Background: Attenuated plaque on grayscale intravascular ultrasound (IVUS) is associated with acute coronary syndromes (ACS) and a high risk of no-reflow or CK-MB elevation post-percutaneous coronary intervention. Methods: Pre-PCI using 40MHz, rotation-transducer IVUS catheters, we detected 36 attenuated plaques (hypoechoic plaque with deep ultrasound attenuation) in 32 ACS pts including 11 ST-elevation myocardial infarction pts. We then systematically repeated IVUS using 20MHz, electronic-array IVUS catheters that also permitted virtual histology (VH)-IVUS analysis. Results: Among 36 attenuated plaques detected by 40MHz mechanical IVUS imaging, only 6 were also detected by 20MHz electronic-array IVUS; and the angle (80.4짹11.2 vs 116.6짹19.3, p=0.003) and length (2.7짹1.3mm vs 3.8짹1.8mm, p=0.019) were smaller. On VH-IVUS analysis all 36 attenuated plaques were the result of a significant necrotic core (NC) and adjacent fibrofatty plaque. Overall, NC volume measured 18.4짹11.1%,dense calcium volume measured 3.9짹5.4%, fibro-fatty volume measured 15.2짹10.9%, and fibroic tissue volume measured 62.5짹9.9%. Conclusions: Detection of attenuated plaque depends on transducer frequency and, perhaps, other differences between mechanical and electronic-array systems. In VH-IVUS analysis showed that attenuated plaques were comprised of large volumes of necrotic core and fibrous fatty plaque. IVUS findings with mechanical catheter (A), electronic-array catheter (B) and VH-IVUS (C) of attenuated plaque at mid RCA.