Purpose: When brain angiography was performed, we devised the calculation theory for rectifying magnifying power, and produced the auxiliary tool. Method: We produced the frame with a size of 300×300×15mm from the thin board made from carbon. The frame is around arranged for every 1 cm in Pb-ball of the diameter of 1 mm. We named this the measuring-chart. The measuring-chart has been arranged so that head may be surrounded. A photograph of a measuring-chart and head was taken simultaneously. The projected mark used for computing the magnifying power of a brain aneurysm. Magnifying power was used in order to measure the size of an aneurysm. Results: In the basic experiment using the stainless-steel ball whose size is known, the standard error was -2.29%±5.61%. A measurement value and true value showed high correlation (R^2=0.9986). Conclusion: The measuring-chart did not need a proofreading factor peculiar to equipment. This method had high accuracy. This method has high flexibility. It suggested that this method was useful on clinical use.
