Correlating Noninvasive Optical Imaging to Mechanical Function in Pelvic Tissues

Objectives: The lack of noninvasive assessments of tissue integrity impedes the diagnosis and treatment of reproductive diseases. Raman spectroscopy (RS) is a label-free, non-destructive optical technique that extracts a molecular fingerprint indicative of the extracellular matrix content. This study aimed to utilize RS as a noninvasive assessment of the local vaginal composition and evaluate its correlation to the mechanical functionality of the tissue. The findings of this study are promising and display significant potential for clinical translation. Within the implementation of an in vivo RS analysis of the vaginal wall there exists a possibility of reducing the need for invasive examination techniques that risk disease progression.

Methods: Vaginal specimens were collected from the distal anterior walls of five cadavers (56-65 years old). Light-based benchtop measurements of the resected tissue were performed followed by measurements of tissue strength. A near infrared laser in a handheld probe design was utilized to acquire ten spectral replicates in twenty unique locations in a grid on the tissue to capture local vaginal composition. Spectra from a pure sample database, including total collagen, actin, glycogen, cholesterol, and lipid, were input into a regression model to acquire scores associated with the approximate percentage of each compound. A heatmap of the coefficients corresponding to the local vaginal composition was created for comparison against mechanical properties.
Immediately following spectral analysis, planar biaxial testing was performed to cyclically and simultaneously stretch the specimen along circumferential and longitudinal directions and evaluate its local extensibility and compliance (resistance to changes in shape). The respective heatmaps were then visually analyzed to correlate the regional differences in composition to mechanical integrity (Figure 1).

Results: Correlation analysis was performed between collagen (0.48±0.17), actin (0.42±0.28), glycogen (0.10±0.08), extensibility (0.11±0.07), and compliance (122.97±82.47kPa) measured at all thirty points. Vaginal extensibility negatively correlated with the actin coefficients (R=-0.40, p=0.03), and no significant correlations were observed between tissue compliance and composition. Additionally, collagen negatively correlated with actin (R=-0.44, p=0.02) and positively correlated with the glycogen coefficients (R=0.76, p< 0.01).

Conclusions: This study highlights the potential utility of RS to track tissue quality and strength. The correlation of the anterior distal vaginal wall composition to the local tissue function demonstrates potential for clinical tissue analysis without biopsies. The actin content showed a direct relationship to tissue function, consistent with the understanding that smooth muscle actin contributes significantly to the quality of the vaginal tissue. This work represents the first steps towards noninvasively quantifying the correlation of compliance and composition in the vagina.