Photo‐magnetic imaging: a new functional imaging modality for more accurate photothermal therapy planning

Abstract

In photothermal therapy, heat generated from absorbed light energy is used to treat cancerous tissue. Thus, determining laser parameters that control temperature elevation prior to therapy is crucial for an effective outcome. These parameters can be defined by modeling the expected temperature increase via numerical simulations. The quality of these simulations highly depends on the accurate knowledge of optical properties of the treated tissue. Multi-wavelength Photo Magnetic Imaging (PMI) utilizes four laser wavelengths from the near infrared (NIR) window to induce a relatively low temperature increase, while measuring the laser-induced temperature increase using Magnetic Resonance Thermometry. These measured temperature maps are then used by the PMI image reconstruction algorithm to provide high spatial resolution absorption maps at these wavelengths. These absorption maps are then processed to recover the concentration of the main chromophores of the tissue, and consequently obtain its total optical absorption spectrum at any wavelength in the NIR region based on the Beer-Lambert law. In this paper, PMI was used to recover the absorption coefficient of a gelatin tissuesimulating phantom at four wavelengths. These spatially-resolved absorption values were used to successfully recover the concentration of the chromophores of the phantom and calculate its total absorption spectrum in the NIR spectral window with an error as low as ∼ 2.3%. Therefore, applications of photothermal therapy applied in NIR window can benefit from the absorption spectrum recovered by PMI to achieve accurate simulations and determine important laser parameters, which are key for accurate therapy planning.