lecture notes hemispheres

Key Elements for MRI
- Strength: Refers to the intensity of the magnetic field.
- A stronger magnetic field amplifies spin effects.
- Higher field strength increases the signal-to-noise ratio, facilitating high resolution images.
- Example:
  - 3 Tesla scanner produces an image with a spatial resolution of 1 cubic mm.
  - 7 Tesla scanner reduces spatial resolution to 0.5 cubic mm.
- Homogeneity: Refers to the uniformity of the magnetic field.
- A more homogeneous field allows precise localization of gradient coil effects.
- Gradient coils create predictable distortions; thus, understanding the static magnetic field's changes is crucial.

The MRI magnet generates a strong magnetic field, but does not create the magnetic resonance signal used for imaging.
- The signal is produced by Radio Frequency (RF) Coils that send RF pulses.
- Transmitter Coils:
- Send RF waves to the body, knocking protons out of alignment and causing them to spin at the resonant frequency of the atomic nuclei within the magnetic field.
- Receiver Coils:
- Detect the signal from protons returning to alignment, contributing to image formation.
- Each atomic nucleus resonates at a different frequency; e.g., hydrogen resonates at 42,500,000 Hz.
Specialized coils are used for different body parts that conform to their contour for closer proximity during scanning.
Essential components of MRI setup include a powerful computer and patient table for positioning.

The static magnetic field is always active, but the RF waves are pulsed in a standardized sequence.
Hydrogen atoms, primarily found in body water, originally arrange randomly.
In a strong magnetic field, they align either up or down, with some more likely to orient upwards enhancing magnetic field strength.
The RF coil creates disturbances, exciting hydrogen protons and affecting their alignment.
Pulses:
- 180-degree pulse: Flips protons in the opposite direction.
- 90-degree pulse: Flips protons perpendicular to the initial direction.
When pulsing stops, protons return to equilibrium, emitting energy that is detected to form the raw MR signal, akin to weighing and measuring energy absorption.

In fMRI, participants perform tasks while scanned, such as a finger-tapping sequence.
Active Trials: Prompted tapping sequence.
Control Trials: No specific action commanded (i.e., relax fingers).
Typical brain areas activated include:
- Primary Motor Cortex: Activates contralaterally to movement.
- Primary Somatosensory Cortex: Responds to touch sensation during the task.
- Visual Cortex: Activates equally during both active and control conditions due to visual prompts.
fMRI measures changes in the BOLD signal (Blood Oxygen Level Dependent) using MR physics discussed prior.
Activation maps generated to indicate significantly higher activity compared to control conditions.

Cross-disciplinary Collaboration: MRI requires input from physicists, engineers, radiologists, and cognitive neuroscientists.
Understanding and analyzing results relies on coordination across various specialties.
BOLD Response & Stimulus Duration:
- Initial dip in signal post-stimulus onset reflects oxygen depletion from increased neuronal activity before an influx of oxygenated blood occurs.
- Signal peak typically occurs 6-8 seconds after stimulus onset.

DTI is distinct from standard MRI, focusing on the diffusion of water molecules in the brain.
Anisotropic Diffusion: In white matter regions, bundled axon fibers restrict water movement directionally due to constraints (i.e., fibers run in one direction).
- Principal Diffusion Direction correlates to fiber orientation.
- Increased compactness and directional movement occurs in white matter versus isotropic diffusion in gray matter.
Color Maps:
- DTI creates vector maps indicating diffusion directions with colors (red for left-right, green for anterior-posterior, blue for superior-inferior.)
- White matter fiber tracts can be mapped using this color approach.
- 3D renderings visualize individual fiber tracks compared to anatomical scans.

Lateralization: Each brain hemisphere managing different functions and cognitive tasks.
Structural Asymmetry: Notable differences exist between the left and right hemispheres in areas such as the Sylvian fissure and planum temporalis.
Neurochemical Asymmetry: Increased dopamine receptors in the left hemisphere suggesting a readiness for action localized there.
Functional Asymmetry: Greater activity in one hemisphere during specific tasks.
- Language functions are largely controlled by the left hemisphere (Broca's and Wernicke's areas).
Key Theories of Specialization:
- Direct Access Theory: Processing occurs where information first arrives.
- Collosal Relay Theory: Information is sent to the hemisphere better equipped for handling the task.
- Activating Orienting Model: Engagement in specific processes heightens activation in one hemisphere.
Broca's Aphasia: Damage causes agrammatical and disfluent speech while comprehension can remain relatively intact.
Wernicke's Aphasia: Individuals speak fluently yet struggle to retrieve meaningful language; coherent output is minimal.

Electroencephalography (EEG): Functional measure; detects electrical activity of neurons near the brain's surface.
- Excellent temporal resolution but poor spatial resolution.
Magnetic Resonance Imaging (MRI): Structural; measures hydrogen flipping, not functional timing aspects.
Functional MRI (fMRI): Measures BOLD, effective spatial resolution, but temporal resolution limited compared to other methods.
Diffusion Tensor Imaging (DTI): Structural; assesses water molecule movement in neurons with good spatial resolution.
Transcranial Magnetic Stimulation (TMS): Functional; induces brain activity changes, not directly measuring it.
Neurophysiology Techniques: Invasive functional measure with excellent spatial and temporal resolution but limited generalizability to whole-brain activity.

This comprehensive understanding of MRI and its functional counterparts provides insights into brain structure, connections, and functions and identifies various methodologies in use for study.
Knowing the advantages and limitations of each technique aids researchers in selecting the most suitable method for their study of brain activity and neural processes.
Discussion of lateralization emphasizes the importance of understanding hemispheric specialization in cognitive tasks, further enriching the field of neuroscience and neuropsychology.
Exams: Quiz performance closely relates to exam difficulty; emphasis on practical application of the discussed methodologies is encouraged for assessments.