Area x-ray beam equalization for digital angiography

An area beam equalization technique has been investigated in order to gener ate patient-specific compensating filters for digital angiography. An initi al image was used to generate the compensating filter, which was fabricated using a deformable compensating material, containing CeO2 and an array of square pistons. The CeO2 attenuator thicknesses were calculated using the g ray level information from the initial unequalized image. The array of pist ons was pressed against a uniform thickness of attenuating material to gene rate a filter for x-ray beam equalization. The filter was subsequently inse rted into the x-ray beam for the final equalized radiograph. It was positio ned close to the focal spot (magnification of 8.0) in order to minimize edg e artifacts from the filter. The equalization of x-ray transmission across the field exiting from the object significantly improved the image quality by preserving local contrast throughout the image. The contrast-to-noise ra tio (CNR) in the equalized images was increased by up to fivefold. Phantom studies indicate that equalized images using a relatively small array of pi stons (e.g., 8X8) produce significant improvement in image quality with neg ligible perceptible artifacts. Animal studies showed that beam equalization significantly improved fluoroscopic and angiographic image quality. X-ray beam equalization produced an image with a relatively uniform scatter-glare intensity and it reduced the scatter-glare fraction in the previously unde rpenetrated region of the image from 0.65 to 0.50. Also, x-ray tube loading due to the mask assembly itself was negligible. In conclusion, area beam e qualization reduces the scatter-glare fraction and significantly improves C NR in the previously underpenetrated region of the image. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)02212-9].