A structured system to evaluate urethral support anatomy in magnetic resonance images

OBJECTIVE: To develop a systematic method for analyzing the normal magnetic resonance imaging location and appearance of structural features involved in urethral support. STUDY DESIGN: Multiplanar proton density magnetic resonance images of 50 nu lliparous women were made at 0.5-cm intervals. The arcuate pubic ligament w as the chosen reference point in all views. Structural features were locate d by identification of the magnetic resonance images in which they appeared . The presence or absence of urethral support structures in each scan level relative to the arcuate pubic ligament was evaluated and recorded as a gra phic display. Support structures examined were the arcus tendineus fasciae pelvis, the perineal membrane, the pubococcygeal levator ani muscle and its vaginal and bony attachments, and the pubovesical muscle. Structural defin itions were developed on the basis of established periurethral anatomy. Two examiners independently assessed 10 scans for interobserver concordance. T his system was used in nulliparous women to quantify the location of magnet ic resonance visible structures. RESULTS: Because all levels were 0.5 cm apart, slice identification represe nted the distance above the arcuate pubic ligament (eg, 2 images above the arcuate pubic ligament or 1.0 cm). Interobserver concordance was 88% in ide ntification of structure location. In the axial plane, specific structural features and relationships were localized. The frequency at which a specifi c structure was observed at a specific image level in all women was depicte d as a gray density within the graphic display. These frequencies indicated where we would expect each structure to be located in healthy women. Relat ionships between structures and their attachments became apparent. Consider ation of expected structural locations proven by nulliparous scans will ena ble us to quantitatively define abnormal structures in parous women. CONCLUSIONS: This systematic magnetic resonance evaluation allows, for the first time, quantification of the normal anatomic location of urethral supp ort structures. It can be used to help identify the difference between stru ctural abnormalities and normal variation in parous women.