MRI-induced stimulation of peripheral nerves: dependency of stimulation threshold on patient positioning

Fast gradient systems enable the imaging of moving organs but also restrict imaging possibilities by stimulating the peripheral nerves. Several studies have determined the stimulation thresholds of the peripheral nerves. However, no study has explicitly examined whether stimulation thresholds are dependent on the position of the patient in the MR imager. In this study, combinations of one, two and three gradient axes were switched with sinusoidal wave forms (rise time 300 micros). Stimulation thresholds were determined for five patient positionings: gradient isocenter at the bridge of the nose, jugular fossa, 10 cm cranial of the umbilicus, umbilicus, and 10 cm caudal of the umbilicus. Experiments were performed in two volunteer groups, one for triple gradient switching (17 male, 14 female) and the other for single and double gradient switching (14 male, 11 female). The dependence of the stimulation threshold on the position of the volunteers in the MR imager was statistically significant for most of the gradient combinations, with the highest stimulation thresholds for positions umbilicus+10 and umbilicus at isocenter. Similar patterns of position dependence were obtained for gradient pairs whose axes were axially symmetric to the human body. Mean stimulation thresholds of male volunteers were lower than those of female volunteers. These findings indicate that stimulation thresholds should be defined according to the position of the patient in the MR scanner.