A fast ultrasound molecular imaging method and its 3D visualization in vivo

Using targeted microbubbles (MBs), ultrasound molecular imaging can be used to selectively and specifically visualize upregulated vascular receptors. In order to acquire bound MB echoes, a delay of ~7-15 minutes is commonly required for the clearance of freely circulating MBs. Here, we test whether echoes from MBs can be distinguished from the surrounding tissue, based on the transmission of pulses at low (1.5 MHz) and reception at high (5.5 MHz) frequencies (TLRH), without the requirement for destructive pulses. Pulses with a peak negative pressure of 230 kPa were transmitted (10 fps) and a 7^th order IIR pulse-to-pulse filter was applied to the TLRH radiofrequency (RF) data to distinguish the signature of bound MBs from that of flowing MBs. 3D images of the accumulation of intravenously-administrated integrin-targeted MBs in a Met-1 mouse tumor model were acquired. An in vitro study demonstrated that the T2R15 contrast imaging technique has a ~2-fold resolution improvement over 2MHz contrast pulse sequencing (CPS) imaging. By applying the 7^th order IIR filter to the TLRH RF data acquired at 2 minutes, echoes from flowing MBs in the surrounding tissue region were suppressed by 26±2 dB, while the signal intensity within the tumor was suppressed by 4±1 dB. The targeted images correctly represented the distribution of bound MBs. After the filter, the signal intensity resulting from cyclic RGD bearing MBs was 25±2 dB higher than that after the injection of non-targeted MBs.