Comparison of synthesized and acquired high b -value diffusion-weighted MRI for detection of prostate cancer.

Background: High b -value diffusion-weighted images (DWI) are used for detection of clinically significant prostate cancer (csPCa). To decrease scan time and improve signal-to-noise ratio, high b -value (>1000 s/mm ² ) images are often synthesized instead of acquired.
Purpose: Qualitatively and quantitatively compare synthesized DWI (sDWI) to acquired (aDWI) for detection of csPCa.
Study Type: Retrospective.
Subjects: 151 consecutive patients who underwent prostate MRI and biopsy.
Sequence: Axial DWI with b =0, 500, 1000, and 2000 s/mm ² using a 3T clinical scanner using a 32-channel phased-array body coil.
Assessment: We synthesized DWI for b =2000 s/mm ² via extrapolation based on monoexponential decay, using b =0 and b =500 s/mm ² (sDWI ₅₀₀ ) and b =0, b =500, and b =1000 s/mm ² (sDWI ₁₀₀₀ ). Differences between sDWI and aDWI were evaluated within regions of interest (ROIs). The maximum DWI value within each ROI was evaluated for prediction of csPCa. Classification accuracy was also compared to Restriction Spectrum Imaging restriction score (RSIrs), a previously validated biomarker based on multi-exponential DWI.
Statistical Tests: Discrimination of csPCa was evaluated via area under the receiver operating characteristic curve (AUC). Statistical significance was assessed using bootstrap difference (two-sided α=0.05).
Results: Within the prostate, mean ± standard deviation of percent mean differences between sDWI and aDWI signal were -46±35% for sDWI ₁₀₀₀ and -67±24% for sDWI ₅₀₀ . AUC for aDWI, sDWI _500, sDWI ₁₀₀₀ , and RSIrs within the prostate 0.62[95% confidence interval: 0.53, 0.71], 0.63[0.54, 0.72], 0.65[0.56, 0.73] and 0.78[0.71, 0.86], respectively. When considering the whole field of view, classification accuracy and qualitative image quality decreased notably for sDWI compared to aDWI and RSIrs.
Data Conclusion: sDWI is qualitatively comparable to aDWI within the prostate. However, hyperintense artifacts are introduced with sDWI in the surrounding pelvic tissue that interfere with quantitative cancer detection and might mask metastases. In the prostate, RSIrs yields superior quantitative csPCa detection than sDWI or aDWI.