BACKGROUND AND PURPOSE:Voxel size/shape of diffusion tensor imaging (DTI) may directly affect the measurement of fractional anisotropy (FA) in regions where there are crossing fibers. The purpose of this article was to investigate the effect of voxel size/shape on measured FA by using isotropic and nonisotropic voxels.

Sensorimotor fibers of the lower extremity form an axis of rotation, around which the pyramidal fibers rotate anteriorly and the sensory fibers rotate posteriorly.

Purpose: To identify reproducible anatomical landmarks that would allow predicting the course of the pyramidal tract (PT) through centrum semiovale. Materials and Methods:A total of 20 normal volunteers (12 males, eight females) with a mean age of 34 years (range, 20 -59 years) were scanned using a 1.5-Tesla clinical MR unit to assess the trajectory of the PT. Neuroanatomical characteristics of the PT passing through the centrum semiovale were assessed by two independent observers. Tractography data of 10 consecutive patients with brain tumor were used to test the precision of anatomy-based prediction of the tract location.Results: On sagittal view, 95% of the PT depicted on tractography displayed a completely straight or primarily straight course through the supratentorial brain. In 98% of tracts, the bending point of the PT was identified Յ3 mm from the level of the anterior commissure-posterior commissure (AC-PC) plane. In 80% of PT, the intersection with the AC-PC plane occurred midway between the AC and the PC as seen on the sagittal view. Evaluation of the PT in 10 patients with brain tumor revealed that the anatomy-based prediction of PT on the contralesional hemisphere was not substantially deviated from the actual tractography depicted PT. PT on the lesional hemispheres, however, had deviations of various degrees. Conclusion:The course of the PT through supratentorial brain can be predicted based on easily identifiable landmarks. This anatomy-based prediction can be clinically applied for cases without substantial mass effect from a space occupying lesion. THE PRIMARY MOTOR CORTEX and pyramidal tract (PT) are among the most important eloquent regions of the brain that are connected to the lower motor neurons and control muscular movement. Thus, being able to determine whether a surgical brain lesion (e.g., tumor) is located near the motor system would be of major clinical importance. The PT, by definition, consists of fibers that travel longitudinally through the ventromedial part of the medulla oblongata, known as the pyramid. Fibers from the precentral gyrus (Brodmann's area 4) of the primary motor cortex make up the major part of this fiber bundle. The precentral gyrus is relatively easily identified on computed tomography (CT) and MRI using well established neuroradiological method (1-4). The PT at the level of the internal capsule is also reliably identifiable on MRI, since focal hyperintensity on T2-weighted images and focal hypointensity on T1-weighted images are apparent in the posterior third quarter of the posterior limb of the internal capsule. These signal characteristics have been attributed to lower axonal and myelin densities (5).Assessing the location of the PT at the centrum semiovale is much more challenging, since no landmark architecture are present that help identify its position. However, it is now possible to identify such vital structures using diffusion-tensor imaging (DTI) based tractography (6 -11). We have used tractography in our institute to study a number of normal volunte...

BACKGROUND AND PURPOSE:The purpose of this work was to test the feasibility of using high angular resolution diffusion imaging (HARDI)-based multitensor tractography to depict motor pathways in patients with brain tumors.

Purpose: Although previous studies have revealed the mechanisms of changes in diŠusiv-ity (apparent diŠusion coe‹cient [ADC]) in acute brain infarction, changes in diŠusion anisotropy (fractional anisotropy [FA]) in white matter have not been examined. We hypothesized that membrane permeability as well as axonal swelling play important roles, and we therefore constructed a simulation model using random walk simulation to replicate the diŠusion of water molecules.Materials and Methods: We implemented a numerical diŠusion simulation model of normal and infarcted human brains using C＋ ＋ language. We constructed this 2-pool model using simple tubes aligned in a single direction. Random walk simulation diŠused water. Axon diameters and membrane permeability were then altered in step-wise fashion. To estimate the eŠects of axonal swelling, axon diameters were changed from 6 to 10 mm. Membrane permeability was altered from 0z to 40z. Finally, both elements were combined to explain increasing FA in the hyperacute stage of white matter infarction.Results: The simulation demonstrated that simple water shift into the intracellular space reduces ADC and increases FA, but not to the extent expected from actual human cases (ADC approximately 50z; FA approximately ＋20z). Similarly, membrane permeability alone was insu‹cient to explain this phenomenon. However, a combination of both factors successfully replicated changes in diŠusivity indices.Conclusion: Both axonal swelling and reduced membrane permeability appear important in explaining changes in ADC and FA based on eigenvalues in hyperacute-stage white matter infarction.