Abstract
The authors present a well-designed study in which they performed both standard dose CT and reduced dose CT scans on the same patient who presented with a presumed diagnosis of urolithiasis. Thirty-six stones were identified in this cohort. HU were obtained of the stones on both the standard dose and reduced dose CT images and compared with the measured density of the iliac crest that was used as internal validation. They conclude that there is an increase in the measured HU of urinary stones when measured on reduced dose CT scans compared with standard dose CT scans.
Their data show a consistent slight increase in density when comparing the 80 kV with the 120 kV cohort, yet there were only 18 stones in this group. The data for the 100 kV vs the 120 kV show a more mixed result with several stones measured on the 100 kV showing lower densities than on the 120 kV. However, none of the differences in the later cohort were statistically significant. This study is certainly limited by its small numbers, yet, Alikhani and colleagues also found that when measuring bone density on CT scans obtained with a lower voltage, the measured values were slightly higher. 1
On a practical level, there are challenges with applying these data to urologic practices. In many institutions, the radiology department has a variety of CT scanners with variations in CT scanning protocols for each scanner creating significant heterogeneity in image acquisition technique. Radiology reports may not mention the voltage used for a particular study; therefore, it is incumbent on the urologist to find this information on the image viewing software, or to inquire about this information from the institution performing the study.
One of the other challenges is how urologists use radiographic stone density for surgical decision-making. Stone density clearly affects treatment efficacy for shockwave lithotripsy, yet how we obtain these numbers is quite variable. 2 There is significant heterogeneity in the composition of stones that creates problems when measuring only certain areas of the stone for density. 3 Furthermore, there is significant interobserver variability when comparing the measured length of stones. 4 It is likely that density measurements also suffer from similar interobserver variability, which further complicates comparisons. When using stone density for surgical decision-making, urologists should consider these factors and may benefit from allowing a small buffer of the measured stone density.
Reducing radiation exposure is a critical part of optimizing imaging for patients with urolithiasis, yet it poses several challenges when interpreting the imaging data. This article highlights that voltage reduction may increase the measured density of stones, which should be considered when engaging in informed and shared decision-making with patients regarding urinary stone disease.