Urinary Sonography: A Good Place to Start

A 5.0-10 MHz transducer may be used for medium to large dogs while a 7.5 MHz or higher transducer is best suited for smaller dogs and cats. Use of higher frequency transducers allows the best resolution and image detail, the trade-off being depth discrimination. Linear array transducers provide excellent anatomic detail of superficially located kidneys in smaller patients. The hair coat is clipped and acoustic gel is applied to the skin to properly acoustically couple the transducer to the patient. Kidneys have three distinct anatomic regions; the renal cortex (intermediate echogenicity), the medulla (hypoechoic to nearly anechoic) and the renal pelvis/sinus (hyperechoic). Renal cortical and medullary tissues are routinely evaluated for relative echogenicity and sonographic distinction between them. The cortex is hyperechoic relative to the medulla and a distinct demarcation between them should be present. Often an echogenic rim separates cortical from medullary tissue. This is seen in normal and abnormal kidneys. The medulla is hypoechoic, sometimes to the degree that inexperienced sonographers may mistake its echogenicity as anechoic and make an incorrect diagnosis of hydronephrosis. Normal kidneys have a smooth contour bordered by a thin echogenic capsule. In comparison to other organs, the canine renal cortex is usually less echogenic than the splenic parenchyma, and may be hypoechoic to isoechoic relative to the liver. Ultrasound determination of canine kidney size and correlation with body weight or surface area is limited primarily due to large variation in kidney length and volume among normal dogs and difficulty in accurate mensuration. The normal cat kidney is approximately 4 cm long. Larger male cats (especially intact males) have the largest kidneys, small spayed females the smallest.