Chapter 27 - Medical Physics Flashcards
X-ray wavelengths
10^-13 –> 10^-8 m
Producing x-rays
Eject electrons from a cathode in thermionic emission with a p.d. of 50 to 200kV
They are attracted to an anode (tungsten), reducing the kinetic energy of electrons and converting to photons
Bremsstrahlung
A continuous spectrum of energies of x-rays emitted as electrons lose kinetic energy
Characteristic x-rays
Emitted from the anode as electrons move between energy levels in the anode
Shortest wavelength of x-rays
λ = hc/eV
Hard and soft x-rays
Hard (0.10 to 0.01nm) are more penetrative than soft (10 to 0.1nm)
An aluminium sheet is used to filter out ionising but less useful soft x-rays
Collimated x-rays
A parallel beam
μ
The linear attenuation coefficient (the percentage reduction in intensity per unit length)
Attenuation
The decrease in intensity of EM radiation
X-ray intensity formula
I = I0 e^-μx
Simple scattering
X-rays <20keV cannot remove electrons from atoms so they are scattered and intensity is reduced
Soft x-rays are filtered so not significant in radiography
Photoelectric effect
If hf > Φ, electrons absorb a photon and are emitted, meaning the intensity is reduced as photons are lost
Compton effect
0.5 to 50 MeV energy
A photon with momentum collides with an electron, knocking it out of the nucleus, reducing its own momentum
The energy is reduced and likely no longer an x-ray photon
Pair production
Energy >= 1.02 MeV
A photon exchanges its energy for mass, producing an electron-positron pair
Contrast media
Compounds with high atomic numbers create scattering in soft tissue
μ ∝ Z^3
e.g. barium
Used to detect the boundaries between media
CAT scan
Computerised Axial Topography
An x-ray tube is mounted on a ring with a sensing detector array opposite
The tube and detector rotate around the ring and a patient moves slowly through it to create a “spiral” of slices
The images are overlayed by a computer
CAT scan advantages and disadvantages
+ much higher contrast resolution than x-rays, much more precise than x-rays
- much more ionising than x-rays, take longer
Tracers
Small amounts of radioactive material into the body to follow blood flow or to investigate metabolic pathways
Tracer uses
To replace a stable atom and act as labels to follow the path of a compound
Tc-99
Tc-99m is a gamma emitter with a 6 hour half life
(m is for metastate before gamma is emitted)
Stable Tc-99 then emits beta minus
Gamma camera
Detects gamma photons from the body and a collimator ensures only photons from the body are detected
A scintillator converts the gamma photons to visible photons and a photomultiplier converts photons to many electrons
Photomultipliers
- Photon emits a photoelectron
- Increasing pds attract these to release electrons from dynodes
- The amount of electrons continually increases
PET scan
Positron Emission Topography
Uses F-18, a beta plus emitter
PET scan process
The F-18 decay will release a positron that will travel a few mm before annihilating with an electron in the body. This releases a pair of gamma photons in opposite directions so this and the time difference tells you the location
PET scan F-18 production process
O-18 + proton –> F-18 + neutron
Must be done on site due to the short half life
Ultrasound frequencies
Above the limit of human hearing 20 kHz
1-3 MHz are used in scans
Piezoelectric effect
Piezoelectric crystals produce a net p.d. across its sides as it is stretched or compressed
An imbalance of charge causes a net charge on opposite sides
Passing an A/C causes the crystal to stretch and compress as the charge on each side changes, choose a p.d. and frequency to generate ultrasound
Ultrasound transducer
A piezoelectric crystal between electrodes, connected to a computer by a cable
Surrounded by damping material
An acoustic window focuses the sound waves
A-scans
Surfaces between media of different densities partially reflect ultrasound
The reflected ultrasound is detected by the transducer and causes the crystal to change shape
average speed x distance gives twice the distance from the scanner
B-scans
A 2D image built up from A-scans
Each A-scan produces a series of dots between boundaries between tissues
Acoustic impedance
Z = ρc
Acoustic impedance = density x speed of sound in a material
Reflected intensity formula
I(r)/I(0) = (Z2 - Z1)^2/(Z2 + Z1)^2
Intensity-reflection coefficient
I(r)/I(0)
Impedance matching
99.9% of the ultrasound should be reflected by the skin based on the acoustic impedance difference to air
A coupling gel with a similar acoustic impedance to skin must be used to remove the air gap
Doppler ultrasound
If an ultrasound wave is reflected from red blood cells, the reflected wave has a different frequency to the initial due to the doppler effect. Only possible for blood vessels with fast blood flow
A computer can colour-code the images
Doppler ultrasound formula
Δf/f = 2vcosθ/c
θ is the angle between the pulse and the motion of the blood
c is the speed of sound in the medium
