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Influence of Body Size and Section Level on Calcium Phantom Measurements at Coronary Artery Calcium CT Scanning¹

¹ From the Colleges of Medicine (W.S., T.L.B., B.H.T., R.M.L., L.T.M.) and Public Health (T.L.B., J.D.W., R.M.L., L.T.M.), University of Iowa, 200 Hawkins Dr, 3896 JPP, Iowa City, IA 52242. Received July 1, 2002; revision requested August 28; final revision received March 14, 2003; accepted May 19. L.T.M. supported by National Heart, Lung, and Blood Institute grant R01 HL48050 "CT Vascular Calcium—An Epidemiologic Study in the Young." Address correspondence to W.S. (e-mail: william-stanford@uiowa.edu).

View larger version (63K): [in a new window]   Figure 1. Transverse CT image obtained in a 22-cm-diameter polyurethane tissue-equivalent body phantom positioned on the mattress directly over the calcium hydroxyapatite phantoms. After imaging, the attenuation value for each calcium hydroxyapatite phantom was determined and served as the reference standard against which the subject data were compared.

View larger version (20K): [in a new window]   Figure 2a. Measured mean phantom attenuation (Mean Density) in Hounsfield units by BMI quartile and phantom in (a) women and (b) men. Phantom 1 consisted of 0 mg/mL calcium hydroxyapatite; phantom 2, 75 mg/mL calcium hydroxyapatite; and phantom 3, 150 mg/mL calcium hydroxyapatite. With increasing BMI, the attenuation increased (ie, the measured values moved further away from the reference standard values as the BMI increased).

View larger version (20K): [in a new window]   Figure 2b. Measured mean phantom attenuation (Mean Density) in Hounsfield units by BMI quartile and phantom in (a) women and (b) men. Phantom 1 consisted of 0 mg/mL calcium hydroxyapatite; phantom 2, 75 mg/mL calcium hydroxyapatite; and phantom 3, 150 mg/mL calcium hydroxyapatite. With increasing BMI, the attenuation increased (ie, the measured values moved further away from the reference standard values as the BMI increased).

View larger version (19K): [in a new window]   Figure 3a. Measured mean phantom attenuation (Mean Density) in Hounsfield units by section level and phantom in (a) women and (b) men. Section level 0 was the first section wherein the left main and/or left anterior descending coronary artery was visualized; section level 10, the section 10 sections below section level 0; section level 20, the section 20 sections below section level 0; and section level 30, the section 30 sections below level 0. With increasing section level, there was less attenuation (ie, the measured values more closely approximated the reference standard values).

View larger version (19K): [in a new window]   Figure 3b. Measured mean phantom attenuation (Mean Density) in Hounsfield units by section level and phantom in (a) women and (b) men. Section level 0 was the first section wherein the left main and/or left anterior descending coronary artery was visualized; section level 10, the section 10 sections below section level 0; section level 20, the section 20 sections below section level 0; and section level 30, the section 30 sections below level 0. With increasing section level, there was less attenuation (ie, the measured values more closely approximated the reference standard values).

View larger version (28K): [in a new window]   Figure 4. Graph illustrates the estimation of a subject- and section level-specific fitted regression line (solid line) and the corresponding estimated attenuation (Density). In this example, a measured value of 130 HU has an adjusted value of 140.3 HU (dotted line). A phantom data-adjusted value (Gold Standard) for a mean attenuation of 130 HU actually corresponds to a measured value of only 120.3 HU (dashed line). = actual data point.