Chapter 27 - Medical Physics Flashcards
Describe the basic structure of an x-ray tube, and explain how it produces x-ray photons.
The two electrodes - a hot filament and target metal are in a vacuum, with a large P.D. across them. Electrons are emitted from the filament and hit the target metal at high speed, their deceleration causing x-ray photons to be emitted. The photons pass through a window out of the tube.
Define attenuation.
The decrease in intensity of electromagnetic radiation as it passes through matter/space.
What are the four x-ray attenuation mechanisms?
Simple scatter, photoelectric effect, Compton effect, pair production.
Describe and explain simple scatter.
The x-ray photon interacts with an electron, but does not have the required energy to release the electron, so bounces off.
Describe and explain the photoelectric effect.
The x-ray photon is absorbed by an electron, and its energy is used to release the electron from the atom.
Describe and explain Compton scattering.
The x-ray photon interacts with an electron, the electron is ejected and the photon is scattered with reduced energy.
Describe and explain pair production.
A high-energy x-ray photon interacts with the nucleus of an atom, and its energy is used to create an electron and a positron.
Describe the relationship between x-ray intensity and the thickness of the material it is travelling through.
Intensity decreases exponentially with thickness.
What is a contrast medium?
A substance inserted into a patient to improve the visibility of internal structures in soft tissues.
Describe Computerised Axial Tomography (CAT) scanning.
-An x-ray tube and detector array rotate 360° around the patient.
-Scan creates a 3D mapping of the area.
-This can be viewed in 2D ‘slices’.
What are the pros & cons of using a CAT scan over an x-ray scan?
-Better at distinguishing soft tissues.
-Slower & more expensive.
-Longer exposure to x-rays (so more harmful).
-Patients must stay still for a clear image.
What is a medical tracer?
A compound labelled with a radioisotope that can be traced inside the body with a gamma camera.
What features are important for a medical tracer to have?
-Short half-life
-Can penetrate through the patient
-Not very ionising
Give two tracers that are commonly used for the gamma camera.
Technetium-99m and flourine-18.
Roughly describe the structure of the gamma camera.
-Gamma photons enter at the collimator, which makes them parallel.
-Photons hit the scintillator, which produces many light photons.
-Photomultiplier tubes convert light into electrical pulses.
How can medical tracers be used to diagnose patients?
They target and build up in certain areas of the body, e.g. where there is a blockage or a particular type of cell.
How does fluorine-18 produce gamma photons.
It emits positrons, which annihilate surrounding electrons, producing a pair of gamma photons travelling in opposite directions.
Describe a positron emission tomography (PET) scan.
-Gamma detectors are in a ring around a patient.
-They pick up radiation from all angles.
-These pulses are used to create a 3D mapping, viewed in 2D slices.
-Brightness/colour on the image indicates concentration of tracer.
Give some positives of using a PET scan.
-Non-invasive (doesn’t require surgery.
-Can be used to diagnose different types of cancers or observe brain function.
Give one negative of using a PET scan.
-Expensive due to the facilities needed to produce medical tracers.
Define ultrasound.
Sound with a frequency greater than 20kHz (beyond human hearing range).
What is the piezoelectric effect?
The effect that causes some crystals to:
-Produce an e.m.f. when stretched/squashed.
-Stretch/compress as a result of a P.D. across it
What is an ultrasound transducer?
A device used to transmit & receive ultrasound using the piezoelectric effect.
Explain the ultrasound A-scan.
-A transducer records along a straight line through the patient.
-Each pulse is partially reflected & transmitted at the boundary between tissues.
