Chapter 27 - Medical Imaging Flashcards
Acoustic impedance (Z)
Property of a material that determines the intensity of ultrasound reflected at a boundary with another material.
Compton effect
The effect whereby X-rays deflected off particles have a longer wavelength than their initial wavelength.
CAT scan
Computerised Axial Tomography - process using X-rays and computers to produce an image of a slice through the body.
Many slices are used to produce 3D images.
Pair production
Particle-antiparticle pair created from a high energy photon e.g. X-ray -> electron + positron.
Piezoelectric effect
Change in volume of certain crystals when a p.d. is applied across them
OR
production of e.m.f. when certain crystals are placed under stress.
PET scan
Positron emitting tomography - detects gamma photons produced when positrons annihilate with electrons inside the body.
Used to map out active areas in the body.
Simple scatter
Deflection of charged particles off other charged particles e.g. alpha particle and nucleus of gold atoms.
Transducer
Device that converts non-electrical signal into an electrical signal e.g. microphone.
Ultrasound
Sound waves above the audible range of human hearing i.e. greater than 20kHz.
Characteristics of X-rays
Can be polarised, diffracted by atoms in crystals.
Extremely short wavelengths.
EM waves - travel through vacuum at speed of light.
Harmful to living cells, can kill them.
How are X-rays produced?
- Large p.d. between cathode and anode.
- Cathode produces electrons by thermionic emission.
- Electrons accelerated towards anode (made of target metal such as tungsten).
- Deceleration of electrons produces X-ray photons, which are emitted through a window.
Attenuation coefficient
A measure of absorption of X-ray photons by a substance. SI unit: /m
What are the four attenuation mechanisms?
Simple scatter
Photoelectric effect
Compton scattering
Pair production
What is the purpose of a contrast medium?
Improves invisibility of internal structures in X-ray images.
Able to see outline of certain organs.
Examples of contrast media
Iodine or barium compounds.
Components of CAT scan
Rotating X-ray tube that produces thin fan-shaped X-ray beam.
Ring of detectors.
Computer software and display.
Advantages of CAT scan
Can create a 3D image.
Can distinguish between soft tissues of similar attenuation coefficients.
Disadvantages of CAT scan
Longer and more expensive than simple X-ray.
Prolonged exposure of ionising radiation.
Patients must remain still during scan.
Examples of medical tracers
Technetium-99 and fluorine-18.
Components of gamma camera
Collimator
Scintillator
Photomultiplier tubes
Computer and display
Advantages of PET scans
Non-invasive technique
Can be used to diagnose different cancers, plan heart surgery, observe brain function.
Used to assess effect of new drugs on organs.
Disadvantages of PET scans
Very expensive (esp. to make medical tracers) so usually found at larger hospitals.
A scan (ultrasound)
Single transducer used to record along a straight line through patient.
Used to determine thickness of bone or distance between lens and retina in eye.
Total distance travelled by ultrasound pulse is 2L.
B scan (ultrasound)
Output of transducer connected to computer. For each position, computer shows rows of dots on screen (brightness show intensity of reflected pulse).
2D image produced.
What does coupling gel do?
Has similar acoustic impedance to skin.
Fills air gaps between transducer and skin.
Ensures that almost all ultrasound enters body and not reflected at air-skin boundary.
Doppler effect in ultrasound
Ultrasound reflected off of moving blood cells returns with changed frequency.
Doppler shift in frequency directly proportional to speed of blood flow.
Transducer must be held at angle as cos90 = 0 (no observed change in f).
Use of colour Doppler scans
Transducer connected to computer to show colour-coded image of direction and speed of blood flow.
