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A Prototype Intraoral Periapical Sensor with High Frame Rates for a 2.5D Periapical Radiography System

A Prototype Intraoral Periapical Sensor with High Frame Rates for a 2.5D Periapical Radiography... Hindawi Applied Bionics and Biomechanics Volume 2019, Article ID 7987496, 9 pages https://doi.org/10.1155/2019/7987496 Research Article A Prototype Intraoral Periapical Sensor with High Frame Rates for a 2.5D Periapical Radiography System 1,2 2 3 2 2 Che-Wei Liao , Ker-Jer Huang, Jyh-Cheng Chen , Chih-Wei Kuo, Yin-Yi Wu , 4,5 and Jui-Ting Hsu Graduate Institute of Biomedical Sciences, China Medical University, Taichung 404, Taiwan Materials & Electro-Optics Research Division, National Chung-Shan Institute of Science & Technology, Taoyuan City 407, Taiwan Department of Biomedical Imaging and Radiological Sciences, National Yang-Ming University, Taipei 112, Taiwan School of Dentistry, College of Dentistry, China Medical University, Taichung 404, Taiwan Department of Bioinformatics and Medical Engineering, Asia University, Taichung 413, Taiwan Correspondence should be addressed to Yin-Yi Wu; davidwu0801@gmail.com and Jui-Ting Hsu; jthsu@mail.cmu.edu.tw Received 14 February 2019; Accepted 2 April 2019; Published 24 April 2019 Guest Editor: Yuan-Chiao Lu Copyright © 2019 Che-Wei Liao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. X-ray radiography is currently used in dentistry and can be divided into two categories: two-dimensional (2D) radiographic images (e.g., using periapical film, cephalometric film, and panoramic X-ray) and three-dimensional (3D) radiographic images (e.g., using dental cone-beam computed tomography (CBCT)). Among them, 2D periapical film images are most commonly used. However, 2D periapical film compresses 3D image information into a 2D image, which means that depth cannot be identified from the image. Such compressed images lose a considerable amount of information, reducing their clinical applicability. A 2.5D periapical radiography system prototype was developed by our research team. Our previous study indicated that this prototype could be used to capture images at different depths of an object. However, the prototype was limited by its commercially available intraoral periapical sensor, which had a low temporal resolution and could not capture multiple images in a short period of time. Therefore, the total time required for image capture was too long for practical clinical application. The present study developed a high-frame-rate intraoral periapical sensor with a sensor imaging speed of up to 15 Hz. The primary components of the developed intraoral periapical sensor include a scintillator, complementary metal oxide semiconductor chip, component circuit board, and video processing board. The external dimensions of the sensor are 41 × 26 × 6 6 mm . The performance of the developed high-frame-rate intraoral periapical sensor was verified through qualified and quantified analyses using line pairs. The results showed that the resolution of the developed intraoral periapical sensor could reach 18 lp/mm. The sensor was further installed in our 2.5D periapical radiography system to conduct image capturing. The results indicated that the developed sensor could be used for high-frame-rate imaging to incorporate tomosynthesis to obtain reconstructed slice images of different depths. The developed sensor has the potential for clinical dentistry applications in the future. 1. Introduction digitalized [2], and digital radiography can now be divided into three categories, namely, computed radiography (CT), Since X-ray was discovered by Wilhelm Conrad Röntgen indirect digital radiography, and direct digital radiography more than 100 years ago, it has been widely applied in med- [3, 4]. In dentistry, the most common application of X-rays icine. X-ray can be used for noninvasive medical examina- is periapical film, which has advantages such as high resolu- tions and is one of the methods often used to assess hard tion, easy operation, and low costs [5, 6]. However, periapical film can only provide two-dimensional (2D) images; specifi- tissue before surgery [1]. X-ray imaging requires a sensor to capture the image. In the early days of X-ray technology, cally, three-dimensional (3D) tissue compressed projection radiographic films were applied to capture images. In the past can only be displayed in a 2D image. In such 2D images, 30 years, sensors for X-ray imaging have gradually been 3D tissues with different depths are relatively difficult to 2 Applied Bionics and Biomechanics Intraoral digital sensor Electric rotating platform X-ray generator Intraoral digital sensor Central incisor (a) (b) Figure 1: Prototype of the intraoral digital tomosynthesis system: (a) entire view and (b) close view (figure reproduced with permission). distinguish, and image distortion may be caused in the requires a high temporal resolution sensor that can continu- ously capture multiple images in a short period. compression process, thereby often limiting the clinical applicability [7, 8]. Currently, high-frame-rate sensors are used in technol- To overcome the shortcomings of image compression in ogy such as CT, CBCT, and micro-CT [5, 17, 18], all of which periapical film, scholars have conducted experiments using employ large sensors that cannot be placed in the mouth. tomosynthesis in imaging [9]. In 2013, Li et al. [7] conducted Therefore, this study developed a high-frame-rate intraoral a laboratory experiment with a pig’s mandible. A film was periapical sensor capable of frame rates of up to 15 Hz. The placed on the posterior mandible, and the X-ray tube was developed sensor was applied to the 2.5D periapical radiogra- rotated at a limited angle of ±30 . These 2D projection images phy system developed by this research team. were reconstructed through tomosynthesis to obtain numer- ous slice images of different depths. Shan et al. [10] and 2. Materials and Methods Inscoe et al. [11, 12] used a carbon nanotube X-ray source array to develop stationary digital tomosynthesis for dental In our previous study [13], we developed a prototype 2.5D imaging. In their studies [10–12], the feasibility of stationary periapical radiography system (Figure 1) combining a com- intraoral tomosynthesis was demonstrated. They built proto- mercially available intraoral periapical sensor, an X-ray tube, type stationary intraoral tomosynthesis imaging systems, and a supporting frame. A total of 121 canine 2D projection which were evaluated and found to meet all the manufac- images were taken at a limited angle of ±60 . The images were turers’ specifications. In addition, our team built a prototype reconstructed using tomosynthesis to obtain slice images of of a 2.5D periapical radiography system in 2018 [13]. Adopt- the canine at different depths. However, the commercial ing the tomosynthesis approach, we placed a canine in front intraoral periapical sensor used in the prototype requires of a commercially available intraoral periapical sensor and cooling for a few seconds between takes to avoid overheating. rotated the X-ray tube (±60 ) to obtain various 2D projection The process of capturing a set of images in the range of ±60 images. The images were reconstructed using tomosynthesis. therefore took tens of minutes to complete, making the Our results proved that tomosynthesis can be applied in system unsuitable for clinical applications. The present study dentistry to obtain slice images of different depths [14–16]. therefore designed a high-frame-rate intraoral periapical However, in the experiment, we also found that the commer- sensor capable of frame rates of up to 15 Hz. The proposed cially available intraoral periapical sensor possessed relatively sensor could greatly shorten the shooting interval, thereby low temporal resolution and that the system could not con- enhancing its clinical applicability. tinuously capture images multiple times in a short period. Because the original purpose of the commercially available 2.1. Components of the High-Frame-Rate Intraoral Periapical intraoral periapical sensor was not continuous capture of Sensor. The intraoral component of the proposed high- high-frame-rate images, it took tens of minutes to capture a frame-rate intraoral periapical sensor can be divided into set of images, making it unsuitable for clinical application. four parts, namely, the scintillator, complementary metal Therefore, in order for our proposed 2.5D periapical radiog- oxide semiconductor (CMOS) chip, component circuit raphy system to be used as a diagnostic tool in dentistry, it board, and video processing board (Figure 2). The top layer Applied Bionics and Biomechanics 3 Scintillator Fiber optic board CMOS chip Component circuit board (a) (b) Figure 2: (a) The major components of the high-frame-rate intraoral periapical sensor and (b) the high-frame-rate intraoral periapical sensor, video processing board, and control computer. 100 휇 m 100 휇m ×300 39 mm (a) (b) Figure 3: Scanning electron microscope images of the thin-film scintillator: (a) top view and (b) cross-sectional side view. template can increase the residence time of the X-ray in the is the scintillator. The scintillator converts the X-ray intensity to visible light of different grayscales, and the visible light CsI(Tl) crystal and improve the photoelectron conversion signals are then converted from photon signals to electric efficiency of the scintillator. signals through the coupling layer and complementary metal In this study, we used the thermal evaporation method oxide semiconductor (CMOS) sensor array. Subsequently, and adjusted the controlling process parameters to grow a columnar CsI film. After the CsI(Tl) powder was uniformly the electric signals from the CMOS sensor array are transmit- ted to a video processing board (outside the subject’s mouth) mixed, the powder was made into a compressed tablet by through a USB 3.0 cable. The electric (analog) signals of the using a tablet press machine and sintered to remove moisture CMOS sensor array are then converted into digital signals, and impurities in the powder through annealing. The powder which are transmitted to the computer for imaging can be made more compact through the compressing pro- (Figure 2). cess, thereby reducing the amount of air in the tablet. The purpose of the tablet compression was to prevent the air from 2.1.1. Process of Scintillator. CsI scintillator columns exhibit being rapidly expanded due to the heat during the evapora- high absorption capacity for X-rays. These columns can be tion process, which may have caused powder splash that applied in scintillation detectors for capturing simultaneous would affect the quality of the scintillator. Annealing using short-wave images as well as for digitalizing images. A pure different temperature parameters can effectively eliminate CsI scintillator has an extremely short luminescence decay thin-film cracking, formation of voids on the film, and a dis- time (3.5 ns); however, incorporation of a thallium activator ordered structure of the interface (Figure 3). can greatly improve the crystal luminescence efficiency, thereby facilitating coupling between the optical emission 2.1.2. CMOS Chip and Component Circuit Board. Once the and photomultiplier. In addition to enhancing the photoelec- scintillator produces visible light, the optical signals are tron conversion efficiency of the scintillator, changing the transmitted to the CMOS image sensor by the continuous refractive index of the X-ray in the anodic aluminum oxide shooting fiber optic board. The CMOS is a mixed integrated 4 Applied Bionics and Biomechanics XRAY_READY Digital timing control HSR HSR V V V 504X1500 504X1500 V M M CMOS APS CMOS APS S S Input U U array array R R light X X Digital output APS CDS V-Amp Analog ciruit output Video Video CDS CDS Cell stage Output stage Column stage AMP AMP Analog Analog Video_L Video_R (a) (b) Figure 4: (a) CMOS image sensor signal processing flow chart and (b) architecture of the chip export signals. circuit that includes an analog circuit and a digital circuit. output format is TIFF, the image size is 1000 × 1496 pixels with a pixel size of 20 μm, and the image format is 14 bits. The analog circuit processes optical signals and includes four electric circuit modes, namely, a photon-to-electron transfor- Another function of the video processing board is to syn- mation circuit (active pixel sensor (APS)), pixel signal collec- chronously control the X-ray tube. Because the video pro- tion circuit (correlated double sampling (CDS)), pixel signal cessing board controls the X-ray tube exposure, the board amplification circuit (video output amplifier (V-Amp)), and also controls the synchronization timing of the chipset analog-to-digital conversion circuit (ADC). CMOS image exposure, meaning that the chipset and X-ray tube can be sensing utilizes the photoelectric effect to excite electrons in synchronously activated to accurately control the chipset the silicon crystal from the valence to conduction bands, and begin receiving signals. Table 1 lists the characteristic and the optical signal strength is measured by the amount features of the developed high-frame-rate intraoral periapical of photocurrent generated during the process. The CMOS sensor. image sensor uses the N+-to-PSUB PN interface as a light sensor. Figure 4(a) shows a flow chart of the signal processing 2.2. Quantitative Performance of the High-Frame-Rate of the CMOS image sensor. The architecture of the CMOS Intraoral Periapical Sensor. The developed intraoral periapi- chip signal export is illustrated in Figure 4(b). To improve cal sensor was also used to capture images of two phantoms the image export rate, the chip signal is synchronously to verify image quality. The phantoms were line pairs. From processed by a dual-channel setting and exported through the images of the line pairs, the calculated modulation Analog Video_L and Analog Video_R. Regarding the circuit transfer function (MTF) could be used to measure the actual of the AD9244 analog-to-digital converter in the video pro- resolution of the sensor. cessing board, V_Out1 is the chip output signal of one of the channels; however, it is also the input signal for the video processing board. After being transmitted through the 2.3. Applying the High-Frame-Rate Intraoral Periapical sample-and-hold circuit, the signal is transmitted to the Sensor in the Prototype 2.5D Periapical Radiography System. AD9244BSTZ-40 chip, and the final output digital image We installed the high-frame-rate intraoral periapical sensor on our previously developed 2.5D periapical radiography data format is 14 bits. system prototype (Figure 5) [13]. The image sample was a 2.1.3. Video Processing Board. The chip imaging architecture human third molar. For details regarding the prototype, refer requires a matching video processing board. The primary to our previous study [13]. The scanning parameters were as function of the matched video processing board is to output follows: distance between the X-ray source and rotation axis was 350 mm; distance between the sensor and rotation axis signals to the A/D converter chip on the external video pro- cessing board through the serial transmission of chipsets. was 5 mm; the voltage was 80 kVp; the current was 5 mA; Subsequently, the video processing board converts the signals exposure time was 0.2 second; the angle of the X-ray tube ° ° into digital signals and transmits them through CameraLink, was ±30 ; and images were taken every 2 . A total of 31 a high-speed data transmission interface, to output the image images were taken, and these 31 2D projection images were information to a computer for data storage. The image used to reconstruct images through tomosynthesis. Applied Bionics and Biomechanics 5 Table 1: The characteristic features of the high-frame-rate intraoral periapical sensor. Measurements of trabecular bone microarchitectural parameters based on the micro-CT and dental CBCT images. Number Item Specification 1 Process UMC 0.35 μm CIS with stitching (8 inches) 1008 × 1500 2 Frame resolution 3 Sensitive area 20 16 mm × 30 mm 4 Pixel size 20 μm 5 Output type Serial 6 Interface (chip to video processing board) Analog 7 Interface (video processing board to system) CameraLink 8 Color Gray 9 Frame rate (max) ≤15 Hz 10 Pixel data rate 15 MHz 11 Pixel sampling resolution 16384 (14 bits) 12 Voltage 3.3 V 13 Power of chip 165 mW 14 Number of pads 68 15 Chip size 20 68 mm × 32 92 mm 19 lp/mm (Figure 6(b)); thus, the resolution of the developed intraoral periapical sensor was 18 lp/mm. 3.2. The Performance of the 2.5D Periapical Radiography System Using the High-Frame-Rate Intraoral Periapical Sensor. The high-frame-rate intraoral periapical sensor was installed in the 2.5D intraoral periapical sensor prototype designed by our research team. The X-ray tube scanned the +30° 0° 2D projection images of the human third molar at an angle of 30 . Figure 7 illustrates that the more the X-ray tube devi- ates from the orthogonal axis (0 ) of the sensor, the more −30° severe the deformation of the 2D projection images becomes. ° ° At ±30 , an image was captured every 2 , for a total of 31 images taken in approximately 4 s. Clear outlines of the den- Artificial head tin and enamel of the third molar can be observed in each of the 2D projection images; thus, the outline was not distorted or blurred when shooting at a high frame rate. The 2D projection images captured by the X-ray tube at were equivalent to the images captured using clinical peri- apical film. In images captured using periapical film, 3D images of tissue are compressed into a 2D image (Figure 8(a)). Figures 8(b)–8(d) present the reconstructed images utilizing the 31 2D projection images taken of the third molar at different depths. These images display the Figure 5: 2.5D periapical radiography system and X-ray tube anteroposterior relationship between different parts of the scanning ranges. molar, and the structure of the internal tissue, such as the dentin, enamel, and pulp cavity, is also present in the images. These images provide more information regarding the molar than the 2D periapical film image does (Figure 8(a)). 3. Results 3.1. The Performance of the High-Frame-Rate Intraoral 4. Discussion Periapical Sensor. This high-frame-rate intraoral periapical sensor utilized two methods for image quality evaluation, Our research team previously conducted in vitro tests to ver- namely, line pairs and an aluminum step wedge. From the ify the feasibility of our 2.5D periapical radiography system line pair images (Figure 6(a)), image quality was quantized [13]. By using an X-ray tube, an intraoral periapical sensor, using the MTF, the value of which was lower than 0.09 at a supporting frame, and electronic control equipment, we 6 Applied Bionics and Biomechanics (a) (b) 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 02468 10 12 14 16 18 20 (lp/mm) (c) Figure 6: (a) The line pair phantom, (b) the image of the line pair phantom, and (c) the curve of modulation transfer function. X-ray source (c) (b) (d) (e) (a) (a) (b) (c) Tooth Rotation axis (d) (e) Digital X-ray sensor Figure 7: X-ray tube captures 2D projection images of the third molar at different angles. captured multiple 2D projection images of a tooth. These available intraoral periapical sensor was low and the sensor projection images were used to reconstruct images by adopt- required a few seconds between each shot, resulting in an overall shooting time of tens of minutes. Therefore, the pres- ing tomosynthesis to obtain slice images of different depths of the tooth. Images captured using this method provided ent study developed a high-frame-rate intraoral periapical more image information than those captured with conven- sensor with a frame rate of up to 15 Hz. The preliminary test tional periapical film, and the images from the 2.5D periapi- results indicated that the system could perform a tomosynth- cal radiography system were not affected by the image esis scan of ±30 in only 4 s, which greatly improves the superposition that occurs with a 2D periapical film. However, potential for clinical application of our 2.5D periapical radi- we also found that the 2.5D periapical radiography system ography system. prototype was unsuitable for current clinical applications Periapical film has the advantages of high resolution, easy because the temporal resolution of the commercially operation, and low costs [6]; however, its ultimate limitation Modulation transfer function Applied Bionics and Biomechanics 7 (a) (b) (c) (d) (b) (c) (d) Figure 8: (a) Periapical radiology image of the third molar and (b–d) the reconstructed slice images at different depths from the sensor surface. is that it can only capture 2D images. Our research team pre- Currently, high-frame-rate sensors with imaging speeds of up to 10 Hz are available [21, 22]; however, these are viously showed the feasibility of a 2.5D periapical radiogra- phy system [13] using a commercial sensor (RVG6200- mostly used in CT, dental CBCT, or micro-CT [23–25]. SIZE1, Carestream Dental, Stuttgart, Germany) and an X- Moreover, these sensors are at least 12 × 7 or 15 × 15 cm , ° ° ray tube of ±60 to obtain 2D projection images every 1 . which means they are too large to be placed in the mouth. The obtained 2D projection images of a canine were then Intraoral periapical sensors used in modern dentistry are used to reconstruct images of the canine at different depths. not designed for high-frame-rate capture; thus, manufac- The reconstructed images obtained using tomosynthesis turers do not provide information on maximum sensor were similar to those obtained by using an X-ray tube to per- frame rates. Our previous study found that interval between form 360 scanning incorporating the background projection shots of less than 5 s using a commercial sensor led the sys- method; the dentin and enamel outlines could be distin- tem to overheat. Therefore, development of a high-frame- rate intraoral periapical sensor is necessary if our 2.5D peria- guished in the images. In a previous study [13], we demon- strated that tomosynthesis could be applied to dentistry. As pical radiography system is to be used for future clinical early as 1996, Webber et al. [19] had already used an intraoral applications. In addition to use in our 2.5D periapical radiog- CCD X-ray transducer to indicate that tomosynthesis might raphy system, the high-frame-rate intraoral periapical sensor be applicable to dentistry. In 2013, Li et al. [7] constructed developed in the present study can be applied to a micro-CT a desktop intraoral digital tomosynthesis system in the machine [13] or used in industrial quality management that laboratory and conducted an experiment on a pig mandible. requires capturing high-frame-rate images. By placing a sensor in the posterior mandible, the results of The line pair phantom was used in this study to quantify their study also indicated the feasibility of tomosynthesis in the image quality. According to the analysis results, the reso- dentistry. Currently, tomosynthesis is mostly applied to lution of the intraoral periapical sensor was 18 lp/mm. In our mammography [15] [20], with no commercially available previous study [13], we employed the RVG6200-SIZE1 products found in dentistry. A possible reason for this is that commercial intraoral digital sensor (Carestream Dental, no high-frame-rate intraoral periapical sensor currently Stuttgart, Germany) to build a prototype 2.5D periapical exists. Therefore, this study sought to develop such a sensor. radiography system. The line pair phantoms were used to 8 Applied Bionics and Biomechanics between ±30 measure the resolution of the commercial sensor, with a ), resulting in the scanning width being less result of 18 lp/mm, which was similar with the developed than approximately 80% of the 2D periapical X-ray. Further- high-frame-rate intraoral periapical sensor. Figure 7 displays more, the computer operation interface of the high-frame- 2D projection images of the third molar captured by the X- rate intraoral periapical sensor is relatively complicated; the ray tube at different angles. Because the high-frame-rate interface should be improved to increase user friendliness intraoral periapical sensor was fixed in place, the larger the in the future, making system operation easier for dentists shooting angle of the X-ray tube was, the more severe the and medical image radiologists. image distortion of the teeth became. However, the outlines and boundaries of the dentin and enamel on these distorted 5. Conclusion images could still be identified. These projection images were taken by the moving X-ray tube, and due to the high-frame- The developed high-frame-rate intraoral periapical sensor requires further improvement for use in capturing images rate capture, each image was blur free. In this study, the images reconstructed from the 31 2D in a patient’s mouth. However, the sensor can greatly reduce shooting time using our 2.5D periapical radiography system projection images using tomosynthesis revealed slice images of the third molar at different depths (Figures 8(b)–8(d)). to less than 5 s, proving its potential for use in future clinical Compared with 2D periapical film images (Figure 8(a)), the applications. images captured using the new method obtained more infor- mation from the tooth, and the images were unaffected by Data Availability compression of the 3D structure of the tissue into a 2D peri- The datasets generated during the current study are available apical film image (Figure 8(a)), meaning that the images could still show the anteroposterior relationship of the tooth. from the corresponding author on reasonable request. Mammography also uses tomosynthesis to capture images by scanning at limited angles to obtain reconstructed images Conflicts of Interest [26, 27]; these limited angles mean that the reconstructed The authors declare that they have no competing interests. slice images are not always clear. The farther from the rota- tion axis the slice images are taken, the more blurred the slice images become. Authors’ Contributions In this study, the milliampere seconds per projection was The fifth author and sixth author equally contributed to this 1.0 mAs for a total exposure of 31 mAs, which is twice of the work. 15.75 mAs achieved by the previous stationary intraoral dig- ital tomosynthesis system developed by Shan et al. [10]. However, the X-ray tube output for the total exposure in Acknowledgments the present study (31 mAs) was half that of another digital The authors would like to thank Dr. Tzu-Hung Lin, Mrs. Yi- tomosynthesis system developed by Ziegler et al. [9], which Fu Tang, Ching-Hsing Chang, and Ruei-Teng Wang (Indus- was 67.2 mAs. Regardless, the X-ray tube output of all trial Technology Research Institute, Material and Chemical intraoral digital tomosynthesis systems should be much less Research Laboratories, Taiwan) for their assistances and sug- than that of dental CBCT. gestions regarding the experimental setup. This study was The frame rate of a CMOS sensor can be affected by partially supported by China Medical University (CMU many factors, such as pixel size, X-ray output power, and 107-S-09), Taiwan. design of the sensor (e.g., fill factor, quantum efficiency, and signal processing). The intraoral periapical sensor devel- oped in the present study could capture images at 15 Hz, References which was sufficient for our 2.5D periapical radiography sys- [1] N. Shah, N. Bansal, and A. Logani, “Recent advances in imag- tem. 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A Prototype Intraoral Periapical Sensor with High Frame Rates for a 2.5D Periapical Radiography System

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Copyright © 2019 Che-Wei Liao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Hindawi Applied Bionics and Biomechanics Volume 2019, Article ID 7987496, 9 pages https://doi.org/10.1155/2019/7987496 Research Article A Prototype Intraoral Periapical Sensor with High Frame Rates for a 2.5D Periapical Radiography System 1,2 2 3 2 2 Che-Wei Liao , Ker-Jer Huang, Jyh-Cheng Chen , Chih-Wei Kuo, Yin-Yi Wu , 4,5 and Jui-Ting Hsu Graduate Institute of Biomedical Sciences, China Medical University, Taichung 404, Taiwan Materials & Electro-Optics Research Division, National Chung-Shan Institute of Science & Technology, Taoyuan City 407, Taiwan Department of Biomedical Imaging and Radiological Sciences, National Yang-Ming University, Taipei 112, Taiwan School of Dentistry, College of Dentistry, China Medical University, Taichung 404, Taiwan Department of Bioinformatics and Medical Engineering, Asia University, Taichung 413, Taiwan Correspondence should be addressed to Yin-Yi Wu; davidwu0801@gmail.com and Jui-Ting Hsu; jthsu@mail.cmu.edu.tw Received 14 February 2019; Accepted 2 April 2019; Published 24 April 2019 Guest Editor: Yuan-Chiao Lu Copyright © 2019 Che-Wei Liao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. X-ray radiography is currently used in dentistry and can be divided into two categories: two-dimensional (2D) radiographic images (e.g., using periapical film, cephalometric film, and panoramic X-ray) and three-dimensional (3D) radiographic images (e.g., using dental cone-beam computed tomography (CBCT)). Among them, 2D periapical film images are most commonly used. However, 2D periapical film compresses 3D image information into a 2D image, which means that depth cannot be identified from the image. Such compressed images lose a considerable amount of information, reducing their clinical applicability. A 2.5D periapical radiography system prototype was developed by our research team. Our previous study indicated that this prototype could be used to capture images at different depths of an object. However, the prototype was limited by its commercially available intraoral periapical sensor, which had a low temporal resolution and could not capture multiple images in a short period of time. Therefore, the total time required for image capture was too long for practical clinical application. The present study developed a high-frame-rate intraoral periapical sensor with a sensor imaging speed of up to 15 Hz. The primary components of the developed intraoral periapical sensor include a scintillator, complementary metal oxide semiconductor chip, component circuit board, and video processing board. The external dimensions of the sensor are 41 × 26 × 6 6 mm . The performance of the developed high-frame-rate intraoral periapical sensor was verified through qualified and quantified analyses using line pairs. The results showed that the resolution of the developed intraoral periapical sensor could reach 18 lp/mm. The sensor was further installed in our 2.5D periapical radiography system to conduct image capturing. The results indicated that the developed sensor could be used for high-frame-rate imaging to incorporate tomosynthesis to obtain reconstructed slice images of different depths. The developed sensor has the potential for clinical dentistry applications in the future. 1. Introduction digitalized [2], and digital radiography can now be divided into three categories, namely, computed radiography (CT), Since X-ray was discovered by Wilhelm Conrad Röntgen indirect digital radiography, and direct digital radiography more than 100 years ago, it has been widely applied in med- [3, 4]. In dentistry, the most common application of X-rays icine. X-ray can be used for noninvasive medical examina- is periapical film, which has advantages such as high resolu- tions and is one of the methods often used to assess hard tion, easy operation, and low costs [5, 6]. However, periapical film can only provide two-dimensional (2D) images; specifi- tissue before surgery [1]. X-ray imaging requires a sensor to capture the image. In the early days of X-ray technology, cally, three-dimensional (3D) tissue compressed projection radiographic films were applied to capture images. In the past can only be displayed in a 2D image. In such 2D images, 30 years, sensors for X-ray imaging have gradually been 3D tissues with different depths are relatively difficult to 2 Applied Bionics and Biomechanics Intraoral digital sensor Electric rotating platform X-ray generator Intraoral digital sensor Central incisor (a) (b) Figure 1: Prototype of the intraoral digital tomosynthesis system: (a) entire view and (b) close view (figure reproduced with permission). distinguish, and image distortion may be caused in the requires a high temporal resolution sensor that can continu- ously capture multiple images in a short period. compression process, thereby often limiting the clinical applicability [7, 8]. Currently, high-frame-rate sensors are used in technol- To overcome the shortcomings of image compression in ogy such as CT, CBCT, and micro-CT [5, 17, 18], all of which periapical film, scholars have conducted experiments using employ large sensors that cannot be placed in the mouth. tomosynthesis in imaging [9]. In 2013, Li et al. [7] conducted Therefore, this study developed a high-frame-rate intraoral a laboratory experiment with a pig’s mandible. A film was periapical sensor capable of frame rates of up to 15 Hz. The placed on the posterior mandible, and the X-ray tube was developed sensor was applied to the 2.5D periapical radiogra- rotated at a limited angle of ±30 . These 2D projection images phy system developed by this research team. were reconstructed through tomosynthesis to obtain numer- ous slice images of different depths. Shan et al. [10] and 2. Materials and Methods Inscoe et al. [11, 12] used a carbon nanotube X-ray source array to develop stationary digital tomosynthesis for dental In our previous study [13], we developed a prototype 2.5D imaging. In their studies [10–12], the feasibility of stationary periapical radiography system (Figure 1) combining a com- intraoral tomosynthesis was demonstrated. They built proto- mercially available intraoral periapical sensor, an X-ray tube, type stationary intraoral tomosynthesis imaging systems, and a supporting frame. A total of 121 canine 2D projection which were evaluated and found to meet all the manufac- images were taken at a limited angle of ±60 . The images were turers’ specifications. In addition, our team built a prototype reconstructed using tomosynthesis to obtain slice images of of a 2.5D periapical radiography system in 2018 [13]. Adopt- the canine at different depths. However, the commercial ing the tomosynthesis approach, we placed a canine in front intraoral periapical sensor used in the prototype requires of a commercially available intraoral periapical sensor and cooling for a few seconds between takes to avoid overheating. rotated the X-ray tube (±60 ) to obtain various 2D projection The process of capturing a set of images in the range of ±60 images. The images were reconstructed using tomosynthesis. therefore took tens of minutes to complete, making the Our results proved that tomosynthesis can be applied in system unsuitable for clinical applications. The present study dentistry to obtain slice images of different depths [14–16]. therefore designed a high-frame-rate intraoral periapical However, in the experiment, we also found that the commer- sensor capable of frame rates of up to 15 Hz. The proposed cially available intraoral periapical sensor possessed relatively sensor could greatly shorten the shooting interval, thereby low temporal resolution and that the system could not con- enhancing its clinical applicability. tinuously capture images multiple times in a short period. Because the original purpose of the commercially available 2.1. Components of the High-Frame-Rate Intraoral Periapical intraoral periapical sensor was not continuous capture of Sensor. The intraoral component of the proposed high- high-frame-rate images, it took tens of minutes to capture a frame-rate intraoral periapical sensor can be divided into set of images, making it unsuitable for clinical application. four parts, namely, the scintillator, complementary metal Therefore, in order for our proposed 2.5D periapical radiog- oxide semiconductor (CMOS) chip, component circuit raphy system to be used as a diagnostic tool in dentistry, it board, and video processing board (Figure 2). The top layer Applied Bionics and Biomechanics 3 Scintillator Fiber optic board CMOS chip Component circuit board (a) (b) Figure 2: (a) The major components of the high-frame-rate intraoral periapical sensor and (b) the high-frame-rate intraoral periapical sensor, video processing board, and control computer. 100 휇 m 100 휇m ×300 39 mm (a) (b) Figure 3: Scanning electron microscope images of the thin-film scintillator: (a) top view and (b) cross-sectional side view. template can increase the residence time of the X-ray in the is the scintillator. The scintillator converts the X-ray intensity to visible light of different grayscales, and the visible light CsI(Tl) crystal and improve the photoelectron conversion signals are then converted from photon signals to electric efficiency of the scintillator. signals through the coupling layer and complementary metal In this study, we used the thermal evaporation method oxide semiconductor (CMOS) sensor array. Subsequently, and adjusted the controlling process parameters to grow a columnar CsI film. After the CsI(Tl) powder was uniformly the electric signals from the CMOS sensor array are transmit- ted to a video processing board (outside the subject’s mouth) mixed, the powder was made into a compressed tablet by through a USB 3.0 cable. The electric (analog) signals of the using a tablet press machine and sintered to remove moisture CMOS sensor array are then converted into digital signals, and impurities in the powder through annealing. The powder which are transmitted to the computer for imaging can be made more compact through the compressing pro- (Figure 2). cess, thereby reducing the amount of air in the tablet. The purpose of the tablet compression was to prevent the air from 2.1.1. Process of Scintillator. CsI scintillator columns exhibit being rapidly expanded due to the heat during the evapora- high absorption capacity for X-rays. These columns can be tion process, which may have caused powder splash that applied in scintillation detectors for capturing simultaneous would affect the quality of the scintillator. Annealing using short-wave images as well as for digitalizing images. A pure different temperature parameters can effectively eliminate CsI scintillator has an extremely short luminescence decay thin-film cracking, formation of voids on the film, and a dis- time (3.5 ns); however, incorporation of a thallium activator ordered structure of the interface (Figure 3). can greatly improve the crystal luminescence efficiency, thereby facilitating coupling between the optical emission 2.1.2. CMOS Chip and Component Circuit Board. Once the and photomultiplier. In addition to enhancing the photoelec- scintillator produces visible light, the optical signals are tron conversion efficiency of the scintillator, changing the transmitted to the CMOS image sensor by the continuous refractive index of the X-ray in the anodic aluminum oxide shooting fiber optic board. The CMOS is a mixed integrated 4 Applied Bionics and Biomechanics XRAY_READY Digital timing control HSR HSR V V V 504X1500 504X1500 V M M CMOS APS CMOS APS S S Input U U array array R R light X X Digital output APS CDS V-Amp Analog ciruit output Video Video CDS CDS Cell stage Output stage Column stage AMP AMP Analog Analog Video_L Video_R (a) (b) Figure 4: (a) CMOS image sensor signal processing flow chart and (b) architecture of the chip export signals. circuit that includes an analog circuit and a digital circuit. output format is TIFF, the image size is 1000 × 1496 pixels with a pixel size of 20 μm, and the image format is 14 bits. The analog circuit processes optical signals and includes four electric circuit modes, namely, a photon-to-electron transfor- Another function of the video processing board is to syn- mation circuit (active pixel sensor (APS)), pixel signal collec- chronously control the X-ray tube. Because the video pro- tion circuit (correlated double sampling (CDS)), pixel signal cessing board controls the X-ray tube exposure, the board amplification circuit (video output amplifier (V-Amp)), and also controls the synchronization timing of the chipset analog-to-digital conversion circuit (ADC). CMOS image exposure, meaning that the chipset and X-ray tube can be sensing utilizes the photoelectric effect to excite electrons in synchronously activated to accurately control the chipset the silicon crystal from the valence to conduction bands, and begin receiving signals. Table 1 lists the characteristic and the optical signal strength is measured by the amount features of the developed high-frame-rate intraoral periapical of photocurrent generated during the process. The CMOS sensor. image sensor uses the N+-to-PSUB PN interface as a light sensor. Figure 4(a) shows a flow chart of the signal processing 2.2. Quantitative Performance of the High-Frame-Rate of the CMOS image sensor. The architecture of the CMOS Intraoral Periapical Sensor. The developed intraoral periapi- chip signal export is illustrated in Figure 4(b). To improve cal sensor was also used to capture images of two phantoms the image export rate, the chip signal is synchronously to verify image quality. The phantoms were line pairs. From processed by a dual-channel setting and exported through the images of the line pairs, the calculated modulation Analog Video_L and Analog Video_R. Regarding the circuit transfer function (MTF) could be used to measure the actual of the AD9244 analog-to-digital converter in the video pro- resolution of the sensor. cessing board, V_Out1 is the chip output signal of one of the channels; however, it is also the input signal for the video processing board. After being transmitted through the 2.3. Applying the High-Frame-Rate Intraoral Periapical sample-and-hold circuit, the signal is transmitted to the Sensor in the Prototype 2.5D Periapical Radiography System. AD9244BSTZ-40 chip, and the final output digital image We installed the high-frame-rate intraoral periapical sensor on our previously developed 2.5D periapical radiography data format is 14 bits. system prototype (Figure 5) [13]. The image sample was a 2.1.3. Video Processing Board. The chip imaging architecture human third molar. For details regarding the prototype, refer requires a matching video processing board. The primary to our previous study [13]. The scanning parameters were as function of the matched video processing board is to output follows: distance between the X-ray source and rotation axis was 350 mm; distance between the sensor and rotation axis signals to the A/D converter chip on the external video pro- cessing board through the serial transmission of chipsets. was 5 mm; the voltage was 80 kVp; the current was 5 mA; Subsequently, the video processing board converts the signals exposure time was 0.2 second; the angle of the X-ray tube ° ° into digital signals and transmits them through CameraLink, was ±30 ; and images were taken every 2 . A total of 31 a high-speed data transmission interface, to output the image images were taken, and these 31 2D projection images were information to a computer for data storage. The image used to reconstruct images through tomosynthesis. Applied Bionics and Biomechanics 5 Table 1: The characteristic features of the high-frame-rate intraoral periapical sensor. Measurements of trabecular bone microarchitectural parameters based on the micro-CT and dental CBCT images. Number Item Specification 1 Process UMC 0.35 μm CIS with stitching (8 inches) 1008 × 1500 2 Frame resolution 3 Sensitive area 20 16 mm × 30 mm 4 Pixel size 20 μm 5 Output type Serial 6 Interface (chip to video processing board) Analog 7 Interface (video processing board to system) CameraLink 8 Color Gray 9 Frame rate (max) ≤15 Hz 10 Pixel data rate 15 MHz 11 Pixel sampling resolution 16384 (14 bits) 12 Voltage 3.3 V 13 Power of chip 165 mW 14 Number of pads 68 15 Chip size 20 68 mm × 32 92 mm 19 lp/mm (Figure 6(b)); thus, the resolution of the developed intraoral periapical sensor was 18 lp/mm. 3.2. The Performance of the 2.5D Periapical Radiography System Using the High-Frame-Rate Intraoral Periapical Sensor. The high-frame-rate intraoral periapical sensor was installed in the 2.5D intraoral periapical sensor prototype designed by our research team. The X-ray tube scanned the +30° 0° 2D projection images of the human third molar at an angle of 30 . Figure 7 illustrates that the more the X-ray tube devi- ates from the orthogonal axis (0 ) of the sensor, the more −30° severe the deformation of the 2D projection images becomes. ° ° At ±30 , an image was captured every 2 , for a total of 31 images taken in approximately 4 s. Clear outlines of the den- Artificial head tin and enamel of the third molar can be observed in each of the 2D projection images; thus, the outline was not distorted or blurred when shooting at a high frame rate. The 2D projection images captured by the X-ray tube at were equivalent to the images captured using clinical peri- apical film. In images captured using periapical film, 3D images of tissue are compressed into a 2D image (Figure 8(a)). Figures 8(b)–8(d) present the reconstructed images utilizing the 31 2D projection images taken of the third molar at different depths. These images display the Figure 5: 2.5D periapical radiography system and X-ray tube anteroposterior relationship between different parts of the scanning ranges. molar, and the structure of the internal tissue, such as the dentin, enamel, and pulp cavity, is also present in the images. These images provide more information regarding the molar than the 2D periapical film image does (Figure 8(a)). 3. Results 3.1. The Performance of the High-Frame-Rate Intraoral 4. Discussion Periapical Sensor. This high-frame-rate intraoral periapical sensor utilized two methods for image quality evaluation, Our research team previously conducted in vitro tests to ver- namely, line pairs and an aluminum step wedge. From the ify the feasibility of our 2.5D periapical radiography system line pair images (Figure 6(a)), image quality was quantized [13]. By using an X-ray tube, an intraoral periapical sensor, using the MTF, the value of which was lower than 0.09 at a supporting frame, and electronic control equipment, we 6 Applied Bionics and Biomechanics (a) (b) 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 02468 10 12 14 16 18 20 (lp/mm) (c) Figure 6: (a) The line pair phantom, (b) the image of the line pair phantom, and (c) the curve of modulation transfer function. X-ray source (c) (b) (d) (e) (a) (a) (b) (c) Tooth Rotation axis (d) (e) Digital X-ray sensor Figure 7: X-ray tube captures 2D projection images of the third molar at different angles. captured multiple 2D projection images of a tooth. These available intraoral periapical sensor was low and the sensor projection images were used to reconstruct images by adopt- required a few seconds between each shot, resulting in an overall shooting time of tens of minutes. Therefore, the pres- ing tomosynthesis to obtain slice images of different depths of the tooth. Images captured using this method provided ent study developed a high-frame-rate intraoral periapical more image information than those captured with conven- sensor with a frame rate of up to 15 Hz. The preliminary test tional periapical film, and the images from the 2.5D periapi- results indicated that the system could perform a tomosynth- cal radiography system were not affected by the image esis scan of ±30 in only 4 s, which greatly improves the superposition that occurs with a 2D periapical film. However, potential for clinical application of our 2.5D periapical radi- we also found that the 2.5D periapical radiography system ography system. prototype was unsuitable for current clinical applications Periapical film has the advantages of high resolution, easy because the temporal resolution of the commercially operation, and low costs [6]; however, its ultimate limitation Modulation transfer function Applied Bionics and Biomechanics 7 (a) (b) (c) (d) (b) (c) (d) Figure 8: (a) Periapical radiology image of the third molar and (b–d) the reconstructed slice images at different depths from the sensor surface. is that it can only capture 2D images. Our research team pre- Currently, high-frame-rate sensors with imaging speeds of up to 10 Hz are available [21, 22]; however, these are viously showed the feasibility of a 2.5D periapical radiogra- phy system [13] using a commercial sensor (RVG6200- mostly used in CT, dental CBCT, or micro-CT [23–25]. SIZE1, Carestream Dental, Stuttgart, Germany) and an X- Moreover, these sensors are at least 12 × 7 or 15 × 15 cm , ° ° ray tube of ±60 to obtain 2D projection images every 1 . which means they are too large to be placed in the mouth. The obtained 2D projection images of a canine were then Intraoral periapical sensors used in modern dentistry are used to reconstruct images of the canine at different depths. not designed for high-frame-rate capture; thus, manufac- The reconstructed images obtained using tomosynthesis turers do not provide information on maximum sensor were similar to those obtained by using an X-ray tube to per- frame rates. Our previous study found that interval between form 360 scanning incorporating the background projection shots of less than 5 s using a commercial sensor led the sys- method; the dentin and enamel outlines could be distin- tem to overheat. Therefore, development of a high-frame- rate intraoral periapical sensor is necessary if our 2.5D peria- guished in the images. In a previous study [13], we demon- strated that tomosynthesis could be applied to dentistry. As pical radiography system is to be used for future clinical early as 1996, Webber et al. [19] had already used an intraoral applications. In addition to use in our 2.5D periapical radiog- CCD X-ray transducer to indicate that tomosynthesis might raphy system, the high-frame-rate intraoral periapical sensor be applicable to dentistry. In 2013, Li et al. [7] constructed developed in the present study can be applied to a micro-CT a desktop intraoral digital tomosynthesis system in the machine [13] or used in industrial quality management that laboratory and conducted an experiment on a pig mandible. requires capturing high-frame-rate images. By placing a sensor in the posterior mandible, the results of The line pair phantom was used in this study to quantify their study also indicated the feasibility of tomosynthesis in the image quality. According to the analysis results, the reso- dentistry. Currently, tomosynthesis is mostly applied to lution of the intraoral periapical sensor was 18 lp/mm. In our mammography [15] [20], with no commercially available previous study [13], we employed the RVG6200-SIZE1 products found in dentistry. A possible reason for this is that commercial intraoral digital sensor (Carestream Dental, no high-frame-rate intraoral periapical sensor currently Stuttgart, Germany) to build a prototype 2.5D periapical exists. Therefore, this study sought to develop such a sensor. radiography system. The line pair phantoms were used to 8 Applied Bionics and Biomechanics between ±30 measure the resolution of the commercial sensor, with a ), resulting in the scanning width being less result of 18 lp/mm, which was similar with the developed than approximately 80% of the 2D periapical X-ray. Further- high-frame-rate intraoral periapical sensor. Figure 7 displays more, the computer operation interface of the high-frame- 2D projection images of the third molar captured by the X- rate intraoral periapical sensor is relatively complicated; the ray tube at different angles. Because the high-frame-rate interface should be improved to increase user friendliness intraoral periapical sensor was fixed in place, the larger the in the future, making system operation easier for dentists shooting angle of the X-ray tube was, the more severe the and medical image radiologists. image distortion of the teeth became. However, the outlines and boundaries of the dentin and enamel on these distorted 5. Conclusion images could still be identified. These projection images were taken by the moving X-ray tube, and due to the high-frame- The developed high-frame-rate intraoral periapical sensor requires further improvement for use in capturing images rate capture, each image was blur free. In this study, the images reconstructed from the 31 2D in a patient’s mouth. However, the sensor can greatly reduce shooting time using our 2.5D periapical radiography system projection images using tomosynthesis revealed slice images of the third molar at different depths (Figures 8(b)–8(d)). to less than 5 s, proving its potential for use in future clinical Compared with 2D periapical film images (Figure 8(a)), the applications. images captured using the new method obtained more infor- mation from the tooth, and the images were unaffected by Data Availability compression of the 3D structure of the tissue into a 2D peri- The datasets generated during the current study are available apical film image (Figure 8(a)), meaning that the images could still show the anteroposterior relationship of the tooth. from the corresponding author on reasonable request. Mammography also uses tomosynthesis to capture images by scanning at limited angles to obtain reconstructed images Conflicts of Interest [26, 27]; these limited angles mean that the reconstructed The authors declare that they have no competing interests. slice images are not always clear. The farther from the rota- tion axis the slice images are taken, the more blurred the slice images become. Authors’ Contributions In this study, the milliampere seconds per projection was The fifth author and sixth author equally contributed to this 1.0 mAs for a total exposure of 31 mAs, which is twice of the work. 15.75 mAs achieved by the previous stationary intraoral dig- ital tomosynthesis system developed by Shan et al. [10]. However, the X-ray tube output for the total exposure in Acknowledgments the present study (31 mAs) was half that of another digital The authors would like to thank Dr. Tzu-Hung Lin, Mrs. Yi- tomosynthesis system developed by Ziegler et al. [9], which Fu Tang, Ching-Hsing Chang, and Ruei-Teng Wang (Indus- was 67.2 mAs. Regardless, the X-ray tube output of all trial Technology Research Institute, Material and Chemical intraoral digital tomosynthesis systems should be much less Research Laboratories, Taiwan) for their assistances and sug- than that of dental CBCT. gestions regarding the experimental setup. This study was The frame rate of a CMOS sensor can be affected by partially supported by China Medical University (CMU many factors, such as pixel size, X-ray output power, and 107-S-09), Taiwan. design of the sensor (e.g., fill factor, quantum efficiency, and signal processing). The intraoral periapical sensor devel- oped in the present study could capture images at 15 Hz, References which was sufficient for our 2.5D periapical radiography sys- [1] N. Shah, N. Bansal, and A. Logani, “Recent advances in imag- tem. 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