ACTIVE ENERGY SELECTIVE IMAGE DETECTOR FOR DUAL-ENERGY COMPUTED RADIOGRAPHY

A new energy selective detector for dual energy computed radiography h as been developed that combines many of the advantages of x-ray tube v oltage switching and single exposure double screen detectors. The new active detector utilizes electro-optical modulation of the response of the storage phosphor screens to allow dual exposure acquisition with no motion of the screens. Electrooptical modulation can be done rapidl y so the detector can acquire the voltage switched images in a short e nough time to minimize patient motion artifacts. Voltage switching pro duces effective detected energy spectra that result in much lower nois e for a given patient dose than the effective spectra of double screen detectors. In this paper the new concept is' described, optimal volta ges and filter materials are determined by computer simulations, and t he active detector performance is compared to other energy selective d etectors. The new detector provides over 30 times better signal to noi se ratio squared (SNR(2)) for the same dose and over five times better SNR(2) for the same x-ray tube loading than a double screen detector. The effects of incomplete erasure of the x-ray exposures are determin ed quantitatively. With achievable erasing fractions, the SNR(2) is ov er 20 times better for the same dose and over three times better for c onstant tube loading than a double screen detector. The active detecto r is also compared to mechanical screen switching. Mechanical switchin g provides somewhat better SNR(2) for the same dose, approximately 1.1 times the active detector SNR(2) at optimal x-ray tube voltages. The performance is compared with highly absorbing back screens. If these a re used in both the active and passive detectors, both detectors' qual ity factors increase, but the advantage of the active detector over th e passive detector decreases with large back screen thicknesses. (C) 1 996 American Association of Physicists in Medicine.