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Lecture 8

MEDRADSC 2Z03 Lecture Notes - Lecture 8: Hemoglobin, Magnetic Susceptibility, Paramagnetism

Medical Radiation Sciences
Course Code
Dawn Danko

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In addition to providing strikingly detailed anatomical information about the brain, MRI
has the ability to measure brain function. That is, we can observe changes in brain tissue
resulting from neuronal activity that may include sensory, motor or cognitive processes.
fMRI detects changes in signal due to these processes and localises the specific function
being tested to the regions of the brain responsible for performing it. Using fMRI it is
possible to investigate:
How the healthy brain responds during functional tasks such as sight, hearing, decision
making, facial recognition etc.
How the damaged brain may change in its performance of these tasks following stroke,
disease or injury
How the brain recovers function following injury (plasticity).
How is it that MRI can detect neuronal activity?
fMRI takes advantage of the coupling between neuronal activity and blood flow:
as a region of the brain is being worked to perform a function, there is an increase
in blood flow to that region via the extensive cerebral vascular network.
Arterial intracortical vessels deliver new oxygenated blood to activated brain
tissue and the deoxygenated blood is removed via the venous microvasculature.
Oxygen is delivered bound to haemoglobin in the blood.
Oxygenated haemoglobin is diamagnetic and does not have a large effect on the
local magnetic environment.
However, deoxygenated haemoglobin is paramagnetic and causes distortions to
the local magnetic environment through changes in magnetic susceptibility.
This causes water protons in nearby brain tissue to dephase more rapidly when in
the transverse plane, in a similar manner to B0 field inhomogeneity as we
discussed in the section on MR Signal, causing a signal reduction. If the level of
oxygen increases, the paramagnetic effect is reduced, leading to an increase in
This phenomenon is called the Blood Oxygenation Level Dependent (BOLD)
As shown in figure 1 below, in the resting state the ratio of oxyhaemoglobin to
deoxyhaemoglobin is constant.
When activity occurs, increased blood flow delivers more oxyhaemoglobin and so
a signal increase is observed for nearby tissue.
This is the origin of the fMRI signal.
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Figure 1: The BOLD effect
In an fMRI experiment, the subject is asked to undertake a task or series of tasks
delivered while they are lying in the scanner. A number of scans is acquired while the
subject is resting and attempting to minimise active thought, then another series of scans
occurs while the subject performs the task – this may be as simple as looking at a flashing
chequer board, undertaking a recognition task, performing a motor function such as
finger tapping or some other cognitive stimulus. The two sets of images are subtracted
and statistically analysed to look for changes in regions of the brain that are associated
with performing the task. These tasks, or paradigms, have to be carefully designed to
ensure that the function being tested is truly correlated to the fMRI signal. The resulting
fMRI signal, which is a map of neural activity can be overlayed onto a structural MRI
image of the brain of the subject acquired separately to show the anatomical location of
the activity, as depicted in figure 2 below. In this case, the subject was asked to tap his
finger constantly during the scan and that part of the motor cortex responsible for hand
movement is visualised.
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