Shadpour Demehri MD, Presenter: Nothing to Disclose

Mannudeep Karanvir Singh Kalra MD, Abstract Co-Author: Research grant, General Electric Company

Michael Lally Steigner MD, Abstract Co-Author: Nothing to Disclose

Frank John Rybicki MD, PhD, Abstract Co-Author: Speakers Bureau, Toshiba Corporation Advisory Board, Toshiba Corporation Speakers Bureau, Siemens AG Speakers Bureau, Bracco Group Advisory Board, Bracco Group Advisory Board, Vital Images, Inc

E. Andres Houseman DPHIL, Abstract Co-Author: Nothing to Disclose

Stuart George Silverman MD, Abstract Co-Author: Consultant, Galil Medical Ltd, Yokneam, Israel Consultant, Siemens AG

Optimal care of patients with urinary stones requires an accurate assessment of stone volume. Current diameter measurements from axial CT images are insufficiently accurate, and even in aggregate do not reflect stone burden.

Thirty four stones were imaged in an anthropomorphic abdominal phantom with 64-channel MDCT (120 kVp, 200 mAs, 0.6 mm collimation). Stone volumes were measured using open-source software (OsiriX) with fixed (FTM) and variable (VTM) threshold methods. For FTM, stones were segmented with 0.6 mm sections using thresholds of 130 HU (similar to coronary calcium scoring) and 570 HU (half of mean attenuation of all stones, 1140 HU). Using VTM, stones were segmented at a threshold determined by half of the individual stone attenuation. The phantom was also scanned with variable tube currents (150, 100, and 50 mAs) and kilovoltages (100 and 80 kVp; maintaining a constant CTDI). Normalized [(measured/gold standard-1) x 100%] Bland-Altman analysis was performed to assess the mean (bias) and SD (precision) of the difference between CT and fluid-displacement, considered the gold standard. In addition, a retrospective study was performed in 17  patients (M/F=1.6, mean age=55) after IRB-approval with urinary calculi. Two independent readers measured stone volumes using VTM and FTM to assess inter-observer agreement..

Using FTM (0.6 mm images) stone volumes were 113+/-92 mm3 (bias (B) =147%, precision (P)=79%; p<0.0001) using a threshold of 130 HU and 54+/-56 mm3 (B=-10%, P=30%) using a threshold of 570 HU. Using VTM, stone volumes were 55+/-51 mm3 (B= 2%, P=20%), 54+/-49 mm3 (B=5%, P=28%), and 64+/-59 mm3 (B=43%, P=83%; p<0.001) with 0.6, 1, and 3 mm sections, respectively. Using VTM, no difference was seen in bias and precision of volume measurements at various tube currents and potentials. The correlation coefficient for inter-observer agreement in the clinical study was >0.99 and 0.94 using FTM and VTM, respectively (p<0.0001).

Individually derived thresholds yield more accurate and precise volume measurements when compared to standard, fixed thresholds. Thinner collimation offers superior volume measurements and can be achieved with lower dose protocols.

CT-based methods can be used to assess urinary stone volume with high accuracy and inter-observer agreement. Routine use may improve patient triage when compared to diameter measurements.

Demehri, S, Kalra, M, Steigner, M, Rybicki, F, Houseman, E, Silverman, S, Novel CT-based Method for Determining Urinary Stone Volume: Phantom and Clinical Studies.  Radiological Society of North America 2009 Scientific Assembly and Annual Meeting, November 29 - December 4, 2009 ,Chicago IL. http://archive.rsna.org/2009/8001358.html