M6.5 Medical Physics Flashcards
decribe the structure of an x ray tube
a high voltage power supply connected to a heater which accelerates electrons
electrons are shot at a tungsten target
some of their KE is converted to EM waves
explain simple scattering
when the x-ray has less than less energy than the work function
photon is absorbed and the electron jumps up an energy level then drops releasing an x ray
describe compton scattering
incoming x-ray has enough energy to remove electron
some of the photons energy is converted to the electrons Kinetic enegry
electron is removed
and photon is scattered with less energy
explain pair production
high energy interacts with the nucleus
electron and positron is produced annihilate each other produce a pair of smaller energy photons travelling in opposite directions
photoelectric effect
x ray is absorbed by electron
electron escapes atom with some KE
when using an intensity against wavelength how to find maximum potential difference
use eV = hc/lambda
use minimum wavelength on the graph
What happens to an intensity against wavelength graph when accelerting p.d is increased
graph shifts left but k lines stay in the same place
(theyre dependent on the meterial)
why are contrast mediums used
because lots of soft tissues have similar attenuation coefficients
what are the two commonly used contrast mediums
barium and iodine
why do contrast mediums absorb x rays more than soft tissues
because attenuation coefficient is directly proportional to Z^3 (number of protons)^3
how do CAT scans work
x ray tube around patient
detector opposite
thin beam of xrays used
gets cross sectional image
pros and cons of CAT scan
produce 3d image
slower than xray
longer time exposed to raidation
define a medical tracer
a radioactive substance injected into the patient
what are the properties of a medical tracer
gamma source
must have a long enough half life to be detectable
and short enough so they dont stay in the system
what are two main medical tracers used
And their half lives
fluorine - 18
half life of 110 mins
technitium 99m
half life of 6 hours
what are the components of a gamma camera
collimator
scintilator
photomultiplier
computer
what does a collimator do
thin lead tubes
only allows the gamma rays parallel to the camera to pass through
what doe the scintilator do
converts the gamma rays to multiple visible light photons
what does the photo multipler do
convert the visible light photons to electic signals
what does the computer do
generates an image based on the electric signals
pros and cons of a PET scan
pros
non evasive
can produce real time images
cons
radioactive source is used
patient is exposed to activity
expensive
how are sounds produced and what affects their frequency
vibrations
greater the frequency of the oscilation the greater the frequency of the sound produced
describe an ultra sound A-scan
ultrasound ray is sent out and some is reflected back
and it uses 2distanc = speed/time
(there and back)
decribe an ultrasound B-scan
MANY A-SCANS
and the intensity of the reflected sound is used to generate a light of a matching intensity to produce an image
what causes a high proportion of the ultrasound wave to be reflected
A high differenence in acoustic impedence
what is impedance matching
things like coupling gel to avoid the sound traveling through air as air and skin and very different acoustic impdednace
so a gel of simiar Z can be used to ensure no air gets between so all/most of the ultrasound are absorbed by skin
in the equation
Δf= 2vcos(θ)
f c
what is theta and c
theta is the angle between the blood vessel and the transducer
c is the speed of sound in blood
explain how ultrasound is used ot measure the speed of blood flow in an artery
pulses are aimed at the moving blood cells in an artery
transducer is places at an angle usually 60 degrees
the detected frequency is different to the emitted frequency
ratio of frequencies enables the speed of blood flow to be calculated
photon energy where simple scattering is an appropriate attenuation method
1-20 eV
Photon energy where the photo electric effect is an appropritae attenuation method
30 eV
Photon energy where compton scattering is an appropritae attenuation method
0.5 - 5.5 MeV
Photon energy where Pair Production is an appropritae attenuation method
1.1 MeV+
name some diagnostic applications of a gamma camera
Detection of cancer
scans of organs (in exam maybe name one)
scans of tissue
bone scans
observing the functionality of an organ
in an x ray tube what is the cathode and what is the anode?
cathode is the heated filament
anode is the tungsten target
