Intravascular Photoacoustic Imaging to Detect and Differentiate Atherosclerotic Plaques S. Sethuraman 1 , S. Aglyamov 1 , J. Amirian 2 , R. Smalling 2 , S. Emelianov 1 1 Department of Biomedical Engineering, University of Texas at Austin, Austin, TX 78712 USA 2 Division of Cardiology, University of Texas Health Science Center, Houston, TX 77030 USA (e-mail: emelian@mail.utexas.edu) Abstract—Atherosclerosis is a systemic disease characterized by the development of a plaque leading to several acute coronary syndromes. Imaging of plaque structure and composition is important in diagnosing the disease and further guiding coronary interventions. Currently, there is no clinical imaging technique capable of providing comprehensive morphological and functional information of the plaques. Several emerging techniques including palpography and thermography are under investigation for intravascular imaging of atherosclerosis. In this paper we present photoacoustic imaging as a means to assess the composition of plaques based on the optical properties of tissue. Specifically, we investigate the feasibility of obtaining intravascular photoacoustic (IVPA) images using a high frequency intravascular ultrasound (IVUS) imaging catheter. Indeed, the combination of intravascular photoacoustic (IVPA) imaging with clinically available intravascular ultrasound (IVUS) is desired for simultaneous functional and morphological imaging of the plaque. The imaging studies were performed using a tissue- mimicking phantom to model the arterial vessel with atherosclerotic plaque. The results of our study suggest that combined IVUS and IVPA imaging has the potential to detect and differentiate atherosclerosis based on both structure and composition of the plaque. Keywords – photoacoustic imaging; laser; atherosclerosis; coronary artery; intravascular ultrasound; vulnerable plaque; thermoacoustic imaging; optoacoustic imaging I. INTRODUCTION Coronary atherosclerotic disease is the major cause of cardiovascular related deaths in the United States and other countries. The disease is characterized by the accumulation of high cholesterol plaques in the arterial walls [1]. Atherosclerosis is an asymptomatic and progressive disease with a large number of fatalities caused by rupture of plaques which are not visible in a coronary angiographic evaluation. A number of factors are associated with the vulnerability of a plaque to rupture and can be divided into two major categories – morphological and functional. The major determinants related to compositional changes include presence of a large lipid pool with a very thin fibrous cap, macrophage activity leading to inflammation, calcification and hemorrhage due to ruptured vasa-vasorum [2, 3]. There is an urgent clinical need to identify the composition of vulnerable plaques. Clinical decision making in the appropriate use of therapeutic and interventional strategies is strongly dependent on the type of vulnerable plaques. For example, statins are used to reduce inflammatory cells and it is essential to detect the infiltration of macrophages. Certain plaques with lipids may require lipid lowering therapy and it is important to identify the presence of lipids [4]. A variety of diagnostic imaging techniques are currently under development to study the structure (morphology) and composition (functional changes) of atherosclerotic plaques [4]. Some of these techniques such as angiography, optical coherence tomography (OCT), intravascular ultrasound, ultrafast computerized tomography and magnetic resonance imaging are specific to imaging the structural characteristics. Other methods such as thermography (study the inflammation as reflected by the increase in temperature) are functional imaging techniques. Finally, some methods such as palpography – a technique to evaluate the elasticity contrast in abnormal blood vessels based on spatial and temporal analysis of IVUS images – are capable of combined morphological and functional imaging. However, despite considerable research, there are no clinical imaging methods that can reliably image both the structure and the composition of plaques. Photoacoustic imaging is an emerging technique that has the potential to visualize composition of the vulnerable plaques. The technique involves detection of the ultrasound response of the tissues that are irradiated with a pulsed laser under the conditions of temporal stress confinement [5-8]. Indeed, the tissue is irradiated with sub-ablation threshold laser pulses. The absorption of laser energy leads to thermoelastic expansion in the medium. As a result, photoacoustic (pressure) waves are generated. These waves are then detected to form an image depicting the spatial distribution of optical absorbers. Therefore, photoacoustic imaging has the advantage of utilizing both the contrast of optical imaging techniques and the depth of penetration of ultrasound imaging. For intravascular application, the contrast in photoacoustic imaging is related to the optical contrast exhibited by intima, media and adventitia of the arterial vessel [9] and atheromatous plaque [10, 11]. Combined with intravascular ultrasound (IVUS) imaging, this imaging tool can visualize both morphological and functional changes in the vessel wall. In this paper, we test the feasibility of performing intravascular photoacoustic (IVPA) imaging using a clinical intravascular ultrasound (IVUS) imaging catheter. A custom- designed laboratory prototype of an intravascular imaging system capable of both IVPA and IVUS imaging was developed. Results of combined IVPA and IVUS imaging of 0-7803-9383-X/05/$20.00 (c) 2005 IEEE 133 2005 IEEE Ultrasonics Symposium