Structural heterogeneity and CT-based classification of circumferential coronary calcification

Background Circumferential (360°) coronary calcification is traditionally regarded as a homogeneous plaque morphology on intravascular imaging. However, its structural characteristics on coronary CT angiography (CCTA) remain incompletely defined. Objective To evaluate structural heterogeneity of circumferential coronary calcification using CCTA. Methods Between May 2014 and January 2025, patients undergoing rotational atherectomy, orbital atherectomy, or intravascular lithotripsy with pre-procedural CCTA and intravascular ultrasound (IVUS) were screened. Circumferential calcification was defined as a 360° calcium arc identified by both modalities. For each CCTA frame corresponding to IVUS-confirmed segments, calcium density distribution was extracted. Structural heterogeneity was quantified using Shannon entropy derived from density values. Calcium was categorised into 351–649 HU, 650–999 HU, and ≥1000 HU, and dominant density proportion was calculated. Principal component analysis (PCA) incorporating entropy, dominant proportion, and total calcified area was performed, followed by k-means clustering to identify structural phenotypes. Results Among 246 patients (267 vessels) screened, 47 patients (53 vessels) met inclusion criteria for circumferential calcification on both CCTA and IVUS. A total of 385 matched CCTA frames were analysed. Mean age was 73.5 ± 8.2 years (median 74 [71.5–79.5]); 36 patients (76.6%) were male. Of 385 frames, 345 were from LAD and 40 from non-LAD vessels. PCA identified three distinct structural phenotypes (mean silhouette coefficient 0.41): a large-area, high-entropy phenotype, an intermediate phenotype, and a small-area, low-entropy phenotype. All three clusters demonstrated significant differences in entropy, dominant proportion, and total calcified area (all p < 0.001). Cluster classification was largely independent of clinical background variables, although a modest association with renal function was observed for the large-area phenotype. Vessel type was significantly associated with cluster distribution (p < 0.001). Conclusion Circumferential coronary calcification demonstrates substantial structural heterogeneity on CCTA. CT-based structural classification is feasible and largely independent of clinical background characteristics, while showing vessel-specific distribution. This framework may provide refined structural characterization of complex calcified plaques.