Diffusion Weighted Imaging

Diffusion-weighted imaging (DWI) relies on the detection of the Brownian motion of water molecules between the intra-cellular and extracellular spaces in the brain. Such motion through tissue is a random event, the speed and direction of which is dictated by the presence of barriers such as macro-molecules, cell membranes, and cellular organelles. Contrast is generated on DWI through background suppression and changes in signal intensity between images obtained at different gradient strengths (b values) that are sensitive to diffusion. Apparent diffusion coefficient (ADC) maps are then generated as the slopes of the lines derived from plotting the natural log of the signal intensity (SI) versus gradient strength. These apparent diffusion coefficient maps are essential for the visual evaluation of diffusion because the signal intensity (SI) of DWI is prone to T2 shine-through effects from heavy T2 weighting. ADC maps are independent of T1 and T2 effects, with decreased ADC values indicative of decreased diffusion.3

DWI has been evaluated for its potential in the differentiation of necrotic brain tumors from abscesses, of infiltrat figure 27.4. 10 year-old patient with leukemic meningeal infiltration. (A) FLAIR shows increased signal in the sulci at the brain convexity, more on the right side (arrowheads). (B) Corresponding enhanced T1-weighted images show marked meningeal enhancement compatible with the diagnosis of diffuse leukemic involvement.

figure 27.4. 10 year-old patient with leukemic meningeal infiltration. (A) FLAIR shows increased signal in the sulci at the brain convexity, more on the right side (arrowheads). (B) Corresponding enhanced T1-weighted images show marked meningeal enhancement compatible with the diagnosis of diffuse leukemic involvement.

figure 27.5. Enhanced Tj-weighted images, DWI images, and ADC maps of (A) right parietal abscess and (B) left parietooccipital glioblastoma multiforme (GBM). Note restricted diffusion in the abscess cavity (increased signal on DWI and decreased signal on ADC maps) in A compared to nonrestricted diffusion of the necrotic tumor (decreased signal on DWI and increased signal on ADC maps) in B.

figure 27.5. Enhanced Tj-weighted images, DWI images, and ADC maps of (A) right parietal abscess and (B) left parietooccipital glioblastoma multiforme (GBM). Note restricted diffusion in the abscess cavity (increased signal on DWI and decreased signal on ADC maps) in A compared to nonrestricted diffusion of the necrotic tumor (decreased signal on DWI and increased signal on ADC maps) in B.

ing tumor from vasogenic edema, and in tumor grading. Resembling free water, necrotic or cystic portions of tumors display high ADCs whereas abscess cavities, consisting of necrotic debris, neutrophils, and bacteria, which impede free water diffusion, tend to have low ADCs12 (Figure 27.5). However, although Dorenbeck et al. found overlapping ADC values between tumors and abscesses,13 multiple other studies demonstrated the opposite. In a case-control designed study, Guzman et al. found that the ADC values in patients with brain abscesses were significantly lower than those in patients with neoplastic lesions (P less than 0.05).14 Similar confirmatory results were reached by other investigators.15-17 Besides the differences seen within the cystic/necrotic portions, Chan et al. found that the tumor wall of cystic or necrotic brain tumors had significantly lower ADCs relative to those of the abscess wall (P less than 0.005).18

In a more quantitative study in which the authors calculated ADC values based on eight gradient (b) values, the specificity of DWI in differentiating tumor from abscess was 100% using a threshold ADC value of 1.10 x 10-3mm2/s. Unfortunately, those results cannot currently be applied clinically because most available commercial systems calculate ADC based on two b values only.19

Another application of DWI is the differentiation of epi-dermoid tumors from arachnoid cysts, a classic diagnostic problem on conventional MRI sequences. Because cysts contain more free water than solid masses, they tend to have more restricted diffusion and higher ADC values (see Figure 27.3C,D), which proved to be the case for arachnoid cyst versus epidermoid, as proven in two preliminary studies.20,21 DWI was found to provide the best lesion conspicuity of epidermoid in comparison to FLAIR and conventional sequences.8 By implementing both FLAIR (see foregoing) and DWI, epidermoid and arachnoid cysts can be fairly easily dif ferentiated without cisternography, the previous clinical standard, in the majority of cases.3

The ability of DWI to differentiate between high- and low-grade tumors has been evaluated by several groups. Sugahara et al. found that the cellularity of a variety of histologically verified gliomas correlated well with the minimum calculated ADC value of these tumors (P = 0.007) but not with the signal intensity on T2-weighted images.22 They hypothesized that the highly cellular (higher-grade) gliomas would have smaller intercellular space than tumors of lower cellularity and consequently would display lower ADCs; this is similar to the case for lymphoma and medulloblastoma, both being highly cellular CNS tumors and known to display low ADC values.23 Further support for the utility of DWI in tumor grading comes from the work of Bulakbasi et al., who evaluated 49 patients with malignant tumors. They found that ADCs were effective for grading malignant tumors (P less than 0.001) but not for distinguishing different tumor types with the same grade. In this study, high-grade malignant tumors had significantly lower ADC values than did low-grade malignant and benign tumors.24 Two more studies further supported the previous results, showing that ADC values are significantly higher in low-grade than in high-grade tumors.25,26 As far as tumor extension is concerned, however, DWI provided no clear advantage over the conventional methods. ADC values could not separate high-grade gliomas from surrounding edema.25,27,28

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