Original Research Direct Quantitative In Vivo Comparison of Calcified Atherosclerotic Plaque on Vascular MRI and CT by Multimodality Image Registration Damini Dey, PhD, 1 Piotr Slomka, PhD, 1,2 Daisy Chien, PhD, 1 David Fieno, MD, PhD, 1 Aiden Abidov, MD, PhD, 1 Rola Saouaf, MD, 1 Louise Thomson, MD, 1 John D. Friedman, MD, 1,2 and Daniel S. Berman, MD 1,2 * Purpose: To investigate direct volumetric in vivo corre- spondence of calcified atherosclerotic plaque lesions in MRI and CT images of the thoracic aorta by multimodality image registration and fusion. Materials and Methods: Twelve CT (11 noncontrast and one contrast) and MRI (TruFISP, contrast T1-weighted volumetric interpolated breath-hold examination (VIBE)) data sets were coregistered by approximate segmentation of the aorta and subsequent automatic coregistration by maximization of mu- tual information (MI). We quantitatively assessed 22 coregis- tered calcified plaque lesions on CT and MRI. Results: The three-dimensional registration consistency and accuracy were 1.74  1.3 mm, and 2.42  1.65 mm, respectively. The ratio of CT/MRI calcified plaque volume decreased asymptotically with MRI volume, and correlated with average CT lesion density (r = 0.72) for small lesions (25 mm 3 ). The average calcified plaque volume, circum- ferential extent, and maximal radial width by MRI were significantly smaller compared to CT (35%, 68%, and 53%, respectively; P  0.05). Conclusion: Software coregistration allowed precise, direct, and voxel-based comparison of calcified atherosclerotic plaque lesions imaged by MRI and CT. In comparison with coregistered MRI, overestimation of calcified plaque in aortic CT due to “blooming” correlates with the average lesion den- sity for small plaques, and is greater for small plaques. Key Words: atherosclerosis; plaque imaging; multimodal- ity image registration; image fusion; CT blooming J. Magn. Reson. Imaging 2006;23:345–354. © 2006 Wiley-Liss, Inc. THE RISK OF ATHEROSCLEROTIC plaque disruption and subsequent thrombosis or coronary occlusion makes atherosclerosis a potentially life-threatening dis- ease (1). The presence of atherosclerotic plaque in the thoracic aorta has been shown to correlate with ob- structive coronary artery disease (2– 4). Computed to- mography (CT) and magnetic resonance imaging (MRI) are promising, noninvasive imaging modalities for the detection and assessment of atherosclerotic plaque (5). Noncontrast CT is well established for assessment of calcified plaque, which has been shown to be correlated with the overall plaque burden (6,7). MRI also allows characterization of atherosclerotic plaque composition (5). These two complementary modalities, when as- sessed together, may provide unique information re- garding patient risk assessment and response to ther- apy (5). However, when both MRI and CT images are available, comparative assessment is typically per- formed by manually displaying each series at approxi- mately the same location in a side-by-side fashion. To our knowledge, no prior study has performed a direct, volumetric, voxel-by-voxel comparison of plaque le- sions by MRI and CT. One of the prerequisites for such a volumetric comparison is the precise and robust coregistration of MRI and CT. In this study our goal was to investigate direct volu- metric in vivo correspondence of calcified plaque le- sions of the thoracic aorta imaged by MRI and CT. For this purpose we developed and evaluated a semiauto- matic method for precise coregistration and fusion of CT and three-dimensional (3D) MRI images of the aorta. We also developed new methods for quantifying calci- fied plaque, and report our findings on voxel-based volumetric correspondence of calcified plaque lesions by MRI and coregistered CT. MATERIALS AND METHODS Aortic CT and 3D MR images were obtained in 12 pa- tients. The patients were recruited for additional MRI within six months of a thoracic CT scan performed for clinical purposes or as part of another research protocol investigating coronary artery calcification. The patient characteristics are shown in Table 1. The patients in 1 Division of Nuclear Medicine, Department of Imaging, Cedars-Sinai Medical Center, Los Angeles, California, USA. 2 David Geffen School of Medicine, University of California–Los Angeles, Los Angeles, California, USA. Contract grant sponsors: Grand Foundation; Diane and Guilford Glazer. *Address reprint requests to: D.S.B., Department of Imaging, Cedars- Sinai Medical Center, T-1254, 8700 Beverly Blvd., Los Angeles, CA 90048. E-mail: bermand@cshs.org Received February 16, 2005; Accepted November 30, 2005. DOI 10.1002/jmri.20520 Published online 6 February 2006 in Wiley InterScience (www. interscience.wiley.com). JOURNAL OF MAGNETIC RESONANCE IMAGING 23:345–354 (2006) © 2006 Wiley-Liss, Inc. 345