[BUG] Silent field overwrite in `_rename_fields` — conflicting PostLabelingDelay sources silently lost

[Devguru-codes]

Location

package/src/pyaslreport/modalities/asl/processor.py:236-246

Description

field_mappings maps both InversionTime and InitialPostLabelDelay to PostLabelingDelay. If a session contains both fields with different values, the loop silently overwrites the first mapping with the second, losing the original value with no warning. In medical imaging, silently changing physical scan properties like PLD compromises data integrity.

Reproduction (on main branch)

Script

from pyaslreport.modalities.asl.processor import ASLProcessor

proc = object.__new__(ASLProcessor)

# Both fields present with different values
session = {
    "InversionTime": 1.5,
    "InitialPostLabelDelay": 2000.0,
    "RepetitionTime": 4000.0,
    "MagneticFieldStrength": 3.0,
    "MRAcquisitionType": "3D",
    "Manufacturer": "Generic"
}
proc._rename_fields(session)
# Result: PostLabelingDelay = 2000.0, InversionTime value 1.5 safely LOST

Processor / API Normalized Response

When the frontend sends the above JSON payload to the FastAPI /api/report/process/bids endpoint, the ASLProcessor normalizes it before generating the BIDS metadata. Due to the bug, the API produces this state where 1.5 is entirely lost:

{
  "MagneticFieldStrength": 3.0,
  "MRAcquisitionType": "3D",
  "Manufacturer": "Generic",
  "RepetitionTimePreparation": 4000.0,
  "PostLabelingDelay": 2000.0
}

Automated Results on main branch

#	Test	Status	Detail
1	InversionTime only -> PostLabelingDelay	PASS
2	InitialPostLabelDelay only -> PostLabelingDelay	PASS
3	Both fields with diff values -> silent overwrite	FAIL	PLD=2000.0 (1.5 silently lost)
4	Both fields with same value -> no data loss	PASS

Summary: PASSED=3 FAILED=1