Traditional analog X-ray imaging systems use specialized photographic 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 that can take several minutes, thus delaying the time to begin treatment of the patient. In addition, after the development process is complete, the medical team may find that the image needs to be re-ingested due to incorrect X-ray exposure. After the film processing is completed, it 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 chemicals used in the development process have a limited life span and must be stored carefully and must be destroyed once they have reached the end of their life. If you use Direct Radiography (DR), all of these challenges are gone. Direct X-ray photography is a digital X-ray imaging technology that is gaining more and more adoption.

As the initial cost of ownership declines and its advantages become more apparent, the momentum of traditional X-ray imaging to migrate to direct X-ray photography continues to increase. With direct X-ray photography, a few seconds after taking a picture of the patient, an X-ray image is available, and the image can be sent to the world immediately to consult a medical professional at any location. The patient's X-ray image is digital and can be archived and retrieved on a small hard drive without the need for a large file cabinet. The popular direct X-ray method uses a flat panel detector to capture the 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 wirelessly send images to the control unit for viewing, archiving and distribution. With a flat panel detector, you don't have to buy, store, or destroy chemicals related to film processing. Perhaps most importantly, two studies in Europe have shown that the X-ray dose required to archive a DR image of comparable quality to a simulated photographic film will be reduced by 30% to 70%. Some flat panels are designed to deliver illumination in real time to the X-ray source, ensuring a properly exposed image and extremely low radiation dose. Lower X-ray doses can improve the safety of patients and nearby health care professionals who may subsequently encounter scattered X-ray particles.

To produce images, many direct X-ray systems use a full-frame flat panel detector consisting of a CMOS sensor that covers a scintillation layer. This scintillation layer converts the wavelength of the incident X-ray into a wavelength at which the silicon material can better absorb. CMOS sensors are often favored for manufacturing processes that are compatible with mixed-signal and logic architectures, helping to create more integrated solutions. Improvements in manufacturing technology for 200mm and 300mm silicon wafers have further contributed to the shift to direct X-ray photography. Larger wafers allow fewer CMOS sensor modules to be bonded together, resulting in an X-ray flat panel sensor that is the same size as a 35cm x 43cm (14" x 17") 1.5cm thick ISO standard X-ray film. This type of film is used by hospitals around the world. 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 achieve all potential advantages, attention must be paid to electronic noise, heat and size issues. It is also a key goal to maintain a high signal-to-noise ratio (SNR) while reducing the X-ray dose applied to the patient. Although the noise performance of the sensor itself has received great attention, the noise injected by the power supply deserves careful consideration.

The power 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 can inject noise into the image. X-ray CMOS sensor manufacturers claim to have implemented 14-bit or even 16-bit A/D conversion to support a wide range of contrast levels, resulting in very detailed images. To complicate matters, image sensors, A/D converters, and/or instrumentation amplifiers must function properly. In addition to requiring a stable positive voltage, a regulated -3.3V to -7V negative rail 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 (tens of mV), high operating efficiency, and low spontaneous heat.

Many new X-ray imaging units, including sensor plates, are mobile for patient comfort and convenience. The power supply for the sensor panel often selects a rechargeable battery with a nominal voltage of 12V. In order to charge and capture hundreds of images at a time, high work efficiency is required, which prompts people to use switching regulators. Unfortunately, the switch mode regulator is an electromagnetic interference (EMI) source that increases the noise level of the system. In addition, some X-ray sensor panels have wireless data transfer capabilities to help maintain a safe boundary between medical staff and patients. Higher EMI levels may cause distortion of the captured image and/or interfere with wireless data transfer to the user terminal. Perhaps more troublesome is that EMI radiation levels may exceed the values ​​allowed by government regulators, making medical products inaccessible to the market, which is discussed later in this article.

A second goal is to require higher efficiency, that is, to strive to maintain a high signal-to-noise ratio (SNR). The dark current inside the CMOS sensor increases exponentially with increasing temperature. The dark current is formed by the movement of the charge and is present before the X-ray exposure. According to an X-ray CMOS sensor manufacturer, the dark current is roughly doubled for every 8 °C rise in temperature. Although post processing can remove some dark current artifacts from the image, the higher operating temperatures and the cumulative damage accumulated by repeated X-ray exposures increase the dark current. Eventually, the dark current will drown the charge deposited by the incident X-ray particles on the sensor, and the flat panel detector will have to be replaced. In addition, because medical devices often come into contact with human tissue, if the heat is not controlled, in addition to shortening the working life of the device, it may cause discomfort or burns to the patient.

Fight against heat

As mentioned earlier, higher operating temperatures reduce the signal-to-noise performance of CMOS sensors and shorten the life of such sensors. In addition, higher operating temperatures also pose a safety risk for the patient. To maintain excellent image resolution, the X-ray flat panel detector is in direct contact with the patient's body. When the temperature reaches 40 ° C (100 ° F), the human skin begins to burn. Therefore, the outside of any medical device that is likely to come into contact with human skin must remain below this temperature limit. Therefore, high work efficiency and the ability to dissipate heat generated over 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 increasing, and the available space to load so many components to support more functions has not increased. In the case of flat-panel X-ray detectors, the existing hospital infrastructure has been configured with a fixed-size slot called the “Grid Socket Slot”, which was originally used to place analog X-ray film cymbals. of. These film concealers generally follow the ISO4090 guidelines and can have an external dimension of 46cm x 38.6cm x 1.5cm with an allowable X-ray image size of 43cm x 35cm (14" x 17"). Power management solutions must be compact and efficient to meet such limited size requirements and minimize operating temperature increases.

Regulatory regulations

As part of US and European regulatory requirements, medical devices must demonstrate compliance with CISPR11 (also known as EN55011) regulations. Because switching regulators radiate electromagnetic fields, designers must fully understand the effects of switching regulators on EMI compatibility, or must choose a power solution that has been tested to meet manufacturers' EMI emissions limits. Otherwise, in order to achieve compliance with relevant standards, it may be necessary to perform a large amount of time-consuming product iterative design work. The most stringent radiated EMI limits are specified for medical devices intended for use in office buildings. The radiation limits for Group1 – Class B equipment are equivalent to EN55022 Class B (CISPR22 Class B) for office buildings and household information technology equipment. Limit.

Long product life

For medical devices, it is necessary to prove the reliability of the power solution. For X-ray flat panel sensors, the image must be obtained correctly at one time, otherwise the patient and the medical staff will unfortunately face the radiation again. At the very least, treatment delays can be caused by delays in diagnosis, which is unacceptable according to modern medical standards.

Another factor to consider is how long will the delivery time of selected electronic components last? After undergoing a lengthy regulatory approval process and certification by CE, UL, IEC, and FDA, each medical electronic device should be manufactured for a long period of time - more than 15 years. This length of time is much longer than the period of consumer products, and the consumer product market is the main market for many power management semiconductor manufacturers. Re-certification of products only due to component elimination is a heavy burden for engineering resources and company revenue.

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