Gradient system providing continuously variable field characteristics

Abstract

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 ...

Ultrashort shielded gradient coil design with 3D geometry

“…[12] with respect to the coefficients a n ( p) , a n (s) allows one to determine a solution for the current densities.…”

“…However, in the design process, the RECE can be controlled in the same fashion as for a transverse coil. In designing an axial gradient coil we use the method of Lagrange multipliers by minimizing the following functional: [12] where the notations are similar to those used in Eq. [6].…”

“…In (10), the concept of 3D geometry was used to design a gradient coil for an open vertical MRI system. In (11,12), the concept of the 3D geometry was applied to a horizontal MRI system where the gradient primary and the shield coils have equal lengths. In our approach, the step (a) allows the uniform control of the RECE over the imaging volume to any desired but practical level.…”

Gradient coil design and intra-coil eddy currents in MRI systems

“…In order to reduce the performance loss, some designs use a three-dimensional (3D) coil that connects the primary coil and the secondary coils using an additional annular surface. The additional surface allows the wire paths to link the inner and outer cylindrical surfaces and provide extra contribution to the magnetic field, thus improving the coil performance [58][59][60].…”

“…Improved performance has been shown for ultra-short whole-body gradient coils, where coils are connected at both the patient and service ends. Benefiting from this connected structural design, the gradient sensitivity could be improved by 10-15% [58], and save up to 50% of space compared to a conventional coil design. This reduces the localized regions of Joule heating in the coil and allows the design of coils easier to manufacture.…”