Object ID: 00000018WIA30E45970GYZ
Topic ID: id_40022634 Version: 1.2
Date: May 26, 2022 9:39:51 AM

PRESS CSI

PRESSCSI is a version of the PRESS sequence that acquires a double spin echo from a localized volume. You can prescribe the volume manually or graphically. The standard sequence consists of three slice selective RF pulses (a 90° and two refocusing pulses), and two sets of crusher gradients. The crusher areas are compensated to include the slice refocus, any phase encoding, and the readout-dephase gradients.

The acquisition of the spin echo rather than the stimulated echo gives PRESS a theoretical two-fold SNR advantage over STEAM for the same acquisition parameters. The SNR advantage can be used to decrease the examination time, or the volume of the prescribed voxel. PRESS CSI is the sequence of choice for all hydrogen single voxel, spectroscopy data acquisitions with TE values equal to or greater than 35 ms.

Figure 2. CSI storage image of 256, 32x32 CSI images. Bright regions correspond to the metabolite peaks in the spectra.

Consider this information when modifying PRESS CSI scan parameters. For specific scan parameter values, select a protocol from your GE or Site library.

Scan selections: MRS Mode, Spectroscopy family, Press CSI pulse.
For a 2D PRESS CSI acquisition, see the PROBE 2D CSI Spectroscopy procedure.
For a 3D PRESS CSI acquisition, see the PROBE 3D CSI Spectroscopy procedure.
For a Single Voxel PRESS acquisition, see PROBE-P edit procedure.

Phase cycling

The PRESS CSI sequence supports phase cycles of 1, 2, or 8. In the two-step cycle, the first RF pulse (90°) is phase-alternated by 180°. In the eight-step cycle, all three RF pulses are phase alternated. The NEX selection in the Acquisition Timing area determines the phase cycle used. By default, the largest phase cycle that evenly divides NEX is used.

Sequence timing

PRESS CSI sequences use RFA RF refocusing pulses to increase the effective bandwidth (to 1384 Hz; for comparison, the bandwidth of the 90° pulse is 2367 Hz) of the refocusing pulses, to decrease the power requirements of the refocusing pulses, and to decrease the minimum TE time of the sequence. The table below shows the minimum echo times calculated for the acquisition of a spectrum at a spectral width of 2500 Hz, and for the acquisition of an image of the voxel.

Table 1. Minimum echo times
Pulse type	90° Pulse width	180° Pulse width	Base	Hi or Echo Speed
Spectra	3600 µs	5200 µs	37 ms	25 ms
Voxel images	3600 µs	5200 µs	40 ms	31 ms

Crusher pulse widths are calculated for each acquisition, subject to the minimum and maximum gradient pulse width limitations. The equation used is (all values are in µs): PW_GC12 = (TE/2 - FILTER_DELAY - 7*PW_RAMP - PW_RF1/2 - PW_RF2/2 - PW_RF3/2 - 200)/2,

PW_GC12 is the pulse width of the crusher gradient.
TE is the echo time.
FILTER_DELAY varies depending on the spectral width and filter prefill (the default value for the 2500 Hz spectral width is 1016 µs).
PW_RAMP is the width of a gradient ramp.
PW_RF1 is the width of the 90º RF pulse.
PW_RF2 and PW_RF3 are the widths of the RFA RF pulses.

Experience has shown that optimal voxel profiles and crusher gradient areas are delivered at a TE of 35 ms - the recommended short echo time for the PRESS CSI sequence.

Gradient ordering for artifact reduction

The PRESS sequence is subject to signal artifacts arising from local susceptibility differences in one or more of the three planes that serve to define the prescribed voxel from which the double spin echo signal is acquired (see Th. Ernst and L. Chang, Magnetic Resonance in Medicine, 36, 462-468 (1996)). While these artifacts may be reduced by using eight-step phase cycling, it has been found that a change in the order in which the three slices are excited can greatly reduce these susceptibility-induced artifacts. The slice excitation and gradient order is:

Y Logical gradient axis – RF1 (90° pulse)
X Logical gradient axis – RF2 (refocusing pulse)
Z Logical gradient axis – RF3 (refocusing pulse)

The X, Y, and Z labels correspond to the logical gradient axes, or to the frequency, phase, and slice axes, respectively. The Y, X, Z gradient ordering is best for voxels prescribed in an axial scan plane. The Freq DIR selection in the Acquisition Timing area can be used to swap the order of the X and Y gradients. In addition, to reduce artifacts and to accommodate system specific optimization, a primer-crusher gradient pair is applied on all three gradient axes for the second refocusing RFA pulse. This means that the gradients are applied before and after the third RF pulse for all acquisitions.

Probe-P quantitative analysis

The PROBE-P sequence is a version of PRESS. The special PROBE-P reconstruction process includes complete spectrum processing, display, and storage; and a quantitative analysis of five chemical species that are usually present in human brain spectra. Refer to Quantitative Analysis of the Data in PROBE/SVQ for details of the algorithms used in the reconstruction and analysis.

Auto Prescan

PRESS CSI supports APS for all hydrogen data acquisitions. The APS process for the PRESS CSI and PROBE-P sequences adjusts TG, R1, R2, and the center frequency; optimizes water suppression; and invokes a voxel localized Autoshim routine to optimize the shim through a volume around the prescribed voxel. A constant TR (1500 ms) is used during APS to prevent system time-outs or excessively long prescan times when long repetition times are prescribed.