HR-pQCT has become standard practice when quantifying volumetric BMD (vBMD) in vivo. Yet, it is only accessible to peripheral sites, with small fields of view and lengthy scanning times. This limits general applicability in clinical workflows. The goal of this study was to assess the potential of photon counting CT (PCCT) in quantitative bone imaging. Using the European Forearm Phantom, PCCT was calibrated to hydroxyapatite (HA) density. Eight cadaveric forearms were scanned twice with PCCT and once with HR-pQCT. The dominant forearm of two volunteers was scanned twice with PCCT. In each scan, the carpals were delineated. At bone level, accuracy was assessed with a paired measurement of total vBMD (Tt.vBMD) calculated with PCCT and HR-pQCT. At voxel-level, repeatability was assessed by image registration and voxel-wise subtraction of the ex vivo PCCT scans. In an ideal scenario, this difference would be zero; any deviation was interpreted as falsely detected remodeling. For clinical usage, the least detectable remodeling was determined by finding a threshold in the PCCT difference image that resulted in a classification of bone formation and resorption below acceptable noise levels (<0.5%). The paired measurement of Tt.vBMD had a Pearson correlation of 0.986. Compared to HR-pQCT, PCCT showed a bias of 7.46 mgHA/cm3. At voxel-level, the repeated PCCT scans showed a bias of 17.66 mgHA/cm3 and a standard error of 96.23 mgHA/cm3. Least detectable remodeling was found to be 250 mgHA/cm3, for which 0.37% of the voxels was incorrectly classified as newly added or resorbed bone. In vivo, this volume increased to 0.97%. Based on the cadaver data, we conclude that PCCT can be used to quantify vBMD and bone turnover. We provided proof of principle that this technique is also accurate in vivo, hence, that it has high potential for clinical applications.
Keywords: BMD; HR-pQCT; bone remodeling; photon counting CT; repeatability.
In QCT, bone images have gray values that reflect the local bone mineral content within each voxel. Aggregated over large bone regions, a total BMD can be calculated, which helps in identifying weak bones and fracture risk. At small scales, QCT can detect where bone is being formed, and thus the bone mineral content increases, and where bone is being removed, and thus the bone mineral content decreases. These measurements are typically done with HR-pQCT. However, HR-pQCT can only scan small regions of the arms and legs, for which a long scanning time is needed. This makes it challenging to use HR-pQCT in a clinical context. Photon counting CT (PCCT) is a new CT device that can scan bone with an image quality similar to HR-pQCT, yet it can scan faster and cover a larger area. Used at the large scale, our results indicate that PCCT and HR-pQCT can be used interchangeably for the quantification of BMD in large bone regions. Used at small scales, our results indicate that both technologies can detect changes in bone mineral content with similar sensitivity. These results demonstrate that PCCT enables the use of these QCT analyses in a clinical context.
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