Chapter 16: Diagnostic Methods In Medicine

Question 1

Why are medical tracers gamma emitters

Answer 1

1. Least ionising

2. Most penetrating so can be detected outside body

Question 2

How are medical tracers used

Answer 2

Patient injected with radioisotope; radiation is detected outside body using a gamma camera

Question 3

Why do medical tracers have a short half life

Answer 3

1. Gives out radiation quickly so only a small amount is required to produce an image
2. Reduces exposure to ionising radiation

Question 4

What’s a radiopharmaceutical

Answer 4

Particular radioactive isotopes are combined with other elements so a substance is produced which will be taken up by tissue of interest
Cancerous cells absorb large amounts so can clearly be seen on a trace

Question 5

Name 4 radioisotopes and their uses

Answer 5

fluorine-18: bone imaging
technetium-99m: bone growth/blood circulation(lungs/brain/liver)/function of heart/liver
iodine-123: function of thyroid/kidney
xenon-133: function of lungs

Question 6

Draw a diagram of a gamma camera

Answer 6

See page 240

Question 7

Describe the main components of a gamma camera

Answer 7

1. Collimator: Gamma photons that are parallel travel to scintillator (~10%)
2. Scintillator: Gamma ph converted to many 1000s of vis. light photons
3. Photomultiplier tubes: electrical pulse/electrons produced from vis. light photons
4. Computer: signals from photomultiplier tubes used to produce image where 1 pixel = output from 1 tube

Question 8

Draw a diagram of a photomultiplier tube

Answer 8

See page 241

Question 9

Describe how a photomultiplier tube works

Answer 9

1. Each vis. light photon from scintillator releases a single electron from cathode by photoelectric effect
2. Electrons accelerated to next electrode where more electrons are released

Question 10

Uses of a gamma camera

Answer 10

1. Bone scan - static study

2. Kidney scan - dynamic study

Question 11

Describe the principles of positron emission tomography

Answer 11

1. Beta+ emitting tracer injected into patient (e.g. fluorine-18)
2. Positron annihilates with an electron inside the patient
3. This produces 2 gamma photons travelling in opp. directions to each other
4. The patient is surrounded by a ring of gamma detectors which compare the times the pair arrive at the detector
5. Delay time indicates location
6. 3D image created by computer

Question 12

Uses of PET scans

Answer 12

1. Showing up cancerous tissue

2. Bodily functions e.g. brain activity

Question 13

Outline the principles of magnetic resonance imaging

Answer 13

1. Protons/nuclei have spin -> behave like tiny magnets
2. P/N precess about magnetic field provided by STRONG electromagnet
3. Transmitting coils provide pulses of radio waves with freq. equal to Lamor freq./frequency of precession
4. P/N absorb radiowaves causing them to resonate and flip into a higher energy state
5. When radiowaves turned off, P/N flip back into lower energy state and emit radio photons
6. Radio waves picked up by receiving coils and the signal is processed by a computer -> 3D image
7. Relaxation time depends on surrounding tissue
8. Gradient coils alter mag. flux density through body so location of P/N can be determined

Question 14

Write down equation for Larmor frequency

Answer 14

See page 245

Question 15

Draw an MRI scanner diagram

Answer 15

See page 246

Question 16

What does relaxation time depend on for biological materials

Answer 16

Water content

1. Water/watery tissue = few seconds
2. Fatty tissues = few hundred milliseconds
3. Cancerous tissues = intermediate

Question 17

Describe the main components of an MRI scanner

Answer 17

1. Large superconducting magnet: produces external mag. field up to 2T; cooled to 2.4K using liquid helium
2. Radio freq. transmitting coil
3. Radio freq. receiving coil
4. Longitudinal gradient coils: vary mag. field so only 1 section @ resonant freq. so can precisely locate source of radio signal
5. Computer: controls gradient coils and RF pulses; stores and analyses received data

Question 18

Advantages of MRI

Answer 18

1. Does not use ionising radiation which causes a hazard to patients/staff
2. No moving mechanisms
3. Patient feels nothing during a scan and there’s no after effects
4. Gives better soft tissue contrast than a CAT scan
5. Computer images can show any section through patient or a 3D image

Question 19

Disadvantages of MRI

Answer 19

1. Patient with metallic object cannot be scanned e.g. surgical pins/pacemakers
2. Does not show bone as clearly as CAT scan
3. Confined space -> difficult for patients with claustrophobia

Question 20

Describe how image intensifier works

Answer 20

Intensifier used or X-rays would pass through film
Converts 1 Xph to many vis phs
Therefore the patient is exposed to a lower amount of ionising radiation

Question 21

Describe how contrast media are used

Answer 21

Iodine/Barium are used as the contrast material
Have a large attenuation coefficient/Z number to improve image contrast
They are ingested/injected into the body
Scan shows outline of soft tissue