Ultrasound Assisted NIR imaging for Breast Cancer Detection Nan Guang Chen, Puyun Guo, Shikui Yan, Daqing Piao, and Quing Zhu Department of Electrical and Computer Engineering, University of Connecticut, Storrs, CT06269 ABSTRACT We have constructed a nearly real-time combined imager suitable for simultaneous ultrasound and near infrared (NIR) diffusive light imaging and co-registration. The imager consists of a combined probe and associated electronics for data acquisition. A two-dimensional ultrasound array occupies the center of the combined probe, while 12 dual wavelength laser source fibers (780 nm and 830 nm) and 8 optical detector fibers are deployed in the periphery. We have experimentally evaluated the effects of missing optical sensors in the middle of the combined probe upon the accuracy of the reconstructed optical absorption coefficient, and assessed the improvements of reconstructed absorption coefficient with the guidance of the co-registered ultrasound. The results have shown that when the central ultrasound array area is in the neighborhood of 2x2 cm2, which corresponds to the size of most commercial ultrasound transducers, the quality of optical images will not be degenerated. In addition to the acoustic information for cancer discrimination, NIR image reconstruction becomes much easier and more reliable. According to our results, the iterative inversion algorithm converges very fast with the guidance of a priori target temporal and spatial distributions. Only one iteration is needed to reconstruct accurate optical absorption coefficient. 1. INTRODUCTION Ultrasound is used extensively for differentiation of cysts from solid lesions in breast examinations and it is routinely used in conjunction with mammography. Ultrasound can detect breast lesions a few mm in size,1 however its specificity in breast cancer detection is not high as a result of overlapping characteristics of benign and malignant lesions.23 Optical imaging based on diffusive near- infrared (NIR) light has the great potential to differentiate tumors from normal breast tissue through determination of tissue parameters, such as blood volume, blood 07 saturation, tissue light scattering, water concentration, and the concentration and lifetime of exogenous contrast agents.4'1 As a potential diagnostic tool, however, NIR diffusive light imaging suffers from low spatial resolution and lesion location uncertainties due to the intense light scattering in tissue. Most NIR imaging reconstruction algorithms are based on tomographic inversion techniques.13'9. Reconstruction of tissue optical properties in general is underdetermined and ill-posed because the total number of unknown optical properties always exceeds the number of measurements and the perturbations produced by the heterogeneities are much smaller than the background signals. In addition, the inversion reconstruction algorithms are very sensitive to measurement noise and model errors. Our group and others have introduced a novel hybrid imaging method that combines the complementary features of ultrasound and near-infrared diffusive light imaging.2023'27 The hybrid imaging obtains co-registered ultrasound and NIR diffusive light images through simultaneously deployment of an ultrasound array and NIR source detector fibers on the same probe.2021'23 Co-registration permits joint evaluation of acoustic and optical properties of breast lesions, and enables the use of lesion morphology provided by high-resolution ultrasound to improve the lesion optical property estimate. With the a priori knowledge of lesion location and shape provided by co-registered ultrasound, NIR imaging reconstruction can be localized within specified spatial and temporal regions. As a result, the reconstruction is over-determined because the total number of unknown optical properties is reduced significantly. In addition, the reconstruction is less sensitive to noise because the convergence can be achieved within small number of iterations. The clinical use of the combined diagnosis relies on the co-registration of both ultrasound and NIR sensors at the probe level. Conventional ultrasound pulse echo imaging requires an imaging transducer be located on top of the target, while NIR diffusive light imaging is feasible when the optical source and detector fibers are distributed at the peripheral of the ultrasound transducer. However, the effects of missing optical sensors in the middle of the combined probe upon the accuracy of the reconstructed optical properties have to be evaluated. In addition, the improvements of reconstructed optical properties with the guidance of the co-registered ultrasound need to be quantitatively assessed. Furthermore, real-time data acquisition is necessary to avoid errors in co-registration caused by patient motion during the clinical experiments. In this Optical Tomography and Spectroscopy of Tissue IV, Britton Chance, Robert R. Alfano, Bruce J. Tromberg, Mamoru Tamura, Eva M. Sevick-Muraca, Editors, Proceedings of SPIE Vol. 4250 (2001) © 2001 SPIE · 1605-7422/01/$15.00 546