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

Abstract

A theoretical investigation of factors limiting the detective quantum efficiency (DQE) of active matrix flat-panel imagers (AMFPIs), and of methods to overcome these limitations, is reported. At the higher exposure levels associated with radiography, the present generation of AMFPIs is capable of exhibiting DQE performance equivalent, or superior, to that of existing film-screen and computed radiography systems. However, at exposure levels commonly encountered in fluoroscopy, AMFPIs exhibit significantly reduced DQE and this problem is accentuated at higher spatial frequencies. The problem applies both to AMFPIs that rely on indirect detection as well as direct detection of the incident radiation. This reduced performance derives from the relatively large magnitude of the square of the total additive noise compared to the system gain for existing AMFPIs. In order to circumvent these restrictions, a variety of strategies to decrease additive noise and enhance system gain are proposed. Additive noise could be reduced through improved preamplifier, pixel and array design, including the incorporation of compensation lines to sample external line noise. System gain could be enhanced through the use of continuous photodiodes, pixel amplifiers, or higher gain x-ray converters such as lead iodide. The feasibility of these and other strategies is discussed and potential improvements to DQE performance are quantified through a theoretical investigation of a variety of hypothetical 200 microm pitch designs. At low exposures, such improvements could greatly increase the magnitude of the low spatial frequency component of the DQE, rendering it practically independent of exposure while simultaneously reducing the falloff in DQE at higher spatial frequencies. Furthermore, such noise reduction and gain enhancement could lead to the development of AMFPIs with high DQE performance which are capable of providing both high resolution radiographic images, at approximately 100 microm pixel resolution, as well as variable resolution fluoroscopic images at 30 fps.

Flat-detector computed tomography (FD-CT)

“…up to 60 images per second with 4×4 binning, but at the expense of resolution (see Spatial resolution in the next section). For CT purposes, an increase would be of interest, but only if the characteristics of the absorber are also adequate [20]. Caesium iodide was abandoned in the 1980s in clinical CT since its temporal lag phenomena classified it insufficient for fast CT [13,21,22].…”

High‐DQE EPIDs based on thick, segmented BGO and CsI:Tl scintillators: Performance evaluation at extremely low dose

“…2 Given the low quantum efficiency ͑QE͒ of such detectors at radiotherapy energies ͑ϳ2% at 6 MV͒, the detective quantum efficiency ͑DQE͒ of conventional EPIDs is only ϳ1%, compared to ϳ40% -80% for kV AMFPIs. [3][4][5] The low DQE of conventional EPIDs constrains the practical execution of volumetric imaging techniques, such as MV cone-beam computed tomography ͑CBCT͒ 6-13 and cone-beam digital tomosynthesis ͑CBDT͒. 14,15 These techniques are under examination for providing 3D visualization of soft tissues -information that could help ensure accurate execution of advanced treatment plans for 3D conformal radiotherapy 16 and intensity modulated radiotherapy ͑IMRT͒, 17 in which dose delivery is precisely shaped to the tumor treatment volume.…”

A Monte Carlo investigation of Swank noise for thick, segmented, crystalline scintillators for radiotherapy imaging

“…As a result, the detective quantum efficiency ͑DQE͒ for conventional MV AMFPIs is only ϳ1%, 3 which is much lower than that for kilovoltage AMFPIs ͑ϳ40% to 80%͒. [4][5][6] In order to significantly improve portal imaging performance, 7 as well as reduce the dose requirement for MV CBCT imaging, [7][8][9] it is necessary to substantially increase the DQE of MV AMFPIs.…”

“…7,[16][17][18][19] Among these approaches, segmented crystalline scintillators offer significantly improved QE, with only limited loss in spatial resolution, and possibly no substantial increase in noise compared to conventional AMFPIs. In order to examine this strategy, 1D segmented arrays incorporating zinc tungstate ͑ZnWO 4 ͒ and cadmium tungstate ͑CdWO 4 ͒ scintillators, [17][18][19] as well as 2D segmented matrices employing bismuth germanate ͑BGO͒, thallium-doped cesium iodide ͑CsI:Tl͒ and CdWO 4 scintillators, 7,[20][21][22] have been investigated by different research groups. Recently, a 40 mm thick segmented CsI:Tl detector, with an element-toelement pitch of 1.016 mm and an area of 16.25 ϫ 16.25 cm 2 , has been developed and evaluated by our group.…”