6.5 Medical Imaging Flashcards
(44 cards)
Structure of X-ray tubes - heater
An external power supply connected to a cathode causes electrons to be produced by thermionic emission
Structure of X-ray tubes - anode (4)
The electrons are accelerated towards the positive anode
It is made from the target metal (e.g. tungsten) which has a high melting point
The electrons are decelerated by hitting the anode, producing X-ray photons which are directed towards a window
The anode is usually rotated to keep it cool as about 99% of the kinetic energy of the incident electrons is converted to thermal energy
How is the wavelength from an X-ray tube related to the accelerating PD?
Wavelength is inversely proportional to PD
What are K-lines?
Lines produced on a characteristic spectrum when ionisation occurs in target metal
How does changing current affect the characteristic spectrum of an X-ray?
At a higher current, the peak is higher but at the same photon energy
How does changing voltage affect the characteristic spectrum of an X-ray?
At a higher voltage, the peak moves up and to the right
Define attenuation
The decrease in the intensity of an electromagnetic radiation as it passes through matter
Attenuation mechanisms - simple scatter (2)
Energy range - 1-20 keV
A photon with insufficient energy interacts with an electron in the atom so the photon is scattered elastically
Attenuation mechanisms - photoelectric effect (2)
Energy range - <100 keV
The photon is absorbed by an electron in the atom, causing it to escape the atom
Attenuation mechanisms - Compton scattering (3)
Energy range - 0.5-5.0 MeV
The photon has more energy than it needs so after it interacts with an electron, causing it to leave the atom, the photon is scattered with reduced energy
Both energy and momentum are conserved
Attenuation mechanisms - pair production (2)
Energy range - >/ 1.02 MeV
The photon interacts with the nucleus of the atom causing it to disappear, creating an electron and a positron
Why is simple scatter insignificant for hospital radiography?
X-ray machines use PDs greater than 20keV
What does the transmitted intensity of X-rays depend on? (2)
The energy of the photons
Thickness and type of substance
What is a collimator used for?
It produces parallel X-rays by filtering out non-parallel beams, causing the image to be clearer
Why is a glass lens impractical for producing parallel X-rays?
The wavelength of X-rays is too short
Why are contrast media used?
To improve the visibility of internal structures because soft tissues have low absorption coefficients
Contrast medium - barium (2)
Barium sulphate used to image digestive systems
White liquid mixture (barium meal) that is swallowed before image is taken
Contrast medium - iodine (2)
Used to image the circulatory system
It is injected into blood vessels to diagnose blockages
What is a CAT scan?
A series of cross-sectional X-ray images from different angles, used to produce a three-dimensional image
CAT scanner components - X-ray tube (2)
The tube produces a fan-shaped beam and rotates opposite to an array of X-ray detectors
The thin beam irradiates a thin slice of the patient, producing a single 2D image
CAT scanner components - computer and display
Used to combine the 2D slices into a three-dimensional image
What are the advantages of a CAT scan over an X-ray? (2)
Used to create a 3D image
Can distinguish between soft tissues of similar attenuation coefficients
What are the disadvantages of a CAT scan compared to an X-ray? (3)
X-rays are quicker and cheaper
CAT scans are longer so there is more exposure to ionising radiation
Patients have to remain very still in a CAT scan
Why is technetium-99 used as a medical tracer? (4)
It decays by releasing gamma photons
It has a half life of around 6 hours
It can be injected into the bloodstream or ingested
it can be easily incorporated into different molecules to monitor a variety of organs
