This study investigates the potential use of combining near infrared (NIR) diffuse light and ultrasound methods for early detection of small breast tumors. A novel imaging system which combines these two methods has been introduced. A NIR phased array positioner capable of providing the line-of-sight of a small heterogeneity in a scattering medium is incorporated into an ultrasound imaging system. This combined system has been tested using models consisting of few mm emulated tumors buried in a scattering phantom. In each test, the combined system was mechanically translated to scan for a small object. At each scanning position, optical amplitude and phase data and an ultrasound image were acquired. The object line-of- sight information was coded into ultrasound images after scanning. Ultrasound image without and with line-of-sight information were evaluated by two radiologists. ROC curves using ultrasound and combined methods are measured and the areas under the average ROC curves of ultrasound and combined methods are 49% and 82%, respectively. Thus the combined method affords a significant improvement in detection sensitivity and specificity compared with conventional ultrasound in this phantom study.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.