“Number needed to read”—How to facilitate clinical trials in MR-angiography

PURPOSE: To evaluate the effect of the number of readers on the statistical results in peripheral MRA. MATERIALS AND METHODS: 40 patients with peripheral arterial occlusive disease were included as a sample dataset in this study, randomly separated into two matched groups with n = 20 patients (group 1—gadobutrol, group 2—gadoterate meglumine) who underwent a continuous table movement MRA of the peripheral vessels at 3 T. Image quality (IQ) of 17 vessel segments was evaluated by 5 independent readers. The effect of the number of readers on significance and statistical power was statistically analyzed. RESULTS: Image quality in group 1 (gadobutrol) ranks significantly higher compared to group 2 (gadoterade meglumine) with a diagnostic IQ in 97% vs. 78% (p < 0.0001). For the diagnostic/non-diagnostic IQ assessment significance was reached with one reader 1/5 times (20%), with two readers in 4/10 (40%), with three readers in 6/10 (60%), with four readers in 4/5 (80%), with five readers in 1/1 (100%). Power considerations showed considerable gain when increasing the number of readers. CONCLUSION: Increasing the number of readers in a diagnostic MRA-study can be used to achieve a higher power or to decrease the number of subjects included with maintained statistical validity.