Volume localized shift selective ¹³C spectroscopy using pulsed rotating frame transfer sequences with windows (PRAWN)

Abstract

Some novel techniques for volume localized, chemical shift selective 13 C spectroscopy are described in this work. These techniques are based on rotating frame J cross polarization and are reported for both direct and indirect modes of 13 C detection. The performance of two selective mixing sequences, viz., pulsed rotating frame transfer sequences with windows (PRAWN) and PRAWN-p has been studied systematically with different liquid and gel phantoms. Two different front-end modules are used for volume localization, viz., point resolved spectroscopy (PRESS) and localized distortionless enhancement by polarization transfer (LODEPT). It is shown experimentally that both the selective J cross polarization sequences can operate efficiently with very low radiofrequency duty cycle; further, they have considerable tolerance to Hartmann-Hahn mismatch. A simple theoretical analysis is also presented to understand J cross-polarization dynamics at low RF field amplitudes. Key words: rotating frame coherence transfer; shift selective volume localized 13 C magnetic resonance spectroscopy; PRAWN; PRAWN-p; low RF duty cycle; Hartmann-Hahn mismatch tolerance; direct detection; indirect detection; PRESS; LODEPT 13 C nuclear magnetic resonance spectroscopy is a powerful tool to study metabolic events in living organisms (1-3). The advantage of this technique is that one can get very high spectral resolution (less crowding of peaks compared to proton nuclear magnetic resonance); however, this is countered by low natural abundance and poor sensitivity, which makes 13 C nuclear magnetic resonance (or magnetic resonance spectroscopy) methodology much more challenging than 1 H and 31 P magnetic resonance spectroscopy. Heteronuclear polarization transfer (PT) (4,5) from abundant spins such as 1 H provides a way to enhance the sensitivity of 13 C detection. The sensitivity of 13 C spectroscopy can be further improved by indirect detection, i.e., selective detection of 1 H spins that are coupled to 13 C. Some PT studies have been reported in the literature, based primarily on laboratory frame PT protocols (6-12). Among them, the most frequently used sequences are based on distortionless enhancement by polarization transfer (DEPT) (13) and insensitive nuclei enhanced by polarization transfer (INEPT) (14), e.g., localized DEPT (LODEPT) and localized INEPT (LINEPT) (15), and their semi-adiabatic versions (11). Heteronuclear J cross polarization (JCP) using spin-locking radiofrequency (RF) fields (4) is an important technique for many solid state and liquid state nuclear magnetic resonance experiments to enhance the sensitivity of detection of rare nuclei; however, this technique is not widely used for in vivo experiments. The application of rotating frame PT for 13 C localized spectroscopy and for 13 C editing has been reported in a series of papers by , using the MOIST mixing sequence (19,20) or adiabatic RF fields (21). One technique involves ''forward'' and ''reverse'' PT under spin lock conditions, which typically result in reduced motio...