Physics of magnetic resonance imaging

Magnetic resonance imaging (MRI) is a widely used, non-ionizing medical imaging modality that provides exceptional soft-tissue contrast. MRI enables the non-invasive visualization of anatomical structures and physiological processes in both clinical medicine and biomedical research.

The present course focuses on the fundamental physical principles of MRI. It covers the physics of MRI signal generation, the principles of spatial encoding and image formation, the mechanisms affecting image contrast, and the mathematical foundations of image reconstruction. In addition, the course examines the key factors that influence image quality, common imaging artifacts and their underlying causes, as well as state-of-the-art techniques for accelerating MRI acquisitions.

By the end of the course, students will have a solid understanding of the physical principles underlying MRI signal acquisition, image formation and image reconstruction. They will be able to relate these principles to image quality, recognize common sources of image artifacts, and understand the fundamental concepts behind modern MRI methods used in biomedical imaging applications.

The course follows the outline below:

1. Introduction of MRI applications and basics of MRI hardware

2. Introduction to signal generation: nuclear magnetic moments, bulk magnetization, spin precession, chemical shift, resonance condition for radiofrequency (RF) excitation, RF pulse characteristics

3.  Signal generation: rotating frame of reference, Bloch equation, on resonance excitation, rotation of magnetization vector after an RF pulse

4. Signal detection: free precession, introduction to relaxation, detection principles, signal expression, free induction decay

5. Signal characteristics: RF echo, gradient echo

6. Signal localization I: slice selection, spatial encoding (frequency and phase encoding)

7. Signal localization II: k-space encoding, 1D imaging, 2D imaging, 3D imaging

8. Image contrast: introduction to image contrast, saturation recovery sequence, inversion recovery sequence, basic spin echo imaging, basic gradient echo imaging

9. Sampling and image reconstruction: sampling of k-space, basic theory of image reconstruction, infinite sampling, finite sampling

10. Reconstruction, resolution and SNR: reconstruction from FT samples, point spread function, image noise and SNR

11. Artifacts: Gibbs ringing, aliasing, chemical shift artifact, motion artifacts

12. Fast imaging acquisition techniques: turbo spin echo imaging, echo planar imaging

13. Acceleration techniques: parallel imaging, compressed sensing, machine learning

14. Optimization of MRI measurements: protocol optimization considering resolution-SNR-scan time trade-off in specific applications