Genitourinary Radiology > Collecting System > Calyces & Pelvis > Kidney Stones


Kidney Stones

 

Urinary stones are the most common cause of acute ureteral obstruction, occurring in up to 12% of the population. They occur in caucasians more than afroamericans, and males more than females. Peak age of occurrence is 40-60 years. Clinical presentation includes colicky flank pain with intermittent hematuria. Common types of stones include, in order of decreasing radiopacity: calcium(75-85%) , struvite (10-20%), uric acid (5-10%) and cystine (1%). Calcium stones include mixed (50%), calcium oxalate (40%), and calcium phosphate/apatite (10%) Causes include idiopathic (85%), hyperparathyroidism (10-15%), sarcoidosis, renal tubular acidosis, immobilization, and hyperoxaluria. Struvite stones are caused by urea-splitting bacteria: Proteus, Klebsiella, and Pseudomonas. When the stones are large and branched, they are termed staghorn calculi. Uric acid stones are cause by acidic, concentrated urine, small bowel disease, gout, and cell lysis.

In a patient with suspected renal obstruction as a cause for acute renal failure, ultrasound is a quick and inexpensive imaging modality to image hydronephrosis.  If the cause is not seen on US, additional imaging may be performed with CT.

CT has replaced IVU as the imaging modality of choice for detection of urinary stones. Sensitivity and specificity of CT are above 95%. Nearly all stones are radiopaque on CT while only about 50% are radiopaque on plain film.  Findings include: stones in ureter, enlarged kidneys (hydronephrosis), perinephric fluid, ureteral dilatation, soft-tissue rim sign.

Common locations include: uretero-pelvic junction, ureterovesical junction, and the point where ureter crosses iliac vessels. Phleboliths (calcified pelvic veins) can be distinguished from stones by showing a central lucency. Additionally, stones may be surrounded by a rim of soft tissue, while phleboliths may have comet-tail shaped soft tissue. Plain radiographs may be used to follow migration of the stone.

Gray scale ultrasound of right kidney shows dilated renal pelvis (p) from obstructing ureteropelvis junction (UPJ) stone (S). Note echogenic interface between calcified stone and fluid filled renal pelvis.

Stone protocol CT (done without IV or oral contrast so as to not obscure finding a stone) shows bilateral renal calculi (arrrows). The collecting systems are dilated bilaterally (p) possibly from dilated obstructing stones.

Delayed CT scan with (1) soft tissue windows and (2) bone windows shows normal excretion of contrast from left kidney (LK) but dilated collecting systemin the right kidney (arrows) with delayed excretion. Bone windows reveal a stone obstructing the right kidney at the UPJ.

KUB and 3D reconstruction of stone protocol CT in patient status post lithotripsy or left staghorn calculus. There are stone fragments lining up in the distal ureter causing obstruction. This line of stones is known as "stein strasse" or "stone street" in German.

Stone protocol CT shows tiny stone obstructing left distal ureter (arrow). Notice soft tissue around the stone indicating this stone is in fact in the ureter (soft tissue rim sign) and not a calcified pelvic vein (i.e., a phlebolith).

Stone protocol CT shows calcification in left pelvis. The soft tissue (arrow) extending towards the calcification represents a pelvic vein. This finding is called the "tail" sign and is consistent with a pelvic phlebolith and not a ureteral calculus.


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