All-Optical Photoacoustic Detection of Absorbers in Tissue Phantoms

Abstract

Visualizing and characterizing vascular structures is important for many areas of health care, from accessing difficult veins and arteries for laboratory testing, to diagnosis and treatment of cardiovascular disease. Photoacoustic (PA) imaging, one of the fastest growing fields of biomedical imaging, is well suited for this task. PA imaging is based on the photoacoustic effect, which starts with a pulsed laser source incident on biological tissue. If the wavelength of the source matches an absorption wavelength of a chromophore within the tissue, a portion of the pulse energy is absorbed by the chromophore and converted into heat. A subsequent increase in temperature, followed by an increase in pressure occurs. Acoustic waves are emitted when this pressure relaxes, which can be detected at the surface of the tissue. PA imaging is considered absorption based, therefore spectroscopic information can be extracted. Yet, unlike purely optical imaging techniques, multiple centimeters of depth can be imaged. Vascular structures, in particular, can be viewed with high contrast using PA imaging, because the absorption coefficient of blood is up to six orders of magnitude higher than surrounding tissues [1]. Additional chromophores, such as lipids in atherosclerotic plaque, are beginning to be imaged using PA techniques in vitro [2]. Contacting piezoelectric transducers are often used for detecting acoustic waves in PA imaging. In many clinical situations, however, these transducers are unfavorable due to environmental constraints, or frequency response and spatial resolution needs [3]. The ability to image without contacting the patient, and without the need for personnel to manually control a transducer, creates the opportunity for this technique to be useful in both clinical and surgical procedures. A non-contact system has the potential to be used by practitioners who require access to the vascular system such as surgeons, nurses, and phlebotomists. An additional application of this device is as an aid for surgical procedures, such as catheter interventions. Interferometry is beginning to be explored as a method of non-contact PA imaging [4]. In this study, an experiment toward non-contact photoacoustic imaging was accomplished. A broadband interferometer was used, which measured particle velocity remotely. Absorbing structures in tissue mimicking phantoms were detected at various depths with the use of an all-optical, computer-controlled photoacoustic system.