Medical Physics Flashcards
How are x-rays produced
- Cathode (heater) produces electrons by thermionic emission
- Electrons are then accelerated to the anode
- The anode is made from a metal such as tungsten
- Electrons hitting the target metal decelerate
- X-rays are produced
Simple scatter
X-ray interacts with an electron in the atom but has less energy required to move the electron, so the X-ray photon is scattered
Photoelectric effect attenuation
X-ray photon is absorbed one of the electrons in the atom and uses the energy to escape from the atom.
Compton scattering
X-ray photon interacts with an electron causing it to eject from the atom. X-ray is scattered with reduced energy.
Pair production
X-ray photon interacts with the nucleus of the atom. The X-ray disappears and an electron and position are produced
Contrast mediums
They improve visibility of internal structures in X-ray images as they have a high attenuation coefficient (absorption coefficient). Examples are barium and iodine.
How does a PET scan work
- Fluorine-18 is injected into the patient
- Fluorine-18 decays releasing a positron which annihilates with an electron inside the patients body
- Two gamma photons are produced travelling in opposite directions
- A ring of detectors around the patient detects the photons.
- The time of arrival of these photons can be used to locate the area of activity
How does a CAT scan work
- X-ray tube produces a fan shaped beam of X-rays
- X-rays are attenuated by different amounts by different tissues
- Detectors on the opposite side record the intensity of transmitted X-rays, which is sent to a computer
- Multiple slices are assembled to produce a 3D image
Advantages and disadvantages of CAT scans
Advantages:
- CAT scans can produce 3D images
- Can distinguish between soft tissues with similar attenuation coefficients
Disadvantages:
- X-rays are ionising and CAT scans can be prolonged
- longer and more expensive than tradition X-rays
- have to remain very still
Gamma camera
Detects gamma photons emitted from medical tracers.
- A collimator prevents gamma photos from arriving at an angle
- The gamma photon reaches the scintillator which produces photons of light
- Photons hit a photomultiplier tube which converts light into an electrical impulse
- Photomultiplier tubes are connected to a computer which produces and image
What is ultrasound
It is a longitudinal wave with a frequency greater than 20khz
Technetium-99m
It is a medical tracer that targets the brain and emits gamma photons
Piezoelectric effect
- When quartz is compressed and emf is induced.
- When an external p.d is applied across the opposite face the crystal can stretch and compress
- When a high natural frequency is applied across the crystal ultrasound is produced.
A-scan
- A single transducer is used to record a straight line through the patient.
- Each pulse of ultrasound will be partly reflected and partly transmitted at the boundary of 2 different tissues.
B-scan
Produces a 2d image
- The transducer is run along the patients skin, the output is connected to a computer
- The computer produced a row of dots on the screen with different brightness showing intensity of ultrasound reflected. Each dot shows a boundary
- The collection of dots from different positions produce an image
