---
title: "diffsim"
description: "Biophysical MRI simulation engine — generates synthetic MR imaging data from tissue microstructure for training/stress-testing inverse models."
image: "https://docs.virdx.dev/img/virdx-social-card.png"
---

> Documentation Index
> Fetch the complete documentation index at: https://docs.virdx.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# diffsim

Goal: build a simulation engine that generates realistic MR imaging data
from a comprehensive biophysical representation of tissue (architecture +
parameters). This enables cheap synthetic training data for `ve2e`
(inverse) models, exploration of the space of possible MR images,
biophysical inductive biases, and MR sequence optimization.

## Status at a glance (as of the last filed report, January 2026)

- Core simulator: sequence objects (pulseq-compatible), adaptive
  time-stepping, and an ensemble Kalman inversion (EKI) procedure for
  biophysical parameter estimation/calibration are implemented; EKI has been
  validated on toy domains, not yet on real tissue/phantom data.
- Multi-coil MRI reconstruction and imaging-noise modeling were
  theoretically systematized; a novel magnetization propagator for diffusion
  MRI was derived from the Feynman path-integral formalism.
- No report was filed for February 2026 (explicit "no report filed" stub);
  no diffsim report exists for March 2026 either (no file at all — an
  undocumented gap, unlike February's explicit stub) — treat diffsim status
  as frozen at January 2026 until a newer report or `inbox/` session shows
  up.

## Priorities (per the last Q1 plan)

1. Ship a first usable simulator version + basic verification/validation/UQ
   (VVUQ, per the ASME standard).
2. Acquire and scan an ex-vivo phantom; finalize EKI parameter estimation on
   real data.
3. Run joint histology–MRI + simulator sensitivity-analysis experiments
   (coordination point with the `histo` workstream on domain-representation
   labeling requirements).

## Source material

- [sources/monthly-reports/](https://github.com/virdx/knowledge/blob/main/sources/monthly-reports/index.md) —
  filed as `<MM>-2026_diffsim.md`. Only January 2026 has real content; Feb
  2026 is an explicit "no report filed" stub; no file at all exists for Mar
  2026 onward (undocumented gap, not a stub).

No dedicated `projects/` or `sops/` subdirectories exist yet for this
workstream — add them here as diffsim work gets synthesized from `inbox/`.

Source: https://docs.virdx.dev/knowledge/wiki/workstreams/diffsim/index.mdx