RENAL TRACT STONES ULTRASOUND

RENAL TRACT CALCIFICATON
Calcification within the kidney usually occurs in the form of stones. Smaller foci of calcium, which do not shadow on ultrasound, are associated with conditions such as tuberculosis, xanthogranulomatous pyelonephritis, nephrocalcinosis or some neoplastic tumours.

Renal tract stones
Renal calculi are a common finding on ultrasound. They may be an incidental discovery in an asymptomatic patient; alternatively they may be present in patients with acute renal colic and complete or partial obstruction of the ipsilateral renal tract. They may be the cause of haematuria and can also be associated with urinary tract infections.

The composition of calculi can vary. The common types include:
● Calcium stones are the most common type and are frequently associated with patients who have abnormal calcium metabolism.
● Struvite (triple phosphate) stones have a different composition of salts and are associated with urinary tract infections. They may form large, staghorn calculi.
● Uric acid stones are rare, and tend to be associated with gout.
● Cystine stones are the rarest of all and result from a disorder of amino acid metabolism—cystinuria.

Ultrasound appearances
Most renal calculi are calcified foci located in the collecting system of the kidney. Careful scanning with modern equipment can identify over 90% of these. Most stones are highly reflective structures which display distal shadowing. The shadowing may, however, be difficult to demonstrate due to the proximity of hyperechoic sinus echoes distal to the stone, or due to the relatively small size of the stone compared to the beam width.

The identification of reflective foci in the kidney is complicated by the fact that the normal renal sinus echoes are of similar echogenicity. This means that small stones may be missed on ultrasound. Differentiation of stones from sinus fat and reflective vessel walls is dependent upon careful technique and optimal use of the equipment. The operator must adjust the technique to display the distal shadow by using a variety of scanning angles and approaches and by ensuring that the suspected stone lies within the (narrowest) focal zone of the beam. The higher the frequency used, the better the chances of identifying the stone. Clearly the identification of large calculi is normally straightforward; however, for many of the reasons above, identification of small calculi can be difficult, especially in a patient with pain. Both false-positive and false-negative studies are well recognized. Although traditionally the plain film, that is kidneys, ureters, bladder (KUB), is often the first-line investigation for patients with suspected calculi, it is now being accepted that CT IVU is the best and most reliable diagnostic test for calculi detection.

Ultrasound still has a major role, however, not just in calculus detection but in identifying the secondary effects, that is, hydronephrosis, and where necessary, guiding renal drainage. The PCS may be obstructed proximal to the stone. Obvious hydronephrosis may be present and a dilated ureter may be apparent when the stone has travelled distally. The stone can sometimes be identified in the dilated ureter, but this is unusual as the retroperitoneum is frequently obscured by overlying bowel. Plain X-ray and/or IVU are traditional essential adjuncts to investigating renal colic in these cases; however CT IVU is rapidly becoming accepted as one of the mainstream investigations. Early obstruction occurs before the PCS can become dilated, making the diagnosis more difficult on ultrasound. Occasionally there will be mild separation of the PCS to give a clue, but sometimes the kidney appears normal. Doppler ultrasound can help to diagnose obstruction in a non-dilated kidney, as discussed previously; however this may not always be definitive.

Staghorn calculi
These large calculi are so called because they occupy a significant proportion of the collecting system, giving the appearance of a staghorn on X-ray. They may be less obvious on ultrasound than on X-ray, casting a dense shadow from the PCS which may obscure any associated dilatation and can, in small, atrophied kidneys, be misinterpreted as shadowing from bowel gas. Because of the lobulated shape of the calculus it may appear as several separate calculi on ultrasound. A coronal section may therefore be more successful in confirming a staghorn calculus than a sagittal section.

Cystinuria
This rare metabolic disease causes crystals of cystine to precipitate in the kidneys and be excreted in the urine. Cystine stones form in the kidneys and may result in obstruction.

Nephrocalcinosis
This term is used to describe the deposition of calcium in the renal parenchyma. It is most often related to the medullary pyramids and is frequently associated with medullary sponge kidney. It may also be seen in papillary necrosis and in patients with disorders of calcium metabolism, e.g. hyperparathyroidism.

Ultrasound appearances
Nephrocalcinosis may affect some or all of the pyramids. A regular arrangement of hyperechoic pyramids are seen which may shadow if large calcific foci are present, but not if the foci are numerous and tiny, as they are smaller than the beam width. Less frequently, calcification is seen in the renal cortex.

Hyperparathyroidism
The (normally) four parathyroid glands in the neck regulate calcium metabolism in the body. Patients with primary hyperparathyroidism (due to an adenoma or hyperplasia of one or more of the parathyroid glands) have hypercalcaemia, which makes them prone to nephrocalcinosis or stones in the kidneys. Secondary hyperparathyroidism is associated with chronic renal failure; hypocalcaemia, which results from the chronic renal failure, induces compensatory hyperplasia of the parathyroid glands. There is a high incidence of hyperparathyroidism secondary to chronic renal failure in patients on dialysis; scintigraphy may demonstrate the region of increased activity and ultrasound is particularly suitable for demonstrating the enlarged parathyroid, guiding a diagnostic aspiration and, if necessary, ablating the gland with ethanol as an alternative to surgical removal. Alcohol ablation is generally reserved for those patients deemed to be a poor surgical risk.