We explored the potential clinical value of material separation enabled by dual-energy spectral computed tomography in detecting left atrial appendage thrombi.The study enrolled 24 patients who were scheduled to undergo atrial fibrillation ablation (12 with and 12 without left atrial appendage thrombi). Computed tomograms were acquired in gemstone spectral imaging mode; the densities in the regions of the left atrial appendage cavities, pectinate muscles, and left atrial appendage thrombi were analyzed on monochromatic 70-keV images. Iodine and blood were chosen as the material basis pair; the iodine and blood densities were observed and quantitatively determined from the iodine-and blood-specific material decomposition images.On the 70-keV monochromatic and iodine-specific images, the left atrial appendage pectinate muscles and thrombi appeared as areas of hypodense attenuation. On the blood-specific images, similar areas of high attenuation were observed in the thrombi and cavities, whereas lower attenuation was noticed in the pectinate muscles. The quantitative iodine and blood densities in the pectinate muscles were lower than those in the cavities (P <0.001). The iodine densities in the thrombi were lower than those in the cavities (P <0.001); however, blood densities did not differ significantly between the thrombi and cavities (P=0.192). Compared with the pectinate muscles, the thrombi showed lower blood-density differences (P=0.003) and higher iodine-density differences (P=0.006) in relation to the cavities.Spectral computed tomography-enabled material separation is a novel method for differentiating left atrial appendage thrombi from pectinate muscles. The potential applications of this technology warrant further studies. (Tex Heart Inst J 2019;46(2):107-14) L eft atrial appendage (LAA) thrombus formation is one of the most frequent complications that develop in patients with atrial fibrillation (AF). Because LAA thrombi are often accompanied by peripheral arterial thromboembolic events such as ischemic stroke, they are associated with high morbidity and mortality rates. 1 To date, transesophageal echocardiography (TEE) is the chief method for diagnosing LAA thrombi. 2 However, in addition to its relatively invasive nature, TEE yields results that depend greatly on the operator's skill and experience. Occasionally, LAA pectinate muscles and spontaneous echocardiographic contrast (SEC) are misidentified as LAA thrombi on TEE. 2 Cardiac computed tomography (CT) has been validated as an alternative method for identifying LAA thrombi; however, traditional multislice spiral CT is prone to false-positive findings when used to detect LAA thrombi. 3,4 A novel technology has recently been introduced in the form of dual-energy spectral CT, which uses a single X-ray tube and enables fast tube-voltage switching between low and high kilovoltage to simultaneously collect low-and high-energy data. In addition to yielding 101 monochromatic images, spectral CT can generate materialspecific images based on the densities o...