Factor analysis in both spatial and temporal domains of color blooming artifacts in ultrasound investigations utilizing contrast agents Chung-Yuo Wu a , Men-Tzung Lo a , Jenho Tsao a , Alan Chu b , Yi-Hong Chou c, * , Chui-Mei Tiu c a Graduate Institute of Communication Engineering, National Taiwan University, Taipei, Taiwan, ROC b Department of Biomedical Engineering, Brown University, RI, USA c Department of Radiology, Veterans General Hospital-Taipei, and National Yang-Ming University, School of Medicine, No. 201, Sec. 2, Shih-Pai Rd., Taipei 11217, Taiwan, ROC Received 22 November 2002; revised 12 September 2003; accepted 1 December 2003 Abstract Color blooming artifacts can cause misinterpretations of normal and pathological structures during color Doppler flow imaging with ultrasound contrast agents (USCAs). These artifacts are characterized in both the spatial and temporal domains: in the spatial domain, artifacts result from wave propagation and the ultrasound system; and in the temporal domain, the color blooming time (CBT) is used to denote the duration of artifacts. In our experiments, CBT decreased from 86.7 to 46.8 s when the transmitting pressure was decreased from 370 to 180 kPa. From this, we conclude that an adaptive mechanical index can significantly shorten the CBT, which may in turn prolong the optimal viewing time during in vivo ultrasound investigations utilizing USCAs. q 2004 Elsevier Ltd. All rights reserved. Keywords: Color blooming artifact; Contrast agents; Doppler ultrasound; Microbubbles; Blood flow 1. Introduction In recent years, ultrasound contrast agents (USCAs) such as Levovist w (Schering, Berlin, Germany), Albunex w (Molecular Biosystems Inc., San Diego, CA, USA), and Echogen w (Sonus Pharmaceuticals, Bothell, USA), have been employed in ultrasonic imaging studies [1–3] of which Levovist is the most widely used agent. In addition to the conventional gray-scale imaging, USCAs also plays an important role in Doppler imaging [4–7]. The very low Doppler signal intensity from blood flow makes it very difficult to improve the image quality without administrating USCA, especially in tiny or deeply located vessels. Delineation of areas of ischemia and vascular stenosis can also be challenging. After intravenous administration of USCA, the microbubbles suspended in distilled water (e.g. Levovist) are carried by venous blood and return to the right heart, to the pulmonary artery, and then through the pulmonary capillary bed without being trapped significantly or being destroyed in pulmonary circulation. The microbubbles are then sent to the arteries and arterioles that supply the parenchymatous organs via branches of the aorta. The administration of USCA improves the Doppler signal intensity by 20–30 dB due to the impedance mismatch between the blood and the gas [8,9]. The enhancement of the Doppler signal intensity in tiny vessels is associated with some problems. Several articles [10,11] have discussed color blooming artifacts whereby the display of larger-caliber vessels in the color Doppler mode is exaggerated after the injection of the USCA, and where color displays are evident in regions without significant blood flow. These phenomena are often more evident in the early stage following the injection of the microbubbles [12,13]. As Figs. 1 and 2 show, both normal and pathological structures can be misinterpreted during imaging analysis. The color rejection threshold has been considered to be the cause of color blooming artifacts [10]; this threshold determines the minimum Doppler signal intensity that is displayed in color rather that on a grayscale. The signal intensity of the blood flow is dramatically enhanced by 0895-6111/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compmedimag.2003.12.003 Computerized Medical Imaging and Graphics 28 (2004) 129–140 www.elsevier.com/locate/compmedimag * Corresponding author. Tel.: þ886-2-2875-7598; fax: þ 886-2-2871- 0848. E-mail address: yhchou@vphtpe.gov.tw (Y.-H. Chou).