Innovation in imaging technology
The traditional analog X-ray imaging system uses a special photosensitive film as a medium to convert the passing X-rays into visible images. In order to accomplish this task, the film must undergo a chemical development process, which may take several minutes, thus delaying the start of treatment for the patient. In addition, after the development process is completed, the medical team may find that the image needs to be taken again due to incorrect X-ray exposure. After the film processing is completed, someone must be sent to the attending doctor and then stored in the patient's medical file. In the hospital, the patient's medical file may occupy a large number of storage cabinets. In addition, the chemical used in the development process has a limited service life and must be carefully stored, and once it exceeds the life span, it must be destroyed. If you use Direct Radiography (DR), all these challenges are gone. Direct X-ray photography is a digital X-ray imaging technology that is being increasingly adopted.
As the initial cost of ownership has decreased and the advantages have become increasingly apparent, the migration of traditional X-ray imaging to direct X-ray photography has continued to increase. When using direct X-ray photography, after taking a picture of the patient for a few seconds, the X-ray image can be obtained, and the image can be immediately sent to all parts of the world to consult a medical expert in any place. The patient's X-ray image is in digital form and can be archived and retrieved on a small hard drive without the need for a large file cabinet. The popular direct X-ray photography method uses a flat panel detector plate to acquire passing X-rays. The flat panel detector can display different photographing angles without moving or manually moving to take a variety of images, and the sensor-image size ratio is 1: 1. Newer flat panel X-ray detectors can send images to the control unit wirelessly for viewing, archiving and distribution. With a flat panel detector, there is no need to purchase, store, or destroy chemicals related to the processing of film. Most importantly, two studies in Europe have shown that the X-ray dose required to archive a DR image of comparable quality to analog photographic recording film will be reduced by 30% to 70%. Some flat panel designs can transmit the exposure rate to the X-ray source in real time, thereby ensuring a correctly exposed image and extremely low radiation dose. Lower X-ray doses can improve the safety of patients and nearby healthcare professionals, who may subsequently encounter scattered X-ray particles.
To produce images, many direct X-ray imaging systems use a full-frame flat panel detector, which consists of a CMOS sensor covered with a scintillation layer. This scintillation layer converts the wavelength of incident X-rays to a wavelength that the silicon material can better absorb. CMOS sensors are often favored because of the manufacturing process. These sensors are compatible with mixed-signal and logic architectures, thus helping to form a more integrated solution. Improvements in 200mm and 300mm silicon wafer manufacturing technology have further promoted the trend towards direct X-ray photography. Larger wafers allow fewer CMOS sensor modules to be combined, making the resulting X-ray flat panel sensor the same size as a 35cmx43cm (14 "x17") 1.5cm thick ISO standard X-ray film cassette, while the world Hospitals everywhere use this type of film cassette. Not surprisingly, the hardware design of the system has a direct impact on the image quality, form factor, personnel safety, and working life of such products, and plays an important role. However, does this important hardware design include power management components?
Hard struggle with electronic noise
In order for direct X-ray photography to realize all potential advantages, attention must be paid to electronic noise, heat, and size issues. A high signal-to-noise ratio (SNR) must be maintained, while reducing the X-ray dose applied to the patient is also a key goal. Although the noise performance of the sensor itself has received great attention, the noise injected by the power supply is also worth careful consideration.
Power supply architecture has a direct impact on signal-to-noise performance. The voltage ripple on the power rail is fed to the image sensor, and the A / D converter may inject noise into the image. X-ray CMOS sensor manufacturers claim that they have achieved 14-bit or even 16-bit A / D conversion, which can support a wide range of contrast, which in turn produces very detailed images. To complicate matters further, image sensors, A / D converters and instrumentation amplifiers need to work properly. In addition to a stable positive voltage, a regulated negative voltage rail of -3.3V to -7V is often required. In addition, the battery pack or AC / DC power supply may only provide an unregulated positive voltage. Therefore, the intermediate DC / DC converter must have low output ripple performance (several tens of mV), high operating efficiency, and low spontaneous heat.
For patient comfort and convenience, many new X-ray imaging units (including sensor tablets) are mobile. The power supply of the sensor panel often selects a rechargeable battery with a nominal voltage of 12V. In order to shoot and transfer hundreds of images on one charge, higher work efficiency is required, which prompts people to use switching regulators. Unfortunately, the switch-mode regulator is a source of electromagnetic interference (EMI) radiation, which increases the noise level of the system. In addition, to help medical staff and patients maintain a safe boundary, some X-ray sensor panels have wireless data transmission capabilities. A higher EMI level may cause the captured image to be distorted and interfere with the wireless data transmission to the user terminal. Perhaps more troublesome is that the level of EMI radiation may exceed the value allowed by government regulatory agencies, thus preventing medical products from entering the market.
There is a second purpose that requires higher work efficiency, that is, efforts to maintain a high signal-to-noise ratio (SNR). The dark current inside the CMOS sensor increases exponentially with increasing temperature. Dark current is formed by the movement of electric charges and exists before X-ray exposure. According to an X-ray CMOS sensor manufacturer, each time the temperature rises by 8 ° C, the dark current approximately doubles. Although post-processing can remove some dark current artifacts from the image, the higher operating temperature and the damage accumulated by repeated X-ray exposure accelerates the dark current. Eventually, the dark current will overwhelm the charges deposited by the incident X-ray particles on the sensor, and the flat panel detector must be replaced. In addition, because medical equipment often contacts human tissues, if the heat dissipation is not controlled, in addition to shortening the working life of the equipment, it may also cause patient discomfort or burns.
The fight against heat
As mentioned before, the higher operating temperature reduces the signal-to-noise ratio performance of CMOS sensors and shortens the life of such sensors. In addition, the higher operating temperature also poses a patient's safety risk. In order to maintain excellent image resolution, the X-ray flat panel detector will be in direct contact with the patient's body. When the temperature reaches 40 ° C (100 ° F), the human skin begins to get burned. Therefore, the exterior of any medical device that may come into contact with human skin must be kept below this temperature limit. Therefore, high work efficiency and the ability to dissipate the heat generated in a large area are critical to many aspects, such as sensor life, image clarity, and patient safety.
Maintain a compact form factor
From surgical system accessories to hand-held inspection tools, the complexity of next-generation medical devices is getting higher and higher, and the available space for incorporating so many components to support more functions has not increased accordingly. As far as the flat panel X-ray detector is concerned, the existing hospital infrastructure has been equipped with a fixed-size slot called the “wire grid slotâ€, which was originally used to place analog X-ray film cassettes of. These film cassettes generally follow the ISO4090 guidelines and can have an external size of 46cmx38.6cmx1.5cm. The allowed X-ray image size is 43cmx35cm (14 inches x 17 inches). Power management solutions must be compact and efficient to meet such limited size requirements and minimize the rise in operating temperature.
Regulatory regulations
As part of the regulatory requirements in the US and Europe, medical devices must demonstrate compliance with CISPR11 (also known as EN55011) regulations. Because switching regulators radiate electromagnetic fields, designers must fully understand the impact of switching regulators on EMI compatibility, or must choose a power supply solution that has been tested to meet the manufacturer's EMI emission limits. Otherwise, in order to achieve compliance with relevant standards, it may be necessary to carry out a large amount of time-consuming product iterative design work. The medical equipment intended to be used in office buildings has the most stringent radiated EMI limits. The emission limits of Group1–Class B equipment are equivalent to EN55022 Class B (CISPR22 Class B) for information technology equipment used in office buildings and homes. Limit.
Long product life
For medical equipment, it is necessary to prove the reliability of the power solution. For the X-ray flat panel sensor, the image must be obtained correctly once, otherwise the patient and the medical staff will regret to face the radiation again. At the very least, treatment delays can also be caused by delayed diagnosis, which is unacceptable according to modern medical standards.
Another factor to consider is: How long can the delivery time of selected electronic components last? After undergoing the long regulatory approval process of CE, UL, IEC and FDA and obtaining certification, each medical electronic device should be able to be manufactured for a long time-more than 15 years. This length of time is much longer than the cycle of consumer products, and the consumer product market is the main market for many power management semiconductor manufacturers. Re-identification of products only due to component elimination is a heavy burden on engineering resources and company revenue.
Solution: Advanced DC / DC switching regulator
To help design engineers meet the electronic noise, heat and size challenges in medical applications, Linear Technology offers more than 50 different miniature module (? Module) power products, providing customers with a wide range of options. Each of these products is a high-efficiency, fully integrated DC / DC switching power management solution in a compact surface-mount package (Figure 1). These switch-mode regulators have been carefully designed to operate with low output ripple in negative and positive output voltage circuit configurations, as shown in Figures 2 and 3. A sub-category of micro-module power products is the micro-module voltage regulator that has passed EN55022 Class B certification, which is an ideal solution to overcome the EMI challenges found in medical applications. These switching regulators have been certified by independent laboratories such as TUV to meet the radiated EMI requirements of industry standard EN55022 Class B (equivalent to CISPR11 / EN55011 Group 1–Class B) when the output current is up to 8A. The results of testing with the respective standard demonstration circuit boards have been publicly available online. Part of the test situation is shown in Figure 4. Choosing a compatible and fully integrated step-down solution, such as a micro-module regulator, can save design time and reduce the risks associated with common switching regulators or controllers when meeting these requirements.
The risks associated with high output ripple and radiated EMI should not be underestimated. Both of these factors affect the product's ability to function properly and meet strict government regulations for the first time. As for the impact of these two factors on the work of the X-ray tablet, if the product design controls the output ripple and EMI radiation well, the product can provide a high signal-to-noise ratio, which can provide high-quality, high-resolution images. The correct image can be obtained at one time to avoid treatment delay and unnecessary repeated exposure to radiation, but also provide reliable wireless communication and accelerate EMI compatibility testing. For these reasons, Linear Technology has done a lot of work to ensure that these devices pass the certification of independent test laboratories such as TUV and publicly provide test results online. After overcoming noise and EMI problems, a proper power management solution needs to address efficiency, reliability, and heat dissipation issues.
The micro-module power supply products are very efficient switching regulators, which are surface-mounted LGA or BGA packages made of thermally conductive plastic, and the top of the package is flat. The top of a flat package covers the entire power management solution, which is conducive to heat dissipation measures to minimize the temperature rise at any point outside the medical device (Figure 5). As mentioned before, keeping the operating temperature low can improve patient safety, signal-to-noise ratio, and equipment life. The size of the largest micro-module power supply product is 15mmx15mmx5mm, the smallest is 6.25mmx6.25mmx2.3mm, so the use of micro-module power supply products helps to free up space for more important functions, such as increasing the battery size to charge twice Work longer between.
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