B ₁ Insensitive Adiabatic RF Pulses

Abstract

Inhomogeneities in the B ₁ field pattern of RF transmitters are often the cause of major problems in applications of the NMR phenomenon. This simply follows from the fact that NMR experiments in general rely on rotation of magnetization vectors about the B ₁ field through an angle γ ∫ B ₁(t)dt, where γ is the gyromagnetic ratio. Consequently, even very simple tasks such as spin-excitation becomes non-uniform over the sample and this deleterious effect gets compounded in more complex and demanding pulse sequences employed in contemporary NMR applications ranging from high resolution studies of molecular structure to imaging and localized spectroscopy in vivo. This problem is particularly pronounced in the human and intact animal investigations in vivo where geometrical constraints frequently require the use of transmitters and detectors, such as surface coils, which generate extremely non-uniform B ₁ field patterns.