Adaptive statistical iterative reconstruction (ASIR) reduced radiation dose 33 percent compared with routine-dose CT with filtered back projection (FBP), according to a study published in the September edition of American Journal of Roentgenology.
Although researchers have successfully developed and implemented CT dose reduction techniques such as tube current modulation, low tube voltage and noise reduction filters, additional dose reductions have been difficult. The standard FBP reconstruction algorithm used on all CT scanners is limited and associated with increased image noise and degraded image quality in dose reduction attempts.
ASIR (GE Healthcare) uses a statistical model to reduce image noise, and a pilot study produced image quality superior to FBP, noted lead author Yoshiko Sagara, MD, of the department of radiology at Oita University Faculty of Medicine in Oita, Japan. Sagara and colleagues undertook the study to compare image quality of low-dose contrast-enhanced abdominal CT reconstructed with ASIR to routine-dose contrast-enhanced CT reconstructed with FBP.
Researchers recruited 53 patients (average age, 60.8 years) who had undergone contrast-enhanced, venous-phase abdominal CT with FBP between December 2008 and January 2009. Patients underwent a second low-dose scan at 40 percent ASIR within an average of 244 days of the first study. A previous study showed 40 percent ASIR is well matched to the noise content of a full-dose FBP study, noted Sagara.
After a radiologist not involved in grading images selected and matched representative images from each exam, two radiologists compared and graded the images. Liver images showing the main portal vein were assessed for image noise, diagnostic acceptability and artifacts, wrote Sagara; and images of the aorta below the superior mesenteric artery were assessed for image sharpness of the aortic wall. In addition, radiologists assessed image quality of the 24 CT datasets with identical slice thickness for the ASIR and FBP studies.
Low-dose CT with ASIR averaged 17 mGy volume CT dose index compared with 25 mGY for routine-dose CT with FBP, reported Sagara. The ASIR images were graded as having significantly less image noise than the routine-dose with FBP images, continued Sagara. In the subset of 24 comparisons with identical slice thickness, low-dose CT with ASIR produced decreased noise, but artifacts and diagnostic acceptability were not statistically different from FBP images.
“The measured and observed image noise, however, varied by the size of the patients because of the more aggressive dose reduction that occurred in smaller patients,” wrote Sagara. Patients with a body mass index (BMI) of less than 20 had an average reduction of 66 percent versus 23 percent for patients with a BMI of 25 or greater.
Similarly, changes in BMI seemed to impact image sharpness with smaller patients who underwent the greatest dose reduction linked with the worst image sharpness.
“[A] low-dose CT with ASIR protocol can achieve significant reductions in radiation dose without compromising diagnostic acceptability compared with routine-dose CT with FBP,” concluded Sagara and colleagues, who used the study to support a shift to full integration of ASIR into the practice’s abdominal CT protocol.