Slice timing correction - multiband acquisition

From Bangor Imaging Unit
Jump to navigation Jump to search

1. SPM

“Multi-band acquisition, also known as POMP (GE), Simultaneous excitation or SMS for simultaneous multi-slice (Siemens), Multi-slice (Philips), Dual-slice (Hitachi), and QuadScan (Toshiba), is a technique that allows the acquisition of a few slices simultaneously[27]. In practice, if you look at the slice timing, you will see that multiple slices have the same slice timing, which you will never see with the slice timing of a non-simultaneous EPI acquisition. It is very difficult to check if you have multi-band acquisition enabled on your sequence, as the printout may not reveal this information. One reliable way is to check the slice timing in the scanner console, or also in the DICOM if the private vendor field is available (see above)[28] ·[29]: if two values are the same in the slice timing, then your acquisition is multi-band. For newer machines such as Siemens Magnetom VIDA, the DICOM can include the private fields (0018,9077) Parallel Acquisition and (0018,9078) Parallel Acquisition Technique to describe the use of multiband. Note that these two tags will only report if multiband is enabled ("(0018,9077) CS [YES]" and "(0018,9078) CS [SMS]") not the acceleration factor. Alternatively, the field (0021,1009) will report the in-plane (e.g. iPAT, SENSE, GRAPPA) and between slice (e.g. SMS) acceleration (for example "(0021,1009) LO [p2 s4]" would suggest multiband 4). If you use multi-band acquisition, you cannot use the slice order as an input to slice timing correction, since a slice order cannot represent multiple slices acquired at the same time (if it was a matrix it would be possible, but SPM only accepts a vector). However, you can use the slice timing instead of slice order when using a multi-band EPI acquisition[30] ·[31]. If you do know your slice order but not your slice timing, you can artificially create a slice timing manually, by generating artificial values from the slice order with equal temporal spacing, and then scale the numbers on the TR, so that the last temporal slices timings = TR - TR/(nslices/multiband_channels).”

2. Brain Voyager: File:BrainVoyagerSliceTimingCorr.png “The slice time table is also visible in the Slice Scanning Order dialog: the timing value printed below each slice (see below) is the value extracted from the header and not calculated as in the conventional case from the TR and inter slice timing values. The screenshot above shows that the Acquisition time option is automatically turned on in the Slice order specification mode field in case that the slice time table is available. Note that the Standard slice scanning order and Multiband fields are disabled since all necessary information is directly used from the slice time table. Value "2" in the Factor spin box of the Multiband field indicates that in this data a multiband sequence with acceleration factor 2 has been detected. The red square in the snapshot indicates the begin of the first and second "band", i.e. it is the first pair of slices that is recorded simultaneously at time point 0. Note that in case of an available slice time table, no settings have to be changed in this dialog.”