medical physics Flashcards
What is the wavelength range of Xrays
10^-8 to 10^-13, typically 10^-10
outline the production of xrays
- An xray tube consists of an evacuated tube containing two electrodes.
- the vacuum is necessary so electrons don’t interact with the gas atoms
- An external power supply of 50kV is connected
- The cathode (negative) emits electrons via thermionic emission, usually a filament.
- These electrons are accelerated towards the positive anode
- The anode is the target metal usually made out of tungsten which has a high melting point
- The electrons hit the target metal and decelerate very quickly
- 1% of the KE energy of the accelerated electrons hitting the anode is converted into X-Rays.
- The remaining 99% is thermal, which is why a coolant is needed, or a rotating anode
- There is an x-ray transparent window which the x-rays are pointed through.
- The rest of the machine is covered in lead to prevent damage to radiographers
Outline how to find the minimum wavelength of a emitted xray
- KE of the accelerated electrons = eV
- due to the conservation of energy the max energy of the emitted xray = the max KE of the inital electron due to the one on one interaction
- max energy = max frequency = minimum λ
- E=hf = hc/λ
- λ = hc/eV where eV is the KE of the original particle
define attenuation
decrease in intensity of an electromagnetic radiation as it passes through matter
Outline the simple scatter model
incident x-ray photon interacts with an electron in the atom, but has less energy that required to remove the electron and so just bounces off with no change to its energy
Outline the photoelectric effect for xray absorption
incident x-ray photon is absorbed by one of the electrons in the atom, the electron uses this energy to escape the atom with the KE left over, normally seen in hospitals due to the incident x-ray photon energy
Outline Compton scattering
incident x-ray photon interacts with an electron in the atom, the electron is ejected from the atom and because of the high energy of the incident x-ray, it doesn’t disappear but also is ejected with lower energy
Outline pair production
incident x-ray interacts with the nucleus of the atom and disappears, ejecting an electron positron pair which conserve momentum
why are contrast medium used
- to increase absorption coefficients
- soft tissues have low absorption
- used to model blood flow to spot clots
- barium meal for digestive systems
What does CAT stand for
computerised axial tomography
- axial meaning cross sections
- tomography meaning slices
outline the function of a CAT scanner
- patient lies om back
- slides into a ring with detectors on one side and x-ray tubes on the other
- xray tubes produce fan like beams of xrays
- each different tissue absorbs different amounts of xrays and so detectors pick up different intensities
- every 360 degree rotation a slice is made
- these can be manipulated by computers to make a 3D image
outline the advantages of a CAT scan
- creates a 3D image
- can distinguish between different tissues of similar attenuation coefficients
outline the disadvantages of CAT scans
- have to stay still for a long time
- relatively more expensive that a normal x-ray machine
- x-rays are ionising and can be harmful in high exposure long time periods
important things to think about when choosing a medical tracer
- high activity so only a small amount is needed
- short half life so patient isn’t exposed to radiation a long time after the process
- gamma emitter has it is less ionising and highly penetrative
What can we use to target cells in the brain
combine Tc-99 with sodium and oxygen to target cells in the brain
outline the process of a gamma camera
- medical tracer is injected into the patient
- travels to targeted cells
- gamma photons are emitted toward the camera
- firstly the collimator is a series of long thin lead tubes designed to eliminate all gamma photons incident not from the targeted area. i.e. coming out from an angle
- parallel gamma photons pass down the collimator to the scintillator
- this is made of sodium iodide and when a gamma photon hits it, produces many photons of visible light
- about 1 in 10 gamma photons interacts with the scintillator
- photomultiplier tubes converts these flashes into electrical pulses
- these pulses are sent to a computer which constructs, using software a detailed image of the point and which the gamma photons left the body
what is different in a gamma camera image to an x-ray image
gamma camera shows functions and processes of the internal structures, rather than anatomy
name the isotope in the main medical tracer used in PET scans and outline why it is used
FDG with fluorine-18
- positron emitter
- with a short half life that decays into oxygen, a positron, a neutrino and a gamma photon
what is the downside to using flourine-18
has to be produced on sight due to its half life
how is flourine-18 produced
proton + oxygen-18 = flourine-18 + neutron
what does FDG stand for
flourodeoxyglucose
One advantage of using FDG
- body treats it like glucose so can easily model the respiratory system and in tissues with high rates of respiration
outline the process of PET scans
- patient lies on a table and is surrounded by a ring of gamma cameras
- the patient is injected with FDG
- when the FDG emits a positron from beta+ decay the positron instantly annihilates with a nearby electron
- this annihilation produces two gamma ray photons which travel in opposite directions to eachother to conserve momentum
- The computer determines the point of annihilation by analyzing the time difference between the two gamma ray photons being received.
- the two gamma photons are picked up by two detectors on exactly opposite sides of the patient
- the computer is fed impulses created by the gamma cameras and makes an image using the time difference
- this image has different colour and brightness
Outline the advantages of a PET scanner
- non invasive
- can assess effectiveness of new drugs
- can diagnose different types of cancers
