Conventional X-ray production involves the excitation of tungsten metal to release photons. A cathode ray is used to direct energy into the rotating tungsten filament anode. The resultant photons released can be absorbed or transmitted through the body to provide information on the amount of attenuation. These attenuation gradients are used to reconstruct an image by mapping the amount of primary or returning photons that hit a photosensitive detector plate and produce a planar image. Other fates of photons may include scattering, in which photons are deflected away from the detector.
Alternatively, photons can be completely absorbed into the tissue. Physical tissue properties may determine whether energy is more easily absorbed, attenuated, or scattered. In particular, tissue density, thickness, and atomic number alter the trajectory and absorption of X-rays. Increased atomic number, thickness, and density can cause photons to be attenuated, absorbed, and scattered to higher degrees. These properties create contrast among different tissues within the body, allowing for a separation of intensity values and evaluating potential pathology.
X-ray image production procedures focus on optimizing settings to produce the appropriate contrast among the anatomy of interest while limiting noise and artifacts that may detract from the evaluation of the image. It is often a trade-off to optimize imaging parameters while keeping ionizing radiation exposure as low as reasonably achievable (ALARA). Important aspects of determining appropriate protocol in clinical settings include X-ray tube voltage (kVp), current (mA), and exposure time (seconds).
Issues of Concern
The kilovoltage peak (kVp) is the difference in potential applied to the X-ray tube. kVp is directly proportional to the average energy of the X-ray spectrum produced, referred to as X-ray quality. kVp plays a role in adjusting the amount of penetration and exposure in an acquisition. Penetrance is characterized by the number of photons reaching the image receptor to discern differences between structures. For example, in underexposed chest X-ray acquisitions in which the diaphragm cannot be visualized to the intersection of the spine, kVp can be increased to mitigate this issue. An adequate penetrance ensures the ability to separate definable structures of interest; recent advances have allowed altering digital windowing levels to achieve the same effect. Changes in kVp affect radiation dose, exposure, and contrast. Also, dose increases proportionally with higher kVp. Exposure doubles in intensity for every 15% increase in kVp, whereas contrast decreases with increases in kVp.
In contrast, this decrease is primarily due to an increased proportion of Compton scatter at higher kVp. Compton scatter is one of two primary methods in which X-rays interact with matter, the other being the photoelectric effect. An increased ratio of Compton scatter introduces excess photons that reach the image receptor. As a result, the image becomes overexposed. To achieve the best possible results, kVp is increased for sufficient exposure but kept low to minimize overexposure and radiation dose.
Milliamperes (mA) is a unit representing the amount of current passed through the X-ray tube. Current determines the number of photons produced by the X-ray tube, also known as X-ray quantity. Another contributing factor toward X-ray quantity is the total exposure time, measured in seconds. Current and exposure time are often reported together such that: current(mA) x time(s) = milliampere-seconds (mAs). Changes in mAs affect radiation dose, signal-to-noise ratio (SNR), and contrast. Increasing mAs produces more electrons in an X-ray tube and subsequently increases the amount of radiation exposure. High mAs will increase SNR but will decrease image contrast. X-ray imaging protocols are designed to optimize SNR while maintaining adequate contrast and limiting radiation dose.
Factors that Influence Image Quality
Effectively determining anatomy and suspected pathologies rely on identifying and separating different tissue types and boundaries. In X-ray imaging, contrast describes the number of relative photons that can pass through a tissue comparative to another. This is determined by the amount of tube voltage (kVp) and filtration used. Conversely, increasing the mA does not improve or worsen contrast and contributes to the amount of noise in the image. Selecting parameters with lower kVp will allow for the best separation in a given spectrum of intensities and consequently improve contrast. However, this is always balanced with achieving enough exposure and penetration. Another technique utilized to improve contrast to noise is using a grid to reduce scatter. The choice of the grid is based on imaging modality (breast, abdomen, skull) and grid spacing ratios contribute to the amount of noise reduction.
Beam profiles and paths of the photons also influence the quality and characteristics of an image. X-ray divergence patterns can be described by photons directed linearly towards the center, whereas those on the periphery tend to splay out more radially. As a result, the anatomy located on the periphery of the beam profile and lateral to the center will suffer some degree of distortion. However, some commonly manipulatable factors can limit the amount of distortion in an image: centering, source image receptor distance (SID), and object image receptor distance (OID). Centering refers to positioning the anatomical portion of interest in line with the central point of the X-ray. SID is the distance between the X-ray tube and the image receptor and is inversely proportional to magnification/distortion. In other words, the greater the SID, the less magnification/distortion will be apparent in the image. The standard SID (ref) utilized is set to be 100 cm. OID is the distance between the object (e.g., femur, abdomen) and the image receptor, which is directly proportional to magnification. The greater the OID, the greater the magnification. Taking all three factors into account, the most optimal positioning for X-ray imaging would be to have the anatomy of interest in the center of the X-ray beam, the beam sufficiently distanced from the image receptor, and the image receptor as close as possible to the anatomy being imaged.
Also known as quantum noise, mottle is noise due to random distribution and an uneven number of photons reaching the image detector. Mottle is the largest contributor to noise in plain X-ray; it is mostly a consequence of images acquired with low radiation doses. The noise generates graininess in an image, thus disrupting the uniformity of the image. Mottle can be reduced by using a higher mA, which will increase the average number of photons and the SNR.
Another factor considered in image quality is the spatial resolution, which is determined by measuring the smallest distinguishable space between two distinct lines or landmarks. The smaller the distance between line pairs relates to discerning boundaries and colloquially defined sharper or better resolution images. One of the changeable features that may influence spatial resolution is anode angle. Anode angle is the relationship between the slant of the tungsten anode and the incident cathode ray. The degree of the anode angle significantly contributes to the size of the focal spot generated. Lower amounts of anode angle relate to a smaller focus and image with better spatial resolution.
Beam filtration refers to the use of X-ray absorbing material (e.g., copper, aluminum, titanium) placed between the X-ray beam and patient to increase the average photon energy by absorbing lower energy photons. These low energy photons detract from image quality by increasing the amount of scatter and unnecessarily contributing to increased patient dose. Filtration reduces Compton scatter and has the effect of decreasing X-ray quantity and increasing X-ray quality. The clinical effects of beam filtration include increased image contrast at the cost of increased patient exposure.
Scatter reduction is primarily addressed with the use of grids. A grid is placed between the patient and the receptor and is composed of X-ray absorbing material (e.g., lead) interspaced with low attenuating material (e.g., carbon fiber). The amount of scatter reduction a grid provides is directly proportional to the ratio between the height of the grid and the interspacing, also known as the grid ratio. The greater the grid ratio (e.g., 10:1, 12:1), the greater the amount of scatter reduction, which also increases image contrast and patient exposure dose. The ratio of increase in image contrast and the patient dose is referred to as the contrast improvement factor and the Bucky factor, respectively.
Anode Heel Effect
The anode heel effect describes the phenomenon of the gradient of X-ray emission relative to the angle of X-ray toward the cathode. The number of X-rays emitted is inversely proportional to the angle of emission relative to the cathode. This difference in photon production is a consequence of tungsten excitation beneath the surface of the anode. The X-rays produced within the anode must travel through the material before being emitted. As a result, fewer X-rays are produced in areas where more material must be traversed. X-rays are produced in a gradient, with the highest beam strengths found closest to the cathode. This effect is more pronounced at lower anode angles; angles < 6o are not recommended in clinical practice due to this phenomenon.
X-ray image production procedures utilize a balance of image optimization, contrast, distortion, noise, and patient dose. However, these are only a few of many factors technicians and radiologists must consider optimizing when scanning patients and selecting appropriate protocols. Often these settings offer improvement of one imaging parameter at the cost of another. For example, devices (e.g., filters, grids) enhance image contrast at the cost of increased patient dose. Understanding these parameters is central to the understanding and use of X-ray imaging in diagnostic medicine. These understandings demonstrate further utility with interventionalists in radiology, surgery, and pain medicine who use X-ray imaging in real-time to guide therapeutic interventions.