Reliability and accuracy of MRI scanogram in the evaluation of limb length discrepancy

The purpose of this study was to compare MRI scanogram with traditional radiographic methods for measurement of limb length. The authors hypothesized that MRI scanogram would be as reliable and accurate as radiographic scanogram in measurement of limb length without exposing patients to ionizing radiation. Twelve cadaveric femurs were measured using AP conventional radiographic scanogram, CT scanogram, MRI scanogram, and electronic caliper. Three orthopaedists performed two separate measurements using each technique. Intraobserver and interobserver variability was assessed for each of the three radiographic techniques. Accuracy was assessed by comparison of radiographic measurements to electronic caliper measurements of femur length. The reliability of all three radiographic limb length measurement techniques was excellent (ICC > 0.99). The accuracy of plain radiographic scanogram was slightly superior to CT scanogram and MRI scanogram. The mean absolute differences for radiographic, CT, and MRI scanograms compared with the gold standard, direct caliper measurement, were 0.52 mm, 0.68 mm, and 2.90 mm, respectively. All three scanogram techniques showed excellent reliability and accuracy. Radiographic scanogram remains the gold standard for leg length measurement. MRI scanogram is slightly less accurate compared with radiographic scanogram, but it does not use ionizing radiation. MRI scanogram merits clinical study and comparison with the traditional radiographic scanogram method for measurement of limb length.