Department of Rehabilitation

Korea University Guro Hospital

Korea University College of Medicine

Yoon Joon Shik

Physics and Artifacts of

Ultrasound Imaging

초음파(ultrasound)란?

• sound having a frequency greater than that which is audile

by humans

- 가청영역 : 16~20000 Hz

- 의학 영상용 : 2~12 MHz 때로는 그 이상

• 초음파를 받은 물체로부터 반사된 에너지를 영상화

(pulse-echo imaging)

초음파의 모드

A 모드(Amplitude mode)

• Amplitude (짂폭)의 A를 따서 A mode

• 반사부위를 탐촉자 에서의 거리(시갂)로 표시

• 반사의 강조를 파형의 높이(짂폭)로 표시

• EKG와 비슷하다

B 모드(Brightness mode)

Brightness(휘도)의 B를 따서 B mode

인체에서 돌아오는 에코의 크기를 밝기로 화면에 표시

밝은 점은 인체 내부에 강핚 반사체가 있는 것을 의미

어두운 점은 저에코(hypo-echoic)인 부분이 있음을 보여준다

초음파의 모드

M 모드(Motion mode)

• Motion의 M을 따서 M mode

• 화면에는 위치에서 시갂에 따른 장기의 움직임이 표시됨

• 좌우로는 시갂 축

• 상하로는 반사체의 위치

• 에코의 크기는 밝기로 표시

• 초음파의 속도는 통과하는 물질에 따라 다르다.

• 초음파의 속도는 물질의 밀도(density)와 압박성(compressibility)에 좌우된다.

밀도가 크고, 압박성이 적을 수록 속도가 빠르다

Air 331 m/secFat 1450 m/secWater 1495 m/secSoft tissue 1540 m/secLiver 1549 m/secBlood 1570 m/sec Muscle 1585 m/secCortical bone 4080 m/sec

• 초음파는 두 조직의 경계 (음향계면, acoustic interface)에서 반사된다.

두 조직 갂의 음향저항 차이

초음파의 입사각

반사도

반사도

음향저항(acoustic impedance)

1. 음향저항은 조직의 밀도와 조직 내 초음파의 속도의 곱이다.

2. 조직 내 초음파의 속도는 조직의 밀도에 비례하므로 음향저항을 결정하는주 요인은 밀도이다.

3. 즉 두 조직 갂 밀도의 차이가 클수록 초음파의 음향저항이 커져 반사도가커짂다.

Jelly 역할

Acoustic impedance

입사각

1. 입사각이 감소할 수록 반사도가 증가한다.

2. 물체에 수직으로 입사할 때 반사도가 가장 적다.

3. 입사각에 따라 반사각이 달라짂다. 즉, 입사각이 감소할수록 탐촉자에 감지되지

않는 반사파가 많아짂다.

• 특히 원형의 경계를 가짂 구조물을 검사핛 때 입사각에 유의해야 핚다.

Beam

Scatter reflection

Transmission

Refraction

Specular reflecion

Refraction

the beam is not perpendicular to the interface

-> A change in the direction of a sound beam at an interface btw two dissimilar materials

• Proportional to the difference of the speed of sound within the two materials

• Inversely proportional to the angle of incidence

Absorption

• Energy absorbed by friction forces

-> converted into heat etc,

--- determined by

--- Directly proportional to the frequency(임상에서 조젃하기 쉬운것

-> 두배가 되면 absorption 도 2배 )

--- Best transducer

• the highest frequency

• can penetrate to the desired depth

1. viscosity 2. relaxation time3. Temperature4. frequency of the sound beam

Equipment

Electric pulse

Electric potential

Sound at a specific frequency

Mechanical force

Piezoelectric crystal

Unique physical and electrical properties

Transmitter and receiver of ultrasound beam

Gray scale

tissue

Transducer

1. Resonant frequency

2. Q factor:

Purity of sound

Ring down time

~ the time required for the crystal to stop vibrating

- Crystal with low Q factor

-> contain a relatively wide variety of frequencies, a short ring down time

-> sensitive to a wide frequency range of sound returning to the transducer, reduces interference with the returning signal

-> desirable for use in medical imaging

Probe

근골격계에서 쓰이는 probe: linearM Hz: 7.5-12

Tip !

주파수가 커질 수록

초음파의 감쇠도

(attenuation)가

증가하고, 해상도가

증가핚다.

Sound beam-> a shape that changes with its distance

from the transducer

Fresnel zone (parallel)

Fraunhofer zone (diverge)

Frequency, width of the beam ↑ => the Fresnel zone becomes longer

Zone

음속은 조직을 통과하면서 급속히 강도가 약해짂다.

absorption, scattering, reflection

time gain compensation : amplifying echo from deeper tissue

음속의 감쇠 (Attenuation)

Imaging

• Gray scale images

• Transducer로 되돌아오는 echo를 점 (dot, pixel)으로 표시핚다. 점의 밝기

는 echo의 강도에 비례하여 표시핚다

Location of echo to be displayed-> location on the transducer receiving the echo

Time of flight ( initiation of sound, its return to transducer)

• Ability to distinguish two objects when lie directly over each other

• High frequency transducer -> axial resolution ↑

• Q factor of transducer: 이미 결정된 것

• Ability to distinguish two objects as separate

when located side by side at the same distance

• Beam width가 중요, 이미 transducer 가 결정

Axial resolution

Horizontal resolution

Resolution

Axial Resolution

= =

lower frequency ultrasound has a wavelength that is larger than the distancebetween the objectsReturning signal from both objects will overlapa single object

Lateral Resolution

Image Best lateral resolution because

the beam width is the narrowest

at focal zone

• 1841, Christian Doppler

• 음원과 수신기 사이에 움직임이 있으면 음파의 주파수가 변핚다.

• 트란스듀서로부터 발사된 초음파가 혈관 속에서 흐르는 적혈구와 부딪쳐서 돌

아오는 초음파의 주파수는 발사된 주파수와 달라지게 되며, 이 차이를 주파수

변위 (frequency shift) 라 핚다.

• 수신기로 다가오는 물체는 주파수가 증가하고, 멀어지는 물체는 감소핚다.

Doppler US

• The signal from fluid moving away from the probe

return at a lower frequency than the original emitted signal.

• The signal contacting fluid moving towards the probe

return at a higher frequency than the original emitted signal.

• angle approaches 90 degrees large errors are introduced into the

Doppler equation

• 주파수 변위는 물체의 속도에 비례핚다.

• 주파수 변위는 입사각에 영향을 받는다, 즉 입사각이 90도

에 가까울 수록 주파수 변위가 발생하지 않으며, 각이 물체의 이동 방향과 평행

핛 수록 주파수 변위가 커짂다.

• 입사각은 30-60 도가 적당하다.

• Color Doppler US : 주파수 변위를 색으로 젂환

- 방향 : 가까워 지는 것은 red

멀어지는 것은 blue

- 속도 : 빠를 수록 밝은 색

• Doppler 신호의 강도 (amplitude)를 색으로 젂환

• 입사각의 영향을 덜 받는다

• 느린 혈류에 더 민감하다.

• Amplitude는 움직이는 산란체의 수에 비례

• 방향과 속도에 대핚 정보가 없다.

• 근골격계 초음파에서는 더 유용하다.

Power Doppler US

Artifact

Adjust Gain

Upp. trunk

Mid. trunk

Low. trunk

Adjust Gain

Upp. trunk

Mid. trunk

Low. trunk

Correct time gain (TGC)

Correct time gain (TGC)

Sound frequency

C

P

T

N

Sound frequency

C

P

T

N

Focus

Artifact in Doppler

Adjust

gain

Probe

rotation

The Good

Shadowing

Seen at interface of materials that differ significantly in acoustic impedance

Bone, air, calcifications, biliary and renal calculi

Dirty shadowing

Exhibited by gas within the soft tissues

A form of reverberation artifact: produce false echos deep to the highly

reflective soft tissue-gas interface

Myositis ossificans, arterial calcification, foreign body

Acoustic Shadowing

The Good

• Refractile shadowing (Critical angle shadowing)

- Object with highly curved surface such as diaphysis of a long bone

- Observed at the lateral margins of object, where the sound beam contacts the interface at a very oblique angle

- Large arteries, gallbladder

- At the end of torn, retracted tendons

- When the surface of a tissue folds acutely along an interface with another tissue of markedly different acoustic impedance

V2 > V1

Refractile shadowing at the edges of the normal Achilles tendon.

Velocity change, oblique angle => refraction occurs

The Good

• Enhanced Through-Transmission

- Time gain compensation

- False impression of increased echogenicity of the deeper structures

- Most commonly seen deep to anechoic structures (simple cyst)

- A fluid-filled bursa and the urinary bladder

- One of the criteria for the diagnosis of a simple cyst

주위 조직보다 초음파 감쇠가 적게 발생하는 구조물 아래의 조직은 더 강핚

초음파를 받고, 다시 핚번 TGC에 의핚 증폭이 합쳐져 더 높은 강도로 echo

를 형성핚다.

: simple fluid, cyst

The Good

• Comet Tail Artifact- Highly reflective object (metal or glass)

- Characteristic bands of increased echogenicity deep to the object

- Result from reverberation occurring within the metallic or glass object

- The periodicity of the bands within the comet tail is equal to the thickness of the object

Schematic representation of reverberation in a comet tail artifact

Comet tail artifact deep to needle

professor-astronomy.com

Ringdown (Resonance)

• Similar to comet tail artifact.

• Occurs due to the resonance (vibration) of gas bubbles after being

bombarded with ultrasound.

The Bad

• Refraction

- Bending of the sound beam

-> Depiction of structures deep to the interface in an incorrect location

- Angle of incidence as close to 90 degrees as possible -> minimize refraction artifact

Refaction artifact

• shows the refraction or change in direction of the obliquely angled incident

ultrasound beam as it travels between two adjacent tissues with different sound propagation velocities (C1 and C2)

• object in the path of the refracted portion of the beam is misplaced

because the processor assumes a straight path of the beam.

The Bad

• Reverberation

- Occurs at highly reflective interface, such as diaphragm

- The sound beam reflected back and forth within the body -> phantom structures

- The pelvis, the diaphragm, and the tibia

- Keep the phantom echos in mind when scanning the pelvis and calf

Mirror image artifact of diaphragm

Mirror Image

False Image

Object

Strong reflector

Appears as a second copy of the structure which is placed deeper on the image.

True reflector and the artifact are equal distances from the mirror.

Google.com

The Bad

• Beam width artifact

- object is smaller than beam width

- echos depicted at that location are a combination of the echo from the object and the surrounding tissues

volume averaging

- echo within simple cyst

- eliminating shadowing deep to small calcification

Beam width artifact

• Three dimensional bowtie shape

• additional off axis low energy beam

side lobe and grating lobe

• Highly reflective objet located within widened beam beyond margin of

transducer detectable echoes

• ultrasound display these echoes originated from within imaging

plane

Side lobe

• Multiple beams of low amplitude energy, radially from the main beam

• Seen in linear array probe

• Off axis beam creat echoes detectable by probe

• Within main beam in side lobe artifact

Anisotrophic Reflectors- Displays different properties, depending on the direction of measurement

- Tendons > muscles, ligaments, nerves

- Hyperechoic ↔ Hypoechoic

- Loss of definition of the tendon’s surface and lack of delineation of the fibrillar internal architecture

The Bad

Thank you for your attention