From this study, the ALL is an independent structure in the anterolateral compartment of the knee and may serve a proprioceptive role in knee mechanics.

Peripheral nerve stimulation (PNS) caused by time-varying magnetic fields has been studied both theoretically and experimentally. A human volunteer study performed on three different body-size gradient coils and one head-size gradient coil is presented in this work. The experimental results were used to generate average PNS threshold parameters for the tested gradient systems. It was found that the average stimulation threshold increases while gradient-region-of-uniformity size decreases. In addition, linear relationships between PNS parameters and diameter of homogeneous gradient spherical volume (DSV) were discovered: SR min and ⌬G min both vary inverse linearly with DSV. More importantly, the chronaxie value was found to vary inversely linearly with the DSV. This finding indicates that, contrary to the general understanding, the parameter "chronaxie" in the commonly accepted simple stimulation models cannot be considered to be a single-value, nerve-specific constant. A modified linear model for gradient-induced PNS based on these results was developed, which may permit, for the first time, Key words: gradient coils; peripheral nerve stimulation (PNS); diameter of homogeneous spherical volume (DSV); chronaxieHigh-strength, high-slew-rate gradient coils are desired in modern MRI for better image quality and faster imaging speed. However, rapidly time-varying magnetic fields produced by gradient coils induce electric fields in tissue, which can cause peripheral nerve stimulation (PNS). This significantly limits our ability to take full advantage of these high-performance gradient systems. Despite considerable research in this area in recent years, the relationship between PNS thresholds and gradient coil linear region dimensions or gradient performance parameters is still not well understood. In this study, we investigated this relationship.The fundamental law of electrostimulation is written as (1)where E stim is the threshold electric field to cause stimulation, is the interval over which this electric field is applied, E r is the electric field rheobase, and c is the chronaxie. The rheobase E r is defined as the minimum electric field to produce stimulation, and chronaxie c is defined as the stimulus duration for which the threshold E stim is twice the value of the rheobase E r . Currently accepted gradient-induced PNS models utilize these two parameters (rheobase E r and chronaxie c ) to define PNS threshold curves. It has been assumed that these two parameters are functions only of underlying nerve properties. Recently, Chronik and Rutt (2) reported different rheobase and chronaxie values for a head-size coil and a body coil, and attributed these differences to different nerve sensitivities in the head and body. On the other hand, various studies have reported different values of rheobase and chronaxie even within one class of gradient coils, such as whole-body coils (1-8). We hypothesized, therefore, that these "nerve-specific" PNS model parameters are actually dependent in a predictable way on gradient c...

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Gradient coil magnetostimulation thresholds were measured in a group of 20 volunteers in both a whole-body gradient coil and a head/neck gradient coil. Both coils were operated using both x and y axes simultaneously (xy oblique mode). The waveform applied was a 64-lobe trapezoidal train with 1-ms flat-tops and varying rise times. Thresholds were based on the subjects' perception of stimulation, and painful sensations were not elic- The mean threshold curves were combined with the gradient system performance curves to produce operational limit curves. The operational limit curves for the head/neck coil system were verified to be higher than those of the whole-body coil; however, the head/neck system was also found to be physiologically limited over a greater range of its operation than was the body coil. Subject thresholds between the two coils were not well correlated. Magn Reson Med 46: 386 -394, 2001.