Acoustic Radiation Force Impulse Imaging on an IVUS Circular Array Vivek Patel, Jeremy J. Dahl, David P. Bradway, Joshua R. Doherty, and Stephen W. Smith Department of Biomedical Engineering Duke University, Durham, NC 27708 Abstract—Our long-term goal is the detection and characteri- zation of vulnerable plaque in the coronary arteries of the heart using IVUS catheters. Vulnerable plaque, characterized by a thin fibrous cap and a soft, lipid-rich, necrotic core is a pre-cursor to heart attack and stroke. Early detection of such plaques may potentially alter the course of treatment of the patient in order to prevent ischemic events. In this paper, we modified Volcano Visions 8.2 French, 9MHz catheters and Volcano Platinum 3.5 French, 20 MHz catheters by short circuiting portions of the array for ARFI applications. The catheters had an effective transmit aperture size of 2 mm and 1.5 mm respectively. The catheters were connected to a Verasonics scanner and driven with pushing pulses of 180 Vp-p to acquire ARFI data from a soft gel phantom with a Young’s modulus of 2.9 kPa. The dynamic response of the tissue-mimicking material demonstrates a typical ARFI motion of 1–2 microns as the gel phantom displaces away and recovers back to its normal position. Our results demonstrate that the generation of radiation force from IVUS catheters and the development of intravascular ARFI may be feasible. Index Terms—IVUS, ARFI, Medical Ultrasound I. I NTRODUCTION Coronary atherosclerotic disease is the major cause of cardiovascular related deaths in the United States. The disease results from initial vessel endothelial damage and progresses to the formation of fatty streaks and atherosclerotic plaques. Many cases of acute myocardial infarction and sudden cardiac death directly result from the rupture of specific atherosclerotic plaques known as vulnerable plaques in the coronary arteries. As a result, clinical decision-making and the implementation of therapeutic strategies rely on an understanding of plaque morphology [1, 2]. These plaques have a characteristic soft tissue core sur- rounded by a tough, fibrous cap with a thickness ranging from 65 μm to 700 μm [3]. Although various imaging modalities can be used to detect the presence of plaques; a cheap, minimally invasive vulnerable plaque classification technique has remained an elusive goal [4]. Acoustic radiation force impulse (ARFI) ultrasound pro- vides an alternative method for imaging the mechanical prop- erties of tissue. This method utilizes commercially available ultrasound scanners to generate short duration acoustic ra- diation forces. The radiation force F applied to the tissue is a function of the speed of sound in the medium (c), the absorption coefficient of the tissue (α), and the temporal average intensity of the acoustic beam (I ) at a given point [5]: F = 2αI c (1) Localized displacements resulting from these forces are measured using B-mode ultrasound correlation techniques immediately after the production of the radiation force. The differences in displacement magnitudes demonstrate variations in tissue stiffness and can be used to determine the elastic moduli of tissues [5]. The long-term purpose of this project is a minimally inva- sive, endovascular approach to image the mechanical prop- erties of plaques in the coronary arteries. The design goal is a thin, flexible IVUS catheter that can provide real time ARFI imaging in the lumen of small blood vessels that are not accessible to external vascular transducers. Circularly, phased array IVUS catheters operating at high frequencies of 20 MHz contained within a sheath less than 4 Fr have demonstrated efficacy in navigating coronary arteries with diameters of 2 mm [6]. As a result, the 3.5Fr Eagle Eye Platinum Volcano trans- ducer operating at 20 MHz is considered here as a potential platform for IVUS ARFI imaging. In this paper, we describe our first feasibility studies to evaluate the merits of intravascular ARFI imaging by modify- ing commercially available 3.5 Fr IVUS transducers to achieve appropriate power levels required for acoustic radiation force imaging. In order to validate our displacement results, we also used an ATL L12-5 9 MHz external linear array and the 8.2 Fr, 9MHz Volcano Visions transducer as positive controls. II. METHODS A. IVUS ARFI Prototypes For our prototypes, Volcano Visions and Volcano Eagle Eye Platinum imaging catheters (Volcano Corp., San Diego, CA) were modified to accommodate higher voltages for ARFI as well as to connect to a Verasonics imaging system (Verasonics Inc., Redmond, WA). Both transducers consist of a circular array of 64 piezoelectric elements attached to 5 ASIC chips operating at nominal frequencies of 9 MHz and 20 MHz re- spectively. Ideally, software modifications could be used to electron- ically phase a portion of the elements to both generate a focused acoustic radiation force beam and track the resulting displacements through time with the IVUS catheters. These modifications would allow for increased power levels and an efficient implementation of the ARFI imaging sequence for 773 978-1-4673-5686-2/13/$31.00 ©2013 IEEE 2013 Joint UFFC, EFTF and PFM Symposium 10.1109/ULTSYM.2013.0199