Diffusion Weighted Imaging

DWI is a technique that applies a series of increasingly strong magnetic gradients during acquisition. Water molecules that are restricted in their motion will not wander over these gradients during the acquisition and so will tend to retain signal; whereas tissues with highly diffusing water molecules will quickly lose signal with the

Figure 5 Advantage of FLAIR for detecting parenchymal lesions. Occasionally, FLAIR can depict parenchymal lesions better than other methods, although this is unusual. Left to right: T1-weighted MRI without contrast, T2-weighted MRI, T1-weighted MRI after intravenous contrast, and FLAIR after contrast. Note that two subtle lesions, one in the midbrain and one in the frontal lobe, are best seen on the FLAIR sequence.

Figure 5 Advantage of FLAIR for detecting parenchymal lesions. Occasionally, FLAIR can depict parenchymal lesions better than other methods, although this is unusual. Left to right: T1-weighted MRI without contrast, T2-weighted MRI, T1-weighted MRI after intravenous contrast, and FLAIR after contrast. Note that two subtle lesions, one in the midbrain and one in the frontal lobe, are best seen on the FLAIR sequence.

application of progressively stronger gradients as the signal is dissipated by the gradients (22). The loss of signal as a function of diffusion gradient strength can be quantified by the apparent diffusion coefficient (ADC) which becomes a convenient measure of diffusion.

DWI has primarily been employed in evaluating early brain infarcts where the lower temperature within ischemic strokes decreases water diffusion, and therefore results in higher signal even before changes in tissue T1 and T2 can be detected (23-25). Therefore, DWI is used as a method of confirming the presence of a suspected brain infarct in its early hours. Its role in cancer imaging is more controversial. It is thought that intact tumors with their dense cellularity and intact cellular membranes have restricted diffusion due to these structural barriers. However, after treatment, as cell membranes become more disrupted and permeable, water is less restricted and is able to diffuse further, a process detectable as signal loss on DWI. This property of DWI makes it attractive as a method of demonstrating early responses to therapy (26). For instance, lymphoma with its tightly packed cells usually has restricted diffusion and is relatively bright on DWI compared to the detection of other tumors. Following treatment, however, the signal within lymphoma will drop on DWI indicating cellular membrane breakdown. Additionally, when attempting to differentiate between an abscess and a glioblastoma, DWI is often used because the water molecules within infected pocket of an abscess exhibit restricted diffusion (cellularity, mucus, proteinaceous debris, cytokines), and thus abscesses tend to be much higher in signal than do glioblastomas (27,28). However, ADC maps generated using the images obtained with different gradient field strengths can clarify the nature of the lesion (Fig. 6).

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