The Siemens scanner will automatically add one or more “dummy scans” or “prep scans” to the start of each BOLD acquisition. The number of prep scans is determined by the TR. The purpose of these scans is to allow T1 values to reach a steady state. Note, however, that the presence of these scans does not necessarily assure that you will have achieved steady state if your flip angle is less than 90 degrees. Each prep scan has a duration of 1 TR. To determine the number of prep scans that will be inserted at the beginning of your run, use the following chart:
- TR > 3000: 1 prep scan (TR)
- 1500 < TR ≤ 3000: 2 prep scans (TRs)
- 1000 < TR ≤ 1500: 3 prep scans (TRs)
- TR ≤ 1000: 4 prep scans (TRs)
Thus, for example, if your run has a TR of 2500 and 100 reps your total acquisition time will be:
(100 * 2500) + (2 * 2500) = 102 * 2500 = 255000 ms = 255 seconds.
NOTE: No data are saved for the dummy scans; you will collect only as many volumes as you specify for the number of repetitions even though your acquisition time will be extended by 1-4 TRs depending on the number of prep scans (according to your TR). N.B. Acquisition times reported by the scanner are always rounded up to the nearest second.
NOTE: If you use the trigger box, no triggers are generated for the prep scans.
NOTE: If your acquisition time is 1 sec longer than you expect, be sure that the “Introduction” option is disabled for your sequence.
NOTE: If you are using parallel imaging — either iPAT, SMS/Multiband, or both — then there are additional reference scans that are also acquired prior to scanning, each lasting 1 TR and for which there is no trigger pulse. The specific number of reference scans depends on the parallel imaging factor chosen and is somewhat complex to determine in advance. If you use parallel imaging and need to determine how much time will elapse from the start of your scan until the first trigger is received and data are collected, it is usually best to determine this empirically by subtracting the expected acquisition time (number of acquisitions x TR) from the acquisition time (TA) reported by the scanner for your sequence, which will include all reference and dummy scans.