Heteronuclear spectral editing with adiabatic pulses

Abstract

Spectral editing pulse sequences have greatly expanded the capabilities of NMR spectroscopy in many different areas of research. In the field of in vivo NMR, editing is frequently employed to extract spectral information which may otherwise be concealed by broad and overlapping resonances. Heteronuclear ( i3C‘H) spectral editing (1-6) provides the additional capability to monitor the metabolism of i3C-labeled compounds while exploiting the high sensitivity of ‘H observation. Recent studies have shown that heteronuclear editing of 13C signals has many advantages, including a more than lo-fold increase in signal-to-noise, over direct 13C observation with nuclear Overhauser enhancement ( 7). Existing spectral editing pulse sequences are based upon conventional radiofrequency pulses which induce flip angles that are directly proportional to the RF field strength, B, . This hip-angle dependence on Br can result in substantial signal losses when an inhomogeneous RF coil, such as a surface coil, is used for pulse transmission. Even with homogeneous RF coils, minor misadjustment of flip angles can cause significant signal losses in multiphase editing sequences since the net error accumulates in a multiplicative manner with each pulse. In this Communication, we describe a pulse which achieves uniform heteronuclear spectral editing above a threshold Br where the pulse operates adiabatically. This Brinsensitive spectral editing pulse (BISEP) selectively excites ‘H spins engaged in heteronuclear J modulation. Uncoupled spins are initially rotated into the transverse plane, but then are returned to the longitudinal axis by the end of the pulse. Theoretically, spectral editing is complete in a single acquisition. BISEP is based on our adiabatic pulse, BIR-4, which can induce any desired flip angle with compensation for Br inhomogeneity and resonance offset (8, 9). The flip angle generated by BIR-4 is determined by two discontinuous phase shifts, Ab, and A&, which take place at specific times during the pulse when the magnetization has been adiabatically rotated onto the transverse plane. For an arbitrary flip angle 8 to be induced at the completion of the pulse, the discontinuous phase shifts are A4, = ?r + 0 / 2 and A& = ( 7r + 6 / 2 ) . These expressions can be equated to give A4 1 A& = 2a + 0 = 8. If A& = A$,, this latter equation predicts that BIR4 will execute a B,-insensitive identity transformation (fl = 0) provided the pulse is operating adi-