New directions in medical imaging of cancer. Magnetic resonance methods and single photon emission computed tomography. Magnetic resonance methods and single photon emission computed tomography (SPECT) are developing technologies that provide both functional and anatomic information. Their role in the diagnosis and monitoring of cancer is the subject of current clinical research. Magnetic resonance imaging (MRI) delineates organs and tissue heterogeneities using differences in the relaxation parameters of water and fat protons; both protons and other nuclei can be imaged or studied by magnetic resonance spectroscopy (MRS) to provide information on the state of naturally occurring or infused molecules. SPECT quantifies the distribution of radiolabeled agents in tissues and organs; labeled monoclonal antibodies provide highly specific imaging of tumors. Spatial resolution is the limiting technologic factor. Proton MRI provides the highest current resolution, better than 1 mm in vivo in deep tissues, whereas the resolution of MRS and SPECT is limited to several cubic centimeters. Recent advances in these technologies have significantly increased their specificity and ability to detect small, deep lesions. 