5.2 Magnetic Resonance Imaging (MRI)

Magnetic Resonance Imaging (MRI) is a powerful medical imaging technique that uses magnetic fields and radio waves to create detailed images of the body's internal structures. It relies on the principles of nuclear magnetic resonance, exciting hydrogen atoms in tissues to produce signals that are converted into images.

MRI offers exceptional soft tissue contrast without using ionizing radiation, making it safer than X-rays or CT scans. Various pulse sequences allow for versatile applications, from anatomical imaging to functional studies, though longer scan times and higher costs can be limiting factors.

Fundamentals of Magnetic Resonance Imaging (MRI)

Principles of MRI

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• Utilizes nuclear magnetic resonance (NMR) phenomenon
  • Atomic nuclei with spin property (hydrogen) align parallel or anti-parallel in strong magnetic field (B0)
  • Nuclei precess around magnetic field at Larmor frequency ($\omega = \gamma B0$, $\gamma$ = gyromagnetic ratio)
• Radiofrequency (RF) pulses excite nuclei
  • RF pulses at Larmor frequency cause nuclei to absorb energy and flip magnetization vector
  • Nuclei return to equilibrium state through relaxation after RF pulse turned off
• Relaxation times characterize rate of nuclei returning to equilibrium
  • T1 relaxation (spin-lattice) describes recovery of longitudinal magnetization
  • T2 relaxation (spin-spin) describes decay of transverse magnetization
  • Varying T1 and T2 times in different tissues provide basis for MRI contrast

Components of MRI scanners

• Superconducting magnet
  • Generates strong, uniform magnetic field (1.5 or 3 Tesla)
  • Aligns nuclear spins and enables NMR phenomenon
• Gradient coils
  • Create linear variations in magnetic field strength along x, y, and z axes
  • Allow spatial encoding of MR signal by altering Larmor frequency and phase of spins
• RF coils
  • Transmit RF pulses to excite nuclear spins
  • Receive MR signal emitted by relaxing spins
• Patient table
  • Supports patient and moves them into scanner bore
• Computer system
  • Controls scanner components and coordinates image acquisition process
  • Processes received MR signal to reconstruct final images

MRI Pulse Sequences and Applications

Types of MRI pulse sequences

• Spin echo (SE) sequences
  • Use 90° RF pulse followed by 180° refocusing pulse to generate echo
  • Provide good T1 and T2 weighting
  • Suitable for anatomical imaging and detecting pathologies
• Gradient echo (GRE) sequences
  • Use single RF pulse with flip angle less than 90°
  • Offer faster imaging times and better T1 weighting
  • Useful for dynamic imaging (perfusion, functional MRI)
• Inversion recovery (IR) sequences
  • Begin with 180° inversion pulse followed by 90° excitation pulse
  • Provide strong T1 weighting and excellent fluid suppression
  • Used in FLAIR sequences for brain imaging
• Echo planar imaging (EPI)
  • Acquire multiple lines of k-space data after single RF excitation
  • Enable very fast imaging times (less than 100 ms per slice)
  • Used in diffusion-weighted imaging (DWI) and functional MRI (fMRI)

MRI vs other imaging modalities

• Advantages of MRI
  • Non-ionizing radiation, safer for patients than X-ray and CT
  • Excellent soft tissue contrast for better visualization of anatomy and pathology
  • Multiplanar imaging in any plane without patient repositioning
  • Versatile sequences for various applications (DWI, fMRI, MR spectroscopy)
• Limitations of MRI
  • Longer acquisition times than X-ray and CT, potential for motion artifacts
  • Contraindications for patients with certain metallic implants, pacemakers, or claustrophobia
  • Higher costs for scanner installation, maintenance, and operation
  • Limited availability due to need for specialized facilities and trained personnel