Clinical Neuroscience: Brain Imaging Methods

Brain Imaging Methods: Structure vs. Function

Introduction

  • This lecture focuses on brain imaging methods at the macro-scale.
  • Links micro-scale neuroscience (neurons, drugs, receptors) discussed in Weeks 1-3 to macro-scale brain imaging.
  • Weeks 6 & 9 will bridge the gap between micro- and macro- scales using models, theory, and mathematics.

Spatial Scales in Neuroscience

  • Micro-scale: genes, brain cells, cytoarchitecture & receptor systems
  • Meso-scale: connectivity
  • Macro-scale: systems, behaviour, mental disorder

Overview of Brain Imaging

  • Brain imaging is used extensively in research, with numerous publications annually.
  • Applications span a wide range, and brain images are often used in media to validate claims (neurosensationalism).

Structural vs. Functional Imaging

Structural Imaging
  • Provides high-resolution images of the brain's structure.
  • Clinically useful for identifying tumors, hematomas, etc.
  • Used in morphological research and to study connectivity (e.g., DTI for white matter maps).
Functional Imaging
  • Examines how different brain regions function and when they are active.
  • Non-invasive.
  • Used in basic neuroscience research.
  • Some clinical applications, such as pre-surgical mapping before epilepsy surgery.

Phineas Gage: An Early Case of Brain Mapping

  • Phineas Gage's case demonstrated the impact of brain damage on personality and behaviour.
  • An iron rod through his left frontal lobe led to significant changes in his personality.

Structural MRI (sMRI)

  • Three planes: axial, coronal, sagittal
Anatomical Terminology
  • Anterior: in front
  • Posterior: behind/back
  • Lateral: one side (left/right)
  • Medial: towards the middle
  • Superior: above
  • Inferior: below
How sMRI Works
  • Patient is placed in a strong magnetic field.
  • Hydrogen nuclei align like tiny compasses.
  • Nuclei spin on an axis.
  • Radio waves pulse, causing protons to spin at a specific frequency (Resonance).
  • When the radio waves stop, nuclei release energy, acting like miniature radio stations.
  • A coil picks up this energy, sending signals to a computer to create an image (Imaging).
sMRI in Psychiatric Disorders
  • Currently used to rule out organic causes of psychiatric symptoms (e.g., tumors, inflammation).
  • Potential for identifying morphological changes to predict psychiatric disorders and guide treatment.
  • Example: Ventriculomegaly (enlarged ventricles) and reduced brain volume in twins with schizophrenia.

Functional MRI (fMRI) - BOLD

How fMRI Works
  • Measures brain activity through oxygen uptake (Blood Oxygen Level Dependent).
  • Based on hemoglobin, which is diamagnetic when oxygenated (oxyhemoglobin) and paramagnetic when deoxygenated (deoxyhemoglobin).
  • Differences in oxygenation levels affect the MR signal.
fMRI in Psychiatric Disorders
  • Functional neuroimaging holds significant promise for psychiatry.
  • Can identify differences in brain systems related to psychiatric symptoms, treatment response, and risk factors.
  • Meta-analysis of resting-state fMRI data from 537 studies of various disorders.

Direct Measures of Function: MEG & EEG

Magnetoencephalography (MEG)
  • Measures magnetic fields produced by brain activity.
  • Highly sensitive, capturing changes in brain activity millisecond by millisecond.
Electroencephalography (EEG)
  • Electrodes placed on the scalp measure average electrical activity.
  • Used for research into brain function, diagnosing epilepsy and sleep disorders.
  • Non-invasive and risk-free.
  • Lower spatial accuracy compared to fMRI or MEG.
MEG and EEG in Psychiatry
  • Examples include:
    • Reduced high-frequency oscillations in schizophrenia during visual attention tasks.
    • Modulation of hippocampal theta by ZNF804A genotype (a genetic risk factor for schizophrenia and bipolar disorders).
    • Long-range functional connectivity in autism spectrum disorder (ASD).

Functional Imaging of Specific Receptor Systems: PET

How PET Works
  • Positron Emission Tomography: A nuclear medical imaging technique.
  • Short half-life radioactive isotopes (tracers) are injected into the body.
  • PET detects radiation emitted from these tracers.
  • Gamma rays converted to photons of light and electrical signals.
  • Signals converted into image slices.
Applications of PET
  • Shows glucose metabolism in the brain.
  • Detects cancer, dementia, and seizures.
  • Maps brain function.
Limitations of PET
  • Low resolution and slow compared to MRI and MEG.
  • Invasive due to the injection of radioactive substances.