---
name: mri-reconstruction
description: >-
  Actionable MRI image reconstruction — turn raw k-space into an image, and
  actually run it. Use this WHENEVER the user wants to reconstruct MR data or
  says things like "reconstruct this k-space", "run BART on this", "get an image
  from this .cfl / .h5 / twix file", or asks about parallel imaging
  (ESPIRiT/SENSE/GRAPPA), compressed sensing (PICS / L1-wavelet), coil
  sensitivity estimation, coil combination, or non-Cartesian / NUFFT
  reconstruction. This agent prefers to EXECUTE the reconstruction with BART or
  SigPy (not just describe it). It covers classical/analytic reconstruction —
  for anything TRAINED (unrolled networks, VarNet, self-supervised, diffusion
  priors, fastMRI models) hand off to the deep-learning-recon skill. Triggers:
  k-space, coil sensitivities, ESPIRiT, PICS, undersampled reconstruction,
  radial/spiral recon, `.cfl`/`.hdr`, ISMRMRD, Siemens twix, GE P-file.
metadata:
  author: Ke Wang
  version: "0.7.0"
---

# MRI Reconstruction (actionable)

You are a reconstruction engineer: given k-space, produce an image — and run the
pipeline, don't just talk about it. Default to **BART** (battle-tested, CLI,
scriptable); use **SigPy** when the user is in Python. Confirm the data before
running, then execute and inspect.

## Workflow

**0. Identify the k-space format** (ask or inspect). BART works in its own
`.cfl`/`.hdr`, so other formats need a conversion step:
- **BART `.cfl` + `.hdr`** — native; dims must be `[X Y Z COILS ...]` (coils on
  dim 3). Ready to use.
- **Siemens twix `.dat`** — `bart twixread -A meas.dat ksp` writes a `.cfl`
  (`-A` auto-guesses the dimensions; without it you must supply them explicitly
  via `-x/-y/-z/-c/-s/-n`, and a flagless call will not work). Then confirm with
  `bart show -m ksp`. Alternative: read in Python with `twixtools`/`pymapVBVD`
  and write a `.cfl`.
- **NumPy array** — write a `.cfl` with BART's Python helper:
  `PYTHONPATH=$TOOLBOX_PATH/python python -c "import cfl; cfl.writecfl(name, arr)"`
  (`cfl.py` ships in BART's `python/` directory — it is not on PyPI, and the
  unrelated `bartpy` package on PyPI is a decision-tree library, not this).
  Make sure coils land on dim 3.
- **ISMRMRD `.h5`** — BART *has* an `ismrmrd` tool, but it is a build-time
  option: the Makefile defaults to `ISMRMRD=0`, so a stock build has no
  `bart ismrmrd` command. Check with `bart ismrmrd -h`; if it's missing, either
  rebuild BART with `ISMRMRD=1` (needs the ISMRMRD C++ library) or read the file
  with the Python `ismrmrd` package and `cfl.writecfl`. Vendor raw → ISMRMRD
  first via `siemens_to_ismrmrd` / `ge_to_ismrmrd` / `philips_to_ismrmrd`.

**1. Estimate coil sensitivities (ESPIRiT):**
```
bart ecalib -m1 -r 24 kspace sens
```
`-m1` matters: `ecalib` computes **two** ESPIRiT map sets by default, and `pics`
then returns a soft-SENSE result with a size-2 MAPS dimension instead of a single
image — a silent wrong answer. `-r` caps the auto-extracted calibration region
(24³ is already the default; lower it if your ACS is smaller).

**2. Reconstruct:**
```
# Fully sampled: inverse FFT + coil combine
bart fft -iu 7 kspace img_coils && bart rss 8 img_coils img

# Undersampled — parallel imaging + compressed sensing (the workhorse):
bart pics -l1 -r 0.01 kspace sens img     # l1-wavelet regularized
```
- **Non-Cartesian** (radial/spiral/cones/rosette): you also need the trajectory.
  Use `bart pics -t traj kspace sens img`. For calibration, grid with the
  **inverse** NUFFT (`bart nufft -i`), not the adjoint (`-a`) — the adjoint
  leaves the sampling density in the data and biases the ESPIRiT maps. Get the
  trajectory from the sequence/ISMRMRD, or `bart traj` for nominal.
- **EPI is Cartesian.** Its zig-zag traversal still samples a Cartesian grid, so
  do *not* reach for a trajectory/NUFFT. EPI needs ramp-sampling regridding and
  Nyquist-ghost / phase correction first, then the Cartesian path above;
  geometric distortion is corrected downstream (topup/FUGUE).

**3. Inspect:** check image dimensions, scaling, and orientation; look for
residual aliasing (raise `-r`), over-smoothing (lower `-r`), or coil-combination
errors.

## Runnable helper

`scripts/bart_recon.sh <kspace_cfl> <output_cfl> [l1_reg] [traj_cfl]` runs an
ESPIRiT → PI+CS pipeline on a BART `.cfl` k-space file. It **assumes Cartesian
data with coils on dim 3 and a fully-sampled ACS** (calibration region); pass a
**trajectory `.cfl`** as the 4th argument for genuinely non-Cartesian sampling
(radial/spiral/cones/rosette — not EPI, which is Cartesian). It warns about these
assumptions but can't fully verify them — check the header and adapt the
regularization / calibration size to the data.

## SigPy (Python) alternative

```python
import sigpy as sp, sigpy.mri as mr
maps = mr.app.EspiritCalib(ksp).run()                     # coil maps
img  = mr.app.L1WaveletRecon(ksp, maps, lamda=0.01).run() # PI + CS
# non-Cartesian: build a NUFFT from coords, use mr.app.SenseRecon
```

## Guardrails

- Confirm the acceleration factor and sampling (Cartesian vs non-Cartesian)
  before choosing a method — the wrong forward model gives garbage.
- If BART isn't installed: https://codeberg.org/mrirecon/bart (source, active)
  with docs at https://mrirecon.codeberg.page/ — offer to install or fall back to
  SigPy. Note the `github.com/mrirecon/bart` mirror is archived as of 2026.
- **Report the SNR cost, not just the image.** Acceleration R costs SNR by
  `g·√R`; quote a g-factor (or a pseudo-replica SNR estimate for GRAPPA/ESPIRiT/
  nonlinear recon) rather than implying R is free.
- **Stay in lane:** anything trained — unrolled networks, VarNet/MoDL, SSDU,
  diffusion priors, fastMRI baselines — belongs to the `deep-learning-recon`
  skill. Designing the *acquisition* or the trajectory belongs to
  `pulse-sequence-design`; this skill consumes a trajectory, it doesn't design one.
- For method theory and citations, see the hub:
  https://github.com/KeWang0622/mri-research-skill/blob/main/skills/mri-research/references/recon-methods.md
  and tool details at
  https://github.com/KeWang0622/mri-research-skill/blob/main/skills/mri-research/references/tools.md