Medical imaging Flashcards
Production of X-rays (Bremsstrahlung)
Hot filament produces electrons by thermionic emission
Electrons are accelerated through a potential difference to the target metal(the anode)
For some of the electrons(<1%) the kinetic energy is converted into an X-ray photon (Bremsstrahlung)
The rest heat up the target metal
How is target metal chosen
Metal must have a high boiling point
When electrons bombard metal, electrons are removed from metal atoms
Electrons drop from higher levels to fill gap, releasing photons in the process
Material should be chosen so that photons are in the X-ray region
Why are X-rays produced in a vacuum
So electrons don’t interact with gas atoms
Simple scatter
X-ray photon interacts with atomic electron and is absorbed and re-emitted in a random direction
Photoelectric effect
X-ray photon is absorbed by electron and electron is emitted
Compton scattering
X-ray photon interacts with atomic electron
Electron is ejected and remaining energy is released as a new photon with higher wavelength
Pair production
X-ray photon interacts with nucleus and electron-positron pair is produced
Attenuation coefficients
μ
Measure of how much X-ray is absorbed
I= I0e^(-μx)
Contrast medium
Used to improve visibility of internal structures by having a large difference in μ
Must have high z value as z is proportional to μ^3
Iodine is injected to see blood flow
Barium sulfate is eaten to see digestive system
CAT scan
Patient lies on their back in an X-ray tube
X-ray tube produces fan shaped beam and the table moves slowly through tube to create image
2-D images are put together to form a 3-D image
Pros and cons of CAT scan
+ 3D images
Can distinguish between soft tissues
-Expensive
Time consuming
More ionising radiation
Have to remain completely still
Gamma tracers
Gamma cameras detect gamma photons emitted from isotopes injected into patients
Gamma emitting sources are used because they are weakly ionising and stronglypenetrating
Isotope used in gamma tracers/how is it produced
Technetium-99m
Produced from beta minus decay or molybdenum-99
To reach correct organ/tumour, isotope is chemically combined with elements which will reach it to form a radiopharmaceutical
How does a gamma camera work
Scattered gamma rays pass to collimator -long metal tubes that only allow parallel waves
The gamma photons reach the scintillator - 1 photon striking scintillator produces thousands of visible light photons (happens to 1/10 photons)
Light enters photomultiplier tubes
How do photomultipliers work
Photon enters photomultiplier tube and an photoelectron is emitted
Electron is accelerated through a pd so that it knocks off 4 electrons
This continues until many electrons are released and a noticeable current is produced
PET scans
Patient lies on horizontal table surrounded by ring of gamma detectors
Patient is injected with with FDG
Positron is emitted via beta plus decay and almost immediately annihilates with an atomic electron, emitting two gamma photons in opposite directions
Time difference in receiving photons can be used to locate position of annihilation
Isotope used in PET scanners
Fluorine-18 is used
It has a very short half life so must be made in a particle accelerator
Most PET scanners use fluorodeoxyglucose which the body treats as glucose
Pros and cons of PET scanners
+ non-invasive
good for diagnosis
-very expensive
requires particle accelerator
Piezoelectric effect
Some crystals(e.g. quartz) produce an emf when they are compressed or stretched
This effect is reversable - when pd is applied to opposite faces of crystal, the crystal will stretch/compress
How to transducers work
To generate ultrasound, high frequency alternating pd is applied across crystal, causing it to oscillate at frequency of pd
Transducer is also used to detect ultrasound - incident ultrasound causes crystal to oscillate, producing alternating pd
Frequency of medical ultrasound
Frequency of ultrasound = 5MHz
Frequency of pulses being emitted = 5kHz
A-scans
Single transducer is used to record along a straight line through the patient
Some ultrasound is reflected at each boundary and reflected pulse is received by transducer
Distance to boundary can be calculated
L=vt/2
B-scans
Transducer is moved over patients skin
Each position makes a row of dots on the computer which represent boundaries
These dots are compiled to form a 2-D image
Acoustic impedance(z)
The product of density and the speed of the ultrasound through substance
Ir/I0 = (Z2-Z1)^2/(Z2+Z1)^2
