Imaging Low-Concentration Metabolites in the Presence of a Large Background Signal

Abstract

A pulse sequence (CD-CHESS) is described that addresses the challenge of imaging low-concentration metabolites whose chemical shifts are close to that of water. This sequence avoids the problems of the basic chemical- shift- selective (CHESS) imaging sequences, which produce an image contaminated by residual signal from the undesired component. Several signal suppression schemes are combined with the basic 2D CHESS imaging sequences and examined using a specially constructed chemical-shift phantom. The degree of suppression for the new enhanced CHESS sequences is compared with that of the basic CHESS sequences and the SUBMERGE-CHESS sequence. The most effective sequence incorporates cw irradiation and soft-pulse saturation with the basic CHESS sequence. The CDCHESS imaging sequence produces a suppression ratio of better than 1000 to 1 when the desired and undesired peaks are 1 ppm apart. The effectiveness of the CD-CHESS sequence is demonstrated by imaging the sugar protons in a phantom composed of 10% (w/v) sucrose in water. The CD-CHESS sequence is used for the selective imaging of sugars in peas, demonstrating that it can be extended to the broad lines normally encountered in biological specimens.