Radiography should be performed when possible as an elective procedure to ensure an empty gastrointestinal tract, as a food-filled stomach or faeces-filled colon may severely hamper interpretation. Lateral and ventrodorsal (VD) positions are the standard. The choice between right lateral recumbent (RLR) and left lateral recumbent (LLR) is a personal one, but whichever is chosen it should be used consistently to familiarize the user with its normal appearance. The differences between the two lateral views are described in Table 9.1. Collimation should include the whole diaphragm and the pelvic inlet. In animals with urinary obstruction, the radiograph should extend caudally to include the whole perineal and urethral area; in large dogs, this may require two separate radiographs. Exposure should be made on expiration to avoid motion blur. Left lateral decubitus views can be used to detect free abdominal gas (see 9.7).

For digital radiography, an appropriate algorithm should be used to combine resolution and contrast. For conventional film radiography, a fast film–screen combination should be used to minimize motion blur, and a grid should be employed if the abdomen is > 10 cm thick or in smaller, obese dogs. A short-scale contrast technique (low kV, high mAs) will improve the naturally low contrast of the abdominal organs. Increased abdominal detail can be obtained by trying to maximize organ separation. This can be achieved by placing the dog in dorsal rather than sternal recumbency, positioning the pelvic limbs perpendicular to the spine and making the exposure on expiration.

Optimal evaluation of abdominal radiographs requires a systematic approach, which involves assessing radiographic technique, signalment (breed type, age, sex, body condition), extra-abdominal structures (superficial soft tissues, osseous structures, caudal thorax and diaphragm) and intra-abdominal structures, and then re-evaluating abnormalities and areas in the light of the history and clinical examination. Intra-abdominal evaluation may be done on a system basis: gastrointestinal, urinary tract, reproductive tract, parenchymatous organs (liver and spleen), endocrine organs (adrenals and pancreas), peritoneal cavity and retroperitoneal space, and lymphatic system (sublumbar lymph nodes). Alternatively, a quadrant approach can be used. For viewing abdominal radiographs, the convention is that lateral radiographs are examined with the diaphragm facing to the left, and VD radiographs with the diaphragm uppermost and the left side of the patient on the right side of the computer screen or light box. Abdominal fat in smaller dogs and in cats helps to outline abdominal structures, as the lower fat opacity contrasts well with the soft tissue opacity of the majority of abdominal organs. However, in large obese dogs this advantage is lost, as the large amounts of fat result in excessive amounts of scatter, which degrades the image despite the use of a grid.

If an organ that is normally visible is not seen despite good-quality radiographs, pathology is implied. Similarly, if an organ that is normally not seen becomes partly or completely visible, disease is also likely. Structures normally visible and not visible on abdominal radiograph are listed in Box 9.1. There are some radiographic anatomical differences between the dog and cat, and these are given in Table 9.2.

Interpretation is similar to that of ultrasonography of the soft tissues of the thoracic wall (see 8.24).

All causes of increased intra-abdominal radiopacity result in loss of serosal detail by obscuring intra-abdominal fat, which normally provides contrast with soft tissues. A diffuse and homogeneous increase in intra-abdominal opacity and loss of serosal detail is sometimes referred to as a ground glass appearance and is usually due to free abdominal fluid. Increase in opacity may also be patchy or mottled.

The medial iliac lymph nodes lie close to the abdominal aorta and CdVC at their caudal bifurcation. They may be visible in normal animals as well-defined, elongated, hypoechoic structures. The lumbar lymph nodes extend along the paralumbar tissues but are usually recognized ultrasonographically only when enlarged. It is difficult to differentiate malignant from reactive lymph nodes. Neoplastic lymph nodes are usually rounded and hypoechoic but can be heterogeneous. Doppler blood flow patterns can be helpful, with predominantly hilar flow strongly suggestive of a benign process.

The aorta lies dorsal and to the left of the CdVC in the retroperitoneal space. Pulsations of both vessels may be evident, due to referred aortic pulsation affecting the CdVC. The CdVC is more easily compressed by pressure from the transducer. Doppler ultrasound allows definitive differentiation between the two.

Generalized changes in liver size may be inferred from the position of the stomach (see 10.2). Liver enlargement may be due to primary liver disease or secondary to disease in another organ system; it usually has to be severe and/or extensive before changes can be detected radiographically. On lateral radiographs, generalized hepatomegaly leads to projection of the ventral part of the liver further beyond the costal arch, caudodorsal displacement of the gastric axis and an increase in the hepatic angle with rounding of the liver margins. The position of the diaphragm, stomach and spleen allows evaluation of the size of the left side of the liver. Assessment of the right side is more difficult on the lateral radiograph, although gross enlargement will displace the pylorus, cranial duodenum and right kidney caudally. Right-sided hepatomegaly is better seen on VD radiographs, displacing the stomach to the left.

Reduction in liver size may be hard to differentiate from normal deep-chested canine conformation. Signs of reduced liver size include cranial displacement of the gastric axis and transverse colon, reduced distance between the stomach and diaphragm and loss of the hepatic angle.

Normal radiographic liver size does not preclude significant liver disease.