A total of 364 optical source-detector pairs were uniformly deployed over a 9 x 9 cm2 probe area initially, and then the total pairs were gradually reduced to 60 in experimental studies. For each source-detector configuration, 3D images of a 1 cm diameter absorber of different contrasts were reconstructed form the measurements made with a frequency domain system. The results have shown that more than 160 source-detector pairs are needed to reconstruct absorption coefficient within 60% of the true value and appropriate spatial and contrast resolution. However, the error in target depth estimated from 3D images was more than 1 cm in all source-detector configurations. With the a priori target depth information provided by ultrasound, the accuracy of the reconstructed absorption coefficient has been improved by 15% and 3% on average for high and low contrast cases, respectively. The speed of reconstruction has been improved by 10 times on average.