Medical Physics Flashcards
(32 cards)
What does the collimator do in a gamma camera?
Collimator is a grid of parallel lead tubes. Only gamma photons travelling along the axis of the tube will pass through.
What does the scintillator do in a gamma camera?
Absorbs a gamma photon and converts the energy into thousands of visible photons.
What do the photomultiplier tubes do in a gamma camera?
Photo-cathode is struck by visible photons and electrons are produced by the photoelectric effect. The number of electrons is multiplied in the tube by a series of dynodes to produce a short plus of current at the end of the tube.
What does the computer do in a gamma camera?
Processes pulses of current to produce an image on the display. The brightness of each pixel in the image is linked to the number of gamma photons in each tube of the collimator.
Define non-invasive
There is no surgical cutting and no risk of infection.
How can you reduce the dangers from ionising radiation of a medical tracer?
Use a pure gamma emitter.
Define a medical tracer
A material enters the body and its position in the body can be detected or function of an organ can be determined.
What is the ideal isotope for medical tracing?
Technetium 99-m
Why is Technetium 99-m the ideal isotope for medical tracing?
It is a pure gamma emitter and has a half-life of 6 hours.
Why do ultrasound waves which are used is medical diagnosis have frequencies which are MHz?
It gives a short wavelength so better resolution.
Definitely the piezoelectric effect
A piezoelectric material generates a voltage when it is contracted or expanded or will contract and expand if a voltage is applied.
What do each of the letters represent in the equation z=ρc
Z - acoustic impedance
ρ - density of tissue
C - speed of ultrasound in tissue
If Ir/Io = 0 what does this mean?
There is acoustic matching all ultrasound is transmitted.
If Ir/Io = 1 what does this mean?
Virtually all ultrasound is reflected.
Define the Doppler Effect
The change in observed frequency (or wavelength) due to relative motion between the source of a wave and the observer.
What is true about x-ray photons?
They have zero rest mass and zero charge, each photon has high frequency and energy, they travel at c=3x10^8 in a vacuum.
What equation is used for electrons in an x-ray tube?
QV = 1/2 mv^2
What is the equation for fmax in an x-ray tube?
Fmax = QV / h
What is the equation for λmin in an x-ray tube?
λmin = hc /QV
What happens in a simple X-ray image?
An X-ray beam passes through a patient and strikes a photographic plate that is white initially. X-ray photons darken the plate.
Define attenuation
The reduction in intensity due to absorption in a material.
What is the main attenuation process for medical imaging?
The photoelectric effect.
What is the use of a gamma camera?
To detect gamma photons emitted from inside a patient. It is important to detect the photons and identify their point of emission.
Why is a pure gamma emitter used as a medical tracer?
Only gamma photons will be emitted which will leave the body and so the ionisation in the patient is minimised.
