fMRI Flashcards
What does an fMRI measure?
chemical changes in the brain that assess sensory, motor, and cognitive processes (neurobiological behaviours)
What does a standard MRI measure?
structural changes
Explain the four steps in an MRI (affect on protons)
- water protons spin around their axes, creating individual magnetic fields with random directions
- when a vertical magnetic field is applied to the tissue, the protons align with it to creare a net magnetic field that is also vertical but very small and difficult to detect
- a radio frequency pulse applied in a second (horizontal) direction makes the protons wobble (change in time -> generate current) or precession, around their vertical axes
- summed across all individual water protons, this creates a net magnetic field that changes in time and gives rise to an electric current that is ultimately measured in MRI
Explain the 3 steps in MRI once measurment begins
- begins my placing the subject in a vertical magnetic field. With the protons alignes vertically, a horizontal radio frequency pulse is applied to top the porton so that they rotate in the horizaontal plane synchronously, or “in phase” with one another
- the horizontal pulse is then turned off, and the rotating protons begin to move out of phase with one another - they “dephase”
- dephasing occurs relatively quickly and leads to a decrease or decay in the measured current - after withdrawal of the horizontal pulse the protons realign with the vertical magnetic field
What is the BOLD effect?
Blood Oxygen Level Dependent signal
*fMRI based on
What is the result of stimulation of the visual cortext with a reversing annular checkerboard when compared with a simple fixation crosshair experiment?
dramatic increases in blood flow and glucose use in the visual cortex that are unaccompanied by similar increases in oxygen use
the result is an increase in the local oxygen availability because the increased supply of oxygen by flowing blood exceeds the increased local demand for oxygen
What does the BOLD signal reflect?
changes in the ratio of oxyhemoglobin to deoxyhemoglobin, the levels of which vary with perfusion and metabolism
- deoxyhemoglobin is pragmatic (attracted to magnetic field), while oxyhemoglobin is not
Why is there increased blood flow to activated brain regions?
supplied more oxygenated blood that in immediately necessary for local metabolism
What does a decrease in deoxyhemoglobin concentration cause as a result of increased neuronal activity?
dephasing to occur more slowly and hence slowing down the decay of the measured electic current
the result is an fMRI image of the locations of metabolic activity as revealed by changes in deoxyhemoglobin concentration
What is neurovascular coupling?
the active process linking local neuronal activity to an orchestrated increase in local blood flow
How is blood flow distribution measured across the vascular network?
oxygen sensitive two photon microscopy use to measure the BOLD relevant microvascular physiology occuring within a typical fMRI voxel to quantify the contribution of individual vascular compartments to the BOLD signal
What is the typical stimulus evoked functional hyperemia response?
increase in total hemoglobin corresponds to vessel dialtion and in increase in the number of red blood cells per unit volume of cortex, consistent with an increase in blood flow
oxyhemoglobin increases, whil deoxyhemoglobin decreases, indicating a net over oxygenation of the region
- positive BOLD corresponds to the decrease in deoxyhemoglobin
What are the coupling mechanisms for how blood flow changes?
direct action of neuronally derived substances such as glutamate and nitric oxide on the vasculature
cellular mediators of neruovascular coupling, indicating astrocytes, interneurons, and pericytes
afferents from the basal forebrain are known to modulate regional blood flow via ACh release
the release of vasoactive substances by cortical interneurons inlcuding VIP and NO
more conservative models suggest that interneurons may fine-tune local hemodynamics, with astrocytes or pericyets perhaps acting intermediaries
What is retrograde propagation of vasodilation?
rapidly propagated retrograde vasodilation mechanism during hyperemia mediated vua endothelial hyperpolarization
During retrograde propagation of vasodilation where does the hyperpolarization occur?
can propagate electrically within vascular endothelium itself, travelling >1mm with limited attenuation and causing self-dilation via myoendothelial coupling to encircling smooth muscle cells via myoendothelial gap junction or some endothelium-derived hyperpolarizing factor
What does EDHF-type propagetd vasodilation provide during retrograde propagation of vasodilation?
an elagant mechanism to explain the rapid dialtion of distant pial arteries
What could retrograde propagation of vasodilation explain?
the possibility that endothelial signalling is intiated at the capillary level, close to active neurons, and travels retrograde also an integrative vascular route would explain the selective recruitment of specific arterial branches and the generation of an optimally localized increase in blood flow
