The Hemodynamic Response & BOLD fMRI

Purpose

The purpose of this tutorial is to discuss the biological basis of functional MRI – the hemodynamic response.

Objectives

  1. Define hemodynamic response.
  2. Describe how fMRI measures the hemodynamic response.
  3. Describe the strengths and weaknesses of using the hemodynamic response as a measure of neural activity.

What is the hemodynamic response?

Roy & Sherrignton, 1890

  • Stimulated sciatic nerve of a dog.
  • Observed
    • Increased brain volume.
    • Increased arterial pressure synchronous with stimulation.

Ngai, Ko, Morii & Winn, 1988

Brain Metabolism

  • The brain relies primarily on glycolysis for ATP production.
  • Uses 20-25% of total glucose and O2 supply.
  • No energy storage.
  • Energy must be supplied on demand.

Blood Supply

  • Rapidly adapting vascular system.
  • Feedback
    • Byproducts of glycolysis stimulate increased blood flow.
    • Adenosine, CO2, lactic acid, increased extracellular K+
  • Feedforward
    • Neurons directly stimulate increased bloodflow via Glutamatergic synapses on astrocytes and pericytes.

Watering the garden for the sake of a flower

  • Compensatory blood is excessive.
  • Astrocyte gap junctions propagate increases in blood flow beyond the borders of the active region.

How is it measured?

BOLD fMRI

  • Oxygenated hemoglobin is diamagnetic because all electrons are paired. It weakly opposes the magnetic field.
  • Deoxygenated hemoglobin is paramagnetic and is attracted to the magnetic field.
  • In T2* fMRI, oxygenated Hgb has a stronger signal intensity.
  • Active regions will have excessive blood flow (have a high concentration of oxygenated hemoglobin) and have higher signal intensity.

Resolution

  • Refers to the degree of precision for a test/method.
  • Spatial resolution – the degree to which test results are geometrically accurate.
  • Temporal resolution – the degree to which the test results are measured across time.

Spatial resolution

  • Very accurate.
  • Determined by voxel size (usually about 3 mm but can be smaller or larger).

Temporal resolution

  • Ok.
  • Data is gathered every few seconds.
  • High speed/rare/short duration events are difficult to measure.
  • Augmentation with secondary measures can help.

Next time

  • Do the questions at the end of the blog post.
  • Complete the quiz.
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