Cone-beam computed tomography with a flat-panel imager: Effects of image lag

A system for cone-beam computed tomography (CBCT) has been developed based upon the technology of active matrix flat-panel imagers (FPIs), and the sys tem has demonstrated the potential for fully three-dimensional volumetric i maging with high spatial and contrast resolution. This paper investigates t he effects of image lag (arising from charge trapping and release in the FP I pixels) upon CBCT reconstructions. Hypotheses were derived based upon a s imple, geometrical/physical model, suggesting that image lag in the project ion data results primarily in two artifacts: a spatial blurring artifact in the direction opposite to the direction of rotation (called a ''comet'') a nd a line artifact along the direction of the first few projections (called a "streak"). The hypotheses were tested by means of computer simulations a nd experimental measurements that yielded CBCT images of a simple cylindric al water phantom containing an attenuating rod of varying size and composit ion. The computer simulations generated projection images based upon analys is of the system geometry and a simple model of the FPI that allowed free a djustment of the image lag. Experimental measurements involved CBCT scans o f the phantom under various conditions and modes of acquisition followed by examination of the resulting CBCT axial slices for lag artifacts. Measurem ents were performed as a function of exposure level, position and contrast of the rod, and for three modes of acquisition designed to isolate and/or m inimize the two hypothesized artifacts. The results clearly illustrate the comet and streak artifacts, particularly in relation to high-contrast objec ts imaged at high exposure levels. The significance of such artifacts under clinical conditions is expected to be small, considering the magnitude of the effect relative to the morphology and composition of typical anatomy. T he artifacts may become appreciable, however, in the presence of high-contr ast objects, such as marker BBs, dental fillings, and metal prosthetics. A procedural method of reducing lag artifacts is demonstrated. (C) 1999 Ameri can Association of Physicists in Medicine. [S0094-2405(99)02012-X].