Imaging-Data Skill

The dataset decides more of a study than the architecture does, and it decides it first. Phases 1–3 establish what the data is and what it will not support, while that is still cheap; Phases 4–7 design and audit the preparation pipeline so it is leakage-safe before /model-scaffold builds the repo. Describe-and-audit only: never modify, resample, reorient, split or write image data, never run preprocessing on real patient data, and wire MONAI / TorchIO transforms by reference rather than writing a new normalisation or resampling implementation.

Elsewhere: tabular/clinical variables → /generate-codebook, /clean-data; auditing the split table, held-out metrics, calibration, subgroup results → /model-assessment; choosing an architecture → /model-selection; building the repo → /model-scaffold.

Workflow

Phase 1 — Profile every case

python3 ${CLAUDE_SKILL_DIR}/scripts/profile_imaging_dataset.py \
    --split train:imagesTr:labelsTr \
    --split test:imagesTs \
    --dataset "MSD Task09 Spleen" \
    --declared-labels 0=background,1=spleen \
    --target-label 1 \
    --plan resample=true,reorient=false,loss=dice_ce,metrics=dice+hd95 \
    --out eda/profile.json

One record per case: grid, spacing, orientation, intensity percentiles, the label values actually present, foreground fraction, and target volume in mL. A --split given no label directory is recorded as unlabelled — itself a finding. Requires nibabel + numpy; the gate does not. Every profile figure comes from opening the files — never from a dataset's README, a similar dataset, or memory. A README can be wrong about its own label indices; the labels cannot.

--target-label on a multi-structure atlas. Foreground defaults to every non-zero index — the whole annotated anatomy. Measured on the AMOS22 CT cases, that pools to 3.2 % instead of the spleen's 0.20 %, so the pooled figure sits above the 1 % imbalance threshold while the target sits far below it and the imbalance verdicts go quiet exactly where the risk is. Naming the target also makes LABEL_EMPTY mean this case has no spleen. Pass --target-label all for a genuinely multi-class study; leave it out on a multi-structure atlas and the gate raises TARGET_LABEL_UNDECLARED.

Phase 2 — Gate the profile against the declared plan

python3 ${CLAUDE_SKILL_DIR}/scripts/check_dataset_profile.py --profile eda/profile.json \
    --out qc/dataset_profile.json --strict

Stdlib-only, so the audit re-runs anywhere the JSON travels. Never report a profile "pass" without running it.

Verdict	Severity	Fires when
LABEL_SHAPE_MISMATCH	Major	label grid ≠ image grid
LABEL_EMPTY	Major	a labelled case has zero foreground
LABEL_VALUE_UNEXPECTED	Major	label values outside the declared set
TEST_SET_UNLABELLED	Major	a split named test/held-out/external/eval carries no labels
ACCURACY_UNDER_IMBALANCE	Major	accuracy is planned while the target is a sliver of the volume
LABEL_MISSING	Minor	a case in a labelled split has no label file
SPACING_HETEROGENEOUS	Minor	spacing spans ≥ ratio on an axis and no resampling is declared
ORIENTATION_MIXED	Minor	>1 orientation code and no reorientation declared
INTENSITY_SCALE_INCONSISTENT	Minor	some cases on the HU scale, others not
EXTREME_IMBALANCE	Minor	median foreground below the threshold with no Dice-family loss
TARGET_LABEL_UNDECLARED	Minor	>1 structure declared, no target named, so foreground pools them all

The gate flags an undeclared decision, not variability: 5× spacing spread and two orientation codes pass once resampling and reorientation are declared (the clean challenge fixture proves this). --spacing-ratio (default 2.0) and --imbalance-frac (default 0.01) are screening defaults, not published cut-points — never present them as such; the values applied are printed in the output and belong in the Methods. A split the profile shows unlabelled is never a held-out test set, however the directory is named.

Phase 3 — Turn the profile into research decisions

Write these decision notes into the study record, so /design-study, the preparation phases below, and /write-paper inherit them instead of re-deriving them:

1. Resampling target — from the spacing distribution, not a tutorial default (carried into Phase 5).
2. Loss and metric family — from the foreground fraction. Segmentation reports Dice and a boundary metric per structure (/model-assessment); accuracy is not on the list.
3. Pre-specified subgroups — from the clinical spread the profile shows (target volume, slice thickness, modality). Pre-specifying them here is what separates a subgroup finding from a post-hoc one.
4. Where the held-out set comes from — especially when the shipped "test" directory is unlabelled.
5. What the cohort cannot support — n, single-source acquisition, absent subgroups: the seed of the Limitations paragraph, written before results can bias it.

Phase 4 — Inventory the preparation steps and fix fit scope

Collect the modality, the data manifest (one row per image/slice with a patient_id), the resample spacing, the intensity transform (fixed HU window vs a fitted z-score / min-max / histogram match), and the augmentation plan. Read ${CLAUDE_SKILL_DIR}/references/preprocessing_guide.md for the modality-aware normalisation, physiology-preserving vs -breaking augmentation, and MONAI / TorchIO wiring.

• Fit dataset-level normalisation on the training split only — never all/full/test.
• Run any data-fitted transform after the split; before it there is no train/test distinction.
• Prefer per-image (per-sample) normalisation where clinically appropriate — leakage-free even before the split.
• Keep augmentation train-only; augmenting val/test folds undisclosed test-time augmentation into the metric.
• Split at the patient level, then map slices to their patient's split.

Phase 5 — Emit the preprocessing manifest

Write preprocessing_manifest.json, which /model-scaffold consumes and the gate checks. Every value comes from the real data manifest and the declared pipeline — never invented patient IDs or split assignments.

{
  "split_seed": 42,
  "transforms": [
    {"name": "hu_window", "type": "clip", "fit_scope": "none", "stage": "before_split"},
    {"name": "train_zscore", "type": "standardize", "fit_scope": "train", "stage": "after_split"},
    {"name": "flip_rotate", "type": "augmentation", "stage": "after_split", "applies_to": ["train"]}
  ],
  "split_assignment": [
    {"patient_id": "P001", "unit_id": "P001_s1", "split": "train"}
  ]
}

fit_scope: train (OK) · all/full/dataset/test (leak) · sample/per_image/none/fixed (not data-fitted). stage: before_split / after_split. The fields must describe what the code actually does — never tag a dataset-fitted transform per-sample to clear the gate; that hides the leak. A declared dataset-level fit_scope is judged whatever the type is, so a library class name (HistogramStandardization, NormalizeIntensityd) fit on all is a leak like standardize would be.

Declare the fit scope of resampling too. A target spacing chosen in advance is fit_scope: fixed and never leaks. A target derived from the cohort does: nnU-Net sets its target spacing from a percentile of the dataset fingerprint, so a resample fitted over every case carries held-out geometry into the training grid exactly as an intensity statistic would. The fingerprint's scope decides which you have, not the word "resample".

Phase 6 — Gate the manifest

python3 ${CLAUDE_SKILL_DIR}/scripts/check_preprocessing_leakage.py --manifest preprocessing_manifest.json \
    --out qc/preprocessing_leakage.json --strict

Verdicts: PREPROCESS_BEFORE_SPLIT, NORMALIZATION_LEAKAGE, PATIENT_CROSS_SPLIT (Major); AUGMENTATION_ON_EVAL, UNSPECIFIED_FIT_SCOPE, MISSING_SEED (Minor), reproduced by set arithmetic and rule on the manifest. A green gate is the precondition for handing the manifest to /model-scaffold; its split_assignment is the same patient-level split /model-assessment later re-verifies. Never report a pass without running it.

Phase 7 — Before inference on a new cohort: check the normaliser's domain

Phase 6 asks whether a transform was fit on the right scope. Before running a trained model on a cohort it was not trained on, ask whether that cohort sits in the intensity domain the trained normaliser assumes:

python3 ${CLAUDE_SKILL_DIR}/scripts/check_normalizer_domain.py \
    --profile eda/<cohort>_profile.json \
    --contract work/nnUNet_results/.../plans.json \
    --splits external_mri --out qc/normalizer_domain.json --strict

Its challenge card holds a cohort in the contract's own domain that must come back clean, an arbitrary-unit cohort that must raise a Major, and an unreadable contract that must refuse rather than pass.

Outputs and hand-off

• eda/profile.json (Phase 1), qc/dataset_profile.json (Phase 2), and the decision notes (Phase 3).
• preprocessing_manifest.json with the augmentation-appropriateness and normalisation fit-scope notes (Phases 4–5), qc/preprocessing_leakage.json (Phase 6), qc/normalizer_domain.json (Phase 7).

The manifest feeds /model-scaffold, which also reads the qc/ reports: keep them in qc/ beside the manifest (or ../qc/), or pass them with --imaging-qc. An unresolved Major there refuses the scaffold until it is fixed and re-gated or acknowledged with a stated reason (--ack-qc); Minor and Flag claims are carried into the repo's IMAGING_QC.md, and a missing report is recorded as not assessed. Re-run a gate after fixing its finding — a stale report still blocks. The manifest documents the CLAIM 2024 / TRIPOD+AI data-preprocessing items for /check-reporting; /self-review's model_development probe looks for exactly this pipeline in a finished manuscript. Regression: bash ${CLAUDE_SKILL_DIR}/scripts/check_dataset_profile_challenge/verify.sh, bash ${CLAUDE_SKILL_DIR}/scripts/check_preprocessing_leakage_challenge/verify.sh, bash ${CLAUDE_SKILL_DIR}/scripts/check_normalizer_domain_challenge/verify.sh, bash ${CLAUDE_SKILL_DIR}/tests/test_dataset_profile.sh, bash ${CLAUDE_SKILL_DIR}/tests/test_preprocessing_leakage.sh.