CEST: Signal Drift

[firasm]

In Kim's thesis, there is a plot that shows a bi-exponential decay of S/So vs. time. This image is acquired while saturating at a frequency much far off-resonance (20 kHz) - ideally this saturation pulse should have no effect on the signal.

Here's our signal drift:

Here's Kim's signal drift plot from the Bruker scanner in Toronto:

[DrSAR]

Were you using higher order shims?

[firasm]

Yep, I used the global shim and Andrew Yung showed me how to set up the higher order local shim using a PRESS scan/voxel

[DrSAR]

If this can be avoided without severely deteriorating the signal, it might be interesting to see whether the drift looks the same with those shims. I think Piotr/Andrew were surmising that there is some coil heating caused by higher order shims and if that is so, it could be less drifty without the shim currents on.

[firasm]

*you mean without those shims?

Hmm interesting, I think without the shims on, we saw wave-like distortions within the image.

I'll try it again without the higher order shims set to see if it makes a difference.

Also, that plot above is for a single voxel in one of the 5 vials (I added an image of the phantom with a cross)

[DrSAR]

yes without - from what you're saying that might not be a feasible test if you can't get a decent image. S

firasm wrote:

*you mean without those shims?

Hmm interesting, I think without the shims on, we saw wave-like distortions within the image.

I'll try it again without the higher order shims set to see if it makes a difference.

Also, that plot above is for a single voxel in one of the 5 vials (I added an image of the phantom with a cross)

— Reply to this email directly or view it on GitHub https://github.com/firasm/analysis/issues/4#issuecomment-67608247.

[firasm]

I wonder if 3% over an hour is manageable or if it's something we need to address. I've added Andrew to the conversation, perhaps he can comment as well.

[DrSAR]

3% I didn't pay enough attention to that. Yes - in Kim's thesis it was more like 10% of I remember correctly.

[firasm]

Updated issue with Kim's plot.

Looks like it's 4% and she did account for the change.

[andrewcyung]

I think the wave like distortions in the phantom without higher order shims were due to the B0 inhomogeneity, which will naturally be worse without the higher order shims. The temperature effects during higher order shimming that Piotr and I referred to may still be there, but it will be a lot better than before we got our new gradient set which combines the gradient and shim coils (which are therefore water-cooled together, whereas our old setup had the shims in the large gradients, which would not be water cooled if our old medium gradients were being used).

As to whether or not 3% is significant, I suppose it depends on how large the amide/amine peaks. Maybe you can run the same stability analysis, but use the offset frequency where the CEST peak is?

[firasm]

Good idea Andrew!

In a sample closer to that of interest, it would be useful to do a test acquiring signal over time to see how much signal we lose.

Will add that to my todo list

[firasm]

I analyzed a more recent dataset trying to see how the Signal Drift over time.

This is actually for 200 kHz saturation offset

FYI: scn = sarpy.Scan('HPCoil1.tQ2/14')