Adiabatic RARE imaging

Abstract

The practical implementation of ultra-fast spin-echo, or RARE imaging with adiabatic RF pulses and surface coil transmission at 7 T is described. Despite the large RF inhomogeneities, the adiabatic character of the 180 degrees BIR-4 refocusing pulses ensures optimal sensitivity and minimal image artifacts. An internal 'phase-cycle' is used to remove spurious unwanted coherences. The short T(2) relaxation times in rat brain at 7 T demand a centric, rather than a linear coverage of k-space in order to avoid excessive signal loss. T(2) relaxation during k-space coverage also leads to image blurring, which can be counteracted by interleaved k-space sampling. The coverage of k-space in four acquisitions provides high-quality anatomical images comparable to conventional spin-echo images. A two-scan RARE implementation provides sufficient spatial and temporal resolution for most applications. Quantitative mapping of T(1) relaxation and cerebral blood flow changes during forepaw stimulation in the rat are demonstrated.