B₁-insensitive heteronuclear adiabatic polarization transfer for signal enhancement

A new B₁-insensitive pulse sequence, BINEPT (for B₁-insensitive nuclear enhancement through polarization transfer) is introduced that accomplishes signal enhancement for low-sensitivity nuclei through heteronuclear polarization transfer. Previously described methods for polarization transfer through J-ordered states such as INEPT [D. P. Burum and R. R. Ernst, J. Magn. Reson. 39, 163 (1980)] require homogeneous B₁ fields and carefully calibrated pulses to impart π 2 and π rotations in order to achieve the maximum possible signal enhancement. In contrast, BINEPT, which is based on recently introduced adiabatic plane-rotation pulses, can induce the desired enhancement over a very large range of B₁ magnitude and can be used even with surface coils that generate highly inhomogeneous B₁ fields. BINEPT is not simply the INEPT sequence with the conventional pulses replaced with their adiabatic plane-rotation pulse analogs. Rather, it is based on the variable-angle, adiabatic BIR-4 pulse [M. Garwood and Y. Ke, J. Magn. Reson. 94, 511 (1991)] and the principle that the evolution of spin states during intrapulse delays inserted in BIR-4 can determine the final rotation induced on magnetization vectors. The BINEPT technique is described theoretically in the rotating frame. Its performance is compared experimentally with INEPT using coils that generate homogeneous and inhomogeneous B₁ fields.