Introduction/Purpose: Urolithiasis is a common problem in veterinary medicine and minimally invasive treatment with lithotripsy is being used with increasing frequency. Not all stone types can be fragmented with lithotripsy and the treatment is not without potential side effects so in vivo determination of stone composition would be advantageous to facilitate optimal treatment selection. Dual energy CT exploits the differences in the probability of the photoelectric and Compton interactions and the variability of k-edges between different tissues resulting in the relative linear coefficients being different at different energies. Images are acquired at both a high and low kV and the image data is combined in to a dual energy measurement. There is some support in the literature that dual energy measurements are superior to Hounsfield units in determining stone composition. The objective of this study was to determine whether dual energy measurements of canine stones in a phantom model are able to differentiate stone materials.
Methods: Thirty bladder stones from the canine urolithiasis bank previously determined to be greater than 70% pure composition were evaluated with dual energy CT. Stones were suspended in an agar phantom and two axial scans were acquired using 140 kV, 100 mAs and 80 kV, 100 mAs with a 0.625mm slice thickness. Hounsfield units from the high and low energy scans were obtained from manually drawn regions of interest. Dual energy number (low HU – high HU) and dual energy ratio (low HU / high HU) were calculated. Low and high energy HU, dual energy number and dual energy ratio were compared to the gold standard lab analysis and results were evaluated with an ANOVA with significance set at 0.05.
Results: No single measure differentiated between all stone types. Dual energy ratio could differentiate between urate stones and all others. Dual energy number differentiated six stone pairs and low energy HU differentiated 7 stone pairs. If all three measures were used together all stones could be differentiated except for struvite and cysteine.
Discussion/Conclusions: Dual energy CT provides additional information about stone composition but no one CT measurement can consistently differentiate all stone materials so clinical application of this technique is not recommended. Further work is required to determine whether dual energy CT can be used to predict stone fragility.