6.5 Medical Physics (Scanning Techniques) Flashcards
Pros and Cons of taking a conventional X-ray image
+ Cheap and quick
- Only provides a 2D image
- Cant distuinguish overlapping bones or different soft tissues
What does a Computerised Axial Tomography (CAT) scan do?
It records a large number of 2D x ray images, then assembles them into a 3D image with the help of computer software.
Pros and Cons of CAT scans
+ 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.
Describe the construction of a CAT scanner
- X Ray tube that generates a fan shaped beam
- Ring of electronic detectors opposite to detect the x ray beam intensity
How does a CAT scan work?
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.
What is a medical tracer?
A radioactive substance that is either injected or digested. It can be used for diagnosis or treatment.
What type of radiation is most useful in medical tracers? Why?
Gamma. It is the least ionising and most penetrative.
Medical tracers have high activity and a short half life. Why?
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
Which specific source is commonly used in PET scans?
Fluorine 18
Which specific source is used to monitor many major organs?
Technetium 99m
What do gamma cameras do?
They detect gamma photons emitted from medical tracers within the body.
Why is tracing the location of gamma ray emission within the body difficult?
They travel in every direction.
What are the 5 key pieces in a gamma camera?
Collimator
Scintillator
Photocathode
Photomultiplier Tube
Computer
What does a collimator do?
It allows only gamma photons travelling perpendicular to the tube to pass.
What is a collimator made up of?
Thin long lead tubes. They are parallel and honeycomb shaped.
What would happen to a gamma photon travelling at an angle into a collimator?
It is absorbed.
A beam of gamma rays which has passed through a collimator is said to be…?
Collimated.
What is a scintillator crystal?
A material that will emit visible light photons when it is struck by gamma photons.
What does a photocathode do?
It produces an electron for each visible photon detected.
What does a photomultiplier tube do?
It amplifies the signal by generating a cascade of electrons when hit by an initial electron from the photocathode.
What can be done with the signal from a photomultiplier tube?
It can be detected by a computer and displayed on a screen.
What is a PET scanner?
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.
What series of steps occurs in the production of a 3D image by a PET scanner?
- 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.
Pros and Cons of PET scanning
+ Non Invasive
+ Produces real time images
- Very expensive
- Patient will be exposed to some radiation.
