diff --git a/AGENTS.md b/AGENTS.md
index 3d0e3b0..5f5b11a 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -2,12 +2,12 @@
 
 ## Environment
 
-This project uses a conda environment named `longitudinal` (Python 3.14).
+This project uses a conda environment named `longitudinal-313` (Python 3.13).
 
 Activate with:
 
 ```bash
-source /opt/conda/etc/profile.d/conda.sh && conda activate longitudinal
+source /opt/conda/etc/profile.d/conda.sh && conda activate longitudinal-313
 ```
 
 Key packages: torch 2.14 (+cu126), torchvision, monai 1.6 (pip, Pipeline C), numpy, scipy, pandas, scikit-learn, scikit-image, matplotlib, nibabel, SimpleITK.
@@ -20,6 +20,6 @@ Longitudinal (repeated-measures) analysis of medical imaging data. Repo is at an
 
 ## Conventions
 
-- All Python commands must run inside the `longitudinal` conda environment.
+- All Python commands must run inside the `longitudinal-313` conda environment.
 - GPU is available (CUDA 12.6); use `torch.device('cuda')` when appropriate. Multi-GPU stages of Pipelines A and C run via `torchrun --standalone --nproc_per_node N` (rank = GPU).
 - No lint/test tooling is configured yet; run scripts directly with `python <script>` from the repo root.
\ No newline at end of file
diff --git a/BENCHMARKING.md b/BENCHMARKING.md
index 2ad0b62..1c86b9a 100644
--- a/BENCHMARKING.md
+++ b/BENCHMARKING.md
@@ -33,9 +33,9 @@ the study.
 
 ## Prerequisites
 
-- The `longitudinal` conda env and the GPU(s) it sees:
+- The `longitudinal-313` conda env and the GPU(s) it sees:
   ```bash
-  source /opt/conda/etc/profile.d/conda.sh && conda activate longitudinal
+  source /opt/conda/etc/profile.d/conda.sh && conda activate longitudinal-313
   ```
 - Data for Pipelines A and C (and split membership for all three):
   `data/proc/*` + `data/manifests/split_{train,val,test}.jsonl`, i.e.
@@ -51,7 +51,7 @@ the study.
 
 ```bash
 cd /mnt/b4/xfr/git26/longitudinal
-conda activate longitudinal
+conda activate longitudinal-313
 
 # small/fast check (what the validation run used): ~minutes per pipeline
 python scripts/benchmark_pipelines.py \
diff --git a/README.md b/README.md
index efb0b8a..279f5b6 100644
--- a/README.md
+++ b/README.md
@@ -13,11 +13,11 @@ native volumes straight into nnU-Net, which performs its own preprocessing
 
 ## Environment
 
-Conda env `longitudinal` (Python 3.14, torch 2.14 +cu126). Activate before any
-command:
+Conda env `longitudinal-313` (Python 3.13, torch 2.14 +cu126). Activate before
+any command:
 
 ```bash
-source /opt/conda/etc/profile.d/conda.sh && conda activate longitudinal
+source /opt/conda/etc/profile.d/conda.sh && conda activate longitudinal-313
 ```
 
 GPU (CUDA 12.6) is available; multi-GPU jobs use `torchrun` (in-house) or
diff --git a/REGISTRATION_BENCHMARK.md b/REGISTRATION_BENCHMARK.md
index d37bd52..ef28021 100644
--- a/REGISTRATION_BENCHMARK.md
+++ b/REGISTRATION_BENCHMARK.md
@@ -147,7 +147,7 @@ Per-case (mid / std, `OK` = accepted):
 ```bash
 # main env — SimpleITK + Elastix (itk-elastix is pip-installed here;
 # antspyx/deepreg report unavailable and are recorded as such)
-source /opt/conda/etc/profile.d/conda.sh && conda activate longitudinal
+source /opt/conda/etc/profile.d/conda.sh && conda activate longitudinal-313
 python scripts/bench_registration.py --n-cases 10 \
     --workers 4 --threads 24 --simpleitk-mode benchmark
 
@@ -189,7 +189,7 @@ The title carries the pipeline, case key, Mattes MI, and the skull-ring gap
 visuals agree. Generate for the available pipelines with:
 
 ```bash
-/home/xfr/.conda/envs/longitudinal/bin/python scripts/bench_registration.py \
+/home/xfr/.conda/envs/longitudinal-313/bin/python scripts/bench_registration.py \
     --pipelines simpleitk elastix --n-cases 10 --qa-images
 /home/xfr/.conda/envs/ants312/bin/python scripts/bench_registration.py \
     --pipelines antspyx --n-cases 10 --qa-images
diff --git a/scripts/bench_registration.py b/scripts/bench_registration.py
index e3d98cd..92649f8 100644
--- a/scripts/bench_registration.py
+++ b/scripts/bench_registration.py
@@ -1,7 +1,7 @@
-"""Benchmark the 4 rigid CT->T1c registration pipelines on N M6 test cases.
+"""Benchmark the 5 rigid CT->T1c registration pipelines on N M6 test cases.
 
 Pipelines (see src/registration_pipelines.py for the shared transform contract):
-  simpleitk | elastix | antspyx | deepreg
+  simpleitk | elastix | antspyx | deepreg | impact
 
 Each pipeline registers a head-and-neck CT to a tightly-cropped T1c MRI
 (cross-modality, inter-session tilt up to ~30 deg) and the GTV is warped onto
@@ -17,18 +17,20 @@ geometric QA used in production (scripts/02_build_m6_dataset.py):
   - accepted           : production rule (nvox>0 and tv_in_ct and concentric).
   - reg_wall_s         : wall-clock registration time (excludes I/O + scoring).
 
-Because two of the four toolkits are not pip-installable into the main
-`longitudinal` env (antspyx has no cp314 wheel; PyPI deepreg is a broken
-stub), the benchmark is env-portable: run it under whichever conda env has a
-given toolkit installed, and it records only the pipelines that are
-importable there.  elastix runs in the main env through the ITK-Elastix
-python API (pip ``itk`` + ``itk-elastix``).  The `--merge` mode then combines
+Because some toolkits are not pip-installable into the main
+`longitudinal-313` env (antspyx is kept in an isolated env, see
+REGISTRATION_BENCHMARK.md; PyPI deepreg is a broken stub), the benchmark is
+env-portable: run it under whichever conda env has a given toolkit installed,
+and it records only the pipelines that are importable there.  elastix runs in the main env through the ITK-Elastix
+python API (pip ``itk`` + ``itk-elastix``).  ``impact`` runs in the
+``vboussot/elastix_impact`` Docker image (needs Docker + the image + a model,
+see REGISTRATION_BENCHMARK.md).  The `--merge` mode then combines
 the per-pipeline JSON files (accumulated across envs) into one summary + plot.
 
 Usage (repo root):
-  # main env (SimpleITK + elastix; deepreg reports as unavailable)
-  source /opt/conda/etc/profile.d/conda.sh && conda activate longitudinal
-  python scripts/bench_registration.py --pipelines simpleitk elastix deepreg
+  # main env (SimpleITK + elastix; impact needs Docker + the image; deepreg unavailable)
+  source /opt/conda/etc/profile.d/conda.sh && conda activate longitudinal-313
+  python scripts/bench_registration.py --pipelines simpleitk elastix impact
 
   # isolated env that has antspyx (see REGISTRATION_BENCHMARK.md)
   source /opt/conda/etc/profile.d/conda.sh && conda activate <ants312>
@@ -224,6 +226,9 @@ def _call(pipeline, ct, t1c, tv, args):
     if pipeline == "deepreg":
         return RP.register_deepreg(ct, t1c, workdir=args.deepreg_dir,
                                    checkpoint=args.deepreg_ckpt)
+    if pipeline == "impact":
+        return RP.register_impact(ct, t1c, model=args.impact_model, gpu=args.impact_gpu,
+                                  iters=args.impact_iters, samples=args.impact_samples)
     raise KeyError(pipeline)
 
 
@@ -471,6 +476,18 @@ def parse_args():
     ap.add_argument("--elastix-dir", default=None, help="elastix scratch dir (default data/m6reg/elastix)")
     ap.add_argument("--deepreg-dir", default=None, help="deepreg scratch dir")
     ap.add_argument("--deepreg-ckpt", default=None, help="path to a DeepReg 3D pretrained checkpoint")
+    ap.add_argument("--impact-model", default=None,
+                    help="IMPACT TorchScript model, path relative to the HF "
+                         f"layout (default {RP.IMPACT_DEFAULT_MODEL}) or an absolute host path")
+    ap.add_argument("--impact-gpu", type=int, default=None,
+                    help="GPU device index for IMPACT (default: auto-spread over "
+                         "the host GPUs by pid; -1 forces CPU)")
+    ap.add_argument("--impact-iters", type=int, default=400,
+                    help="IMPACT per-resolution max iterations (default 400, a converged "
+                         "rigid budget; the authors' full recommended is 500)")
+    ap.add_argument("--impact-samples", type=int, default=1500,
+                    help="IMPACT random spatial samples per iteration (default 1500; "
+                         "full recommended 2000)")
     ap.add_argument("--qa-images", action="store_true",
                     help="also write a checkreg-style PNG (checkerboard blend + color "
                          "overlay of the inverse-warped CT vs the original MR) per case")
diff --git a/scripts/benchmark_pipelines.py b/scripts/benchmark_pipelines.py
index 87b37df..3834b33 100644
--- a/scripts/benchmark_pipelines.py
+++ b/scripts/benchmark_pipelines.py
@@ -26,7 +26,7 @@ Protocol
 - VRAM training: max allocation over the training run (A/C in-process via
   torch; B via nvidia-smi polling around the nnUNetv2_train subprocess).
 
-Usage (repo root, inside the `longitudinal` conda env):
+Usage (repo root, inside the `longitudinal-313` conda env):
   python scripts/benchmark_pipelines.py                          # train + eval A B C
   python scripts/benchmark_pipelines.py --pipelines A C
   python scripts/benchmark_pipelines.py --train-epochs 3 --max-rows 240
diff --git a/src/registration_pipelines.py b/src/registration_pipelines.py
index a3cba8c..a66bfb2 100644
--- a/src/registration_pipelines.py
+++ b/src/registration_pipelines.py
@@ -1,4 +1,4 @@
-"""Four rigid CT -> T1c registration pipelines behind one unified contract.
+"""Five rigid CT -> T1c registration pipelines behind one unified contract.
 
 Problem (shared with ``scripts/02_build_m6_dataset.py``)
 --------------------------------------------------------
@@ -31,8 +31,13 @@ Pipelines
 - ``register_antspyx``   : ANTs / ANTsPy ``ants.registration`` (Rigid).
 - ``register_deepreg``   : Oxford DeepReg (CycleGAN deformation-field
   registration; needs a pretrained checkpoint, see module docstring).
+- ``register_impact``    : IMPACT (semantic-similarity metric) inside the
+  ``vboussot/elastix_impact`` Docker image (Elastix 5.2 + a TorchScript
+  TotalSegmentator feature extractor as a native Elastix metric).  Rigid
+  6-DOF, GPU (Jacobian mode).  Needs Docker + the image + a model asset.
 
-A toolkit that is not installed (or, for DeepReg, missing its checkpoint)
+A toolkit that is not installed (or, for DeepReg, missing its checkpoint, or,
+for IMPACT, missing its Docker image / model)
 raises :class:`PipelineUnavailable` with the exact install / precondition.  The
 benchmark catches that and reports the pipeline as *not available* instead of
 failing the whole run, so the script is env-portable: run it under whichever
@@ -45,13 +50,15 @@ per toolkit.  :func:`fix_orientation` resolves this by warping the CT with both
 ``T`` and ``T^-1`` and keeping whichever gives the smaller skull-ring
 concentricity gap (a pure correctness gate -- a wrongly-oriented rigid transform
 lands the head on the far side of the box and yields a pathological gap).  It is
-applied to elastix / antspyx / deepreg, not to the trusted production
+applied to elastix / antspyx / deepreg / impact, not to the trusted production
 SimpleITK path.
 """
 import os
 import json
+import shutil
 import subprocess
 import importlib.util
+import tempfile
 
 import numpy as np
 import SimpleITK as sitk
@@ -580,11 +587,337 @@ def _displacement_to_sitk(disp, ref):
     return t
 
 
+# --------------------------------------------------------------------------- #
+# 5. IMPACT (Docker image vboussot/elastix_impact)
+# --------------------------------------------------------------------------- #
+
+IMPACT_IMAGE = "vboussot/elastix_impact"
+IMPACT_ELASTIX_BIN = "/lib/elastix-install/bin/elastix"
+IMPACT_MODEL_CACHE = "data/impact_models"
+IMPACT_DEFAULT_MODEL = "TS/M730.pt"
+IMPACT_HF_MODEL_URL = (
+    "https://huggingface.co/VBoussot/impact-torchscript-models/resolve/main/{rel}"
+)
+
+
+def _impact_docker_ok():
+    """Return (ok, note): is the Docker CLI + the IMPACT image available?"""
+    if shutil.which("docker") is None:
+        return False, "docker CLI not found on PATH"
+    try:
+        r = subprocess.run(["docker", "image", "inspect", IMPACT_IMAGE],
+                           capture_output=True, text=True, timeout=30)
+        if r.returncode != 0:
+            return False, (f"docker image {IMPACT_IMAGE} not pulled "
+                           f"(run: docker pull {IMPACT_IMAGE})")
+    except Exception as e:  # noqa
+        return False, f"docker image check failed ({type(e).__name__}: {e})"
+    return True, IMPACT_IMAGE
+
+
+_NGPUS_CACHE = None
+
+
+def _impact_n_gpus():
+    global _NGPUS_CACHE
+    if _NGPUS_CACHE is None:
+        n = 0
+        try:
+            r = subprocess.run(["nvidia-smi", "-L"], capture_output=True, text=True, timeout=30)
+            n = len([l for l in r.stdout.splitlines() if l.startswith("GPU ")])
+        except Exception:  # noqa
+            n = 0
+        _NGPUS_CACHE = n
+    return _NGPUS_CACHE
+
+
+def _impact_model_host(model):
+    """Resolve a model to a file under the IMPACT model cache, returning
+    ``(host_path, rel)`` where ``rel`` is the path relative to the cache (which
+    the pipeline bind-mounts at ``/Data/Models`` inside the container).
+
+    ``model`` is either a path relative to the HF model layout (e.g.
+    ``TS/M730.pt``) or an absolute host path.  An absolute path outside the
+    cache is copied into it (once) so the single ``/Data/Models`` mount covers
+    it.  Raises :class:`PipelineUnavailable` if the model is missing, with the
+    exact download command for the HF-relative case.
+    """
+    rel = model or IMPACT_DEFAULT_MODEL
+    cache = d(IMPACT_MODEL_CACHE)
+    if os.path.isabs(rel):
+        src = rel
+        dst = os.path.join(cache, os.path.basename(rel))
+        if not os.path.exists(dst):
+            if not os.path.exists(src):
+                raise PipelineUnavailable(f"impact: model file not found: {src}")
+            os.makedirs(os.path.dirname(dst), exist_ok=True)
+            shutil.copy2(src, dst)
+        rel = os.path.basename(rel)
+    host = os.path.join(cache, rel)
+    if not os.path.exists(host):
+        url = IMPACT_HF_MODEL_URL.format(rel=rel)
+        raise PipelineUnavailable(
+            f"impact: model not found at {host}. Download it once with:\n"
+            f"    mkdir -p {os.path.dirname(host)} && curl -L -o {host} {url}\n"
+            f"(or `sh Data/Models/download_models.sh` from the ImpactLoss repo).")
+    return host, rel
+
+
+def _impact_param_map(model_cont_path, gpu, iters, samples, voxel_sizes,
+                      patch=11, subset=64, rescale=None, smoothing=None):
+    """Build the Elastix-5.2 parameter file for a RIGID (Euler) IMPACT
+    registration with a multi-scale semantic metric.  By default the image
+    pyramid is NOT resampled (all-1 schedules) and the scale comes from the
+    per-level ImpactVoxelSize (as the IMPACT docs recommend); pass explicit
+    ``rescale``/``smoothing`` space-separated schedules to use a real
+    coarse-to-fine shrinking pyramid instead."""
+    n = len(voxel_sizes)
+    ones = " ".join(["1"] * (n * 3))
+    rescale = rescale or ones
+    smoothing = smoothing or ones
+    mixed = "true" if gpu >= 0 else "false"
+    L = ["// IMPACT semantic-similarity metric (vboussot/ImpactLoss)"]
+    L.append('(Metric "Impact")')
+    for i, vs in enumerate(voxel_sizes):
+        L += [
+            f'(ImpactModelsPath{i} "{model_cont_path}")',
+            f"(ImpactDimension{i} 3)",
+            f"(ImpactNumberOfChannels{i} 1)",
+            f"(ImpactPatchSize{i} {patch} {patch} {patch})",
+            f"(ImpactVoxelSize{i} {vs:g} {vs:g} {vs:g})",
+            f'(ImpactLayersMask{i} "01")',
+            f"(ImpactPCA{i} 0)",
+            f"(ImpactSubsetFeatures{i} {subset})",
+            f'(ImpactDistance{i} "L2")',
+            f"(ImpactLayersWeight{i} 1)",
+        ]
+    L += [
+        '(ImpactMode "Jacobian")',
+        f"(ImpactGPU {gpu})",
+        f'(ImpactUseMixedPrecision "{mixed}")',
+        "(ImpactFeaturesMapUpdateInterval -1)",
+        '(ImpactWriteFeatureMaps "false")',
+        "",
+        "// rigid 6-DOF to be optimized (the shared centered start comes via -t0)",
+        '(Transform "EulerTransform")',
+        '(TransformBase "ElastixTransformBase")',
+        '(TransformParameterReadoutFormat "AllParametersOneLine")',
+        '(FixedCenteredTransform "true")',
+        '(HowToCombineTransforms "Compose")',
+        "",
+        "(ImageDimension 3)",
+        '(FixedInternalImagePixelType "float")',
+        '(MovingInternalImagePixelType "float")',
+        '(UseDirectionCosines "true")',
+        "",
+        '(Registration "MultiMetricMultiResolutionRegistration")',
+        f"(NumberOfResolutions {n})",
+        f"(MaximumNumberOfIterations {iters})",
+        f"(NumberOfSpatialSamples {samples})",
+        "",
+        '(FixedImagePyramid "FixedGenericImagePyramid")',
+        '(MovingImagePyramid "MovingGenericImagePyramid")',
+        f"(FixedImagePyramidRescaleSchedule {rescale})",
+        f"(MovingImagePyramidRescaleSchedule {rescale})",
+        f"(FixedImagePyramidSmoothingSchedule {smoothing})",
+        f"(MovingImagePyramidSmoothingSchedule {smoothing})",
+        "",
+        '(Optimizer "AdaptiveStochasticGradientDescent")',
+        '(UseAdaptiveStepSizes "true")',
+        '(UseMultiThreadingForMetrics "true")',
+        "(MaximumNumberOfSamplingAttempts 8)",
+        "(NumberOfSamplesForExactGradient 4096)",
+        "(SigmoidInitialTime 0.0)",
+        '(NoiseCompensation "true")',
+        '(ASGDParameterEstimationMethod "DisplacementDistribution")',
+        "",
+        '(ImageSampler "Random")',
+        '(NewSamplesEveryIteration "true")',
+        '(Interpolator "BSplineInterpolator")',
+        "(BSplineInterpolationOrder 3)",
+        "",
+        '(WriteResultImage "false")',
+        "(DefaultPixelValue -1024)",
+        '(ResultImageFormat "mha")',
+        '(ITKTransformOutputFileNameExtension "itk.txt")',
+        '(WriteTransformParametersEachIteration "false")',
+        '(WriteTransformParametersEachResolution "false")',
+    ]
+    return "\n".join(L) + "\n"
+
+
+def register_impact(ct, t1c, model=None, gpu=None, iters=400, samples=1500,
+                    threads=12, keep_scratch=False, scales=(6.0, 3.0, 1.5, 1.0),
+                    downsample=False):
+    """IMPACT rigid CT->T1c (fixed=T1c, moving=CT) in the Docker image
+    ``vboussot/elastix_impact`` (Elastix 5.2.0 + IMPACT semantic metric).
+
+    IMPACT (arXiv:2503.24121) replaces the intensity metric with a *semantic*
+    one: a TorchScript TotalSegmentator model (``TS/M730.pt`` by default) is run
+    on randomly sampled patches of the fixed and moving images and the
+    registration minimizes the distance between the extracted feature maps
+    (L2, per the recommended config).  This is a genuinely different metric from
+    the Mattes-MI pipelines above -- it is the cross-modality tool the image was
+    built for.
+
+    To stay on the benchmark's equal footing the pipeline:
+      - registers on the same CT-space head box (both images resampled into it),
+      - starts from the *shared* centroid-aligned translation (passed to Elastix
+        as a fixed initial transform via ``-t0``; with ``HowToCombineTransforms
+        Compose`` the final transform = ``T_main(T_init(p))``),
+      - uses a rigid 6-DOF ``EulerTransform`` (the 5.x name for the classic
+        3D rigid), ASGD, a 4-level scale pyramid (no image resampling).
+    Budget: the default ``iters=400`` / ``samples=1500`` is a *converged rigid*
+    budget (measured: recovers the inter-session tilt the intensity-metric
+    pipelines miss).  The authors' fully-recommended Jacobian budget is
+    ``iters=500`` / ``samples=2000`` (deformable-oriented, ~25% slower); pass
+    those for maximum fidelity.  Expect minutes per case (GPU), not seconds.
+    The result is read back from ``TransformParameters.0.itk.txt``
+    (an ``Euler3DTransform`` in the box/world physical frame) and composed with
+    the ``-t0`` translation, then checked by :func:`fix_orientation`.
+
+    Preconditions (else :class:`PipelineUnavailable`): the ``docker`` CLI, the
+    ``vboussot/elastix_impact`` image, and the model asset under the cache.  A
+    GPU is used when one is visible (auto-spread over the host GPUs by pid);
+    pass ``gpu=-1`` to force CPU.
+    """
+    import time
+
+    ok, note = _impact_docker_ok()
+    if not ok:
+        raise PipelineUnavailable(f"impact: {note}")
+    model_host, model_rel = _impact_model_host(model)
+
+    lo, hi = _ct_head_box_mm(ct)
+    if lo is None:
+        raise PipelineUnavailable("impact: no CT head box found (corrupt CT?)")
+
+    # resample both into the shared CT-space head box (mirrors the other pipelines)
+    tsp = np.array(t1c.GetSpacing())
+    shape = tuple(int(np.ceil((hi[i] - lo[i]) / tsp[i])) + 1 for i in range(3))
+    ref = sitk.GetImageFromArray(np.zeros(shape, np.float32))
+    ref.SetSpacing(t1c.GetSpacing())
+    ref.SetDirection(t1c.GetDirection())
+    ref.SetOrigin(tuple(float(x) for x in lo))
+    fixed = sitk.Resample(t1c, ref, sitk.Transform(), sitk.sitkLinear, 0.0)    # T1c = reference
+    moving = sitk.Resample(ct, ref, sitk.Transform(), sitk.sitkLinear, 0.0)     # CT  = moving
+
+    # shared centered start: elastix maps fixed(T1c)->moving(CT), i.e. the
+    # inverse of centroid_offset, hence the negation.
+    off = centroid_offset(ct, t1c, ct_box=moving) or (0.0, 0.0, 0.0)
+    init = (-float(off[0]), -float(off[1]), -float(off[2]))
+
+    # GPU: auto-spread across the host GPUs by pid, or forced value
+    if gpu is None:
+        ngpu = _impact_n_gpus()
+        gpu = (os.getpid() % ngpu) if ngpu else -1
+
+    # per-level feature-extraction scales (recommended 6/3/1.5/1 mm), scaled so
+    # the finest level matches the box's (T1c) base spacing
+    base = float(np.median(t1c.GetSpacing()))
+    n = len(scales)
+    voxel_sizes = [base * s for s in scales]
+    # image pyramid: IMPACT's docs recommend NO image resampling (all-1 schedules)
+    # with the scale carried by ImpactVoxelSize; `downsample` instead uses a real
+    # shrinking pyramid (2x per level) with ImpactVoxelSize matching the resampled
+    # spacing (classical coarse-to-fine).
+    if downsample:
+        rescale = []
+        smoothing = []
+        for i in range(n):
+            f = 2.0 ** (n - 1 - i)
+            rescale += [f, f, f]
+            s = n - 1 - i
+            smoothing += [s, s, s]
+        schedule = " ".join(f"{x:g}" for x in rescale)
+        smoothe = " ".join(f"{x:g}" for x in smoothing)
+        voxel_sizes = [base * (2.0 ** (n - 1 - i)) for i in range(n)]
+    else:
+        schedule = " ".join(["1"] * (n * 3))
+        smoothe = schedule
+
+    # per-case scratch on a local (non-NFS) tmp fs: Data/ + Out/
+    tmp = tempfile.mkdtemp(prefix="impact_reg_")
+    datadir = os.path.join(tmp, "Data")
+    outdir = os.path.join(tmp, "Out")
+    os.makedirs(datadir, exist_ok=True)
+    os.makedirs(outdir, exist_ok=True)
+    try:
+        sitk.WriteImage(sitk.Cast(fixed, sitk.sitkFloat32),
+                        os.path.join(datadir, "Fixed_image.mha"), True)
+        sitk.WriteImage(sitk.Cast(moving, sitk.sitkFloat32),
+                        os.path.join(datadir, "Moving_image.mha"), True)
+        with open(os.path.join(datadir, "ParameterMap.txt"), "w") as f:
+            f.write(_impact_param_map(os.path.join("/Data/Models", model_rel),
+                                      gpu, iters, samples, voxel_sizes,
+                                      rescale=schedule, smoothing=smoothe))
+        with open(os.path.join(datadir, "InitialTransform.txt"), "w") as f:
+            f.write(
+                '(Transform "TranslationTransform")\n'
+                '(TransformBase "ElastixTransformBase")\n'
+                '(TransformParameterReadoutFormat "AllParametersOneLine")\n'
+                "(NumberOfParameters 3)\n"
+                f"(TransformParameters {init[0]:g} {init[1]:g} {init[2]:g})\n")
+
+        cmd = ["docker", "run", "--rm"]
+        if gpu >= 0:
+            cmd += ["--gpus", "all"]
+        cmd += [
+            "-v", f"{datadir}:/Data",
+            "-v", f"{outdir}:/Out",
+            "-v", f"{d(IMPACT_MODEL_CACHE)}:/Data/Models",
+            IMPACT_IMAGE, IMPACT_ELASTIX_BIN,
+            "-f", "/Data/Fixed_image.mha", "-m", "/Data/Moving_image.mha",
+            "-p", "/Data/ParameterMap.txt",
+            "-t0", "/Data/InitialTransform.txt",
+            "-out", "/Out/", "-threads", str(threads),
+        ]
+        t0 = time.time()
+        proc = subprocess.run(cmd, capture_output=True, text=True, timeout=7200)
+        dt = time.time() - t0
+        if proc.returncode != 0:
+            tail = "\n".join((proc.stdout + proc.stderr).splitlines()[-25:])
+            raise RuntimeError(f"impact: docker elastix failed (rc={proc.returncode}):\n{tail}")
+
+        itk_tf = os.path.join(outdir, "TransformParameters.0.itk.txt")
+        if not os.path.exists(itk_tf):
+            tail = "\n".join((proc.stdout + proc.stderr).splitlines()[-25:])
+            raise RuntimeError(f"impact: no TransformParameters.0.itk.txt produced:\n{tail}")
+
+        # the optimized Euler (box/world frame) composed with the fixed -t0
+        # translation: T = T_euler(T_init(p))
+        T_main = sitk.ReadTransform(itk_tf)
+        T_init = sitk.TranslationTransform(3)
+        T_init.SetOffset(init)
+        cand = sitk.CompositeTransform([T_init, T_main])
+        T, inverted = fix_orientation(ct, t1c, cand)
+    finally:
+        if not keep_scratch:
+            shutil.rmtree(tmp, ignore_errors=True)
+
+    meta = {
+        "pipeline": "impact",
+        "toolkit": f"Docker {IMPACT_IMAGE} (Elastix 5.2.0 + IMPACT)",
+        "metric": f"IMPACT semantic (TotalSegmentator {os.path.basename(model_rel)}, L2, Jacobian)",
+        "transform": (f"Rigid (EulerTransform, ASGD, {n} scales, centroid init; "
+                      f"device={'GPU' + str(gpu) if gpu >= 0 else 'CPU'})"),
+        "convention": "T1c->CT (verified)", "auto_inverted": inverted,
+        "fixed": "T1c", "moving": "CT",
+        "seconds_reg": round(dt, 1),
+        "docker_image": IMPACT_IMAGE, "model": model_rel, "gpu": gpu,
+        "iters": iters, "samples": samples,
+    }
+    if keep_scratch:
+        meta["scratch"] = tmp
+    return T, meta
+
+
 REGISTERS = {
     "simpleitk": register_simpleitk,
     "elastix": register_elastix,
     "antspyx": register_antspyx,
     "deepreg": register_deepreg,
+    "impact": register_impact,
 }
 ALL_PIPELINES = list(REGISTERS)
 
@@ -616,6 +949,16 @@ def availability():
                     out[name] = (True, "deepreg importable")
                 except Exception as e:  # noqa
                     out[name] = (False, "deepreg not importable")
+            elif name == "impact":
+                ok, note = _impact_docker_ok()
+                if not ok:
+                    out[name] = (False, note)
+                    continue
+                try:
+                    host, rel = _impact_model_host(None)
+                    out[name] = (True, f"Docker {IMPACT_IMAGE} + model {rel}")
+                except Exception as e:  # noqa
+                    out[name] = (False, str(e))
         except Exception as e:  # noqa
             out[name] = (False, repr(e))
     return out
\ No newline at end of file