The goal of the proposed work is to develop a fast, quantitative, non- invasive method for estimating the fractional volume of bone marrow occupied by blood cells. In hematology, this parameter is known as the "percentage cellularity" and is used to gauge the total active bone marrow volume in a wide variety of clinical situations. Presently, the only way to measure the percentage cellularity is via a needle aspirate or biopsy. In addition to causing patient discomfort, biopsies and aspirates are limited to superficial areas of bone, such as the posterior iliac crest. There is presently no direct method for assessing the percentage cellularity at many important sites of blood cell production, such as the vertebrae. We have recently completed a pilot study using proton magnetic resonance spectroscopy which demonstrated the feasibility of non-invasively measuring the percentage cellularity in bone marrow. We now propose to develop an image based assay for the percentage cellularity which makes use of the relative concentrations of water and lipid in marrow as well as nuclear relaxation times and self-diffusion coefficients of water. The successful completion of this project would yield a fast method for assessing the percentage cellularity which eliminates the pain associated with the needle biopsy and provides information on blood cell production at previously inaccessible anatomical sites. With these new techniques it will be possible to 1) quantify expansion or contraction of the active marrow volume due to disease or after the administration of blood cell growth hormones, and possibly 2) identify areas of residual hematologic disease present after chemotherapy.