6.5 Medical Physics (Scanning Techniques) Flashcards

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1
Q

Pros and Cons of taking a conventional X-ray image

A

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

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2
Q

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

A

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

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3
Q

Pros and Cons of CAT scans

A

+ 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.

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4
Q

Describe the construction of a CAT scanner

A
  • X Ray tube that generates a fan shaped beam
  • Ring of electronic detectors opposite to detect the x ray beam intensity
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5
Q

How does a CAT scan work?

A

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.

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6
Q

What is a medical tracer?

A

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

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7
Q

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

A

Gamma. It is the least ionising and most penetrative.

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8
Q

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

A

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

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9
Q

Which specific source is commonly used in PET scans?

A

Fluorine 18

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10
Q

Which specific source is used to monitor many major organs?

A

Technetium 99m

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11
Q

What do gamma cameras do?

A

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

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12
Q

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

A

They travel in every direction.

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13
Q

What are the 5 key pieces in a gamma camera?

A

Collimator
Scintillator
Photocathode
Photomultiplier Tube
Computer

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14
Q

What does a collimator do?

A

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

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15
Q

What is a collimator made up of?

A

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

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16
Q

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

A

It is absorbed.

17
Q

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

A

Collimated.

18
Q

What is a scintillator crystal?

A

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

19
Q

What does a photocathode do?

A

It produces an electron for each visible photon detected.

20
Q

What does a photomultiplier tube do?

A

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

21
Q

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

A

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

22
Q

What is a PET scanner?

A

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.

23
Q

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

A
  • 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.
24
Q

Pros and Cons of PET scanning

A

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