Optical spin-dynamic measurements in a high-mobility n-doped GaAs/AlGaAs quantum well show oscillatory evolution at 1.8 K consistent with a quasi-collision-free D'yakonov-Perel'-Kachorovskii regime. Above 5 K evolution becomes exponential as expected for collision-dominated spin dynamics. Momentum scattering times extracted from Hall mobility and Monte Carlo simulation of spin polarization agree at 1.8 K but diverge at higher temperatures, indicating the importance of electron-electron scattering and an intrinsic upper limit for the spin-relaxation rate.

Peripheral nerve stimulation limits the use of whole-body gradient systems capable of slew rates > 80 T/m/s and gradient strengths > 25 mT/m. The stimulation threshold depends mainly on the amplitude of the induced electric field in the patient's body, and thus can be influenced by changing the total magnetic flux of the gradient coil. A gradient system was built which allows continuous variation of the field characteristics in order to permit the use of full gradient performance without stimulation (slew rate 190 -210 T/m/s, G max 32-40 mT/m). The system consists of a modular six-channel gradient coil designed with a modified target field method, two three-channel amplifiers, and a six-channel gradient controller. It is demonstrated that two coils on one gradient axis can be driven by two amplifiers in parallel, without significant changes in image quality. Scaling of the field properties and stimulation threshold according to the current polarity and ratio of both coil sets was verified in both phantom and volunteer studies. Since the release of the second generation of clinical MRI scanners in the early 1990s, application demands on gradient hardware have significantly increased. Conventional systems were capable of switching 15-20 mT/m gradient amplitude within 600 s, whereas today's cardiac and neuroimaging sequences make use of gradient rise times of 100 -200 s and amplitudes up to 40 mT/m to provide high temporal and spatial resolution. The highest performance demands are made by diffusion-weighted sequences, which require fast gradient switching, 40 -60 mT/m peak amplitudes, high duty cycles, and excellent shielding of eddy fields.System performance depends on many factors, including peak gradient amplitude, slew rate, linearity volume, and free bore diameter. As a consequence, gradient coils optimized for whole-body applications with a large linearity volume and inductance require high-voltage power supplies to provide slew rates suitable for cardiac/neuro imaging. This posed a problem in the late 1980s, as gradient coil and amplifier technology was limited to several hundred volts. Partitioning of the coil windings and parallel driving with two or more amplifiers was considered at that time to increase slew rate performance (1).Advances in gradient system hardware have been particularly driven by the requirements of the ultrafast imaging method EPI (2). Sensory perception of induced effects from time-varying magnetic field gradients were first reported at the end of the 1980s (3,4), although they had been predicted by Budinger (5) as early as 1979. Several theoretical models of magnetostimulation have been established in MRI: the Mansfield and Morris (6) model based on Hodgkin, the Reilly et al. (7) model based on Huxley, and the Irnich (8) model based on Weiss. All three models predict stimulation on the basis of the induced electric field (E) in biological tissue. Reilly et al. (7) and Mansfield and Morris (6) fit their results to exponential strengthduration curves, whereas Irnich used a hyperbolic ...

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Since the first observations of peripheral nerve stimulation in MRI, it has been clear that the underlying mechanism is the activation of the nervous system by induced electric fields. However, compared to experimental investigations little work has been done on calculating these electric fields with adequate accuracy. In this article a numerical analysis of the electric fields induced by a complete whole body gradient system is presented. The calculations were carried out on three human body models of different complexities. The numerical results correlate better to the experimental observations with a body model that resembles the human body. Applying a model with inhomogeneous conductivity, numerical stability was not reached. The results were compared to the limits given in the upcoming IEC 60601-2-33 standard. The comparison shows that the derived peak electric fields depend substantially on the body model used, which dictates that limits have to refer to a body model that is exactly defined.Magn Reson Med 48: 731-734, 2002.

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