. Author manuscript; available in PMC: 2022 May 1.

Published in final edited form as: NMR Biomed. 2020 Feb 21;34(5):e4257. doi: 10.1002/nbm.4257

Table 3.

Recommendations for preprocessing operations to reduce dimensionality.

Name of operation	Recommendation
RF coil combination	- Most vendors provide a data output option in which an acceptable RF coil combination has already been performed. - However, if the raw data is provided with coils uncombined, the user must perform coil combination. - Coil combination should include appropriate complex weights (phase and amplitude). - Complex weights should be determined using an unsuppressed water scan. - The amplitude weighting should be generally determined by signal/noise² as per Hall et al.⁴⁶, although more sophisticated approaches may yield improved results for certain coil designs and voxel locations.
Signal averaging	- We recommend combining transients using the arithmetic mean (the sum of all transients divided by the number of transients). Although less robust to instabilities across transients than the median, we suggest dealing with these instabilities through removal of motion corrupted averages and frequency/phase drift correction prior to averaging. -If motion corrupted transients have been removed, divide only by the number of transients that were retained. - Avoid combining averages using the simple sum of the acquired transients (i.e., the sum should always be divided by the number of transients). - Likewise, subtraction operations used in difference spectroscopy should also be treated as an averaging operation, i.e., when subtracting sub-spectra of a MEGA-PRESS difference editing experiment or a SPECIAL localization experiment, the difference spectrum should always be divided by 2 (the number of transients involved in the subtraction) following subtraction.

Name of operation

Recommendation

RF coil combination

- Most vendors provide a data output option in which an acceptable RF coil combination has already been performed.
- However, if the raw data is provided with coils uncombined, the user must perform coil combination.
- Coil combination should include appropriate complex weights (phase and amplitude).
- Complex weights should be determined using an unsuppressed water scan.
- The amplitude weighting should be generally determined by signal/noise² as per Hall et al.⁴⁶, although more sophisticated approaches may yield improved results for certain coil designs and voxel locations.

Signal averaging

- We recommend combining transients using the arithmetic mean (the sum of all transients divided by the number of transients). Although less robust to instabilities across transients than the median, we suggest dealing with these instabilities through removal of motion corrupted averages and frequency/phase drift correction prior to averaging.
-If motion corrupted transients have been removed, divide only by the number of transients that were retained.
- Avoid combining averages using the simple sum of the acquired transients (i.e., the sum should always be divided by the number of transients).
- Likewise, subtraction operations used in difference spectroscopy should also be treated as an averaging operation, i.e., when subtracting sub-spectra of a MEGA-PRESS difference editing experiment or a SPECIAL localization experiment, the difference spectrum should always be divided by 2 (the number of transients involved in the subtraction) following subtraction.