PROBE 2D CSI

General

• Typical examination times range from 3 to 15 minutes.
• The 2D CSI SNR depends on TE, TR, NEX, Freq, Phase, and Voxel Thickness.
• One cubic centimeter (1 cc), nominal CSI volumes provide reasonable SNR and spatial resolution when using the head coil. If the FOV = Freq = Phase, each edge of the CSI pixel is 1 cm in length. A 1 cm voxel thickness gives a volume of 1 cc.
• The 2D CSI reconstruction process creates 256 small chemical shift images that are stored in a standard MR image in a separate series. The 256 images span the frequency range from 4.3 to 0.49 PPM. Spectra and metabolite maps are extracted from the individual CSI images with READY View .
• PROBE-P is compatible with surface coil acquisitions. You may also use the body coil but low SNR will often be a problem.
• All imaging coils can be used to acquire hydrogen spectral data.
• 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. For example, Prescan values R1:11 R2:29 AX:63855957 LnWdth:9 Flip Ang:145 Supp Lvl: 98.
• The LnWdth 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 linewidth value is a measure of the voxel homogeneity; the smaller the linewidth, the better the homogeneity. For 2D CSI acquisitions, a linewidth value less than 0.2 ppm is acceptable. Remember however, that the linewidth will usually increase as the CSI phase encoding volume increases. If the linewidth is greater than 0.2 ppm, recheck the position of the voxel and, if necessary, reposition the voxel to avoid magnetically in-homogeneous regions of the anatomy. Then click Auto Prescan again. When you are satisfied with the Auto Prescan results, click Scan.

Scan parameters

• Frequency/Phase: Allowed Freq and Phase choices are 8 to 24 in steps of 2. Increasing the frequency and phase values increases both spatial resolution and scan time. Setting both to 16 provides reasonable spatial resolution and scan time.
• ROI: Each of the PRESS ROI dimensions must be less than or equal to the FOV. If you prescribe an ROI that is larger than the FOV along either in-plane dimension, an error message is posted on the Scan desktop. The ROI must be resized before any other inputs or command selections are allowed.
  • It is possible to acquire CSI spectra from volumes smaller than 1 cc if there is sufficient SNR, as may be possible with a surface coil. For example, if Voxel Thickness = 1 cm, Freq = Phase = 16, and FOV = 8, then the CSI volume = 0.25 cc = 8/16 × 8/16 × 1.
• Scan plane: If the spectroscopy acquisition is an axial, then you must deposit the Graphic Rx ROI cursor on the axial localizer. You can adjust the ROI cursor position from any viewport, but you must initially deposit it on an image plane that matches the acquisition plane.
• Slice thickness: The CSI Slice Thickness entry is irrelevant for 2D CSI prescriptions, so set Spacing = Voxel Thickness for consistency.
• TE: Use TEs > 100 ms. For 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 of 1000 ms is a reasonable compromise between signal saturation and longer scan times. You may wish to use a TR of 1500 or 2000 ms to enable the comparison with spectra acquired with PROBE-P in its single voxel mode. Whatever your choice, it should be used routinely so that you can compare spectra acquired at different times or in different patients.
  • Ideally the TR should be 3 to 5 times as the longest T1 of the chemical species contributing to the spectrum, but this is impractical for clinical CSI acquisitions.
• Voxel thickness: It is the depth of the FOV.
  • The FOV divided by Freq/Phase determines the lengths of the in-plane edges of the CSI voxel. The length of the third edge is the same as the Voxel Thickness of the PRESS VOI.
• FOV/ Phase/ Frequency: For PROBE 2D CSI acquisitions, the FOV, frequency, and phase choices not only affect the data acquisition but they also determine the number of zero-fills applied during the reconstruction process, and the aspect ratios of the reconstructed images. The allowed frequency and phase values are 8, 10, 12, 14, 16, 18, 20, 22, and 24. No zero-fill is applied for the 8 and 16 selections, that is, the number of reconstructed CSI voxels is 8 or 16 along that dimension. The 10, 12, and 14 selections are zero-filled to 16, and the 18, 20, 22, and 24 selections are all zero-filled to 32 during the reconstruction.

  Table 1. Frequency and phase selections with the resulting CSI image size.

  Frequency (F)/Phase (p)	P:8	P:10	P:12	P:14	P:16	P:18	P:20	P:22	P:24
  F: 8	8×8	8×16	8×16	8×16	8×16	8×32	8×32	8×32	8×32
  F: 10	16×8	16×16	16×16	16×16	16×16	16×32	16×32	16×32	16×32
  F: 12	16×8	16×16	16×16	16×16	16×16	16×32	16×32	16×32	16×32
  F: 14	16×8	16×16	16×16	16×16	16×16	16×32	16×32	16×32	16×32
  F: 16	16×8	16×16	16×16	16×16	16×16	16×32	16×32	16×32	16×32
  F: 18	32×8	32×16	32×16	32×16	32×16	32×32	32×32	32×32	32×32
  F: 20	32×8	32×16	32×16	32×16	32×16	32×32	32×32	32×32	32×32
  F: 22	32×8	32×16	32×16	32×16	32×16	32×32	32×32	32×32	32×32
  F: 24	32×8	32×16	32×16	32×16	32×16	32×32	32×32	32×32	32×32

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.

• Automatic Water Suppression Optimization
• Number of Points
• ROI Edge Mask SAT
• Scan mode
  • For Voxel imaging (Scan Mode = 0 or -1), select the shortest allowed value for TE and TR.
• Spectral Width
• Total number of scans

Post-process tasks