What is MRI? Magnetic resonance imaging (MRI) is a young, developing technology used to create images with extraordinary detail of body tissue or the brain by applying nuclear magnetic resonance phenomena. The distribution of hydrogen nuclei of water and fat in the body depends on the tissue type and whether or not the tissue is healthy or diseased. The MRI technique uses a pulse of radio-frequency energy to excite the protons in the region of interest. Because these protons are in a stable magnetic field, they will only absorb a characteristic energy. The image brightness is a complex function of properties in the region of interest, which include parameters of spin density (concentration of protons) and the relaxation times of the protons. Manipulating these properties is accomplished by varying the experiment (pulse sequence) used at the time of examination to yield an image that contains high contrast. Contrast provides the optimum difference between light and dark regions of the tissue or organ to help the physician detect lesions, such as a tumor. Although MRI is normally a noninvasive technique, contrast agents can be administered to a subject to enhance a region of interest. Intrinsic contrast differences caused by oxygen metabolism, which produce different ratios of oxy- and deoxyhemoglobin, are the basis for intense interest in cognitive studies using MRI, an emerging technology called functional MRI (fMRI).
How is MRI used? Soft tissue, such as human organs, is relatively transparent to X-rays, limiting the practical application of other imaging modalities such as computed tomography. MRI, however, has excellent sensitivity for these tissues and the added benefit of not exposing the patient to ionizing radiation. The magnetic resonance phenomenon has been steadily gaining in vitro application in the fields of chemistry, biochemistry, and the medical life sciences since its inception in 1946. The technique was first extended to a live animal by Jasper Jackson in 1967, and the first two-dimensional MR image was generated in 1972 by Lauterbur. Common to the first MR image and the images of today are the use of nonuniform magnetic fields, such as a linear variation of field strength along one spatial coordinate. Because the field strength varies over a distance, MR signals originating from different spatial locations can be discriminated.
Modern techniques in MRI take advantage of contrast between tissues to identify the location of structures emitting a specific MR signal. Extensive work in the field has focused on methods for creating techniques to enhance the contrast between normal and pathologic tissues. In addition to anatomical images, MRI can be used to generate additional information through what is termed localized spectroscopy or magnetic resonance spectroscopy (MRS). MRS is the only noninvasive technique capable of directly measuring chemicals within the body. MRS can be used to monitor the metabolic state of diseased and normal tissues by quantitating metabolites using phosphorous (31P) NMR. Other chemicals including lactic acid, produced during exercise, can be measured using hydrogen (1H) MRS. 31P MRS has been used as a method for the noninvasive investigation of brain metabolism as well as ischemia, brain infarction, and stroke. Both 31P and 1H MRS have been used to study seizures, including those of epilepsy, and to study tumor growth. In vivo spectroscopy is a relatively new technique in the imaging field and is rapidly developing in importance and utility. Rapid-scanning techniques, which allow the acquisition of an MR image in a time frame of 30 to 100 msec, have recently opened new applications in MRI including cardiac studies, blood flow, diffusion, drug therapy, and metabolic activity. In the last two years, these rapid imaging techniques have allowed studies in human functional brain imaging. Researchers have shown localized changes in signal intensity in images of the human brain during task-specific brain stimulation. The current use of MRI to diagnose many disease states without X-rays or surgery is continuing to expand. The potential future development of areas including cancer diagnosis and assessment of treatment efficacy and human brain mapping makes this dynamic field extremely interesting and exciting.
The MRI Division has two magnetic resonance imagers, a GE chemical shift imager with a 45-cm bore which can accommodate animals up to the size of baboons; and an Elscint imager with a 100-cm bore for human studies. Both instruments operate at a magnetic field of 2 tesla, and a large variety of volume and surface coils for imaging or localized spectroscopy are available. Facilities for constructing unique or dedicated radiofrequency coils are also available. MRI projects at the RIC include: developing computer programs to automatically measure the volume and segmentation of grey and white matter and cerebral spinal fluid in MR images and for multimodality spatial registration and normalization of MR images; developing an MRI method to detect periodontal disease; using MRI to analyze the efficacy of a biodegradable bone bracing material; using MRI to assess cancer suppression of the retinoblastoma gene using gene therapy in a mouse model; developing an in vivo method to measure cerebral energetics during exposure to hyperbaric oxygen; and using MRI to develop unique melanin-based MRI contrast agents including blood-pool, site-directed, and immunospecific imaging agents.