Removal of harmonic artifacts from synchrotron radiation coronary angiograms

摘要

Intravenous coronary angiograms of human subjects are now available from the National Synchrotron Light Source (NSLS). The high (4.4-T) magnetic field used under normal operation of the superconducting wiggler magnet on the X-17 beam line at NSLS yields a high critical energy (18.7 keV) and a large harmonic contamination of the desired X-ray flux at the K-edge of iodine (33.169 keV). The 66- and 99-keV contamination is a factor of 2.29 and 5.32 worse than for the human angiograms taken at the Stanford Synchrotron Radiation Laboratory, where a critical energy of 13.0 keV was obtained at beam line IV-2. This increased harmonic contamination produces severe artifacts in the images, especially where the patient's ribs cross the left ventrical and aorta, which are filled with contrast agent. An algorithm was developed to remove these artifacts by solving the two simultaneous transcendental equations for the fractional X-ray transmissions through iodine and through water. The parameters are derived from measurements on a Lucite step-wedge phantom. Human angiograms taken at 4.4 T are shown before and after the correction algorithm is applied. These corrected angiograms are compared to uncorrected angiograms taken at 3.0 T.