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Stone volume measuring methods: Should the CT based three-dimensional-reconstructed algorithm be proposed as the gold standard? What did the three-dimensional printed models show us?

OBJECTIVES
We researched to detect the optimal method for evaluating stone volume, by comparing the ellipsoid formula and 3D reconstructed volume to gold standard of water displacement volume using 3D printed models.


METHODS
We retrospectively analyzed out patient database and evaluated 27 patients who had percutaneous nephrolithotripsy. From the DICOM data of patient's preoperative CT images, stone structures were obtained using an image identification system. All DICOM files were saved in the stereolithography file format and a 3D printed model was created. True stone volume was found by water displacement method using this model and this volume compared with ellipsoid formula and 3D-reconstructed volume.


RESULTS
Stone volume on water displacement ranged from 0.38-29.8cm3 with a median of 17.5cm3, stone volume on ellipsoid formula ranged from 0.46-34.7cm3 with a median of 19.6cm3 and stone volume on CT based 3D-reconstructed ranged from 0.48-31.8cm3 with a median of 18.9cm3. There was a significant difference between ellipsoid formula and water displacement ( p <   0.0001 ); however, there was no difference between 3D-reconstructed volume and water displacement ( p = 0.051 ).


CONCLUSION
Stone volume calculation using CT based 3D-reconstructed algorithm improves the accuracy of stone volume estimation and this measurement is superior to ellipsoid formula.