Novel CT-based Method for Determining Urinary Stone Volume: Phantom and Clinical Studies

1. Novel CT-based Method for Determining Urinary Stone Volume: Phantom and Clinical Studies S Demehri1, M.K Kalra2, M.L Steigner1, F.J Rybicki1, M.J. Lang,3, S.G Silverman 1. • Department of Radiology, Brigham & Women's Hospital , Harvard Medical School • Department of Radiology, Massachusetts General Hospital, Harvard Medical School • Department of Biological Engineering, Massachusetts Institute of Technology • Presented RSNA 2009

2. Why Stone Volume? • Urolithiasis : chronic and largely recurrent disease • CT used to depict size, number, and location of urinary calculi to assess metabolic activity: • Stone growth or new stone formation in one year • Stone size has been typically described using maximal axial diameter, which is a poor indicator of stone volume

3. Attenuation Threshold-based CT Methods • Attenuation threshold-based CT methods can quantify volume of calcium in the coronary arteries : Coronary calcium score • Such methods could be applied both at diagnosis and follow-up of urinary calculi to determine if medical therapy is necessary, and to monitor the success of medical treatment. • In this project, we described fixed (FTM) and variable (VTM) attenuation-threshold based methods for quantification of volume of urinary stones

4. Material & Methods:Gold Standard for Stone Volume • Thirty six calcium oxalate stones were obtained from a human stone bank (de-identified samples). • Gold standard for stone volumes : Fluid-displacement technique • With repeated measurements of a flask mass filled with double-distilled water, the standard error of this method was estimated to be 4 mm3.

5. Material & Methods:Phantom Experiment Stones embedded in ground meat (approximately 30 HU) and placed in dosimetric holes in abdominal slices of Alderson-Rando Phantom. Phantom was scanned using a CT protocol (120 kVp, 200 mAs) in our clinical practice for patients with acute flank pain

6. Material & Methods:Phantom Experiment • All images reconstructed at 0.6, 1, and 3 mm thick sections with no overlap. • The phantom rescanned using lower tube currents (150, 100, and 50 mAs; other parameters held constant) and tube potentials (80 and 100 kVp maintaining a constant CTDI).

7. Material & Methods:Phantom Experiment • Open source software (OsiriX, Mac OS X Version 3.3.2) used to segment the urinary stones. • Using FTM, stones were segmented using two fixed attenuation thresholds: • 130 HU (the threshold used to determine coronary calcium volume) • 575 HU i.e., attenuation value equal to half of the mean attenuation of all stones (1150+/-290, mean+/-SD). • Using VTM, stones segmented using individual attenuation-thresholds equal to half of the attenuation of each stone.

8. Material & Methods:Clinical Study-Interobserver Agreement • A retrospective study in 17 patients (M/F=1.6, mean age=55) after IRB-approval with urinary calculi. • Two independent observers measured the maximum axial diameter of urinary stones stone volumes using both the FTM and the VTM.

9. Material & Methods:Statistical Analysis • Normalized (%) Bland-Altman analysis used to evaluate the mean difference (i.e., bias) and standard deviation of difference (i.e., precision) between each CT method and the gold standard • To examine the effect of CT parameters (section thickness, mAs, and kVp) on the accuracy and precision of the stone volume measurements: one-way analysis of variance (ANOVA). • To estimate interobserver agreement: Bland-Altman limits of agreement for the paired measurements for each method.

10. Normalized Bland-Altman Difference Plot CT–derived Stone Volume Measurement Using FTM, Threshold Level at 130 HU Measurements compared with the volume measurement using fluid displacement technique, the gold standard.

11. FTM with Threshold Level at 575 HU- Half of the Mean Attenuation of All Stones

12. VTM- Threshold Level Set at the Half of the Attenuation of Each Stone

13. 134.5 mm3 220.6 mm3 a b Fixed (FTM) and Variable (VTM) threshold methods in phantom study. Using FTM, with segmentation threshold is set at 130 HU (as in coronary calcium score) (a) and at 575 HU (b), respectively. Using VTM (c) with segmentation threshold set at 699 HU (half of the stone attenuation (1398 HU)). The gold standard stone volume was 119.9 mm3. 114.9 mm3 c

15. 4.9 mm3 5.5 mm3 a b The stone volume was measured using variable threshold method (VTM) in 0.6 mm (a), 1 mm (b), and 3 mm (c) section widths. The stone volume using fluid displacement technique as gold standard was 4.1 mm3. 9 mm3 c

17. Study Limitations • Phantom study was limited to calcium oxalate stones • Imaging of stone in phantom is an idealized clinical scenario ie., no motion, no radio-opaque surrounding structure (e.g., nephrostomy tube) • In phantom study, the observer was blinded to gold standard stone volume but not the CT parameters. • The gold standard, has measurement error which could affect the bias and precision of CT based measurements.

18. Novel CT-based Stone Volume Measurement • Attenuation threshold-based CT method can accurately and precisely quantify urinary stone volumes • Individually-derived attenuation-thresholds are superior to fixed thresholds • Thinner collimations yield more accurate volume measurement • Measurements can be performed using low radiation dose CT protocols

19. “The sum of each stone’s volume could be used to derive a cumulative stone volume or “stone score” (akin to coronary calcium score), which may help the radiologist obtain an accurate and precise method of quantifying individual as well as cumulative stone burden and metabolic activity. “

20. Thank you