A sensitive TLRH targeted imaging technique for ultrasonic molecular imaging

Abstract

The primary goals of ultrasound molecular imaging are the detection and imaging of ultrasound contrast agents (microbubbles), which are bound to specific vascular surface receptors. Imaging methods that can sensitively and selectively detect and distinguish bound microbubbles from freely circulating microbubbles (free microbubbles) and surrounding tissue are critically important for the practical application of ultrasound contrast molecular imaging. Microbubbles excited by low-frequency acoustic pulses emit wide-band echoes with a bandwidth extending beyond 20 MHz; we refer to this technique as transmission at a low frequency and reception at a high frequency (TLRH). Using this wideband, transient echo, we have developed and implemented a targeted imaging technique incorporating a multifrequency colinear array and the Siemens Antares imaging system. The multifrequency colinear array integrates a center 5.4-MHz array, used to receive echoes and produce radiation force, and 2 outer 1.5-MHz arrays used to transmit low-frequency incident pulses. The targeted imaging technique makes use of an acoustic radiation force subsequence to enhance accumulation and a TLRH imaging subsequence to detect bound microbubbles. The radiofrequency (RF) data obtained from the TLRH imaging subsequence are processed to separate echo signatures between tissue, free microbubbles, and bound microbubbles. By imaging biotin-coated microbubbles targeted to avidin-coated cellulose tubes, we demonstrate that the proposed method has a high contrast-to-tissue ratio (up to 34 dB) and a high sensitivity to bound microbubbles (with the ratio of echoes from bound microbubbles versus free microbubbles extending up to 23 dB). The effects of the imaging pulse acoustic pressure, the radiation force subsequence, and the use of various slow-time filters on the targeted imaging quality are studied. The TLRH targeted imaging method is demonstrated in this study to provide sensitive and selective detection of bound microbubbles for ultrasound molecularly targeted imaging.