Enhanced image clarity, less noise, and half the radiation exposure for patients: sound good? It’s true. It’s iterative reconstruction, and it’s a mounting wave in multidetector CT technology. There’s only one catch—it’s time-consuming unless you happen to have the proprietary software of the single vendor that has come to market with a workflow-ready product at this point.
W. Dennis Foley, MD, FACR
W. Dennis Foley, MD, FACR, chief of digital imaging and a professor of radiology at the Medical College of Wisconsin in Milwaukee, has been championing iterative reconstruction since putting the technique to the test in a pilot demonstration in 2008. Foley and his colleagues judged iterative reconstructions against the same exams reconstructed in the more common way, using filtered back projection (FBP). FBP, Foley explains, involves “point-source radiation from a focal spot on the x-ray tube passing through as a single, thin line through the center of a voxel that goes to the center of each detector cell.”
That’s what supposed to happen, anyway, but as Foley puts it, “That’s not the reality.” Instead, the x-ray beam is not a thin line, and the detector receives radiation across its face at various angles to the voxel, Foley says, noting that this can occur randomly. When this happens, image noise is created that must be overpowered by increasing the radiation exposure, so FBP uses a higher radiation dose than iterative reconstruction.
“There’s a direct relationship between radiation dose and noise,” Foley says. “Noise is a statistical variation in the attenuation values not reflecting the underlying anatomy. Noise means that you don’t see a clear edge or clear contrast between two adjacent tissues. It gets blurred. Noise is a model that blurs anatomic features of an image.”
Noise can happen because of variations in detector sensitivity, or it can occur through variations in electronics, Foley says. With FBP, dosage strength is used to minimize noise, but a new focus on patients’ radiation exposure from CT has placed a premium on being able to lower the dose—and this is what iterative reconstruction can do, without the gain in noise associated with FBP.
“The problem is that most medical radiation the population receives now comes from CT, and the utilization of CT has increased markedly in the past 10 years,” Foley says. “There is some concern about excessive radiation of the population increasing the incidence of cancer, even though the risk is very slight.”
The Software Approach
While FBP takes a signal-to-detector approach to image creation, iterative reconstruction uses complex software, algorithms, mathematics, and modeled anatomical inputs to create the image by passing over the exam data from the modality again and again. It is the thousands of repeated iterations—each time, filling in data—that give the technique its name. Iterative-reconstruction software is continuously refining and building, using models and nearby anatomy to guess at missing data, until the image becomes the completed exam that the radiologist interprets.
“Iterative reconstruction is done by a statistical modeling process. Lowering dose is more important than getting noise down,” Foley says. Nonetheless, he adds, iterative reconstruction gives both lower dose and equal or better structural detail in the completed image. Iterative reconstruction doesn’t have to overpower noise: It eliminates noise by refashioning data. What would be noisy in FBP is less noisy with iterative reconstruction.
“The noise in the reconstructed image is fundamentally determined by the noisiest projection data,” Foley says. “If you can smooth out the noise differential by statistical modeling in projection data, you can make an image that is much smoother that still demonstrates the anatomy with equal fidelity.”
ASIR and MBIR
The first iterative-reconstruction software on the market that is fast enough to match the workflow of FBP, Foley says, is Adaptive Statistical Iterative Reconstruction (ASIR) from GE Healthcare, Waukesha, Wisconsin. The ASIR software works with any GE CT750HD scanner. With ASIR, most, if not all, CT studies can be completed at a lower radiation dose. Foley notes that ASIR is particularly useful for younger adults or people having repeated exams. It is also useful in studies looking for renal stones, in Crohn disease, for CT angiography, and for abdominal aortic and iliac studies in patients with stent grafts—all