Medical Imaging
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Describe the structure of an X-ray tube.
A vacuum tube has a heated filament (cathode) and the tungsten anode with a high potential difference of 200kV.
Describe how X-ray photons are produced from an X-ray tube.
• Electrons are emitted from heated filament via thermionic emission. • Electrons accelerate through high voltage towards anode. • Electrons hit the anode and decelerate. • KE is converted into high frequency X-rays.
What is the attenuation of X-rays?
The decrease in intensity of electromagnetic radiation.
X-rays attenuate as they travel through matter. What are the four mechanisms that explain why they attenuate?
• Simple scatter • Compton effect • Pair production • Photoelectric effect
Name three factors that affect X-ray attenuation.
• Intensities before and after material (I) • Amount of radiation absorbed (μ) • Thickness of material (x) I = Io e^-μx
How is contrast media used in X-ray imaging?
Materials that have large atomic numbers (eg Iodine and Barium) absorb a lot of X-rays. They are injected in the bloodstream to make internal structures more identifiable.
What is the relationship between the attenuation coefficient and the proton number?
μ ∝ z^3
Describe the simple scatter mechanism.
• X-ray photons have energies 1-20keV • X-ray interacts with electron in atom but does not have enough energy to remove electron. • X-ray photons are scattered Not used in hospitals as X-ray machines use p.d greater than 20kV.
Describe the photoelectric effect mechanism.
• X-ray photons have energies < 100 keV • X-ray is absorbed by electron and electron escapes Used in hospitals as X-ray machines use 30-100kV
Describe the Compton scattering mechanism
• X-ray photons have energies of 0.5MeV to 5MeV • Electron absorbs some of the X-ray and is removed from atom. • Left over X-ray becomes scattered X-ray photons.
Describe the pair production mechanism.
• X-ray photons have energies of 1.02MeV or greater • Energy is absorbed by nucleus of atom. • Positron and electron are ejected.
Describe the components of a CAT scan.
X-ray tube on one side and detectors opposite rotate around the patient.
How does a CAT scan work?
• X-ray tube generates a fan shaped beam towards the patient. • Beam produces a thin cross section of the patient. • Tube and detectors rotate around the patient. • X-rays attenuated by tissues in the body. • Transmitted X-rays are absorbed by the detectors • Computer creates an image using the slices of images.
Name advantages and disadvantages of CAT scan over X-ray.
Advantages: - 3D image can be used to assess shape, size and different tissues. Disadvantages: • Exposed to ionising radiation for longer • Takes longer • Must stay still for ages
Why are gamma sources used as medical tracers?
They are least ionising and can penetrate through the patient (so can be detected externally).
What are the characteristics of radioisotopes and why are they important?
• Short half-life • High activity To limit exposure to harmful radiation, to retrieve a clear image quickly and to limit the amount of radioactive substance used.
Name two radioisotopes used as medical tracers and when they are used.
Fluorine-18 -> PET scans Technetium-99m -> Major organs (heart, brain etc)
Why are radioisotopes made at hospitals?
They have a very short half-life so cannot be stored.
Describe the gamma camera and its components.
Collimator- lead tubes that allow photons to travel in one direction. Scintillator- photons hit scintillator and photons of visible light are emitted (1/10 chance gamma hit scintillator) Photomultiplier tube- VL photons enter photomultiplier tube and electrical pulse is sent. Computer- produces high quality image of concentrations of medical tracers in patient.
State one advantage of the gamma camera over the X-ray.
Gamma camera produces an image showing the FUNCTIONS and PROCESSES of the body.
Describe the structure of the PET scan
A ring of gamma detectors that contain a photomultiplier tube and sodium iodide scintillator to produce a voltage pulse when gamma photons are incident.
Explain how a PET scan works
• Decaying radioactive medical tracer emits positron • Positron annihilates with electron • Two gamma photons are emitted, travelling in opposite directions • Detected by gamma cameras • Arrival time of photons indicates location of tissue / tumour • Computer produces a 3D image.
Name the medical tracer used in PET scans
FDG which has fluorine-18 nuclei
How do you produce fluorine-18 nuclei?
Collide protons with oxygen-18 nuclei to form fluorine-18 and neutrons.