Diffusion-weighted imaging assesses the diffusion of water molecules within different tissues. Normal prostate glandular tissue has a higher water diffusion rate than cancer tissue owing to restricted diffusion in tightly packed cancer cells. DWI is an inherently T2-weighted sequence but, unlike conventional T2-weighted imaging, prostate cancer frequently demonstrates increased signal intensity on standard DWI scans, making the tumor difficult to visualize within the normal high-signal peripheral zone. To reduce the effect of this T2 'shine through' effect, the apparent diffusion coefficient (ADC) is calculated, which corresponds to the difference of diffusion. Prostate cancer appears as an area of high signal on DWI images, but as a low-signal region on ADC mapping, owing to restricted diffusion .
The advantages of DWI are high-contrast resolution between normal prostate and cancerous tissue, and its short acquisition time. The disadvantages of DWI are its low spatial resolution (due to low signal:noise ratio), increased susceptibility to artifacts and overlap of diffusion values between benign and malignant lesions.There is no ADC threshold value that will reliably differentiate between benign and malignant lesions. In addition, there are age-related changes in diffusion within the prostate and intersubject variability.
Recent studies have demonstrated that DWI, when used as an adjunct to T2-weighted MRI, improved sensitivity and specificity for prostate cancer detection to 84 and 85%, respectively.[68,69] The sensitivity of DWI is better in the peripheral zone than the central gland.[70] DWI has also been shown to be helpful in the identification of prostate cancer in patients with previous negative biopsies and persistently elevated PSA.
Diffusion-weighted imaging is also helpful in differentiating between low-, intermediate- and high-risk patients as there is a significant correlation between the Gleason score of the tumor and the ADC value. There is also an inverse relationship between the ADC value and the percentage of tumor involvement in core biopsies. Pre- and post-radiotherapy ADC values can also be used to assess response to treatment. In patients managed by active surveillance, the reduction of the ADC value by 10% or more indicates disease progression.
A recent study has demonstrated that DWI can be used to accurately differentiate between benign and malignant pelvic nodes in patients with prostate cancer and the ADC value is significantly superior to size criteria
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