Is the basis set for MEGA-sLASER interchangeable with that of MEGA-PRESS as for detecting GABA?

Analysis / Modeling Basis Sets
1

Hi everyone,

When using Osprey, I noticed the file in the folder fit/basissets/7T/siemens/mega/slaser/gaba74, named basis_siemens_megapress_gaba74.mat.
I would like to understand under what constraints the basis set for MEGA-sLASER can be interchangeable with that of MEGA-PRESS.

Best,
Yushan

2

Hi Yushan,
I would assume that this is not possible, because:

  1. different influence of the chemical shift displacement error, but I canā€™t say if this is considered in the basis set and how high this influence is. For PRESS, this error would vary anyway.
  2. different evolution of the j-coupling, because adiabatic pulses can prevent it.
  3. besides the pulses you have to take the timings into account, in PRESS you have to look at TE1/TE2, in sLASER TE1/ā€¦/TE4 (+MEGA pulse position)
  4. maybe some points moreā€¦
    All in all, every point has influence on the evolution of the j coupling and in MEGA youā€™re looking at scalar coupled systems. But nevertheless, you can try. Maybe it works.

Best,
Heiner

3

Hi Heiner,

Thank you very much for your response and detailed explanation!
I am not familiar with the relationship between adiabatic pulses and evolution of the j-coupling. I would like to read some relevant materials. Do you have any recommendations?

Best,
Yushan

4

Hi Yushan,
Off the top of my head, I canā€™t think of papers with relevant information. Only for example https://cds.ismrm.org/protected/07MProceedings/PDFfiles/03163.pdf whereas the spin locking properties of the adiabatic pulses are mentioned. But to my knowledge, adiabatic pulses have similar spin locking properties than CPMG pulse trains which are used for example in CPRESS (https://onlinelibrary.wiley.com/doi/epdf/10.1002/mrm.1910370603). The conditions therefore are equation 29 and 30 in Analysis of Carrā€”Purcell Spinā€Echo NMR Experiments on Multipleā€Spin Systems. I. The Effect of Homonuclear Coupling | The Journal of Chemical Physics | AIP Publishing. For sLASER, I would expect similar conditions and because of equation 30 an influence of the magnetic field strength compared to 3T.
Also some papers which treat the results of this effect: https://onlinelibrary.wiley.com/doi/10.1002/mrm.21302 and https://onlinelibrary.wiley.com/doi/epdf/10.1002/jmri.21999.

Best,
Heiner

1 Like
5

I donā€™t know how to express my gratitude. These are very helpful for me. Thank you, Heiner!

6

Basis sets for PRESS and sLASER generally shouldnā€™t be interchangeable, and certainly even less so for MEGA-edited sequences, where the effects of chemical shift displacement on the refocusing are even more substantial.

Since itā€™s 7T (where few people use PRESS), my guess is that this specific basis set is indeed for sLASER (i.e. in the correct folder, just with an incorrect filename). @Helge uploaded that basis set a few years ago, Iā€™m tagging him to see if he can clarify.

7

I see. Thank you very much, Georg!