Week 4 - MRI & fMRI

Introduction to MRI and fMRI

  • Lecturer: Dr. Melanie Rose Burke

  • Reading this week: fMRI primer

Lecture Objectives

Part One

  • Provide a history of MRI

  • Understand how MRI works

    • Recognize T1 and T2 weighted images

Part Two

  • Record neuronal activity in the brain using fMRI

    • Understand the "BOLD" signal and fMRI

  • Recognize capabilities and limitations of MRI

  • Comprehend spatial and temporal resolution of the technique

Introduction to Magnetic Resonance Imaging (MRI)

  • MRI has undergone dramatic advances.

  • Key figures:

    • Isidor Isaac Rabi: Nobel Prize in Physics 1944

    • Paul C. Lauterbur: Nobel Prize in Physiology or Medicine 2003

    • Sir Peter Mansfield

Electromagnetic Spectrum

  • Medical imaging modalities utilize electromagnetic waves:

    • MRI uses long waves

    • X-Ray, PET, CT utilize short waves

Understanding MRI

  • Basic principle: MRI makes hydrogen-1 nuclei resonate to image the properties of that resonance.

  • Different tissues (e.g., white vs grey matter) resonate differently.

Physical Basis of MRI

Components of MRI

  • Two components:

    • Induction of resonance

    • Recording of the induced resonance signal

Induction of Resonance

The Spin

  • Protons and neutrons of atomic nuclei exhibit spin.

  • Nuclei with even mass numbers (e.g., carbon-12) do not exhibit a net spin and are undetectable.

  • Hydrogen-1 has a net spin; it is prevalent due to its presence in water (H2O).

Precession in a Magnetic Field

  • Application of a magnetic field causes atomic nuclei to undergo circular motion called "precession."

  • The frequency of this precession is the Larmor frequency; analogous to gravitational pull on a spinning top.

Quantum States of Hydrogen-1

  • In a magnetic field, hydrogen-1 can be aligned in two states:

    • Spin up: Aligned with the magnetic field

    • Spin down: Aligned against the magnetic field

Net Magnetization and Magnetic Field Strength

  • Detection requires a large number of nuclei to align and create net magnetization.

  • Strong magnetic fields (1.5 to 3 Tesla) help achieve this alignment.

Flipping into the Transverse Plane

  • Application of a radiofrequency (RF) pulse flips the net magnetization from the longitudinal to the transverse plane.

  • This allows for recording the resonance signal.

Relaxation Processes

T1 and T2 Relaxation

  • T1 (longitudinal) relaxation involves recovery of magnetization along the Z axis.

  • T2 (transverse) relaxation describes the decay of the transverse magnetization; involves interactions with neighboring atoms.

  • Solid materials cannot be imaged with MRI due to rapid relaxation.

Image Acquisition

  • MRI images are formed by activating and encoding voxels using gradient magnets, allowing for spatial resolution of the brain's structure.

T1 vs. T2 Weighted Images

T1 Images

  • Used for brain structure reconstruction; fluid appears dark, white matter bright, grey matter medium.

T2 Images

  • Fluid appears bright; grey matter brighter than white matter.

Clinical Applications of Structural MRI

  • Utilized in various fields including neurology, orthopedics, cardiology, and oncology.

Safety and Risks of MRI

  • Generally considered safe; no harmful radiation exposure.

  • Special precautions for pregnancies (during organogenesis) due to possible effects of contrast agents.

Summary of MRI

  • MRI is a non-invasive technique that utilizes magnetic resonance to image internal structures, based primarily on hydrogen's magnetic properties.

  • The process involves several steps from inducing resonance to generating images that represent the anatomy and physiology of the brain.

Part Two: Recording Neuronal Activity with fMRI

  • fMRI studies brain function by detecting changes in blood flow associated with neuronal activity.

Functional Imaging with fMRI

  • Enables visualization of brain activity during task performance.

  • Measures intrinsic signals related to neuronal activity, primarily the BOLD signal (Blood-Oxygenation Level Dependent).

The BOLD Signal

Characteristics

  • Variations in blood oxygenation alter T2 or T2* times, producing measurable signals.

  • Initial dip, peak, and post-stimulus undershoot are key phases of the BOLD response.

Visual Representation

  • Signal change during specific activities can be quantitatively assessed (% change).

Limitations of fMRI

  • Expensive to perform and requires subject immobilization, which can be challenging for certain populations (e.g., children, epileptics).

  • Masked temporal dynamics due to slower hemodynamic response compared to neuronal firing.

Advantages of fMRI

  • Non-invasive and repeatable imaging allows for extended observation of brain function.

  • Provides a comprehensive view of brain areas involved in various tasks.