6.5 Medical Physics (Scanning Techniques)

Question 1

Pros and Cons of taking a conventional X-ray image

Answer 1

+ Cheap and quick
- Only provides a 2D image
- Cant distuinguish overlapping bones or different soft tissues

Question 2

What does a Computerised Axial Tomography (CAT) scan do?

Answer 2

It records a large number of 2D x ray images, then assembles them into a 3D image with the help of computer software.

Question 3

Pros and Cons of CAT scans

Answer 3

+ Image has greater resolution
+ The scan can distinguish between differing soft tissues
- Can take a significantly longer time
- Exposes the patient to a greater dose of ionising radiation.

Question 4

Describe the construction of a CAT scanner

Answer 4

• X Ray tube that generates a fan shaped beam
• Ring of electronic detectors opposite to detect the x ray beam intensity

Question 5

How does a CAT scan work?

Answer 5

The x ray tube and detectors rotate about the patient, and move up and down their length. The images of each slice are stitched together to create a full 3D image of the patient’s body. The image is displayed on a computer monitor.

Question 6

What is a medical tracer?

Answer 6

A radioactive substance that is either injected or digested. It can be used for diagnosis or treatment.

Question 7

What type of radiation is most useful in medical tracers? Why?

Answer 7

Gamma. It is the least ionising and most penetrative.

Question 8

Medical tracers have high activity and a short half life. Why?

Answer 8

High activity
- To allow it to be detected from outside the body

Short Half Life
- So that it doesn’t stay in your system for too long, but long enough to be detected.
- Only small amounts of the source are needed
- Patient is exposed to minimal dosage of harmful radiation

Question 9

Which specific source is commonly used in PET scans?

Answer 9

Fluorine 18

Question 10

Which specific source is used to monitor many major organs?

Answer 10

Technetium 99m

Question 11

What do gamma cameras do?

Answer 11

They detect gamma photons emitted from medical tracers within the body.

Question 12

Why is tracing the location of gamma ray emission within the body difficult?

Answer 12

They travel in every direction.

Question 13

What are the 5 key pieces in a gamma camera?

Answer 13

Collimator
Scintillator
Photocathode
Photomultiplier Tube
Computer

Question 14

What does a collimator do?

Answer 14

It allows only gamma photons travelling perpendicular to the tube to pass.

Question 15

What is a collimator made up of?

Answer 15

Thin long lead tubes. They are parallel and honeycomb shaped.

Question 16

What would happen to a gamma photon travelling at an angle into a collimator?

Answer 16

It is absorbed.

Question 17

A beam of gamma rays which has passed through a collimator is said to be…?

Answer 17

Collimated.

Question 18

What is a scintillator crystal?

Answer 18

A material that will emit visible light photons when it is struck by gamma photons.

Question 19

What does a photocathode do?

Answer 19

It produces an electron for each visible photon detected.

Question 20

What does a photomultiplier tube do?

Answer 20

It amplifies the signal by generating a cascade of electrons when hit by an initial electron from the photocathode.

Question 21

What can be done with the signal from a photomultiplier tube?

Answer 21

It can be detected by a computer and displayed on a screen.

Question 22

What is a PET scanner?

Answer 22

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

Question 23

What series of steps occurs in the production of a 3D image by a PET scanner?

Answer 23

• A positron emitter is injected.
• The positron annihilates with an electron within the body, producing a pair of gamma photons
• The gamma photons are emitted in opposite directions.
• They are detected by the ring, and their arrival times are recorded.
• Using the arrival times, the exact location of the annihilation can be calculated.
• This is repeated until a 3D model of the tracer locations can be produced.

Question 24

Pros and Cons of PET scanning

Answer 24

+ Non Invasive
+ Produces real time images
- Very expensive
- Patient will be exposed to some radiation.