Dynamic MRI of the pelvic floor muscles in an upright sitting position

Since the pelvic floor muscles are situated inside the pelvis, the actual function is difficult to observe. Magnetic resonance imaging (MRI) is a new method that may prove to give additional information about pelvic floor muscle function during contraction. The purpose of the present study was to assess pelvic floor muscle function during contraction and straining in an upright sitting position by use of MRI. Sixteen women, nine continent and seven with urodynamically and clinically proven genuine stress incontinence participated in the study. MRI was performed in an open-configured GE Signa SP, 0.5 T Tesla magnet. With the participants sitting in an upright position on a pelvic RF-coil, sagittal, coronal and axial T1 weighted spin echo images of the pelvic floor were obtained. During contraction and straining a scan time of 2 seconds per image for 150-250 images, was performed in a mid-sagittal plane. The results showed that the mean inward lift during contraction was 10.8 mm (SD 6.0) for all women. During straining the mean downward movement was 19.1 mm (SD 7.4). The coccyx moved in a ventral, cranial direction during contraction and was pressed in a caudal, dorsal direction during straining. It is concluded that contraction of the pelvic floor muscles is concentric, moving the coccyx in a ventral, cranial direction. The movement measured by MRI in upright sitting position is less than that concluded after clinical observation in supine position. The coccyx is pressed dorsally during straining.