Clinical Magnification Error in Lateral Spinal Digital Radiographs

Abstract

Observational. The objectives of this study were to determine the range of clinical magnification error in lateral spinal digital radiographs, and to determine the effect of body mass index (BMI) on this error. The magnification error in plain radiographs is often estimated at 15% to 30%. The variability of this error in digital spinal radiographs has not been assessed. An analysis of 250 patients with digital radiographs and computed tomography (CT)/magnetic resonance images (MRIs) was performed. Digital imaging software was used to measure the anteroposterior vertebral body dimensions at C2, C5, L1, and L4. Magnification values were determined in comparison to CT/MRI. CT measurements were also compared with MRI. BMI for each patient was obtained by chart review. The mean magnification at the cervical spine (C2 and C5 combined) was 1.22 +/- 0.01, with a range of 1.06 to 1.57 (n = 198, STDEV = 0.08); at the lumbar spine (L1 and L4 combined) it was 1.31 +/- 0.01, with a range of 1.09 to 1.63 (n = 300, STDEV = 0.08). The difference between the mean anteroposterior vertebral body dimensions as measured on CT and MRI was < 0.1 mm (n = 135, P < 0.2514, paired t test). There was a significant positive correlation between BMI and magnification at both the cervical and lumbar spines by linear regression (Cervical: n = 99; P = 0.0019; Lumbar: n = 150; P < 0.0001). There was a significant difference in magnification between nonobese and obese patients at both the cervical and lumbar levels. Cervical: 1.19 +/- 0.01 magnification for nonobese (n = 144), versus 1.26 +/- 0.01 for obese (n = 39) (P < 0.0001). Lumbar: 1.28 +/- 0.01 (n = 208), versus 1.38 +/- 0.01 (n = 78) (P < 0.0001), respectively. Linear clinical measurements obtained on digital radiographs are subject to significant magnification errors at both the cervical and lumbar spines. This error correlates to the patient's BMI. Consequently, clinical decision-making that is based on linear measurements obtained from radiographs that do not account for this error is invalid. In a scenario where this measurement is crucial (e.g., dynamic radiographs), this error can be corrected by comparison to morphometric data from a CT/MRI.