Radiation dose and magnification in pelvic X-ray: EOS™ imaging system versus plain radiographs.

BACKGROUND: In plain pelvic X-ray, magnification makes measurement unreliable. The EOS™ (EOS Imaging, Paris France) imaging system is reputed to reproduce patient anatomy exactly, with a lower radiation dose. This, however, has not been assessed according to patient weight, although both magnification and irradiation are known to vary with weight. We therefore conducted a prospective comparative study, to compare: (1) image magnification and (2) radiation dose between the EOS imaging system and plain X-ray.

HYPOTHESIS: The EOS imaging system reproduces patient anatomy exactly, regardless of weight, unlike plain X-ray.

MATERIAL AND METHOD: A single-center comparative study of plain pelvic X-ray and 2D EOS radiography was performed in 183 patients: 186 arthroplasties; 104 male, 81 female; mean age 61.3±13.7years (range, 24-87years). Magnification and radiation dose (dose-area product [DAP]) were compared between the two systems in 186 hips in patients with a mean body-mass index (BMI) of 27.1±5.3kg/m2 (range, 17.6-42.3kg/m2 ), including 7 with morbid obesity.

RESULTS: Mean magnification was zero using the EOS system, regardless of patient weight, compared to 1.15±0.05 (range, 1-1.32) on plain X-ray (P<10-5 ). In patients with BMI<25, mean magnification on plain X-ray was 1.15±0.05 (range, 1-1.25) and, in patients with morbid obesity, 1.22±0.06 (range, 1.18-1.32). The mean radiation dose was 8.19±2.63dGy/cm2 (range, 1.77-14.24) with the EOS system, versus 19.38±12.37dGy/cm2 (range, 4.77-81.75) with plain X-ray (P<10-4 ). For BMI >40, mean radiation dose was 9.36±2.57dGy/cm2 (range, 7.4-14.2) with the EOS system, versus 44.76±22.21 (range, 25.2-81.7) with plain X-ray. Radiation dose increased by 0.20dGy with each extra BMI point for the EOS system, versus 0.74dGy for plain X-ray.

CONCLUSION: Magnification did not vary with patient weight using the EOS system, unlike plain X-ray, and radiation dose was 2.5-fold lower.

LEVEL OF EVIDENCE: 3, prospective case-control study.