Utility of Depth Correction Calculator for Split Renal Function Studies utilizing prior CT or MRI

Introduction: Nuclear medicine split renal function is typically needed to assess for renal function asymmetry prior to nephrectomy, partial nephrectomy, kidney donation, or to evaluate congenital malformations, or sequelae of urinary obstruction. This is commonly done using Tc-99m MAG-3, Tc-99m-DTPA, and Tc-99m DMSA tracers by measuring regions of interest over the kidneys on posterior imaging.

The split renal function results are often incorrect due to small asymmetric differences in the distances between the skin and the kidneys. Such depth asymmetries are common. For example, a 2 cm difference in depth would cause a falsely asymmetric measurement of 57.2% - 42.8% due to asymmetries in photon absorption and scattering, even if the true function of both kidneys were exactly equal 50% - 50%. Many patients have prior CT or MRI, which can be used to obtain measurements which could be used to compute a depth-correction to the measured raw photon counts or percentages.

Methods: Depth-correction of the measured split renal function was performed using the Radiology Universe Institute Depth-Correction calculator version 1.0 at https://radiology-universe/calculator/kidney-depth-correction with input parameters as mentioned below. Distance from skin to superficial cortex and deep cortex measured from posterior and anterior aspects of bilateral kidneys on axial images. Measured kidney scintigraphy ROI counts or percentages. Background counts are optional and if percentages are taken background counts will be taken as 0. Percentage of muscle in the overlying tissues roughly estimated by volume was taken as 25% as it typically has only small effect. Thickness of cortex is by default taken as 1 cm instead of average cortical thickness and found to have negligible effect for depth correction. Anatomical imaging modalities included CT, MRI and ultrasound.

Results: Retrospectively 36 nuclear medicine MAG3 studies performed for various indications including infection, nephrolithiasis, renal neoplasms, renal graft were included in the study. A significant split renal function difference of at least 10% after depth correction and difference in kidney depths exceeding 2 cm was noted in 10 cases. In a case of potential renal donor, seemingly symmetric split renal function was noted, however, the kidneys were at different depths resulting in significant differences in split renal function after accounting for depth correction of 50.6% (uncorrected), 31.8% (depth-corrected) and 49.4% (uncorrected), 68.2% (depth-corrected) on the contralateral side. Another example is a 78-year-old male with history of left renal mass underwent MAG3 study for presurgical assessment with Split renal function of Right kidney 53.97% (depth-corrected), 25% (uncorrected). Split function per cubic centimeter of renal tissue is 30.1% per cc of cortex, and 28.4% per cc of whole kidney. Cortical volume is 106.2 cc. Kidney volume is 137.4 cc. Cortical thickness is 1 cm. Right kidney spans 15.8 to 19 cm from skin surface, with overlying tissue estimated as 25% muscle, and 75% fat, implying total photon penetration fraction of 0.07946. Left kidney: 46.03% (depth-corrected), 75% (uncorrected). Split function per cubic centimeter of renal tissue is 69.9% per cc of cortex, and 71.6% per cc of whole kidney. Cortical volume is 39 cc. Kidney volume is 46.4 cc. Cortical thickness is 1 cm. Left kidney spans 6.3 to 11 cm from skin surface, with overlying tissue estimated as 25% muscle, and 75% fat, implying total photon penetration fraction of 0.2795.

Conclusions: In a substantial fraction of cases, renal scintigraphy split renal function results may be significantly inaccurate without depth-correction. An understanding of this would be crucial to improve the accuracy of determining split renal function and may potentially play a large role in the evaluation and management of various clinical situations.