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fMRI User Guide
6
3.2 fMRI Acquisition
3.2.1 Trigger Out for Synchronization
The EPI sequences generate an optical TRIGGER_OUT at the beginning of each volume (each measurement).
The TRIGGER_OUT pulse is of 10µs duration. This optical trigger signal is available at the U3 connection (only
applicable for Tim™ Systems) on the LWL-Int board in the cabinet. Please ask your service engineer for assistance
in locating this output. Typically, one needs to convert the optical signal to a TTL pulse in order to use it for
synchronizing scanner activity with other devices. An optical to TTL "fMRI Trigger Converter" may be purchased
from Siemens for this purpose. The output from this box is via a BNC or serial port. There are two different modes
of TTL output on this box – impulse mode or toggle mode. Refer to the converter box manual for details. Please
contact your local Inside sales representative to purchase this equipment.
3.2.2 Gating an EPI Sequence
The EPI sequence can be gated using either standard ECG or pulse triggering available through the scanner, or by
means of an external trigger from another device. The external trigger can be fed into the scanner via the cinch
jack trigger input on the left side of the front of the magnet cover. Of the two jacks, connect the external trigger
signal to the jack that has the following symbol:
For specification on the desired shape and amplitude of the trigger signal, please refer to the System Manual,
under "External Triggering."
Note: The measurement is triggered by the rising edge of the external signal.
3.2.3 Interleaved Slice Reordering in EPI
If TOTAL # SLICES = EVEN, then the excitation order when "interleaved" is EVENS first, ODDS second, e.g.
2,4,6,8..... and then 1,3,5,7.....
If TOTAL # SLICES = ODD, then the excitation order when "interleaved" is ODDS first, EVENS second, e.g.
1,3,5,7..... and then 2,4,6,8......
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