NIKOS II—a system for noninvasive coronary angiography with synchrotron radiation (abstract)

During the last years attempts have been underway at several synchrotron radiation laboratories to employ the abrupt change of absorption at the iodine K edge (33.17 keV) to the imaging of coronary arteries. The subtraction of two images taken with photon energies just above and below the edge strongly surpresses background contrast like bone and soft tissue and allows for amplification of iodinated structure contrast. At HASYLAB, for these studies, a beamline at a bending magnet was only available until 1987. At this beamline the system NIKOS I was tested to obtain information on its essential components like monochromator and detector by in‐vivo investigations of dogs.The basic feature which differs from the approaches in other laboratoriesis simultaneous registration of two stationary beams in a two‐line detector without any beam switching. Based on the experience with this system during the last year all its components have been improved and changed to dimensions capable of taking a 12‐cm‐wide image of the human heart. At present the new system NIKOS II is under testing at the new wiggler beamline HARWI. The new monochromator comprises two pairs of a thin Si Laue case crystal and a Ge Bragg crystal, each, with inherent focussing of the beam to a horizontal line focus of about 0.5 mm height and 12 cm width. The advantage of the Laue crystals which are optimized in thickness (30 μm) for maximum reflectivity, is the low heat absorption and the full use of the white beam in the production of two monochromatic beams. The crystals are cooled in a stream of helium. Compared to the NIKOS I version5 the detector was changed in the following points: The two detection lines are enlarged to 125 mm width and the phosphor at the entrance is exchangeable to be either made of powderlike $Gd_2O_2S$:Tb or of single crystals like $CdWO_4$ or $CaF_2$:Eu separated by thin walls for optimum spatial resolution. The numerical aperture of the glass fiber optics guiding the light via image intensifiers to the photodiode arrays is higher. The gain of the intensifiers is raised to 1000.Now two photodiode arrays are needed for the simultaneous readout of the two entrance lines. The readout time is halved by reading the odd and even number diodes by two ADCs per array. All these measures will increase the detective quantum efficiency from 0.2 to 0.6. The performance of the new system will be tested again by investigations of dogs before moving to investigations on humans, expected at the end of this year