Diffusion-weighted and diffusion tensor imaging of the brain, made easy

Abstract

Diffusion-weighted and diffusion tensor imaging (DWI/DTI) has revolutionized clinical neuroimaging. Pathology may be detected earlier and with greater specificity than with conventional magnetic resonance imaging sequences. In addition, DWI/DTI allows exploring the microarchitecture of the brain. A detailed knowledge of the basics of DWI/DTI is mandatory to better understand pathology encountered and to avoid misinterpretation of typical DWI/DTI artifacts. This article reviews the basic physics of DWI/DTI exemplified by several classical clinical cases.

Figure 1

Graphical display of water molecules moving at different rates through the gray matter and cerebrospinal fluid (CSF). The effective distance that water molecules travel in gray matter is smaller than in CSF (represented by the magnitude of the red arrow). The difference in travelled diffusion distance versus time is displayed in the lower graph. The faster the molecules move, the more distance is travelled, the more signal loss will occur if diffusion gradients are applied. Consequently the signal loss in the CSF is higher (hypointense) compared with the signal loss in the gray matter (hyperintense relative to the CSF).

Figure 2

Sample of an axial DWI, ADC, FA and color-coded (cFA) image of the brain. On DWI the cortex is slightly more intense than the white matter because of residual T2 effects (T2 shine through). The CSF is DWI hypointense. On the ADC map the cortex and white matter are equally intense because the T2 effect has been cancelled out. The gray and white matter are ADC hypointense compared with the CSF because diffusion is restricted within the brain and high in the CSF. The FA map shows high degrees of anisotropic diffusion along white matter tracts in the corpus callosum and internal capsule. A low degree of anisotropic diffusion is seen in the cortical and central gray matter (areas of isotropic diffusion). The color-coded FA maps display the predominant direction of diffusion, with left to right diffusion in the corpus callosum (red), superior-inferior diffusion in the internal capsule (blue) and anterior-posterior diffusion in the frontal white matter (green).

Figure 3

Anisotropic diffusion (a) resembles a three-dimensional ellipsoid in space with predominant diffusion of molecules along the main axis of the ellipsoid and restricted diffusion perpendicular to the ellipsoid. Isotropic diffusion (b) can be represented by a sphere with equal diffusion in all directions in space. The arrows represent the motion of individual molecules.

Figure 4

(a) Raw DWI data acquired with diffusion-encoding gradients applied along the three principal axes in space (Dxx, Dyy, Dzz). The anisotropy of diffusion in the brain can be recognized by the differences in signal intensity of the various white matter tracts in relation to the applied diffusion gradient. The fourth image is the averaged trace of diffusion or DWI image that is used for clinical routine. (b) For diffusion tensor imaging, diffusion gradients are applied in six directions to fully sample the diffusion tensor in space. Consequently, six individual diffusion-weighted images are generated that are again averaged to render the trace of diffusion or diffusion-weighted image.

Figure 5

Graphical display of the range of isotropic towards anisotropic diffusion as can be observed in the various regions of the brain. An FA value of zero represents complete isotropic diffusion (perfect sphere); an FA value of one represents the hypothetical case of complete anisotropic diffusion (narrow ellipsoid).

Figure 6

(a) DTI allows the shape of the diffusion tensor in space (e.g. ellipsoid) to be calculated but also gives the orientation of the tensor in space. The tensor is represented by the three principal eigenvectors both in direction (orientation of the vector) and magnitude (length of the vector). Various orientations of the diffusion tensor are displayed, which provide information about the orientation of white matter tracts in the brain. (b) Based on the DTI data, fiber tracts can be reconstructed from the DTI data based on the identified direction and magnitude of the anisotropic diffusion within the individual voxels. This graph shows a crossing of descending corticospinal tracts (blue) and anterior-posterior (green) running tracts. Various left to right connecting tracts are seen in red.

Figure 7

In multitensor DTI, a significantly higher number of diffusion-encoding gradients are applied along multiple directions in space. This provides more detailed information about the course and crossing of white matter within the brain. In this example, 36 directions have been sampled. In the bottom row the various DTI maps that can be calculated are displayed.

Figure 8

Example of a 6-year-old boy with an acute bilateral, right dominant ischemic infarction. DWI shows several areas of restricted diffusion with matching ADC hypointensity (usually irreversible cytotoxic edema) in the distribution of the middle and anterior cerebral arteries. On the FA maps a reduced anisotropic diffusion is seen especially in the right middle cerebral artery infarction indicating ongoing loss of fiber tract integrity and cytotoxic edema.

Figure 9

Example of a 13-year-old boy with a large frontal and epidural brain abscess. T2-weighted and contrast-enhanced T1-weighted images reveal a large, centrally cystic lesion with peripheral enhancement and extensive perifocal white matter edema. The contents of the lesion are DWI hyperintense and ADC hypointense indicating restricted diffusion confirming a large abscess. The perifocal white matter edema is ADC hyperintense compatible with (usually reversible) vasogenic edema. The FA maps show the displacement of adjacent white matter tracts as well as increased isotropic diffusion in the areas of the vasogenic edema.

Figure 10

Example of a 10-year-old female with a left cerebellar pilocytic astrocytoma. The T2- and contrast-enhanced T1-weighted images reveal a large peripherally solid, partially contrast enhancing, centrally necrotic/cystic tumor in the left cerebellar hemisphere. DWI and ADC maps show an increased diffusion within the cystic component excluding abscess formation. The FA maps show a lack of internal diffusion directionality as characterized by the FA hypointensity. The green-encoded white matter tracts in the middle cerebellar peduncle are compressed; the internal fiber architecture of the brainstem is preserved but mildly displaced.

Figure 11

Example of a 13-year-old female patient with an epidermoid in the left parapontine cistern. The lesion is T1- and T2-isointense with the CSF, limiting detection of the exact extent of the lesion. The epidermoid can easily be recognized on the DWI maps because of the restricted diffusion within the lesion due to the high cellularity (DWI hyperintense, ADC hypointense, FA slightly hyperintense).