الفهرس 
SPINAL CORD ANATOMYOnly 14 pages are availabe for public view
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Abstract
T
raumatic injuries of the cervical spine are potentially catastrophic. When associated with neurologic damage, they can result in devastating medical, social, emotional, and financial consequences. Fractures with bony retropulsion, disk extrusions, and epidural hematomas can result in cord compression. Spinal cord injury may be complete or incomplete.
Spinal cord injury (SCI) results in drastic functional disabilities in patients. Due to the interference of spinal shock with functional assessment of SCI patients in the clinical setting, it is important to develop a noninvasive imaging technique for early evaluation of spinal cord integrity after injury.
X-ray and better CT are useful in delineating bony details and are important in preoperative planning. The cord is vulnerable to transection if the applied forces are sufficient. It should be noted that diagnosis of cord transection cannot be made, even with a severe fracture dislocation evident on CT.
Magnetic resonant imaging (MRI) is the method of choice for detection and diagnosis of many disorders in the spine because of its inherent sensitivity to subtle soft tissue changes and its capability to displaying long segment of vertebral column in one examination. In the context of trauma MRI can detect ligamentous injury and internal derangement of the spinal cord.
Diffusion weighted MRI (DWI) promises to add to the diagnostic specificity of MRI in the spine. Based on its ability to depict the microscopic motion of water protons, DWI can be used to sensitize image contrast to microstructural changes and thus can provide important information complimentary to regular MRI sequences.
DWI provides important biological information about the composition of tissues, their physical properties, their microstructure, and their architectural organization. This information is available non invasively and without contrast administration. DWI generates images that are based on the molecular motion of water, which is altered by disease.
Water molecules held in a container outside the body are in constant random motion. This uninhibited motion of water molecules is what’s called free diffusion. By contrast, the movement of water molecules in biologic tissues is restricted because their motion is modified and limited by interactions with cell membranes and macromolecules. The degree of restriction to water diffusion in biologic tissue is inversely correlated to the tissue cellularity and the integrity of cell membranes.
Water diffusion in the white matter is defined based on axonal alignment. Water diffuses preferentially in a direction parallel to the axon’s longitudinal axis, but diffusion is relatively restricted in the perpendicular axis.
Diffusion tensor imaging (DTI) is an application of DWI that provides unique quantitative information about the structural and orientational features of central nervous system tissue. This method offers in vivo localization of neuronal fiber tracts, which was previously impossible.
Two DWI methods, diffusion tensor imaging (DTI) and diffusion tensor tractography (DTT) are used to study the white matter tracts in the central nervous system.
Fractional anisotropy depends on how the water molecules diffuse in the living body depending on the nature of the local environment, and this variation is called (anisotropic diffusion). The white matter fibers constituting the spinal cord are highly anisotropic, and visualization of their anisotropy should delineate axonal arrangement. An image representing anisotropy two-dimensionally is called an (anisotropy map) or an (FA map). In a color FA map, different colors are assigned to different axes; thus, fibers can be distinguished from each other by using different colors according to the direction of their arrangement.
DTT (diffusion tensor tractography) is an imaging technique in which the direction of maximum anisotropy for each voxel is traced. Before spinal DTT can be applied clinically, it is indispensable to conduct detailed analyses to determine the extent to which DTT reflects each tissue type, and the reliability with which DTT depict axonal information.
Unlike conventional MRI, which depicts the injured spinal cord only as changes in signal intensity on T1 and T2 weighted images, DTT allows visualization of the injury in the form of interrupted white matter fibers.
Spinal cord DTI in humans still has a number of limitations. Adequate spatial resolution remains a problem, and it is difficult to visualize the individual funiculi on diffusion-weighted images, particulary in the lower thoracic cord, DTI of these segments is affected more by artifacts arising from cardiac and respiratory motion and cerebrospinal fluid pulsation ,the use of faster imaging techniques such as parallel imaging and single-shot echoplanar imaging and the use of cardiac pulse gating have helped to reduce these artifacts.
However, scan acquision time is still a limitation for patients with acute SCI because these patients often cannot withstand additional scanning time in the MRI suite. In addition, the signal-to-noise ratio is not uniform throughout the cervical spinal cord and is significantly decreased in caudal segments.
A low signal-to-noise ratio can lead to overestimation of anisotropy measures, particularly in low-anisotropy tissues such as the central gray matter, the use of 3-T MR scanners improves the signal-to-noise ratio but they are still not used universally.