BenInglis
commented
8 years ago Tests on 10/26/2015
SETUP: Birdcage coil, Siemens cylindrical phantom laying on the coil cradle but phantom removed as soon as EPI acquisition commenced, TMS coil taped onto a fiberglass beam cantilevered with heavy blocks to isolate it from the birdcage coil and phantom. Biphasic TMS pulse of duration 250 usec, set at 70% amplitude, one TMS pulse per TR incremented 0.1 ms each time. Timing of TMS pulses and EPI sequence as listed in Issue #5. The EPI acquisition was initiated and then the phantom was pushed out of the birdcage coil so that only noise images would result for the remainder of the time series. In the total image time series it is possible to see the displacement of the phantom as well as a drop in total noise after the magnet room door is closed. In some runs one or more images of the mosaic show structure at the moment the door is closed.
DATA FILE: tmsMRI_sequence_70pcTMS_1for1TR, scan 6.
RESULTS: A complete set of 490 TR periods was acquired even though TMS pulses end at about volume number 370. This was to enable a long period of noise without TMS pulses. We observed birdies and a general increase in noise in the slice acquired following a slice with TMS pulses applied, whereas the noise remained constant in the slice prior to the TMS'd slice. No birdies were observed for very late acquired slices, either.
CONCLUSION: There is residual movement likely inside of the TMS coil that causes low level RF interference in the few slices following a TMS pulse. Most likely the torque experienced in the TMS coil due to a pulse has a relaxation period of some tens to hundreds of milliseconds, during which the TMS coil's internal components create friction and/or small regions of electric field that discharge.
SUMMARY: Tested TMS coil for RF noise and birdies to determine whether noise floor increases in the slice targeted by TMS, and perhaps in slices immediately following a TMS pulse.
This issue developed out of observations made during testing for issue #10.