A simple method for the measurement of Pelvic Incidence   on standard lumbar  CT and MRI studies

Purpose: This study aimed to describe a simple method for the measurement of pelvic incidence on standard lumbar CT and MRI studies not involving the femoral heads. Methods: Three groups of asymptomatic participants were recruited according to radiological modalities (X-ray [n = 50], CT [n = 54], and MRI [n = 38]). In each group, the hip axis was found and PI was measured. Then the intersection point (X point) of a line connecting the midpoint of the sacral endplate to the posterior pubic end and a line connecting the anterior pubic edge to the inferior border of the S4 segment was determined and referred to as the new hip axis. Using this point, PI was measured again. The distances between the hip axis and the X point were measured in each group and the corresponding PI values were compared. Results: The average distance between the hip axis and X point and the average PI difference were 4.39 ± 3.9 mm and 1.91°, 4.14 ± 3.2 mm and 0.80°, and 4.65 ± 2.94 mm and 0.23° ^{in the X-ray, CT, and MRI groups. Likewise, the mean absolute difference between PI values in the X-ray, CT, and MRI groups was 2.4°, 2.9°, and 1.3°, respectively. Conclusion: Determining the hip axis on standard spinal CT and MRI studies allows almost accurate measurement of PI (difference 1°–2 o ) without any additional time or cost. This method can be used routinely to measure PI on standard CT or MRI in clinical practice and research.}