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.

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

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