Investigation of the signal behavior at diagnostic energies of prototype, direct detection, active matrix, flat-panel imagers incorporating polycrystalline HgI2

Abstract

Active matrix, flat-panel x-ray imagers based on a-Si:H thin-film transistors offer many advantages and are widely utilized in medical imaging applications. Unfortunately, the detective quantum efficiency (DQE) of conventional flat-panel imagers incorporating scintillators or a-Se photoconductors is significantly limited by their relatively modest signal-to-noise ratio, particularly in applications involving low x-ray exposures or high spatial resolution. For this reason, polycrystalline HgI2 is of considerable interest by virtue of its low effective work function, high atomic number and the possibility of large-area deposition. In this study, a detailed investigation of the properties of prototype, flat-panel arrays coated with two forms of this high-gain photoconductor are reported. Encouragingly, high x-ray sensitivity, low dark current and spatial resolution close to the theoretical limits were observed from a number of prototypes. In addition, input-quantum-limited DQE performance was measured from one of the prototypes at relatively low exposures. However, high levels of charge trapping, lag and polarization, as well as pixel-to-pixel variations in x-ray sensitivity are of concern. While the results of the current study are promising, further development will be required to realize prototypes exhibiting the characteristics necessary to allow practical implementation of this approach.

Figure 1

Microphotographs of the top surface of: (a) an ND10 pixel, and (b) an MD88 pixel. These images were acquired prior to the deposition of the materials comprising a HgI₂ detector (i.e., the barrier layer, the photoconductor, the top electrode, and an encapsulation layer). The dashed lines superimposed on the photos indicate the periphery of the signal collection electrode, (c) Schematic cross-sectional view of an ND10 pixel with a HgI₂ detector - where the drawing has not been made to scale in order to better illustrate various features of the pixel designs. The cross-sectional view for an MD88 pixel would generally look the same except that the collection electrode also serves as the top electrode of the pixel storage capacitor and this electrode is connected by a via to the drain contact of the TFT. Note that, in this drawing, the depression illustrated in the otherwise planar collection electrode corresponds to an ~40×40 μm² large, ~2.6 μm deep depression located on the surface of the bare ND10 pixel - a feature that is located at the position of the via indicated in (a).

Figure 2

Block diagram representing the various stages in the serial cascaded systems analysis performed in this paper for prototype PVD#4. The x-ray spectrum (Boone and Seibert 1997) used in the calculations corresponds to that used in our measurements and is characterized by a fluence of ${\stackrel{‒}{q}}_0$ = 262,410 × rays/mm²/mR. The HgI₂ photoconductor is described by a quantum efficiency ${\stackrel{‒}{g}}_1$ = 0.643 with a variance of 0.229. It is also described by a photoconductive gain represented by ${\stackrel{‒}{g}}_2$ = 11,246 (mean number of electron-hole pairs generated in the photoconductor per interacting x ray) with a variance of 9.86×10⁶ - as determined via EGS4 Monte Carlo simulations (El-Mohri et al 2007) assuming a thickness of 210 μm for HgI₂, a density of 5.72 g/cm³ and an internal ionization energy of 4.2 eV.(Schieber et al, 1997; Alexiev et al, 2004) The stochastic spatial spreading is characterized by T₃ obtained from the empirically determined MTF divided by the sine function for a square pixel aperture of 127 μm. The gain ${\stackrel{‒}{g}}_4$ = 0.392 with a variance of 0.238 corresponds to the collection efficiency of charges created in the photoconductor. The value of ${\stackrel{‒}{g}}_4$ was obtained by fitting the calculated x-ray sensitivity to the empirically determined value, with ${\stackrel{‒}{g}}_4$ taken as a free parameter. The array pixels are characterized by a spatial spreading represented by the aforementioned sine function, T₅, and an additional stage (labeled a_pix) representing the sampling of the signal. The additive noise of the electronic acquisition system is characterized by σ_ADD, which was empirically determined from dark NPS measurements to be ~2000 e (rms). Note that, for simplicity, k-fluorescent interactions have not been separated as a parallel branch in the model since their effect on the MTF, as determined from Monte Carlo simulations that include such interactions, was found to be negligible.

Figure 3

(a) Pixel x-ray signal and (b) pixel dark current as a function of electric field up to 0.7 Vμm. The results, correspond to averages over more than a thousand pixels in a largely defect-free region for prototypes PVD#4, PVD#16 and PIB#2. For each prototype, the dashed vertical arrow in (a) corresponds to the selected electric field strength at which properties for this array were measured. Note that the right hand side scale in (b) corresponds to normalization of the dark current to unit photoconductor area, based on a 127 × 127 μm² pixel area.

Figure 4

Data from individual array pixels illustrating charge trapping, charge release (i.e., lag) and polarization effects. Pixel signal data measured initially in the absence, then the presence, and finally the absence of radiation, are plotted as a function of consecutive frame number. Data is shown for prototypes (a) PVD#1 at an equilibrium signal level of ~10% of pixel saturation; (b) PVD#4 at an equilibrium signal level of ~8%; and (c) PVD#16 at equilibrium signal levels of ~3%, 6% and 15% (indicated by cross, circle and triangle symbols, respectively).

Figure 5

X-ray pixel signal response averaged over a large number of properly functioning pixels, plotted as a function of exposure, for prototypes (a) PVD#4 and (b) PVD#15. The degree of non-linearity in this x-ray response data is plotted in (c) for PVD#4 and (d) for PVD#15.

Figure 6

Pixel response linearity measured in radiographic mode, presented in the form of a bar chart. For each prototype, linearity is reported in terms of the pixel signal range (expressed in units of percent of pixel signal saturation) over which the degree of non-linearity is less than 1%.

Figure 7

Histograms of pixel x-ray signal for prototypes (a, b) PVD#16, (c, d) PVD#12, (e, f) PVD#4 and (g, h) PIB#2. Results are shown for 4 exposures. The graphs on the left correspond to pixel data which has been corrected only by a dark signal subtraction. The graphs on the right correspond to the same data, after the application of a gain correction which primarily accounts for pixel-to-pixel variation in x-ray signal response. (Antonuk et al 1992a) This correction is based on 5 image frames acquired independently under the same conditions. The solid lines imposed on these graphs represent histograms to be expected based on x-ray statistics.

Figure 8

Summary of the non-uniformity in x-ray signal response measured from a ROI of several thousand pixels for each prototype. Results are shown in the form of a bar chart for (a) the lowest exposures, ranging from 0.058 to 0.136 mR, and (b) the highest exposures, ranging from 0.206 to 0.380 mR. In each chart, for a given array, the non-uniformity before and after the application of a gain correction to the data is indicated by the top of the lighter and darker shaded portion of the bar, respectively. In addition, the non-uniformity to be expected, based only on considerations of the x-ray statistics of the image frame from which the data for this prototype was obtained, is indicated by a horizontal line across the corresponding bar.

Figure 9

Pixel x-ray signal sensitivity measured in radiographic mode and the corresponding W_EFF obtained from blocks of hundreds tow-free regions of the prototypes, presented in the form of a bar chart. For a given array, the W_EFF and the corresponding sensitivity are indicated by the top of the darker and lighter shaded portions of the bar, corresponding to the left and right scales, respectively.

Figure 10

Presampled MTF measured from four prototype arrays, PVD#4 (thin solid line), PVD#12 (dashed line), PVD#16 (dot-dashed line) and PIB#2 (dotted line). The graph also shows the sine function corresponding to the 127 μm pixel pitch of the arrays (thick solid line). The MTF of a direct detection AMFPI with a 1000 μm thick a-Se detector (plus symbols), along with the MTF for an indirect detection AMFPI with a CsI:Tl detector (cross symbols), are also shown. The a-Se and CsI:Tl results are based on reported MTFs, (Hunt et al 2004, Granfors et al 2003) modified so as to correspond to the results expected for 127 μm pitch arrays with signal collection fill factors of 100% and 80%, respectively.

Figure 11

MTF values obtained from those prototype arrays that offered a block of properly functioning pixels sufficiently large for measurement of this quantity. Results are shown in the form of a bar chart. For a given prototype, the MTF at 2 and 4 lp/mm is indicated by the top of the lighter and darker shaded portions of the bar, respectively. Values are also shown for a sine function with an aperture of 127 μm, corresponding to the theoretical upper limit of MTF imposed by the design of the array. In addition, MTF values for a 127 μm pitch AMFPI with a-Se and CsI:T1 detectors, derived from published results, and corresponding to the curves shown in figure 10, are included.

Figure 12

NPS results obtained empirically at several exposures for prototype PVD#4. Note that a gain correction, of the same type used in the non-uniformity studies, has been applied in the data analysis.

Figure 13

Frequency-dependent DQE results for prototype PVD#4 at 3.4, 10 and 20 μR (circle, square and diamond symbols, respectively). These results correspond to DQE values determined experimentally using the NPS values reported in figure 12. For clarity of presentation, each plotted data point represents an average of the measured DQE over a frequency interval of ~0.16 lp/mm. The dashed and solid lines correspond to cascaded systems calculations of the DQE at 3.4 and 20 μR, respectively. Finally, the cross and plus symbols correspond to DQE values reported for a 300 and 200 μm pitch direct and indirect detection AMFPIs with a-Se and CsI:Tl detectors, measured with an x-ray spectrum similar to that used in the present study, at 4.4 and 3 μR, respectively. (Hunt et al 2004, Granfors et al 2003)