Controlled E-field gradient coils for MRI

Peripheral neural stimulation is a major problem in current gradient coil designs. Induced current problems in patients relate directly to gradient strength and modulation frequency. Present designs of gradient coil tend to limit ultra-high-speed imaging methods such as echo-planar imaging (EPI) and echo-volumar imaging (EVI) because of the effect of induced currents in the patient which produce neural stimulation resulting in tingling sensations and involuntary muscle twitch. Neural stimulation could also trigger epileptic fits and/or cardiac fibrillation. For reduction of induced currents, an important aspect is the coil geometry. It is desirable to design the gradient coil in such a way as to prevent closed loop circulating currents within the body. Preliminary results using a four-sector gradient coil with a rectangular geometry, operating in a low mutual coupling mode and using passive E-field control, indicate significant reduction of the E-field within the subject volume of the coil, leading to a reduction of induced currents in a patient. Such a reduction allows safer operation using higher magnetic gradient strengths together with faster scans. Currently very fast scans are prohibited by virtue of the neural stimulation effects produced in present standard coil geometries.