Transcript Methods for Brain Imaging

Brain Imaging Methods
The Amazing Brain: 2011-2012
Electroencephalogram (EEG) - measures electric activity (sum of action potentials of
brain at specific locations on the scalp.
Advantage: very fast, real-time recordings
Disadvantage: not very specific – difficult to interpret
The Electroencephalogram (EEG)
Advantage: easy and inexpensive to set up, measures in “real time”
Disadvantage: not very specific to brain regions, difficult to interpret
Principle of a CAT scan macine
Computer Axial Tomography
The device takes numerous X-ray pictures of the brain from
many different angles. The data may then be called up from
the computer data bank in “slices” from any chosen direction.
Computed Tomagraphy (CT Scan)
Advantages: provides fairly clear images to detect tumors, hemorrhage, plan surgeries
Disadvantages: somewhat expensive, static (single pictures), no real-time images, X-rays
MRI (magnetic resonance imaging) When protons (here brain protons) are
placed in a magnetic field, they become capable of receiving and then
transmitting electromagnetic energy. The strength of the transmitted energy is
proportional to the number of protons in the tissue. Signal strength is modified
by properties of each proton's microenvironment, such as its mobility and the
local homogeneity of the magnetic field. MR signal can be "weighted" to
accentuate some properties and not others.
You've probably seen MRI chambers on TV. The machine surrounds the
patient, who lies still on a pallet inside a narrow cavern. A powerful electromagnetic
field, considered to be harmless, is generated around the person. The
electromagnetic field causes the nuclei of the body's hydrogen atoms (each a single,
positively-charged proton) to stop their random spinning and align like compass
needles. Precise radio waves are then slammed into the flipped nuclei, making the
nuclei snap back to their original configurations. As they do this, they release energy
in the form of radio waves that, echo-like, can be picked up by a detector and sorted
out by a computer. Regions dense with hydrogen atoms will emit more radio waves,
allowing the computer to generate a high-resolution, three-dimensional density map
of the body.
Magnetic Resonance Imaging (MRI)
Advantages: sharper, more detailed pictures than CT scan, no X-rays
Disadvantages: static images (no real-time), expensive equipment
MRI used
To guide
Surgical
Removal of a
Tumor.
What resembles an odd marriage between Trojan battle gear and Medusa is actually part
of the most powerful brain scanner ever made. The invention of biophysicists Graham
Wiggins and Lawrence Wald of Massachusetts General Hospital in Boston, this $250,000
helmet could enable earlier detection of brain diseases such as Alzheimer's.
Positron Emission Tomography (PET scan)
Uses specially labeled molecules (usually glucose)
To demonstrate Areas of Increased Blood Flow
Advantages: Shows changes in brain function and metabolism, real-time images
Disadvantage: Requires uses of expensive, radio-labeled, injected substances
Requires highly trained personnel and very expensive equipment
and materials
MAGNETOENCEPHALOGRAPHY (MEG)
MEG measurements capture brain action in real time with temporal resolution in the
range of tens of milliseconds.
A magnetoencaphalogram (MEG) detects
the neuromagnetic brain signals of a subject
by bringing a set of magnetic sensors,
preferably SQUIDs (superconducting
quantum interference device), close to the
scalp of the subject.
Advantages: more comfortable, much
faster
Disadvantage: very expensive and very
rare.
What does FMRI measure?
Oxygen is delivered to neurons by haemoglobin
in capillary red blood cells. When neuronal
activity increases there is an increased demand
for oxygen and the local response is an increase
in blood flow to regions of increased neural
activity.
Haemoglobin is diamagnetic when oxygenated but paramagnetic when deoxygenated. This
difference in magnetic properties leads to small differences in the MR signal of blood
depending on the degree of oxygenation. Since blood oxygenation varies according to the
levels of neural activity these differences can be used to detect brain activity. This form of
MRI is known as blood oxygenation level dependent (BOLD) imaging.
One point to note is the direction of oxygenation change with increased activity. You might
expect blood oxygenation to decrease with activation, but the reality is a little more complex.
There is a momentary decrease in blood oxygenation immediately after neural activity
increases, known as the “initial dip” in the haemodynamic response. This is followed by a
period where the blood flow increases, not just to a level where oxygen demand is met, but
overcompensating for the increased demand. This means the blood oxygenation actually
increases following neural activation. The blood flow peaks after around 6 seconds and then
falls back to baseline, often accompanied by a “post-stimulus undershoot”.
Image Credits
•Diagram of the BOLD effect - Courtesy of Stuart Clare, FMRIB.
Magnetoencephalography (MEG)
(research only)
Advantage: very rapid (millisecond resolution)
Disadvantage: very expensive, only a few machines available
Functional MRI (fMRI)
Advantage: takes movies of MRI relatively quickly
Functional MRI adds another dimension to static MRI. When neurons (nerve
cells) are active, their metabolism increases significantly, requiring increased
blood flow to supply oxygen and carry away metabolic waste. Blood that's
carrying oxygen to the neurons has different magnetic properties than
deoxygenated blood; as oxygen is rushed to active neurons, it causes a
temporary increase in MRI signal that a computer can detect and amplify,
giving a four-dimensional map in time and space of brain activity.
By taking rapid MRI’s, one after another, the changes in blood flow in the
brain may be monitored due to the changes in oxygenated/deoxygenated
blood MRI’s.
Changes in MRI during a Visual Experiment
Each image moving left to right and top to bottom represents
a change in time of a second.
A computer can tell with 78 percent accuracy when someone is thinking
about a hammer and not pliers.
By Sharon Begley
NEWSWEEK
Updated: 2:07 PM ET Jan 12, 2008
Now research has broken the "content" barrier. Scientists at Carnegie
Mellon University showed people drawings of five tools (hammer, drill and
the like) and five dwellings (castle, igloo …) and asked them to think about
each object's properties, uses and anything else that came to mind.
Meanwhile, fMRI measured activity throughout each volunteer's brain. As
the scientists report this month in the journal PLoS One, the activity
pattern evoked by each object was so distinctive that the computer could
tell with 78 percent accuracy when someone was thinking about a hammer
and not, say, pliers. CMU neuroscientist Marcel Just thinks they can
improve the accuracy (which reached 94 percent for one person) if people
hold still in the fMRI and keep their thoughts from drifting to, say, lunch.