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Measuring Coronary Calcium on CT Images Adjusted for Attenuation Differences¹

¹ From the Ctr for Health Studies, Group Health Cooperative, Metropolitan Park East, Suite 1600, 1730 Minor Ave, Seattle, WA 98101 (J.C.N.); Dept of Biostatistics, Univ of Washington, Seattle, Wash (J.C.N., R.A.K.); Dept of Radiology, Wake Forest Univ School of Medicine, Winston-Salem, NC (J.J.C.); Dept of Radiology, David Geffen School of Medicine at UCLA, Los Angeles, Calif (M.F.M.G., J.G.G.); Heart Disease Prevention Program, Div of Cardiology, College of Medicine, Univ of California, Irvine, Calif (N.D.W.); Div of Epidemiology and Clinical Applications, National Heart, Lung, and Blood Institute, Bethesda, Md (C.M.L.); Dept of Medicine, Div of Preventive Medicine, Univ of Alabama at Birmingham, Birmingham, Ala (O.D.W.); and Harbor-UCLA Research and Education Inst, Div of Cardiology, Los Angeles, Calif (R.D.). Received Mar 18, 2004; revision requested May 27; revision received Jun 25; accepted Jul 27. Supported by NHLBI contracts N01-HC-95159 through N01-HC-95165, N01-HC-95169, N01-HC-95095, N01-HC-48047 through N01-HC-48050, and N01-HC-05187. Address correspondence to J.C.N. (e-mail: nelson.jl@ghc.org).

PURPOSE: To quantify scanner and participant variability in attenuation values for computed tomographic (CT) images assessed for coronary calcium and define a method for standardizing attenuation values and calibrating calcium measurements.

MATERIALS AND METHODS: Institutional review board approval and participant informed consent were obtained at all study sites. An image attenuation adjustment method involving the use of available calibration phantom data to define standard attenuation values was developed. The method was applied to images from two population-based multicenter studies: the Coronary Artery Risk Development in Young Adults study (3041 participants) and the Multi-Ethnic Study of Atherosclerosis (6814 participants). To quantify the variability in attenuation, analysis of variance techniques were used to compare the CT numbers of standardized torso phantom regions across study sites, and multivariate linear regression models of participant-specific calibration phantom attenuation values that included participant age, race, sex, body mass index (BMI), smoking status, and site as covariates were developed. To assess the effect of the calibration method on calcium measurements, Pearson correlation coefficients between unadjusted and attenuation-adjusted calcium measurements were computed. Multivariate models were used to examine the effect of sex, race, BMI, smoking status, unadjusted score, and site on Agatston score adjustments.

RESULTS: Mean attenuation values (CT numbers) of a standard calibration phantom scanned beneath participants varied significantly according to scanner and participant BMI (P < .001 for both). Values were lowest for Siemens multi–detector row CT scanners (110.0 HU), followed by GE-Imatron electron-beam (116.0 HU) and GE LightSpeed multi–detector row scanners (121.5 HU). Values were also lower for morbidly obese (BMI, 40.0 kg/m²) participants (108.9 HU), followed by obese (BMI, 30.0–39.9 kg/m²) (114.8 HU), overweight (BMI, 25.0–29.9 kg/m²) (118.5 HU), and normal-weight or underweight (BMI, <25.0 kg/m²) (120.1 HU) participants. Agatston score calibration adjustments ranged from –650 to 1071 (mean, –8 ± 50 [standard deviation]) and increased with Agatston score (P < .001). The direction and magnitude of adjustment varied significantly according to scanner and BMI (P < .001 for both) and were consistent with phantom attenuation results in that calibration resulted in score decreases for images with higher phantom attenuation values.

CONCLUSION: Image attenuation values vary by scanner and participant body size, producing calcium score differences that are not due to true calcium burden disparities. Use of calibration phantoms to adjust attenuation values and calibrate calcium measurements in research studies and clinical practice may improve the comparability of such measurements between persons scanned with different scanners and within persons over time.

© RSNA, 2005

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