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
Coupling gel
Coupling gel fills the air gaps between the transducer and skin ensuring that almost all the ultrasound enters the body. As air reflects 99.9% of ultrasound. Has the same impedance to the skin.
Doppler ultrasound
Frequency of ultrasound changes when reflected off a moving object.
- The frequency increases if it is moving towards the transducer and decrease when moving away
- Change in frequency is directly proportional to velocity
