Magnetic resonance imaging anatomy of the female urethra: a direct histologic comparison

To define the urethral structures visible on magnetic resonance imaging (MRI) relevant to stress urinary incontinence.

The urethra and surrounding tissues were harvested from 13 female cadavers (ages 21-81) and fixed in 10% buffered formalin. High-resolution T1- and T2-weighted images were obtained at 1.5 tesla. Mallory trichrome-stained histologic sections were prepared in corresponding planes from the cadaveric specimens. Immunohistologic stains for smooth muscle (actin) and vascular endothelium (CD-34 and factor VIII) were obtained on two specimens. Histology and MRI were compared using side-by-side correlation of projected images and by superimposing projected images. Comparison was also made to a non-cadaveric urethral MRI of a 29-year-old woman and to the MRI of another specimen imaged pre- and post-fixation.

Distinct layers of the cadaveric urethra were seen best on proton density and T2-weighted images. From the center to the periphery, a series of concentric rings were visible: an inner bright ring, the mucosa; a dark ring, the submucosa; an outer bright ring, the smooth muscle of the urethra in a loose connective tissue matrix; and a peripheral dark ring, the striated urogenital sphincter muscle of the urethra in dense connective tissue. No significant alterations were caused by fixation. These cadaveric images matched the non-cadaveric MRI of the 29-year-old woman.

The internal urethral anatomy visible on high-resolution MRI can be identified and confirmed histologically, and these findings may form the basis for future anatomic investigation of stress urinary incontinence and other urethral abnormalities.