Preoperative membranous urethral length (MUL) prior to radical prostatectomy (RP) is a prognostic patient anatomical factor that affects the recovery of continence following surgery(1). MUL is important because the structure and function of the membranous urethra is inherently associated with the urinary sphincter complex containing the smooth muscle fibres of the internal sphincter and surrounded along its entire length by the rhabdosphincter. Acquiring MUL measurements in routine urological clinical practice is often limited to facilities that are able to refer patients to time- and resource-intensive MRI radiological services. The development of an alternative, non-invasive imaging method that is reliable, clinically accessible, less resource intensive and has good agreement with the established, gold standard MRI methodology for the measurement of MUL is warranted.  Transperineal ultrasound (TPUS) is a well-established non-invasive imaging modality used in outpatient urological clinical practice, including for the assessment of male pelvic floor muscle function(2, 3). TPUS has the potential to visualise the anatomical landmarks required for MUL measurements. Our aim was to compare the measurement of MUL using the gold standard MRI methodology with a novel 2D transperineal ultrasound (TPUS) methodology.

MUL was prospectively measured in 18 male patients prior to radical prostatectomy using MRI and in two different patient positions using TPUS; the patient supine with the knees extended (Supine) and supine with the knees flexed to 70 degrees (Supine KF). Agreement between TPUS and MRI measurements of MUL was assessed using Bland-Altman method comparison techniques and a two-way mixed effects single measures intraclass correlation (ICC).

The mean difference in MUL measurements between MRI and i) TPUS Supine was -0.8mm (95% limits of agreement (LOA):  -3.2, 1.7) and ii) TPUS Supine KF was -0.8mm (95% LOA: -3.5, 1.9). ICC indicated a point estimate of excellent agreement between MRI and TPUS Supine ICC 0.93 (95% CI: 0.76, 0.98) and TPUS Supine KF ICC 0.91 (95%CI 0.74, 0.97).

To our knowledge, our investigation is first to report excellent agreement and small differences between MRI and TPUS measurements of MUL in two supine patient positions. The excellent agreement and small differences between the MRI and TPUS measurements of MUL were consistent in both the Supine and Supine KF patient positions. TPUS imaging, while providing less delineation of the anatomical structures of the lower urinary tract compared to MRI, permitted the reliable measurement of MUL. Since ultrasound imaging quality can be affected by operator and patient factors, individual preference may be given by clinicians to select a particular patient positon.

Preoperative MUL can be reliably used to measure MUL using TPUS in two supine positions, demonstrating excellent agreement with gold standard MRI measurements of MUL. TPUS provides clinicians with an accessible non-invasive alternative to MRI for the measurement of MUL that can be used in outpatient settings and when MRI is contraindicated.

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