6.5 - Particles - Medical Imaging Flashcards
How are X-rays produced
Rapidly decelerating or accelerating charged particles
How are Gamma rays produced
By radioactive decay and during particle collisions with a mass defect
How do you make X-rays (diagram)
Electrons are accelerated in a circuit shooting an electron beam towards a copper anode which is attracted to with most of the kinetic energy from the beam being transferred to thermal whilst some is used to product the x ray beams
What happens after the electron beam hits the copper anode
The x rays are emitted in every direction and a collimated beam can be used to focus on a specific parts of the patient
What is the X ray intensity equation
I = Io . e^-μx
I = attenuated intensity
Io = initial intensity
x = thickness that it passes through
μ = medium
What is attenuation coefficient
It shows how well a medium absorbs x rays (bone has higher than muscle)
What are the 4 absorption mechanisms
Simple scattering
Photoelectric effect
Compton effect
Pair production
What is simple scattering
X ray photon is scattered by electron in new direction but same amount of energy
What is photoelectric effect (absorption)
The incident x ray photon disappears and removes an electron from the atom
What is the compton effect
The x ray is scattered with longer wavelength (loses energy) and the electron is ejected from the atom
What is pair production
The x ray photon turns into a positron which annihilates with an electron emitting energy in opposite directions
What is a CAT scan
(Computerised Axial Tomography)
X ray emitter goes around the body shotting through the body into detectors to create a large number of 2d images which can be made into a 3D image and allows for differentiation between different tissues.
A fan shaped beam is used
What are the disadvantages of CAT scans
Takes a long time, exposes patient to ionising radiation
What are the advantages of CAT scans
Resolution is greater than that of normal xrays
What is a common medical tracer and why is it used
Flourine-18 (F18) as it has a short half life but long enough to be used in procedures
How is F18 traced
It undergoes beta plus decay releasing a positron which annihilates with an electron forming a pair of gamma photons which are detected with the photons being detected and that is used to determine the location inside the patient
What are gamma cameras used for
They detect gamma photons emitted from medical tracers
What are the parts of a gamma camera
Collimator
Scintillator
Photomultiplier tubes
What is a collimator used for
It ensures that the gamma photons emitted are only accepted if they are traveling parallel to the tubes (directly into them)
What does the scintillation crystals do
After having the gamma photons be incident on them they will emit photons
What does a scintillator do
Produces visible photons which are directed to a photocathode which produces an electron for each visible photon detected
What does a photo multiplier do
It contains dynodes which are kept at a high voltage so the cascade of electrons amplifies the signal which is detected by a computer and displayed on a screen
What is a PET scanner
(Position Emission Tomography) Is a ring of gamma cameras which move around the patient to product a 3d image
How do PET scans locate issues
The arrival times of the annihilation event can be found as we know the speed of the gamma photons
