Multi-wavelength Photo-Magnetic Imaging: a novel high-resolution diffuse optical imaging modality
The functional state of biological tissue alters the optical properties of its constituents; therefore, studying the distribution of tissue chromophores is beneficial as it provides a contrast between abnormalities and normal surrounding tissue. Diffuse optical tomography (DOT) is one of the optical imaging modalities that is used noninvasively to quantify tissue chromophore concentrations such as water, lipid, oxy- and deoxy- hemoglobin by employing near infrared (NIR) light at multiple optical wavelengths. It has been demonstrated that these optical measurements have the ability to differentiate between diseased and normal tissues. Although DOT can obtain valuable information, its spatial resolution is very poor resulting in very low quantitative accuracy.Extensive efforts have been made to enhance the performance of DOT. Its combination with anatomical imaging modalities has been widely demonstrated to improve its spatial resolution through the use of structural information. Yet, this approach is limited to a few cases due to technical challenges required for such incorporation. Furthermore, the optical and anatomic imaging systems in those multi-modalities are working independently from each other. To overcome these limitations, our lab introduced a new technology termed, “multi-wavelength Photo-Magnetic Imaging (PMI)” in the recent years. PMI is a hybrid modality that synergistically utilizes NIR light and Magnetic Resonance Imaging (MRI). Unlike DOT where measurements are acquired only at the boundary, PMI utilizes MRI to noninvasively acquire the laser-induced temperature measurements internally. Utilizing multiple wavelengths PMI, endogenous and exogenous chromophore concentrations are recovered. In contrast to other conventional multi-modality approaches, optical and MR modalities work together in harmony by interacting with each other in PMI. However, PMI developmental efforts have been limited to a basic single-wavelength prototype system up to now. This thesis presents the development of the novel multi-wavelength PMI system including the instrumentation and its associated control software. This system can noninvasively elevate the internal temperature of biological tissue by a few degrees utilizing five wavelengths ranging between 760 nm and 980 nm, where the laser-induced temperature is internally measured using MRI. Two image reconstruction approaches are developed and implemented to recover the chromophore concentrations from PMI multi-wavelength temperature measurements. The feasibility of the multi-wavelength PMI technique for providing chromophore concentrations with high spatial resolution and high quantitative accuracy is validated using simulation studies and experimentally demonstrated on homogenous and heterogeneous phantoms, using several newly introduced PMI reconstruction methods.