Medical Physics

Question 1

How are X-rays produced?

Answer 1

When charged particles are rapidly decelerated (or accelerated), kinetic energy is transformed into high frequency photons.

Question 2

What is the Piezoelectric Effect?

Answer 2

A material generates a voltage as it contracts and expands or will contract/ expand when a voltage is applied.

Question 3

What is acoustic impedance?

Answer 3

the product of density of the material and speed of sound in that material

Question 4

Does sound travel faster in denser or less dense mediums?

Answer 4

Denser

Question 5

What do Piezoelectric materials tend to be made from?

Answer 5

Quartz, Polymeric or Ceramic materials

Question 6

Which radioactive isotope gives out beta plus radiation, which can be used in PET scans?

Answer 6

Fluorine-18

Question 7

What is the half life of flourine 18?

Answer 7

110 mins

Question 8

Which element gives off gamma radiation as a radio isotope?

Answer 8

Technetium 99m

Question 9

What is the half life of technetium-99 m?

Answer 9

67 hours

Question 10

What device is Technetium 99-m used in?

Answer 10

Gamma Cameras

Question 11

What role does a collimator play in a gamma camera?

Answer 11

Only rays moving directly upwards are detected by the detectors behyond this.

Question 12

The Gamma Camera: What is a scintillator?

Answer 12

Crystal that gives out a small flash of visible light when hit by a gamma photon.

Question 13

What is the job of a photomultiplier in a gamma camera?

Answer 13

Causes a cascade of electrons from a singular electron, producing a measurable current/voltage

Question 14

Name the parts of the gamma camera.

Answer 14

Collimator, Scintillator, Light guide, Photomultiplier.

Question 15

Why is the collimator in a gamma camera made of a high density material such a lead?

Answer 15

To ensure gamma radiation is absorbed.

Question 16

What is a PET scanner?

Answer 16

A ring of gamma cameras placed around a patient so that a 3D image can be generated from the emission of gamma photons.

Question 17

What are disadvantages of PET scans?

Answer 17

Expensive, requires tracers to be synthesised on site.

Question 18

In a pet scan how many photons are produced in each electron-positron annihilation?

Answer 18

2

Question 19

How is the site of a tracer estimated in a PET scan?

Answer 19

A pair of gamma photons are emitted from the electron-positron annihilation. These travel in opposite directions (conservation of momentum) so are detected at opposite sides of the ring. The speed of these photons is known allowing us to locate the annihilation event.

Question 20

How much energy is each gamma photon produced by beta plus and beta minus annihilation in a PET scan?

Answer 20

0.51 MeV

Question 21

Gamma Rays and X-rays may have the same frequency, how do we tell which a wave is?

Answer 21

Depends on how they are formed, X-rays formed through decelerating electrons, Gamma released from an atomic nucleus.

Question 22

In X-ray production electrons are accelerated from the cathode to the

Answer 22

anode

Question 23

In X-ray production electrons are accelerated in a vacuum towards a metal anode, what is an example of a metal the anode may be made from?

Answer 23

Tungsten

Question 24

In X-ray production when electrons hit the anode how do they loose their energy?

Answer 24

In the form of heat and as X-rays.

Question 25

How do you work out max energy of X-ray photons assuming no energy released as heat?

Answer 25

eV=hf

Question 26

What is the most common way X-rays interact with matter?

Answer 26

The photoelectric effect

Question 27

Describe the Compton Effect.

Answer 27

High energy photon bounces off an electron, this changes the direction and energy of the photon (lower energy X-ray photons are emitted)

Question 28

What kind of voltage is needed for pair production?

Answer 28

Above 1 million volts.

Question 29

What is pair production?

Answer 29

An electron-positron pair is spontaneously produced via mass-energy relation.

Question 30

Describe simple scattering

Answer 30

X-rays reflect off layers of atoms when they do not have enough energy for more complex processes.

Question 31

What are the four attenuation mechanisms?

Answer 31

Simple Scattering, Photoelectric Effect, Compton effect, Pair Production.

Question 32

What are contrast media? (Used in X-ray imaging)

Answer 32

High attenuation materials with large proton numbers. (Far better at absorbing X-rays than soft tissues).

Question 33

Give two examples of contrast media used in X-ray imaging.

Answer 33

Iodine - Used in liquids (Eg. observing blood flow). Barium Sulphate - Used in the digestive system.

Question 34

For the photoelectric effect, the attenuation coefficient is proportional to the proton number…

Answer 34

cubed.

Question 35

What relationship does the intensity of a collimated beam have with thickness?

Answer 35

Decreases exponentially. (I= Io e^ux)

Question 36

What is an ultrasound B scan?

Answer 36

A series of A scans stitched together to form a 2D image, accomplished by moving the transducer over the skin.

Question 37

With materials with similar acoustic impedances a small amount of the sound is…

Answer 37

reflected.

Question 38

What is the Doppler technique used for?

Answer 38

Measuring the speed of blood flow (aspirin can reflect waves back to the transducer).

Question 39

What is the Doppler effect?

Answer 39

Changing frequency of a wave due to relative motion of the observer and emitter,

Question 40

Describe the principles of PET scanning.

Answer 40

Body surrounded by a ring of gamma cameras. F-18 decays releasing positrons which annihilate electrons Annihilation of a positron and electron releases gamma rays travelling in opposite directions. Delay time between gamma rays is used to determine the location of F-18. Computer is connected to detectors an image is formed from these.

Question 41

How would you define attenuation?

Answer 41

The decrease in intensity of EM radiation as it travels through matter/space

Question 42

What are the important diagnostic or therapeutic uses of X rays?

Answer 42

Diagnosis of blood flow issues, digestive system imaging, killing cancerous cells