Bidirectional digital miniature wireless endoscope system technology

This paper proposes a new digital two-way miniature wireless endoscope system, which has the functions of real-time observation of the patient's digestive tract images, full digestive tract examination, and providing three-dimensional depth image data.

Endoscopy is currently the most commonly used and most direct and effective method for the examination of digestive tract diseases. It plays an extremely important role in the diagnosis of digestive tract diseases. However, existing common endoscope systems have to be equipped with a guide cannula, which brings inconvenience to the operation of the system and also causes great pain to the patient. In addition, the examination site is limited and the small intestine cannot be examined. With the development of microelectronics technology, Israelis have developed a wireless endoscope system [1]. Its development is still in its infancy and has some limitations, such as insufficient image resolution, only a one-way data communication system, the inability of doctors to observe the patient's digestive tract image on site in real time, the inability to control the working status of the capsule in the body, the limited battery power supply time in the capsule (6 to 8 hours), the inability to examine the large intestine, and only the acquisition of two-dimensional images. In addition, Park and Nam [2] from South Korea also proposed a wireless endoscope system based on analog circuits. Their main contribution is the introduction of the concept of two-way communication in the wireless endoscope system.

1 System Features

Table 1 is a comparison of the main performance indicators of the three systems introduced above. As shown in Table 1, compared with the other two wireless digestive endoscopy systems, this system has the following characteristics: (1) It uses a low-power CMOS image sensor with digital image output, and the image size can reach up to VGA size, which is about four times larger than the image of the small intestinal capsule endoscope developed by the Israelis; (2) It can observe the patient's digestive tract image in real time, and the image frame rate is 2 frames/second; (3) It uses the management and control of various energy sources to achieve full digestive tract examination; (4) It provides three-dimensional deep endoscopy image data; (5) It uses two-way data communication; (6) The compression rate and image size of the endoscopy image are controllable; (7) The working state of the circuit in the digital endoscopy capsule camera device that wirelessly transmits and receives in the body can be controlled externally to extend the life of the battery in the body; (8) The digital wireless endoscopy system provides three optional system working modes [3]: online working mode, offline working mode, and online and offline combined mode. The schematic diagram of the digital micro wireless endoscopy system is shown in Figure 1.

Table 1 Performance comparison of three wireless endoscope systems

Note: “-” indicates undisclosed

2 System Hardware Structure

As shown in Figure 2, the hardware structure of the entire system consists of three parts: (1) the in-vivo capsule part: including all circuits in the digital endoscope capsule camera device with wireless transceiver; (2) the in-vitro portable part: including all circuits in the portable wireless receiving and data transmission device; (3) the in-vitro workstation part, i.e., the computer control and processing device: including the computer and the in-vitro wireless transceiver and data storage circuit board. The circuit structures of these three parts are analyzed below.

2.1 Some hardware circuits in the body

The hardware circuit in the capsule is the core part of the entire wireless endoscope system. Its function is to complete the acquisition of endoscope color images and transmit the images to the outside of the body wirelessly. At the same time, it can receive control commands from outside the body and adjust the working state and working parameters of the hardware in the capsule according to the control commands. Its key technologies are: acquisition of high-definition two-dimensional and three-dimensional endoscopic images reflecting the condition of digestive tract lesions, efficient wireless transmission of acquired images, low-power design of circuits, and system energy management. The circuit in the capsule mainly includes the following three parts.

2.1.1 Image acquisition, processing and control part

This part includes CMOS image sensors with digital image output, image compression modules, MEMS micromotors, LEDs that emit white light and have two different infrared wavelengths (to collect three-dimensional depth image data), etc.

This part of the circuit not only determines the quality of the endoscopic image, but also its low power design is critical. Therefore, obtaining high-quality endoscopic images that meet medical clinical requirements and low power design are necessary for the design of the internal circuit. Based on this, the hardware part of the solution adopts the following design:

(1) The image acquisition front end uses a low-power CMOS image sensor with digital color image output. This image sensor does not have any image post-processing functions, but instead places these processes in an external computer, greatly reducing power consumption;

(2) In order to provide an image that accurately reflects the condition of the lesion, the system uses a spectral method to form a three-dimensional depth image, that is, using two different wavelength LEDs and a white light LED as the illumination source to obtain a three-dimensional depth endoscopic image;

(3) The digital image output format of the CMOS image sensor does not use RGB, but directly uses the original color Bayer format, so that a better compression ratio can be obtained in the lossless image compression module [5-6], thereby reducing the communication bandwidth and the total transmission energy of the wireless transceiver. Even if this method is not compressed, the maximum original signal data bit rate of the communication is 640×480×8×2=4915200 bits/second at a maximum rate of 2 frames/second, which is only 1/3 of the bit rate of the RGB format;

(4) The system adopts the digital image processing flow shown in Figure 3(b). There are only two modules in the capsule: compression and wireless modulation, which is three modules less than the general digital image processing flow shown in Figure 3(a), reducing the area and power consumption of the circuit in the capsule;

Figure 3 CMOS image sensor/output digital image processing flow chart

(5) Depending on the patient's condition, the CMOS image sensor can be controlled to output images of different sizes at different times, and the compression ratio and frame rate can be adjusted to reduce power consumption and communication bandwidth.

2.1.2 Wireless transmission part

This part includes channel coding, wireless transceiver, RF power amplifier and antenna. As a communication system, it has three main characteristics: (1) ultra-short distance communication, because the capsule inside the body and the receiver outside the body are separated by only a layer of human tissue (including muscle, fat and skin), and the maximum communication distance is tens of centimeters; (2) the attenuation of the communication channel is very large, because human tissue has a great absorption and reflection effect on radio waves (especially electromagnetic waves in the UHF band and above) [5]; (3) the communication is mainly the transmission of a large amount of image data from inside the body to outside the body, and the transmission from outside the body to inside the body is to send a few bytes of control commands according to clinical needs. Considering the volume, power consumption, antenna, circuit implementation complexity and system communication characteristics, the system adopts a half-duplex communication mode, and the transmission and reception share one antenna. As a communication system, the first two important parameters to be determined are the communication frequency and modulation mode. 2.4GHz in the ISM band is used as the communication frequency. In terms of wireless modulation mode, the system uses FSK modulation mode for transmission from inside the body to outside the body, and OOK mode for receiving control commands from outside the body inside the body.

In this scheme, a circuit structure of a wireless transceiver without a frequency synthesizer is proposed, as shown in Figure 4. This ensures low power consumption of the entire circuit at the high-level stage of circuit design.

The antenna part mainly solves the contradiction between the miniaturization and efficiency of the antenna, because the antenna must be placed inside the wireless endoscopy capsule (whose size is 11mm×27mm) and there must be enough space for other parts. The design of the system antenna must adopt the design method of micro-antenna to increase the effective length of the antenna.

2.1.3 Energy supply

This part includes the battery and energy management circuit [6]. It is one of the most critical parts of the entire internal hardware circuit, because the energy supply of the internal part is a necessary condition to ensure the realization of the whole digestive tract examination. In order to extend the battery life, the system mainly takes the following three measures:

(1) Low power consumption design for high-level and low-level circuits;

(2) A system dynamic energy management strategy that combines the physical characteristics of the battery itself, which greatly extends the battery life;

(3) “Communication-based” energy management strategy, which is an energy management strategy that adjusts the operation of each system module based on the system-level communication structure. It is much better than conventional energy management strategies in extending battery life.

2.2 Portable wireless receiving and data transmission device outside the body

The function of the portable wireless receiving and data transmission device outside the body is mainly to divide the endoscopic image data received by the antenna receiving array into two paths, one path is sent to the capsule positioning module to obtain the capsule positioning information, and the other path is sent to the connected wireless receiver, and then the positioning information and the image are stored in a portable storage or forwarded to a computer control and processing device. The key technologies involved are: (1) Based on radio positioning technology, the position of the capsule in the human body is located by the angle and strength of the signal received by the antenna receiving array; (2) Design technology of efficient antenna array; (3) Low-power circuit design technology; (4) ASIC design technology of wireless receiver with high sensitivity, low power consumption and high-speed FSK demodulation, etc.

2.3 Computer control and processing device

The computer control and processing device mainly includes a wireless transmission card, a computer, a high-definition monitor and related processing software. Its key technologies mainly include: (1) the design of high-speed wireless transceiver (OOK modulation, FSK demodulation); (2) image processing technology based on original Bayer color image data; (3) three-dimensional depth image reconstruction technology, etc.

The wireless endoscope system proposed in this paper is a new digital miniature wireless endoscope system solution based on the characteristics of Israeli small intestinal capsule endoscopes and South Korea's research results in this field. This solution not only improves the quality of image acquisition, but also provides real-time observation of patient digestive tract images, full digestive tract examination, two-dimensional and three-dimensional endoscopic image data acquisition and other functions. In addition, according to the different conditions of the patients, the system can provide three different system working modes (i.e., diagnostic methods). At present, the digital circuit module part of this system has been verified by FPGA.