Chapter 27 - Medical Imaging

Question 1

Acoustic impedance (Z)

Answer 1

Property of a material that determines the intensity of ultrasound reflected at a boundary with another material.

Question 2

Compton effect

Answer 2

The effect whereby X-rays deflected off particles have a longer wavelength than their initial wavelength.

Question 3

CAT scan

Answer 3

Computerised Axial Tomography - process using X-rays and computers to produce an image of a slice through the body.

Many slices are used to produce 3D images.

Question 4

Pair production

Answer 4

Particle-antiparticle pair created from a high energy photon e.g. X-ray -> electron + positron.

Question 5

Piezoelectric effect

Answer 5

Change in volume of certain crystals when a p.d. is applied across them

OR

production of e.m.f. when certain crystals are placed under stress.

Question 6

PET scan

Answer 6

Positron emitting tomography - detects gamma photons produced when positrons annihilate with electrons inside the body.

Used to map out active areas in the body.

Question 7

Simple scatter

Answer 7

Deflection of charged particles off other charged particles e.g. alpha particle and nucleus of gold atoms.

Question 8

Transducer

Answer 8

Device that converts non-electrical signal into an electrical signal e.g. microphone.

Question 9

Ultrasound

Answer 9

Sound waves above the audible range of human hearing i.e. greater than 20kHz.

Question 10

Characteristics of X-rays

Answer 10

Can be polarised, diffracted by atoms in crystals.

Extremely short wavelengths.

EM waves - travel through vacuum at speed of light.

Harmful to living cells, can kill them.

Question 11

How are X-rays produced?

Answer 11

1. Large p.d. between cathode and anode.
2. Cathode produces electrons by thermionic emission.
3. Electrons accelerated towards anode (made of target metal such as tungsten).
4. Deceleration of electrons produces X-ray photons, which are emitted through a window.

Question 12

Attenuation coefficient

Answer 12

A measure of absorption of X-ray photons by a substance. SI unit: /m

Question 13

What are the four attenuation mechanisms?

Answer 13

Simple scatter
Photoelectric effect
Compton scattering
Pair production

Question 14

What is the purpose of a contrast medium?

Answer 14

Improves invisibility of internal structures in X-ray images.
Able to see outline of certain organs.

Question 15

Examples of contrast media

Answer 15

Iodine or barium compounds.

Question 16

Components of CAT scan

Answer 16

Rotating X-ray tube that produces thin fan-shaped X-ray beam.
Ring of detectors.
Computer software and display.

Question 17

Advantages of CAT scan

Answer 17

Can create a 3D image.

Can distinguish between soft tissues of similar attenuation coefficients.

Question 18

Disadvantages of CAT scan

Answer 18

Longer and more expensive than simple X-ray.

Prolonged exposure of ionising radiation.

Patients must remain still during scan.

Question 19

Examples of medical tracers

Answer 19

Technetium-99 and fluorine-18.

Question 20

Components of gamma camera

Answer 20

Collimator
Scintillator
Photomultiplier tubes
Computer and display

Question 21

Advantages of PET scans

Answer 21

Non-invasive technique

Can be used to diagnose different cancers, plan heart surgery, observe brain function.

Used to assess effect of new drugs on organs.

Question 22

Disadvantages of PET scans

Answer 22

Very expensive (esp. to make medical tracers) so usually found at larger hospitals.

Question 23

A scan (ultrasound)

Answer 23

Single transducer used to record along a straight line through patient.

Used to determine thickness of bone or distance between lens and retina in eye.

Total distance travelled by ultrasound pulse is 2L.

Question 24

B scan (ultrasound)

Answer 24

Output of transducer connected to computer. For each position, computer shows rows of dots on screen (brightness show intensity of reflected pulse).

2D image produced.

Question 25

What does coupling gel do?

Answer 25

Has similar acoustic impedance to skin.

Fills air gaps between transducer and skin.

Ensures that almost all ultrasound enters body and not reflected at air-skin boundary.

Question 26

Doppler effect in ultrasound

Answer 26

Ultrasound reflected off of moving blood cells returns with changed frequency.

Doppler shift in frequency directly proportional to speed of blood flow.

Transducer must be held at angle as cos90 = 0 (no observed change in f).

Question 27

Use of colour Doppler scans

Answer 27

Transducer connected to computer to show colour-coded image of direction and speed of blood flow.