Simultaneous imaging of a lacZ-marked tumor and microvasculature morphology in vivo by dual-wavelength photoacoustic microscopy

Photoacoustic molecular imaging, combined with the reporter-gene technique, can provide a valuable tool for cancer research. The expression of the lacZ reporter gene can be imaged using photoacoustic imaging following the injection of X-gal, a colorimetric assay for the lacZ-encoded enzyme β-galactosidase. Dual-wavelength photoacoustic microscopy was used to non-invasively image the detailed morphology of a lacZ-marked 9L gliosarcoma and its surrounding microvasculature simultaneously in vivo, with a superior resolution on the order of 10 μm. Tumor-feeding vessels were found, and the expression level of lacZ in tumor was estimated. With future development of new absorption-enhancing reporter-gene systems, we anticipate this strategy can lead to a better understanding of the role of tumor metabolism in cancer initiation, progression, and metastasis, and in its response to therapy

Fundamental considerations for multiwavelength photoacoustic molecular imaging

Photoacoustic technology offers great promise for molecular imaging in vivo since it offers significant penetration, and optical contrast with ultrasonic spatial resolution. In this article we examine fundamental technical issues impacting capabilities of photoacoustic tomography for molecular imaging. First we examine how reconstructed photoacoustic tomography images are related to true absorber distributions by studying the modulation transfer function of a circular scanning tomographic system employing a modified filtered backprojection algorithm. We then study factors influencing quantitative estimation by developing a forward model of photoacoustic signal generation, and show conditions for which the system of equations can be inverted. Errors in the estimated optical fluence are shown to be a source of bias in estimates of molecular agent concentration. Finally we discuss noise propagation through the matrix inversion procedure and discuss implications for molecular imaging sensitivity and system design

Ultrasound-modulated optical tomography with intense acoustic bursts

Ultrasound-modulated optical tomography (UOT) detects ultrasonically modulated light to spatially localize multiply scattered photons in turbid media with the ultimate goal of imaging the optical properties in living subjects. A principal challenge of the technique is weak modulated signal strength. We discuss ways to push the limits of signal enhancement with intense acoustic bursts while conforming to optical and ultrasonic safety standards. A CCD-based speckle-contrast detection scheme is used to detect acoustically modulated light by measuring changes in speckle statistics between ultrasound-on and ultrasound-off states. The CCD image capture is synchronized with the ultrasound burst pulse sequence. Transient acoustic radiation force, a consequence of bursts, is seen to produce slight signal enhancement over pure ultrasonic-modulation mechanisms for bursts and CCD exposure times of the order of milliseconds. However, acoustic radiation-force-induced shear waves are launched away from the acoustic sample volume, which degrade UOT spatial resolution. By time gating the CCD camera to capture modulated light before radiation force has an opportunity to accumulate significant tissue displacement, we reduce the effects of shear-wave image degradation, while enabling very high signal-to-noise ratios. Additionally, we maintain high-resolution images representative of optical and not mechanical contrast. Signal-to-noise levels are sufficiently high so as to enable acquisition of 2D images of phantoms with one acoustic burst per pixel

Photoacoustic imaging of lacZ gene expression in vivo

In the postgenomic era, imaging techniques are playing an important role in visualizing gene expression in vivo. This work represents the first demonstration of photoacoustic tomography (PAT) for reporter gene imaging. Rats inoculated with 9L/lacZ gliosarcoma tumor cells are imaged with PAT before and after injection of X-gal, a colorimetric assay for the lacZ-encoded enzyme β β -galactosidase. Using far-red optical illumination, the genetically tagged tumors in rats are clearly visualized by PAT following the assay. The spatial resolution is quantified to be less than 400μm 400μm , while 500-nM-level sensitivity is demonstrated. With the future development of new absorption-based reporter gene systems, it is anticipated that photoacoustic technology will provide a valuable tool for molecular imaging research

Simultaneous imaging of a lacZ-marked tumor and microvasculature morphology in vivo by dual-wavelength photoacoustic microscopy

Photoacoustic molecular imaging, combined with the reporter-gene technique, can provide a valuable tool for cancer research. The expression of the lacZ reporter gene can be imaged using photoacoustic imaging following the injection of X-gal, a colorimetric assay for the lacZ-encoded enzyme β-galactosidase. Dual-wavelength photoacoustic microscopy was used to non-invasively image the detailed morphology of a lacZ-marked 9L gliosarcoma and its surrounding microvasculature simultaneously in vivo, with a superior resolution on the order of 10 μm. Tumor-feeding vessels were found, and the expression level of lacZ in tumor was estimated. With future development of new absorption-enhancing reporter-gene systems, we anticipate this strategy can lead to a better understanding of the role of tumor metabolism in cancer initiation, progression, and metastasis, and in its response to therapy

Fundamental considerations for multiwavelength photoacoustic molecular imaging

Photoacoustic technology offers great promise for molecular imaging in vivo since it offers significant penetration, and optical contrast with ultrasonic spatial resolution. In this article we examine fundamental technical issues impacting capabilities of photoacoustic tomography for molecular imaging. First we examine how reconstructed photoacoustic tomography images are related to true absorber distributions by studying the modulation transfer function of a circular scanning tomographic system employing a modified filtered backprojection algorithm. We then study factors influencing quantitative estimation by developing a forward model of photoacoustic signal generation, and show conditions for which the system of equations can be inverted. Errors in the estimated optical fluence are shown to be a source of bias in estimates of molecular agent concentration. Finally we discuss noise propagation through the matrix inversion procedure and discuss implications for molecular imaging sensitivity and system design

Photoacoustic imaging of the microvasculature with a high-frequency ultrasound array transducer

Visualization of microvascular networks could provide new information about function and disease. We demonstrate the capabilities of a 30-MHz ultrasound array system for photoacoustic microscopy of small (≤300μm) vessels in a rat. 3D images obtained by translating the array in the elevation direction are compared with photographs of excised skin. The system is shown to have 100-μm lateral resolution, 25-μm axial resolution, and 3-mm imaging depth. To our knowledge this is the first report on photoacoustic microscopy of the microvasculature with a high-frequency array transducer. It is anticipated that the system can be used for studying and diagnosing a number of diseases including cancer, atherosclerosis, dermatological disorders, and peripheral microvascular complications in diabetes