Improved reproducibility of calcium mass score using deconvolution and partial volume correction

Abstract

There are multiple quantitative methods for assessing coronary calcifications with CT including Agatston, mass score, and volume score. Several studies have shown mass score in mg-calcium to be the most reproducible. Since we are interested in tracking changes in individual calcifications over time as a new biomarker of vascular disease, we have analyzed ways to further improve reproducibility. The conventional way to calculate calcium mass score is to sum all voxels above 130- HU and convert to mass score using a calibration constant. However, this does not account for CT system blurring or partial volumes in voxels containing both calcification and soft tissue. To improve coronary calcification measurements, we used Richardson-Lucy deconvolution with a measured impulse response (Philips IQon) and/or partial volume correction processing. At 120 kVp, we imaged a phantom with calcium inserts and calcified cadaver hearts at three rotational orientations at high (0.4883-mm, 0.67-mm-thick) and normal clinical (0.4883-mm, 2.5-mm-thick) resolution. Processing improves accuracy as the absolute difference in conventional and processed results is (12.67 mg, 8.29 mg) for conventional resolution image and (7.09 mg, 5.26 mg) for high resolution image respectively across phantom calcium inserts with known values. Deconvolution also increased contrast (and HU) of small calcifications in cadaver hearts. For low resolution images, across rotation angle, average absolute difference, as compared to high resolution images, was improved with processing by 30.8%. Processing also improves reproducibility across rotation angle. Results were similar in virtual 70 keV images.