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Event-related functional MR imaging of visual cortex stimulation at high temporal resolution using a standard 1.5 T imager

✍ Scribed by Lothar R. Schad; Edzard Wiener; Klaus T. Baudendistel; Edgar Müller; Walter J. Lorenz


Book ID
103910084
Publisher
Elsevier Science
Year
1995
Tongue
English
Weight
755 KB
Volume
13
Category
Article
ISSN
0730-725X

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✦ Synopsis


The authors report the technical feasibility of measuring event-related changes in blood oxygenation for studying brain function in humans at high temporal resolution. Measurements were performed on a conventional wholebody 1.5 T clinical scanner with a nonactive-shielded standard gradient system of 1 ms rise time for a maximum gradient strength of 10 mT/m. The radiofrequency (RF) transmitting and receiving MR unit consists of a commercially available circular polarized bead coil. Magnet shimming with all first-order coils was performed to the volunteer's bead resulting in a magnetic field homogeneity of about 0.1-0.2 ppm. The measuring sequence used was a modified 3D, first-order flow repbased, FLASH sequence with reduced bandwidth = 40 Hz/pixel, TR = 80 ms, TE = 56 ms, flip angle = 40-50°, matrix = 64 x 128, field-of-view = 200-250 mm*, 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-loop for sequential measurement of 128 or 32 consecutive images at the same slice position. This means that event-related functional MRI could be performed with an interscan delay of 80 ms for a series of 128 sequential images or with an interscan delay of 320 ms for a simultaneous measurement of four slices with a series of 32 sequential images for each slice. We used a TTL signal given by the gradient board at the beginning 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 after 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 experiment, respectively. Visual cortex response magnitude measured was about 5-79'0 with an approximate Gaussian shape and a rise time from stimulus onset to maximum of about 3-4 s, and a fall time to the baseline of about 5-6 s after end of stimulus. The reported data demonstrate the feasibility of functional MRI studies at high temporal resolution (up to 80 ms) using conventional MR equipment and measuring sequence.


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