SELECTIVE HEATING OF VASCULAR STRUCTURES TO CREATE CONTRAST IN MEDICAL MID-IR IMAGES
The goal of this study is to develop an enhanced mid-IR imaging modality for mapping vascular structures. This imaging technique is intended for real-time use by medical professionals during surgical procedures such as tumor removal and vascular malformation correction. This novel enhanced thermal imaging method is a combination of IR imaging (8-10 ìm) and selective heating of blood (~0.5 °C) relative to surrounding water-rich tissue using LED sources at low powers. The selective heating provides contrast in the mid-IR images. Post-acquisition processing of these images highlights temporal and spatial changes revealing vascular structures. Initial studies were completed to determine which type of light source was most effective for preferentially heating hemoglobin and thus creating thermal contrast in mid-IR images. Ex vivo tissue samples (porcine blood, skeletal muscle, skin and fat) were illuminated with LEDs that emit at 405 nm and 530 nm (closely matching the wavelengths of blood absorption peaks). Illumination with the 530 nm LED at low powers (irradiance <500 mW/cm2) was more effective at selectively heating blood relative to other tissue types compared to the 405 nm LED. Experiments were conducted to test the ability of our method to map the existing supportive vasculature in porcine heart tissue ex vivo. Blood was injected into a vessel and heated with the LED. We developed an image processing method that highlights temporal and spatial variations in temperature that allowed the mapping of vessels as deep as 0.75 cm below the tissue surface and up to 16 cm from the location of the LED spot. These studies demonstrate that enhanced thermal imaging is a promising imaging modality for medical applications in which determining the location and morphology of vasculature is important. COMSOL Multiphysics, finite element analysis software package, was used to create models of heated vessels embedded in muscle tissue to help develop an algorithm to determine the depth and size of vessels based on the two dimensional thermal images. Gelatin based tissue phantoms were used to verify the algorithm's ability to determine depth and size of heated resistive wires embedded within them. We also used surface temperature profiles and thermal signal arrival time; we were successful at predicting depths and sizes for vessels embedded deeper than 2 mm in muscle tissue. The enhanced thermal imaging technique was used for tumor margin detection in vivo. Fluorescent, enhanced thermal and standard imaging modalities as well as physical caliper measurements were used to estimate breast cancer tumor volumes as a function of time in 19 mice over a 30-day study period. A strong correlation was (R2 >0.9) found between tumor volumes estimated using fluorescent imaging, standard thermal imaging, caliper measurements and enhanced IR images, indicating that enhanced thermal imaging does monitor tumor growth. Further, the enhanced IR images showed a corona of bright emission along the edges of the tumor masses. Histology revealed that the bright corona is associated with the tumor margin. This novel mid-IR imaging technique could be used to estimate tumor margins in real-time during surgical procedures.