MRI Lecture slides

Introduction to MRI

• Magnetic Resonance Imaging (MRI) is a non-invasive medical imaging technique that utilizes strong magnetic fields ($B_0$) and radiofrequency (RF) pulses to produce high-resolution images of internal structures.

• Unlike X-ray or CT scans, MRI does not use ionizing radiation, making it safer for repeated use and sensitive for soft tissue visualization.

Basic Physical Principles

• Proton Alignment: MRI primarily targets hydrogen nuclei (protons) due to their abundance in water ($H_2O$) and fat. Protons possess a property called "spin," which creates a small individual magnetic moment.

• Static Magnetic Field ($B<em>0$): When a patient is placed in the scanner, protons align either parallel or anti-parallel to the external field ($B</em>0$). A slight excess of protons aligns parallel, creating a Net Magnetization Vector ($M_0$).

• Larmor Frequency: Protons precess (wobble) around the axis of the magnetic field. The frequency of this precession is determined by the Larmor Equation:

  • $\omega = \gamma B_0$

  • where $\omega$ is the angular frequency, $\gamma$ is the gyromagnetic ratio (a constant specific to the nucleus), and $B_0$ is the magnetic field strength.

The MRI Imaging Process

• Excitation: A radiofrequency pulse is applied at the specific Larmor frequency, tipping the net magnetization into the transverse plane. This synchronizes the phases of the protons' spins.

• Relaxation Mechanisms:

  1. T1 Relaxation (Longitudinal Recovery): The time it takes for protons to realign with the longitudinal axis. It reflects the energy transfer from protons to the surrounding environment (lattice).

  2. T2 Relaxation (Transverse Decay): The time it takes for protons to lose phase coherence due to interactions between spins (spin-spin relaxation).

• Spatial Encoding: Gradient coils ($G<em>x, G</em>y, G_z$) are used to create linear variations in the magnetic field, allowing the system to determine exactly where a signal originates in 3D space.

Contrast in MRI

• Weighting: By adjusting the Repetition Time (TR) and Echo Time (TE), clinicians can emphasize different relaxation properties:

  • T1-Weighted (T1w): Highlights anatomy; CSF appears black, while fat and white matter appear brighter.

  • T2-Weighted (T2w): Highlights pathology; CSF and edema (water) appear white/bright, while white matter appears darker.

• Brain Maturation: Contrast changes significantly in pediatric imaging as myelination (fatty insulation of neurons) increases, affecting both T1 and T2 signals.

MRI Safety

• Projectile Effect: The magnet is "always on," and any ferromagnetic objects (e.g., oxygen tanks, scissors) can become dangerous projectiles.

• Medical Implants: Objects like pacemakers, cochlear implants, or certain shrapnel can be moved or damaged by the magnetic field or heated by RF pulses.

• Specific Absorption Rate (SAR): This measures the RF energy absorbed by the body. Excessive SAR can lead to internal tissue burns.

Introduction to fMRI

• BOLD Signal: Functional MRI (fMRI) measures the Blood Oxygen Level Dependent signal. It relies on the magnetic difference between oxyhemoglobin (diamagnetic) and deoxyhemoglobin (paramagnetic).

• Hemodynamic Response: When a brain region becomes active, blood flow increases to that area. The system over-supplies oxygenated blood, which decreases the concentration of deoxyhemoglobin, thereby increasing the local MRI signal.

Clinical and Research Applications

• Cognitive Mapping: fMRI identifies brain regions associated with specific tasks, such as the hippocampus for memory or Broca’s area for speech production.

• Structural Assessment: Used to detect tumors, strokes, or neurodegenerative changes (e.g., cortical thinning in Alzheimer’s).

• Clinical Utility: Informs presurgical planning to avoid damaging eloquent cortex (vocal/motor areas) during neurosurgery.