Background: Relative enhanced diffusivity (RED) is a potential biomarker for indirectly measuring perfusion in tissue using diffusion-weighted magnetic resonance imaging (MRI) with 3 b values. Purpose: To optimize the RED MRI protocol for the prostate, and to investigate its potential for prostate cancer (PCa) diagnosis. Study Type: Prospective. Population: Ten asymptomatic healthy volunteers and 35 patients with clinical suspicion of PCa. Sequence: 3T T 2 -and diffusion-weighted MRI with b values: b = 0, 50, [100], 150, [200], 250, [300], 400, 800 s/mm 2 (values in brackets were only used for patients). Assessment: Monte Carlo simulations were performed to assess noise sensitivity of RED as a function of intermediate b value. Volunteers were scanned 3 times to assess repeatability of RED. Patient data were used to investigate RED's potential for discriminating between biopsy-confirmed cancer and healthy tissue, and between true and false positive radiological findings. Statistical Tests: Within-subject coefficient of variation (WCV) to assess repeatability and receiver-operating characteristic curve analysis and logistic regression to assess diagnostic performance of RED. Results: The repeatability was acceptable (WCV = 0.2-0.3) for all intermediate b values tested, apart from b = 50 s/mm 2 (WCV = 0.3-0.4). The simulated RED values agreed well with the experimental data, showing that an intermediate b value between 150-250 s/mm 2 minimizes noise sensitivity in both peripheral zone (PZ) and transition zone (TZ). RED calculated with the b values 0, 150 and 800 s/mm 2 was significantly higher in tumors than in healthy tissue in both PZ (P < 0.001, area under the curve [AUC] = 0.85) and PZ + TZ (P < 0.001, AUC = 0.84). RED was shown to aid apparent diffusion coefficient (ADC) in differentiating between false-positive findings and true-positive PCa in the PZ (AUC; RED = 0.71, ADC = 0.74, RED+ADC = 0.77). Data Conclusion: RED is a repeatable biomarker that may have value for prostate cancer diagnosis. An intermediate b value in the range of 150-250 s/mm 2 minimizes the influence of noise and maximizes repeatability. Level of Evidence: 2 Technical Efficacy Stage: 1

Daniel Chen Billdal (f. 1996) er medisinstudent ved Norges teknisk-naturvitenskapelige universitet og arbeidslivsansvarlig i Norsk medisinstudentforening Trondheim. Forfatter har fylt ut ICMJE-skjemaet og oppgir ingen interessekonflikter. TURID ØKLAND Turid Økland (f. 1991) er medisinstudent ved Aarhus Universitet og arbeidslivsansvarlig i Norsk medisinstudentforening Utland. Forfatter har fylt ut ICMJE-skjemaet og oppgir ingen interessekonflikter.