Physical characterization and performance comparison of active- and passive-pixel CMOS detectors for mammography

Abstract

We investigated the physical characteristics of two complementary metal oxide semiconductor (CMOS) mammography detectors. The detectors featured 14-bit image acquisition, 50 microm detector element (del) size and an active area of 5 cm x 5 cm. One detector was a passive-pixel sensor (PPS) with signal amplification performed by an array of amplifiers connected to dels via data lines. The other detector was an active-pixel sensor (APS) with signal amplification performed at each del. Passive-pixel designs have higher read noise due to data line capacitance, and the APS represents an attempt to improve the noise performance of this technology. We evaluated the detectors' resolution by measuring the modulation transfer function (MTF) using a tilted edge. We measured the noise power spectra (NPS) and detective quantum efficiencies (DQE) using mammographic beam conditions specified by the IEC 62220-1-2 standard. Our measurements showed the APS to have much higher gain, slightly higher MTF, and higher NPS. The MTF of both sensors approached 10% near the Nyquist limit. DQE values near dc frequency were in the range of 55-67%, with the APS sensor DQE lower than the PPS DQE for all frequencies. Our results show that lower read noise specifications in this case do not translate into gains in the imaging performance of the sensor. We postulate that the lower fill factor of the APS is a possible cause for this result.

Experimental measurement of a high resolution CMOS detector coupled to CsI scintillators under X-ray radiation

“…Entrance surface air-Kerma was calculated by relation (1) using the Amptek XR-100T X-ray spectrometer measurements and was found 35.3 mGy for the RQA-5 and 166.9 mGy for the mammographic conditions (W/Rh), respectively. The RQA-5 air-Kerma value is higher than the usual clinical dose ranges for medical imaging systems (Konstantinidis et al, 2012) (Elbakri et al, 2009) and ii) a RadEye HR CMOS sensor coupled to a commercial 33.91 mg/cm 2 Gd 2 O 2 S:Tb scintillation screen (corresponding to 100 mm surface density, assuming a coating weight of 50%) under the representative RQA-M2 (Mo/Mo at 28 KV) beam quality at an exposure level of 20.28mGy (Michail et al, 2011a). All the MTF curves of the present CMOS/CsI:Tl screens combinations are higher than those of the previously published setups.…”

“…6 (Elbakri et al, 2009;Michail et al, 2011a,b). The NNPS of the previously published CMOS sensor/CsI:Tl screen combination shows the lowest noise levels.…”

“…In recent years, complementary metal oxide semiconductor (CMOS) technology replaced charged-coupled device (CCD) technology in almost all the imaging field applications, as well as in Xray imaging (Bohndiek et al, 2008;Elbakri et al, 2009;Cha et al, 2010;Jackson et al, 2013). There are two basic pixel structures that are widely used as CMOS imaging elements: passive pixel sensors (PPSs), which were developed first and the active pixel sensors (APS), which have been developed with the goal of improving the image quality.…”

“…There are two basic pixel structures that are widely used as CMOS imaging elements: passive pixel sensors (PPSs), which were developed first and the active pixel sensors (APS), which have been developed with the goal of improving the image quality. Usually, an APS has three transistors for each pixel, whereas the PPS has only one (Bohndiek et al, 2008;Elbakri et al, 2009;Rocha et al, 2010). With respect to passive pixel, APS technology offers an increased signal-to-noise ratio of the sensor and dynamic range, at the price of a smaller fill factor (Endrizzi et al, 2013).…”

Image Quality Assessment of a CMOS/Gd2O2S:Pr,Ce,F X-Ray Sensor

“…A semiconductor technology that has been used widely recently is complementary metal oxide (CMOS) semiconductors [6][7][8][9]. The initial CMOS pixel structure was in the form of passive pixel sensors (PPSs) and was followed by the active pixel sensors (APS), which have been developed with the goal of improving the image quality [10].…”