Hounsfield Units for nephrolithiasis: predictive power for the clinical urologist

We aim to determine the optimal method for measuring Hounsfield Units (HU) of calculi for the clinical urologist.

We present a single institution prospective study from 2014-2015 for 125 consecutive patients. Demographics, baseline characteristics, imaging, and stone analysis were collected. CT scanners and settings were heterogeneous. Hounsfield units were measured by use of ellipsoid tool and free hand outline by two independent urology graders using Philips iSite PACs.

Stone analysis demonstrated 26 pure calcium oxalate (CaOx) stones, 15 pure calcium phosphate (CaP) stones, and 7 uric acid stones, among other mixed types. Excellent agreement was notable amongst the two graders for ellipsoid and free hand grading, and values were consistent with those previously published with other methods. Mean grades for free-hand versus ellipse differed overall (p = 0.006) as ellipsoid HU measurement was consistently higher than free-hand measurement by an average of 107 units. Either method could differentiate between uric acid stones and any calcium containing stone (p ≤ 0.05). The free-hand method demonstrated statistical difference between pure calcium oxalate and calcium phosphate stones (p = 0.03). Applying either method took less than 6 seconds.

For urologists lacking HU on their radiology reports, free hand or ellipsoid measurement may quickly provide an additional tool to guide management. Both methods differentiate between any calcium containing stone and uric acid stones.