Radiologists have been struggling to balance image noise with radiation dose in computed tomography (CT) scans for decades. But the competition just went up a notch (or perhaps many notches) with the recent FDA approval of GE Healthcare’s Model Based Image Reconstruction (MBIR) technology, Veo. Although it takes this software longer to compute, physicians are using ultra-low radiation doses and getting images with increased clarity.
While MBIR is the most recent of the iterative reconstruction technologies, top manufacturers offer their own software answers to the noise versus dose argument. Here’s a closer look at the offerings.
Iterative Reconstruction Background
First, a little background. Filtered Back Projection (FBP) technology has been around since the beginning of CT in the 70s, said Wendy Kreider, customer development manager for computed tomography at Siemens Healthcare. “It's based on back projection of attenuation line integrals from the radial views that converge on the patient.” While it’s robust and has a fast reconstruction speed, filters accentuate the noise in the image. “This becomes pronounced in really large patients that are dose-starved or when you’re trying to intentionally lower dose, which is what we’ve been trying to do over the years,” she said.
Filters have another problem, according to Eric Stahre, general manager of premium CT for GE Healthcare. “You’re effectively losing information,” he said. “You’re compromising the diagnostic information.” FBP takes only a single pass through the raw data, which accounts for its fast speed.
Along came iterative reconstruction algorithms, which reduce the noise and dose, while increasing the image quality. This is one form of dose reduction, but the companies all offer additional and complementary programs that further reduce radiation exposure.
With each new technology, the image has a different appearance, and some clinicians comment that iterative reconstructive images have a waxy or plastic look. “With each change, when you go from filtered back projection to iterative reconstruction, it has a different looking appearance,” said Stahre. “And to the radiologist who is trained to look at the pathology and image, it takes time to understand the different look. After a few weeks of working with the cases, they become accustomed to the new look and they don’t want to go back.”
GE: ASiR and Veo
GE introduced its Adaptive Statistical Iterative Reconstruction (ASiR) program in 2008, but Veo is the first MBIR technology, and it just received FDA clearance in September.
“Veo is a real disruptive breakthrough, first of its kind, the biggest image clarity improvement in the last 20 years in CT. It fundamentally changes how the images are produced,” said Scott Schubert, general manager for premium CT marketing for GE Healthcare. “With the Veo technology we’re introducing something that takes dose reduction technology to a higher level.”
|Courtesy GE Healthcare|
Schubert said the Veo is an extension of ASiR. “Veo creates an initial image and compares that back to the measure of raw data of the system,” he said. “It uses a model of the entire CT scanner: the tube, the detector, the focal spot of the tube, and the slice thickness, and iterates to create an optimized image of that patient. Through the model based iterations, it can produce the most optimum image in terms of dose and image clarity. It’s the first time it’s been able to do this at the same time, together.” He said that FBP technology has to trade off dose versus image clarity.
As for dose reduction, Stahre said that in Europe, they’re reporting body imaging doses of less than one mSv, and lung cases at the same dose levels of a chest X-ray.
Due to the Veo’s longer processing time, Schubert expects ASiR to continue as their routine mainstream dose reduction technology, since it handles five to 10 cases per hour. “The computational power is significantly more demanding with Veo,” he said, though it has dropped dramatically. He said that two years ago, a study would take 24 hours of mainframe-type computing power, but now it can do about two cases an hour.
The technology is currently used for women of child-bearing age, pediatrics, and those scanned most frequently (e.g. cancer follow-ups), according to Stahre.
In addition to noise and dose reduction, William Shuman, MD, professor of body imaging and director of clinical radiology at University of Washington, said that there are unanticipated fringe benefits: better artifact reduction and spatial resolution. “It has a nice 3D sagittal and coronal reconstruction,” he said.
Shuman said that the reconstruction time is becoming less of an issue. He said he can get pelvic and abdominal reconstructions done on the Veo in about 15 minutes, with 0.6 mSv. For ASiR and FBP, he said the dose would be 3-8 mSv. He anticipated the Veo reconstruction time dropping to around 12 minutes in the near future, making it manageable to use the program on a regular basis for his workflow. At 12 minutes, “we’re in the timeframe where it won’t significantly impact on our workflow, and it can potentially become a routine exam,” he said.
That timeframe would be closer to keeping up with the workflow, especially if he could intermingle shorter exams on the head, with longer chest and abdominal exams. Currently CT exam turnover times are eight to 10 minutes, scanning six to eight patients an hour. He said it takes around 12 minutes to move patients in and out of the scanner, send the exam to PACS, have it show up on the work list and get the radiologist to open and start reading the images.
Currently, he bases his Veo scan decisions on dose considerations. “But with faster reconstruction, we’re at the stage where we can do them all that way, and capture that low radiation dose for everybody. That’s the conceptual breakthrough that occurred recently,” said Shuman.
Siemens Healthcare: IRIS
Iterative Reconstruction in Image Space (IRIS) is Siemens’ approach to reconstruction, approved by the FDA in 2009. “IRIS translates the iterative loop into image domain, thus avoiding the long reconstruction time,” said Kreider. “The starting point of IRIS is a master volume reconstruction that optimally utilizes all measured data and provides all available detail, but at the expense of significant noise in the image.” The master reconstruction cleans up the image in step-by-step iterations in the image domain, to increase speed.
Kreider said that for iterative reconstruction technology to be clinically beneficial, it should have three qualities: noise reduction (which gives the potential for dose reduction), reconstruction time short enough for clinical routine, and preservation or improvement of image quality. “For spatial resolution and texture appearance, you want it to be close to FBP, but lower dose. That’s what the radiologists are used to,” she said. She said that the IRIS quality is closer to FBP. “IRIS doesn’t have that plastic look because the recon image has more detail than the statistical image,” she said.
As for IRIS's image reconstruction time, Kreider said that new systems shipping today reconstruct at 20 images per second. She noted that their systems reconstruct FBP at 40 to 50 images per second.
|IRIS axial image, courtesy Siemens|
|IRIS VRT (volume rendering technique)/MPR (multiplanar reformation) coronal image, courtesy Siemens|