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-related changes in blood oxygenation for studying brain function in humans at high temporal resolution. Measurements were performed on a conventional whole-body 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 head coil. Magnet 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 rephased, 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-view = 200-250 mm2, 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 5-7% 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.
Schad
L R
LR
Department of Radiology, German Cancer Research Center, Heidelberg, Germany.
Wiener
E
E
Baudendistel
K T
KT
Müller
E
E
Lorenz
W J
WJ
D001921:Brain / Q000502:physiology; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D010100:Oxygen / Q000502:physiology; D014793:Visual Cortex / Q000502:physiology*
D010100:Oxygen