Diagnostic Imaging
June 2001

Computed Tomography

Spiral CT replaces IVP and KUB for renal stone disease

Marked improvement in acquisition speed allows a complete data set to be acquired in a single breath-hold

By William E. Brant, M.D.

Single-slice and multislice spiral CT have forever changed the imaging of renal stone disease.1,2 A review of the techniques, findings, complications, and pitfalls involved is timely given that CT is now the imaging method of choice to detect renal stones and diagnose the complications of renal stone disease.3-5

Acute flank pain is a common complaint of patients seeking emergency medical attention. Renal colic is the most common cause and is usually the major consideration for diagnostic imaging. Plain-film radiographs of the abdomen (often called KUB, for kidney-ureter-bladder) and excretory urograms (also called IVP, for intravenous pyelogram) are the traditional imaging methods used in the diagnosis of renal stone disease and its complications.

Plain radiographs have a sensitivity for stones as low as 45%, however, with a specificity of only 77%.6 Noncontrast CT has been shown to be more effective than IVP in precisely identifying ureteral stones and is equally effective in determining the presence or absence of ureteral obstruction.7 Spiral CT has largely replaced plain radiographs and IVP.8 CT for stones requires no contrast and no patient preparation, and the study is routinely completed in less than 90 seconds.

Imaging Techniques

Also known as CT-KUB, renal stone CT is a spiral CT exam of the urinary tract that is used to diagnose the presence of urinary tract calculi and to detect acute urinary obstruction caused by stones. No oral or IV contrast is administered.

Spiral CT is markedly faster than conventional CT, allowing acquisition of a complete data set in a single breath-hold. This speed prevents the misregistration of slice location that is characteristic of conventional CT. Multislice spiral CT further decreases the time of acquisition, allowing for thinner slice collimation and retrospective reconstruction of thin slices to review problematic areas of interpretation.

Data acquisition is continuous from the top of the kidneys through the base of the bladder (mid-liver [T-12] through symphysis pubis) using a maximum of 5-mm collimation with table speed of 5 mm/sec. Slice collimation with multislice CT is usually 2.5 to 3 mm with table speed up to 5 mm/sec.

Multislice technique allows slices as thin as 1 mm to be obtained for problem-solving. Scanning can be performed using 2.5-mm collimation with fusion of images for viewing at 5-mm thickness. The thinner slices can be viewed retrospectively without rescanning the patient. Thin slices allow identification of very small stones that may be overlooked with thicker slices.9

Turning the patient to a prone position permits differentiation of stones impacted at the ureterovesical junction from stones that have already passed into the bladder.10

When noncontrast renal stone CT is equivocal, intravenous contrast may be given to clarify the diagnosis. The pyelogram phase of contrast excretion is of most interest. Optimally, the ureters will be contrast-filled.

An intravenous injection of 100 cc of 60% contrast is given; power injection is not needed. The renal stone protocol outlined above is repeated with a scan delay of three to five minutes after completion of contrast injection. This prolonged scan delay usually results in filling of both collecting systems and ureters. Thin slices (1 to 3-mm collimation) can be obtained through any area in question.

CT Interpretation

CT may detect stones not evident on standard plain radiographs or IVP.11 It may also provide an alternative diagnosis for the patient’s symptoms, including other urinary pathology, acute appendicitis, diverticulitis, pancreatitis, adnexal masses, or leaking aneurysms.12

While only about 85% of urinary stones are seen as calcific densities on plain films, CT detects nearly all calculi. Calcium oxalate and calcium phosphate stones are most common (73%) and typically have a CT attenuation of 800 to 1000 HU. Struvite, or magnesium aluminum phosphate, stones (15%) are seen with chronic infection. Their CT attenuation ranges from 300 to 900 HU. Uric acid stones (8%), which are usually radiolucent on plain film, have an attenuation of 150 to 500 HU. Cystine stones (1% to 4%) are moderately radiopaque because of their sulfur content.13 Calcium may be present in cystine stones, which have attenuation values of 200 to 880 HU, depending on calcium content.9 High CT attenuation makes stones easy to differentiate from other urinary tract filling defects such as tumors, hematoma, fungus balls, or sloughed papilla.

Virtually all stones, even those that are radiolucent on plain-film radiographs, are identified as high-attenuation foci on CT images viewed on soft-tissue windows. Bell reported the mean attenuation of a series of calculi detected on CT as 305 HU with a range of 221 to 530 HU.14

Ureteral calculi are usually geometric or oval in shape (Figure 1) and are seldom completely round.13 This feature is useful in differentiating stones from phleboliths. The positive predictive value of geometric shape in identifying a calculus has been reported as 100%.14 The single exception to the high-density appearance of stones on CT is crystalline stones in the urine related to use of protease inhibitors in the treatment of HIV disease. These stones are nonopaque on CT scans but may cause ureteral obstruction. Retrograde ureterogram, contrast-enhanced CT, or IVP demonstrates these stones as tiny radiolucent filling defects in the ureter.

The burden of stones in the kidneys is easily determined by CT. Stones are seen in the region of the minor calices or medullary pyramids. The stone burden is defined as the number and size of stones present and is used to determine therapy, such as lithotripsy.

The tips of the renal pyramids may show high attenuation, especially when the patient is dehydrated.15 This normal finding of “white pyramids” (Figure 2) should not be interpreted as representing renal stones.

Noncontrast spiral CT has a reported sensitivity of 94% to 98% and specificity of 96% to 98% for acute ureteral obstruction caused by an impacted stone.16 CT evaluation of acute ureteral obstruction caused by stones includes the following:17-19

  • A stone is demonstrated in the ureter (Figure 1).18 The most common locations for stone impaction are at the ureteropelvic junction, where the ureter crosses the pelvic brim, and at the ureterovesical junction. The ureter is followed on consecutive slices until a stone is identified. Scrolling on the CT monitor is the easiest way to follow the course of the ureter. Knowledge of the anatomy of the ureter and adjacent vessels is crucial for accurate interpretation.
  • The size of the stone is measured and its location precisely reported. Stones smaller than 4 mm nearly always pass spontaneously; stones of 6 mm pass about half the time and those larger than 8 mm rarely pass spontaneously.13 Size and location are important factors in determining the treatment of stones that do not pass spontaneously.20 Stones larger than 5 mm and located in the proximal two-thirds of the ureter are more likely to require lithotripsy or endoscopic removal.21
  • To confirm a stone in the ureter, look for a tissue rim sign (present in about 76% of cases).14 The tissue rim sign describes a halo of soft tissue that surrounds stones in the ureter (Figure 1). The soft-tissue rim is the wall of the ureter. The tissue rim sign may be absent because of bloom effect artifact or a very thin ureteral wall.
  • Examination of the CT scout scan is useful for detecting stones and other abnormalities22 and should be included in every CT interpretation. If the stone is visible on the scout scan, plain radiographs can be used to monitor its passage (Figure 3). Calculi not visible on plain radiographs can be followed, when necessary, with unenhanced CT.23
  • Secondary findings of urinary obstruction are common but often subtle. Comparison to the opposite side is highly useful in differentiating preexisting findings from acute obstruction.18
  • The obstructed kidney may be enlarged and slightly decreased in CT density because of edema. The pelvicalyceal system is usually, but not always, mildly dilated. Dilated calyces are best seen at the poles as rounded fluid-filled structures that displace renal sinus fat (Figure 4). Comparison with the opposite kidney is always helpful. Profound dilatation of the collecting system is evidence of chronic, rather than acute, obstruction.
  • Periureteral and perinephric fat stranding occurs secondary to edema produced by obstruction (Figure 5).24 The amount of edema present correlates with the severity of obstruction. Unilateral absence of “white pyramids” on the affected side has been described as a subtle sign of obstruction.25
  • The ureter is mildly dilated to the level of the stone. Normal ureteral peristalsis produces transient focal areas of dilatation and narrowing. This must be differentiated from diffuse dilatation to the level of obstruction (Figure 6). The ureter below the obstructing calculus is not dilated. Moderate or severe hydronephrosis suggests longer-standing obstruction and should cause suspicion of other causes of ureteral obstruction (Figure 5).
  • Focal perinephric fluid collections (Figure 7) may occur secondary to forniceal rupture caused by obstruction and high urine output.
  • Axial plane CT images may be reformatted into coronal plane images that resemble IVP images in problematic cases.26 This procedure is time-consuming and seldom necessary for diagnosis. Some referring physicians may routinely request coronally reformatted images, however, because they resemble the trusted IVP.

Pitfalls In Diagnosis

No imaging test is perfect. A variety of pitfalls complicate interpretation of renal stone CT. An extrarenal pelvis may mimic pelviectasis. Peripelvic cysts can simulate hydronephrosis (Figure 8). Many patients, especially older ones, have preexisting stranding in the peripelvic fat. Comparison with the opposite side is critical to detection of asymmetric stranding.

Phleboliths, which are calcifications that originate in thrombi within pelvic veins, commonly mimic stones. Most phleboliths are found in perivesical veins, in periprostatic veins in men, and in periuterine and perivaginal veins in women. They are occasionally seen in gonadal veins that parallel the course of the ureters.

Most phleboliths are round; they are seldom oval and are never geometric in shape.13 Visualization of a central lucency is highly characteristic of phleboliths but is less often evident on CT than on plain radiographs.27 A tail sign represents a tail of noncalcified vein extending from the phlebolith.28 A tail sign has been reported with 21% to 65% of phleboliths.14,28 Phleboliths are lower density than most stones, with a mean attenuation value of 160 HU and a range of 80 to 278 HU.14 The probability that a calcification represents a phlebolith is 0.03% when mean attenuation is 311 HU or more.14

Atherosclerotic calcifications are occasionally mistaken for ureteral stones. Differentiation is made by carefully examining serial slices and determining if the calcification is in an artery or in the ureter.

It is difficult to differentiate preexisting postobstructive changes from acute obstruction. When signs of ureteral obstruction are present but no stone is evident, consider a recently passed stone, pyelonephritis, stricture or tumor, or protease inhibitor treatment-related stone.25

Stones passed from the ureter may be identified in the bladder or urethra or may not be seen.

Always look for evidence of nonurinary causes of flank pain. Unenhanced CT has been reported to be 94% accurate in the diagnosis of appendicitis.29 Adnexal masses are usually easily detected.

A subsequent contrast-enhanced CT scan may be needed in up to 20% of cases to provide an unequivocal diagnosis.

Indication Creep

The quickness and ease of obtaining noncontrast CT for renal stones has resulted in a broadening of indications by referring physicians, especially emergency department physicians.11 The result is many more studies that are negative for stones but positive for a wider range of other urinary and nonurinary abnormalities.

Noncontrast CT has substantial limitations for the diagnosis of solid masses in the liver, pancreas, and kidneys, as well as for conditions such as visceral ischemia, infarction, and infection. Radiologists may wish to broaden their use of contrast-enhanced CT to follow a negative or equivocal renal stone CT.


References

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© 2001 CMP Media, LLC.
6/1/01, Issue # 2306, page 51.