AJR: ASIR algorithm allows reduced dose without sacrificing image quality

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Reducing radiation dose on CT enterography examinations by lowering tube voltage to 80 kVp in patients weighing less than 160 lbs. will affect image quality, however, with 30 percent adaptive statistical iterative reconstruction (ASIR), the images remain diagnostically acceptable, according to a study published in the May issue of the American Journal of Roentgenology.

Ravi K. Kaza, MD, and colleagues from University of Michigan Hospital in Ann Arbor, noted that CT enterography is the modality of choice for imaging patients with suspected small-bowel disease. However, because many patients with Crohn disease are younger and may be imaged multiple times, radiation exposure is a major concern.

“Newer image reconstruction algorithms that enable noise reduction using CT projection data have the potential to overcome the limitation of increased noise with low-tube-voltage CT techniques by generating images with less noise than images reconstructed with a standard filtered back projection (FBP) algorithm,” wrote the authors.

To test whether reducing tube voltage from 120 to 80 kVp while using the ASIR algorithm maintains adequate image quality, the authors retrospectively reviewed 133 CT enterography exams from 127 patients weighing less than 160 lb. Sixty-four received a 80 kVp exam, while 69 received 120 kVp exams. Image quality for evaluation of the bowel wall, mesenteric vessels and hepatic parenchyma were assessed.

The results showed that the reduced voltage scans scored lower than the 120 kVp exams in terms of image quality. On a scale of one to five, overall image quality for the 80 kVp exams was 3.23 compared with 3.68 for the 120 kVp exams, with specific quality scores for evaluating the bowel wall and liver similarly distributed.

Despite the lower image quality, none of the 80 kVp exams was graded as poor, and all were considered to have acceptable quality. Diagnostic accuracy was comparable between the scan protocols.

Mean volume CT dose index (CTDIvol) and effective dose for the low voltage scans were 6.15 mGy and 4.60 mSv, respectively, while the 120 kVp scans had mean CTDIvol and effective doses of 20.79 mGy and 15.81 mSv, respectively.

“In addition to reduced dose, another principal advantage of lower-tube-voltage CT is the increased attenuation of iodine, which leads to increased contrast of vascular structures and enhanced organs,” wrote the authors. “This increased image contrast could be beneficial in evaluating patients with suspected inflammatory bowel disease because changes of active disease, such as mucosal enhancement and mesenteric hypervascularity, would be readily apparent.”

Although the ASIR method is only available through one vendor, similar iterative image reconstruction methods are available on scanners from other vendors, and the authors noted that investigations using those methods have reported similar results.