no . Gii hn liu c xc lp trn c s xem xt

cc hiu ng sinh hc i vi c th ngi l hiu ng tt nhin v hiu ng ngu

nhin. Hiu ng tt nhin l hiu ng c ngng, nn gii hn liu phi thp hn

cc ngng ny sao cho cc hiu ng tt nhin phi c loi tr. Cc hiu ng

ngu nhin l hiu ng khng c ngng (hiu ng ngng khng). Khng th h

thp gii hn liu loi tr hiu ng ngu nhin m ch t cng thp cng tt

tranh cc hiu ng tt nhin v hn ch cc hiu ng ngu nhin. Cc gii hn liu

chiu c chia thnh 2 loi l gii hn liu chiu x ngh nghip v gii hn liu

cho dn chng.

Chiu x ngh nghip l mi s chiu x i vi nhn vin bc x xy ra

trong cng vic ca h, khng tnh n nhng chiu x c min tr v s chiu

x t nhng cng vic bc x hoc ngun c min tr. Gii hn liu i vi

chiu x ngh nghip c p dng cho chiu x t cc cng vic bc x, loi tr

cc chiu x y t, chiu x tim tng v chiu x t cc ngun phng x t nhin.

i vi nhn vin bc x, ICRP khuyn co rng liu hiu dng tng cng m h

nhn c v phn b u trong sut i lm vic 50 nm ca mnh khng nn vt

qu 1 Sv. Tc liu cho mi nm i vi nhn vin bc x l 20 mSv trong khi mi

ngi u phi chu liu bc x t nhin trung bnh khong 2 mSv/nm. Nh vy

liu gii hn ca nhn vin bc x gp khong 10 ln mc liu bc x t nhin.

Chiu x dn chng l s chiu x i vi cc thnh vin dn chng t cc

ngun bc x, khng k chiu x ngh nghip, chiu x y t v phng bc x t

nhin khu vc bnh thng, nhng c tnh ti chiu x gy ra bi cc ngun bc x

v cc cng vic bc x c cp php v chiu x trong cc trng hp can

thip. Gii hn liu i vi dn chng l 1 mSv/nm ch bng mt na mc phng

t nhin.

Cc gii hn liu cn c quy nh cho cc c quan thng xuyn tip cn

vi ngun phng x l chn tay, da v mt. Gii hn liu tng ng cho thy tinh

- 64 -

th ca mt l 150 mSv/nm, cho da v chn tay l 500mSv/nm i vi nhn vin

bc x. i vi dn chng, gii hn liu tng ng cho thy tinh th ca mt l

15mSv/nm v cho da l 50 mSv/nm.

Bng 2.2. Gii hn liu chiu khuyn co ca ICRP

Nm Nhn vin bc x Dn chng

1928 200 mRem/ngy

1934 100 mRem/ngy

1950 150 mSv/nm 15 mSv/nm

1977 50 mSv/nm 5 mSv/nm

1990 20 mSv/nm 1 mSv/nm

2.3. An ton bc x ti cc c quan y t theo tiu chun Vit Nam

2.3.1. Cc quy ch an ton bc x c ban hnh Vit Nam

- Quy ch tm thi v vic s dng, bo qun v vn chuyn cc cht phng

x do lin b Lao ng, Y t, y ban khoa hc k thut nh nc ban hnh nm

1971.

- Quy phm an ton bc x ion ha (TCVN 439 -87).

- Quy phm vn chuyn an ton cc cht phng x (TCVN 4985-89).

- Php lnh an ton v kim sot bc x nm 1996.

- Ngh nh ca Chnh ph quy nh chi tit vic thi hnh php lnh an ton

v kim sot bc x nm 1998.

- Thng t lin b hng dn vic thc hin an ton bc x y t nm 1999.

Nh vy t nm 19 1 n nay cc tiu chun, quy ch an ton phng x

nc ta hon thin dn cho ph hp vi cc khuyn co ca ICRP.

2.3.2. Tiu chun Vit Nam TCVN 6561:1999 v an ton bc x ion ha

ti cc c s X quang y t

TCVN 6561:1999 do ban k thut tiu chun TCVN/TC 85, vin nng lng