EVENT-RELATED FUNCTIONAL MR-IMAGING OF VISUAL-CORTEX STIMULATION AT HIGH TEMPORAL RESOLUTION USING A STANDARD 1.5-T IMAGER

The authors report the technical feasibility of measuring event-relate d changes in blood oxygenation for studying brain function in humans a t high temporal resolution. Measurements were performed on a conventio nal whole-body 1.5 T clinical scanner with a nonactive-shielded standa rd gradient system of 1 ms rise time for a maximum gradient strength o f 10 mT/m. The radiofrequency (RF) transmitting and receiving MR unit consists of a commercially available circular polarized head coil. Mag net shimming with all first-order coils was performed to the volunteer 's head resulting in a magnetic field homogeneity of about 0.1-0.2 ppm . The measuring sequence used was a modified 3D, first-order flow reph ased, FLASH sequence with reduced bandwidth = 40 Hz/pixel, TR = 80 ms, TE = 56 ms, flip angle = 40-50 degrees, matrix = 64 x 128, field-of-v iew = 200-250 mm(2), slice thickness = 4 mm, NEX = 1, 128 partitions, and a total single scan time of about 10 min. In this sequence the 3D gradient table was removed and the 3D partition-loop acts as a time-lo op for sequential measurement of 128 or 32 consecutive images at the s ame dice position. This means that event-related functional MRI could be performed with an interscan delay of 80 ms for a series of 128 sequ ential images or with an interscan delay of 320 ms for a simultaneous measurement of four slices with a series of 32 sequential images for e ach slice. We used a TTL signal given by the gradient board at the beg inning of every line-loop in the measuring sequence and a self-made '' TTL-Divider-Box'' for the event triggering. This box was used to count and scale down the TTL signals by a factor of 128 and to trigger afte r every 128th TTL signal a single white flash-light, which was seen by the volunteer in the dark room of the scanner with a period of 10.24 s. As a consequence, the resulting event-related scan data coincide at each line of the series of 128 sequential images, which were repeated in 128 x 80 ms or 32 x 320 ms for the single- or four-slice experimen t, respectively. Visual cortex response magnitude measured was about 5 -7% with an approximate Gaussian shape and a rise time from stimulus o nset to maximum of about 3-4 s, and a fall time to the baseline of abo ut 5-6 s after end of stimulus. The reported data demonstrate the feas ibility of functional MRI studies at high temporal resolution (up to 8 0 ms) using conventional MR equipment and measuring sequence.