Medical Physics

Question 1

Explain what is meant by a tracer.

Answer 1

A substance containing radioactive material that is absorbed by the tissue being studied.

Question 2

Explain what is meant by annihilation.

Answer 2

When a particle interacts with its antiparticle so that mass is converted into energy.

Question 3

State the name of the particles involved in the annihilation process.

Answer 3

Electrons & Positrons.

Question 4

Positrons aren’t naturally present in the body.

Explain how positrons come to be present in the body during PET scanning.

Answer 4

Introduction of tracer into the body containing β+ emitter.

Question 5

Explain how positrons cause the emission of gamma radiation from the body during PET scanning.

Answer 5

Positrons emitted by a radioactive tracer interact with electrons, this leads to pair annihilation where the mass of both particles is converted into gamma photons.

Question 6

Explain how the gamma photons are used to produce an image.

Answer 6

Two gamma photons travelling in opposite directions are detected outside the body, based off their arrival times the location of gamma production is determined. This process creates an image of the tracer concentration in the tissue produced.

Question 7

Explain how x-rays are produced for use in medical diagnosis.

Answer 7

Firstly, electrons are accelerated by an applied p.d. Then, these electrons crash into a metal target. Finally, x-rays are produced when the electrons decelerate.

Question 8

Explain why there is a continuous distribution of wavelengths in a typical x-ray spectrum.

Answer 8

X-ray photons are emitted when electrons are suddenly stopped accelerated (decelerated). This acceleration occurs over a range of distances in the metal target, leading to a range of accelerations and consequently a distribution of wavelengths in the emitted X-ray radiation.

Question 9

Explain why there is a sharp cut-off at short wavelength in a typical x-ray spectrum.

Answer 9

Incoming electron is stopped in a single collision so energy is all given to one photon.

(recall energy and wavelength are inversely proportional)

Question 10

Explain why there is a series of peaks superimposed on the continuous distribution of wavelengths in a typical x-ray spectrum.

Answer 10

De-excitation of orbital electrons in metal target after being hit by the incoming electrions.

Question 11

Explain how and why longer wavelength photons are frequently filtered during x-ray imaging of a body.

Answer 11

How: add an aluminum sheet

Why: so they don’t pass through the body

Question 12

State, for an x-ray image, what is meant by “contrast”.

Answer 12

The difference in degrees of blackening.

Question 13

State how, in a modern x-ray tube, the intensity of the x-ray beam and its hardness are controlled.

Answer 13

intensity: vary the potential difference across filament.

hardness: vary the potential difference applied to accelerate the electrons.

Question 14

State the purpose of a computed tomography (CT scanning).

Answer 14

To produce a 3-dimensional image of a structure/body.

Question 15

Outline the principles of CT scanning.

Answer 15

For CT scanning, x-rays are used for scanning, the scanning process is performed in sections. To capture a complete picture, scans from many angles are taken, were the image of each section is 2-dimensional. Finally, the images of the many sections collected is combined together.

Question 16

Outline the use of ultrasound to obtain diagnostic information about internal body structures.

(answer not accurate)

Answer 16

1. ultrasound pulse
2. reflected at boundaries
3. generated and detected by quartz crystal
4. time delay between generation & detection gives information about depth.
5. intensity of reflected wave gives information about nature of boundary.

Question 17

Explain the principles of the generation of ultrasound waves for use in medical diagnosis.

Answer 17

Generating ultrasound waves depends on piezo-electric crystal such as quartz. An alternating p.d. across a quartz crystal is used, this causes it to vibrate. Resonance then occurs because frequency of p.d. matches natural frequency of the crystal. The natural frequency of the crystsal is then in ultrasound range, from which an ultrasound wave can be generated.

Question 18

Explain why ultrasound used in medical diagnosis is emitted in pulses.

Answer 18

1. Allows the reflected signal to be distinguished from the emitted signal.
2. Cannot emitt and detect at the same time hence pulses.

Question 19

Explain the principles of the detection of ultrasound waves used in medical diagnosis.

Answer 19

A piezo-electric crystal such as quartz is used, this type of crystal vibrates when exposed to ultrasound waves, these vibrations produce alternating e.m.f./p.d. across the crystal.

Question 20

State what is meant by the attenuation of an ultrasound wave.

Answer 20

It’s the loss of wave power/intensity as the ultrasound wave travels across a medium.

Question 21

Define specific acoustic impedance.

Answer 21

It’s the product of density and speed of ultrasound in a medium.

Question 22

Describe, qualitatively, how the specific acoustic impedances of two materials affect the intensity reflection coeffecient at a boundary between the materials.

Answer 22

The intensity reflection coefficient at a boundary between materials is determined by the difference between their specific acoustic impedances. The lower the difference is, the lower the reflection coefficient.

Question 23

The magnitudes of the acoustic impedances may be almost equal or very different. State how these differences affect the intensity reflection coefficient at the boundary between the two media

Answer 23

1. If they’re almost equal then the reflection coefficient ‘a’ = 0 means there is little to no reflection at the boundary, which means most ultrasound is transmitted.
2. If they’re very different then reflection coefficient ‘a’ = 1, which means there is a large reflected intensity, which means there’s almost no ultrasound transmitted.

Question 24

Explain why is gel sometimes used during ultrasound medical diagnosis.

Answer 24

The specific acoustic impedances (Z) of the gel and soft tissue is more or less the same, hence gel can be applied on the skin as ultrasound waves would all be transmitted, while without gel, the ultrasound waves are deflected.