Biomedical Instrumentation Unit 15 ReviewMedical Imaging: MRI Fundamentals

Basic Principles of MRI

• MRI utilizes strong magnetic fields and radiofrequency (RF) pulses to generate detailed images of the body's internal structures
• Relies on the magnetic properties of hydrogen atoms, which are abundant in the human body (water and fat)
• Hydrogen atoms have a magnetic moment and align with the external magnetic field
• RF pulses at the Larmor frequency excite the hydrogen atoms, causing them to resonate and generate a detectable signal
• Gradients in the magnetic field allow for spatial localization of the signal
• Fourier transform converts the received signal into an image
• Provides excellent soft tissue contrast without the use of ionizing radiation

MRI Hardware Components

• Main magnet generates a strong, uniform magnetic field (typically 1.5 or 3 Tesla)
  • Superconducting magnets cooled with liquid helium are commonly used
  • Permanent and resistive magnets are less common alternatives
• Gradient coils create linear variations in the magnetic field strength along x, y, and z axes
  • Enable spatial encoding of the MR signal
  • Rapidly switched on and off during image acquisition
• RF coils transmit RF pulses to excite hydrogen atoms and receive the resulting signal
  • Volume coils (e.g., birdcage coil) provide uniform excitation and reception
  • Surface coils offer higher signal-to-noise ratio (SNR) for specific regions
• Patient table moves the patient into the center of the magnet bore
• Faraday cage (RF shielding) prevents external RF interference
• Computer system controls the scanner, processes the data, and reconstructs the images

Magnetic Fields and Gradients

• Main magnetic field (B0) aligns the magnetic moments of hydrogen atoms (protons)
• Protons precess around B0 at the Larmor frequency, which is proportional to the field strength
• Gradients superimposed on B0 create linear variations in the magnetic field
  • Slice-selection gradient determines the location and thickness of the imaging slice
  • Phase-encoding gradient encodes spatial information along one axis
  • Frequency-encoding (readout) gradient encodes spatial information along the other axis
• Gradient strength and duration determine the field of view (FOV) and spatial resolution
• Gradient switching can cause peripheral nerve stimulation and acoustic noise

Radiofrequency Pulses and Signal Generation

• RF pulses at the Larmor frequency excite protons, causing them to absorb energy and flip away from B0
• Flip angle depends on the RF pulse amplitude and duration
  • 90° pulse flips the net magnetization into the transverse plane
  • 180° pulse inverts the net magnetization
• Excited protons emit an RF signal (free induction decay) as they relax back to equilibrium
• T1 relaxation (spin-lattice) describes the recovery of longitudinal magnetization
  • Tissues with short T1 (e.g., fat) appear bright on T1-weighted images
• T2 relaxation (spin-spin) describes the decay of transverse magnetization
  • Tissues with long T2 (e.g., water) appear bright on T2-weighted images
• RF receiver coils detect the emitted signal, which is processed to form the MR image

Image Formation and Reconstruction

• Raw MR data is collected in k-space, which represents spatial frequencies
• K-space is filled by repeating the pulse sequence with different gradient encodings
• Low spatial frequencies (center of k-space) determine image contrast and general shape
• High spatial frequencies (periphery of k-space) determine fine details and edges
• Fourier transform converts k-space data into the final MR image
  • 2D Fourier transform for single-slice imaging
  • 3D Fourier transform for volumetric imaging
• Parallel imaging techniques (e.g., SENSE, GRAPPA) accelerate image acquisition by exploiting the spatial information from multiple receiver coils
• Compressed sensing enables further acceleration by exploiting image sparsity

MRI Sequences and Contrast

• Spin echo (SE) sequences use a 90° excitation pulse followed by a 180° refocusing pulse
  • Provide T1, T2, and proton density (PD) weighting depending on the repetition time (TR) and echo time (TE)
  • Robust to magnetic field inhomogeneities but relatively slow
• Gradient echo (GRE) sequences use a single RF pulse with a flip angle < 90°
  • Offer faster imaging and T2* weighting but are more sensitive to field inhomogeneities
  • Spoiled GRE (e.g., FLASH) and balanced steady-state free precession (bSSFP) are common variants
• Inversion recovery (IR) sequences (e.g., FLAIR, STIR) use a 180° inversion pulse to null the signal from specific tissues
• Diffusion-weighted imaging (DWI) measures the random motion of water molecules and is sensitive to microstructural changes
• Perfusion imaging techniques (e.g., DSC, DCE, ASL) assess blood flow and permeability
• Functional MRI (fMRI) detects changes in blood oxygenation related to neural activity

Safety Considerations in MRI

• Strong magnetic fields can attract ferromagnetic objects, causing projectile hazards
  • Thorough screening of patients and personnel for metallic implants, devices, and foreign bodies is essential
• RF heating can cause tissue damage, especially in patients with metallic implants or tattoos
  • Specific absorption rate (SAR) limits are enforced to prevent excessive heating
• Gradient switching can induce currents in conductive materials, leading to peripheral nerve stimulation and acoustic noise
  • Gradient slew rate and amplitude are limited to minimize these effects
• Cryogens (liquid helium) used in superconducting magnets pose an asphyxiation risk if released into the scanner room
• Claustrophobia and anxiety can be a concern for some patients due to the confined space of the scanner bore
• Gadolinium-based contrast agents may be associated with nephrogenic systemic fibrosis (NSF) in patients with severe renal impairment

Clinical Applications and Future Trends

• Neuroimaging: brain tumors, stroke, neurodegenerative diseases, epilepsy, and psychiatric disorders
• Musculoskeletal imaging: joint disorders, soft tissue tumors, and sports injuries
• Cardiovascular imaging: cardiac function, myocardial viability, and vascular abnormalities
• Abdominal and pelvic imaging: liver, kidney, prostate, and gynecological diseases
• Oncologic imaging: tumor detection, staging, and treatment response assessment
• Interventional MRI guides biopsies, ablations, and other minimally invasive procedures
• Ultra-high field MRI (7T and above) offers increased SNR and spatial resolution but faces technical and safety challenges
• Artificial intelligence and machine learning techniques are being developed for image reconstruction, analysis, and interpretation
• Hybrid imaging systems (e.g., PET/MRI) combine the strengths of multiple modalities for comprehensive assessment