In the multiparametric magnetic resonance imaging (mpMRI) of prostate cancer (PCa), diffusion weighted imaging (DWI) is used to identify suspected malignant tumors based on the movement of water molecules in and between tissues. Normal cells allow relatively unrestricted molecule movement, whereas tumor cells restrict the motion due to the nature of their membranes, biochemical composition, and dense but irregular vascularity (blood supply). This results in a lower apparent diffusion coefficient (ADC) calculated during DWI scanning; the lower the ADC, the greater the likelihood of cancer. In order to increase confidence in interpreting the images, the image operator can “amplify” DWI findings by adding one or more scanning sequences at higher b-value. This means increasing the strength and timing of each field gradient used to “select a region of imaging and to encode the location of MRI signals received from the object being imaged.”[i] The higher the value, the greater the diffusion effects on imaging. There is a trade-off, however. As the b-value is raised, anatomical detail is obscured by “noise” but tissues that restrict water molecule motion show up with greater enhancement. This allows more definitive discrimination of PCa tumors from benign tumors that are picked up by other MR imaging parameters. Thus, image sequences from low to high b-values must be read side by side in order to establish the precise anatomic location of the suspected tumor.

There is no consensus on the optimal b-value. Modern scanners usually allow a range of 0-4000s.mm². Depending on the organ being imaged, b-values typically range from 50-1000s/mm². Two recent studies explored DWI at an ultra high b-value of 2000s/mm².

1. Barral et al. (2015)[ii] designed a study to correlate the accuracy of 3T MRI in which DWI occurred with a b-value of 2000s/mm². Thirty-five PCa patients who were to be treated with radical prostatectomy underwent 3T DWI-MRI without an endorectal coil at the high b-value. Following surgery, the gland specimens were analyzed by a pathologist (histopathology) for the location, size and Gleason grade of all tumor foci (113 foci of PCa). These were matched with the high intensity (b-value) focal results on MRI and correlated for accurate image predictions. Two readers who were blinded to the histopathology reports interpreted the images independently of each other. While the overall image interpretations showed high sensitivity and specificity (averages ranging from 80% to 95+%, one reader missed 20% of tumor foci and the other missed 25% of the 113 foci. However, the majority of missed foci were small and insignificant. The authors concluded, “A normal b-2000 DW-MRI at 3-T may miss small tumors without or with a minor Gleason 4 component,” but the overall performance of ultra high b-value DWI was very good.
2. Kwak et al. (2015)[iii] incorporated ultra high b-value into their trial of a computer-aided diagnosis (CAD) system for PCa that was designed to test whether CAD could improve the “accuracy, reproducibility, and standardization” of mpMRI. The experimental CAD system utilized two sequences: T2-weighted MRI and high b-value (2000s/mm²) DWI. The study involved 244 patients. The CAD system was used to scan 108 patients. Another group of 136 patients underwent two scans using other combinations of MRI sequences in order to compare the CAD performance. They found that the CAD system with ultra high b-value outperformed other MRI sequence combinations, writing, “The novel CAD system is able to detect the discriminative texture features for cancer detection and localization and is a promising tool for improving the quality and efficiency of prostate cancer diagnosis.

Thus, the addition of ultra high b-value can provide additional confirmation of a diagnosis of prostate malignancy, and is being increasingly incorporated into DWI-MRI sequences.

[i] “Gradient magnetic field” from The Free Dictionary by Farlex at http://medical-dictionary.thefreedictionary.com/gradient+magnetic+field.

[ii] Barral M, Cornud F, Neuzillet Y et al.  Characteristics of undetected prostate cancer on diffusion-weighted MR Imaging at 3-Tesla with a b-value of 2000s/mm2: Imaging-pathologic correlation. Diagn Interv Imaging. 2015 Apr 28. pii: S2211-5684(15)00128-X. doi: 10.1016/j.diii.2015.03.004. [Epub ahead of print]

[iii] Kwak JT, Xu S, Wood BJ et al. Automated prostate cancer detection using T2-weighted and high-b-value diffusion-weighted magnetic resonance imaging. Med Phys. 2015 May;42(5):2368-78. doi: 10.1118/1.4918318.