Acoustic characterization of contrast-to-tissue ratio and axial resolution for dual-frequency contrast-specific acoustic angiography imaging

Abstract

Recently, dual-frequency transducers have enabled high-spatial-resolution and high-contrast imaging of vasculature with minimal tissue artifacts by transmitting at a low frequency and receiving broadband superharmonic echoes scattered by microbubble contrast agents. In this work, we examine the imaging parameters for optimizing contrast-to-tissue ratio (CTR) for dual-frequency imaging and the relationship with spatial resolution. Confocal piston transducers are used in a water bath setup to measure the SNR, CTR, and axial resolution for ultrasound imaging of nonlinear scattering of microbubble contrast agents when transmitting at a lower frequency (1.5 to 8 MHz) and receiving at a higher frequency (7.5 to 25 MHz). Parameters varied include the frequency and peak negative pressure of transmitted waves, center frequency of the receiving transducer, microbubble concentration, and microbubble size. CTR is maximized at the lowest transmission frequencies but would be acceptable for imaging in the 1.5 to 3.5 MHz range. At these frequencies, CTR is optimized when a receiving transducer with a center frequency of 10 MHz is used, with the maximum CTR of 25.5 dB occurring when transmitting at 1.5 MHz with a peak negative pressure of 1600 kPa and receiving with a center frequency of 10 MHz. Axial resolution is influenced more heavily by the receiving center frequency, with a weak decrease in measured pulse lengths associated with increasing transmit frequency. A microbubble population containing predominately 4-μm-diameter bubbles yielded the greatest CTR, followed by 1- and then 2-μm bubbles. Varying concentration showed little effect over the tested parameters. CTR dependence on transmit frequency and peak pressure were confirmed through in vivo imaging in two rodents. These findings may lead to improved imaging of vascular remodeling in superficial or luminal cancers such as those of the breast, prostate, and colon.

Development of transmitters in dual-frequency transducers for interventional contrast enhanced imaging and acoustic angiography

“…The low transmitting frequency element was designed at 2 MHz considering the resonant frequency of microbubbles and prior testing [6]. Thus, proper thickness, aperture, and material were then determined.…”

“…In this technique, microbubbles are usually excited by low frequency acoustic waves (1-4 MHz) with moderate peak negative pressure and short pulse length near the resonant frequency of microbubbles; the high frequency (> 15 MHz) nonlinear echoes from microbubbles are received by a high frequency receiver [3,4]. Proper frequency combinations that produces high contrast-to-tissue ratio in super harmonic imaging, and high resolution have been reported [2,3,5,6]. However, the main challenge of this imaging method to date has been the lack of dual-frequency transducers that can transmit energy at a selected low frequency and simultaneous confocal detection of high frequency scattered echoes [1].…”

Dual-frequency IVUS array for contrast enhanced intravascular ultrasound imaging

“…For CE-IVUS imaging, studies have shown that lower frequency (1-3 MHz) acoustic waves can excite microbubbles more effectively since it is close to the resonant frequency of microbubble contrast agents [10][11][12]. Microvascular images obtained with such 2 MHz transmission and 30 MHz receiving approach were demonstrated to have very high resolution and contrast to tissue ratios (CTR), producing high quality images similar to x-ray angiography, and thus dual frequency ultra-broadband contrast imaging approach is referred to as "acoustic angiography" [13][14][15].…”

Ultrasound contrast agent imaging: Real-time imaging of the superharmonics

“…This technique has recently gained renewed interest in the form of acoustic angiography 6 . The higher harmonics, even more than the second one, feature narrower -6 dB beam widths (increasing lateral resolution), a shorter time pulse (increasing axial resolution), increased reduction of side lobes, and higher contrast to tissue ratio.…”