MRI Physics
Overview
The physics underlying MRI is complex, involving a series of processes that differentiate it from other imaging modalities like X-rays, CT, and ultrasound. This overview provides a broad-strokes description of MRI, broken down into key stages:
- Preparation
- Excitation
- Spatial Encoding
- Signal Acquisition
1. Preparation
- Static Magnetic Field: The patient is placed within a static magnetic field generated by the MRI scanner magnet.
- Hydrogen Protons: Abundant in living tissues due to water content; the protons have intrinsic magnetism called 'spin'.
- Larmor Frequency: The frequency at which the spin magnetization vector precesses around the magnetic field.
- Magnetization Alignment: Protons align parallel to the magnetic field, with intensity influencing the degree of magnetization.
- Preparation Sequences: These can manipulate magnetization to affect image contrast (e.g., inversion preparation).
2. Excitation
- RF Pulse: Emitted during image acquisition, tuned to the Larmor frequency to create phase coherence.
- Spin Magnetization: RF pulse tilts spin magnetization perpendicularly to the magnetic field.
- NMR Signal: Generated via Faraday induction in a receiving coil, resulting from transverse magnetization.
- Relaxation Processes:
- T2 (Transverse Relaxation Time): Loss of coherence attenuates the NMR signal.
- T1 (Spin-Lattice Relaxation Time): Magnetization vector relaxes towards equilibrium.
- Image Contrast: Different tissues exhibit varying T1 and T2 times, aiding in soft tissue contrast.
3. Spatial Encoding
- Magnetic Field Gradients: Utilized to encode spatial information based on proton precession rates, causing acoustic noise.
- Lorentz Force: Causes coils to vibrate as gradients are toggled rapidly.
4. Signal Acquisition
- NMR Signal: Contains varying frequencies corresponding to different tissue positions, digitized and stored.
- Fourier Transform: Separates signal contributions into spatial locations, forming the final image.
MRI Exam Protocol
- Image Sets: Multiple sets acquired, protocol varies by anatomy, pathology, and equipment.
- Contrast Medium: Sometimes used for enhanced imaging, typically extends scan time.
- Exams can be ordered with or without contrast; rarely with contrast only.
- Post-Procedure: Includes patient removal from scanner and further instructions on contrast and sedation if applicable.
Additional Information
- Revisions & Contributors: Multiple revisions by various contributors ensure up-to-date information.
- References: Numerous related articles and references enhance understanding of MRI physics.
- Tags and Synonyms: Include terms like 'Physics of MRI', 'Magnetic Resonance Imaging Physics', etc.
These notes aim to provide a comprehensive overview of the MRI physics and related processes, useful as a reference for study and review.