Abdominal radiography

image of Abdominal radiography
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The indications for plain abdominal radiography are numerous. However, for some conditions (especially urinary and gastrointestinal disorders) contrast studies may be required to show abnormalities. The following sections are covered: Indications for radiography; Radiography; Contrast radiography; and Integration with other imaging modalities.

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2.1 Lateral radiograph of a dog with a wet coat. The streaky appearance to the caudal abdomen is due to wet hair and can be mistaken for a small volume of abdominal fluid. Note the streaking extends beyond the boundaries of the abdomen.
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2.2 Characteristic curve for radiographic film. Useful radiographic densities lie between the ‘toe’ (A) and ‘shoulder’ (B) regions of the curve. The regions on the curve below A and above B contain no useful information due to underexposure and overexposure, respectively, where no useful information is present on the film. Radiographic exposures should be chosen to use the relatively straight part of the curve (between A and B) where density is proportional to log exposure. The steepness of the slope is known as the film ‘gamma’, which determines the inherent contrast of the film and also how much latitude there is in choice of exposure.
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2.3 Digital radiographic image of the lumbar vertebrae of a dog. At high magnification the pixels (small squares) that form the image can be seen. With some cheap or poor quality digital radiography systems even small degrees of magnification may result in pixellation.
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2.4 Response curve of a digital radiographic system. There is a linear relationship between density (signal) and exposure. There are no ‘toe’ or ‘shoulder’ regions, which means that useful information is obtained over a much wider range of exposures (wide dynamic range). During post-processing the straight line is usually transformed electronically to produce a sigmoidal curve similar to conventional film but this can be adjusted to produce a range of contrast and brightness. Different post-processing curves are used for different regions to enhance soft tissue or bone.
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2.5 Computed radiograph of a cat’s abdomen. The same image can be manipulated to optimize the assessment of bone or soft tissues. Digital systems can have better soft tissue contrast than a conventional film-based system. The inguinal lymph nodes and lymphatics (arrowed) can be seen in this cat.
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2.6 CR cassette showing the phosphor plate within the cassette. Radiographic technique is the same as for conventional film-based systems. With some CR systems the detector is built into the table, which means some oblique views or horizontal beam studies are not possible.
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2.7 Cassette reader for a CR system. The cassette is automatically unloaded and the latent image on the phosphor plate is read by a laser. With manual CR systems the image can fade if the phosphor is exposed to bright light.
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2.8 Diagnostic workstation for viewing digital radiographs. The monitor should be viewed in a room with dimmable lighting to reduce glare on the screen. The monitor should be calibrated for the ambient light conditions. Viewing digital radiographs on a low-resolution computer monitor in poor conditions should be avoided.
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2.9 Lateral and VD radiographs of a dog with a neoplasm involving the right middle liver lobe. On the lateral view the mass (arrowed) appears to lie caudal to the stomach. The VD view shows that the mass lies lateral to the stomach and displaces the pylorus to the left. If the lateral view alone had been assessed then the mass could have been misinterpreted as being mid-abdominal in location, resulting in an erroneous differential diagnosis list.
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2.11 Lateral radiographs of the abdomen in a dog with a gastric neoplasm. There are marked differences between the right and left lateral views. The left lateral view shows gas within the pylorus and duodenum. The left lateral view also allows better assessment of the degree of pyloric involvement and duodenal dilatation compared with the right lateral view.
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2.12 Lateral radiograph of a dog with septic peritonitis and pneumoperitoneum. Free gas is seen caudal to the diaphragm and adjacent to the liver and bowel. The decubitus lateral view shows the free gas better than the lateral view. This view is the most sensitive for demonstrating small volumes of free abdominal gas. (Courtesy of Cambridge Veterinary School)
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2.13 Positioning for the decubitus lateral view. The animal is placed in left lateral recumbency with the cassette placed adjacent to the thoracolumbar spine. A horizontal beam is used. Care needs to be taken when using a horizontal beam to ensure that personnel are not inadvertently exposed to the X-ray beam. On a radiograph taken with the animal in right lateral recumbency the fundus lies adjacent to the ribcage, making the visualization of gas harder compared with a radiograph taken with the patient in left lateral recumbency.
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2.14 Lateral radiograph of the abdomen in a cat with urolithiasis. Cystic calculi are visible within the bladder. A second radiograph centred on the pelvic canal showed a large number of urethral calculi. It is important that the entire area where pathology may occur is included on the radiograph. (Courtesy of Cambridge Veterinary School)
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2.15 Positioning for lateral abdominal radiography. The X-ray beam is centred just caudal to the last rib. Dorsally the X-ray beam is collimated to the lumbar spine and ventrally to the body wall. In dogs with a deep chest, such as this Boxer, placing a lead sleeve over the cassette ventral to the body wall helps reduce scatter and may improve contrast. Note the pads between the thighs and caudal retraction of the pelvic limbs.
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2.16 Positioning for the VD view of the abdomen. Using a trough and padding under the thighs helps prevent axial rotation, particularly in deep-chested dogs. The hindlimbs should be in a neutral frog-legged position.
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2.18 To ensure a consistently accurate choice of exposure, the depth of tissues can be measured using calipers. For a lateral view the tissue depth is measured just caudal to the ribcage and for a VD view the tissue depth is measured just caudal to the xiphoid. For deep-chested dogs separate radiographs may be required for the cranial and caudal abdomen due to large differences in depth of tissue. It is important that the measurement is made in the same position for each animal to ensure consistency.
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2.22 Lateral radiograph of a dog with contrast medium-induced renal failure. The radiograph was taken 3 days following administration of contrast medium. Note the persistence of opacification of the kidneys and the gallbladder. Absence of these findings does not rule out contrast medium-induced renal failure, which is best diagnosed by biochemical testing and urinalysis. This dog had pre-existing renal disease and cardiac disease, and had been treated with a diuretic for congestive heart failure. Care should be taken when using iodinated contrast media in severely azotaemic animals or hypotensive/dehydrated animals.
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2.23 Lateral radiograph of the caudal abdomen in a dog with prostatic carcinoma and metastases to the lumbar vertebrae (arrowed). Extension of abdominal disease to the spine, body wall, thorax and pelvic canal is often easier to detect radiographically than with ultrasonography. The two modalities give complementary information.
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2.24 Lateral radiograph and sagittal T2-weighted MR image of the cranial abdomen in a dog with cirrhotic liver disease. Although the liver changes (irregular margin and reduced size) are visible radiographically, the MR image gives much better soft tissue contrast. MRI can be used to characterize liver lesions and has greater sensitivity and specificity for many abdominal diseases compared with other modalities. However, it is more expensive and time-consuming, and artefacts may be problematical.
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2.25 Sagittal 3D volume rendered CT angiogram of the thoracic vasculature of a dog with a vascular ring anomaly (abnormal left subclavian and carotid arteries are present in this case). 3D volume rendering does not give additional information compared with cross-sectional images but makes it easier to visualize complex vascular malformations.
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