High-resolution imaging. The NMR microscope

Abstract

Nuclear magnetic resonance imaging has traditionally been applied to macroscopic objects with dimensions in excess of 1 cm. The submillimeter regime may be termed microscopic and to date has been largely unexplored. NMR microscopy is inherently difficult to perform due to the smaller signal which arises from each volume element as the resolution is enhanced. Indeed the problem is dramatically indicated by the dependence of the imaging time on the sixth power of the resolution (I). Some of the early imaging experiments utilized the limited sample space of NMR spectrometers and were performed on smalI scale objects. Transverse resolutions of between 0.2 and 0.3 mm have been reported (2,3). Since that time major developments in imaging techniques have centered on “scaling up” and the NMR imaging literature reports few, if any, examples in which microscopy is the principal objective. Hall et al. (4) have recently coined the term “chemical microscopy” to refer to the localization of chemical-shift information in small-scale samples. Using phantoms consisting of 1.6 mm i.d. capillaries they indicate a transverse resolution of 0.1 mm on long samples without slice selection. In this communication we report microscopic imaging studies carried out at 60 MHz on both phantom and plant stem samples in which the transverse resolution is 25 pm. A slice thickness of 1.5 mm takes advantage of the existing longitudinal symmetry. Plant stem microscopy is of course ideally suited to high resolution in two dimensions. Signal-to-noise considerations are central to understanding limiting resolution. The time-domain signal to noise available in the free induction decay is simply expressed (5) in SI units as