MRI-DTI Assessment of Urethral Internal Sphincter Ring and Prostate Base Morphology

This is a single-center, observational imaging study designed to evaluate the in vivo morphology of the male urethral internal sphincter ring using magnetic resonance diffusion tensor imaging (MRI-DTI) fiber tracking, and to investigate the relationship between sphincter-ring morphology and prostate base morphology.

Post-prostatectomy urinary incontinence remains an important functional complication affecting quality of life in male patients. Although bladder neck preservation and continence-related reconstructive techniques are widely used in prostate surgery, direct in vivo imaging evidence demonstrating whether these approaches preserve or reconstruct the urethral internal sphincter remains limited. Conventional anatomical descriptions generally place the male internal sphincter in the bladder neck and proximal urethral region. However, clinical observations suggest that in men with prostate enlargement, protrusion of the prostate base, or marked changes in the bladder outlet region, a clearly defined ring-like smooth muscle structure may not be identifiable at the bladder neck. This raises the possibility that the location and morphology of the internal sphincter may change in association with prostate growth and remodeling of the prostate base.

MRI-DTI is an imaging technique that uses directional water diffusion to infer tissue fiber orientation and to reconstruct local fiber trajectories. In this study, DTI fiber tracking will be combined with T2-weighted imaging to visualize ring-like muscle fiber structures in the bladder neck, prostate base, and proximal urethral region. T2-weighted imaging will provide anatomical reference for the prostate, bladder neck, and proximal urethra, while DTI data will be used for fiber tracking and three-dimensional reconstruction of the urethral internal sphincter ring.

The study will include male participants undergoing clinically indicated pelvic, seminal vesicle, or prostate-related MRI-DTI examinations. The cross-sectional imaging cohort will include a broad range of male participants rather than only patients with benign prostatic hyperplasia. This cohort may include younger men undergoing MRI for suspected cryptorchidism or suspected seminal vesiculitis, as well as older men with benign prostatic enlargement or prostate base enlargement. This design is intended to capture a wider spectrum of prostate size, prostate base morphology, and urethral internal sphincter-ring morphology. A separate paired follow-up cohort will include patients with prostate cancer scheduled to receive androgen deprivation therapy (ADT), with MRI-DTI performed before treatment and during follow-up after ADT.

All imaging data will be processed using a standardized post-processing workflow. T2-weighted images and DTI datasets will be co-registered. The bladder neck, prostate base, and proximal urethral region will be identified on fused T2WI-DTI images. Two elliptical regions of interest will be placed anterior and posterior to the urethral region, and fiber tracts passing through both regions of interest will be selected to reconstruct the target ring-like fiber structure. The fused images will be used for quantitative measurements, and three-dimensional fiber models will be used for morphological visualization and representative case demonstration.

The main prostate-side morphological parameters will include prostate volume, intravesical prostatic protrusion, prostate base diameter, and prostate base area when technically feasible. Prostate volume will be calculated from T2-weighted images using standard three-dimensional measurements and an ellipsoid formula. Intravesical prostatic protrusion will be measured on sagittal T2-weighted images. Prostate base diameter and area will be measured at predefined anatomical levels based on the bladder neck and prostate base. The main sphincter-ring parameters will include the inner diameter and area of the urethral internal sphincter ring. Exploratory DTI-derived parameters, such as fractional anisotropy and mean diffusivity, may be recorded when the reconstructed fiber structure is technically stable and measurable.

In the cross-sectional cohort, the primary analysis will evaluate the association between prostate base morphology and urethral internal sphincter-ring morphology. Correlations will be assessed between prostate volume, prostate base diameter, prostate base area, intravesical prostatic protrusion, and sphincter-ring inner diameter or area. This analysis is intended to determine whether prostate enlargement or local remodeling of the prostate base is associated with expansion, displacement, or morphological remodeling of the internal sphincter ring.

In the ADT paired cohort, imaging parameters will be compared before and after androgen deprivation therapy. Changes in prostate volume, prostate base diameter, prostate base area, intravesical prostatic protrusion, and sphincter-ring diameter or area will be calculated. The study will further evaluate whether reduction in prostate volume and prostate base morphology after ADT is accompanied by corresponding changes in the urethral internal sphincter ring. Change values and percentage changes will be used to assess the relationship between prostate regression and sphincter-ring remodeling.

This study will not assign treatment or alter the participants' existing clinical diagnostic or therapeutic pathways. Imaging examinations will be performed in participants who undergo MRI for clinical indications or planned follow-up. The study will focus on image post-processing, quantitative measurement, and statistical analysis of MRI-DTI data. Participants with neurological bladder dysfunction, prior pelvic radiotherapy, prior urethral reconstruction or sphincter surgery, contraindications to MRI, or inadequate image quality for sphincter-ring assessment will be excluded.

Continuous variables will be summarized according to their distribution. Normality will be assessed using appropriate statistical tests and visual inspection when applicable. Correlations between prostate-side and sphincter-ring parameters will be evaluated using Pearson or Spearman correlation analysis according to data distribution. In the ADT cohort, paired comparisons will be performed using paired t-tests or Wilcoxon signed-rank tests as appropriate. Associations between changes in prostate morphology and changes in sphincter-ring morphology will also be assessed using correlation analysis. A two-sided P value of less than 0.05 will be considered statistically significant.

The study is expected to provide in vivo imaging evidence regarding the relationship between prostate base morphology and urethral internal sphincter-ring morphology. The findings may improve understanding of age- and prostate-related displacement and remodeling of the male internal sphincter and may provide imaging support for future surgical strategies aimed at identifying, preserving, or reconstructing proximal urinary continence-related structures during prostate surgery.

Inclusion criteria

• Male participants undergoing clinically indicated pelvic, seminal vesicle, or prostate-related MRI-DTI examination.
• Participants with adequate T2-weighted MRI and DTI data for post-processing and quantitative assessment of the bladder neck, prostate base, and proximal urethral region.
• Participants in the cross-sectional imaging cohort may include men undergoing MRI for suspected cryptorchidism, suspected seminal vesiculitis, benign prostatic enlargement, or other clinically indicated pelvic or prostate-related evaluations.
• Participants in the ADT paired follow-up cohort must have prostate cancer and be scheduled to receive androgen deprivation therapy, with MRI-DTI planned before treatment and during follow-up after ADT.
• Participants must be able to comply with MRI examination procedures.
• Written informed consent will be obtained when required by the approved study protocol.

Exclusion criteria

• Contraindications to MRI, including but not limited to incompatible metallic implants, severe claustrophobia, or other conditions preventing MRI examination.
• Known neurogenic bladder dysfunction, including spinal cord injury, multiple sclerosis, Parkinson disease, or other neurological disorders that may significantly affect lower urinary tract function.
• History of pelvic radiotherapy.
• History of urethral reconstruction, sphincter surgery, or other prior surgery that substantially alters the bladder neck, proximal urethra, or prostate base anatomy.
• Severe cardiopulmonary dysfunction, cognitive impairment, psychiatric disorder, or other conditions preventing cooperation with MRI examination or study procedures.
• Use of medications or treatments within the previous 3 months that may substantially affect neuromuscular function of the lower urinary tract, as judged by the investigators.
• Poor image quality or failed post-processing that prevents reliable visualization or measurement of the urethral internal sphincter ring.