Short-wave Infrared Photoacoustic Spectroscopy for Lipid and Water Detection

Lipids play a vital role in brain function; Alzheimer's disease and aging are both associated with changes in lipid composition. Current methods for imaging and monitoring lipid deposits within the brain have poor spatial resolution, sensitivity, and/or penetration depth. Photoacoustic imaging (PA/PAI) allows for deep (>5mm) mapping of optical absorption_properties of biological tissue.; however, water and lipid have relatively weak optical absorption properties in the typical near-infrared window used for PAI (700-1000nm). This study investigates PAI in the short-wave infrared (SWIR) range (>1200nm) as a method to dramatically enhance lipid and water contrast. Due to multiple strong absorption peaks within this optical window, a three wavelength unmixing technique labeled Tri-λ Concentration Mapping (CM) is shown to have the ability to quantify lipid and water content several millimeters into tissue. Values of lipid and water concentration are calculated at voxels within sections of marbled bovine muscular tissue. Investigated regions of muscular meat show a bimodal distribution about 75% and 50% predicted water concentration. Calculated lipid regions at depths of ~5mm display a more precise distribution centered around a lipid concentration of 57.25 ^% . This technique demonstrates the ability of SWIR PAI and Tri-λ CM to identify and characterize lipid content within tissue at depths greater than 5mm. The results of this study show promise of the technique for use in more advanced studies, such as tracking Alzheimer's disease progression within mice.