Ultra-High-Resolution Dual-Source Photon-Counting CT for Expanded Characterization of Pathophysiological Imaging Patterns in Multiple Myeloma

RATIONALE AND OBJECTIVES: Dual-source photon-counting CT (DS-PCCT) combines spectral imaging with ultra-high spatial resolution for whole-body assessment of multiple myeloma (MM). This study assessed axial and appendicular skeletal manifestations at initial diagnosis by exploiting attenuation characteristics of soft tissue and fat in low-keV virtual monoenergetic images (VMI). MATERIALS AND METHODS: In this retrospective study, 84 patients (67.1±10.5 y, 54 men) underwent whole-body DS-PCCT between November 2024 and December 2025. The sample comprised 53 therapy-naïve MM patients and 31 with precursor conditions (monoclonal gammopathy of undetermined significance, smoldering myeloma, solitary plasmacytoma). Three board-certified radiologists measured CT attenuation of osteolytic lesions and femoral bone marrow at 70 and 40 keV. Long bones were evaluated for diffuse high-grade, nodular, pseudonodular, and fatty marrow patterns, as well as endosteal scalloping. RESULTS: Among 53 MM patients, 40 (75.5%) spectral-morphologic analysis identified 3 patient-level imaging phenotypes of osteolytic bone disease: a soft-tissue-attenuation phenotype (n=25, 47.2%), a fat-attenuation phenotype (n=10, 18.9%), and coexistence of both lesion types (n=5, 9.4%). The pseudonodular pattern was the predominant femoral marrow imaging pattern, whereas fatty marrow (30.2%), nodular lesions (18.9%), and diffuse high-grade infiltration (3.8%) occurred less frequently. Endosteal scalloping occurred exclusively in osteolytic MM (18.9%). Precursor conditions mainly showed fatty (58.1%) and pseudonodular (38.7%) patterns. Low-keV VMI demonstrated distinct energy-dependent attenuation across patterns. Inter-reader agreement for Hounsfield unit measurements was good to excellent (ICC >0.82). CONCLUSIONS: DS-PCCT identified a previously undescribed fat-attenuation-predominant imaging phenotype in MM that meets criteria for symptomatic bone disease but is not captured by current International Myeloma Working Group definitions of focal lesions. Spectral-morphologic characterization of marrow involvement may complement existing imaging criteria and aid differentiation between overt MM and nonspecific marrow abnormalities.