It has been suggested that phased-array detection is a highly sensitive method of detecting and locating an inhomogeneity embedded within a scattering medium, e.g., a tumor embedded in breast tissue. No consideration has so far been given to the effects of noise within the system leading to detection errors. A probabilistic approach is used to find the optimum detector threshold and detection criteria. The technique is demonstrated using experimental data obtained from a novel phased-array system designed to eliminate noise due to amplitude fluctuations between the two sources. A brief comparison with single-beam frequency detection using the probabilistic method is also made. The technique has applications in optimizing the detection of signals due to inhomogeneities in the presence of a noisy background. © 1998 Society of Photo-Optical Instrumentation Engineers.