Radiologists address high expectations for kidney stone evaluations

May 22, 2009

Simple measures of kidney stone diameter no longer make the grade for urologists, who now require more comprehensive valuations of stone density and composition to guide treatment decisions.

Simple measures of kidney stone diameter no longer make the grade for urologists, who now require more comprehensive valuations of stone density and composition to guide treatment decisions.

"Expectations have changed," said Dr. Dushyant Sahani, director of CT at Massachusetts General Hospital during a presentation at the 2009 International Multidetector-Row CT Symposium in San Francisco. "Radiologists are now expected to provide more information about stone burden and composition, and we have to be aware of the radiation concerns.

Urolithiasis is a serious healthcare problem. U.S. residents carry about a 10% lifetime risk of developing kidney stones. The relapse rate is about 50% within 10 years and 75% within 20 years. The condition is more prevalent in men than women.

Imaging has traditionally played an important role for diagnosis and triage treatment decisions, Sahani said during a presentation. CT provides a definitive diagnosis with sensitivities and specificities in excess of 95% for stones as small as 1 mm in diameter.

Surprisingly, urinary obstruction is less important for treatment selection than stone size, stone composition, and patient symptoms, Sahani said. Ninety-eight percent of all kidney stones are smaller than 4 mm. They are managed conservatively with painkillers and hydration as they are passed spontaneously.

Extracorporeal shockwave lithotripsy (ESWL) or ureteroscopic lithotripsy is often performed when there is a symptomatic response to moderately sized stones (6 to 15-mm). Percutaneous nephrolithotomy (PCNL) has traditionally been prescribed for stones larger than 15 mm.

Bone window settings (1250 x 250) with CT and fivefold magnification produce the most accurate stone size measurements, Sahani said. Using the bone window is especially important to assure the accuracy of smaller stones. Measurements taken in a soft tissue window can miss stones' true size by several millimeters and lead to the wrong treatment decision.

Magnification also allows the radiologist to examine stone composition. This assessment can, again, affect treatment. Homogenous stones tend to resist lithotripsy, while heterogeneous stones tend to respond better to the procedure.

Maximum diameter may be a poor measure of burden for irregularly shaped stones. In these instances, a measure of stone volume is preferred. At MGH, Sahani uses a computer-aided algorithm that maps these typically dense stones.

Stone composition is also important because certain stones may not require treatment, regardless of their size, in the asymptomatic patient. The most effective treatment varies for other types of stones, which the radiologist identifies by measuring Hounsfield units.

Uric acid stones, for example, are typically handled conservatively with drug treatment and hydration. At 120 kVp, the densities of uric acid stones range from 200 to 400 HU.

ESWL is often the preferred treatment for struvite stones. Their densities range from 600 to 1000 HU. Ureteroscopy or PCNL may be the best choice for calcium phosphate, brushite, and cystine stones with densities of more than 1000 HU.

Dual-energy CT offers a solution for complex mixed composition stones, Sahani said. By imaging at 80 and 140 kVp, analytical software can perform a voxel-by-voxel analysis of density readings at both energies. Voxels with a dual-energy behavior similar to calcium are coded blue. Voxels similar to uric acid are coded red, and voxels with the same density at both energies remain grey.

A study at MGH involving 37 patients found the automated technique is 96% accurate for stones larger than 3 mm. It was deemed inappropriate, however, for smaller stones. Experimental gem spectral imaging using dual kVp switching shows promise, despite ongoing problems, he said.

CT can also detect Randall's plaque, a precursor to stone formation involving calcium salt deposits that appear on CT as whitish buildup in the tip of the renal papilla of patients with nephrolithiasis. No known medical treatment can reverse the process, but lifestyle changes and hydration can stop additional accretion, Sahani said.

Radiation dose is a concern for CT applications for urolithiasis, especially for recurrent stone disease. The effective radiation dose during unenhanced CT is 2.8 to 13.1 mSv for men and 4.5 to 18 mSv for women.

Sahani suggested several ways to reduce radiation exposure. Limiting coverage from the top of the kidney to the superior border of the symphysis pubis can cut dose by 20% to 40%. Another 20% to 40% reduction is possible by increasing slice thickness to 5 mm and using 2.5 to 3-mm coronal reformations. Lowering the noise index and kVp can cut dose substantially. Adaptive statistical iterative reconstruction (ASIR) results in another 35% to 80% reduction without any appreciable increase of noise, Sahani said.