Previous studies have shown that extreme value statistics are useful for quantitative evaluations of streak artifacts on multi-detector computed tomography (MDCT). However, we hypothesized that the scanning direction of the extreme value would affect the quantitative value obtained using the conventional method. In this study, we developed the region of interest rotation method and calculating the extreme value, and we investigated the usefulness of this method in comparison with the conventional approach. For our examination, the high absorber was placed around a water phantom and a head and chest phantom. In the new method, linearity was confirmed in the Gumbel plot of all the phantoms. On the other hand, the value of the location parameter was significantly different according to the scanning direction with the conventional method. In conclusion, compared to the conventional method, the isotropic method of evaluation does not depend on the direction of streak artifact occurrence in the new method.