Advances in Image-Guided Urologic Surgery by Joseph C. Liao & Li-Ming Su

Author:Joseph C. Liao & Li-Ming Su
Language: eng
Format: epub
Publisher: Springer New York, New York, NY

Diffusion-Weighted Imaging

In diffusion-weighted imaging (DWI), image contrast is generated by differences in the Brownian motion of water molecules within different tissues. While water molecules in a container outside the body are in constant, uninhibited motion, the movement of water molecules in biologic tissues is restricted because of interactions with cell membranes and macromolecules [18]. The motion of water molecules is more restricted in tissues with high cellular density and intact cell membranes. The high cellular density and more complex intracellular microstructure of tumors restrict water diffusion in the tissue, a phenomenon that is detectible as a bright signal on DWI.

Diffusion-weighted gradients are applied to water protons, and the resultant signal is related to the ability of water protons to move freely in space. The “b-value” (in s/mm2) of DWI is a function of the strength of the diffusion gradient. Low b-values measure water movement over a large length (i.e., capillary perfusion). High b-values measure water movement over a small length (i.e., within cells). The use of higher b-values (1,000–2,000 s/mm2) has been shown to improve prostate cancer detection [19, 20]. When at least two b-values are acquired, an apparent diffusion coefficient (ADC) map can be generated which displays the ADC value of each voxel. According to Barentsz, minimal requirements for generating ADC maps are b-values of 0, 100, and 800–1,000 and optimally a b-value of 500 would also be included [13]. The ADC map is especially useful for prostate cancer identification. Prostate cancer typically has a high signal intensity (bright) on DWI at high b-value and low signal intensity (dark) on ADC maps (Fig. 13.8).

Fig. 13.8Diffusion-weighted imaging (DWI) and apparent diffusion coefficient map on 3T magnet without endorectal coil in a patient with Gleason 5 + 4 pT3b adenocarcinoma at subsequent prostatectomy. On the axial T2-weighted image (a), a low-signal region (arrow) bulges the capsule. The corresponding apparent diffusion coefficient (ADC) map (b) shows low signal (0.938 × 10−3 mm2/s). The axial high b-value DWI (b = 1,400 s/mm2) image (c) shows asymmetric high signal (arrow) concordant with the ADC map. However, this lesion is less obvious on the lower b-value DWI (b = 800 s/mm2 image) (d)

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