Traditional analog X-ray imaging systems use special photographic film as the medium to convert passed X-rays into a visible image. In order to accomplish that task, the film must undergo a chemical development process that can take several minutes, delaying the start of patient treatment. Moreover after the development process is complete, the medical team could discover that the image needs to be re-taken due to improper X-ray exposure. After processing, the images must be physically handed to the attending physician then stored with the patient’s records, which can accumulate into vast storage closets at medical offices. Additionally, the chemicals used in the development process have a finite useful life and must be carefully stored and disposed of once exhausted. All these challenges disappear with Direct Radiography (DR), a growing form of digital X-ray imaging.
The migration away from traditional X-ray imaging towards Direct Radiography 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 percent-70 percent 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 Direct Radiography 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 favored as the manufacturing process, are compatible with the construction of mixed signal and logic architectures, promoting a more integrated solution. The trend towards Direct Radiography is further incentivized 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 Direct Radiography to realize 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 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.
For patient comfort and convenience, many new X-ray imaging units, including the sensor panels are mobile. A 12V nominal re-chargeable battery is often selected as the power source for the sensor panel. In order to capture and transmit hundreds of images on a single charge, high operating efficiency is required, favoring the use of switching regulators. Unfortunately, switch mode regulators present a source of radiated electromagnetic inference (EMI), increasing the noise level in the system. Additionally, to help medical staff maintain a safe perimeter from the patient, some X-ray sensor panels feature wireless data transmission. High levels of EMI could distort the captured image and/or disrupt wireless data transfer to the user terminal. Perhaps even more troubling, EMI emissions could reach levels above those allowed by government regulatory agencies, preventing the medical product from entering the market, a topic discussed later in this article.
The requirement for high operating efficiency serves a second purpose in the effort to maintain high signal to noise ratio (SNR). The dark current within a CMOS sensor increases exponentially with temperature. Dark current is the movement of charge, which exists prior to X-ray exposure. According to one X-ray CMOS sensor manufacturer, dark current roughly doubles for every 8°C temperature rise. Although post-processing can remove some dark current artifacts from the image, higher operating temperatures and accumulating damage from repeated X-ray exposure hastens the increase in dark current. Eventually, the dark current will overwhelm the charge deposited on the sensor from incident X-ray particles at which point the flat panel detector must be replaced. Furthermore, since medical devices are often in contact with human tissue, unmanaged heat dissipation can lead to patient discomfort or burns in addition to reducing equipment operating lifetime.