Relaxographic imaging

Abstract

A fundamental extension of NMR imaging is described. The distribution of relaxation times, the relaxogram, is considered as the third (or fourth) dimension of a set of 2D (or 3D) image data. There is a relaxographic dimension for each type of relaxation: longitudinal, transverse, rotating frame, etc. It is the formal inverse Laplace transform of the relaxation decay data set. Thus, combined relaxography and imaging (CRI) approaches are defined. CRI data can be displayed in two fundamental ways: localized relaxograms (relaxograms from any part of an image) or relaxographic images (images produced from discrete portions of a relaxogram). Relaxographic images are elemental components of the true spin-density image. The CRI concept is demonstrated with longitudinal relaxation data from samples of yeast cells suspended in media containing the contrast agent (CR) GdDTPA2-. This allows the discrimination of subvoxel intra- and extracellular 1H2O signals in the relaxograms from very small image voxels (about 400 nl). It is possible to isolate the intracellular 1H2O resonance from as few as a million cells. Relaxographic images are shown of the extracellular space (i.e., the distribution space of the CR) and the cytoplasmic space of a cell suspension with a cytocrit gradient. These have important potential applications in the in vivo situation. Also, the extent of equilibrium transcytolemmal water exchange can be detected and quantified.