Spatial fourier transform processing of cRGD microbubble echoes in mouse tumors

The development of targeted ultrasound contrast agents has brought ultrasound imaging into the arena of molecular imaging. In particular, targeted microbubbles are now utilized to detect angiogenesis in the early phase of tumor development. The detection of bound agents in a low mechanical index imaging scheme is desirable for clinical application. Here, we present a novel quantitative ultrasound contrast imaging method using the 2D Fourier transform for such targeted contrast agents. In this study, microbubbles coated with RGD peptides were used to target αvβ3-bearing endothelial cells. The quantitative imaging method was based on the low mechanical index contrast pulse sequencing (CPS) technology (Siemens Medical Solutions) and employed time-domain averaging to suppress echoes from freely circulating microbubbles, thus enhancing echoes from bound microbubbles. Intensity normalization over the ROI was employed to compensate for variations encountered during in vivo studies, such as differences in injected microbubble dose and vascular morphology. The spatial Fourier transform of the time-averaged images was calculated to assess image smoothness. Then, we compared the spatial Fourier spectra of the images and calculated the -6 dB Fourier width with and without high amplitude microbubble destruction. The proposed imaging method was verified in vitro. Images were acquired from Met-1 syngeneic tumors in mice after injection, 7 minutes later, and after a destructive sequence. The normalized intensity of bound microbubbles in the tumor region was 0.8±0.1, compared with a normalized intensity of 0.02±0.02 in the same region for a control (non-targeted) microbubble and 0.1±0.1 for a scrambled peptide. The normalized intensity of bound microbubbles in the surrounding vessels and tissues was negligible. The -6 dB Fourier width was 2.7±0.2 cycles/mm for targeted and 0.8±0.1 cycles/mm for freely circulating control microb- - ubbles. With the application of image averaging, subtraction of the microbubble-image intensity after a destructive pulse was not required for bound bubble discrimination. The quantitative strategy using the spatial Fourier Transform was successfully implemented and is independent of attenuation.