posted January 10, 2014
Recently, the manufacturer of a hydrazine valve for a NASA spacecraft found that the valve was leaking, but there was no obvious reason why.
The manufacturer turned to Aerospace Distinguished Scientist Dr. Gary Stupian of the Electronics and Photonics Laboratory and Neil Ives of the Microelectronics Technology Department (MTD) for help. With a Civil and Commercial Operations contract quickly put into place, Stupian and Ives were able to determine that the cause of the leak was a piece of perforated mesh backing on the valve seal that had come loose and lodged in the valve seat.
It was another example of how, among NASA program offices and others in the space industry, Aerospace has become known as the “go to” organization for quick turnarounds on high quality X-ray computed tomography (CT) investigations and inspections.
Much like medical CAT scans, the Aerospace scanning technique produces 3-D images of an object in astounding detail. But it’s more than having the right equipment that makes a lab stand out for performing challenging X-ray CT analyses. Dr. Martin Leung, associate director of the MTD who has worked with Stupian and Ives for 30 years, noted that “the skill of the scientists makes all the difference” in the quality of the CT image, or even whether it is possible to have a useable image.
“The key is having a lot of gray scale dynamic range in the image,” said Ives, a senior scientist in MTD. This involves knowing how much power to use and the length of the exposure, among other parameters. “With the right settings and an experienced operator, you can image a low-density object inside of a higher-density object.”
Aerospace experience in X-ray imaging of space parts goes back nearly twenty years to 1994 when the company purchased a Feinfocus 2-D X-ray system for the MILSATCOM program office. For several years, Aerospace had the only real-time X-ray facility in the space community. As a result of the company’s success in the area, “Feinfocus” became the common term for any X-ray inspection.
But the real turning point came around the turn of the century when Stupian and Ives, with encouragement from Leung, set about modifying their original Feinfocus system to add an X-ray CT scanning capability.
The essence of a CT scanner is that it is an X-ray machine that takes a series of 2-D X-ray images of its subject – hundreds or even thousands of individual projections of the subject. A computer program then takes these 2-D images and assembles them into a 3-D rendering of the subject, whether that is a person’s chest or the valve of a rocket engine.
One difference between CT scanners for space equipment and people is that the scanners for inanimate objects can use much higher doses of radiation. Another is that for people, the X-ray equipment rotates around the person, while with an object, the object can be rotated and the X-ray source held immobile.
Ives and Stupian built their first CT scanner from the 2-D Feinfocus platform. Although CT was already an established technique in medicine, it took a good deal of effort to take a 2-D X-ray system and successfully turn into a CT scanner. They modified the computer codes to command the system to rotate the test object, turning it 1 degree for each X-ray image. They then put together the necessary CT rendition software for creating the 3-D image. This homegrown CT scanner provided considerable experience in and the scientific understanding of the X-ray CT scanning technique for the Aerospace lab. This unique expertise was used in a number of root cause investigations. Image quality and spatial resolution, however, were limited by the focal spot size of the X-ray tube and mechanical instabilities in the system designed for 2-D imaging only.
In the meantime, realizing the importance of CT imaging, commercial X-ray equipment vendors addressed these issues and began building and selling their own systems designed to provide high-quality CT analysis.
To maintain Aerospace’s leadership position in X-ray analysis within the space community, the lab bought two new X-ray CT scanners in 2010 and 2011 to handle nano and micro scaled objects, that is, a nano scanner for objects smaller than about half a centimeter, and a micro scanner for things that are about half a centimeter to 12 cm long.
Since there is a constant demand for performing CT analysis on space components larger than what the current systems can handle, Stupian hopes the next purchase will be a large-format CT scanner, which he says will further augment Aerospace’s ability to provide the highest-quality X-ray products for the company’s national security space and civil and commercial customers.