27, Medical Imaging Flashcards
(33 cards)
How are X-rays produced?
A high voltage is placed across a vacuum with a hot filament (cathode) at one end and a sheet of metal at the other (anode), this creates a sort of electron gun which hit the metal sheet, slowing them down abruptly which releases X-ray photons (X-rays are also produced at a specific wavelength when electrons are knocked out of the shell and become replaced), these then leave the X-ray tube through a small window.
Why are very high P.D.’s required to produce X-rays?
Because the electrons need to have very high K.E., especially since most is transferred to thermal energy at the anode.
How could you find the wavelength of X-ray emitted from an X-ray tube?
Equate E = hf to E = QV (= eV) and use f = c/wavelength.
What is attenuation?
The decrease in intensity of EM radiation as it passes through matter.
What are the four mechanisms for attenuation of X-rays?
Simple scattering, photoelectric effect, Compton scattering and pair production.
What is simple scattering?
When a low energy X-ray bounces off an electron in an atom.
What is the photoelectric effect?
When an electron absorbs the X-ray and escapes from the atom.
What is Compton scattering?
When an electron absorbs the X-ray, escapes the atom and releases a lower energy photon.
What is Pair production?
When a very high energy X-ray interacts with the nucleus of an atom and an electron-positron pair is produced.
What does the equation I = I_0 e^μx show?
Intensity = initial intensity * e^ attenuation/absorption coefficient * thickness of substance.
What are the units of the attenuation coefficient?
m^-1.
How can soft tissues like blood vessels be seen in X-ray images?
By using a contrast medium, a harmless substance with a high Z number such that the X-rays become absorbed by the tissue where the substance is present.
How can X-rays be used in therapy?
Can be used to target specific groups of cells to kill cancers etc.
What substances can be used as a contrast medium?
Iodine in blood and barium can be used to view the gut.
What is a CAT scan?
Where an X-ray tube spins around the object with a detector array at the other end to produce 3D images of the object.
What are the advantages and disadvantages of X-rays and CAT scans?
X-rays are much cheaper but CAT scans can produce 3D images which can be more useful. CAT scans also expose the patient to much more radiation and require the patient to remain very still for an extended period of time, both have the advantage of being non-intrusive.
What makes an ideal radioisotope?
One which has a very short half-life, decays into a stable isotope, emits mostly gamma rays and is safe inside humans.
What is a tracer and how does it work?
You chemically bind it to the radioisotope and the compound accumulates in the area you want to investigate.
What is a gamma camera?
The device used to find decays of the radioisotope.
What components make up a gamma camera and what do they do?
First there is the collimator which ensures that only photons travelling perpendicular to the camera are detected, then there is a scintillator which attempts to have as many gamma rays interact with it as possible producing a burst of visible light. Then the visible light travels down a light guide (think ethernet cable) into a photomultiplier which uses the photons to induce a current (by letting it interact with charged plates) causing a pulse of current which can be detected by the computer to create a display.
What is a PET scanner?
A device similar to a CAT scanner which uses a ring of gamma detectors to produce a 3D image from the gamma rays produced by the annihilation of positrons produced by radioactive materials in the human.
Why is the fact that two gamma photons are emitted in opposite directions important for PET scans?
Because the decay itself will also produce gamma photons and these need to be ignored so only photons which appear at opposite sides of the scanner at the same time are used, also the slight time difference between the photons can be used to find the origin of the decay.
What is the issue with setting up PET scans in hospitals?
It is very expensive to buy PET scanners and also the tracers used need to be produced on site since they have very short half-lives, this requires a particle accelerator which is very expensive.
What is ultrasound?
Any sound with a frequency beyond the point that humans can hear ~20kHz.
