Fasciculations are brief spontaneous contractions affecting a small number of muscle fibers. Schick et.al. reported how surprisingly these motions had been detected using low b value diffusion sequences^1,2. In our study we imaged this muscle behavior in five healthy volunteers as well as in an agarose phantom and the measurements were quantified.

A 3 T General Electric HDxt clinical scanner with a T/R knee coil was used to image the mid-tibial section of 5 healthy volunteers' legs. Multiple repetitions of a single shot SE (EPI read out) diffusion weighted sequence were acquired with the following parameters: single or multislice 5 mm thick slices, b=100 s/mm2, 128 x 128 acquisition matrix, TR=1000, TE=50ms. The volunteers were instructed to lie quietly and the sequence was repeated for 200 s. The 200 images were reviewed as cine loops, allowing direct visualization of regions undergoing tissue displacements as highly attenuated signal intensity patches within individual frames. Segmentation of images using a threshold of 80% intensity decrease provided crisp overlays that were superimposed on anatomic images, clearly illustrating the effect. Comparison of affected regions to total muscle area on congruent anatomical images allowed quantification in the form of a fasciculation index parameter. Directional properties of tissue motion were explored by varying the diffusion gradient direction. Additional TRs, matrix size, and multislice measurements were employed to determine if muscle changes were generated through peripheral nerve stimulation caused by the switched gradients themselves.No change in diffusion image appearance was observed when the diffusion sensitization direction was changed. The loss of intensity on these diffusion weighted images was attributed to intra-voxel incoherent-like (opposing) motion with displacements of tissue along all three dimensions. Fig. 1 presents an image frame from the 200 image series which displays two regions of tissue having an 80% reduction of signal intensity (overlay on an anatomic image). Color coded overlays were used to represent how often each pixel was activated resulting in a fasciculation frequency map (Fig. 2). Higher duty cycle of the gradients did not have a significant effect on the observed fasciculations (Fig. 3), ruling out scanner gradient switching as the cause of this effect. Scans of an agarose phantom which was mechanically tapped with a plastic rod produced results that were consistent with our understanding of the physics in these measurements.Bulk tissue displacements cause phase accumulation in motion sensitized sequences and is the basis for phase contrast angiography. Incoherent motions within voxels result in signal loss in the same sequence, which is relevant for diffusion imaging. Muscle contractions involve both processes. The contraction of a muscle involves significant intra-voxel tissue motion with displacements of spins towards each other along the muscle fiber direction, and an accompanying displacement in the orthogonal directions where spins move apart (Fig. 4). We have labelled this “incoherent-like” behavior. It helps explain why the direction of motion sensitization does not have a significant effect on these measurements. Our diffusion sequence generates magnitude images, discarding phase information arising from bulk motion. The loss of signal intensity remains, however, as the phases cancel in spins moving away/toward each other. This macroscopic displacement effect is much larger than occurs in water diffusion. Attenuations are very significant when a snapshot image is obtained just as a muscle fiber undergoes a fasciculation. We demonstrate that muscle fasciculations are easily observed with MR imaging in the lower leg and propose methods to quantify the effects. We present an explanation of the mechanism that generates signal attenuation in these motion sensitized images.