Medical Electronics


Medical imaging gets boost from µModule regulators


Afshin Odabaee, product marketing manager, µModule power products, Linear Technology Corporation, talks about the change in X-ray imaging moving towards Direct Radiography


T Imaging technology innovations


he migration away from traditional X- ray imaging towards Direct Radiography (DR) is gaining


momentum as the initial ownership costs decrease and benefits become more apparent. The DR X-ray image is available within seconds after exposure of the patient and can be distributed immediately around the globe for consultation with medical specialists anywhere. In a digital format, patient images can be archived and retrieved quickly in small hard disk drives instead of large file closets. A popular DR method involves a flat panel detector plate, capturing the passed X-rays. The flat panel detector enables multiple images, showing different angles to be shot without ever having to move or touch the panel and without lens distortion with a 1:1 sensor to image size ratio. Newer flat panel X-ray detectors can wirelessly transmit the image to the control unit for viewing, archiving and distribution. No longer do the process chemicals associated with film have to be purchased, stored or discarded. Perhaps most importantly, two European studies indicate a 30-70 per cent reduction in the X-ray dosage required to achieve a DR image quality comparable to that of analog film. Some flat panel designs communicate the exposure rate to the X-ray source in real time guaranteeing a properly exposed image with the minimum radiation dosage. A lower X-ray dose improves the safety of the patient and the health care professional in the vicinity who may be subsequently hit by scattered X-ray particles. To create the image, many DR systems use a full frame flat panel detector constructed of CMOS sensors covered with by a scintillating layer. This layer converts the incident X-rays to a wavelength better


absorbed by silicon. CMOS sensors, often favoured as the manufacturing process, are compatible with the construction of mixed signal and logic architectures, promoting a more integrated solution. The trend towards DR is further incentivised by improvements in 200mm and 300mm silicon wafer manufacturing. Larger wafers enable fewer CMOS sensor modules to be combined to form an X-ray flat panel sensor conforming to the 1.5cm thick ISO- standard 35cm x 43cm (14 x 17 inch) X-ray film cassette size used in hospitals worldwide. It’s no surprise that the hardware design of the system plays a significant role with a direct influence on image quality, form factor, human safety and operating lifetime of these products. But does that include the power management components?


The tough battle against electronic noise In order for DR to realise all potential benefits, attention must be paid to the issues of electronic noise, heat and size. A high signal to noise ratio (SNR) must be maintained, while reducing the X-ray dosage applied to the patient is a key goal. Although the noise performance of the sensor itself gets much of the attention, noise injection from the power supply also deserves careful consideration. The power supply architecture has a


direct influence on SNR performance. Voltage ripple on the power supply rail fed to the image sensor and the A/D converter can inject noise into the image. X-ray CMOS sensor makers are touting 14-bit and even 16-bit A/D conversion, supporting a wide contrast range to generate highly detailed images. Complicating matters further, a regulated


Figure 2: LTM4613 meets EN55022 Class B (CISPR11 Group 1 Class B) requirements with more than 6dBµV/m margin (12VIN


to 5VOUT at 8A)


negative rail between -3.3V to -7V is commonly required in addition to a regulated positive voltage to operate the image sensor, A/D converter and/or instrumentation amplifiers. Still, the battery pack or AC/DC power supply may only provide a single unregulated positive voltage. Therefore, the intermediary DC/DC converter must have a low output ripple performance in the tens of millivolts, high operating efficiency and low self heating.


The battle against heat High operating temperatures degrade the SNR performance of the CMOS sensor and reduce its lifetime. Moreover, high operating temperatures pose a risk to patient safety. To maintain superior image resolution, the X-ray flat panel detector is placed in direct contact with the patient’s body. Human skin starts to suffer burns at temperatures as low as 40°C (100°F). Therefore, the exterior of any medical device that could potentially come in contact with skin must stay below this limit. Thus, high operating efficiency and the ability to spread the heat that is generated over a wide area is critical in multiple areas: sensor lifetime, image clarity and patient safety.


Figure 1: µModule power products are a complete DC/DC switching solution in a thermally enhanced LGA or BGA package, providing a convenient means to dissipate heat through the top & bottom of the package


www.cieonline.co.uk


Regulatory compliance As part of the regulatory requirements in the US and Europe, medical devices must be proven to be compliant with CISPR11, also known as EN55011. Since switching regulators radiate electromagnetic fields, the designer must gain a full understanding of the switching regulator’s impact on EMI compliance or select a power solution that is tested to meet radiated EMI limits by the manufacturer. Otherwise, expensive and time consuming product iterations could result in order to achieve compliance. The most stringent radiated EMI limits are assigned to medical equipment intended for use in office buildings, Group1 – Class B devices whose radiated limit is identical to EN55022 Class B (CISPR22 Class B) limits assigned to information technology equipment intended for use in office buildings and homes.


Solution: advanced DC/DC switching regulators To assist design engineers address the challenges of electronic noise, heat and size in medical applications, Linear Technology offers a broad selection of over 50 different 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 were carefully designed to operate with low output ripple in both negative (inverting) output voltage and positive output voltage circuit configurations. A sub-group of µModule power products, the EN55022 class B certified µModule regulators, present an ideal solution to overcome the EMI challenges found in medical applications. These switching regulators were certified by independent laboratories such as TUV to meet the industry’s EN55022 class B (equivalent to CISPR11 / EN55011 Group 1 - Class B) standard for radiated EMI at output currents up to 8A. The test results using their respective standard demo boards are publicly available online. An excerpt is shown in Figure 2. Selecting a known compliant fully integrated step-down solution such as a µModule regulator, reduces design time and risk associated with common switching regulators or controllers in meeting these requirements.


µModule power products are highly


efficient switching regulators offered in a surface mount LGA or BGA package constructed of thermally conductive plastic with a flat top. A single flat top covering a complete power management solution is conducive to heat sinking techniques to minimise the temperature rise at any particular point of the medical device’s exterior. As mentioned previously, maintaining a cool operating temperature improves patient safety, signal to noise ratio and equipment operating lifetime.


www.linear.com Linear Technology,


now part of Analog Devices Tel: 01628 477066


Components in Electronics May 2017 25


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