Diffusion-weighted MRI

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

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

To visualize the anisotropy of water diffusion in tissue, diffusion-tensor MRI is used.

In the diffusion-tensor MRI on the orientation of diffusion ellipsoids in voxels, the nerve fibers forming the nerve tracts are determined, connecting the eigenvectors of the diffusion tensor with each other. Algorithms of connection are rather complicated, therefore various methods of calculation are used, allowing to "draw" the course of the set of nerve fibers that form the nerve tract. Because of this, tensor MRI is often called a tractography - a method of visualizing the course of the nerve tracts. In its simplest form, partial diffusion anisotropy is color coded, and the visualization of the directions of the diffusion motion of water molecules in tissues is performed by staining with a certain color of pixels depending on the orientation of their own vector (red in the X axis, green in the Y axis, blue in the Z axis).

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

Clinical application of diffusion-weighted and diffusion-tensor MRI. Reduction of the measured diffusion coefficient in brain tissues is a sensitive indicator of ischemic disorders and severity of ischemia. To date, the use of diffusion-weighted images is one of the fastest and highly specific methods of 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 on diffusion-weighted images, the brain lesion area has a typically high MP signal, whereas normal brain tissues look dark. On the maps of the measured diffusion coefficient, the opposite is true. Maps of the measured diffusion coefficient became a means of diagnosing ischemia and dynamic observation of the development of acute cerebrovascular accident and subsequent chronic tissue degeneration caused by ischemia. Non-invasiveness and rapid application of diffusion-weighted images predetermine the main importance of the method in the primary diagnosis of ischemic brain damage.

All diffusion studies are carried out without the introduction of a contrast agent, which is important for severe patients and for specialized studies of brain development in children, beginning with the intrauterine period. In the latter case, diffusion MRI allows to obtain additional qualitative (visualization) and quantitative tissue characteristics, opens new possibilities for studying the microstructure of brain tissues in the process of its development.

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

Invaluable information for such a short scan time, the use of diffusion-weighted images is given in the diagnosis of inflammatory lesions of the brain and spine (eg, brain abscess, empyema). Purulent abscess content is characterized by high MP signal and is easily visualized at any stage of treatment, including postoperative. The peculiarities of the structural organization of some neoplasms of the brain, in particular meningiomas and neurin, make it possible to predict the histological type of the tumor even before the operation when applying diffusion-weighted images with high reliability. Based on the data of this method, the epidermoid and arachnoid cysts are precisely differentiated.

Traktografiya - a new and promising technique that allows non-invasive "see" the conductive ways of the brain. Despite the still existing technical difficulties, the first results in the application to the tasks of neurosurgery seem promising. It became possible with the help of diffusion-tensor MRI, knowing the location of the conducting pathways and taking into account their interest in the pathological process (displacement / deformation or invasion and damage), to plan operational access and the volume of surgical removal of intracerebral tumors.

[1], [2], [3], [4], [5], [6]