MRI Mapping of Heat Dissipation from Polydopamine Particles under Infra-Red Irradiation

Polydopamine (PDA) is a new nanomaterial with promising properties for photothermal therapies. We investigated heating of phantoms made of agar gel with different concentrations (from 1 to 200 mg/ml) of PDA 100 nm particles. Spectrophotometry measurements show significant absorption in near-infrared range with increases below 300 nm. The laser beam (808 nm, 5W) was guided through a fiberglass guide and dispersed by a diffuser. A 25 ml phantom was placed in styrofoam thermal insulation. The temperature of the phantom after 5 minutes of continuous laser irradiation increased dramatically for concentrations above 100 mg/ml. For concentration of 200 mg/ml, an increase of 41 ^oC was achieved, about 14 times higher than for the control pure agar gel. Larger phantoms were studied in MRI settings to obtain heat dissipation maps, i.e. relative change of temperature in the phantom volume for different laser irradiation time and power. Temperature maps were obtained by analyzing the phase change due to proton resonance frequency shift (PRF). PRF results were compared to measurements using four miniature MRI compatible temperature sensors. Results allowed us to determine thermal gradients produced by single point heating which may be useful in MRI guided laser ablations of cancers.