Abstracts

Functional magnetic resonance imaging (fMRI) studies of the temporal lobes are hampered by signal loss caused by magnetic field inhomogeneities. Z-shimming is a technique whereby multiple images are acquired with different acquisition parameters optimised for the regions suffering from signal loss. A final image is created by combining these individual images. This study used z-shimming in an attempt to increase the signal intensity in patients with temporal lobe epilepsy (TLE) with the goal of detecting the generators of interictal epileptiform activity more reliably. Four patients with a clinical diagnosis of TLE underwent continuous EEG-fMRI monitoring using a z-shimming protocol (Constable and Spencer 1999). The fMRI images were acquired in a 1.5T MR scanner (Sonata, Siemens, Germany) using an echo-planar imaging sequence (voxels 5x5x5mm, 12 slices, TE=50ms, TR=3.3s, flip angle 75[ordm]). An angulation along the long axis of the temporal lobes was used to cover as much of the posterior temporal lobes as possible. Three z-shim levels were used (100%, 80% and 120% of the nominal Gz) and composite images formed by taking the sum of squares at each voxel. An anatomical scan was also acquired. EEG data were recorded using a BrainAmp amplifier (BrainProducts, Germany) from 21 MR compatible Ag/AgCl electrodes. The signal intensity in the composite images was compared with that in the nominal images (i.e. those acquired with the nominal Gz), both throughout the brain and specifically in the temporal lobes. For the latter comparison, the temporal lobes were marked bilaterally on the anatomical scan and the corresponding voxels in the functional scan were identified. The overall signal intensity increase in the brain for the composite functional images, averaged across all patients, was 22.7[plusmn]0.4% [range 22.2[ndash]23.1%]. Within the temporal lobes, the increase in signal was 42.3[plusmn]3.8% [range 37.4[ndash]45.7%]. The average increase in the number of voxels above a brain intensity threshold was 295[plusmn]42 [range 253[ndash]343]. An average of 109[plusmn]30 voxels [range 75[ndash]143] were added to the temporal lobes, corresponding to 10.2[plusmn]1.6% of the total number of marked temporal lobe voxels. Using the z-shimming technique leads to a considerable increase in the signal intensity in the temporal lobes. This increases the probability of detecting subtle changes in the fMRI signal as a result of interictal epileptiform activity generated in the temporal lobes.
Constable RT and Spencer DD 1999 Magn Reson Med 42 110 (Supported by CIHR grant MOP 38079. CGB was funded by a CIHR doctoral research award. EK was funded by a Preston Robb Fellowship from the MNI.)