ENH: Improve PET run combination timing and decay metadata handling

[mnoergaard]

What would you like to see added in this software?

PETPrep currently supports --combine-runs for concatenating multiple PET runs, but the combined metadata should better preserve BIDS PET timing semantics when later runs use their own TimeZero.

In datasets where a subject leaves and returns under the same tracer injection, a later run may have FrameTimesStart beginning at 0, while InjectionStart is negative relative to that run’s TimeZero. For example, run 1 may start at injection/scanner time zero, while run 2 starts 7701 seconds later with InjectionStart: -7701. In this case, combined FrameTimesStart should place run 2 frames on the same time scale as run 1, rather than simply appending them after the previous run’s duration.

Requested behavior:

1. Use TimeZero, ScanStart, and/or InjectionStart to reconcile all runs onto a shared timing reference.
2. Merge FrameTimesStart, FrameDuration, and FrameReferenceTime accordingly.
3. Preserve or update frame-wise metadata such as decay correction factors where possible.
4. Consider whether decay-corrected runs need rescaling if each run was reconstructed relative to a different ImageDecayCorrectionTime.
5. Add validation or warnings when the metadata are insufficient to combine runs quantitatively.

Do you have any interest in helping implement the feature?

Yes

Additional information / screenshots

No response