How low can CT dose go? ASIR cuts kidney stone eval dose 84%

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Adaptive statistical iterative reconstruction (ASIR) provided diagnostic quality CT images at a reduced radiation dose of 1.8 mGy in patients with urinary stone disease, according to a study published online in Radiology Aug. 13. These early data suggest that a modified protocol can be introduced into clinical practice for urinary stone evaluation if ASIR or similar tools are available.

CT has emerged as the imaging modality of choice for evaluation of urolithiasis, which has a lifetime incidence of 5 to 10 percent in the U.S., and a recurrence rate of up to 75 percent. However, radiation exposure concerns are prominent, prompting a need to develop techniques to reduce radiation dose.

Previous efforts to cut dose, particularly filtered back projection (FBP), have produced images with excessive noise, which can negatively affect diagnostic confidence, according to Naveen M. Kulkarni, MD, from the departments of abdominal imaging and intervention at Massachusetts General Hospital in Boston, and colleagues.

Thus, Kulkarni and colleagues sought to compare CT exams at 80 and 100 kV with 75-150 mA and the effect of ASIR on image quality in patients with urinary stone disease.   

The researchers recruited 25 patients with suspected or known urolithiasis to undergo unenhanced CT, and images were subsequently reconstructed. In 13 of the 25 patients, prior unenhanced CT images were available as the reference standard.

For the 25 patients, three images series were reconstructed: one FBP and two ASIR series, one at 60 percent and one at 80 percent. Two readers independently reviewed the series and graded image quality, image noise and diagnostic confidence, as well as other measures.

“All reference-protocol CT images reconstructed with FBP and modified protocol CT images reconstructed with 60 percent and 80 percent ASIR were rated as diagnostically acceptable, with mean scores of 4.2, 3.2 and 3.4, respectively.”

Mean diagnostic confidence score for the detection of urolithiasis was 3 for both 60 percent and 80 percent ASIR images. The mean score for FBP images was 1.9, and readers could confidently diagnose calculi in seven of 25 patients.

“In addition to urolithiasis detection, it is also imperative to maintain acceptable image quality for the identification of alternate diagnoses in patients who do not have urinary tract calculi,” wrote Kulkarni et al.

The mean dose for the reference-protocol scans was 9.9 mGy, compared with 1.8 mGy. The mean dose reduction was greater for patients weighing less than 200 lbs, compared with those weighing more than 200 lbs.

The researchers noted that the FDA has approved other advanced reconstruction techniques, including model-based iterative reconstruction (MBIR; GE Healthcare) and sinogram affirmed iterative reconstruction (SAFIRE; Siemens Healthcare). These and additional techniques under development could further reduce radiation dose.

Kulkarni and colleagues concluded, “As evidenced by our observations, use of our modified scanning protocol and application of the ASIR technique can enable substantial radiation dose reduction without introducing image-quality concerns for urinary stone diagnosis.”