ENH: PET uptake model

[oesteban]

I still need to test on some data (ha!), but these are the barebones of a PET uptake model.

The idea is a voxelwise fit of a low number of B-Splines (i.e., smooth function) on the data passed into .fit(), which can be the full dataset (if initialized with FullPETModel) or all frames but the one being aligned (leave-one-volume-out).

I will try to test on the dataset Martin shared over this week. If that worked, I think we could draft some abstract for OHBM with this and some further validation, WDTY?

cc/ @effigies and @mgxd , who will love to see B-Splines in one more place (it's ironic, but their feedback is always very much appreciated).

I'm attaching @mnoergaard and @arokem as reviewers, but I don't expect feedback until I test this really works on data and remove the "draft" marking of the PR. Feedback before that will be of course welcome nonetheless.

How to use (edited):

from eddymotion import estimator
from eddymotion.data import pet

data = pet.load("path/to/pet.nii.gz", frame_duration=[20, 20, 20, 60, 60, 60, 120, 120, 120, 300, 300, 600, 600, 600, 600, 600, 600, 600, 600])

# Run one initialization step fitting on the full dataset and a second step
# with a leave-one-volume-out fit/predict loop.
estimator.EddyMotionEstimator.fit(data, models=("FullPET", "PET"), n_jobs=4)

[oesteban]

Making progress. @mnoergaard, I'm using the first subject of https://doi.org/10.18112/openneuro.ds004230.v2.3.1 - I don't know if it is a good example.

[oesteban]

Okay, I have managed to "simulate" a dataset with this model. The only remaining problems:

• The new PET data structure will need the leave-one-volume-out splitter to be implemented as a function that operates on the data object.
• Fitting the full volume is very slow. Most likely, we don't want to fit one model per voxel.

[arokem]

Looks promising! A couple of random thoughts. In particular, cross-check with #98 for work that @teresamg has been doing to work out memory consumption in the DWI case.

[oesteban]

Okay, since the data split is not yet outsourced from the DWI object, this model can be tested this way:

import json
from pathlib import Path
from eddymotion import model
from eddymotion import data

datapath = Path("/data/datasets/ds004230/")

# Load data
metadata = json.loads((datapath / "sub-PS19/ses-baselinebrain/pet/sub-PS19_ses-baselinebrain_rec-DynTOF_pet.json").read_text())
data = pet.load(datapath / "sub-PS19/ses-baselinebrain/pet/sub-PS19_ses-baselinebrain_rec-DynTOF_pet.nii.gz", frame_time=metadata["FrameTimesStart"], frame_duration=metadata["FrameDuration"])

# Initialize and fit model
petmodel = model.PETModel(timepoints=data.frame_time, xlim=data.total_duration)
petmodel.fit(data.dataobj, n_jobs=16)

# Predict fifth frame
predicted = petmodel.predict(data.frame_time[4])

[mnoergaard]

Making progress. @mnoergaard, I'm using the first subject of https://doi.org/10.18112/openneuro.ds004230.v2.3.1 - I don't know if it is a good example.

Hi @oesteban! This is great - thanks for pushing this forward! ds004230 should be an ideal candidate with "only" a size of about 160MB per PET file. The largest PET files I have come across are around 1GB (high resolution scanners). For ds004230, you can also find the current motion correction results for comparison in /derivatives/petsurfer/logs/. The generated confounds file is located in /derivatives/petsurfer/subXX/sesXX/pet/subXX-sesXX_desc-confounds_timeseries.tsv. Furthermore, motion free PET data is also generated/synthesized and can be found here /derivatives/petsurfer/subXX/sesXX/pet/nopvc/yhat.nii.gz.

This code has been used to perform motion correction for these data (https://github.com/mnoergaard/hmcpet/blob/master/init_pet_hmc_wf.py), that includes both a selection of frames (only frames after 2 minutes, because there is no signal before this) and also a smoothing step (10 mm) to increase SNR. These could ideally be included in the eddymotion workflow?

Let me know if we should touch base over zoom soon.

[oesteban]

Hi Martin - responding quickly from my phone.

• re: selection of frames - totally doable within this PR
• re: smoothing - on the roadmap (see Spatial smoothing & disallowing ANTs' subsamplings/smoothings nifreeze#11)

One question about the bigger size of datasets: I assume the larger size responds mostly to the larger number of voxels each volume, rather than substantially more frames. If the latter, I don't think that would be to worry, but it seems the former so model fit will take longer.

One possibility would be to fit a single uptake function on data from high SNR voxels and one scaler parameter of that curve for each voxel.

I believe a coarse brain mask would be necessary to (1) fit fewer voxels and (2) avoid the structured background which ANTs will love to align.

I think a quick call next week would be useful if you're not on holidays already. I'll follow up via email.

[mnoergaard]

Thanks for pushing these efforts forward @oesteban - I think this looks pretty good so far!

[oesteban]

I think I will merge this as a base, and then we can address the frame selection by allowing weights (and fitting with WLS).