Diffusion/perfusion MR imaging of the brain: from structure to function

Article Abstract:

Measuring diffusion (the movement of molecules based on concentration) and perfusion (amount of blood flow) in the brain is important for the diagnosis and treatment of brain disorders. Magnetic resonance (MR) imaging holds promise as one technique for making such measurements. MR imaging relies on the spin motion of elements with differing magnetic fields. This property of MR makes it appropriate for studies involving motion or flow. Molecular displacements (diffusion) can be measured noninvasively with MR. Water diffusion rates vary from tissue to tissue, and these differences result in contrasts that can be imaged and can show tissue and cell structure. Certain problems exist in using MR to measure diffusion. Most clinical MR imaging methods are not sensitive enough to accurately detect diffusion across cells. It has also been found that the direction of measurement can affect diffusion. Studies in brain white matter have shown greater diffusion when MR measures parallel to the fibers than when it measures perpendicular to them. The reasons for this are not yet known, but one possible result of discovering why this is so could be increased knowledge of white matter diseases. MR measurements of diffusion could be useful for the early detection and evaluation of strokes; water diffusion has been shown to decrease quickly after a stroke. In the later stages of stroke, the diffusion increases to well above normal. Measuring perfusion with MR without using contrast agents is technically difficult. One non-contrast MR technique uses gradient sensitization. The importance of this technique is its ability to make perfusion measurements that are quantifiable in new and more useful terms, allowing for a greater understanding of the microcirculation in the brain. One major problem with this technique is artifacts caused by microscopic tissue motion, but recent innovations may solve this problem. MR imaging of diffusion and perfusion appears to be a promising approach for the diagnosis of brain disorders. (Consumer Summary produced by Reliance Medical Information, Inc.)

Cerebral circulation, Diagnosis, Stroke (Disease), Stroke, editorial

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Temperature mapping with MR imaging of molecular diffusion: application to hyperthermia

Article Abstract:

Noninvasive temperature imaging has many potential uses, such as monitoring tissue interactions in surgical and medical laser procedures. Therapeutic hyperthermia (HT) is another important application. HT may be useful, as an adjunct to chemotherapy or radiation, as a treatment for some forms of cancer. Only limited HT applications have been performed due to the lack of adequate temperature control and monitoring. Effectiveness of HT depends on its ability to reach a tumor and maintain tumor exposure to a temperature to a minimum of 42 degrees centigrade. HT temperature must not exceed 42 degrees centigrade in normal tissues that may be exposed to high levels of heat in the course of targeting tumors. Temperature can be measured to within less than 0.1 degree centigrade with thermosensors implanted in heated tissues. Such a measure is invasive and painful and should only be used in limited circumstances. Magnetic resonance (MR) imaging has been proposed as a means of monitoring tissue temperature during HT. Researchers designed an MR phantom (model of the human body) in order to evaluate the effectiveness of MR as a means of monitoring HT for the presence of tissue damage due to overheating. Highly heated molecules will exhibit increased activity (Brownian motion) that MR is capable of demonstrating with standard MR imaging equipment. The results show that MR is able to measure temperature changes by means of molecular diffusion (movement of molecules) imaging. It will be necessary to shorten the experimental MR imaging time (2 to 3.5 minutes) for real-time temperature mapping in human subjects. Finally, only a map of the temperature change is obtained with this method. To determine the absolute temperature after heating, the initial absolute temperature must be known. Blood perfusion imaging may be useful in future HT studies, since blood circulation plays a role in the thermal clearance of normal and diseased tissue.

Evaluation, Body temperature, Medical thermography, Hyperthermia treatment systems (Medical care), Hyperthermia treatment equipment

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Functional magnetic resonance imaging of the brain

Article Abstract:

Functional magnetic resonance imaging (fMRI) may help scientists study how the brain works. A common fMRI technique captures the magnetic changes in the brain caused by the increased blood flow and oxygenation that occur during visual, motor, and language tasks. During fMRI, many high resolution images of the brain are collected every few seconds for several minutes. Because it is a non-invasive technique that does not emit harmful radiation, one person can be studied repeatedly. FMRI can generate detailed maps of the areas of the brain that are used during language, movement, and vision. FMRI has also been used to study brain structure and function in neuropsychiatric patients in comparison to the normal population. Potential uses of fMRI include tracking patients' recovery from stroke or head injury, evaluating the effects of neuropharmaceuticals, studying seizure disorders, and identifying the dominant side of the brain used in language.

Innovations, Brain

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