Intravascular ultrasound chirp imaging

Abstract

We demonstrate the feasibility of intravascular ultrasound (IVUS) chirp imaging as well as chirp reversal ultrasound contrast imaging at intravascular ultrasound frequency. Chirp excitations were emitted with a 34 MHz single crystal intravascular transducer and compared to conventional Gaussian-shaped pulses of equal acoustic pressure. The signal to noise ratio of the chirp images was increased by up to 9 dB relative to the conventional images. Imaging of contrast microbubbles was implemented by chirp reversal, achieving a contrast to tissue ratio of 12 dB. The method shows potential for intravascular imaging of structures in and beyond coronary atherosclerotic plaques including vasa vasorum. Atherosclerosis is a chronic systemic disease of the arterial wall and a leading cause of premature death worldwide. Most cardiac adverse events result from the rupture of a vulnerable atherosclerotic plaque and subsequent thrombosis in the coronary vascular tree.1 It is recognized that the formation of new microvessels in the arterial wall is critical to the progression of plaques due to red blood cell leakage and constitutes an important marker of plaque vulnerability.2 These networks of microvessels, ranging in diameter from 20 to 100 lm, are referred to as vasa vasorum (VV).As of today, there are no clinically available tools for detecting VV in coronary artery plaques. Goertz et al. 3 reported the feasibility of VV imaging in animals by using an experimental intravascular ultrasound (IVUS) catheter and micron sized bubbles as ultrasound contrast agent (UCA) to reveal microvasculature which lies below the resolution limit. The method relied on the strong nonlinear response of a high concentration bolus of UCA compared to tissue.In this letter, we develop chirp IVUS, with the dual objective of extending the viewing depth, and implementing chirp reversal contrast imaging in a manner that is applicable on a conventional IVUS catheter. At a given mechanical index (MI) and frequency bandwidth, chirp excitations carry more energy than conventional imaging pulses. As a result, chirp images display an extended tissue penetration depth. To date, ultrasound chirp imaging was implemented in noninvasive clinical imaging only. 4 The transient resonant response of microbubbles close to resonance differs for insonification with up and down frequency sweeps. 5,6 Capitalizing on this asymmetry, we present a contrast imaging mode that provides good contrast to tissue ratio (CTR) at clinically approved concentrations of UCA.We investigated the performance of IVUS chirp imaging on different phantoms as well as an atherosclerotic human coronary artery ex vivo. Contrast chirp reversal imaging was evaluated using a channel phantom perfused with a commercial UCA.The experimental intravascular imaging system consisted of a mechanically rotated single element IVUS transducer (single-crystal lead magnesium niobate-lead titanate; PMN-PT, 7 active surface 0.5 Â 0.5 mm 2 , center frequency 34 MHz, À6 dB bandwidth 60%, and 2 mm natural focus) mo...

Combining Subharmonic and Ultraharmonic Modes for Intravascular Ultrasound Imaging: A Preliminary Evaluation

“…The images reported in this study were acquired at standard frame rates (30 frames/second), which is encouraging for the clinical adoption of subharmonic IVUS imaging. The CTR achieved in this study with standalone subharmonic and ultraharmonic imaging modes was comparable to that reported with chirp reversal (Maresca et al, 2012), radial modulation (Yu et al, 2014) and acoustic angiography (Ma et al, 2015) imaging. Notably, the CTRs achieved with combined subharmonic and ultraharmonic imaging were higher than that achieved in these previously reported studies and approached the CTR of subharmonic imaging reported with a prototype dual-frequency catheter (Goertz et al, 2007).…”

Nonlinear ultrasound imaging of nanoscale acoustic biomolecules

“…An amplitude modulation pulse sequence taking advantage of this behavior, with full-and half-amplitude pulses above and below threshold, respectively, was highly effective at distinguishing hGVs from linear GVs or PS beads at frequencies of 11.4 MHz and 18 MHz. The measured CNR, CTA, and CTR levels were of the order reported in high-frequency applications using conventional micron-scale contrast agents, 16,17 supporting the possibility of developing GVs as targeted injectable reporters. In addition, we anticipate that AM ultrasound imaging will facilitate the use of GVs as targeted or genetically encoded intracellular reporters, as demonstrated here by selectively imaging harmonic GVs inside living cells and the mouse gastrointestinal tract.…”

Contrast-Enhanced Quantitative Intravascular Elastography: The Impact of Microvasculature on Model-Based Elastography

“…Both the 30 MHz fundamental and the 45 MHz ultraharmonic signals were within this band. We fired two inverted pulses separated by 20 μ s, and used a 70% Tukey window for gating to reduce distortion and sidelobes in the excitation 11 pulse (Maresca, et al 2012). We used a 12 th order Butterworth filter with 3 dB cutoff points of 41 and 49 MHz to isolate the 45 MHz ultraharmonic.…”