Abstract
This study determines the accuracy and feasibility of using localized spin‐lattice (T1) relaxation time measurements from magnetic resonance (MR) images to follow changes in extracellular/intracellular fluid ratios in defined subvolumes of living tissue. A red blood cell suspension was used as a test system and a simple two‐compartment model incorporating fast exchange was found to suffice for the conversion of T1 values to volume ratios. The technique requires the addition of gadolinium‐DTPA to the model system to selectively enhance relaxation in the extracellular fluid space. No detectable amount of gadolinium‐DTPA was found to enter the intracellular fluid space, and all magnetization decay plots obtained from both intracellular constituents and complete RBC suspensions consisted of a single exponential. Both of these results are compatible with assumptions underlying our physical model. The NMR‐determined fluid ratio values were compared to those measured via the microhematocrit technique. Partial saturation image‐mode determinations are strongly correlated to microhematocrit data (R^2^ = 0.945) and indicate that localized cell volume changes may be followed with a sensitivity of ±2.2%. These values compare favorably with those produced when nonimaging inversion‐recovery techniques are used to determine the MR hematocrit (R^2^ = 0.962, sensitivity = ±1.1%). This technique, with modification, should be applicable to the comparison of ratios of extracellular/intracellular fluid volumes in structurally complex tissues where small subvolumes of homogeneous cell structure could be examined. © 1990 Academic Press, Inc.