Imaging the brain Flashcards
What is structural MRI based on?
The behaviour of hydrogen atoms in the magnetic field - most tissue is water based (H2O)
How do structural MRIs work?
Protons have weak random magnetic fields. MRI introduces an external magnetic field, and some protons align themselves with it. Introduction of a brief radio pulse makes the aligned proton change their orientation by 90 degrees (spin). Spin protons produce a change in the magnetic field - MRI signal. Protons relax and return to their aligned state - a new slice is scanned. Repeated for whole brain.
How is tissue distinguished?
Variations in the rate at which protons return to their aligned state are used to distinguish between different tissue types. Structural MRI is static
What are functional MRI’s?
Dynamic measurements which measure temporary changes in brain physiology associated with cognitive processing. Give a BOLD response - blood oxygen-level-dependent contrast
What is the BOLD response for fMRIS?
Oxygenated blood (oxyhaemoglobin) used by neurons in the brain. Oxygen is consumed by neurons. turns oxyhaemoglobin into deoxyhaemoglobin, which has strong paramagnetic properties. This introduces distortions in the local magnetic field. Amount of distortions = amount of deoxyhaemoglobin in the blood
What is the haemodynamic response function (HRF)?
Changes in the BOLD signal over time
What are the steps in fMRI?
Initial dip, overcompensation and undershoot
What is the initial dip?
Neurons consume oxygen causing a small rise in the amount of deoxyhaemoglobin. This causes a reduction in the BOLD signal
What happens in the overcompensation stage?
Increased oxygen consumption in the blood leads to increased blood flow. This allows an even further increase in blood oxygen concentration. Allows an increase in the BOLD signal
What occurs in the undershoot stage?
Blood flow and oxygen consumption dip and return to original levels
What happens to the size of haemodynamic changes in fMRIS?
The magnitude of haemodynamic changes varies with magnetic strength. 1st (1.5T) magnets cause a change of 1-3%. New generation magnets (3T &7T) produce bigger changes
What is the spatial and temporal revolution of fMRI?
Temporal - few seconds
Spatial - 1mm
What is the spatial and temporal revolution of fMRI?
Temporal - few seconds
Spatial - 1mm
What is positron emission tomography (PET)?
Measures blood flow in the brain by using a radioactive isotope. It commonly uses oxygen-15, and is administered as water
How do PET scans work?
Radioactive isotope is injected into the bloodstream. The radiotracer is then broken down, which emits a positron, which emit gamma rays. A greater blood flow means a greater signal is emitted by the tracer. More gamma rays released leads to a higher level of function
