Testing for life's sake
Close cooperation among a global engineering team led to the success of the LightSpeed VCT medical scanner.
Lawrence D. Maloney, Contributing Editor -- Test & Measurement World, 8/1/2005
 |
| WEB-ONLY SIDEBAR: Patients: The real winners in "slice wars" READ OTHER AUGUST ARTICLES: Table of contents, August 2005 AUGUST FEATURES: |
—For doctors dealing with medical emergencies, the speed and accuracy of diagnostic equipment play crucial roles in a patient's chances for survival. Speed is measured in the precious seconds required to obtain high-resolution images of damaged hearts, shattered bones, or ruptured vessels, while accuracy hinges on the ability of sophisticated x-ray detectors to prevent artifacts that could lead to a misdiagnosis. Now, thanks to the work of engineers at the sprawling GE Healthcare Technologies complex just west of Milwaukee, more doctors can tap what is being described as "the quintessential emergency room tool." Introduced in the first quarter of this year, the LightSpeed VCT (volume computed tomography) generates 3-D-quality images that are setting new standards for CT scanners.
In a single 0.35-s rotation, the VCT simultaneously acquires 64 slices of data, each 0.625 mm wide, for a total anatomical coverage of 40 mm. That's four times the coverage of GE's current LightSpeed Pro 16 scanner. As a result, the scanner can capture images of most organs in 1 s, the heart and coronary arteries in just 5 s, and the whole body in 10 s.
The speed and quality of these images have so impressed the medical community that many physicians see the LightSpeed VCT as an alternative to costly invasive procedures, such as angiograms (see "Patients: The real winners in 'slice wars,'" in the online version of this article at www.tmworld.com/archives). By early May, GE Healthcare had already installed 30 of the scanners, priced at about $2 million each, with another 250 orders booked.
"This is the biggest order backlog for a CT scanner that we've ever had," said Scott Schubert, GE Healthcare's global product manager for CT. He added that the scanner's enhanced capabilities will lead to further expansion of CT procedures, already growing at a rate of more than 10% annually. Worldwide, patients undergo 100 million CT exams each year—with half of that number performed in the US alone.
Detector technologyFrom a testing standpoint, some of the biggest challenges presented by the LightSpeed VCT centered on its new detector system, called the V-Res. Along with the x-ray source, situated directly opposite it, the detector revolves around the patient within a donut-shaped gantry.
The V-Res features a proprietary HighLight composite ceramic material in the scintillator, the component that converts the incoming x-ray to visible light. An adjacent photodiode detects the light, converts it to an electrical signal, and transmits it to the data-acquisition system. On the VCT, GE pioneered a backlit diode, which routes the electrical signal from the rear of the diode. This permits a denser, more compact diode array and makes it possible to add more detector elements in future designs.
 |
| The ADC board undergoes a test journey in manufacturing that begins with x-ray and automated inspection in the surface-mount stage and ends with a functional test of two ADC boards sandwiched together in a module. More tests await when the module is mated with other detector components. |
Engineers at GE's Schenectady, NY, R&D center also developed a new analog-to-digital converter (ADC) chip—the Volara—that was fabricated by Idaho-based AMI. Eight of these chips reside on a detector board that includes a heat sink and is about 20% smaller than that used in the previous-generation Pro 16, which has only two ADCs.
Previous detector designs also featured longer flexible circuits for interconnects, which plugged into card-rack backplanes. "Now, we are packaging the electronics in much tighter proximity to the photodiode and optoelectronics to minimize the capacitance and physical distance—and that translates into much lower electronic noise and higher signal isolation," explained Jeff Kautzer, manager of CT detector engineering. "We've moved from conventional card-rack, connector electronics to microelectronics that are closely aligned to the detector elements."
In the final assembly in the scanner's gantry, technicians place the ADC boards, sandwiched together in modules, in an arc-shaped mechanical card rack. An array of digital-interface (DIF) boards, which power and control the ADC boards, occupy another rack. A micropositioning system on the gantry aligns the x-ray input of the scintillator to a collimation system that defines the direction and dimensions of the x-ray beam. The entire assembly spins at 0.35 s per rotation, resulting in forces of 22 g on the detector boards.
 |
| With its 64-channel capability, the LightSpeed VCT can produce high-resolution, 3-D images of most organs in 1 s, the heart in 5 s, and the entire body in 10 s. |
Ensuring the quality of this network of boards on the V-Res detector demanded cooperation among members of a global engineering team. About 20% of the team was involved in building test fixtures, designing and performing tests, and writing software code for boundary-scan, in-circuit test (ICT), and custom testers, according to Jean Marc Delplanque, the engineer in charge of design for manufacture. His group mapped out the test strategy for the detector boards and designed the process flow for board manufacture. They also built custom testers, both for GE's internal manufacturing and for outside suppliers.
Delplanque noted that the complexity of the VCT's electronics was four times that of the previous product, and the design and test teams wanted to boost the reliability compared to that of previous scanners with no increase in cycle time. "The important thing is that we designed our tests and manufacturing processes from the start based on the needs of the product—not on our organizational structure," he said.
With the VCT's design fixed by early 2003, the engineers responsible for the ADC and DIF boards found themselves testing prototype boards to weed out problems as early as possible, while at the same time ramping up the tests that would be needed for volume manufacturing. "I used to be a design engineer, so I know that getting early feedback from test is critical to getting the design right," commented David Sallis, component engineering and test manager at GE's Global Electronics Center (GEC), a sister facility in Milwaukee where the VCT's detector boards are manufactured.
Journey to reliabilityA look at the test roadmap for the ADC boards demonstrates the company's commitment to provide as much critical feedback as possible. The journey begins at the surface-mount level, where GEC will soon introduce automated online inspection machines to assess solder-joint quality during the paste-printing process. To ensure the board electronics meet IPC Class 3 manufacturing standards,GE uses Agilent's 5DX x-ray system to inspect solder joints for voiding and other defects.
"Because of the [VCT] system's high-density electronics, including micro-BGA packaging, we are using x-ray more and more in manufacturing," said detector manager Kautzer.
After the cleaning and washing stages, boards undergo in-circuit tests on an Agilent 3070 bed-of-nails system. Next, the engineers perform a functional test on the ADC board using a custom tester that accommodates eight boards. This test involves operating the ADCs at their expected frequency rate and under controlled temperature. The tester measures a dozen critical-to-quality parameters, including noise levels, leakage, offset, and integral and digital nonlinearity.
Explained Delplanque, "This is a very important test because you are measuring parameters that you cannot measure effectively later on. Repairs also can be made much more easily at this stage, compared to later on in the process, when the boards are sandwiched into a module for another functional test."
The GE engineers also perform extensive accelerated-life and stress-screening tests on the boards, both at the prototype and final manufacturing stages. For example, prototype boards were tested in Thermex-Thermatron ovens at temperatures ranging from –40C to +125C for 8 hrs. The normal operating temperature of the CT scanner is +15C to +45C.
Following stress screening and another functional test, the ADC module faces still more tests when it is mated with the diode assembly and scintillator pack as a complete subsystem. At this stage, the engineers perform at least a dozen tests in a clean room, using a digital modular tester and a slave x-ray source. They monitor the conversion of x-rays to electronics and the optical interface between the scintillator and the photo diode. "Anywhere that you have energy conversion going on calls for more elaborate and more difficult tests," explained Kautzer.
Next, the assembly goes to system manufacturing, where it is loaded onto a gantry equipped with an x-ray tube and a generator. The detector array is optically aligned with the collimation system, and the engineers can test dynamic images, instead of just the static images tested in the earlier digital module test. More tests follow on the entire CT machine in lead roomsat the Waukesha facility, including tests of the spatial and temporal resolution of the system.
Rewards of boundary scanThe test team also made use of boundary scan—using Goepel's Cascon software—for prototype development of the detector's critical boards. "About 30% of our boards have large digital content suitable for boundary scan," said Bobby Compton, the engineering manager who introduced the technology to the VCT platform team. "Within a few days, we were able to get a quick and easy boundary-scan test going, compared to the 10 weeks it can take to build an expensive ICT fixture."
On the DIF boards, for example, the boundary-scan tests checked for functionality and for electrical faults in the onboard power supplies. Without the boundary-scan system, there was a 67% yield of functional DIF boards during the prototype stage, where the engineers used scopes, probes, meters, and logic analyzers to prove out the design. But when they used x-ray inspection and boundary scan, the yield jumped to 98%.
"This was a real eye opener that CT engineering really loved," recalled GEC test manager Sallis, "because they now had a much larger volume of boards to work with. The result was a highly accelerated development process."
Added Russ Hum, the lead development engineer for the VCT's data-acquisition system: "We went from hours debugging a board with bench instruments to just minutes with boundary scan."
Compton and his team say that boundary scan can also play a role in volume production. Team member Larry Beine built a fixture that can simultaneously test four cards used to reconstruct the CT images. "The pin density on these boards is high enough that it is very difficult to design an ICT fixture that can thoroughly test it," explained Compton.
A bit of home brewExamples abound of such test ingenuity throughout the VCT project, which helped speed design and curb costs. For instance, in the development of the DIF boards, Hum's team built a PC-based simulator that allowed engineers to hook up the data-acquisition system to the ADC boards to take simulated scans and collect data. To test the regulators on the DIF boards, Beine used a small ADC chip on the board to measure voltages, eliminating the need for an additional test circuit setup outside the board.
And while the ADC module team had to build custom testers for several key stages in the manufacturing process, the engineers used the same software throughout, changing only the configuration files. Among the key components of these testers: a voltmeter, a PXI chassis and PXI boards, a CAN interface, and a PC.
"These aren't simple Class 1 boards we are dealing with," noted ADC module team leader Joe Block. "The delicacy of the measurements at the front end—in picoamps—challenges us to design testers that are effective and rigorous at those levels so that these boards are highly reliable inside a CT scanner." Such reliability is crucial to the functioning of the scanner. If even one of the 1024 A-to-D channels in the CT's detector array malfunctions, the image is often unusable.
The GE engineers also had to find ways to keep from drowning in all the test data collected during board development, pointed out Delplanque. The solution: save detailed test data on failures or marginal performance, while relying on overall statistics to track the results of passed components. Engineers also designed a visualization tool that lets manufacturing quickly identify patterns in board failures.
An engineering triumphBy all accounts, such innovative test work played a major role in the LightSpeed VCT's success. Despite the need to coordinate numerous design and test teams involved with multiple subsystems, the cycle time for the VCT was much faster then it was for the first LightSpeed CT introduced in 1998, noted Gary Strong, the lead engineering manager on the project.
Recent articles in publications such as Radiology Today, Popular Mechanics,and the Wall Street Journalhave heralded the technical advancements of the machine. And within GE itself, the scanner has garnered the company's coveted "imagination breakthrough" designation. "The word we are getting from within the company is to make more of these machines," said manufacturing test engineer Gary Schilling. "It's a big hit with customers."
Brian Kost, who manages the engineering effort for all of GE's CT scanner products, traces the VCT's success to its ability to answer the needs of the medical community—most notably, greatly enhanced imaging of the heart. "It's a great product to be associated with," said Kost, "and we couldn't meet the demand that we are getting right out of the chute without the critical role of test. It helped us catch problems early in the process, giving us the assurance in final assembly that we are building a quality product."
WAUKESHA, WI
