Improving Medical Imaging with Advanced Point-of-Load Regulators

Innovation in imaging technology

Traditional analog X-ray imaging systems use specialized light-sensitive film as the medium to convert the passing X-rays into a visible image. To accomplish this task, the film must go through a chemical development process that can take several minutes, delaying the start of patient care. Furthermore, after the development process is complete, the medical team may discover that the image needs to be retaken because the X-ray exposure was incorrect. Once the film is processed, it must be sent to the attending physician and then stored in the patient's medical file, which can take up a large storage cabinet in the hospital. In addition, the chemicals used in the development process have a limited lifespan and must be carefully stored and destroyed once their lifespan has expired. All of these challenges are eliminated with Direct Radiography (DR), a digital X-ray imaging technology that is gaining increasing adoption.

The move from conventional X-ray imaging to direct radiography is gaining momentum as initial cost of ownership decreases and the advantages become more apparent. With direct radiography, an X-ray image is available seconds after the patient is photographed and can be sent instantly to any medical specialist around the world for consultation. 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 filing cabinet. The popular direct radiography method uses a flat-panel detector plate to capture the X-rays as they pass by. The flat-panel detector does not need to be moved or manually manipulated, can display different camera angles to produce a variety of images, and has a 1:1 sensor-to-image size ratio. Newer flat-panel X-ray detectors can wirelessly send images to a control unit for review, archiving, and distribution. With flat-panel detectors, there is no need to purchase, store, or destroy the chemicals associated with processing film. Perhaps most importantly, two European studies have shown that the X-ray dose required to archive a DR image of film quality comparable to analog photographic recordings is 30% to 70% lower. Some flat panel designs transmit the exposure rate to the X-ray source in real time, ensuring correctly exposed images and extremely low radiation doses. Lower X-ray doses improve safety for patients and nearby healthcare professionals who may subsequently be exposed to scattered X-ray particles.

To produce images, many direct radiography systems use full-frame flat-panel detectors consisting of a CMOS sensor covered with a scintillator layer. This scintillator layer converts the wavelength of the incoming X-rays to wavelengths that are better absorbed by the silicon material. CMOS sensors are often favored due to manufacturing process reasons, and they are compatible with mixed-signal and logic architectures, thus facilitating more integrated solutions. Improvements in 200mm and 300mm silicon wafer manufacturing technology have further accelerated the trend toward direct radiography. Larger wafers allow fewer CMOS sensor modules to be combined together, resulting in X-ray flat-panel sensors that are consistent with the dimensions of the 35cm x 43cm (14-inch x 17-inch) 1.5cm thick ISO standard X-ray film cassettes used in hospitals around the world. Not surprisingly, the system's hardware design plays a major role in directly affecting the image quality, form factor, personnel safety, and operating life of these products. But does this important hardware design include power management components?

The tough battle against electronic noise

In order for direct radiography to realize its full potential, electronic noise, heat, and size issues must be addressed. A high signal-to-noise ratio (SNR) must be maintained, while reducing the X-ray dose to the patient is also a key goal. While the noise performance of the sensor itself has received the lion's share of attention, power supply injected noise also deserves careful consideration.

The power supply architecture has a direct impact on the signal-to-noise performance. 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 achieved 14-bit or even 16-bit A/D conversion, which can support a wide contrast range and produce very detailed images. To complicate matters, the image sensor, A/D converter and/or instrumentation amplifier often require a regulated -3.3V to -7V negative voltage rail in addition to a stable positive voltage for proper operation. 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 self-heating.

Many new X-ray imaging units, including sensor panels, are mobile for patient comfort and convenience. Rechargeable batteries with a nominal 12V are often the power source of choice for the sensor panel. The need for high efficiency in order to capture and transmit hundreds of images on a single charge has led to the use of switching regulators. Unfortunately, switch-mode regulators are a source of electromagnetic interference (EMI) radiation, which increases the noise level of the system. In addition, to help maintain a safe boundary between medical personnel and patients, some X-ray sensor panels have wireless data transmission capabilities. High EMI levels have the potential to distort the captured image and/or interfere with the wireless data transmission to the user terminal. Perhaps more troublesome, EMI radiation levels may exceed the values ​​allowed by government regulatory agencies, thereby preventing the medical product from entering the market, an issue discussed later in this article.

The need for high operating efficiency has a secondary purpose, which is to strive to maintain a high signal-to-noise ratio (SNR). Dark current inside CMOS sensors increases exponentially with increasing temperature. Dark current is formed by the movement of charge and exists before the X-ray exposure. According to one X-ray CMOS sensor manufacturer, dark current approximately doubles for every 8°C increase in temperature. Although post-processing can remove some dark current artifacts from the image, the higher operating temperature and the accumulated damage from repeated X-ray exposure accelerate the increase of dark current. Eventually, the dark current will overwhelm the charge deposited on the sensor by the incident X-ray particles, at which point the flat-panel detector must be replaced. In addition, because medical devices often come into contact with human tissue, if heat dissipation is not controlled, it may cause discomfort or burns to the patient in addition to shortening the operating life of the device.

The battle with heat

As mentioned previously, higher operating temperatures reduce the signal-to-noise performance of CMOS sensors and shorten the life of such sensors. In addition, higher operating temperatures pose a patient safety risk. To maintain excellent image resolution, X-ray flat panel detectors are in direct contact with the patient's body. Human skin begins to burn when the temperature reaches 40°C (100°F). Therefore, the exterior of any medical device that may come into contact with human skin must remain below this temperature limit. Therefore, high operating efficiency and the ability to dissipate the heat generated over a large area are critical to many aspects, such as sensor life, image clarity, and patient safety.

Maintain compact form factor

From surgical system accessories to handheld examination tools, the complexity of next-generation medical devices has increased without a corresponding increase in the space available to fit so many components to support more functionality. In the case of flat-panel X-ray detectors, existing hospital infrastructure has been configured with a fixed-size slot called a “grid slot” that was previously used to hold analog X-ray film cassettes. These film cassettes generally follow ISO4090 guidelines and can have external dimensions of 46cm x 38.6cm x 1.5cm, allowing an X-ray image size of 43cm x 35cm (14 inches x 17 inches). Power management solutions must be compact and efficient to meet such limited size requirements and minimize operating temperature rise.

Regulatory regulations

As part of regulatory requirements in the United States 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 select a power supply solution that has been tested to meet the manufacturer's EMI emission limits. Otherwise, a large number of time-consuming product iterations may be required to achieve compliance with the relevant standards. The most stringent radiated EMI limits are specified for medical devices intended for use in office buildings, and the emission limits for Group1 – Class B devices are equivalent to the EN55022 Class B (CISPR22 Class B) limits specified for information technology equipment for office buildings and homes.

Long product life

Proving the reliability of power solutions is essential for medical devices. For X-ray flat panel sensors, images must be acquired correctly the first time, or patients and medical staff are sadly exposed to radiation again. At the very least, delayed diagnosis leads to delayed treatment, which is unacceptable by modern medical standards.

Another factor to consider is: How long will the supply of the selected electronic components last? After going through the lengthy regulatory approval process and obtaining certifications from agencies such as CE, UL, IEC and FDA, every medical electronic device should be able to be manufactured for a long time - up to 15 years or more. This length of time is much longer than the consumer product cycle, which is the main market for many power management semiconductor manufacturers. Requalifying products simply because components are obsolete is a heavy burden on both engineering resources and company profits.

Solution: Advanced DC/DC Switching Regulators

To help design engineers address the electronic noise, thermal and size challenges in medical applications, Linear Technology offers a wide range of choices with over 50 different micromodule (µModule®) power products. Each of these products is a highly efficient, fully integrated DC/DC switching power management solution in a compact surface mount package (Figure 1). These switch mode regulators are carefully designed to operate with low output ripple in both negative and positive output voltage circuit configurations, as shown in Figures 2 and 3. A subcategory of µModule power products are EN55022 Class B certified µModule regulators, which are an ideal solution to overcome the EMI challenges found in medical applications. These switching regulators are certified by independent laboratories such as TUV to meet the industry standard EN55022 Class B (equivalent to CISPR11 / EN55011 Group 1 – Class B) requirements for radiated EMI at output currents up to 8A. The results of the tests using their respective standard demonstration circuit boards are publicly available online. Some of the test results are shown in Figure 4. Choosing a compatible and fully integrated step-down solution, such as a µModule 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 factors affect the ability of a product to function properly the first time and meet stringent government regulations. As for the impact of these two factors on the operation of X-ray flat panels, if the product design has good control of output ripple and EMI radiation, the product can provide a high signal-to-noise ratio, thereby providing high-quality, high-resolution images, getting the image right the first time to avoid treatment delays and unnecessary repeated exposure to radiation, as well as reliable wireless communication and expedited EMI compliance testing. For these reasons, Linear Technology has done a lot of work to ensure that these devices are certified by independent test laboratories such as TUV and test results are publicly available online. After overcoming noise and EMI issues, the appropriate power management solution needs to address efficiency, reliability and thermal issues.

Micromodule power products are very efficient switching regulators that are packaged in surface-mount LGA or BGA packages made of thermally conductive plastic with a flat top. A flat top of the package covers the entire power management solution, facilitating heat dissipation to minimize temperature rise at any point outside the medical device (Figure 5). As mentioned earlier, keeping operating temperatures low improves patient safety, signal-to-noise ratio, and the operating life of the device. With the largest micromodule power products measuring 15mm x 15mm x 5mm and the smallest at 6.25mm x 6.25mm x 2.3mm, using micromodule power products can help free up space for more important features, such as increasing the size of the battery to operate longer between charges.

The µModule regulators undergo extensive device-level and board-level testing to ensure reliable operation. To date, the product family has accumulated more than 23.5 million device-hours of power cycles and 3 million device-hours of accelerated life testing without failure. Reliability is further supported by a full set of design, application, and manufacturing guidelines that ensure the performance of these µModules meets product expectations. A complete report listing all reliability tests conducted and the test results is available at www.linear.com.cn/umodule. Linear Technology has been steadfast in its "non-stop production policy" for products with long production life to support medical devices, and µModule power products will continue to uphold this policy, thereby supporting medical devices with long production life.

Figure 1: µModule power products are complete DC/DC switching solutions in thermally enhanced LGA or BGA packages, providing a convenient path for heat dissipation through the top and bottom of the package.

Thermally Enhanced BGA or LGA Package: Thermally Enhanced BGA or LGA Package

Magnetics: Magnetic materials

Compensation Circuitry: Compensation Circuit

Diodes: diodes

Capacitors & Resistors: Capacitors and Resistors

Power Switches & DC/DC Controller: Power Switches and DC/DC Controller

Figures 2a and 2b: LTM8023 schematic, 12VIN to -7VOUT/1A, operates with less than 30mVpp output ripple, minimizing noise injection into CMOS sensor and signal conditioning components, requires a negative voltage source

See Figure 2a to get the component values

Full bandwidth

Figure 3: With only 26mVpp output ripple (12VIN to 5VOUT/5A), the LTM4613 minimizes noise injection into the CMOS sensor and signal conditioning components, providing high-quality images.

Figure 4: The LTM4613 meets EN55022 Class B (equivalent to CISPR11 Group 1 Class B) requirements (12VIN to 5VOUT/8A) with a margin of more than 6dBuV/m.

Figure 5: Without a heat sink, the maximum case temperature of the LTM4613 is 41°C (12VIN to 5VOUT/5A) in natural air convection. If thermally connected to a metal case (no heat sink at extra cost), the maximum temperature drops further to less than 40°C.

AMBIENT TEMPERATURE: ambient temperature

NO FORCED AIRFLOW: No forced airflow

NO HEAT SINK: No heat sink

in conclusion

While this article focuses on the design challenges associated with digital X-ray flat panel detectors, the challenges described are not unique to this product. From surgical system accessories and endoscopes to portable imaging and monitoring systems, medical professionals are constantly seeking the most effective, reliable and smallest tools available. Devices that continue to achieve breakthroughs in these areas have enabled advances in medical practice, minimizing patient discomfort, scarring and recovery time while improving personnel safety. The electronic components in such products enable doctors to perform treatments more accurately, gain a more complete understanding of the condition, and have greater control, even extending the careers of well-trained surgeons. Add to that the commercial enterprise's demand for limited engineering resources, time to market, proven product reliability and matching long production life, and what power supply solutions can meet these business demands? Linear Technology's micromodule power product line has been carefully designed and proven to meet these daunting demands.