Magnetic Resonance Imaging

Volumetric enlargement of brain tissue due to edema is readily apparent on MRI and the use of gadolinium, an MRI contrast agent, enhances regions of altered BBB. Differences in water content may be detected on MRI by variations in the magnetic field generated primarily by hydrogen ions. T2-weighted sequences

Figure 12 Periventricular FLAIR hyperintensity due to hydro-cephalic edema.
Figure 13 Central pontine myelinolysis illustrated as (a) T2-weighted hyperintensity and (b) T1-weighted hypointensity in the pons.

and fluid-attenuated inversion recovery (FLAIR) images reveal hyperintensity in regions of increased water content. Reversible T2 hyperintensities of the white matter may be seen in HACE and hypertensive encephalopathy. FLAIR images eliminate the bright signal from CSF spaces and are therefore helpful in characterizing periventricular findings such as hydro-cephalic edema (Fig. 12). These conventional MRI sequences are more sensitive in the detection of lesions corresponding to hypodensities on CT. MRI is also superior in the characterization of structures in the posterior fossa (Fig. 13). Recent advances in MRI technology make it possible to specifically discern the type of edema based on signal characteristics of a sampled tissue volume. This discriminatory capability resulted from the development of diffusion imaging techniques. The use of strong magnetic field gradients increases the sensitivity of the MR signal to the random, translational motion of water protons within a given volume element or voxel. Cytotoxic edema and cellular swelling produce a net decrease in the diffusion of water molecules due to the restriction of movement, imposed by intracellular structures such as membranes and macromolecules, and diminished diffusion within the extracellular space due to shrinkage and tortuosity. In contrast, the accumulation of water within the extracellular space as the result of vasogenic edema allows for increased diffusion. Diffusion-weighted imaging (DWI) sequences yield maps of the brain, with regions of restricted diffusion appearing bright or hyperintense. The cytotoxic component of ischemic edema has been demonstrated on DWI within minutes ofischemia onset. DWI has also been used to grade the severity of TBI, because cytotoxic injury is thought to accompany severe trauma. Apparent diffusion coefficient (ADC) maps may be generated from a series of DWI images acquired with varying magnetic field gradients. ADC elevations, resulting from vasogenic edema, appear hyperintense on ADC maps, whereas decreases in ADC due to cytotoxic edema appear hypointense (Fig. 14). These maps may be sampled to measure the ADC of a given voxel for multiple purposes, such as differentiating tumor from tumor-associated edema. The vector of water diffusion in

Figure 14 The cytotoxic component of acute cerebral ischemia is demonstrated by ADC hypointensity (a). The ischemic region appears hyperintense on DWI (b), whereas T2-weighted sequences may be unrevealing at this early stage (c).

each of three orthogonal planes also helps to differentiate types of edema. Isotropic diffusion refers to relatively equivalent vectors in all three planes, whereas anisotropic diffusion exists when these effects are disproportionate. Vasogenic and interstitial edema produce anisotropic diffusion patterns. In contrast, cytotoxic edema is less anisotropic. Anisotropic measurements may therefore be helpful in the characterization of cerebral edema.

The recent development of perfusion-weighted imaging (PWI) with MR technology provides parametric maps of several hemodynamic variables, including cerebral blood volume. Elevations in cerebral blood volume associated with cerebral edema are detectable by this technique.

Simultaneous acquisition of multiple MRI sequences enables the clinician to distinguish various forms of cerebral edema. T2-weighted sequences and FLAIR images permit sensitive detection of local increases in water content. Gadolinium-enhanced T1-weighted sequences reveal sites of BBB leakage that may be present surrounding tumors (Fig. 15) or abscesses. DWI localizes abnormal areas of water diffusion, with ADC maps differentiating various forms of edema. PWI can detect regional elevation of cerebral blood volume. The composite interpretation of these studies has revolutionized the diagnosis of cerebral edema. These images often reflect the combined effects of multiple types of edema. For instance, the cytotoxic component of ischemic edema will cause a reduction in the ADC, whereas the vasogenic component will counter this trend. A pseudo-normalization of the ADC may result from these opposing influences. Serial imaging with this noninvasive modality also allows for the temporal characterization of edema evolution. The relative contributions of cyto-

toxic and vasogenic edema with respect to the ADC during acute ischemic stroke and TBI have been investigated in this manner. The main limitations of this technology logistically relate to cost, availability, contraindications, and its restricted use in critically ill individuals.

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