Medical Imaging Flashcards
State the range of wavelengths for X-rays
10^-8 to 10^-13m
State the main property of X-rays and it’s uses
- they have very high energy
- they can kill cells at high exposure
- they can be used to treat cancer
Explain how X-rays can be produced
draw diagram
- the high voltage accelerates the electrons - they gain Ek
- electrical energy -> kinetic energy
- W = QV = eV = 1/2mv²
- when the electrons hit the target metal, Ek becomes the energy of an EM wave
- eV -> 1/2mv² -> hc/λ
- eV = hc/λ
λ = hc/eV
In X-ray imaging what forms the image?
It is the effect of different media on the intensity of the X-rays which forms the image
State and describe the four X-ray attenuation mechanisms
Simple scatter - the X-ray photon is scattered elastically by an electron
Photoelectric effect - the X-ray photon disappears and removes an electron from the atom
Compton scattering - the incident X-ray photon is absorbed by an electron, a lower energy photon is emitted, and the electron is ejected from the atom
Pair production - the X-ray photon disappears to produce and electron-positron pair
At what photon energy does simple scatter occur?
1-20keV
At what photon energy does the photoelectric effect occur?
1-100keV
At what photon energy does Compton scattering occur?
500-5000keV
At what photon energy does pair production occur?
> or equal to 1.02MeV
What is the typical value for a hospital X-ray?
30keV - photoelectric is what dominates
What are disadvantages of X-ray scans?
- cannot resolve overlaps
- cannot distinguish between soft tissues
State the advantages and disadvantages of CAT scans
Advantages:
- gives a 3d image
- can resolve overlaps
- can distinguish between soft tissues of similar attenuation coefficient
Disadvantages:
- patient must stay still
- very high exposure
- expensive
State two common tracers used to monitor processes in the body
Fluorine - 18
Technetium - 99m
State the list of components in a gamma camera in order from closest to patient to furthest away
Collimator
Scintillator
Light guide
Array of photomultiplier tubes (learn the diagram for these)
Circuits
Computer
Display
State the properties of ultrasound waves
They are longitudinal waves with a frequency greater than 20kHz
Describe the piezoelectric effect
Some crystals produce an e.m.f if they are compressed - an effect which is reversible
What does an ultrasound transducer produce?
5000 pulses per second of 5MHz ultrasound
State and describe the two types of ultrasound scan
A-scans - 1 directional
B-scans - multiple A-scans combined to form a 2d image
What is the equation for acoustic impedance of a medium?
Acoustic impedance = density of medium x speed of ultrasound in the medium
z = ρc
What can happen to ultrasound at boundaries between media?
It can be:
- reflected
- refracted
- transmitted
What equation can be used when considering only ultrasound travelling along the normal?
Ir/Io = ((z2-z1)/(z2+z1))²
How can we avoid reflection before the ultrasound reaches the body?
We can use an acoustic matching gel - or coupling gel - with a similar z (acoustic impedance) to skin
What are some of the advantages and disadvantages of using MRI techniques?
- method does not use ionising radiation, so no radiation hazard to patient or staff
- gives better soft tissue congrats than CT scans
- generates data from a 3D volume simultaneously
- information can be displayed on a screen as a section in any direction
- there are no moving mechanisms involved in MRI
- strong magnetic field could draw steel objects into the magnet
- metallic objects may become heated
- cardiac pacemakers may be affected by the magnetic fields
- CT scanners better for viewing bony structures
Define intensity reflection coefficient
The ratio of the reflected intensity Ir to the incident intensity Io (Ir/Io)
