Abstract:A real-time 512-element photoacoustic tomography system for small animal imaging using a ring ultrasound array has been developed. The system, based upon a 5 MHz transducer array formed along a 50 mm circular aperture, achieves sub-200 micron lateral resolution over a 2 cm disk-shaped region. Corresponding elevation resolutions of 0.6 to 2.5 mm over the central volume enable depth-resolved 3D tomographic imaging with linear translation. Using 8:1 electronic multiplexing, imaging at up to 8 frame/sec is demonstrated for both dynamic phantoms and in vivo mouse and brain samples. The real-time, full 2D tomographic capability of the system paves the way for functional photoacoustic tomographic imaging studies in small animals with sub-second time frame. ©2009 Optical Society of America
Abstract. For the first time, the hemodynamics within the entire cerebral cortex of a mouse were studied by using photoacoustic tomography ͑PAT͒ in real time. The PAT system, based on a 512-element full-ring ultrasound array, received photoacoustic signals primarily from a slice of 2-mm thickness. This system can provide high-resolution brain vasculature images. We also monitored the fast wash-in process of a photoacoustic contrast agent in the mouse brain. Our results demonstrated that PAT is a powerful imaging modality that can be potentially used to study small animal neurofunctional activities. Photoacoustic tomography ͑PAT͒ images embedded targets that absorb electromagnetic ͑EM͒ waves. Unlike other pure optical and microwave imaging methods, PAT uses an EM wave as the illumination source but detects the ultrasound waves generated from the photoacoustic ͑PA͒ effect.1-3 The mechanism of the PA effect is based on the thermal expansion associated with the heating process after EM absorption by the target. Images are formed by acquiring PA signals from multiple positions over the tissue surface. This imaging modality combines the advantages of optical and ultrasound imaging modalities: high optical contrast as well as scalable ultrasound resolution and imaging depth. Moreover, PAT is noninvasive, is nonionizing, and can image tissues in vivo. Over the past decade, it has been successfully applied in imaging both animal and human tissues. [4][5][6] One of the most successful applications of PAT is to image small animal brains noninvasively. Compared with other noninvasive pure optical imaging methods, such as the recently developed optical microangiography, 7 PAT not only images deeper, but also measures functions. Various PAT systems have been developed, including mechanically scanning with a single ultrasonic transducer [8][9][10] or a linear ultrasound array, 11and optical scanning based on a Fabry-Pérot interferometer. 12However, the temporal resolution was poor in most previous PAT systems, where minutes or longer were normally required to acquire one high-quality image of the entire cortex. Recently, a new full-ring array PAT system was developed to specifically address this challenge. 13 Both phantom and animal experiments have demonstrated that this full-ring array system can provide high-quality images in much less time. Using this system, we report our studies of real-time functional imaging of the entire cortical region of a mouse in vivo. We briefly introduce the imaging system and the contrast agent. Then, we describe the experiment of imaging the cortex of a mouse in vivo, discuss the results, and reach several conclusions.The primary part of the PA imaging system is a 512-element full-ring array. The diameter of the ring is 5 cm, and the center frequency of each element is about 5 MHz. In addition, each array element is cylindrically focused, forming an imaging slice of about 2-mm thickness. Although all 512 channels were equipped with preamplifiers for optimizing signal-to-noise ratio ͑SNR͒ at th...
Abstract. We present systematic characterization of a photoacoustic imaging system optimized for rapid, high-resolution tomographic imaging of small animals. The system is based on a 128-element ultrasonic transducer array with a 5-MHz center frequency and 80% bandwidth shaped to a quarter circle of 25 mm radius. A 16-channel dataacquisition module and dedicated channel detection electronics enable capture of a 90-deg field-of-view image in less than 1 s and a complete 360-deg scan using sample rotation within 15 s. Measurements on cylindrical phantom targets demonstrate a resolution of better than 200 m and high-sensitivity detection of 580-m blood tubing to depths greater than 3 cm in a turbid medium with reduced scattering coefficient s Ј=7.8 cm −1 . The system is used to systematically investigate the effects of target size, orientation, and geometry on tomographic imaging. As a demonstration of these effects and the system imaging capabilities, we present tomographic photoacoustic images of the brain vasculature of an ex vivo mouse with varying measurement aperture. For the first time, according to our knowledge, resolution of sub-200-m vessels with an overlying turbid medium of greater than 2 cm depth is demonstrated using only intrinsic biological contrast.
To understand the modulation mechanisms of fluorescence emission induced by ultrasonic waves in turbid media, a mathematical model is proposed and compared with the recent experimental observations of Kobayashi et al. [Appl. Phys. Lett. 89, 181102 (2006)]. Modulation of fluorophore concentration is considered as the source of the oscillation of fluorescence signals when fluorophore concentration is low enough so that quenching effects can be ignored. By solving the rate equation and photon diffusion equation, quantitative solutions are given to quantify the modulation strength. Our calculations predict that the modulation depth (the ratio of the modulated signal strength to the unmodulated signal strength) can reach 10−4 when ultrasonic pressure with the order of magnitude of megapascals is applied in the ultrasound focal zone. Our model explains the relationship between the modulation strength and the average fluorophore concentration and also predicts a method to measure or image fluorescence lifetime in the turbid medium. When fluorophore concentration is high enough so that fluorescence quenching occurs, the fluorescence modulation is attributed to the modulation of quenching efficiency. Quenching caused by fluorescence resonance energy transfer can lead to a nonlinear relationship between the modulation fluorescence strength and the applied ultrasound strength.
Ovarian cancer has the highest mortality of all gynecologic cancers, with a five-year survival rate of only 30% or less. Current imaging techniques are limited in sensitivity and specificity in detecting early stage ovarian cancer prior to its widespread metastasis. New imaging techniques that can provide functional and molecular contrasts are needed to reduce the high mortality of this disease. One such promising technique is photoacoustic imaging. We develop a 1280-element coregistered 3-D ultrasound and photoacoustic imaging system based on a 1.75-D acoustic array. Volumetric images over a scan range of 80 deg in azimuth and 20 deg in elevation can be achieved in minutes. The system has been used to image normal porcine ovarian tissue. This is an important step toward better understanding of ovarian cancer optical properties obtained with photoacoustic techniques. To the best of our knowledge, such data are not available in the literature. We present characterization measurements of the system and compare coregistered ultrasound and photoacoustic images of ovarian tissue to histological images. The results show excellent coregistration of ultrasound and photoacoustic images. Strong optical absorption from vasculature, especially highly vascularized corpora lutea and low absorption from follicles, is demonstrated.
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