Innovative CT Rejuvenates a Mature Technology

Computed tomography (CT) scanning has contributed to rapid and accurate medical diagnosis for almost three decades. In that time, basic CT scanner design has changed little. Most scanners in use today weigh more than 2500 lb. They often occupy a dedicated, climate-controlled room or suite with specially shielded walls, and they require high-power electrical supplies.

Determined to make CT technology more universally available, an Analogic Corp. design team, led by CEO Bernard Gordon, set out to develop a low-cost scanner that could be moved easily and that would be unencumbered by traditional environmental and power requirements. The resulting lightweight CT scanner, developed by Analogic and marketed as the Tomoscan by Philips Medical Systems, is the first of a new generation of CT machines.

By employing advanced structural materials and ensuring that every component and subsystem met strict cost, size, and weight criteria, the design team developed a system that weighs less than 1500 lb. and costs less than most conventional systems. The lightweight scanner can be moved by one person and can pass through a standard 3-ft-wide doorway. With an operational footprint of less than 200 sq. ft. and minimal X-ray scatter, the system can be used in small locations such as operating rooms and intensive care units. The gantry can translate 7 in. forward or back from center (14 in. total), while the patient remains stationary.

Moreover, while most scanners will fail if the ambient temperature rises above 80°F, this machine will operate in temperatures between 58° and 104°F. This system, designed to operate on less than 1500 W, can be plugged into any standard outlet or portable generator. It is impervious to voltage fluctuations and, in the event of a complete power disruption, can complete a patient study before performing a controlled shutdown.

The design team determined that to achieve this seemingly impossible power specification it was necessary to develop a very-low-power x-ray system. Basic physics provided the simple solution—move the x-ray source closer to the body. By reducing the x-ray geometry and incorporating a more efficient detector design the innovative system requires only one-third the x-ray power (120 mAs) to create images comparable to those of conventional systems. This high mAs efficiency results in less heat being generated in the tube. The compact geometry also makes it possible to employ a smaller, lighter gantry ring, which in turn requires smaller, lighter motors to drive it, further reducing the total system weight and power requirements. Yet, at 5 to 6 kW, the electrical demands of the new x-ray design still exceeded the power available at ordinary wall outlets.

According to Analogic's Gordon, "Once again we went back to the fundamentals. We determined the number of x-ray photons received at our high-efficiency detectors. Then we calculated the number of joules of energy needed to power the x-ray tube. We concluded that more than enough power to run several patient studies could be stored in a relatively compact bank of commercially available batteries. As simple as the solution may sound, it still required designing a highly regulated high-voltage power supply that is accurate to a few hundreths of one percent."

The highly efficient, solid state x-ray detectors collect about 99% of the available photons. Even so, with a full-scale current of only about 100 nA, shielding the detectors was a prime design consideration. Because they are also temperature sensitive, they are mounted onto an isothermal shield that is monitored for variations to 1/50°C. The scanner automatically compensates for signal deviations caused by temperature changes.

All CT systems—where the least significant bit in a data word corresponds to approximately 500 electrons—require low noise, highly stable data acquisition systems (DASs). Analogic, having advanced DAS design for over two decades and supplying DASs for about 85% of the CTs in service today, has gained significant expertise in this area. However, the design team discovered that employing an older rather than newer technique used less power. Therefore the lightweight system incorporates a multiple integrating filter whose output is multiplexed into a single 20-bit A/D converter.

According to Gordon, "The DAS/detector combination is one of the most important and most expensive links in the CT scanning chain. It is a sophisticated, highly stable, wide-dynamic range subsystem that must be capable of high speed and reliable operations over prolonged periods, while subjected to a broad range of environmental conditions.

"While the functions of the DAS/detector subsystem may be considered mundane or routine electronic functions, the need to achieve an image of high spatial resolution, low noise, and minimal artifact content makes the accurate implementation of these functions anything but routine. The low-level nature and the wide dynamic range of the signals, coupled with the precision and accuracy required by the mathematics of the reconstruction process, impose extreme requirements on the electronic circuitry."

Hundreds of these new systems are already in service around the world. The innovative application of proven technology, driven by a vision of how things can be improved, is adding new life to what has been considered a "mature" medical technology. Besides making possible new medical applications for CT in such areas as surgical theatres and emergency rooms, the lightweight scanner is opening new non-medical markets for CT in the areas of nondestructive testing and quality assurance.

Analogic Corp. custom designs and manufactures advanced systems and subsystems sold to major medical, industrial, and telecommunications OEMs. For more information, contact Analogic at 978/977-3000.