style="font-size:15px;"> Bit depth determines the number of possible shades of grey that can be applied to the imaging systems output. Most medical imaging devices range from 10 to 14 bit depth thus having the capability of recording 1024, 4096, or 16 384 shades of grey. This is beyond the limits of most digital displays and human resolution. The conversion of the image from a 10 bit depth image (1024) shades of grey to something more useable occurs by the application of a lookup table that determines the displayed greyscale values relative to the recorded greyscale value.
Contrast‐to‐noise ratio (CNR) and signal‐to‐noise‐ratio (SNR) are computations that describe the relationship between the important image quality parameters and the noise of the image. CNR computes the difference in signal between the object and background, divided by the background noise. SNR computes the integrated signal of the object (pixel signal minus noise), on a per pixel basis, independent of size and homogeneity, divided by the background noise. The SNR is a useful metric that is closely related to lesion conspicuity or the observer's ability to detect the lesion. Even without a numerical (computed) value for SNR, it becomes a visual cue to experienced observers whereby image degradation due to a low SNR is readily evident (Figure 5.1).
Accuracy and precision are necessary for reliable interpretation of images. These, in turn, depend not only on image quality but also the ability and experience of the observer to identify true positives and true negatives.
Image Interpretation
Principles of radiographic interpretation can be applied to all diagnostic imaging modalities, and the classic Roentgen findings of variations in size, shape, opacity, number, margination and position can be modified as appropriate for each of the modalities, or in the case of MRI, for each of the sequences. Accurate identification and description of the imaging signs are critical to interpretation and should not only include the bone or joint in question but also the surrounding soft tissues.
Negative Studies
Imaging findings will be determined not only by the time since injury but also on associated tissue status, e.g. where on the spectrum of osseous remodelling the patient is. Depending upon the modality being used and the patient's signalment and anamnesis, the response to a negative study will vary. This may lead to repeat imaging at a later time point or utilizing a different modality. The observer must take an active role in the assessment of risk to the patient that a negative study affords and the level of rigour that should be applied for additional or different imaging.
Figure 5.1 T1W 3D dorsal plane standing MRI images of a front foot. (a) With a slice thickness of 3 mm. (b) With a slice thickness of 0.7 mm. The narrower slice thickness produces a decrease in SNR with resultant image degradation.
Radiography
General Principles
In practice, radiography is the most commonly used imaging modality and remains a cost‐effective screening test for fracture identification. Radiographs principally provide structural information and are considered to have high specificity but carry the risk of false negative studies. Currently available portable generators have the output to produce excellent studies of the appendicular skeleton from the carpus and tarsus distally and parts of the head in all sizes of patient. Radiographs of the upper limb, and axial skeleton in larger patients, can succumb to image degradation through attenuation and scatter, and a higher output gantry‐mounted generator together with selective use of a grid improves image quality. This is particularly important in cases in which the radiographic features are subtle and susceptible to being obscured by low or limited energy transferred to the imaging plate. Utilizing air‐filled anatomic structures such as the trachea to reduce attenuation can also be beneficial to highlight lesions in shoulder and cranial thoracic locations (Figure 5.2) or the caudal lung over the thoracic vertebral bodies.
Figure 5.2 Medial to lateral radiograph of the cranial thorax made with the limb closest to the detector extended craniad. The position of the trachea provides a window of reduced attenuation allowing improved visualization of the rib fracture.
The radiographic technique utilized should provide excellent bone detail but allow for evaluation of adjacent soft tissues. Digital radiography, which includes computed radiography (CR) and direct digital radiography, also referred to as digital radiography, has superseded film/screen systems. While not without inducible artefacts, these are much more forgiving of exposure errors than traditional film/screen combinations. The fundamentals of patient preparation, source–image distance, collimation, positioning, appropriate beam angle and minimizing motion are prerequisites irrespective of the system used. When using digital formats, there are a plethora of post‐processing possibilities including alteration of window width and level, image sharpening, edge enhancement, noise reduction and smoothing filters which allow images to be optimized [2]. It should be noted that new information is not generated by image processing; it helps the information to be more readily perceived, thus increasing the detection rate for abnormalities [2].
Technical Considerations
Projections
Following clinical localization, a standardized approach to image acquisition is usually the most rewarding and strongly recommended. For adequate assessment of common distal limb fractures, a minimum of four orthogonal projections in addition to lesion‐oriented oblique projections are recommended. This not only enables identification and mapping of the fracture but also detection of additional factors that may affect case management. As a two‐dimensional representation of a three‐dimensional object, small adjustments from the standard projections may be required to produce parallel alignment between the X‐ray photon beam and the fracture plane (Figure 5.3). When this occurs, the resultant lack of attenuation by the fracture results in a relative increase in energy to the imaging plate and a radiolucent line on the processed radiograph.
Figure 5.3 Parasagittal fracture of a right forelimb proximal phalanx. (a) Dorsopalmar radiograph (lateral to the left). Two fine linear radiolucencies (white arrows) can be appreciated in the proximal third of the bone corresponding to the fractures in dorsal and palmar cortices. (b) Dorsal 10° lateral–palmaromedial oblique of the same limb. The dorsal and palmar fracture lines and X‐ray photon beam are aligned. A discrete continuous fracture line is now evident (white arrows) extending from the metacarpophalangeal joint to the distal aspect of the medullary cavity. The nutrient foramen is identified by a yellow arrow.
The shape of the structure being imaged also requires consideration. For example, the distal aspect of the equine third metacarpal/metatarsal bone differs both between medial and lateral
