Noninvasive photoacoustic imaging maps collagen and muscle remodelling in bladder outlet obstruction.

OBJECTIVES

Our objective was to characterize regional bladder remodelling in a rat model of partial bladder outlet obstruction (pBOO) and to evaluate collagen-specific photoacoustic (PA) imaging as a noninvasive tool for detecting early structural and collagen remodelling of the bladder wall.

MATERIALS AND

METHODS

Female Sprague-Dawley rats underwent nerve-sparing midurethral obstruction or sham surgery and were evaluated at 4 and 8 weeks. Two independent cohorts of 30 rats each were studied. Cohort 1 bladders underwent ex vivo multispectral PA imaging (680-970 nm, 40 MHz) and histologic analysis. A custom spectral unmixing algorithm quantified collagen density across bladder regions (dome, midsection, and neck) and compared it with histologic collagen-to-muscle area ratios. Cohort 2 underwent molecular and biochemical analyses, including hydroxyproline quantification and assessment of collagen- and matrix-associated gene expression.

RESULTS

pBOO resulted in significant bladder enlargement, with increased absolute bladder mass and bladder-to-body mass ratios at 4 and 8 weeks (P < 0.05). Metabolic cage analysis demonstrated an increased frequency of small-volume voids at 8 weeks, despite preserved mean, maximum, and total voided volumes. Region-specific analysis identified the midsection as the site of greatest collagen accumulation with even greater detrusor muscle expansion, leading to a reduction in the collagen-to-muscle ratio. Molecular markers of collagen and matrix remodelling, along with hydroxyproline content, were significantly elevated, particularly in the midsection. Ex vivo PA imaging quantitatively mapped collagen distribution and demonstrated a strong correlation with histologic collagen-to-muscle ratios (r = 0.75), supporting its validity as a quantitative marker of regional remodelling.

CONCLUSION

Bladder remodelling in pBOO is spatially heterogeneous, involving coordinated alterations in collagen deposition and detrusor muscle architecture that are most pronounced in the midsection. Collagen-specific PA imaging enables quantitative assessment of these structural changes and represents a promising noninvasive tool for early detection and longitudinal monitoring of obstruction-associated bladder wall remodelling.