Body Size, Radiation Dose and Exposure Index in Conventional Radiography: A Multicentre Prospective Study

Background/Objectives: Exposure index (EI) reflects detector exposure rather than patient dose, but its behaviour as patient size increases remains incompletely characterised across radiographic examinations. We evaluated continuous associations of body mass index (BMI) and regional body thickness proxy with dose–area product (DAP), entrance surface dose (ESD), and EI, and tested whether associations differed by examination and institution. Methods: This prospective multicentre observational study included 572 exposures from 331 adults undergoing nine projections at three institutions. Generalised estimating equations with a Gamma distribution and log link accounted for repeated exposures. Total-association models adjusted for examination, institution, age, and sex; technical-adjusted models additionally included log mAs and kVp, with SSD included only in BMI-based models. Results: In total-association models adjusted for examination, institution, age, and sex, each 1 kg/m2 increase in BMI was associated with 4.31% higher DAP, 3.25% higher estimated ESD, and 3.45% lower EI (all p < 0.001). Each 1 cm increase in geometrically derived regional-thickness proxy was associated with 3.03% higher DAP, 2.97% higher estimated ESD, and 2.51% lower EI (all p < 0.001). Technical adjustment substantially attenuated the associations with DAP, whereas inverse associations with EI persisted. Conclusions: Body-size associations were outcome- and examination-specific. Direct EI–dose associations were generally positive but substantially attenuated after accounting for exposure technique and regional thickness, indicating that their relationship is strongly influenced by patient attenuation and technical exposure factors. These findings support further examination- and system-specific evaluation of EI behaviour alongside body size, technical factors, and patient-dose indicators.