Current timing parameters

[DobyRahnev]

Just noting the down the details of the current sequence that we're using (I'm reporting these from memory, will double check and update early next week):

• head coil: birdcage
• TR = 2000.025 ms (shows as 2000 on the Siemens scanner)
• number of slices: 36
• acquisition: descending
• acquisition direction: P->A
• TE = 24 ms
• flip angle = 50 degrees
• field of view = 224 mm
• matrix = 64x64
• slice thickness = ? (3x3x3 mm?)
• inter-slice gap = ?
• fat sat included

Here is the relevant information for TMS targeting:

• for convenience, I'm defining the beginning of the first slice of a volume to be at the TTL pulse
• timing of each slice = 55.5563 ms (there may be a tiny gap at the end of each volume but at least for the current sequence, this gap appears to be negligible)
• here is the (simplified) composition of a slice:
  • each slice starts with an RF excitation pulse; TMS during this period can result in large artifacts that continue for several slices and are very hard to clean; to be on the safe side, avoid the first 4 ms of each slice
  • k-space readout; TMS during this period will make the current slice unusable but will not affect other slices; this period lasts until ~42.5 ms after the beginning of the slice
  • crusher gradient - fat sat pulse - crusher gradient; TMS landing on the fat sat pulse will cause the same large artifacts as with the RF pulse; however, TMS during either of the crusher gradients won't cause any artifacts [WE ARE STILL CONFIRMING THIS EXPERIMENTALLY]; the approximate timing here is (see oscilloscope readings in the picture below, TTL pulse is seen in yellow preceded by the fat sat pulse and followed by the RF excitation pulse):
  • 1st crusher: duration = ~3.4 ms, approximately from -11.4 +/- 1.7 ms (42.5 - 45.8 ms from beginning of slice)
  • fat sat pulse: duration ~ 5 ms, approximately -7.2 +/- 2.5 ms (45.8 - 50.9 ms from beginning of slice)
  • 2nd crusher: duration = ~3.4 ms, approximately from -3 +/- 1.7 ms (50.9 - 54.2 ms from beginning of slice
• the TMS pulse last only about 215 microseconds (.215 ms), and the time from the trigger being sent to the MagPro to the leading edge of the TMS pulse is about 75 microseconds
• based on the above, an ideal spot to target is 43.4 ms after the beginning of a slice (this is hitting the first crusher)

[BenInglis]

From Daniel, for MB-EPI (CMRR version R012b):

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Checked the placement of the TTL pulse relative to RF events today in the MB-EPI sequence today.

Under Zipser I found mb_bold_mb6_20mm_AP_PSN which I assumed is using the MB sequence and approximate parameters we want.

Here's what I found with the oscope (to within 0.1 ms when estimating the length of RF events):

TTL pulse located 7.0 ms after fat sat RF and 5.5 ms before multislice excite RF. Length of fat sat RF: 5.0 ms Length of multislice excite RF: 6.0 ms

(Note: timing from TTL to RF pulses is center-to-center. Also, length of excite RF is a visual estimate. Specified duration on the sequence card is 10.5 ms, commensurate with the ~5.5 ms timing of the TTL to the center of the excite RF.)