Medical Physics

Question 1

Two different ways X-rays are produced?

Answer 1

Rapid deceleration of electrons at anode, knocked electrons in target material emitting characteristic rays

Question 2

Four ways X-rays are attenuated?

Answer 2

Simple scattering, photoelectric effect, compton effect, pair production

Question 3

What is simple scattering?

Answer 3

<20keV, X-rays absorbed by electrons in all directions

Question 4

What is the Compton effect?

Answer 4

0.5MeV-5MeV, X rays ionise electrons but leftover scattered

Question 5

What is pair production?

Answer 5

X-rays interact with nucleus to form electron-positron pair

Question 6

How to image soft tissue using X-rays?

Answer 6

Ingest barium sulphate/inject iodine whihc absorb X-rays strongly CONTRAST MEDIUMS

Question 7

CT scan advantages

Answer 7

3D image produced, good differentiation between soft tissue

Question 8

CT scan disadvantages

Answer 8

Exposure to ionising radiation, time consuming

Question 9

What is a CT scan?

Answer 9

fan shaped beam irradiates thin slice of patient, rotated around body

Question 10

Four stages of Gamma Camera?

Answer 10

Collimator, Scintillator, Photomultiplier tubes, Computer

Question 11

What is a PET scan?

Answer 11

Inject B-plus emitter, positrons annihilate electrons, gamma emitted

Question 12

PET scan advantages

Answer 12

non-invasive, can diagnose different types of cancers

Question 13

PET scan disadvantages

Answer 13

expensive, ionising radiation

Question 14

How to improve resolution of an ultrasound image?

Answer 14

Shorter wavelength

Question 15

What is ultrasound?

Answer 15

A sound wave above the audible range >20kHz

Question 16

What crystal is used in a transducer?

Answer 16

lead zirconate titanate (PZT)

Question 17

What is the purpose of the backing material in a transducer?

Answer 17

Damps oscillations, prevents backwards propagation so no reflected waves

Question 18

What is the pulse repetition frequency?

Answer 18

the frequency at which ultrasound pulses are emitted

Question 19

What is the formula for ultrasound distances?

Answer 19

d = (s x t)/2

Question 20

What is acoustic impedence?

Answer 20

The product of the speed of sound through a given
medium and the density of the medium.

Question 21

What is an A scan?

Answer 21

Straight line of ultrasound, intensity shows boundary type, time delay shows thickness

Question 22

What is a B scan?

Answer 22

Dots on screen build image, brightness proportional to intensity

Question 23

What is a Doppler scan?

Answer 23

For blood flow, c is speed of sound in substance, v is velocity of blood, angle is measured from beam to blood

Question 24

What effect does increasing velocity have?

Answer 24

Higher Doppler frequency

Question 25

What effect does increasing angle have?

Answer 25

Lower Doppler frequency

Question 26

Why is gel used for ultrasound?

Answer 26

There would be a high reflection at dry skin-air boundary due to large difference in acoustic impedence

Question 27

What are the energy transfers when producing X-rays?

Answer 27

Electric potential to kinetic, kinetic to thermal mostly, some kinetic to X-rays

Question 28

Why do we need to filter out low-energy X-rays?

Answer 28

They will be absorbed by the skin and could cause damage as they are ionising

Question 29

What affects how X-rays are attenuated?

Answer 29

Z of medium

Question 30

How does an X-ray scan work?

Answer 30

X-rays penetrate patient, attenuated differently, shadow effect, intensity proportional to darkness of film

Question 31

What is the purpose of the intensifying layer?

Answer 31

photons hit atoms in layer, fluorescence occurs, emit visible light photons, so extra contrast; film is more sensitive to visible light