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Medical imaging drives ADI’s data conversion evolution

Responding to a burgeoning demand for faster, higher performance, yet more affordable CT scanning capabilities, Analog Devices Inc. (ADI) has introduced a new current-to-digital converter chip that will enable CT systems to provide clearer, more detailed X-ray images of the human body for medical analysis and diagnosis.

Explains Jan-Hein Broeders, European healthcare business development and applications engineer for ADI: “The key to improving CT scanner performance is to enable the capture of moving images in real-time, with a high degree of accuracy and detail, and to minimising the amount of time taken to do this.” He adds: “In the past, single slice machines could only scan a small part of the tissue under examination. To scan a complete heart muscle or a lung involved making a lot of rotations and meant exposing patients to more X-rays. More recently, multislice machines have evolved that only need to make one rotation around the patient to scan a much wider area of the body. In addition, if the images can be captured quickly enough, a sharp image of a moving body part, such as a pumping heart, can be obtained.”

The ADAS1128 integrates a 24bit resolution ADC (analog-to-digital converter) with 128 simultaneously sampled data converter channels. With higher slice count CT systems requiring an increase in the number of data acquisition channels necessary to process images, the ADAS1128's 128 data conversion channels provide 'four times more channels than any other integrated converter solution available on the market today', according to ADI. Speed has also been increased, with this part providing selectable sample rates from 6kS/s (kilosamples per second) up to 20kS/s. The 1cm2 single-chip solution also has an on-chip temperature sensor and reference buffer.

Broeders notes that high integration is important because of the number of these devices needed in a typical high end CT system: “One slice is typically built from 1000 channels. Hence, for a 256 slice machine, we are talking about 250,000 channels, or 250,000 photodiode currents. The problem in the past has been how to build a machine with such high slice counts – if you built it using discretes, mechanical conditions limited the number of channels that it was feasible to integrate.” A 64 slice system requires 64,000 channels, necessitating the integration of more than 600 of these chips. However, the most advanced CT scanners nowadays have 250,000 channels or more, demanding some 2000 such chips per system! Broeders predicts that the number of slices will only increase in the future.

The ADAS1128 is said to consume less than half the power of other solutions (4.5 mW/channel versus 10 mW/channel at full speed). Power is important because with more power typically generating more heat, a key design challenge has been to remove heat from the system. The photodetector array in a CT scanner is very sensitive to temperature changes, hence the need for as low a power solution as possible.

According to Broeders, there has been a reduction in spending on medical imaging systems in the US that has had an impact on the number of CT scanners presently sold (just several thousand CT scanners per year). However, with healthcare facilities generally face a growing need for better quality, faster, and more affordable diagnostic imaging equipment, he believes that ADI is well placed to help: “The CT machines bought nowadays are typically high-end systems – 64 slice counts and upwards. Our new chips enable CT systems that can be used in emergency situations, where a high sampling rate allows scans to be taken very quickly. This results in less patient preparation time needed and more people being scanned.” As a result, he believes the CT market has strong growth potential.

ADI foresees strong growth generally for the medical imaging and monitoring market, with consumer medical monitoring equipment likely to experience considerable expansion. Trends driving this, notes Broeders, include more patients being encouraged to recover at home, as well as the elderly remaining in their homes for longer, increasing demand for home monitoring equipment. The medical sector accounted for approximately 5% of Analog Devices' total 2008 revenues of $2.6billion, with A/D converters and amplifiers providing a strong source of these sales. The company claims that it already has two thirds of the high end CT scanner market. “By 2013,” predicts Broeders, “we'll have more of a foothold in the home healthcare market,” proving that there is indeed a silver lining to every cloud.

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