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35 NOVEMBER/DECEMBER 2017 ■ TVPJOURNAL.COM IMAGING ESSENTIALS anechoic fluid can occasionally be seen; in the transverse plane, the pelvic width can measure up to 2 mm in dogs and 1.6 mm in cats. 6 Ureters The proximal and mid portions of the ureters extend along the retroperitoneal space lateral to the aorta and caudal vena cava. When examining the urinary bladder, the distal ureters, if seen, can be evaluated as they enter the ureteral papillae in the region of the trigone. Ureteral jets into the bladder lumen can often be detected when scanning the area of the papillae, and detection of the jet may be enhanced by use of color Doppler. SCANNING TECHNIQUE To optimize images of the urinary tract, the patient's hair must be clipped and ultrasonic gel applied to the skin before examination. As discussed in "A Tour of the Abdomen: Part 1" (January/February 2016), the urinary tract is examined in a clockwise fashion, beginning with the left kidney, urinary bladder, proximal urethra, and then right kidney. Each kidney is evaluated in its long and short axis; these often correspond to the long and short axis of the patient, although, in some cases, the kidneys are oriented obliquely relative to the patient's sagittal and transverse planes, requiring some adjustment of the ultrasound imaging plane to obtain true renal sagittal and transverse images. There are 3 renal imaging planes: 2 long axis (dorsal and sagittal) and 1 short axis (transverse). Within the dorsal plane ( Figure 1 ), the renal pelvis is located in the far field of the image, and within the sagittal plane, the long axis of the kidney is seen but the renal pelvis is not. Typically, imaging the kidney from the lateral abdominal wall creates the dorsal plane, and imaging the kidney from the ventral aspect of the abdomen creates the sagittal plane. The transverse plane can be created by rotating the transducer 90°. Mild renal pelvic dilation is usually best seen in the transverse plane when centered on the renal hilum. Be sure to evaluate each kidney from medial to lateral or dorsal to ventral in one of the long axis imaging planes and from cranial to caudal in the transverse imaging plane. Any abnormalities should be documented with static images or short video clips. Left Kidney The left kidney is easier to localize than the right kidney because of its lateral location along the mid abdomen. Occasionally, it is seen deep to the spleen (located in the near field). The transducer is initially placed in the left cranial abdomen and is moved medially and caudally until the left kidney is visible in a long axis. Right Kidney The right kidney is typically more difficult to image due to its craniodorsal location in the abdomen in dogs; it is especially difficult in deep-chested dogs. The transducer is placed in the right dorsolateral subcostal region to visualize the right kidney. However, a lateral approach through the 11th or 12th intercostal space might be needed, especially in deep-chested dogs. The presence of gas and gastrointestinal contents within the stomach, descending duodenum, and/or the ascending colon can affect the image of the right kidney. 7 URINARY TRACT ABNORMALITIES Congenital Abnormalities of the Kidneys Renal Agenesis or Hypoplasia Unilateral renal agenesis is rare, but has been reported in dogs and cats. 8 If only one kidney exists, compensatory hypertrophy of that kidney may occur. In this instance, the kidney is enlarged but retains normal internal architecture. Congenital Renal Dysplasia This defect can be hereditary or result from neonatal infection, such as feline panleukopenia virus or canine herpesvirus. Renal dysplasia is defined as the disorganized development of renal parenchyma leading to misshapen and fibrosed kidneys. 9 These kidneys can be normal in size or small, irregular, and hyperechoic with reduced corticomedullary distinction. Renal Ectopia Congenital malposition of one or both kidneys may occur. The etiology is unknown. Classification is based on the ectopic kidney's anatomic position: