Edit 9/14/2015: This post is superceded by today's post.
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 emphasised 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.
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 emphasised 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. In the new patches for releases 5.1.2, 6.1.7 and 5.1.8, we have added a buffer-averaging feature to this stabilization. Offset updates are based not on a single FS measurement, but on the average of the previous n measurements (where n is set by the parameter Frequency Stabilization window, which appears when FS is turned ON). The idea being that the original implementation's main failing is that it is noisy, and averaging helps address that. In a way MEGA-PRESS is the ideal application for an averaged approach, since we do not want to update the frequency every TR. This approach seems to be beneficial, but is still being investigated. If you would like to apply it in your experiments, some suggested parameters are given below.
- In the presence of moderate instability, MM suppression is improved by locking the frequency.
- This method is more accurate than the Philips standard FS (which can still be used by changing the window to 1).
- The FS feature can turn off without warning (although possibly less now that the averaging removes spikes).
- This is largely untested.
- The averaged window approach tends to lag behind in the case of drift, and respond more slowly in the case of sudden movement.
Weaknesses
Parameters
Frequency Stabilization: yes (in the motion tab)
Frequency Stab window: 16 (same tab). This is a trade-off between averaging benefit and lag...
+freq stab odd dyn: yes (in the dyn/ang tab). This makes sure the frequency only updates on the first TR of an odd dynamic scan, helping minimize subtraction artefacts.
