Rotation extension

[mcencini]

Hi all,
thanks for this amazing work! I wanted to discuss an undocumented Pulseq extension (i.e, rotation), I hope this is the appropriate place for it.

Motivation
Pulseq format does a fantastic job in keeping the size of the seqfile small when the sequence mostly consists of repetitions of scaled waveforms (which is true for most Cartesian imaging). However, I feel the base format is not so efficient when it comes to Non Cartesian sequences (pulseq/pulseq#31). Luckily, many Non Cartesian trajectories can be efficiently described as rotated versions of a single waveform (see, for instance, this), potentially enabling a significant reduction of memory footprint and faster loading times, even in older scanner platforms.

Apparently, C++ implementation of Pulseq already defines a Rotation event extension, representing the rotation of the gradient matrix for each block. This extension, as far as I know, has never been exposed to MATLAB or Python codebases, perhaps because the main Siemens interpreters do not support it? I am not a Siemens user myself, so I cannot verify this.

Proposal
I exposed the rotation extension in my own PyPulseq fork, since the GE interpreter potentially supports this kind of waveform manipulation already (@jfnielsen). I also adapted the plotting tools and helpers routines extracting gradients, waveforms and so on. Before opening a pull request, I would like to know if that sounds like a good idea. Personally, I do not see this as a too much intrusive modification, as per Pulseq specification extensions are explicitly designed to add new features without having to go through a major release. As an example, code for sequences aiming to be compatible with different interpreters (supporting rotation or not), could contain this kind of code snippet for the gradient rotation:

if use_rot_ext:
    # Create rotation event
    rot = pp.make_rotation(create_rot_matrix(phi, axis="z"))

    # Add gradient and rotation event
    seq.add_block(grad, rot)
else:
    # Add rotated gradient
     seq.add_block(*pp.rotate(grad, angle=phi, axis="z"))

Thanks again!
Matteo

[btasdelen]

@namgyunlee This maybe of interest to you.

[namgyunlee]

@btasdelen Thank you for letting me know! Hope this feature is realized in the Siemens interpreter as well :)