A statistical technique is described for analyzing the image of a contrast-detail phantom acquired by a radiotherapy electronic portal imaging device. By using Student's t test, the image analysis algorithm computes which holes in the contrast-detail phantom can be objectively resolved from the background signal. The specific pattern of the resolved holes and the total number of the resolved holes characterize the performance of the electronic portal imaging device. Image processing techniques are used to align the holes automatically. Because the use of this algorithm is completely automated, the procedure removes operator subjectivity from the evaluation.