Diffusion-weighted MRI.

Diffusion is the main physical process occurring during metabolic reactions of the cell. The first diffusion-weighted MR image was constructed in 1985. Diffusion MRI came into clinical practice together with MRI scanners of the III generation. To obtain diffusion-weighted tomograms, echoplanar pulse sequences "spin echo" EPI with two diffusion gradients of the same amplitude and duration are used. To quantitatively assess the diffusion properties of water in tissue, parametric diffusion maps are constructed, on which the color of each pixel corresponds to the measured diffusion coefficient. On the diffusion map, tissues with a high diffusion rate of water are colored in red and white tones, tissues with a low diffusion rate are colored in blue and black.

The dependence of the diffusion capacity of molecules on direction is called diffusion anisotropy. In the white matter of the brain, water molecules easily diffuse along nerve fibers, but their movement across the fibers is limited by the impermeable myelin sheath.

Diffusion tensor MRI is used to visualize the anisotropy of water diffusion in tissue.

In diffusion tensor MRI, the orientation of diffusion ellipsoids in voxels is used to determine the course of nerve fibers that form nerve tracts by connecting the diffusion tensor's eigenvectors to each other. The connection algorithms are quite complex, so various calculation methods are used to "draw" the course of many nerve fibers that form a nerve tract. As a result, tensor MRI is often called tractography - a method for visualizing the course of nerve tracts. In its simplest form, partial diffusion anisotropy is color-coded, and the directions of diffusion movement of water molecules in tissues are visualized by coloring pixels in a certain color depending on the orientation of their eigenvector (red - along the X axis, green - along the Y axis, blue - along the Z axis).

Diffusion tensor MRI allows us to detect structural connections between parts of the brain, which is especially important in volumetric processes and diseases that distort the anatomical structure or destroy white matter (tumors, TBI, demyelinating diseases, etc.).

Clinical application of diffusion-weighted and diffusion tensor MRI. A decrease in the velocity of the measured diffusion coefficient in brain tissue is a sensitive indicator of ischemic disorders and the severity of ischemia. Today, the use of diffusion-weighted images is one of the fastest and most specific methods for diagnosing ischemic cerebral infarction in the early stages of its development (up to 6 hours), when there is a "therapeutic window" for the use of thrombolysis and partial or complete restoration of blood flow in the affected brain tissues. In the acute phase of cerebral stroke, the area of the brain lesion on diffusion-weighted images has a typically high MP signal, while normal brain tissue appears dark. The opposite is true on the maps of the measured diffusion coefficient. Maps of the measured diffusion coefficient have become a means of diagnosing ischemia and dynamically monitoring the development of acute cerebrovascular accident and subsequent chronic tissue degeneration caused by ischemia. The non-invasiveness and rapidity of application of diffusion-weighted images determine the primary importance of the method in the primary diagnosis of ischemic brain damage.

All diffusion studies are performed without the introduction of a contrast agent, which is important for seriously ill patients and for specialized studies of brain development in children, starting from the intrauterine period. In the latter case, diffusion MRI allows obtaining additional qualitative (visual) and quantitative tissue characteristics, opens up new possibilities for studying the microstructure of brain tissue during its development.

Diffusion-weighted images and diffusion maps provide additional diagnostic information for differentiating brain tumors with similar manifestations on T1 and T2 MRI (gliomas, tumors with ring-shaped accumulation of contrast agent), peritumoral edema (vasogenic or cytotoxic), provide data on the presence or absence of intratumor cysts, etc.

Invaluable information in such a short scanning time is provided by diffusion-weighted images in diagnosing inflammatory lesions of the brain and spine (e.g., brain abscesses, empyema). The purulent contents of the abscess are characterized by a high MP signal and are easily visualized at any stage of treatment, including postoperative. The structural organization of some brain tumors, in particular meningiomas and neurinomas, makes it possible to predict the histological type of tumor with high reliability when using diffusion-weighted images even before surgery. Based on the data from this method, epidermoid and arachnoid cysts are accurately differentiated.

Tractography is a new and promising technique that allows non-invasively "seeing" the brain's conduction pathways. Despite the technical difficulties that still exist, the first results in application to neurosurgical tasks seem promising. It has become possible with the help of diffusion tensor MRI, knowing the location of the conduction pathways and taking into account their interest in the pathological process (displacement/deformation or invasion and damage), to plan the surgical approach and the volume of surgical removal of intracerebral tumors.

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