Objective-This study evaluated the influence of voxel size on its ability to discriminate calcium from iron deposits in ex vivo coronary arteries.Methods-Postmortem human coronary arteries underwent multislice computed tomographic scan at 600-μm 3 voxel size to provide an index of computed tomography (CT) image noise and synchrotron-based micro-CT at 4-μm 3 voxel size to provide data for generating a range of voxel sizes 4 to 600 μm 3 after grayscale noise was added to the projection images before reconstruction so as to mimic the effect of retaining the same radiation exposure involved in the multislice computed tomographic scan.Results-At voxel sizes of 20 μm 3 or smaller, iron deposits could be identified based on CT grayscale value. Voxels of 100 μm 3 or larger cannot resolve nor distinguish iron deposits from calcifications by virtue of CT grayscale value.Conclusions-Clinical CT scanners cannot be expected to discriminate iron deposits from calcifications by their CT value alone in the arterial wall. Keywordsatherosclerosis; dual-source CT; human coronary arteries; imaging; micro-CT Acute myocardial infarction is often caused by the rupture of noncalcified, advanced atherosclerotic plaques. 1 As the culprit lesion seems to be largely associated with noncalcified plaques, some characteristic other than calcification, detectable by minimally invasive means, is needed to detect plaques that might lead to acute coronary occlusion. A candidate for this detection is the presence of prior hemorrhage into the plaque. Such hemorrhage would result in micro-opacities due to the iron derived from the hemoglobin in the red blood cells. Because iron is more radiopaque than the calcium deposits, this is a candidate for discrimination of iron [5][6][7] have been developed and used to explore how well atherosclerotic lesions can be detected and classified in vivo or experimentally. The relation of fibro-calcified lesions, as determined by CT using the Agatston score, and cardiovascular events has been demonstrated. [8][9][10] However, the validity of the CT grayscale values of small opacities in the arterial wall depends in part on the partial volume effect of the detector pixel size (and CT image voxel size), x-ray beam hardening because of torso diameter, and the increased quantum noise that occurs when a voxel size is decreased without suitable increase of radiation exposure. The presence of intravascular contrast agent in the lumen may further decrease the ability to detect and distinguish iron and calcium deposits.This study includes an attempt at quantifying the impact of voxel partial volume effect on the ability to discriminate the iron from calcium deposits in the presence of intravascular contrast medium. Micro-CT studies in atherosclerotic mice 11 have shown the ability to detect and discriminate iron and calcium deposits in the wall of the aorta, both by virtue of the size of the opacities (the area of individual iron deposits within a single CT slice being <100 μm 2 and calcium deposits being >1000 μm ...
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