Monday, 14 September 2015

Frequency Stabilization II

We have written a couple of papers on subtraction artefacts (Evans et al. JMRI 2013) and field drift (Harris et al. MRM 2014).  Obviously as a subtraction technique, stability is important for MEGA-PRESS.  However, one aspect that we have not emphasized is the impact of drift on editing efficiency. As the field drifts, the offset of editing pulses moves from their intended target and the editing efficiency of GABA (and MM) changes.  This is especially important for MM-suppressed editing, where small drifts can rapidly lead to positive or negative MM signals bleeding back in.  MM suppression relies upon editing pulses that are symmetrically disposed about the MM signal to properly null MM.  This symmetry is destroyed by the field drift that occurs due to scanner heating/cooling and subject movement.

The Philips scanner has a Frequency Stabilization (FS) function programmed in. Basically, it performs a whole-slice gradient echo measurement to determine changes in the offset. This quick, small-flip-angle water measurement, which is interleaved within the TR, is strongly recommended not to be used for proton spectroscopy, largely because it is inaccurate.  We tried modifying this approach to buffer and average, but this was unsuccessful.

The basic problem with a proton-based frequency stabilization is that is relatively difficult to localize a signal from just the voxel, using small flip angle excitation.  Therefore, any within-TR approach has to either include signal from outside the voxel (which does not work well), impact the acquired signal due to non-negligible saturation, or use a heteronuclear approach.  We have therefore sought to acquire localized signal (for frequency determination) from the PRESS volume without increasing scan time or saturating the acquired signals. The solution we have come up with is to reorder the acquisition.

We typically acquire 16 TRs of water reference data - 16 scans not because it is needed for SNR, but so as to acquire a full phase cycle and therefore have the same quality of localization on the reference and water-suppressed data.  In the new FS implementation, we now acquire these scans distribute through the acquisition time (every 1/16th of the total time), FFT them on the fly and determine the water frequency to feed back any changes to the RF F0.  This seems to work well, and reduces the effect of linear drift by a factor of 16 and of movement potentially more.  Importantly, by juggling the order of acquisition of the data, we get this benefit without increasing scan time or reducing SNR.

Recommendations:

  • If you are using any form of frequency stabilization, we recommend you check over some GannetLoad outputs for erratic behavior, and stop using it.
  • If you have started a study without FS, continue and adopt the new FS when you start the next study (especially if you plan an MM-suppressed measurement).
  • If you plan an MM-suppressed study, contact us for a new patch with the distributed reference feature.



6 comments:

  1. We are working on porting this new functionality to various Philips releases.

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