Automated and Accurate Assessment of the Distribution, Magnitude, and Direction of Pincushion Distortion in Angiographic Images

Abstract

RATIONALE AND OBJECTIVES. Pincushion distortion continues to be a potential problem for the accurate assessment of arterial and catheter dimensions from x-ray angiograms. The authors investigate whether the distortion of state-of-the-art intensiflers is yet small enough to be neglected, and whether the rotation/angulation of the x-ray system plays a significant role. METHODS. The location and degree of distortion from x-ray images of a centimeter grid, which is positioned against the input screen of the image intensifier, are assessed automatically using image processing techniques. A value for the maximum amount of change in the distortion vector field is derived that allows the estimation of the maximum relative error associated with a diameter measurement uncorrccted for pincushion distortion. RESULTS. The accuracy of the algorithm itself was assessed by rotating and translating the centimeter grid under the image intensifier at anteroposterior position. For the distortion vector length, the standard deviation in the measurement of the distortion areas was found to be 3.7 cm² (1.3% of the total area). For the gradient values, the standard deviation was 2.2 cm² or 0.75% of the total image intensifier area. In the second evaluation study, the centimeter grid was fixed onto the input screen of the image intensifier, and the gantry was rotated to span all possible positions of the system. In this case, the changes in measured areas were often much larger (up to 51.25 cm2 for a 9-inch image intensifier, equivalent to 15.6% of the total image intensifier area) than the standard deviations that had been found in the first evaluation study. CONCLUSIONS. The distortion is highly dependent upon the actual spatial position of the image intensifier, and correcting for pincushion distortion may therefore introduce larger errors than leaving the measurements uncorrectcd.