The first and perhaps most important phase of a surgical procedure is the insertion of an intravenous (IV) catheter. Currently, this is performed manually by trained personnel. In some visions of future operating rooms, however, this process is to be replaced by an automated system. We previously presented work for localizing near-surface veins via near-infrared (NIR) imaging in combination with structured light ranging for surface mapping and robotic guidance. In this paper, we describe experiments to determine the best NIR wavelengths to optimize vein contrast for physiological differences such as skin tone and/or the presence of hair on the arm or wrist surface. For illumination, we employ an array of NIR LEDs comprising six different wavelength centers from 740nm to 910nm. We capture imagery of each subject under every possible combination of illuminants and determine the optimal combination of wavelengths for a given subject to maximize vein contrast using linear discriminant analysis.Keywords: NIR venous imaging, medical robotics, segmentation and rendering, image-guided therapy
DESCRIPTION OF PURPOSEThe ultimate goal of our work is to develop an image-guided robotic system for automated catheterization. In previous work, 1 we described benchtop hardware and algorithms to image subcutaneous veins and map the surface topography of the arm or wrist. This system employed NIR illumination for venous imaging and an NIR structured light ranging system for surface mapping. Our purpose in the work described here is to determine the optimal NIR wavelengths for optimizing vein contrast and to determine if and how those optimal wavelengths vary between subjects with different physiological characteristics (e.g., skin tone and/or hair).
METHODS
Previous research2 has investigated the propagation of light in tissue as a function of wavelength and, to some extent, the variation in venous image contrast as a function of (monochromatic) illumination wavelength in the NIR range.3 We have constructed a benchtop optical system, as illustrated in Fig. 1(a), that allows us to illuminate an area of the forearm or the hand with any combination of the six different wavelength-centered NIR LEDs. For each subject, we acquire 63 NIR-illuminated images, at video frame rates, corresponding to all possible combinations of the six wavelengths (2 6 − 1). We also acquire an RGB image of the skin surface with a color calibration target to quantify skin tone and visualize any surface hair. For this experiment, apparent veins are identified either manually or automatically from the NIR and/or RGB images. Each vein and background pixel is then represented as a 63-dimensional feature vector corresponding to the (normalized) intensity under each of the 63 illumination conditions. To find the optimal linear combination of illuminants, we perform two-class linear discriminant analysis (LDA) 4 for the vein vs. background problem. Vein segmentation and identification is based on a line detection algorithm developed in remote sensing for r...