Researchers used a BMI-adjusted dose reduction approach to cut effective radiation dose at CT colonography (CTC) to 1.41 mSv at 15 mAs and 2.81 mSv at 30 mAs for normal BMI and overweight patients, according to a study published in the April issue of Academic Radiology.
Although the typical radiation exposure with CT hovers in the 5.7 mSv range, both the U.S. Preventive Services Task Force and Centers for Medicare & Medicaid Services have cited concerns about increasing exposure via multiple screening studies in their refusal to endorse CTC.
Michael Ginsburg, MD, from the department of radiology at The University of Chicago Medical Center, and colleagues devised a study to evaluate the association between BMI and CTC image quality to assess the utility of BMI-based dose reduction.
The researchers enrolled 96 consecutive patients undergoing CTC between January 2009 and January 2012. Normal BMI and overweight patients were scanned at 120 kVp by alternating between effective 30 mAs or 15 mAs. Patients’ body weight and height were recorded at the time of the exam.
After 3D reconstruction of CTC data, researchers recorded a quantitative assessment of image noise and three observers rated conspicuity on a 0- to 3-point scale, with 0 corresponding with no mucosal details with excessive floaters and 3 with excellent mucosal detail and no floaters.
Supine and prone series for 92 patients yielded 192 series for review; 102 series were scanned at 30 mAs and 90 at 15 mAs. The mean combined effective dose at 15 mAs was 1.41 mSv and at 30 mAs was 2.81 mSv.
The final analysis focused on 91 patients: 34 normal weight patients (BMI < 25), 28 overweight patients (BMI ≥ 25 and < 30) and 29 obese patients (BMI ≥ 30).
The noise analysis revealed a statistically significant difference between 15 and 30 mAs in the abdomen and pelvis, with 30 mAs producing less noise. BMI correlated with noise at both 15 mAs and 30 mAs, but the link was stronger at 15 mAs.
“There was a significant negative relationship between BMI and conspicuity scores both in the abdomen and pelvis after adjusting for mAs.” More patients had scores of 3 at 30 mAs than at 15. The researchers observed a strong inverse relationship between BMI and conspicuity scores, and wrote, “it is likely that lower conspicuity scores, and therefore high BMI are also likely to hinder polyp detection and confidence of interpretation.”
The results indicate that CTC can be performed using 15 mAs or 30 mAs, rather than 50 mAs, and provide adequate conspicuity, depending on BMI. In general, conspicuity was nearly perfect at 15 mAs in normal-weight patients. Radiation dose of 30 mAs provided perfect conspicuity scores in both the abdomen and pelvis for 93 percent of overweight patients.
Obese patients proved more complex. The use of 30 mAs provided high conspicuity in the abdomen in 88 percent of patients, but conspicuity in the pelvis remained suboptimal for a majority of patients. “For this reason, conventional higher doses of radiation may be required for obese patients.”
Ginsburg et al referred to additional possibilities beyond tube current modulation for further dose reduction. These include noise reducing reconstruction algorithms and adaptive statistical iterative reconstruction. They proposed BMI-adjusted dose reduction for CTC screening exams for normal BMI and overweight patients, and also recommended further evaluation of other dose-reduction techniques, including their effects on image quality and polyp detection.