What is the role of CT scanning in the assessment of stone density, composition and size during the evaluation of nephrolithiasis?

Updated: Jun 21, 2018
  • Author: Chirag N Dave, MD; Chief Editor: Bradley Fields Schwartz, DO, FACS  more...
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Answer

Currently, CT scans can be used to estimate the relative stone density and composition to some extent, although the results have not replaced the formal stone chemical composition analysis. However, this information can still help to plan therapy. Low-density stones are more amenable to shockwave lithotripsy, whereas higher-density stones may require ureteroscopy.

For example, a lucent stone that is not visible on the KUB radiograph that is clearly visible on the CT scan may indicate a uric acid calculus. This suggests a different diagnosis and therapy (urinary alkalinization) than for a calcium stone. For these reasons, many institutions routinely perform KUB radiography whenever renal colic noncontrast CT scanning is performed.

The Hounsfield unit density of the calculus on CT scanning can also be useful in predicting whether the stone is composed of uric acid. In a study of the unenhanced CT scans of 129 patients with renal stones, researchers from the University of Wisconsin concluded that the peak Hounsfield attenuation level of a kidney stone, used either by itself or divided by the size of the calculus in millimeters, may be a useful indicator of the stone’s chemical composition.

An attenuation-to-size ratio of 80 or greater was found to be highly suggestive of calcium oxalate stone material, especially in larger calculi. Uric acid stones have relatively low peak attenuation levels, and their attenuation-to-size ratios were generally below 80. In this Wisconsin study, uric acid stones averaged a mean peak Hounsfield reading of 344 HU, while the mean for calcium oxalate calculi was 652 HU.

Calculating the peak attenuation level and attenuation-to-size ratio adds no financial cost, patient morbidity, or time delay. While this study and similar reports are interesting and suggestive, the precise clinical role of CT scans in predicting stone fragility and chemical composition remains unclear.


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