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.
Department of Radiology, German Cancer Research Center, Heidelberg, Germany.
L R
LR
Schad
E
E
Wiener
K T
KT
Baudendistel
E
E
Müller
W J
WJ
Lorenz
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