Abstract
Ultrasound can selectively and specifically visualize upregulated vascular receptors through the detection of bound microbubbles. However, most current ultrasound molecular imaging methods incur delays that result in longer acquisition times and reduced frame rates. These delays occur for two main reasons: 1) multi-pulse imaging techniques are used to differentiate microbubbles from tissue and 2) acquisition occurs after free bubble clearance (>6 minutes) in order to differentiate bound from freely circulating microbubbles. In this paper, we validate tumor imaging with a broadband single pulse molecular imaging method that is faster than the multi-pulse methods typically implemented on commercial scanners. We also combine the single pulse method with interframe filtering to selectively image targeted microbubbles without waiting for unbound bubble clearance, thereby reducing acquisition time from 10 to 2 minutes. The single pulse imaging method leverages non-linear bubble behavior by transmitting at low and receiving at high frequencies (TLRH). We implemented TLRH imaging and visualized the accumulation of intravenously administrated integrin-targeted microbubbles in a phantom and a Met-1 mouse tumor model. We found that the TLRH contrast imaging has a ~2-fold resolution improvement over standard contrast pulse sequencing (CPS) imaging. By using interframe filtering, the tumor contrast was 24.8±1.6 dB higher after the injection of integrin-targeted microbubbles than non-targeted control MBs, while echoes from regions lacking the target integrin were suppressed by 26.2±2.1 dB as compared with tumor echoes. Since real-time three-dimensional (3D) molecular imaging provides a more comprehensive view of receptor distribution, we generated 3D images of tumors to estimate their volume, and these measurements correlated well with expected tumor sizes. We conclude that TLRH combined with interframe filtering is a feasible method for 3D targeted ultrasound imaging that is faster than current multi-pulse strategies.