A High-Frequency Ultrasound-Guided Fluorescence Tomography System for Protoporphyrin-IX Quantification within Subcutaneous Tumors

A high frequency ultrasound-coupled fluorescence tomography system, which was primarily designed for imaging protoporphyrin IX production in skin tumors in vivo, is demonstrated for the first time. Protoporphyrin IX is an endogenous, fluorescent molecule produced naturally in the heme biosynthesis pathway, with enhanced production in many cancerous tumors. The design couples non-contact fiber-based spectral sampling of the protoporphyrin IX fluorescence emission with high frequency ultrasound imaging, allowing the fluorescence intensity of multiple regions to be quantified. The system measurements are obtained by serial illumination of 4 linear source locations, with parallel detection at each of 5 interspersed detection locations, providing 20 overlapping measures of sub-surface fluorescence from both superficial and deep locations in the ultrasound field. Tissue layers are defined from the segmented ultrasound images, and diffusion theory is used to estimate the fluorescence in these layers. The system calibration is presented with simulation and phantom validation of the system in multi-layer regions. Pilot in vivo data are also presented, showing recovery of subcutaneous tumor tissue values of protoporphyrin IX in three subcutaneous tumor lines, which all display less fluorescence than the skin. All results have been validated with ex vivo fluorescence measurements in situ. Future use of this device involves in vivo imaging of skin tumors to allow visualization of the structure and protoporphyrin IX production.