Abstract Body (Do not enter title and authors here): Background: Radiofrequency (RF) ablation is the most common modality used to treat cardiac arrhythmias, yet real-time lesion characteristics remain unknown, leading to arrhythmia recurrence. Intraprocedural imaging coupled with ablation biomarker identification would improve procedural efficacy. Photoacoustic imaging (PAI) offers the unique ability to localize and quantify ablation biomarkers, such as hemoglobin (Hb) and other chromophores in tissue. PAI delivers non-ionizing laser pulses to tissue, resulting in absorption and thermoelastic expansion, leading to transient ultrasonic (US) transmission at depths up to 6 cm. Spectral PAI can differentiate between various tissues states with high-resolution, high-optical-contrast. Successful development and implementation of PAI would provide cardiologists with the ability to intra-operatively identify target areas for treatment or intervention with immediate outcomes to confirm success.

Hypothesis: We propose epicardial PAI can quantify lesion biomarkers to discriminate between heathy and ablated tissue.

Methods: PAI data were acquired from the left ventricle (LV) in three open-chest porcine. Ablation lesions (n=3) were created over the LV. Post-ablation, anatomically landmarked myocardial sites were subjected to both B-mode US and PAI using a custom PAI coupler adaptor. The acquired US and PA images underwent post-processing segmentation, co-registration, linear unmixing, and masking to generate high-resolution 2D ablation maps. To validate map accuracy, lesion cross-sections were stained with 2,3,5-Triphenyltetrazolium Chloride (TTC), for direct comparison with PAI.

Results: PAI revealed discernible spectral changes in myocardium pre- and post-ablation, notably reduced intensity at 760 nm, corresponding to a unique “ablated” hemoglobin. This spectral change is attributed to thermal heating, which induces conversion of hemoglobin to methemoglobin. Lesions dimensions via PAI correlated with gross pathology by depth (4.36±1.16mm vs. 4.24±0.57mm, p=0.77) and width (7.00±1.81mm vs. 7.64±1.44mm, p=0.36 (Fig. 1). Validation with gross pathology also confirmed accurate necrosis assessments with high average area overlap (94.13 ± 0.7 %).