X-ray machines contain an X-ray tube that produces X-ray photons that pass through the patient to the detection plate (below the thing being X-rayed). An X-ray tube consists of an evacuated tube containing two electrodes (it is evacuated so electrons can pass through the tube without interacting with gas atoms). A large p.d. is created between the electrodes by an external power supply. The negative electrode (cathode) is a heater which produces electrons by thermionic emission. These electrons accelerate towards the positive electrode (anode). It is important to note that the anode is made from a metal known as the target metal (e.g tungsten). This must have a high melting point. This is because the X-ray photons are produced when the electrons are decelerated by hitting the anode, the energy output fo the X-rays is less than 1% of the kinetic energy of the incident electrons and the remainder of the energy is transformed into thermal energy of the anode (hence a high melting point). In many X-ray tubes oil is circulated to cool the anode/the anode is rotated to spread the heat over a larger surface area. The anode is shaped so the X-rays are emitted in a desired direction with lead to shield the radiographer from sporadic X-rays emitted in a different direction...
hf = eV
hc/λ = eV
λ = hc/eV
The term attenuation is used to describe the decrease in the intensity of electromagnetic radiation as it passes through matter (e.g bone attenuates X-rays more than tissue would). There are four attenuation mechanisms by which X-ray photons interact with atoms (each reduces the intensity of the collimated/parallel/ beam in the original direction of travel):
- Simple scatter
- the X-ray photon is scattered elastically (kinetic energy is conserved) by an electron
- This mechanism is important for X-ray photons with energy in the range of 1-20keV
- The X-ray photon interacts with an electron in the atom but has less energy than the energy required to remove the electron (work function) so the X-ray photon bounces off without a change to its energy
- Photoelectric effect
- the X-ray photon disappears and removes an electron from the atom
- This is significant for X-ray photons with energy less than 100keV
- the X-ray photon is absorbed by one of the electrons in the atom, the electron uses this energy to escape from the atom
- Attenuation of X-ray photons by this type of mechanism is dominant wen an X-ray image is taken as hospital X-ray machines use 30-100kV supplies
- Compton scattering
- the X-ray photon is scattered by an electron, it's energy is reduced, and the electron is ejected from the atom
- This is significant for X-ray photons with energy in the range of 0.5-5MeV
- The X-ray photon interacts with an electron in the atom and is ejected from the atom with reduced energy
- Pair production
- the X-ray photon disappears to produce an electron-positron pair
- This only occurs when X-ray photons have energy equal or greater than 1.02MeV
- An X-ray photon interacts with the nucleus of the atom, it disappears and the electromagnetic energy fo the photon is used to create an electron and a positron
The transmitted intensity of X-rays depends on the energy of the photons and on the thickness and type of the substance (e.g bones will attenuate X-rays more than soft tissue will). For a given substance and energy of photons the intensity falls with the thickness of the substance. Transmitted intensity can be calculated using the following equation:
I = I0e-xμ
(NOTE: the μ is meant to be like μ (as in superscript) but for some reason it wouldn't format properly)I0: the initial intensity before absorption
x: the thickness of the substance
μ: the attenuation/absorption coefficient (the larger the μ the better absorber the substance is). SI unit: m-1.
Soft tissues have a low absorption coefficient so a contrast medium is sue to improve the visibility of their internal structures in X-ray images. The most common are barium sulphate and iodine. They have relatively large atomic number (Z) which is good as μ ∝ Z3:
- iodine is used as a contrast medium in liquids (e.g to view blood flow). An organic compound of iodine is injected into blood vessels so that doctors can diagnose blockages in the blood vessels and structure of organs (e.g the hears) from X-ray images.
- Barium sulphate is used for digestive systems. It is given to the patient in the form of a white liquid mixture which is swallowed.
CAT scans
A CAT scanner records a large number of X-ray images from different angles and assembles them into a 3D image (with sophisticated software). In essence, a patient lies horizontally on their back and slide in/out of a gantry/large vertical ring. The gantry houses an X-ray tube on one side and an array of electronic X-ray detectors on the other side. These rotate within the gantry. The X-ray tube produces a fan-shaped beam of X-rays (~1-10mm thick). This thin beam irradiates a thin 'slice' of the patient, the X-rays are attenuated by different amounts by different tissues. The intensity of the transmitted X-rays is recorded by the detectors which send electrical signals to a computer. A 2D slice is acquired each time the X-ray tube and detectors complete one full rotation. The slices can be manipulated to produce a 3D image of the patient.
Advantages:
- CAT scans can be used to create 3D images - this can help doctors to assess the shape/size/position of disorders (e.g cancers)
- CAT scans can distinguish between soft tissues of similar attenuation coefficients
Okay so the spec doesn't say we need to know the disadvantages but they'll probably be useful to know:
- A traditional X-ray is quicker and cheaper
- The X-rays are harmful as they are ionising radiation - some CAT scans can be quite prolonged so expose the patients to a radiation dose equivalent to several years of background radiation
- Patients have to remain very still during the scanning process (any movement will blur the slice) - super tricky with young patients
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