Recent interest in the use of dual modality imaging in the field of optical Near Infrared (NIR) Tomography has increased, specifically with use of structural information, from for example, MRI. Although MRI images provide high resolution structural information about tissue, they lack the contrast and functional information needed to investigate physiology, whereas NIR data has been established as a high contrast imaging modality, but one which suffers from low resolution. To this effect, the use of dual modality data has been shown to increase the qualitative and quantitative accuracy of clinical information that can be obtained from tissue. Results so far have indicated that providing accurate apriori structural information is available, such dual modality imaging techniques can be used for the detection and characterization of breast cancer in-vivo, as well as the investigation of brain function and physiology in both human and small animal studies. Although there has been much interest and research into the best suitable and robust use of a-priori structural information within the reconstruction of optical properties of tissue, little work has been done into the investigation of how much accuracy is needed from the structural MRI images in order to obtain the most clinically reliable information. In this paper, we will present and demonstrate the two most common application of a-priori information into image reconstruction, namely soft and hard priori. The effect of inaccuracies of the a-priori structural information within the reconstructed NIR images are presented showing that providing that the error of the a-priori information is within 20% in terms of size and location, adequate NIR images can be reconstructed.