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Biology 202
2000 Third Web Report
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Magnetic Resonance Imaging

Mary Ferrell

MRI is a procedure, in wide use since the 80s, to see the anatomy of the internal organs of the body. It is based on the phenomenon of nuclear magnetic resonance (NMR), first described in landmark papers over fifty years ago (Rabi et al. 1938; Rabi, Millman, and Kusch 1939; Purcell et al. 1945; Bloch, Hansen, and Packard 1946) (4 ). . The MRI is a valuable diagnostic and research tool with also practical applications for surgical planning and conquering diseases. This imaging procedure is painless and non-invasive although sometimes discomforting as the patient lies down in a body tube that surrounds them. For many years, closed MRI units have been the standard in helping physicians make a diagnosis. These closed MRI units featured a long tube that the patient would be placed inside during their procedure. This was often uncomfortable for many patients due to the "closed in" feeling and was especially stressful for patients who suffer from claustrophobia. The newest generation of MRI units is now open on all four sides which completely alleviates the "closed in" feeling, while still providing the physician with the most accurate information possible to aid in diagnosis (2).. A patient does not see or feel anything. A faint knocking sound may be heard as the machine processes information. Patients may choose to listen to music -- even having the option of bringing their own CDs to listen to. Most MRI procedures take less than an hour. MRI technology is based on three things: magnetism, radiofrequency and computers. The magnetic resonance machine, is a big and strong magnet. When the body is inside, every proton of the body is oriented in the same way (for instance, with the positive pole up). Water molecules in the body naturally align themselves. This means that the body becomes a magnet, but with lots of diversity because the amount of water in each part of the body varies with the specific characteristics of the organ, the layer, the location, and even the types of cells. So, the human body can be pictured as a 3D-map of changes of magnetic field with a human body form (1). . Radiofrequency gives us the density of those changes and we then obtain the images of the internal anatomy.

Radiofrequency gives energy to the spin of the protons, increasing the amplitude of their turns without changing the frequency. Now the 3D-magnetic map becomes a 3D-map of energy. Each particular point in the body has a particular energy (or intensity, in terms of radiofrequency). If radiofrequency is no longer applied to the body, the proton-spins recover the original state and in that moment release radiofrequency waves. Now we have a 3D-map of radiofrequency and this radiofrequency can be registered with coils.

The rest of the procedure is done by the computers that convert the signal intensity, the signal phase, and the signal location into a matrix of dots with different values. Each value is represented with a tone of gray. The minimum value is black, and the maximum value is white, and in between is a scale of gray. At this point We have a MRI image. The MRI provides both structural and chemical information and distinguishes moving blood from static brain tissue (2 ). . In functional magnetic resonance imaging we employ two components: a task and a result. The task is an action or activity that the subject does in order to produce a particular activation of the brain. For example, moving the right hand fingers continuously are a motor task that "activates" the brain cortex in the left frontal lobe. The result in fMRI is an image, which depicts this activation. The task can be of any type. Motor, feeling a sensation, having a perception, thinking in abstract words, attending to a changing stimulus, listening to music, comprehending a story, and many others.

The task will produce this sequence of events: There will be increases in metabolism, blood volume, oxygen, and the local magnetic field (as discussed earlier) of the brain area involved in the task. In brief, the task elicits activity of a region of the brain, and this activity changes the intensity of the radiofrequency coming from that part. This is a very small change. But, if we repeat the task several times we are able to sum the changes to get a significant result that can be registered (1). .We compare a person's brain activity during a particular task with their activity level during a resting state. However, the brain works continuously, even during the resting state. For that reason the resting state is a baseline of the background activity of the brain. Hundreds of images of the region targeted are taken during a few minutes. Meanwhile, the subject is alternating between periods of activity (performing the task) and periods of rest. In this manner there is a group of images of a region taken during the task, and the same number of images of the same region during rest. Two averaged images are obtained corresponding to two conditions in which one is "activated". Images are the result of the values of the signal intensity, encoded on a gray scale. These values can be added, subtracted, etc. Well, that is exactly the next step. Powerful computers subtract the baseline values from the activated ones. These values of activation are then transformed into a map of colors. Usually the scale of this map varies from blue to red in an increasing manner. Finally, the colors are overlaid upon anatomical images, in a similar way that the weather maps are overlaid upon geographical pictures.

There are many practical applications for the MRI and fMRI, among them-pre-surgical planning. In brain surgery, it is very important to know which brain functions are in which area. For example, if a lesion is going to be surgically removed, it is important to know which brain functions are in the area adjacent to the lesion. In lesions formed before birth, such as blood vessel abnormalities and some tumors, the areas of language and other mental process can be misplaced. The fMRI shows the actual location of the functions and their spatial relationship with those lesions (3). . The imaging shows in colors the area used for the movement of the body parts and language capability when these tasks are performed by the patient during surgery. This enables the neurosurgeon to extract the lesion carefully.

Brain functions can be divided into basic and complex. Basic functions require few areas of the cortex, while complex functions could be spread over one hemisphere, or even in both. Basic brain functions are consistently represented in the same places across different people, while the complex functions tend to vary from person to person. Language is a typical complex function. In most people, language is represented in the left hemisphere, but it is completely normal to find representation in the right hemisphere or divided between them. Prior to brain surgery, it is quite important to know the side in which the language is represented. In epilepsy surgery this has a crucial importance. Some times it is necessary to remove part of the temporal lobe involved in the generation of seizures. The extension of the surgery must be restricted if it is found that this lobe is involved in language processing (1). .

Other applications for MRI are for scientific learning. Behaviors rely upon collection of brain cells that comprise highly organized interconnected circuits. Historically, our knowledge of functional roles of brain regions came from clinicopathological correlations in individuals suffering irreversible damage to individual regions or the pathways that connect them. Now there is the successful application of diffusion weighted MRI (DWMRI) to provide 3D views of pathway trajectories, such as has been done in a formalin-fixed rat brain. The greatest benefit of DWMRI will be realized in detecting long term changes in cerebral connections and particularly in the clinical arena; that is, in studies of neural development and the acute evaluation of diseases that affect white and gray matter (4). .Magnetic resonance imaging has a great potential to provide insight into neurologic and psychiatric disorders. The fMRI has been used to assess childhood stroke, migraine and epilepsy. This procedure has also been applied to children with developmental disorders, including autism, dyslexia, and other learning disabilities. One of the future purposes of fMRI is to predict developmental disorders at very young ages in order to provide effective early treatment interventions (1). .

Alzheimer's disease (AD) is the most common cause of dementia, yet very difficult to diagnose precisely without invasive techniques, particularly at the onset of the disease. Therefore, the reliable diagnostic method of MRI is needed. The hippocampus is a part of the mesial temporal lobe memory system, and known to be affected early in the course of Alzheimer's disease. Recent development of imaging techniques, particularly magnetic resonance imaging (MRI), has made the evaluation of diminutive brain structures, such as the hippocampus, conceivable. There has been a study to focus on the sensitivity and specificity of different approaches of hippocampal imaging by MRI, and their applications for the diagnosis of incipient Alzheimer's disease. A total of 193 subjects were examined: 59 patients with probable AD according to NINCDS-ADRDA criteria; 43 patients with age-associated memory impairment (AAMI) according to National Institute of Mental Health criteria; nine patients with vascular dementia (VaD) according to DSM-III-R criteria; 20 patients with Parkinson's disease, eight of whom were demented, and 62 cognitively normal control subjects of whom 42 were older and 20 younger than 50 years of age (5). .

Hippocampal pathology was evaluated by means of linear, planimetric (hippocampal area) and volumetric measurements gained by the utilization of MRI. The accuracy of hippocampal measurements was compared to that of the amygdala and the frontal lobes. Various procedures for normalization of the data to the head and brain size were compared. Very detailed information is made available using the imaging technique. Bilateral volumetric hippocampal atrophy was a highly sensitive indicator of early Alzheimer's.The best discriminant function analysis resulted in correct classification of 95 % of AD patients versus non-demented age-matched controls. The volume of the hippocampus also correlated with AD severity as assessed by Mini-Mental Status Examination and with tests assessing delayed recall. In contrast, the volume of the hippocampus was not significantly affected either by aging. The specificity of hippocampal atrophy in comparison to other dementias, however, may be limited, since the hippocampus seem to display various patterns of atrophy in VaD and Parkinson's disease with and without dementia as well. The Alzheimer's disease group also invariably showed smaller volumes of the amygdala and frontal lobes, smaller hippocampal areas, longer IUDs and prolonged T2. Yet, evaluation of these measurements did not produce as good an accuracy in correct grouping as did hippocampal volumetry, but was compromised by age-dependence of the variables resulting in substantial overlap between the study groups. In conclusion: volumetric hippocampal atrophy is a highly sensitive indicator in early AD. On the other hand, the specificity compared to other dementias with temporal lobe pathology may be limited. Volume of the hippocampus is not significantly affected by age, which makes its assessment useful in detecting Alzheimer's disease, or rather excluding and differentiating it from benign memory impairment (5). .

Magnetic Resonance Imaging is one of the most accurate imaging modalities available today. It is an application of computer technology that has generated knowledge for the future and for practical application today. The field of imaging continues to expand as avidly pursued new dimensions in the acquisition of physiological and biochemical information occurs.


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WWW Sources

1) Principles of Functional Magnetic Resonance ,

2) Consultants in Radiology ,

3) MIT Encyclopedia of Cognitive Sciences ,

4) Tracking Neural Pathways with MRI ,

5) MRI OF HIPPOCAMPUS IN INCIPIENT ALZHEIMER'S DISEASE ,

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