0848: Spatial Frequency Domain Imaging (SFDI) for Skin Assessment in Systemic Sclerosis: Insights from Histology and Clinical Correlates

Background/Purpose: Assessing skin involvement in systemic sclerosis (SSc) is complex, with no single method capturing all pathological changes. The modified Rodnan Skin Score (mRSS) is subjective, requires experienced assessors, and has notable inter-observer variability. Our initial studies showed that Spatial Frequency Domain Imaging (SFDI), an objective, noninvasive optical imaging modality, correlates with mRSS and has higher inter-rater reliability. However, the specific aspects of skin pathology measured by this technique remain unclear, as does its ability to track longitudinal changes. In this study, we systematically compare SFDI to mRSS, histology (gold-standard), ultrasound (for thickness), and durometry (for stiffness), and evaluate its longitudinal changes against changes in mRSS.

Methods: SSc patients and controls where enrolled under an IRB approved protocol. SFDI parameters (μ_s′ and R_d at 0.2 mm⁻¹ and 851 nm) were recorded at six sites: fingers, hands, and forearms. High-frequency ultrasound, durometry, and skin biopsies were done at the dorsal forearm. Two dermatopathologists evaluated histological sections for dermal thickness, alpha smooth muscle actin(ASMA)-positive fibroblast scoring, and other features. Collagen fiber score and trichrome quantification via ImageJ were also assessed. All measurements were performed blinded to clinical data. SFDI values were analyzed and compared across mRSS-defined groups using linear mixed models; clinical and histologic measurements were compared using one-way ANOVA with post-hoc Tukey tests, and correlations were assessed by Spearman’s method.

Results: 37 SSc patients and 18 controls who had skin biopsies along with SFDI measurements were included in the study. Longitudinal measurements were taken in 16 patients. Histology showed similar dermal and epidermal thickness between groups, while SSc patients had more dermal fibrosis, pigment incontinence, higher collagen and ASMA scores (Figure 1). SFDI parameters (μ_s′ and R_d) showed significant correlations with histology - including trichrome, ASMA, and collagen fiber scores - that closely matched those of mRSS. For clinical measures, SFDI correlated with ultrasound to a similar degree as mRSS, but did not perform as well as mRSS for durometry, though associations remained significant (Table 1). Additionally, SFDI measurements were significantly lower in SSc patients with mRSS = 0 compared to controls and declined further with increasing mRSS. Analysis of longitudinal changes revealed a strong correlation between changes in SFDI (∆R_d and ∆μ_s′) and changes in mRSS (∆mRSS) in Figure 2.

Conclusion: Our study shows that SFDI parameters exhibit face, content, and criterion validity comparable to mRSS when benchmarked against skin biopsy and other clinical metrics, supporting its use as a noninvasive, objective tool. SFDI also tracks longitudinal skin changes in SSc patients, making it versatile for monitoring disease progression and treatment response. Given its objectivity and minimal training requirements, SFDI is promising as a practical clinical tool, but further validation in longitudinal and multicenter studies is needed.