X-ray Image Production Procedures

Issues of Concern

X-ray Production

kVp

The kVp is the difference in potential applied to the X-ray tube.[11][14] kVp is directly proportional to the average energy of the X-ray spectrum produced, referred to as X-ray quality.[14] 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, the dose increases proportionally with higher kVp. Exposure doubles in intensity for every 15% increase in kVp, whereas contrast decreases with increases in kVp.[11] 

In contrast, this decrease is primarily due to an increased proportion of Compton scatter at higher kVp.[7] Compton scatter is 1 of 2 primary ways 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, causing the image to become overexposed. To achieve the best possible results, kVp is increased for sufficient exposure but kept low to minimize overexposure and radiation dose.

mAs

mA represents the amount of current passed through the X-ray tube. Current determines the number of photons the X-ray tube produces, also known as X-ray quantity.[12] Another factor in X-ray quantity is the total exposure time, measured in seconds. Current and exposure time are often reported together in the following way: current (mA) x time (s) = mA-seconds (mAs). Changes in mAs affect radiation dose, signal-to-noise ratio (SNR), and contrast.[14] Increasing mAs produces more electrons in an X-ray tube and subsequently increases the amount of radiation exposure.[11] High mAs increase SNR but do not decrease image contrast. X-ray imaging protocols are designed to optimize SNR while maintaining adequate contrast and limiting radiation dose.

Factors Influencing Image Quality

Contrast

Determining anatomy and suspected pathologies effectively requires 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 compared 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 image's noise.[14] Selecting parameters with lower kVp allows for the best separation in a given spectrum of intensities and improves contrast. However, this is always balanced with achieving enough exposure and penetration. Another technique to improve contrast to noise is using a grid to reduce scatter.[9] The choice of the grid is based on imaging modality (breast, abdomen, skull), and grid spacing ratios contribute to the reduction of noise. 

Distortion

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 toward the center, whereas those on the periphery tend to splay out more radially.[8] As a result, the anatomy located on the periphery of the beam profile and lateral to the center suffers some degree of distortion. However, some commonly manipulatable factors can limit the 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 or distortion is apparent in the image. The standard SID (ref) utilized is set to be 100 cm. OID is the distance between the object (eg, femur, abdomen) and the image receptor, directly proportional to magnification. The greater the OID, the greater the magnification. Considering all 3 factors, 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.[15]

Mottle

Also known as quantum noise, mottle is noise due to random distribution and an uneven number of photons reaching the image detector.[16] Mottle is the largest contributor to noise in plain X-ray; it is mostly a consequence of images acquired with low radiation doses.[14] The noise generates graininess in an image, thus disrupting the uniformity of the image. Mottle can be reduced by using a higher mA, which increases the average number of photons and the SNR.[12]

Spatial resolution

Another factor considered in image quality is spatial resolution, which is determined by measuring the smallest distinguishable space between 2 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 the 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.[11]

Beam filtration

Beam filtration refers to using X-ray absorbing material (eg, copper, aluminum, titanium) placed between the X-ray beam and the patient to increase the average photon energy by absorbing lower energy photons.[17] These low-energy photons detract from image quality by increasing the amount of scatter and unnecessarily contributing to increased patient dose.[11] 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.[17]

Grid

Scatter reduction is primarily addressed with the use of grids. A grid between the patient and the receptor comprises X-ray-absorbing material (eg, lead) interspaced with low-attenuating material (eg, carbon fiber).[11] 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 (eg, 10:1, 12:1), the greater the scatter reduction, increasing image contrast and patient exposure dose. The ratio of the increase in image contrast and the patient dose is referred to as the contrast improvement factor and the Bucky factor, respectively.[14]

Anode heel effect

The anode heel effect describes the phenomenon of the gradient of X-ray emission relative to the angle of the X-ray toward the cathode. The number of X-rays emitted is inversely proportional to the angle of emission relative to the cathode.[11] 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.[12] This effect is more pronounced at lower anode angles; angles less than 6 degrees are not recommended in clinical practice due to this phenomenon.[14]

Clinical Significance

X-ray production procedures utilize image optimization, contrast, distortion, noise, and patient dose. However, these are only a few factors technicians and radiologists must consider optimizing when scanning patients and selecting appropriate protocols. Often, these settings offer improvement of 1 imaging parameter at the cost of another. For example, devices (eg, filters, grids) enhance image contrast at the cost of increasing patient dose. Understanding these parameters is central to using 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.