Reference lines in dynamic magnetic resonance imaging of the pelvic floor

Objective: To compare the variability in two commonly used reference lines in pelvic magnetic resonance imaging (MRI), the pubococcygeal line (PCL) and the sacrococcygeal to inferior pubis (SCIPP) line, with respect to their distance from pelvic floor points of interest. Methods: We obtained pelvic MR images of 20 asymptomatic nulliparous women who are part of an ongoing pelvic floor nerve injury postpartum study. The subjects underwent a high-resolution two-dimensional, T2-weighted sagittal pelvic MRI in the supine position using a GE Signa scanner with a body phased-array coil. We also obtained dynamic T2- weighted sagittal MR images in supine position during Kegel and Valsalva maneuvers. Using the midsagittal image, we measured the length of two reference lines: the PCL and the more cephalad SCIPP line. From each line, we then measured the perpendicular distance to the bladder neck and to the posterior margin of the anorectal angle (M-line). We compared the mean values of all measurements between the two reference lines with paired Student ± tests. Result: The SCIPP line (mean [SD], 11.60 [0.91] cm) is longer than the PCL (mean [SD], 10.54 [0.85] cm) at rest (P < 0.001). There is no significant change in length from resting to Kegel maneuver or from resting to Valsalva maneuver in either reference line. Only the resting to Valsalva maneuver for the M-line was significantly different between the 2 reference lines (P = 0.02). The resting to Kegel for the perpendicular distance to the bladder neck and the M-line was not significantly different between the two lines. Conclusions: Both reference lines remain stable during pelvic floor maneuvers.