• Object ID: 00000018WIA306AB890GYZ
  • Topic ID: id_2036693 Version: 1.1
  • Date: Jun 30, 2022 3:39:48 AM

PROSE homogeneity adjustment procedure

About this task

Use these steps to adjust the homogeneity of a PROSE sequence after a PROSE prescription is saved and the Spectro Prescan button displays.

Step-by-step instructions

  1. Click Spectro Prescan to open the Spectroscopy Prescan screen.
  2. Verify that the Entry Point text box value is single1.
  3. Click on the display waveform menu (bottom left edge of the display window) and choose Magnitude for the top display, and Pure Absorp for the bottom display.
    • Many spectroscopists prefer to use the I Chan Raw display waveform (an FID ) to demonstrate homogeneity changes.
  4. Increase the center frequency by 100 HZ with Center Freq (AX).
    • This change allows you to see an intense water peak in the Magnitude and Pure Absorption windows.
    • The FID in the I Channel Raw window will show a beat pattern that looks like a damped sinusoid
    • Important: Be sure that you reset the center frequency before leaving the Spectroscopy Prescan screen.
  5. Click Start to acquire a spectrum from the entire PROSE volume.
  6. Watch the shape and the height of the water peak (or FID) in the display windows.
    • Look for distortions near the base of the water peak or anywhere in the FID.
    • You want the I Chan Raw signal, the FID, to “ring” out as far as possible.
  7. Record the initial Gradient Shimming values for the X, Y, and Z gradients. You can restore the initial values at any time by clicking Reset.
  8. Click the C at either end of the Gradient Shimming sliders to center them.
    • It is suggested that you start by adjusting the Z gradient current.
    • Note: The AutoShim results are usually good enough that you can start with ±1 or ±2 unit changes rather than the coarse ±10 unit adjustments.
    • Increase the Z value +10 units by moving the slider to the far right of the slider field.
      • It takes at least two window display updates to see the effect of a gradient current change.
      • Examine the shape of the water peak. You want the peak to become taller and narrower, while keeping the bottom of the water peak smooth.
      • If the peak narrows and increases in height, increase the Z gradient value again by +10 units. You can re-center the value by clicking C.
      • Continue to increase the value until the peak starts to distort or fails to narrow. Always return to the last good value.
    • If the peak shape degrades or broadens with a +10 unit change, move the slider all the way to the left of the slider field, i.e., by -20 units (a change from the initial value of -10 units).
      • Remember, you need to wait at least two display updates to see the change.
      • Examine the water line shape. If the shim improves after the -10 unit change, decrease the shim current by another 10 units until the peak stops narrowing or until the shape degrades. Always return to the last good value.
      • Stop at the best shape.
    • Adjust the X gradient current in ±10 unit steps.
    • Adjust the Y gradient current in ±10 unit steps.
    • After adjusting the three gradients with the large ±10 unit steps, you should repeat the adjustments with ±5 unit changes, and then ±1 unit changes.
    • Note: The AutoShim results are usually good enough that you can start with ±1 or ±2 unit changes rather than the coarse ±10 unit adjustments.
    • The best procedure is to use a consistent, reproducible method similar to that described above.
  9. Once you are satisfied with the spectral line widths and shapes, reset the center frequency Center Freq (AX), and click Done to leave the Spectroscopy Prescan screen.
  10. Click Scan.