Thin-section definition in magnetic resonance imaging. Technical concepts and their implementation

Reduced slab boundary artifact in multi-slab 3D fast spin-echo imaging

The three-dimensional multi-slab fast spin-echo (3DFSE) sequence is a time-efficient technique for volume scanning which provides images with a good signal-to-noise ratio, adjustable contrast weighting, and high spatial resolution. Unfortunately, it suffers from slice-to-slice amplitude variation associated with imperfect slab definition. This slab boundary artifact becomes especially apparent when multiplanner reformatting is used to create alternate anatomical views. The shifted interleaved multi-volume acquisition (SIMVA) described here suppresses slab boundary artifact in image space. It displaces each slab (RF excitation) position incrementally along the slice (z) axis, in coordination with the primary phase encode step, so that the slab boundary artifact is converted into a correctable amplitude modulation in the primary phase encode direction (k(Y)-axis). After the Fourier transform in primary phase encoding, the slab boundary artifact is mapped into a different and less severe artifact on a different spatial axis. Preliminary measurements show that SIMVA reduces the slab boundary artifact by an order of magnitude in multiplanar reformatted views. Magn Reson Med 44:269-276, 2000.

Optimal control solutions to the magnetic resonance selective excitation problem

Most magnetic resonance imaging sequences employ field gradients and amplitude modulated RF pulses to excite only those spins lying in a specific plane. The fidelity of the resulting magnetization distribution is crucial to overall image resolution. Conventional RF-pulse design techniques rely on the small tip-angle approximation to Bloch's equation, which is inadequate for the design of 90 degrees and 180 degrees pulses. This paper demonstrates the existence of a selective pulse, and provides a sound mathematical and computational basis for pulse design. It is shown that the pulses are optimal in the class of piecewise continuous functions of duration T. An optimal pulse is defined as the pulse on the interval that achieves a magnetization profile "closest" to the desired distribution. Optimal control theory provides the mathematical basis for the new pulse design technique. Computer simulations have verified the efficacy of the 90 degrees and the 180 degrees inversion and "pancake-flip" optimal pulses.