The geometry of projection transmission imaging is described in Figure 7-2. Magnification can be defined simply as

where L_image is the length of the object as seen on the image and L_object is the actual length of the object. Due to similar triangles, the object magnification can be computed using the source to object distance (a) and the object to receptor distance (b)

The magnification will always be greater than 1.0 but approaches 1.0 when a relatively thin object (such as a hand in radiography) is positioned in contact with the detector, where b ≈ 0. The magnification factor changes slightly for each plane perpendicular to the x-ray beam axis, also known as the central ray, and thus anatomical structures at different depths in the patient are magnified differently. This characteristic means that, especially for thicker body parts, an AP (anterior-posterior) image of a patient will have a subtly different appearance to an experienced viewer than a PA (posterior-anterior) image of the same patient. Knowledge of the magnification in radiography is sometimes required clinically, for example, in angioplasty procedures where the diameter of the vessel must be measured to select the correct size of a stent or angiographic balloon catheter. In these situations, an object of known dimension (e.g., a catheter with notches at known spacing) is placed near the anatomy of interest, so that the magnification can be determined and a correction factor calculated.

Radiographic detectors should be exposed to x-ray intensities within a relatively small range. X-ray technique factors such as the applied tube voltage (kV), tube current (mA), and exposure time will be discussed later, but an important parameter that should be kept consistent is the distance between the x-ray source and the image receptor. For most radiographic examinations, the source-to-image distance (SID) is fixed at 100 cm (40 inches), and there are usually detents in the radiographic equipment that help the technologists to set this distance. Upright chest radiography is an exception, where the SID is typically set to 183 cm (72 inches). The larger SID used for chest radiography reduces the differential magnification in the lung parenchyma. Another consequence of projection of a three-dimensional object on a
two-dimensional plane is that anatomic structures distant from the central ray experience differential magnification from those located near the central ray, even when they are at the same depth in the patient. This geometric distortion is more apparent for large body parts and short SID.

The focal spot in the x-ray tube is very small but is not truly a point source, resulting in magnification-dependent loss of resolution. The blurring from a finite x-ray source is dependent upon the geometry of the exam, as shown in Figure 7-3. The length of the edge gradient (L_g) is related to the length of the focal spot (L_f) by

where b/a represents the magnification of the focal spot. The edge gradient is measured using a highly magnified metal foil with a sharp edge. In most circumstances, higher object magnification increases the width of the edge gradient and reduces the spatial resolution of the image. Consequently, in most cases, the patient should be positioned with the anatomy of interest as close as possible to the image receptor to reduce magnification. For thin objects placed in contact with the image receptor, the magnification is approximately 1.0, and there will be negligible blurring caused by the finite dimensions of the focal spot. In some settings (e.g., mammography), a very small focal spot is used intentionally with magnification. In this case, the small focal spot produces much less magnification-dependent blur, and the projected image is larger as it strikes the image receptor. This intentional application of magnification radiography is used to overcome resolution limitations of the image receptor, and increases spatial resolution.

As x-rays pass through the patient’s anatomy, they can interact by the fundamental physical processes of the photoelectric effect (PE) or Compton scatter, or they can continue on their way to the image receptor without interacting. X-rays that lose some of their energy through Compton scatter also usually change directions and can reach the image receptor divergent from the central ray. Typical image receptors have no means to discriminate between primary x-rays, which carry the important projection information, and secondary x-rays that tend to obscure the projection information. Therefore, methods are employed to restrict or minimize off-focus radiation from reaching the image receptor.

The basic principle of projection x-ray imaging is that x-rays travel in straight lines. However, when x-ray scattering events occur in the patient, the resulting scattered x-rays are not aligned with the trajectory of the original primary x-ray, and thus the straight-line assumption is violated (Fig. 7-4). Scattered radiation that does not strike the detector has no effect on the image; however, scattered radiation emanating from the patient is of concern for surrounding personnel due to the associated radiation dose. If scattered radiation is detected by the image receptor, it does have an effect on the image and can be a significant cause of image degradation. Scattered radiation generates image gray scale where it does not belong, and this can significantly reduce contrast. Contrast can be increased in digital images by window/leveling or other adjustment schemes, so for digital radiographic images, scatter acts chiefly as a source of noise, degrading the signal-to-noise ratio (SNR).

The amount of scatter detected in an image is characterized by the scatter-to-primary ratio (SPR) or the scatter fraction (F). The SPR is defined as the amount of energy deposited in a specific location in the detector by scattered photons, divided by the amount of energy deposited by primary (non-scattered) photons in that same location. Thus,

For an SPR of 1, half of the energy deposited on the detector at that location is from scatter—that is, 50% of the information in the image is largely useless. The scatter fraction is also used to characterize the amount of scatter, defined as

The relationship between the SPR and scatter fraction is given by

The vast majority of x-ray detectors integrate the x-ray energy and do not count photons, which is typical in nuclear medicine imaging. Thus, the terms S and P in this discussion refer to the energies absorbed in the detector from the scattered and primary photons, respectively.

If uncorrected, the amount of scatter on an image can be high (Fig. 7-5). The SPR increases typically as the volume of tissue that is irradiated by the x-ray beam increases. Figure 7-5 illustrates the SPR as a function of the side of a square field of view (FOV), for different patient thicknesses. The SPR increases as the field size increases and as the thickness of the patient increases. For a typical 30 × 30 abdominal radiograph in a 25-cm-thick patient, the SPR is about 4.5—so 82% (the scatter fraction) of the information in the image is essentially useless, if scatter rejection methods are not used.

The principal x-ray technique factors used for radiography include the tube voltage (the kV), the tube current (mA), the exposure time, and the x-ray source-to-image distance, SID. The SID is standardized to 100 cm typically (Fig. 7-10) and 183 cm for upright chest radiography. In general, lower kV settings will increase the dose to the patient compared to higher kV settings for the same imaging procedure and same body part, but the trade-off is that subject contrast is reduced with higher kV. The kV is usually adjusted according to the examination type—lower kVs are used for bone imaging and when iodine or barium contrast agents are used; however, the kV is also adjusted to accommodate the thickness of the body part. For example, in bone imaging applications, 55 kV can be used for wrist imaging since the forearm is relatively thin, whereas 75 to 90 kV might be used for lumbar spine radiography, depending on the size of the patient’s abdomen. The use of the 55-kV beam in abdominal imaging would result in a prohibitively high radiation dose. Lower kVs emphasize contrast due to the photoelectric effect in the patient, which is important for higher atomic number (Z) materials such as bone (Z_calcium = 20) and contrast agents containing iodine (Z = 53) or barium (Z = 56). Conversely, for chest radiography, the soft tissues of the cardiac silhouette and pulmonary anatomy are of interest, and the ribs obscure them. In this case, high kV is used (typically 120 kV) in order to decrease the conspicuity of the ribs by reducing photoelectric interactions. While the anatomy of interest in the chest examination is soft tissue, the rib examination for the same FOV, uses lower kV to emphasize contrast of bony structures. Sometimes higher kV technique is selected irrespective of the loss of subject contrast, in order to increase the output of the x-ray tube and achieve a reasonable exposure at the image receptor in a reasonable exposure time to avoid patient motion during the exposure. In the example above, the 55 kV beam would require at least four times as long an exposure as the 75 kV beam at the same mA setting, without considering any differences in attenuation.

With the SID and kV adjusted as described above, the overall x-ray fluence is then adjusted by using the mA and the time (measured in seconds). The product of the mA and time (s) is called the mAs, and at the same kV and distance, the x-ray fluence is linearly proportional to the mAs—double the mAs, and the x-ray fluence doubles.
Modern x-ray generators and tubes designed for radiography can operate at relatively high mA (such as 500 to 800 mA), and higher mA settings allow the exposure time to be shorter, which “freezes” patient motion. Some examinations, so-called breathing techniques, employ deliberately long exposure times to allow patient motion to blur anatomic features and emphasize features that would otherwise be obscured by overlying anatomy.

Manual selection of x-ray technique, also called “fixed technique,” was used in radiography until the 1970s, whereby the technologist selected the kV and mAs based on experience and in-house “technique charts” (Table 7-1). The technique chart was considered by some to be the radiologist’s prescription for radiation exposure for the specified view. The technique chart was used in conjunction with the “protocol book,” a document that specifies the number and type of radiographic views that constitute an examination, and details of the FOV and anatomic markers of each view. Most examinations consist of one view where the x-ray beam is perpendicular to the anatomy and another orthogonal view. This is intended to allow the radiologist to visualize the anatomy in three dimensions. Examinations may include views from other angles such as oblique or decubitus views in order to present the anatomy of interest. Many institutions required technologists to use large calipers to measure the thickness of the body part to be imaged, to improve the consistency of the radiographic film OD.

Today, radiography usually relies on automatic exposure control (AEC), informally called phototiming because of an early device used for the same purpose. The mA is set to a high value such as 500 mA, and the exposure time (and hence the overall mAs) is determined by the AEC system. In general radiography, an air-ionization chamber is located behind the patient and grid, but in front of the x-ray detector (Fig. 7-10). During exposure, the AEC integrates the exposure signal from the ion chamber in real time until a preset exposure level is reached, and then the AEC immediately terminates the exposure. The preset exposure levels are calibrated by the service personnel, and the sensitivity of the radiographic detector is taken into account in the calibration procedure. For screen-film radiography, the AEC was set to deliver proper film darkening. For a digital radiographic system, the AEC is adjusted so that the exposure levels are both in the range of the detector system and produce images with good statistical integrity based on a predetermined SNR. This means that a trade-off between radiation dose to the patient and noise in the digital image should be reached. For some digital imaging systems, the signal from the detector itself can serve as the AEC sensor.

The reliance of technologists on AEC for exposure factor control gives rise to a number of problems, even for modern digital radiographic systems. First, the technique chart still exists; it is embedded in the modern x-ray operator console. It incorporates many assumptions about the size of the patient, usually segregated into six classes, namely large, medium, and small adult and pediatric patients. The technologist must make a determination of the size category of the patient, the default typically being medium adult. Incorrect assessment of patient size has consequences for the technique factors selected and often the postacquisition digital image processing. For example, if the mA setting is appropriate for a large patient, but the patient is actually small, the exposure time may be shorter than the response time of the AEC system (less than 5 ms), resulting in unintended overexposure. Second, most systems allow the technologist to perform the exposure out of the configuration called for by the technique guide, for example, excluding the antiscatter grid or acquiring at a different SID. This has consequences for image quality that may not be immediately obvious. Third, AEC assumes proper positioning of the patient with respect to the

AEC sensor. If the dense anatomy such as bone is positioned in front of the active AEC sensor, the exposure time may be much longer than intended. If the AEC sensor is uncovered, that is, outside the shadow of the patient, the exposure time may be much shorter than intended resulting in a noisy, non-diagnostic image. There are usually multiple AEC sensors that can be selected independently or in groups. If the technologist activates the wrong sensors for a particular view, inappropriate exposure may be delivered. Finally, the computerized operator console in one room may not contain the same technique settings as an identical room next door. This causes unintended variability in the appearance of images.