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Ionising radiation has had a significant role in medicine ever since the discovery of X-rays and radioactivity at the end of the 19th century. The benefits that is has generated are numerous; however there is a risk of harm from exposure. The aim of this essay is to present information about a number of imaging modalities and treatments using X-rays (a type of ionising radiation) and how these techniques can best be used with minimal risk to the patient.
Generation of X-rays
X-rays are produced when electrons from a heated filament are accelerated by a high voltage towards a target normally composed of metal (such as copper), tungsten or molybdenum. When the electrons strike the atoms and nuclei of the target, electrons are knocked out of the inner shells. These vacancies are filled when electrons drop down from higher energy levels. The well-defined difference in binding energy, characteristic of the material, is emitted as a monoenergetic photon. When detected this X-ray photon gives rise to a characteristic X-ray line (sharp peaks) in the energy spectrum (1).
Bremsstrahlung X-rays are generated with a 50KV voltage, and the accelerated electron (much lighter than the nucleus) comes very close to a nucleus in the target but is deviated by an electromagnetic interaction. This deviation in trajectory causes the electron to lose a great amount of energy and emit an X-ray photon. The energy of the emitted photon can take any value up to a maximum corresponding to the energy of the incident electron giving a broad band emission. This process is known as bremsstrahlung (braking radiation) (1).
X-rays can penetrate substantial thickness of material as they only interact with occasional atoms. This is one of the reasons why they are useful for imaging and why it is important to understand their interaction with matter.
Interaction of X-rays with matter
As an X-ray beam passes through continually thicker layers of a material, the intensity of the X-ray is reduced. This decrease in intensity is referred to as attenuation and two mechanisms contribute to it, namely absorption and scattering.
This is an elastic collision; the photon loses no energy but changes direction.
The x-ray photon is scattered elastically from an atom when its electron cloud is accelerated by the electric filed of the photon.
The accelerated charges re-emit photons in any direction.
If the wavelength of the x-ray is less than the atoms dimensions, every electron re-radiates independently and therefore the sum of scattering from any atom linearly increases with the number of electrons (a.k.a the atomic number Z) (2).
Therefore short wavelength x-ray radiation is scattered far more effectively than longer wavelengths (3).
It is an example of inelastic scattering.
The photon comes in, interacts with electrons in the outermost shells of the atom, producing an X-ray that has lost a certain amount of energy that is dependant only on the scattering angle (4).
Compton scattering decreases with increasing energy, so scatter production decreases with increasing photon energy.
The atom absorbs all the energy from the incident x-ray photon and an electron is emitted.
As the atom as a whole has to absorb momentum from the photon, it becomes less frequent as the photon energy increases.
The possibility of an electron being emitted increases considerably when the atomic number (Z) increases (5).
X-ray source: An x-ray tube running a voltage between cathode and anode of about 100kVp produces the photons.
Filter: Low energy photons that wouldn't get through the patients' body are removed as they wouldn't contribute to the image.
Collimator: Reduces both the total energy absorbed by the patient and the amount of tissue producing Compton scattered photons that hit the film. Since the image is based on shadows that assume photons travel in a straight line from the x-ray source, scattered photons reduce the image quality, so the less of them the better (6).
Anti-scatter grid: Removes more Compton-scattered photons.
Detector: Records the image and measures the quantity of radiation that the patient was exposed to. A film is most commonly used. Its efficiency can be improved by placing it in contact with a fluorescent screen, as this improves x-ray absorption. Each x-ray photon produces large numbers of light photons when it interacts with the screen, so the amount of radiation needed to form an image is reduced by a factor of about 100, reducing the dose to the patient (7).
In an x-ray image the factors that affect contrast are the thickness and attenuation of the target (tissue in the patient's body).
Heavier atoms attenuate x-rays more, which explains why x-rays are good at imaging broken bones; bones are denser and have a different attenuation coefficient (due to a different photoelectric cross section) than tissue as they contain heavier atoms such as Calcium which show up more readily on the x-ray image compared to soft tissue.
Image contrast decreases rapidly with increasing photon energy, so for best contrast low photon energy should be used. However utilising a low energy gives a greater dose of radiation to the patient. Therefore the objective is to select an x-ray beam spectrum that provides the optimum balance between contrast and dose. To select the spectrum of an x-ray beam that will give optimum results the combinations ofÂ the anode material, the filter material and thickness, and the selected KV for the procedure must be considered. Since most x-ray examinations are performed with tungsten anode tubes and have the same amount of filtration (few mm of Aluminium), the first two factors cannot be used to adjust contrast. The exception to this is in mammography when molybdenum and rhodium anode tubes and filters are used to optimize the contrast to dose relationship. As the breast is completely composed of soft tissue it has extremely low contrast so achieving the correct relationship is important for the patient.
Figure : The Optimum Photon Energy Increases with Breast Size and Density
The "moly-moly" spectrum (Figure 2) is most frequently used in mammography as it is very close to the optimum spectrum, especially for smaller and less dense breasts. However the x-ray beam contains the bremsstrahlung spectrum with energies in the range of 24 kV to 32 kV.Â This part of the spectrum is undesirable because of its increased penetration which reduces the contrast.Â That problem is solved by using a molybdenum filter.
Figure : Spectrum Produced with Molybdenum Anode and Molybdenum Filter
A Rhodium filter has a slightly higher atomic number (Z)Â than the molybdenum filter so is used when you want to image denser breast material as it increases the energy spectrum (Figure 3). The additional penetration that it provides improves visualization within the denser tissue areas.Â
Figure : Spectrum Produced with Molybdenum Anode and Rhodium Filter
As the rhodium anode has a higher atomic number (Z) than the molybdenumÂ anode, it produces characteristic x-radiation with higher energies. The rhodium anode is selected always with the rhodium filter as the beam penetration is increased and optimum for imaging dense breast. It does this by extending the spectrum (Figure 4) so "seeing through" some of the more dense areas is easier as the contrast and visibility has improved.Â However, this increased penetration can reduce contrast in other breast environments.
Figure : Spectrum Produced with Rhodium Anode and Rhodium Filter
Therefore the optimum spectra in mammography for various breast sizes and densities are obtained with combinations of molybdenum and rhodium anodes and filters, with energy values in the range of 24 kV to 32 kV (8).
In general for all other x-ray images the only spectrum controlling factor that can be changed by the operator to alter contrast is the energy range. It is typically 17-150KeV, with the higher energies used to image thicker body sections. In this energy range the important photon interactions are the photoelectric effect and scattering
Noise and dose
It is due to statistical fluctuations in the number of x-ray photons detected per unit area (quantum noise). Quantum noise can be reduced by increasing the number of photons used to form the image, but the disadvantage is that it increases the dose to the patient. There is a minimum surface dose that is can be used to see a contrast over an area against a background noise arising purely from quantum noise.
is the mass energy absorption coefficient for the tissue, is the photon energy, is the receptor efficiency, k is the ratio for when the object becomes detectable, x is the depth of tissue, and is the area of tissue.
Therefore it can be seen that the minimum dose required to visualise an object increases as the inverse fourth power of the size of the object, so the larger the object the more dose is required for contrast to still be good (9). It can also be seen that different types of tissues have varying sensitivity to x-ray exposure (due to mass energy absorption coefficient). So the actual risk to different parts of the body from an x-ray procedure varies (10). In x-ray images the overall dose that the patient is exposed to is determined by the film sensitivity of the image receptor, hence why there is development in detectors for x-ray imaging.
Computerised Tomography (CT)
CT scans use x-rays to generate cross-sectional images of the body. It is primarily a soft tissue imaging device. The equipment comprises of an x-ray tube and an array of detectors contained within a gantry which the patient is passed through. Collimated x-rays pass through a section of the patient's body to produce a 1-D set of x-ray attenuation data. The x-ray tube and detectors move in a circle within the gantry around the patient's body (while they pass through the scanner) which gives a large number of data from different directions around the body maximising attenuation information from different parts of the body. Cross-sectional images are then constructed via software from the multiple X-ray projections to show differences in tissue densities (11).
CT images are usually obtained with high x-ray tube energies in the range of 120-140kV. Due to these high energies, Compton scattered photons effect contrast. Filters must therefore be used to remove lower energy x-rays.
The number of x-ray photons is also has an effect on image quality. If the number of photons is too low, then background 'noise' may be too large, preventing small variations in contrast to be seen. The number of photons is determined by x-ray tube output, scanning time and the width of slice being imaged. If noise levels are similar, the x-ray output is large for thinner slices and smaller for thicker slices, but dose must be kept to a minimum.
Variation in image contrast is primarily due to difference in tissue attenuation. Body tissue attenuations have a wide range, so windowing is used to distinguish small changes. The window can be changed to focus on high or low density structures, e.g for the analysis of the structure of lungs, the window is reduced to low attenuation, so all other soft tissues fall outside the visible window and are white (12).
Detectors need to maintain a high precision of their individual x-ray photon to electric current conversion factors throughout the scan to keep the resolution of the image good. Variations in the detectors tension alters the conversion factor so is monitored throughout the scan. This is achieved by the extreme edges of the x-ray beam being purposely arranged to miss the patient and enter special monitoring elements.
To achieve spatial resolution of less than 1mm, the detectors must be small and there must be a sufficient number a measuring points. Minimum resolution images are produced with 500 orientations of the tub-detector with respect to the patient. Maximum resolution images are produced from oversampling, with as many as 3000 orientations (13).
Comparison of CT and X-rays
CT produces much more information than a single x-ray radiograph purely because it combines information from many radiographs. But this means a CT scan exposes the patient to a much larger radiation dose. Its use is therefore restricted to life threating and serious illnesses (e.g. cancer and head injuries). Further development of CT to give a higher spatial resolution thus is effectively blocked by the issue of dose.
X-rays images have a loss of depth information, because the 3-D structure has been projected onto a 2-D film. Thus small differences in x-ray linear absorption coefficient aren't visible in projection radiology without the aid of contrast enhancement. CT solves this and therefore enables diagnosis of many diseases affecting soft tissue to be made easier (13).
In conclusion we have seen that the interaction of x-rays with matter gives rise to the mechanism of contrast in both the x-ray radiograph image and CT image, and that other physical parameters such as noise, resolution and radiation dose can be altered using the instrumentation to improve the quality of the final image produced.
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