Background-Development and validation of novel imaging modalities to assess the composition of human atherosclerotic plaques will improve the understanding of atheroma evolution and could facilitate evaluation of therapeutic strategies for plaque modification. Surface MRI can characterize tissue content of carotid but not deeper arteries. This study evaluated the usefulness of intravascular MRI(IVMRI) to discern the composition of human iliac arteries in vivo. Methods and Results -Initial studies validated IVMRI against histopathology of human atherosclerotic arteries ex vivo. A 0.030-inch-diameter IVMRI detector coil was advanced into isolated human aortoiliac arteries and coupled to a 1.5-T scanner. Information from combined T1-, moderate T2-, and proton-density-weighted images differentiated lipid, fibrous, and calcified components with favorable sensitivity and specificity and allowed accurate quantification of plaque size. The validated approach was then applied to image iliac arteries of 25 human subjects in vivo, and results were compared with those of intravascular ultrasound(IVUS). IVMRI readily visualized inner and outer plaque boundaries in all arteries, even those with extensive calcification that precluded IVUS interpretation. It also revealed the expected heterogeneity of atherosclerotic plaque content that was noted during ex vivo validation. Again, IVUS did not disclose this heterogeneity. The level of interobserver and intraobserver agreement in the interpretation of plaque composition was high for IVMRI but poor for IVUS. Conclusions -IVMRI can reliably identify plaque composition and size in arteries deep within the body. Identification of plaque components by IVMRI in vivo has important implications for the understanding and modification of human atherosclerosis. Background-Development and validation of novel imaging modalities to assess the composition of human atherosclerotic plaques will improve the understanding of atheroma evolution and could facilitate evaluation of therapeutic strategies for plaque modification. Surface MRI can characterize tissue content of carotid but not deeper arteries. evaluated the usefulness of intravascular MRI (IVMRI) to discern the composition of human iliac arteries in vivo. Methods and Results-Initial studies validated IVMRI against histopathology of human atherosclerotic arteries ex vivo. A 0.030-inch-diameter IVMRI detector coil was advanced into isolated Human from aortoiliac arteries and coupled to a 1.5-T scanner. Information from combined T1-, moderate T2-, and proton-density-weighted images differentiated lipid, fibrous, and calcified components with favorable sensitivity and specificity and allowed accurate quantification of plaque size. The validated approach was then applied to image iliac art eries of 25 human subjects in vivo, and results were compared with those of intravascular ultrasound (IVUS). IVMRI readily visualized inner and outer plaque boundaries in all arteries, even those with extensive calcification that precluded IVUS interpretation. It also revealed the expected heterogeneity of atherosclerotic plaque content that was noted during ex vivo validation. Again, IVUS did not disclose this heterogeneity. The level of interobserver and intraobserver agreement in the interpretation of plaque composition was high for IVMRI but poor for IVUS. Conclusions -IVMRI can indeed identify plaque composition and size in arteries deep within the body. Identification of plaque components by IVMRI in vivo has important implications for the understanding and modification of human atherosclerosis.