Signal and Noise Characteristics Induced by Unattenuated X Rays from a Scintillator in Indirect-Conversion CMOS Photodiode Array Detectors

Abstract

We report the measurement results of signal and noise characteristics induced by the direct x-rays in an indirect-conversion CMOS photodiode array detector. In order to isolate the signal and noise due to the direct x-rays from those due to the optical photons, we inserted a light-absorbing blackout material between a phosphor screen and the photodiode array. From the images irradiated with and without the blackout paper, the signal and noise characteristics due to the optical photons emitted from a phosphor screen were estimated. For the analysis of the measurements, we have developed a model describing the signal and noise transfers based on the cascaded linear-systems approach. The measured results show the direct x-ray is very harmful to the detector performances, such noise power spectrum (NPS) and signal-to-noise ratio (SNR). However, from the theoretical estimation, the degradation of NPS and SNR would not be due to the directly absorbed x-ray photons, but we believe that other sources, such as Compton and photoelectric scattered rays from a scintillator, a photodiode passivation layer or bulk substrate, are main causes.Index Terms-Cascaded system analysis, image quality, noise power spectrum, radiography.

Single-shot dual-energy x-ray imaging with a flat-panel sandwich detector for preclinical imaging

“…8(c) and (d), this analysis showed the relative signal contribution from direct interactions in either front or rear detectors, d i pd =ðd i scn þ d i pd Þ, was never more than 5% while the noise contribution, s R pd =ðs R scn þ s R pd Þ, was in the range of 30e70%. Thus, since the x-ray direct-interaction probability in the photodiode is low because the thickness of the active Si layer in the photodiode is only 0.002 mm [22], and the conversion gain for liberation of secondary charge carriers is relatively large due to the low effective w value (3.6 eV for Si), the overall increase in noise and decrease in signal-to-noise ratio (SNR) are significant. The cascaded-systems analysis shows this noise contribution can be reduced to negligible levels only by increasing the product of the optical conversion gain and coupling efficiency, b  h, by a factor of approximately 10.…”

“…Therefore, quantum noise relative to signal in images obtained from the rear detector will increase due to the reduction in the number of x-ray photons, reducing the DQE performance of the rear detector. Since x-ray scatter plays a role as an "additive quantum noise" in the DQE [36], further degradation in DQE performance of the detector is predicted [22]. In order to achieve the theoretical DQE performances, either a higher x-ray exposure should be used or the rear detector be designed to have a higher secondary quantum gain [20].…”

Technical Note: Modification of the standard gain correction algorithm to compensate for the number of used reference flat frames in detector performance studies

“…As mentioned before, the latter method is mainly used in the electro-optical evaluation of digital sensors, 3-6 and can be found only in a small number of x-ray performance characterization studies. 1,2, [26][27][28][29][30] To implement the subtraction-based algorithm in this study, two consecutive raw flat frames are subtracted to exclude the FPN. The difference flat image is divided by the square root of two to compensate for the propagated uncorrelated noise [see Eq.…”

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“…This additional white spectral noise is harmful to DQE at high spatial frequencies. Although the magnitude of direct signal may be small, its variance can be very large, cause a reduction in the total signal-tonoise performance of an image [43]. To reduce or eliminate this harmful effect, we introduced an FOP layer between the phosphor scintillator and the CMOS photodiode array to keep direct x-ray photons from reaching the underlying photodiode array [44].…”