omot-k.e., p.e

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MEDICAL PHYSICS QUESTIONS

a. Explain the following forms of energy and the useful

role each of them plays in production of a goodradiograph:i. Potential energyii. Kinetic energy

iii. Heat energy

iv. Chemical energy

v. Nuclear energy.

b. Explain concisely the purposes served by thepresence of oil surrounding the glass envelope of a

diagnostic X-ray tube.c. Give reasons for the choice of tungsten being used

as X-ray filament in preference to other materials.

8/6/2019 OMOT-K.E., P.E.


POTENTIAL ENERGY

This is a property of an object in a force field. In thiscase, it is electrical force field as electrons arecharged particles.

The force(F) exerted on a charged particle by asecond charged particle is described by theequation:

F= K q1q2

r2where q1 and q2 are the charges on the twoparticles, r is the distance between the twoparticles and k is a constant.

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It is the energy possessed by electrons by

virtue of their positions. The thermionically

emitted electrons from the filament (cathode)

come to the surface from the various energy

levels where they were bounded and without

these electrons emitted, no X-rays could be

produced.

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If electron changes position with respect to theforce field, its potential energy changes.

Electrical potential(voltage) is the difference in the

electrical potential energy of an electrical chargeat two positions, divided by the charge:

Vab = Epa - Epb

q

where Epa is the electrical potential energy of thecharge at location a, Epb is the electrical potentialenergy of the same charge at location b and q isthe amount of charge.

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Potential energy is useful in determining the

final kinetic energy of a charged particle

moving between two electrodes through a

vacuum (X-ray tube).

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KINETIC ENERGY

This is the energy possessed by a body as a

result of its motion and it is defined by:

Ek =1/2 mv2where Ek is the kinetic energy, m is the mass of

the object in motion and v is the velocity of

the object in motion.

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� The thermionically emitted electron must be

accelerated towards the target (anode) in

which it is suddenly stopped to produce the X-

rays with which a radiograph is produced.

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According to the principle of conservation of energy,

the gain in kinetic energy of the charged particle is

equal to its loss of potential energy: Ekfinal

Ekinitial = Epinitial Epfinal

i.e Ekfinal = qV (Ekinitial =0)

For example, the kinetic energy of an electron, initially

at rest, which is accelerated through a potential

difference of 70 kilovolts is

Ek= qV=(1 electron charge)(70kV) = 70 KeV[ Electron Volt is the kinetic energy developed by an

electron accelerated across a potential difference of

1 V].

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HEAT ENERGY

Heat energy is the energy of the movement of

atoms and molecules of any material as a

result of temperature differences only.

Heat is used to dry wet processed

radiographic films in the drying chamber

before it could be read by the radiologist.

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CHEMICAL ENERGY

This is the energy that is locked up in chemicalcompounds and released under certaincircumstances.

To make the latent image formed on the exposedradiographic film visible, it requires chemicalprocess of development (chemical reaction)involving release of chemical energy.

The developer is a water-based solution containingchemicals that will reduce the exposed silverhalide crystals to metallic silver without changingthe unexposed silver halide crystals.

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Chemical components Functions

� Hydroquinone Reducing agent

� Phenidone Reducing agent

� Sodium carbonate Maintains proper pH� Sodium sulfite Preservative

� Potassium bromide Restrainer

� Glutaraldehyde Hardener

� Water Solvent

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The fixer stops the action of developer and removesthe unexposed silver halide crystals from theemulsion:

� Chemical component Functions� Ammonium thiosulfate Clearing agent

� Acetic acid Maintains proper pH

�

Potassium alum Hardener� Sodium sulfite Preservative

� Water Solvent

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NUCLEAR ENERGY

� This is the large amount of energy that is

locked up in the heart of the atom- the

nucleus.

� Nuclear energy does not play any role at all in

the production of a good radiograph as no X-

ray energy is produced from the nucleus.

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b. The purposes served by the presence of oilsurrounding the glass envelope of a diagnostic

X-ray tube are:

i.It acts as a coolant by transferring heat fromthe insert(cathode, anode, rotor assembly and

support structures sealed in a glass under ahigh vacuum) to the housing surface whichencloses and supports the other componentsunto the surrounding air so that the heat is

eventually distributed through the whole X-rayroom by convection.

ii. It attenuates X-ray beam (inherent filtration).

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iii. Since there is usually a high potential

difference (between 40-150 KV) between the

filament and target of a diagnostic X-ray tube,

the oil then provides sufficient electrical

insulation between these two structures.

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c. The reasons for the choice of tungsten beingused as X-ray filament in preference to othermaterials are as follows:

i. It is characterized by high atomic number(Z=74).

ii. It has a high melting point (3,380 degreecentigrade)

iii. It maintains its strength at high temperatureswith copious thermionic emission.

iv. It has a relatively low rate of evaporation.

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THANK YOU.

omot-k.e., p.e

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