Development of a prototype CMOS digital imaging system for X-ray medical applications

Abstract

Digital X-ray imagers have radically altered the otherwise traditional radiographic techniques and enabled further innovative ones. Currently, a great part of detectors is based on the combination of a scintillator with a digital optical sensor. Crucial properties beyond the radiation absorption efficiency and the spatial response include ruggedness, absence of hygroscopicity, low manufacturing cost and complexity. Scintillating screen manufacturing processes involve strict preparation conditions and long production cycles, while the results are sometimes delicate and prone to mechanical or environmental damage. This has an impact on the cost and hinders the broader applications in X-ray imaging. Simpler techniques exist that are targeted to limited productions, research settings and sites lacking specialized instrumentation. One of them is the classic sedimentation method, which however leaves room for improvements – mechanical strength, phosphor coating cohesion and material waste. In this context, it was developed a novel screen preparation technique, aimed to eliminate the aforementioned weaknesses. The novel method was optimized and validated by integrating the new screens in an X-ray detecting system. Its light sensing element was a complementary metal oxide semiconductor (CMOS) active pixel sensor (APS) with an active area of 27 × 36 mm^2 and high resolution (22.5 µm pixel pitch). Screens were fabricated using the widespread in medical imaging Gd2O2S:Tb phosphor. The novel method is based on the concept that a host material encapsulates the active phosphor and provides structure and protection. This material is a mixture of resin and hardener in which it is also added a certain amount of graphite powder. Its intended purpose is to form a light-absorbing layer that will restrict light from propagating to unwanted directions where it could contribute to image blur. The technique takes advantage of the inherent difference in the densities of the phosphor and graphite, so that the respective layers are formed at the desired heights within the final screen. With the intention of ensuring valid comparisons with recent and earlier published data, the IEC 62220-1 international standard was adopted throughout the integrated detector characterization. Established image quality metrics such as the modulation transfer function (MTF), the noise power spectrum (NPS) and the detective quantum efficiency (DQE) were estimated. Moreover, the proposed detector was compared under the same conditions to others reported previously, including a detector of wide commercial circulation. The novel detector exhibited comparable or even better performance in the aforementioned image quality metrics. Specifically, the detector coupled to a 34 mg/cm^2 screen achieved a DQE 15-20% greater than its comparison counterpart (with the widespread Min-R screen), and its limiting resolution was 5.3 cycles/mm. The detector coupled to a 70 mg/cm^2 screen achieved a DQE 10-24% greater than its own counterpart, and its limiting resolution was found 5.4 cycles/mm. Besides, the novel method screens exhibit resistance to strains caused by handling. The scintillator powder remains enclosed in a resin envelope that protects it while offering enhanced rigidity of the screen. During fabrication there is practically no scintillating material loss. The technique does not demand sophisticated equipment and this can lower costs dramatically and promote further scientific and industrial applications.