image of Imaging
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When performed correctly and using the most appropriate modality, imaging should significantly reduce the differential diagnoses for a case and can often allow a specific diagnosis to be made. This chapter provides in-depth guidance on the use and interpretation of radiography, computed tomography, magnetic resonance imaging, ultrasonography and scintigraphy.

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3.2 A simple (stationary or fixed anode) X-ray tube. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission
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3.3 Different types of direct digital radiography systems. Flat panel detectors either directly or indirectly convert X-rays into electric signals; direct detectors use a photodetector to immediately transform X-rays (red arrows) to electric signals (dotted arrows), while indirect detectors use a scintillator to convert X-rays to visible light (yellow arrows) that is then transformed to an electrical charge by a photodiode. Charged coupled devices (CCD) also use scintillators to convert X-rays to light, which is then focused on to the CCD chip via a lens and converted to an electrical charge.
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3.4 Characteristic curves for digital and conventional radiography systems showing their performance over a range of exposures. The curve for conventional film-screen radiography (red) is narrow and sigmoidal, with only the central part being linear. This results in a narrow dynamic range. The equivalent plot for digital systems (blue) is a straight line over a much larger range, meaning that digital systems can display a wider range of exposures correctly (wide dynamic range).
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3.5 (a) Mediolateral and (b) caudocranial views of the stifle of a dog. The mediolateral view shows few abnormalities aside from a likely joint effusion. However, the caudocranial view shows focal soft tissue swelling around the head of the fibula and ill defined amorphous periosteal reaction (arrowed) in the proximal interosseus space between the tibia and fibula. There is cortical lysis of the proximal lateral tibia and subtle lysis of the fibular head. Histopathology confirmed an osteosarcoma.
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3.6 Cranioproximal–craniodistal view of the intertubercular groove of the proximal humerus of a dog, which shows irregular osteophyte formation on the medial aspect of the greater tubercle. Further faint mineralization is visible just proximal to the bicipital groove suggestive of mineralization within/adjacent to the biceps tendon. The groove itself is increased in opacity indicating sclerosis and osteophytosis.
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3.7 Mediolateral flexed view of the elbow joint of a dog. Flexing the joint means that the anconeal process can be viewed unobstructed by the medial supracondylar crest: in this case, subtle osteophytosis (arrowed) along the border of the anconeal process is now evident.
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3.8 Stress radiography to demonstrate instability in a tarsus with multiple injuries, including a comminuted central bone fracture. Rope ties are used to apply shearing force proximal and distal to the tarsus in the direction of the arrows. (a) Stressing the medial aspect of the joint results in no significant change, but (b) stressing the lateral aspect results in widening of the calcaneoquartal joint (arrowhead) and indicates either damage to the collateral ligament or instability due to the collapse of the central tarsal bone buttress as a result of fracture.
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3.9 Left lateral recumbent view of the thorax of the same dog as in Figure 3.5 . Several small pulmonary soft tissue nodules are apparent (most obvious superimposed over the cardiac silhouette); three views of the thorax were obtained and nodules seen on all of them. The finding of pulmonary nodules is significant for the staging of the dog’s disease, and they also increase diagnostic certainty for the stifle lesion being neoplastic.
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3.11 Examples of radiographic positioning aids. Shown are sandbags of various sizes, radiolucent foam wedges and blocks (plastic-covered) and ties.
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3.12 (a) A significantly underexposed digital radiograph (obtained with CR system). The image shows a moderately mottled or grainy appearance. (b) This becomes more obvious when the image is enlarged.
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3.13 Screen shot of a computed radiography reader console. This manufacturer terms the exposure index the ‘S value’. The S value of 492 indicates that the radiograph is correctly exposed according to the system manufacturer’s guidelines.
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3.14 An example of an image converted using an inappropriate look-up table. The soft tissues around the stifle (arrowed) have been clipped from the image and cannot be evaluated.
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3.15 Inadequate collimation has resulted in this radiograph being incorrectly processed by the system, creating a very light image with little contrast.
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3.16 An example of double exposure. The wide latitude means that both images are displayed normally, resulting in a potentially confusing radiograph until the error is recognized.
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3.17 Moiré artefact on a ventrodorsal view of the pelvis. Alternating light and dark bands are present across the image when a static radiographic grid of low frequency is used. Use of a moving grid will eliminate this artefact.
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3.18 Überschwinger artefact. A subtle radiolucent halo is present around many of the screws used in the repair of this radius and ulna fracture. (Courtesy of Gareth Arthurs)
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3.19 The interior of a 4-slice CT gantry (open for servicing). The X-ray tube head (red arrow) is clearly visible, with the detector array (yellow arrow) arranged exactly opposite.
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3.20 Rotation of the X-ray tube around the patient. For simplicity the number of projection positions has been reduced to eight; in reality up to 1000 projections may be obtained. For each projection, a wedge-shaped primary X-ray beam passes through the patient and is recorded by the opposing detector. The circular region where all of these projections overlap is termed the scan field of view. Body parts outside the scan field of view will not be correctly imaged.
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3.21 (a) Soft tissue and (b) bone transverse reconstructions of the stifle of a dog with a periarticular histiocytic sarcoma. The bone reconstruction is useful for demonstrating subtle cortical destruction and bone lysis (L) of the lateral femoral condyle and the soft tissue reconstruction highlights the large soft tissue mass present (S).
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3.22 Multiplanar reconstructions (MPR) in the (a) transverse, (b) sagittal and (c) dorsal planes of the shoulder of a dog with a caudal humeral head osteochondrosis lesion. The lesion (arrowed) is relatively difficult to detect in the dorsal plane, but clearly visualized in the sagittal and transverse planes.
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3.23 (a) Representative transverse images and (b) three-dimensional surface renderings of the bones of the elbow of a dog with severe joint incongruency (the levels of the transverse images are indicated by the dotted lines). There is abnormal morphology of the humeral condyle and moderate lateral subluxation of the radial head. Extensive secondary osteophyte formation is evident. Although the reconstruction is not necessary for the diagnosis, appreciation of the morphological changes is much easier with a three-dimensional representation. The levels of the transverse images 1–3 in (b) relate to the images from left to right in (a).
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3.24 Three-dimensional volume rendering of a pelvis with multiple comminuted fractures. Characterization of the fractures in complex structures such as the pelvis is enhanced by the three-dimensional reconstruction.
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3.25 The medial coronoid process of the ulna (red box) is not well visualized on standard mediolateral and craniocaudal radiographs of the elbow due to superimposition (yellow arrows represent the direction of the primary X-ray beam for these views). A lack of superimposition means that the fragmented coronoid is easily visualized on a transverse CT image of the elbow.
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3.26 A transverse CT image demonstrating why a humeral intracondylar fissure is not well represented on a standard craniocaudal radiograph of the elbow; the primary X-ray beam (A) is not parallel to the fissure. An oblique view (B) will better depict the fissure, but some superimposition of the olecranon will still remain on the radiograph.
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3.27 (a) Dorsal, (b) sagittal and (c) transverse T1-weighted MR images of the shoulder of a dog. Unlike CT, these are not merely reconstructions of pre-existing data; rather, each is a separate acquisition that takes a number of minutes to acquire. In this study, contrast has been further increased by the injection of a gadolinium-based contrast medium into the shoulder joint.
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3.28 Transverse ultrasonographic image of the proximal humeral region. The biceps tendon (B) can be identified in transverse section surrounded by a moderate anechoic tendon sheath effusion. There is a heterogeneous, predominantly hypoechoic region with a small focus of mineralization (arrowed) adjacent to the greater tubercle (GT), representing tendinopathy of the supraspinatus tendon insertion.
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3.29 (a) Dorsopalmar radiograph of the manus showing multipartite/fragmented sesamoids of the metacarpophalangeal joint of the second digit (arrowed). This is a potential cause of lameness in some dogs. (b) The scintigraphic image of the corresponding region shows a region of increased uptake of radiopharmaceutical agent in the third digit (arrowed). (c) Fusion of the two images using appropriate software shows the increased activity to be related to the metacarpophalangeal joint of the third digit, indicating the sesamoid changes are unlikely to be significant in this case. The significance of the apparent increased uptake is in part interpreted by comparison with images from the contralateral limb, but the clinician needs to be aware of the possibility of bilateral pathology.
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3.30 The principal stages in the interpretation of the results of a diagnostic imaging study.
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3.35 Caudocranial radiograph of a dog with a palpable swelling adjacent to the stifle. This swelling can be identified radiographically (delineated by the arrows). An accurate characterization of the lesion in terms of its radiographic signs would read as follows: ‘There is a single large (20 × 60 mm) smoothly marginated and well defined, oval-shaped, mass of fat opacity present in the soft tissues lateral to the stifle joint. No changes are seen in the adjacent bones’. Correct identification of the lesion as being of fat opacity, with non-aggressive characteristics, means that the appropriate differential diagnosis of a benign lipoma is quickly achieved.
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3.37 Mediolateral radiograph of the stifle of a dog. There is severe cranial displacement and compression of the infrapatellar fat pad (P) and caudal displacement of the fascial planes (F) of the caudal musculature. Fat radiopacity allows identification of the infrapatellar fat pad and fascial planes and detection of an increase in synovial mass (likely joint effusion). However, the inability to distinguish soft tissue from fluid limits the definitive conclusion that this represents joint effusion and in fact this was a synovial tumour. Subtle lysis of the distal patella can be appreciated.
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3.38 Mediolateral radiographs of the (a) right and (b) left stifles of a dog. There is an avulsion of the tibial tuberosity of the right stifle and very mild proximal displacement of the tibial tuberosity can be seen. However, establishing the diagnosis is easier when comparison is made with the normal left stifle (the possibility of bilateral disease should be considered). This dog also has a Salter–Harris Type II fracture and minor displacement of the proximal tibial physis as well as a greenstick fracture of the fibula.
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3.42 Types of focal bone destruction (lysis) seen on radiographs. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission
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3.44 Periosteal reactions from those considered least to most aggressive. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission
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3.46 Mediolateral radiograph of the distal crus of a dog. The lesion shows all the signs of aggressive bone disease listed in Figure 3.45 . Additionally, the lesion is monostotic and its location (distal tibial metaphysis) is a predilection site for primary bone tumours. The final diagnosis was osteosarcoma.
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