Quiz 1- Lecture 3: fMRI Flashcards
What is one use of functional MRI (fMRI)?
To show the difference between a normal brain, a brain with mild cognitive impairment, and a brain with Alzheimer’s disease
Conventional MRI is used extensively for radiological diagnosis and produces…
Spatial maps of the properties of mobile hydrogen nuclei (single protons) that are contained mainly in water molecules
Conventional magnetic resonance images portray…
On what order? What dimensions?
Anatomical details with exquisite resolution
On the order of 1 mm or better; 3 dimensions - and differentiate soft tissues very well
What do the contrast within images (Structural MRI) result from?
Variations mainly in the density of water within tissues and in the manner in which water interacts with macromolecules
What does fMRI rely on? What is it associated with?
Detecting small changes in the signals used to produce magnetic resonance images; neuronal activity in the brain
Advantages of fMRI
Safe, noninvasive, and repeatable in adults and children and this has widespread potential uses
What does fMRI detect?
When do the changes arise?
blood oxygen level-dependent (BOLD) changes in the MRI signal
Arise when changes in neuronal activity occur following a change in brain stats (exam produced by a stimulus or task)
What happens in order to meet the larger demand for oxygen and other substrates?
An increase in neural activity in a region of cortex stimulates an increase in the local blood flow
What happens to the change in blood flow and why?
Change in blood flow actually exceeds that which is needed so that (at capillary level) there is net increase in the balance of oxygenated arterial blood to deoxygenated venous blood
What happens to the concentration of deoxyhemoglobin within tissues?
How does this have a direct effect?
The concentration of deoxyhemoglobin within tissues decreases
This decrease has a direct effect on the signals used to produce magnetic resonance images
What color is oxygenated blood (when you are cut)?
What color is deoxygenated blood (when you give blood sample and it’s drawn into a tube without oxygen so you can see this color)?
What color does deoxygenated blood appear as it flows through veins (esp people with fair skin)?
Bright red
Deep purple
Deep purple deoxygenated blood appears blue as it flows through our veins
Why is there a color difference of deoxygenated blood in the skin?
Due to the way that different colors of light travel through skin
(Blue light is reflected in the surface layers of the skin whereas red light penetrates more deeply- dark blood in vein absorbs most red light so we see blue light reflected at skin’s surface)
What organisms truly have blue blood and how?
Snails and crabs; They use copper to transport oxygen
(COVID 19 vaccine example: horseshoe crab)
What is the protein that makes blood red?
What is it composed of?
What binds to these iron atoms?
Hemoglobin
Composed of 4 protein chains, 2 alpha chains and 2 beta chains, each with a ring-like heme group containing an iron atom
Oxygen binds reversible to these iron atoms and is transported through blood
What are each of the protein chains similar in structure to?
What is it?
Myoglobin; The protein used to store oxygen in muscles and other tissues
Hemoglobin is…
What does it use?
A remarkable molecular machine
Uses motion and small structural changes to regulate its action
Oxygen binding…
What happens when the first heme binds oxygen?
What do these changes do?
At the four heme sites in hemoglobin does not happen simultaneously
It introduces small changes in the structure of the corresponding protein chain
These changes nudge the neighboring chains into a different shape, making them bind oxygen more easily
It is difficult to add the first oxygen molecule but…
Binding the second, third and fourth oxygen molecules gets progressively easier and easier
What happens when blood is in the lungs where oxygen is plentiful?
Oxygen easily binds to the first subunit and then quickly fills up the remaining ones
As blood circulates through the body, what happens to the oxygen level and carbon dioxide level?
What happens in this environment?
What happens as soon as the first oxygen molecule drops off?
What does this prompt?
The oxygen level drops while that of carbon dioxide increases
Hemoglobin releases its bound oxygen
The protein starts changing its shape
This prompts the remaining three oxygens to be quickly released
(In this way, hemoglobin picks up largest possible load of oxygen in the lungs and delivers all of it where and when needed)
In this picture, the heme is seen edge-on with the…
You can see the key…
Oxygen does what?
This does what in turn?
Iron atom
Histidine reaching up on the bottom side to bind to the iron atom
Oxygen has bound to the iron, pulling it upwards
Pulls on the histidine below, which then shifts the location of the entire protein chain. (These changes are transmitted throughout the protein, which ultimately causes the big shift in shape that changes the binding strength of the neighboring sites)
What is the result of having lower levels of deoxyhemoglobin present in blood in a region of brain tissue?
What is this small signal increase called that’s recorded in fMRI?
What is it typically around?
The MRI signal from that region decays less rapidly and so is stronger when it is recorded in a typical magnetic resonance image acquisition
Blood oxygen level-dependent (BOLD)
1% or less, though it varies
What does the magnitude of the signal depend on?
Is there a relation…
The changes in blood flow and volume within tissue, as well as the change in local oxygen tension
There is no simple relation between the signal change and any single physiological parameter
What happens as neurons become more active?
Thus…
There is a time delay before the necessary vasodilation can occur to increase flow and for the Wash-out of deoxyhemoglobin from the region to occur
Thus, the so-called hemodynamic response detected by BOLD imaging is delayed and has a duration of several seconds following a stimulating event
In fMRI, a subject is placed… where…
In the magnet of an MRI machine; various kinds of different stimulus may be administered in a controlled fashion
Example of stimulus in fMRI machine
Sounds may be played, Visual scenes may be presented, and Small motor movements or responses can be recorded
What may happen for a visual-stimulation task to localize primary visual areas?
What may happen during that time?
A subject might view a bright flickering checkerboard for 20 secs, followed by a dark screen for 20 secs, with these blocks repeated several times (Ex: 8 pairs of blocks would require a total recording that lasts under 6 minutes)
Images may be recorded for many different parallel slices (10-20) such that each slice is imaged about every 2 seconds. 80 images in this ex. would be acquired for each slice for both conditions (stimulus ON and OFF)
What happens in a block design experiment?
What happens after acquisition?
Sequences of images are acquired in contrasting conditions (I and II)
Those voxels whose signals change in synchrony (i to ii) with the stimulus or task can be identified
fMRI has found applications in both (2)…
Clinical and More basic neuroscience
Appropriate experiments may now be designed to address specific hypotheses regarding…
the nature of the distributed systems responsible for various functional responses
For clinical applications…
Simple mapping of critical sensory and motor functions
can be readily performed with subjects lying in the bore of a magnet where they perform simple tasks or experience sensory stimuli in blocks
Simple mapping of critical sensory and motor functions is the primary…
Approach for evaluations of the brains of patients prior to neurosurgery (ex: for treatment of temporal lobe epilepsy or arterio-venous malformations) or radiation therapy
What activation can you see in fMRI?
Auditory, Motor, Language, Visual
In neurological applications, fMRI may have a role in
Studies of patients recovering from stroke, and in understanding the occult or asymptomatic losses of cortical functions in degenerative disorders such as Alzheimer’s disease
In psychiatry, fMRI is being exploited to delineate the…
Neurobiological bases of various cognitive deficits and aberrant behaviors
What is the classic Stroop test?
In this test, subjects are instructed explicitly to suppress the (automatic) response of reading the words, and instead to identify only the color of the font
The average time course of the responses of two slices of the brain in the…
What happened?
event-related Stroop task
Each image was acquired 1.65 seconds after the previous image, starting at the time of the incongruent word-color pair. Peak activities are apparent in the BOLD responses 5-7 seconds after the event
What is triggered by the Stroop effect?
A widespread pattern of activity is seen in both the frontal and posterior regions
What can hemoglobin do?
Aside from oxygen transport, hemoglobin can bind and transport other molecules like nitric oxide and carbon monoxide
What does nitric oxide affect?
What does this do in turn?
The walls of blood vessels, causing them to relax
Reduces blood pressure
Recent studies have shown that nitric oxide…
Can bind to specific cysteine residues in hemoglobin and to irons in the heme groups
