RENAL ULTRASOUND KIDNEYS RENAL TRACT

THE NORMAL RENAL TRACT
Ultrasound technique
The right kidney is readily demonstrated through the right lobe of the liver. Generally a subcostal approach displays the (more anterior) lower pole to best effect, while an intercostal approach is best for demonstrating the upper pole. The left kidney is not usually demonstrable sagittally because it lies posterior to the stomach and splenic flexure. The spleen can be used as an acoustic window to the upper pole by scanning coronally, from the patient’s left side, with the patient supine or decubitus (left side raised), but, unless the spleen is enlarged, the lower pole must usually be imaged from the left side posteriorly. Coronal sections of both kidneys are particularly useful as they display the renal pelvicalyceal system (PCS) and its relationship to the renal hilum. This section demonstrates the main blood vessels and ureter (if dilated).

As with any other organ, the kidneys must be examined in both longitudinal and transverse (axial) planes. This usually requires a combination of subcostal and intercostal scanning with anterior, posterior and lateral approaches. The operator must be flexible in approach to obtain the necessary results. The bladder should be filled and examined to complete the renal tract scan. An excessively full bladder may cause mild dilatation of the PCS, which will return to normal following micturition.

Normal ultrasound appearances of the kidneys
The cortex of the normal kidney is slightly hypoechoic when compared to the adjacent liver parenchyma, although this is age-dependent. In young people it may be of similar echogenicity and in the elderly it is not unusual for it to be comparatively hyperechoic and thin. The medullary pyramids are seen as regularly spaced, echo-poor triangular structures between the cortex and the renal sinus. The tiny reflective structures often seen at the margins of the pyramids are echoes from the arcuate arteries which branch around the pyramids. The renal sinus containing the PCS is hyperechoic due to sinus fat which surrounds the vessels. The main artery and vein can be readily demonstrated at the renal hilum and should not be confused with a mild degree of PCS dilatation. Colour Doppler can help differentiate. The kidney develops in the fetus from a number of lobes, which fuse. Occasionally the traces of these lobes can be seen on the surface of the kidney, forming fetal lobulations ; these may persist into adulthood.

Normal ultrasound appearances of the lower renal tract
When the bladder is distended with urine, the walls are thin, regular and hyperechoic. The walls may appear thickened or trabeculated if the bladder is insufficiently distended, making it impossible to exclude a bladder lesion. The ureteric orifices can be demonstrated in a transverse section at the bladder base. Ureteric jets can easily be demonstrated with colour Doppler at this point and normally occur between 1.5 and 12.4 times per minute (a mean of 5.4 jets per minute) from each side. It is useful to examine the pelvis for other masses, e.g. related to the uterus or ovaries, which could exert pressure on the ureters causing proximal dilatation. The prostate is demonstrated transabdominally by angling caudally through the full bladder. The investigation of choice for the prostate is transrectal ultrasound; however an approximate idea of its size can be gained from transabdominal scanning. When prostatic hypertrophy is suspected, it is useful to perform a postmicturition bladder volume measurement to determine the residual volume of urine.

Measurements
The normal adult kidney measures between 9 and 12 cm in length. A renal length outside the normal range may be an indication of a pathological process and measurements should therefore form part of the protocol of renal scanning. The maximum renal length can often only be obtained from a section which includes rib shadowing. A subcostal section, which foreshortens the kidney, often underestimates the length and it is more accurate to measure a coronal or posterior longitudinal section. The cortical thickness of the kidney is generally taken as the distance between the capsule and the margin of the medullary pyramid. This varies between individuals and within individual kidneys and tends to decrease with age.

The bladder volume can be estimated for most purposes by taking the product of three perpendicular measurements and multiplying by 0.56:
Bladder volume (ml) = length * width * anteroposterior diameter (cm) * 0.56

Haemodynamics
The vascular tree of the kidney can be effectively demonstrated with colour Doppler. By manipulating the system sensitivity and using a low pulse repetition frequency (PRF), small vessels can be demonstrated at the periphery of the kidney. Demonstration of the extrarenal main artery and vein with colour Doppler is most successful in the coronal or axial section by identifying the renal hilum and tracing the artery back to the aorta or the vein to the inferior vena cava (IVC). The best Doppler signals, that is, the highest Doppler shift frequencies, are obtained when the direction of the vessel is parallel to the beam, and taken on suspended respiration. The left renal vein is readily demonstrated between the superior mesenteric artery (SMA) and aorta by scanning just below the body of the pancreas in transverse section. The origins of the renal arteries may be seen arising from the aorta in a coronal section. The normal adult renal vasculature is of low resistance with a fast, almost vertical systolic upstroke and continuous forward end diastolic flow. Resistance generally increases with age.2 The more peripheral arteries are of lower velocity with weaker Doppler signals, and are less pulsatile than the main vessel.

Assessment of renal function
Blood and urine tests can be useful indicators of pathology. Frequently, the request to perform ultrasound is triggered by biochemical results outwith the normal range. Raised serum levels of urea and creatinine are associated with a reduction in renal function. However, any damage is usually quite severe before this becomes apparent. The creatinine clearance rate estimates the amount of creatinine excreted over 24 h, and is a guide to the glomerular filtration rate (normal glomerular filtration rate 100–120 ml/min). A poor rate of clearance (ml/min) is indicative of renal failure.

Blood in the urine is a potentially serious sign which should prompt investigation with ultrasound. Frank haematuria may be a sign of renal tract malignancy. Microscopic haematuria may reflect inflammation, infection, calculi or malignancy. The urine can be easily examined for protein, glucose, acetone and pH using chemically impregnated strips.

Radioisotope scans
Although the ultrasound scan is invaluable in assessing the morphology of the kidneys, it is not able to assess function. The administration of a radioactive tracer, however, reveals valuable information regarding renal function and an isotope scan may often be performed in addition to ultrasound. A diethylene triaminepenta-acetic acid (DTPA) scan, in which the isotope is intravenously injected as a bolus, can assess renal perfusion, with further data reflecting renal uptake, excretion and drainage during later images.

A dimercaptosuccinic acid (DMSA) scan shows uptake of isotope which is proportional to functioning renal tissue. Relative renal function can be determined between kidneys and localized areas of poor or absent function, such as scars, are clearly demonstrated.