Small animal imaging systems now allow researchers to non-invasively monitor the progression of diseases in living small animals and study the efficacy of drugs and treatment protocols. Magnetic resonance imaging (MRI) is an established imaging modality capable of obtaining high resolution anatomical images which are sensitive to blood volume, blood flow, and metabolic rate of oxygen. Optical tomography, on the other hand, is an emerging imaging modality, which, while much lower in spatial resolution and insensitive to blood flow, can separate the effects of oxyhemoglobin, deoxyhemoglobin, and blood volume with high temporal resolution. We illustrate how these imaging modalities can supplement each other and cross validation can be performed by applying both modalities to imaging of tumors growth & regression in mice that are treated with a vascular endothelial growth factor (VEGF) antagonist.