Photoacoustic imaging of vascular hyperpermeability

Vascular hyperpermeability (VHP), an abnormal increase in the leakiness of the blood vessels, is common in many diseases and disorders, including cancer, infections, and inflammation, among others. This study aims to develop a real-time, non-ionizing photoacoustic method for imaging and quantitative analysis of vascular hyperpermeability. Toward this goal, we investigate the use of Evans blue (EB) as a marker of VHP. The hypothesis is that photoacoustic imaging (PAI) of EB's high-molecular-weight complex with serum albumin can allow a non-invasive, robust, and specific analysis of VHP.

The studies were performed in mice with hepatocellular carcinoma implanted on both legs. Each animal acted as its control (microbubbles only), where ultrasound treatment induced hyperpermeability in one leg. PAI of HCC was performed before and after inducing hyperpermeability with ultrasound. The treatments included ultrasound alone and antivascular ultrasound (AVUS), combining ultrasound and microbubbles. The PA signal was unmixed for EB. The change in EB signal (ΔPA) induced by AVUS was compared with the treatment with ultrasound alone and a control group with no treatment. The image-derived PA measurements were compared with the Miles assay and histology performed on excised tumors.

PAI of the control group showed no difference in ΔPA between treated and untreated tumors: 0.14 ± 0.04 au versus 0.15 ± 0.07 au. The ultrasound-only group showed a small increase in ΔPA between the treated and the untreated groups: 0.14 ± 0.04 versus 0.18 ± 0.05. In contrast, the AVUS group showed a marked increase in ΔPA between the untreated and the treated groups: 0.18 ± 0.02 au versus 0.60 ± 0.19 au. EB concentration measured by Miles assay was highest in the AVUS group: 141 ± 48 ng/mg tissue compared to 61 ± 16 ng/mg tissue and 58 ± 24 ng/mg tissue in the ultrasound-only and the control group. Histology showed vascular dilation, vascular disruption, and extravasation of EB in the AVUS-treated group.

PAI is a promising platform for imaging and quantitative assessment of VHP. Future studies observing a temporal change in EB signal could provide a platform for quantitative permeability assessment in various diseases.