RELAXOGRAPHIC IMAGING

A fundamental extension of NMR imaging is described. The distribution of relaxation times, the relaxogram, is considered as the third (or fo urth) dimension of a set of 2D (or 3D) image data. There is a relaxogr aphic dimension for each type of relaxation: longitudinal, transverse, rotating frame, etc. It is the formal inverse Laplace transform of th e relaxation decay data set. Thus, combined relaxography and imaging ( CRI) approaches are de fined. CRI data can he displayed in two fundame ntal ways: localized relaxograms (relaxograms from any part of an imag e) or relaxographic images (images produced from discrete portions of a relaxogram). Relaxographic images are elemental components of the tr ue spin-density image. The CRI concept is demonstrated with longitudin al relaxation data from samples of yeast cells suspended in media cont aining the contrast agent (CR) GdDTPA(2-). This allows the discriminat ion of subvoxel intra- and extracellular (H2O)-H-1 signals in the rela xograms from very small image voxels (about 400 nl). It is possible to isolate the intracellular (H2O)-H-1 resonance from as few as a millio n cells. Relaxographic images are shown of the extracellular space (i. e., the distribution space of the CR) and the cytoplasmic space of a c ell suspension with a cytocrit gradient. These have important potentia l applications in the in vivo situation. Also, the extent of equilibri um transcytolemmal water exchange can be detected and quantified. (C) 1994 Academic Press, Inc.