Using X-rays (6.5.1)
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X-rays can be artificially produced using X-ray tubes. Electrons are expelled from a heater (cathode) into a vacuum via thermionic emission:
- Thermal energy from the heater liberates electrons from their atomic nuclei.
- The electrons are then rapidly accelerated by a high voltage electric field through the vacuum towards a target metal.
- Finally, the electrons are rapidly decelerated by colliding with a target metal anode (e.g. tungsten).
- Colliding with the anode makes the electrons lose most of their kinetic energy. The lost kinetic energy is converted into X-rays.
It is important to note that the internal vacuum is necessary to prevent electrons from colliding with air molecules. An external power supply creates a potential difference across the cathode and anode. A high-voltage supply is required to generate an electric field strong enough to produce X-rays.
X-rays are produced in every direction from the anode of an X-ray tube. However, in practice, the X-rays need to be guided in one direction towards a sample or object, such as a suspected broken bone. Hence, one side of the X-ray tube has a window made of a material thinner than the lead casing to allow unidirectional emission of X-rays.
The beam is directed into a series of straight, parallel metal tubes (called a collimator) that further confine the beam by absorbing any rays that are not parallel to the axis of the tubes.
Inside an X-ray tube, a heated filament (cathode) emits electrons via thermionic emission. The filament is typically made of tungsten, which can withstand high temperatures. A high external voltage of around is applied between the cathode (negative) and the anode (positive), creating an electric field. This accelerates the emitted electrons towards the anode.
A potential difference translates to a kinetic energy gain of of the electrons. It is important to note the difference in units here. This is due to the equation:
Where:
- is the energy transferred to the particle,
- is the charge (an electron has a charge of one electron volt eV), and
- is the potential difference across the X-ray tube.
Inside an X-ray tube, a filament (cathode) is heated so electrons gain energy; if they gain enough energy to escape the metal surface, this is called thermionic emission. The emitted electrons cross the vacuum and are accelerated towards a metal target (the anode).
When the electrons strike the anode, only about 1% of their kinetic energy is converted into X-ray photons, with the rest transferred to the anode as thermal energy.
When a high-speed electron is incident on the tungsten anode in an X-ray tube, X-rays are produced by two different types of interactions:
Bremsstrahlung radiation: Electrons slow down or brake when they come near the nucleus of the tungsten atoms, they lose energy in the form of X-ray photons. This type of radiation produces a broad spectrum of X-rays.
It is a broad spectrum as the electrons can have a range of kinetic energies as they accelerate through the X-ray tube. This creates a range of kinetic energy losses when they collide with the anode. Bremsstrahlung radiation is sometimes referred to as braking radiation.
Characteristic radiation: Some electrons collide with the inner shell electrons of tungsten atoms, knocking them out of their orbit. When electrons from higher energy levels drop down to fill the vacancy, they release energy in the form of X-ray photons with specific, characteristic energies.
X-rays ionise the matter they pass through, and will lose energy when interacting with atoms or molecules. As the X-ray passes through matter, the intensity (or energy) of the beam will decrease: this is known as X-ray attenuation.
There are four main X-ray attenuation mechanisms that cause X-ray beams to be attenuated and the X-rays to lose energy:
- Simple scattering
- Photoelectric effect
- Compton scattering
- Pair production
The mechanism that dominates is dependent on the energy of the incoming X-rays.
Simple scattering is a type of X-ray attenuation mechanism and is common in the lowest energy X-rays in the range of
An incoming X-ray photon will be reflected by an electron in the material. The X-ray will be elastically scattered and change direction.
This is the simplest attenuation mechanism, and will not affect the energy or wavelength of the X-ray beam. It will not lose energy as the incoming X-ray has insufficient energy to ionise the atom.
The photoelectric effect is a type of X-ray attenuation mechanism that affects X-rays with energy less than but greater than
These X-rays can match the ionisation energy of the atoms in the material. This results in the incoming X-ray being absorbed by an orbiting electron and the emission of a photoelectron (a liberated electron from within the atom).
After the photoelectron leaves the atom, another electron may transition down to fill the now vacant position: this electron movement also releases a scattered low-energy photon as the electron moves from a higher energy state to one of lower energy.
The Compton effect is an X-ray attenuation mechanism which dominates for X-rays with energies between and
An incoming X-ray photon will inelastically scatter off an electron within a material: it is not absorbed by the atom as it has too much energy. Instead, inelastic scattering imparts some of the energy into the atom. The scattered X-ray photon will have decreased energy, and therefore an increased wavelength.
The electron involved (sometimes referred to as a Compton electron) will also be ejected from the atom, scattered in a different direction in order to conserve momentum.
Pair production is an X-ray attenuation mechanism that can occur for the highest energy X-rays over
An incoming X-ray photon will interact with the electric field of an atom and spontaneously transfer all energy into producing an electron–positron pair that travels in opposite directions to conserve momentum. The positron will then quickly collide with a different electron and annihilate. Annihilation of a particle–antiparticle pair will produce photons.
The incoming X-ray photon must have a lot of energy to produce an electron–positron pair; these energies are not used in medical imaging, so pair production normally is not observed in this context.
It is useful to note that Compton scattering is the main cause of X-ray energy loss between roughly 30 keV and a few MeV. This is because its interaction probability decreases slowly. Other processes like photoelectric absorption (at lower energies) and pair production (above 1.02 MeV) also happen, but Compton scattering is simply the most likely to occur in that middle energy range. Remember that these “ranges” overlap; they aren’t strict cut-offs.
X-rays are a form of ionising radiation: as they pass through matter they will lose energy due to attenuation. This causes a gradual decrease in the intensity of the X-ray, where intensity is defined as the power per unit of cross-sectional area and has units of
The penetrating power of the X-ray is dependent on the material it is passing through. An incoming X-ray will be attenuated based on:
- the thickness of the material,
- the density of the material, and
- the atomic number of the material.
A very thick, dense, high atomic number material will attenuate the X-ray a lot more than a thin, less dense, low atomic number material.
Different materials will attenuate X-rays to a different extent; this can be used to distinguish different materials inside the body such as bone and soft tissue.
Applying the varied attenuation abilities of different materials is vital for medical imaging. For example, an X-ray image must discern between muscle, soft tissue, and bone to diagnose potential health issues in a patient.
Bone attenuates X-rays more than soft tissue. Therefore, X-rays propagating through bone are more likely to be absorbed, and the beam is more attenuated on the other side of the bone.
Photographic film will turn from white to black if exposed to a high intensity of X-rays. Therefore, X-rays passing through bone will be attenuated and not turn the areas of the plate immediately behind the bone black. This contrast is evident in the image above. Modern digital detectors can also be used to enhance image processing, store and transfer images.
The intensity of a collimated beam of X-rays decreases exponentially when attenuating. The intensity can be modelled as:
Where:
- the initial intensity before the material is
- is the intensity after the beam passes through a material thickness of (in metres), and
- is the attenuation coefficient (units of ).
The attenuation coefficient (sometimes called the absorption coefficient) depends on the material the X-rays are passing through: it is a measure of how well a material can absorb X-rays. For instance, the attenuation coefficient is much higher for bone than soft tissue. The attenuation coefficient of materials is linked to the proton number,
Contrast media are easily seen on X-ray images; these materials have a high proton number , and hence a high attenuation coefficient. Common contrast materials:
- barium
- iodine (
Soft tissue has a composition of different atoms, which has an average proton number, , and has a low attenuation coefficient.
The correlation between the attenuation coefficient of a material and proton number is due to the photoelectric effect: this is the dominant absorption mechanism at X-ray frequencies of In general there is a proportionality of:
Contrast media are used to distinguish between different tissues in the body that look very similar in normal X-ray imaging.
In medical X-ray imaging, barium and iodine are commonly used as contrast media.
Iodine is used as a contrast medium in liquids, for example, to observe blood flow. An organic iodine compound is injected into the bloodstream. In the X-ray image, low attenuated intensities indicate where the blood is flowing well, and areas with high attenuation (where the iodine is collecting) indicate poor blood flow.
Barium sulphate is used as a contrast medium in the digestive system. A liquid ‘barium meal’ is swallowed by the patient. Similarly, areas where the barium builds up will be highly attenuated and can locate blockages.
A computed axial tomography scan or CAT scan is a medical imaging technique that uses X-rays to analyse a patient’s body structure.
It comprises the following key elements:
- Rotating X-ray source: an X-ray tube rotates around the patient. It produces a thin, fan-shaped X-ray beam that passes through the body, targeting specific slices or sections.
- Ring of detectors: a ring of detectors encircling the patient capture the X-rays after they pass through the body. As the X-ray beam rotates, the detectors measure the amount of X-rays absorbed by different tissues, which varies based on the attenuation ability of different materials.
To generate images on a computer, software and displays are used.
- Computer software: data from the detectors is sent to a computer. Using specialised software, the computer processes this data, performing complex mathematical calculations to generate detailed images of the scanned body part.
- Display: processed data is converted into visual images that can be displayed on a monitor. These images show detailed cross-sections (slices) of the body, allowing healthcare professionals to make accurate diagnosis.
There are advantages of using CAT (computed axial tomography) scans over traditional X-ray medical imaging:
- Cross-sectional imaging: CAT scans provide detailed cross-sectional slices of the body, offering a better view of internal structures. X-ray images are 2D, which can make it difficult to distinguish overlapping tissues.
- Better soft tissue contrast: CAT scans are superior at visualising soft tissues, such as the brain and blood vessels, which are hard to differentiate on X-ray images.
- Diagnostic accuracy: CAT scans can provide more extensive information for healthcare professionals to use for diagnosis, making their conclusions more accurate.
