Whole-body Dual-source Photon-counting CT in Multiple Myeloma-The Value of Virtual Monoenergetic Imaging for Lesion Vitality Assessment.

Dual-source photon-counting CT (DS-PCCT) facilitates an unprecedented combination of spectral information and ultra-high resolution in whole-body imaging of multiple myeloma. This study explored the distinct characteristics of soft tissue, fat, and calcium in virtual monoenergetic images (VMI) with low photon energy, aiming to identify criteria of lesion vitality. This retrospective study included 51 patients with multiple myeloma (67.1±10.1 y, 36 men) who underwent unenhanced whole-body DS-PCCT between October 2024 and February 2025. Three board-certified radiologists measured CT numbers within 169 osteolytic lesions (85 active) and their surrounding tissues. Differences between 40 and 70 keV were compared among active and inactive lesions. In addition, the presence of intralesional fat, calcifications, hypodense rims, homogeneity, and highlighting in color-coded virtual non-calcium maps was assessed subjectively. The attenuation difference between 40 and 70 keV VMI was markedly larger in active than inactive lesions [median 19.3 (interquartile range: 12.7-27.0) vs. -3.8 (-26.1 to 17.0) HU; P<0.001]. Homogenous density (86.3% vs. 2.4%) and conspicuous color-coding (94.9 vs. 68.7%) were more common in active myeloma, whereas intralesional fat (10.6 vs. 72.2%), calcifications (1.6% vs. 40.1%), and hypodense rims (0 vs. 37.3%) were more frequent in inactive lesions (all P<0.001). Interrater measurement reliability was excellent (intraclass correlation coefficient ≥0.95), and agreement for all qualitative criteria was high (Krippendorff α ≥0.85). This investigation on whole-body DS-PCCT demonstrated a significant difference in attenuation changes from 70 to 40 keV VMI across multiple myeloma patients with a therapy response versus initial diagnosis and disease progression. Qualitative characteristics of medullary lesions, such as heterogeneity, partial recalcification, or a hypodense rim ("halo sign"), can serve as additional indicators of therapy response.