Strategies to improve the signal and noise performance of active matrix, flat-panel imagers for diagnostic x-ray applications

A theoretical investigation of factors limiting the detective quantum effic iency (DQE) of active matrix flat-panel imagers (AMFPIs), and of methods to overcome these limitations, is reported. At the higher exposure levels ass ociated with radiography, the present generation of AMFPIs is capable of ex hibiting DQE performance equivalent, or superior, to that of existing film- screen and computed radiography systems. However, at exposure levels common ly encountered in fluoroscopy, AMFPIs exhibit significantly reduced DQE and this problem is accentuated at higher spatial frequencies. The problem app lies both to AMFPIs that rely on indirect detection as well as direct detec tion of the incident radiation. This reduced performance derives from the r elatively large magnitude of the square of the total additive noise compare d to the system gain for existing AMFPIs, In order to circumvent these rest rictions, a variety of strategies to decrease additive noise and enhance sy stem gain are proposed. Additive noise could be reduced through improved pr eamplifier, pixel and array design, including the incorporation of compensa tion lines to sample external line noise. System gain could be enhanced thr ough the use of continuous photodiodes, pixel amplifiers, or higher gain x- ray converters such as lead iodide. The feasibility of these and other stra tegies is discussed and potential improvements to DQE performance are quant ified through a theoretical investigation of a variety of hypothetical 200 mu m pitch designs. At low exposures, such improvements could greatly incre ase the magnitude of the low spatial frequency component of the DQE, render ing it practically independent of exposure while simultaneously reducing th e falloff in DQE at higher spatial frequencies, Furthermore, such noise red uction and gain enhancement could lead to the development of AMFPIs with hi gh DQE performance which are capable of providing both high resolution radi ographic images, at similar to 100 mu m pixel resolution, as well as variab le resolution fluoroscopic images at 30 fps. (C) 2000 American Association of Physicists in Medicine. [S0094-2405(00)01302-X].