Sensor enables medical CT scanners to render sharper images

The AS5900 detector interface chip for CT X-ray scanners sets new performance benchmarks in crucial parameters:

• Ultra-low noise: As little as 3,200 electrons for a full-scale range at an input current of 1μA and 30pF input capacitance. The circuit’s very high signal-to-noise ratio allows the flexible use of any pixel size, enabling it to digitise low-current outputs from the latest high-sensitivity photodiodes. As a result, the CT scanner can render more detailed images. Furthermore, low noise allows CT equipment to maintain high spatial resolution while reducing the X-ray dose to which the patient is exposed.
• Very low power dissipation: Power dissipation as low as 1mW per channel reduces the requirement for cooling capability in the CT scanner, lowering the overall system cost.
• High resolution: The 26-bit sigma delta ADC integrated in the AS5900 produces a richer grayscale palette for sharper and more detailed images.
• High linearity: ±250ppm of Read inputs (±1ppm of full-scale range) to give improved contrast, accurately rendering slight variations in image brightness.

The new ams detector chip provides detailed motion-free medical images while operating at very high acquisition rates: the adjustable integration time can be set as low as 50μs.

The configurable design of the AS5900 gives it the flexibility required for integration into different types of end equipment that use a multiple-slice photodiode array. The chip is suitable for use not only in medical equipment, but also in industrial and security CT scanners. Equipment designers can configure or adjust:

• Gain setting, to configure the input-current full-scale range and the noise floor
• Power modes, to adapt power consumption to the detector requirements
• Clock frequency, to balance the resolution with the required integration time.

The AS5900 integrates all the functions of a current-input ADC to minimise the requirement for external components. It includes an internal reference voltage and temperature sensor. This reduces the bill of materials, thus lowering system cost. The AS5900 also provides a standard LVDS interface to a host FPGA, running at a data rate of up to 80Mb/s for optimised power consumption.

“The ultra-low noise and high resolution of the AS5900 will enable CT scanner OEMs to produce better images at higher full scale ranges, allowing for flexible pixel sizing. Medical practitioners will see a marked difference in the quality and clarity of images from the next generation of CT scanners based on this new, high-performance current-to-digital converter. Its low-noise operation at high speed and low power consumption produces CT images free of motion artefacts even at the highest rotational speeds while exposing the patient to a lower dose of radiation”, said Josef Pertl, Product Marketing Manager at ams.