Active energy selective image detector for dual-energy computed radiography.

A new energy selective detector for dual energy computed radiography has been developed that combines many of the advantages of x-ray tube voltage 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. Electro-optical modulation can be done rapidly so the detector can acquire the voltage switched images in a short enough time to minimize patient motion artifacts. Voltage switching produces effective detected energy spectra that result in much lower noise for a given patient dose than the effective spectra of double screen detectors. In this paper the new concept is described, optimal voltages and filter materials are determined by computer simulations, and the active detector performance is compared to other energy selective detectors. The new detector provides over 30 times better signal to noise ratio squared (SNR2) for the same dose and over five times better SNR2 for the same x-ray tube loading than a double screen detector. The effects of incomplete erasure of the x-ray exposures are determined quantitatively. With achievable erasing fractions, the SNR2 is over 20 times better for the same dose and over three times better for constant tube loading than a double screen detector. The active detector is also compared to mechanical screen switching. Mechanical switching provides somewhat better SNR2 for the same dose, approximately 1.1 times the active detector SNR2 at optimal x-ray tube voltages. The performance is compared with highly absorbing back screens. If these are used in both the active and passive detectors, both detectors' quality factors increase, but the advantage of the active detector over the passive detector decreases with large back screen thicknesses.