At high resolution EPI, the gradients are pushed to their limits and the ramp sampling ratio is particularly large. This means that the ghosting is increased and the Nyquist ghost correction is getting more important. In this post, I describe how to change the Nyquist ghost correction algorithm.
Nyquist ghost correction becomes challenging with large ramp sampling ratios
For a given gradient strengths, any time delay between odd and even lines in k-space refers to a shift in read direction. Hence, without ramp sampling, a given delay can be easilty corrected for with a global realignment of odd and even k-space lines. When the gradient strengths is not constant anymore during ramp sampling, however, time delay cannot be approximated with a a gloabl shift (phase offset). During the ramps, a given time delay refers to different shift distances. Hence, with high ramp sampling factors, the two fit parameters in the phase correction cannot completely correct for all eddy currents.
Additionally, field drifts due to gratient temperature changes can then be translated to additional phase offsets and additional EPI-ghosting.
How to change the Nyquist ghost correction
Changing the phase correction algorithm in the protocol editor
Very few sequences allow the user to change the phase correction algorithm. In this case its very easy to adjust. I have the best experienced with the ‘local’ phase correction scheme. I also found that the ‘local’ phase correction should reduce the changing ghosting over time. Strangely, sometimes, the tSNR is higher for the ‘normal’ phase correction scheme, though the ghosting makes the image completely unusable.
Changing the phase correction algorithm in the sequence code
The phase correction algorithm can be set in the sequence code as follows by setting the parameter pSeqExpo->setOnlinePhaseCorrectionAlgo().
Changing the phase correction algorithm in the Ice-Configurations
You can change the reconstrudtion method via Twix at the scanner. This method is helpful when you don’t have the sequence code. It is also very helpful, when you want to reconstructh the same dataset multiple times with different phase correction methods:
- Start twix (Ctrl+Esc and run twix)
- select the dataset in the left panel
- click the edit sympol
- click on edit
- search for iOnlinePhasCorrectionAlgo
- select iOnlinePhasCorrectionAlgo
- change number to 68
- click save
- recunstruct by clicking Start
There are miltple algorythms implemented that you can choose from. As far as I understand (and I am really not sure here) the assignlemt is as follows:
0 no phase correction: the ghosts look horrible
1 global phase correction with one fiting parameter only: this can be helpfull when you want a time-conserved phase offset. E.g. with funtional susceptibility mapping
2-5 No idea
6 Two paramter fit: results in non-quatitative phase images across time, but reduces ghosting.
7 Pixel-by-pixel fit with one parameter: unusable for me because the ghots are very bad
8 same as 2 I think
68 local phase correction: correlation estimation of odd and even lines (two parameters) multiplied by the magnitude (according to Polimeni).
As usual with sequence stuff. I learned everything from Ben Poser.