Comparison of dual energy detector system performance

Dual energy detector systems are combinations of x-ray detectors, x-ray source spectrum switching, and x-ray filter attenuation that provide two measurements of transmitted flux through the object with different effective spectra. We describe technology independent methods to measure and compare the quantum noise and sensitivity to motion artifacts of these systems. The experimental methods use relatively simple phantoms to measure the parameters in the general mathematical expressions for the noise in the subtracted image. The parameters are used to compute an x-ray energy spectrum quality factor and a subtracted image noise per unit patient dose quality factor. Patient motion causes artifacts in switched spectrum systems, particularly with the heart in chest radiography. We describe a method to measure effective interexposure time using subtracted image data of a uniformly moving object. This parameter measures the sensitivity to patient motion artifacts. We use these methods to compare three examples of systems with different dual energy detector technologies: a passive, "sandwich" detector with two computed radiography plates separated by a copper filter, an "active" detector that uses voltage switching with an electro-optical system and computed radiography plates, and a flat-panel, solid state detector with voltage switching.