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Published on : Mar 14, 2014
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Slide 1 - Brain Imaging Methods The Amazing Brain: 2011-2012
Slide 2 - 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
Slide 3 - The Electroencephalogram (EEG) Advantage: easy and inexpensive to set up, measures in “real time” Disadvantage: not very specific to brain regions, difficult to interpret
Slide 4 - 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.
Slide 5 - 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
Slide 6 - 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.
Slide 7 - 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.
Slide 8 - Magnetic Resonance Imaging (MRI) Advantages: sharper, more detailed pictures than CT scan, no X-rays Disadvantages: static images (no real-time), expensive equipment
Slide 9 - MRI used To guide Surgical Removal of a Tumor.
Slide 10 - 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.
Slide 11 - 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
Slide 12 - MAGNETOENCEPHALOGRAPHY (MEG) MEG measurements capture brain action in real time with temporal resolution in the range of tens of milliseconds. Advantages: more comfortable, much faster Disadvantage: very expensive and very rare. 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.
Slide 13 - 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.
Slide 14 - Magnetoencephalography (MEG) (research only) Advantage: very rapid (millisecond resolution) Disadvantage: very expensive, only a few machines available
Slide 15 - Functional MRI (fMRI) Advantage: takes movies of MRI relatively quickly
Slide 16 - 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.
Slide 17 - 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.
Slide 18 - 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.