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Aqueduct of Sylvius: A canal that communicates between the third and fourth ventricles in a system of four communicating cavities within the brain that are continuous with the central canal of the spinal cord.
The four ventricles consist of the two lateral ventricles, the third ventricle and the fourth ventricle:
* Lateral ventricles: The lateral ventricles are in the cerebral hemispheres. Each lateral ventricle consists of a triangular central body and four horns. The lateral ventricles communicate with the third ventricle through what is called the interventricular foramen (opening).
* The third ventricle is a median (midline) cavity in the brain that is bounded by the thalamus and hypothalamus on either side. Anteriorly (in front) the third ventricle communicates with the lateral ventricles and posteriorly (in back) the third ventricle communicates with the aqueduct of Sylvius (also called the aqueduct of the midbrain).
* The fourth ventricle is the most inferior (lowest) of the four ventricles of the brain. It extends from the aqueduct of the midbrain to the central canal of the upper end of the spinal cord with which it communicates by the two foramina (openings) of Luschka and the foramen (opening) of Magendie.
The ventricles are filled with cerebrospinal fluid, which is formed by structures called choroid plexuses located in the walls and roofs of the ventricles.
How does the procedure work?
MRI is a unique imaging method because, unlike the usual radiographs (x-rays), radioisotope studies and even CT scanning, it does not rely on radiation. Instead, radio waves are directed at protons, the nuclei of hydrogen atoms, in a strong magnetic field. The protons are first "excited" and then "relaxed," emitting radio signals, which can be computer-processed to form an image. In the body, protons are most abundant in the hydrogen atoms of water—the "H" of H2O—so that an MRI image shows differences in the water content and distribution in various body tissues. Even different types of tissue within the same organ, such as the gray and white matter of the brain, can easily be distinguished. Typically an MRI exam consists of two to six imaging sequences, each lasting two to 15 minutes. Each sequence has its own degree of contrast and shows a cross section of the head in one of several planes (right to left, front to back, upper to lower).
How is the procedure performed?
Magnetic Resonance Imaging (MRI) procedureThe patient is placed on a sliding table and a radio antenna device called a surface coil is positioned around the upper part of the head. After positioning the patient with the head inside the MRI gantry, the radiologist and technologist leave the room and the individual MRI sequences are performed. The patient is able to communicate with the radiologist or technologist at any time using an intercom. Also, many MRI centers allow a friend or, if a child is being examined, a parent into the room. Depending on how many images are needed, the exam will generally take 15 to 45 minutes, although a very detailed study may take longer. The patient will be asked not to move during the actual imaging process, but between sequences some movement is allowed. Patients are generally required to remain still for only a few seconds at a time. Some patients will require an injection of a contrast material to enhance the visibility of certain tissues or blood vessels. A small needle connected to an intravenous line is placed in an arm or hand vein. A saline solution will drip through the intravenous line to prevent clotting until the contrast material is injected about two-thirds of the way through the exam.
When the exam is over the patient is asked to wait until the images are examined to determine if more images are needed.
What are the limitations of MRI of the Head?
Bone is better imaged by conventional x-rays, and CT is preferred for patients with severe bleeding, acute trauma or who because of their medical condition are unable to tolerate an MR scan procedure. MRI may not always distinguish between tumor tissue and edema fluid and does not detect calcium when this is present within a tumor. In most cases the exam is safe for patients with metal implants but there are a few exceptions, so patients should inform the technician of an implant prior to the test. The exam must be used cautiously in early pregnancy. MRI often costs more than CT scanning.
Broca's area is the section of the human brain (in the opercular and triangular sections of the inferior frontal gyrus of the frontal lobe of the cortex) that is involved in language processing, speech production and comprehension. Broca's and Wernicke's areas are found unilaterally in the brain.
It comprises of Brodmann's Area 44, and some authorities also include Brodmann's Area 45); Broca's Area is connected to Wernicke's area by a neural pathway called the arcuate fasciculus. The corresponding area in macaque monkeys is responsible for high-level control over orofacial actions.
Functional MRI and AIR
Cohen and his colleagues are helping to shape a newly evolving discipline called cognitive neuroscience. In recent work, they have exploited technology developed during the 1980s that many scientists believe will revolutionize study of the brain. Imaging technology, such as magnetic resonance imaging (MRI) and other techniques, combined with computing power makes it possible, in effect, to peel away the bone and membrane surrounding the brain. Without even touching their human subject, researchers can see what happens inside a living, thinking brain, and they can identify what parts of this intricate, complexly folded, interconnected mass of tissue "light up" during mental activities.
Cohen and colleagues use a technique known as functional MRI to record a view of the functioning brain that is among the most detailed yet reported. While other brain-mapping techniques give what resembles a satellite view of the world, in which cities can be seen and identified, the Pitt/CMU researchers can see streets. With functional MRI, they can map the sites of brain activity to a resolution as fine as one millimeter, comparable to mapping a football field in six-inch units.
Cohen and his colleagues used a conventional MRI machine, like those that became available in many hospitals during the 80s, a big advantage since the research can be accomplished without major new investment in technology. Functional MRI works on the principle that when brain cells (neurons) become active, blood flows to them, and the MRI scanner registers increased oxygen in the area. Because MRI machines used in this way detect changes resulting from biological function, the method got its name.
The technique generates large amounts of data quickly -- a great advantage, says Cohen, and a problem. "It gives a lot of information to work with, but likewise it's a tremendous amount of data to process -- as much as half a gigabyte per experiment." To deal with the data overload, Cohen and his colleagues turned to the Pittsburgh Supercomputing Center's Alpha Cluster, a linked network of 14 DEC Alpha workstations. They used the cluster to address a particular problem of their functional MRI experiments. A human subject stays in the machine for two to three hours as the MRI scanner records data. Though special pillows are used to reduce movement, it's impossible to keep the head perfectly still. Software called automatic image registration (AIR), developed by Roger Wood of UCLA, can correct for head movement, but the sheer number of images -- typically 1,200 per experiment -- creates an imposing demand on computing.
AIR is an ideal application for the Alpha Cluster, notes Cohen, because it is inherently parallel. A single experiment typically records 200 separate images for each of 6 separate scan sites, or slices. "For each slice," says Cohen, "you take as a reference point one of the 200 images and align all the others to it. There's no need for communication back and forth. Each sample can be aligned to the reference independently."
On a high-speed workstation, says Cohen, it took as much as 24 hours computing time to register the images from one experiment. "Often, we run two experiments in an evening, which means the computing can't keep up with the data, and this cripples the research." On the Alpha Cluster, the same computing takes an hour, a radical speedup that overcomes the research bottleneck.
This image shows what regions in a subject's brain were involved in a memory task. This kind of study leads to improved understanding of "working memory", which affects clinical treatment of schizophrenia and amnesias.
This 3-D image of a subject's brain shows that the primary visual cortex becomes activated while the subject, who is inside an MRI scanner, looks at the whirling pattern.
A light changes from green to yellow, and you jam your foot on the brake. It's almost an automatic impulse, a no-brainer, you might say, guided by the knowledge, planted somewhere, that moving-vehicle violations are to be avoided. But where, exactly, is that knowledge planted? How does perception transform in an instant to action? Where's the owner's manual with the wiring diagram of the mind that shows all the connections?
The functional MRI experiments conducted by Cohen investigate a concept known in cognitive psychology as working memory. Each subject's brain is scanned while they perform a working memory task and a control task. In the control task, the subject sees a random sequence of letters one at a time on a visual display. They are instructed to press a button whenever the letter "X" shows on the display. In the memory task, subjects see a similar sequence of letters, but they are instructed to press the button only when a letter repeats after exactly one intervening letter. For example, A-F-A should prompt a response, but not A-A or A-Q-G-A.
Both tasks, explains Cohen, require subjects to visually monitor sequences of letters presented one at a time, to evaluate their identity and respond by pressing a button. The memory task, however, requires in addition that the subject keep in mind both the identity and order of the two previous letters and continuously update this mental record as the sequence progresses.
The MRI machine records data from six slice locations in the prefrontal cortex of each subject (above left). A set of activation images for one subject (panels 1-6 above) shows the brain areas significantly activated during the memory task and not during the control task. Results to date from these studies, says Cohen, "support the idea that the prefrontal cortex becomes engaged when recently presented information must be represented and actively maintained to perform a task."
The basal ganglia is located deep within the cerebral hemispheres in the telencephalon region of the brain. It consists of the corpus stratium,subthalamic nucleus and the substantia nigra.
Spin-Echo Magnetic Resonance Imaging
In spin-echo MRI, gradients and Fourier analysis are used to perform three-dimensional imaging. Other techniques of MRI, such as gradient-echo, are slight variations of spin-echo imaging, so I will only describe spin-echo imaging in detail. The component of the imaging system which allows the spatial localization of the protons is a set of magnetic field gradients, set up by magnetic coils which are turned on and off at appropriate times (Horowitz, 1995).
When hydrogen nuclei relax, the frequency that they transmit is positively correlated with the strength of the magnetic field surrounding them. A magnetic field gradient along the z-axis, called the "slice select gradient," is set up when the RF pulse is applied, and is shut off when the RF pulse is turned off. This gradient causes the hydrogen nuclei at the high end of the gradient (where the magnetic field is strong) to precess at a high frequency (e.g., 65 MHz), and those at the low end (weak field) to precess at a lower frequency (e.g., 63 MHz). When the RF pulse, of a single frequency, is applied, only those nuclei which precess at that frequency will be tilted, to later relax and emit a radio transmission (i.e., the nuclei "resonate" to that frequency). For example, if the magnetic gradient caused hydrogen nuclei to precess at rates from 63 MHz at the low end of the gradient to 65 MHz at the high end, and the gradient were set up such that the high end was located at the patient's head and the bottom part at the patient's feet, then a 63 MHz RF pulse would excite the hydrogen nuclei in a slice near the feet, and a 65 MHz pulse would excite them in a slice near the head. Thus a single "slice" along the z-axis is selected; only the protons in this slice are excited to a higher energy level, to later relax to a lower energy level and emit a radio transmission (Horowitz, 1995).
The second dimension of the image is extracted with the help of a phase encoding gradient. Immediately after the RF pulse ceases, all of the nuclei in the activated slice are "in phase," that is, their magnetic vectors all point in the same direction. Left to their own devices, these vectors would relax. In MRI, however, the phase encoding gradient (in the y-dimension) is briefly applied, in order to cause the magnetic vectors of nuclei along different portions of the gradient to point in different directions (Horowitz, 1995).
After the RF pulse, slice select gradient, and phase encoding gradient have been turned off, the MRI instrument sets up a third magnetic field gradient, along the x axis, called the "frequency gradient" or "read-out gradient." This gradient causes the relaxing protons to be differentially re-excited, so that the nuclei near the low end of the gradient begin to precess at a faster rate, and those at the high end pick up even more speed. When these nuclei relax again, the fastest ones (those which were at the high end of the gradient) will emit the highest frequency of radio waves. The frequency gradient is applied "only when the signal is measured" (Horowitz, 1995).
The second and third dimensions of the image are extracted by means of Fourier analysis. The entire procedure must be repeated multiple times in order to form an image with a good signal-to-noise ratio.
Finally, in spin-echo imaging, there is the problem that the inhomogeneity of the main magnetic field induces variations in the rate of precession of nuclei. To fix this problem, a 180-degree RF pulse is inserted into the cycle, at a time point halfway between the 90-degree pulse and the measurement of the radio transmission signal given off by the relaxing nuclei (Horowitz, 1995).