• Object ID: 00000018WIA30995970GYZ
  • Topic ID: id_40023696 Version: 1.3
  • Date: May 23, 2022 3:44:25 AM

Brain SVQ scan protocol considerations

PROBE - PRESS CSI single voxel 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.

PROBE-P is a version of the PRESS (Point RESolved Spectroscopy) sequence that acquires a spin echo from a localized volume defined by the intersection of three orthogonal slices.

Sixteen non-water suppressed excitations are acquired to create the reference data. Similarly, PROBE/SVQ data processing or reconstruction follows a fixed path:

  • Creatine (Cr) is the chemical species used as the reference for the calculated ratios and for data processing.
  • The reference resonance is used to establish the frequency and line width deviations of the spectrum from a standard frequency and line width. This information is used to preprocess the spectrum before fitting.
  • The spectrum is reconstructed (processed) with water subtraction, and quantitative analysis is performed as described in more detail just below.
  • The frequency of the water resonance is temperature dependent. To correctly locate the four metabolite frequencies relative to the water frequency (a system frequency reference), the frequency differences between water and the four metabolites are defined for 37° C (the normal human temperature). The automatic quantitative analysis of PROBE/SVQ data acquired from a phantom not at 37° C will fail.

Consider these tradeoffs as you change scan parameters for images that are post processed with the Brain SVQ protocol.

Patient setup

  • Secure the head firmly before the study so there will be a minimum voxel displacement. Also, since arm or leg motion will affect the voxel shim, instruct the patient not to fidget. Perform the automated global head shim, center frequency and transmitter power adjustments as normally done with imaging.

Localizer

  • Acquire localizer images in three orthogonal planes directly before the single voxel spectroscopy scan.

Single voxel spectroscopy scan parameters

  • Scan selections: MRS Mode, Spectroscopy family, PROBE-P or PROBE-S pulse.
  • TE considerations:
    • Typical longer echo time selections are 144 ms and 288 ms (these times are related to the 7 Hz coupling between the two peaks in the lactate doublet).
    • Use a TE⋲144 ms and acquire about 128 scans. The later echo time reduces the signal from macromolecules and from lipid contamination while retaining good S/N. For accurate automated analysis it is recommended that the S/N of Cr should be greater than 5.0. Using TE ⋲288 ms would make the baseline flatter, but scan time should be doubled or voxel size increased by 35% to get spectra with the same S/N as at TE⋲144 ms.
    • The use of short TE values (≤35 ms) is not recommended because some tumors contain lipids that can cause inaccurate baseline phase correction. For other pathologies, a short TE will provide more information (e.g., myo-inositol), but metabolite quantification will be more difficult because there are more metabolites, lipids, and macromolecule peaks.
  • For voxel imaging, use a much shorter TR (e.g., < 300 ms).
  • Selecting a Frequency Direction other than the default reorders the slice selection in the pulse sequences by swapping the gradients normally associated with the frequency and phase gradients; e.g., if the default order is X and Y, the swapped order would be Y and X.
  • NEX determines how many excitations are summed to form a single frame of raw data and sets the RF phase cycling method; when the phase cycle is 2, the phase of the first slice selective RF pulse is alternated from excitation to excitation; when the phase cycle is 8, the phase of each of the 3 slice selective RF pulses is alternated during the acquisition of 8 excitations.
  • From the Graphic Rx viewport, position and size the PROBE/SVQ voxel. Ensure that the volume of the voxel under investigation is at least 4.5 cm3. Shim the voxel, either manually or ideally under an automated shim process.
  • Advanced tab selections:
    • Scan Mode = 0 (to acquire an image from the voxel) or 1 (to acquire a spectrum from the voxel),
    • For an 8 cc voxel for PROBE-P, the Total Number of Scans (water suppressed) excitations acquired to create the “signal” data set:
      • 128 at 1.5T
    • Spectral Width is set to fixed values, and can not be changed during the prescription:
      • 2500 Hz at 1.5T
    • The points per raw data frame for the Number of Points is set to fixed values, and cannot be changed during the prescription:
      • 2048 at 1.5T

Auto PreScan considerations

  • The linewidth value is critical to the success of a spectroscopy scan. The shim process (part of Auto PreScan) 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.
  • Depending on round errors, the values for the shim are:
    • for 1.5T it is 7Hz