INEPT-enhanced 13C spectroscopy using double-tuned surface coils

Abstract

The problems of in vivo 13C spectroscopy are threefold, viz., the low sensitivity of the 13C nucleus, the low natural abundance of 13C (1. 1’S), and the low in vivo concentration of metabolites. Despite these difficulties 13C spectra of natural abundance glycogen have been obtained (I), but the experimental duration of such experiments is relatively long. This article describes an attempt to increase the sensitivity of 13C spectroscopy for in vivo applications by means of the INEPT polarization transfer technique (2). INEPT, nuclear Overhauser enhancement (NOE), and DEPT (3, 4) are established techniques for increasing the sensitivity of low-y nuclei, by a transfer of polarization from a J-coupled higher-y nucleus, normally ‘H. The in vivo application of these techniques for i3C spectroscopy is handicapped by the low intrinsic sensitivity of the experiment, which requires that signal reception be via a surface coil. However, if this is also used for transmission, then there is an extremely inhomogeneous Bi field. If signal transmission is by means of a larger coil, with a homogeneous field, then it may be necessary to decouple the coils. There is also a reduction in the effective bore size due to the presence of two coils. A number of schemes for 90” excitation, 180” refocusing, and inversion pulses have been suggested that are largely insensitive to the effects of BI inhomogeneity (5, 6). These in principle provide the requisite pulses for INEPT by surface coil excitation only. The full implementation of DEPT requires RF pulses having a range of flip angles, which is at present unrealistic for surface coil excitation, although DEPT enhancement of the signal from CH groups would be possible in these circumstances. INEPT has the advantage over NOE that the effective T1 is governed by the proton relaxation rate, not the longer ‘H-13C cross-relaxation rates, and the advantage over DEPT that CH, CH1, and CH3 groups are equally enhanced. However, the basic INEPT sequence gives spectra that are difficult to interpret, while refocused INEPT requires more RF pulses than DEPT. The basic INEPT pulse sequence is