Four-dimensional computed tomography, or 4-D CT, is the latest development in the realm of industrial X-ray inspection for the nondestructive testing industry. In the simplest of terms, 4-D CT is a 3-D X-ray computed tomography process that captures movement over a period of time.
Like most other nondestructive imaging modalities, 3-D CT has traditionally been used to study static objects. Historically, motion has been the enemy of sharp image quality because the movement would result in the images appearing blurred. The question asked at North Star Imaging Inc. is “How can we overcome this limitation to study objects that have moving internal parts?” This simple question led to the development of 4-D CT.
In order to appreciate the significance of 4-D CT, one must have a basic knowledge of 3-D CT and its evolution. Industrial CT, commonly known as computed axial tomography in the medical field, is a diagnostic imaging tool that allows 3-D inspection of a wide variety of objects. Computed tomography was first used for industrial nondestructive testing purposes during the late 1980s, roughly 10 to 15 years after the first medical CT scanner was introduced in England. Even though industrial CT has existed for close to 30 years, it hasn’t been a highly practical inspection technique until the last five to seven years. Now, it is quickly becoming the industry standard for many nondestructive testing applications.
Three-dimensional CT is a process of capturing hundreds or thousands of individual 2-D X-ray images around a 360° rotation of an object. These 2-D images are then combined into a single 3-D model through the use of mathematical algorithms in a process called reconstruction. This process is highly taxing on computer hardware and historically took hours or even days to complete. In recent years, rapidly improving computer hardware, combined with innovations such as North Star Imaging’s graphics processing unit reconstruction process, has significantly reduced the amount of time it takes to reconstruct a 3-D CT model. In the last year alone, multiple systems have been delivered with reconstruction times ranging from two to six seconds.
The successful development of 4-D CT was a culmination of multiple innovations at North Star Imaging during the last five years. With this technology, speed and efficiency is crucial. The ability to acquire clear 2-D images while in motion, the capability to reconstruct a 3-D CT model in a matter of seconds, and even optimizing hard drives to write and store data quickly enough, were just some of the challenges that needed to be overcome.
Four-dimensional CT is a 3-D X-ray CT process that combines traditional 3-D CT imaging with motion over a period of time. This results in a truly dynamic 3-D volumetric data set. Four-dimensional CT is accomplished by acquiring continuous 3-D CT reconstructions and combining them into a single 4-D CT model. Depending on the rotation time and frame rate of the detector it is possible to capture full-motion 3-D CT data at 15 to 30 frames per second. The motion captured in the 4-D model can then be played like a movie while the user simultaneously slices through and inspects the internal functionality of the object at any angle. The motion can be viewed in forward or reverse, it can be paused and restarted, and even navigated to a specific point along the timeline. This unique technology allows users, for the first time, to simultaneously study structure, form, and function.
For example, a small plastic rack and pinion gear set was recently examined with 4-D CT. The product had an unknown flaw when the gear reached the end of the rack, causing the gear to jump and occasionally slip. The gear set was sealed and could not be opened without destroying the product. Upon viewing the 4-D CT scan, it was immediately apparent that a foreign object had wedged itself between the teeth, and the user was able to study exactly how this obstruction affected the functionality of the product.
To date, 4-D CT has successfully been used to study the function of moving parts in mechanical gear assemblies, thermal switches, and even hydrodynamic flow. The very nature of 4-D CT lends itself well to studies concerned with product and process, reverse engineering, quality, as well as forensic and failure analysis. Keep in mind that 4-D CT is highly application-dependent at this point; it will not work for every product every time. The most effective scans have been completed on small, low-density objects. Essentially, any industry with products of that nature could potentially benefit from this technology.
Currently, 4-D CT is available in North Star Imaging’s X-ray CT systems (X-View CT) or an upgrade to almost any existing X-ray system, regardless of the manufacturer.
As 4-D CT continues to develop and evolve, more industries and more products will adopt this tool. The technology will likely undergo a very similar life cycle to that of 3-D CT in the last three to five years, eventually developing into the process of choice for many applications.