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Imaging Evidence Path

Chapter 8 — MRI and multimodality foundations

This chapter · ~3 h total across 8 lessons · The minimum mental model for modern MRI papers · Energy: low to medium · Setup: optional 3D Slicer for the contrast lesson

This is not an MRI physics textbook. It is the minimum mental model an experienced technical reader needs to understand modern MRI papers — knee/shoulder/ elbow MRI, TotalSegmentator MRI, OmniMRI, synthetic T1/STIR — without treating MRI as “CT with different colours”. The single most important sentence in the chapter: MRI intensity is not a physical scale.

The thread this chapter pulls

CT maps to Hounsfield Units — an approximately physical, scanner-comparable scale where water is 0 (Ch. 1). MRI does not. MRI voxel intensity is relative, sequence/protocol-dependent, and varies across field strength, vendor, coil, and pulse-sequence parameters. Almost everything that is hard about MRI radiomics, MRI harmonisation, and synthetic MRI follows from that one fact. This chapter builds the signal model that explains it, then traces the consequences for diffusion, k-space, domain shift, and synthesis.

Lessons

  1. Why MRI intensity is fundamentally unlike CT HU — the central contrast.
  2. What happens to hydrogen after the RF pulse — B0, longitudinal/transverse, T1/T2.
  3. How TR and TE make tissue look different — T1/T2/PD; concrete sequence comparisons.
  4. Why STIR suppresses fat — inversion recovery, TI.
  5. What diffusion and ADC actually measure — b-values, the quantitative-ish MRI map.
  6. What k-space is (without an MRI physics degree) — spatial frequency, Fourier reconstruction, sampling.
  7. Why MRI is especially vulnerable to domain shift — vendor, field, coil, protocol, reconstruction.
  8. When synthetic/multimodal MRI asks the model to invent information — connect to Ch. 6.

The sequence-family table, the modality-orientation table, and the further reading are in the MRI reference.

A curated video backbone

For the visually difficult parts (relaxation, contrast, k-space), the chapter points to the Stanford Medicine Body MRI Research Group “MRI Physics Education Materials” (VERIFIED, university source, free for education) — short intuitive overviews originally for radiology residents, organised by topic. Each lesson that uses it names the relevant section on that page; watch the section rather than chasing a specific timestamp.

How this chapter connects

  • It assumes Chapter 1 (HU as a contrast point) and reuses all of Chapter 2 (MRI still lives in physical patient space).
  • It feeds Chapter 6’s synthesis discussion and Chapter 3’s radiomics-fragility discussion.

If you only have an hour, lessons 1, 2 and 6 carry the core: intensity is not a scale, the relaxation mental model, and what k-space actually is.