• Object ID: 00000018WIA30B45970GYZ
  • Topic ID: id_40023686 Version: 1.3
  • Date: May 26, 2022 9:06:43 AM

PROBE-P scan

PROBE-P is a version of the PRESS sequence that acquires a spin echo from a localized volume defined by the intersection of three orthogonal slices. The PROBE-P sequence is the automated PROBE/SVQ version of the PRESSCSI sequence with specialized data acquisition and data analysis capabilities.

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

  • Scan selections: MRS Mode, Spectroscopy family, PROBE-P pulse.

General

  • PROBE-P is the preferred PROBE sequence because of the potential two-fold SNR increase relative to PROBE-S. In any case, use PROBE-P for all long TE acquisitions to gain the SNR advantage.
  • PROBE includes spectroscopy specific Auto Prescan and reconstruction processes. The PROBE Auto Prescan includes voxel localized shim, and AWS an optional CHESS based, automatic water suppression process. The single voxel spectrum reconstruction includes a quantitative analysis of the spectrum that reports the signal intensities of choline, creatine, N-acetyl groups, myo-inositol, and the SNR of creatine. Water reference spectra acquired as part of the acquisition are employed in the reconstruction to determine and apply phase, residual eddy current, and artifact corrections to the water suppressed data. The PROBE spectrum is drawn on a 512×512 image that is stored in the patient database. The results of the quantitative analysis are stored in the image header and are displayed with the Viewer.
  • If Auto Prescan is successful, the receive and transmit gains, the center frequency, the linewidth, the flip angle of the water suppression RF pulses, and the water suppression level are displayed, such as, " R1:11 R2:29 AX:63855957 LnWdth:5 Flip Ang:145 Supp Lvl: 98".
  • Auto Prescan can fail if the voxel volume is comprised of too varied anatomy. If Auto Prescan fails, it is most likely due to the placement of the voxel volume. Try repositioning the voxel volume to a more homogeneous anatomy and click Auto Prescan again.
  • Each viewport of a single voxel VOI spectroscopy graphic prescription is screen saved and stored in a series numbered 5000+. Both the VOI and SAT pulses are saved with the images. When the spectroscopy acquisition is completed, the screen saved series is displayed in the Patient List and the three images can be viewed in the Viewer or Mini Viewer.
  • The linewidth value is critical to the success of a spectroscopy scan. The shim process (part of APS) attempts to improve the homogeneity through the voxel by adjusting the gradient currents. The LnWdth value is a measure of the voxel homogeneity; the smaller the linewidth, the better the homogeneity. For single voxel spectroscopy, a linewidth value less than 0.1 ppm is acceptable. If the linewidth is greater than 0.1 ppm, recheck the position of the voxel and, if necessary, reposition the voxel to avoid magnetically inhomogeneous regions of the anatomy. Then click Auto Prescan again. When you are satisfied with the Auto Prescan results, click Scan. If the FWHM is still > 6 Hz, the scan may result in an unusable spectrum — the peaks are likely to be broader than normal.

Scan parameter

  • FOV: The minimum available FOV is displayed next to the text box and is the default FOV if the voxel is graphically prescribed without entering a value.
    • Voxel thickness is the depth of the FOV and the PRESS volume. Note that if you prescribe a small volume the SNR decreases proportionally.
  • Matrix: For a Single Voxel acquisition, the matrix is 1×1.
  • NEX: The NEX value controls the number of RF phase cycles – the choices are 2 and 8. Set NEX to 8 to best reduce motion and susceptibility artifacts. Phase cycling is the repetition of a pulse sequence and signal acquisition, such that all acquisition parameters except the phase of the RF pulses (and possibly the receiver) remain unchanged from repetition to repetition. Only the RF phase is changed, or cycled, as the repetitions occur. Phase cycling suppresses, or eliminates, undesirable signals while taking advantage of the effects of signal averaging.
  • Scan plane: For graphic prescription of the PROBE volume, the plane of the localizer images must be the same as the prescribed plane. For example, an axial, PROBE ROI is prescribed on an axial localizer.
  • Slice thickness: While no CSI Slice Thickness value need be entered, set it to the voxel thickness value for consistency.
  • TE: Use the short TE (35 ms) to visualize short T2 chemical species such as glutamate, glutamine, and myo-inositol. With PROBE-P at TE 144 ms, the lactate doublet is inverted relative to the long T2 species such as the creatine, choline, and N-acetyl resonances. The lactate doublet is fully refocused at TE 288 ms, but even the signals from the long T2 species experience substantial decay, and therefore the 288 ms choice is not recommended.
  • TR: TR must be > 1200 ms. The long minimum TR is required to accommodate the water suppression portion of the pulse sequence. Whatever your choice, it should be used routinely so that you can compare spectra acquired at different times, and in different patients. Most published spectra have been acquired at TE 1500, 2000, or 3000 ms. Ideally, the TR should be 3-5 times as the long as the longest T1 of the chemical species contributing to the spectrum.

Imaging Options

  • ASSET: If you are using a multi-channel coil, you must first acquire an ASSET calibration scan.
    • If any or all of these scan parameters are selected (ASSET, HyperBand, PURE), and if you select On from the Calibration in Prescan menu, which is located on the Details tab, a calibration scan is acquired during Auto Prescan.
  • ASSET: If you are using a single-channel coil, you cannot acquire an ASSET calibration scan. Instead, a warning message is posted noting that the system will acquire the multi-voxel, multi-channel data, and store and present it on a channel-by-channel basis instead of combining the channels. You must acknowledge this warning before you can continue.
  • For general spectroscopy scan parameter details, see

User CVs

Click the Advance tab to view the available User CVs. The CVs may vary based on the field strength and selected scan and imaging parameters.