Quantitative Evaluation of Computed High b Value Diffusion-Weighted Magnetic Resonance Imaging of the Prostate

OBJECTIVESComputed diffusion-weighted magnetic resonance imaging (cDWI) refers to the synthesizing of arbitrary b value diffusion-weighted images (DWI) from a set of measured b value images by voxelwise fitting. The objectives of this study were to quantitatively analyze the noise and the contrast-to-noise ratio (CNR) in cDWI as a function of b value by numerical simulations and by measurements in patients with prostate cancer and to compare cDWI to directly measured DWI at a b value of 1400 s/mm. MATERIALS AND METHODSNumerical simulations were performed to assess image noise and CNR in both cDWI and regular DWI. Forty-two patients with prostate cancer (age, 51–73 years; prostate specific antigen level, 0.5–30 ng/mL; and biopsy Gleason score, 6–9) received 2 DWI examinations at 3.0 T (one with b values of 100, 500, and 1400 s/mm and another with b values of 0, 100, 400, and 800 s/mm) to create cDWI images at arbitrary b values, both with and without incorporating a b value of 0 s/mm in their calculation. Regions of interest were drawn to compare the scan time adjusted CNR (CNReff) between cDWI and directly measured DWI at b = 1400 s/mm on tumor-suspicious lesions and normal-appearing regions. RESULTSIn the numerical simulations, noise depended strongly on the b value, the diffusion coefficient, and the signal intensity at a b value of 0 s/mm in cDWI but not in regular DWI. The CNR between simulated tumor and normal regions showed a continuous increase with increasing b value. Both these findings were also observed in tumor-suspicious and normal-appearing regions in in vivo data. In vivo prostate DWI at a b value of 1400 s/mm showed a similar CNReff between the tumor-suspicious regions and the normal-appearing tissue in cDWI as in the directly measured DWI (P = 0.395). CONCLUSIONSThe CNReff between tumor-suspicious and normal-appearing prostate tissue in DWI images at a b value of 1400 s/mm is comparable in cDWI and directly measured DWI. Computed DWI at even higher b values, calculated from measured images with b values between 0 and 800 s/mm, yields higher CNReff than measured DWI, which may be of clinical aid in the management of prostate cancer.