Phantomless calibration of CT scans for measurement of BMD and bone strength—Inter-operator reanalysis precision

Abstract

Patient-specific phantomless calibration of computed tomography (CT) scans has the potential to simplify and expand the use of pre-existing clinical CT for quantitative bone densitometry and bone strength analysis for diagnostic and monitoring purposes. In this study, we quantified the inter-operator reanalysis precision errors for a novel implementation of patient-specific phantomless calibration, using air and either aortic blood or hip adipose tissue as internal calibrating reference materials, and sought to confirm the equivalence between phantomless and (traditional) phantom-based measurements. CT scans of the spine and hip for 25 women and 15 men (mean ± SD age of 67 ± 9 years, range 41–86 years), one scan per anatomic site per patient, were analyzed independently by two analysts using the VirtuOst software (O.N. Diagnostics, Berkeley, CA). The scans were acquired at 120 kVp, with a slice thickness/increment of 3 mm or less, on nine different CT scanner models across 24 different scanners. The main parameters assessed were areal bone mineral density (BMD) at the hip (total hip and femoral neck), trabecular volumetric BMD at the spine, and vertebral and femoral strength by finite element analysis; other volumetric BMD measures were also assessed. We found that the reanalysis precision errors for all phantomless measurements were less than 0.5%, which was as good as for phantom calibration. Regression analysis indicated equivalence of the phantom- versus phantomless-calibrated measurements (slope not different than unity, R2 ≥ 0.98). Of the main parameters assessed, non-significant paired mean differences (n=40) between the two measurements ranged from 0.6% for hip areal BMD to 1.1% for mid-vertebral trabecular BMD. These results indicate that phantom-equivalent measurements of both BMD and finite element-derived bone strength can be reliably obtained from CT scans using patient-specific phantomless calibration.

Osteoporosis and Hip Fracture Risk From Routine Computed Tomography Scans: The Fracture, Osteoporosis, and CT Utilization Study (FOCUS)

“…The VirtuOst‐based BCT analysis used in FOCUS also provides DXA‐equivalent BMD T ‐scores at the hip . In two prior studies that analyzed previously taken routine clinical CT scans in which the BCT measurements were blinded to the DXA data, the femoral neck BMD T ‐scores from VirtuOst agreed well with those from DXA.…”

“…Our current results demonstrate general robustness of our approach, consistent with two previous studies that validated this approach versus DXA for CT enterography and CT colonography scans . The interoperator reanalysis precision for our implementation of BCT is also equivalent to that for traditional quantitative CT (using a calibration phantom) . Second, in the current study, we uniquely applied a small adjustment for the presence of any intravenous contrast in the CT scan, based on two independent but consistent observations of such effects in paired sets of CT scans with and without intravenous contrast .…”

“…In analyzing previously taken routine clinical CT scans, our approach to phantomless calibration is noteworthy for two reasons. First, we used each patient's own internal reference tissues as references for calibration, which provides a patient‐specific calibration . This approach differs from the “asynchronous” approach, which utilizes a previously measured calibration and therefore provides a machine‐specific but not a patient‐specific calibration.…”

“…First, the proximal femur is separated from any surrounding tissues and resampled into 1.5‐mm‐sized voxels in a standardized sideways‐fall coordinate system. Second, independent of this process, a patient‐specific phantomless calibration is performed, using the air and hip fat in the CT scan as reference materials . Third, the calibrated BMD values of each voxel in the scan are converted into bone mechanical properties using empirical relations derived from human cadaver studies.…”

“…One effective option to address this care gap would be to expand options for osteoporosis screening while maintaining efficacy for predicting hip fracture. Toward that end, it has been proposed to reanalyze previously taken routine clinical computed tomography (CT) scans that contain the proximal femur to measure a DXA‐equivalent hip BMD, without the need for using an external calibration phantom during patient imaging . Because millions of hip‐containing CT scans are taken annually, the “add‐on” nature of this ancillary approach offers the potential for widespread adoption.…”