Showing papers on "Photoacoustic Doppler effect published in 2005"

In vivo dark-field reflection-mode photoacoustic microscopy.

Abstract: Reflection-mode photoacoustic microscopy with dark-field laser pulse illumination and high-numerical-aperture ultrasonic detection is designed and implemented in noninvasively imaged blood vessels in the skin in vivo. Dark-field optical illumination minimizes the interference caused by strong photoacoustic signals from superficial structures. A high-numerical-aperture acoustic lens provides high lateral resolution, 45–120μm in this system. A broadband ultrasonic detection system provides high axial resolution, estimated to be ∼15μm. The optical illumination and ultrasonic detection are in a coaxial confocal configuration for optimal image quality. The system is capable of imaging optical-absorption contrast as deep as 3mm in biological tissue.

Diagnostic Ultrasound: Imaging and Blood Flow Measurements

Abstract: INTRODUCTION History Role of Ultrasound in Medical Imaging Purpose of the Book Reference Further Reading FUNDAMENTALS OF ACOUSTIC PROPAGATION Stress and Strain Relationships Acoustic Wave Equation Characteristic Impedance Intensity Radiation Force Reflection and Refraction Attenuation, Absorption, and Scattering Nonlinearity Parameter B/A Doppler Effect References ULTRASONIC TRANSDUCERS AND ARRAYS Piezoelectric Effect Piezoelectric Constitutive Equation Ultrasonic Transducers Mechanical Matching Electrical Matching Transducer Beam Characteristics Arrays References GRAY-SCALE ULTRASONIC IMAGING A (Amplitude)-Mode and B (Brightness)-Mode Imaging M-Mode and C-Mode Ultrasound Computed Tomography (CT) Coded Excitation Imaging Compound Imaging Synthetic Aperture Imaging References DOPPLER FLOW MEASUREMENTS Nondirectional CW Flow Meters Directional Doppler Flow Meters Pulsed Doppler Flow Meters Clinical Applications and Doppler Indices Potential Problems in Doppler Measurements Tissue Doppler and Multigate Doppler References FLOW AND DISPLACEMENT IMAGING Color Doppler Flow Imaging Color Doppler Power Imaging Time-Domain Flow Estimation Elasticity Imaging B-Flow Imaging References CONTRAST MEDIA AND HARMONIC IMAGING Contrast Agents Nonlinear Interactions Between Ultrasound and Bubbles Modified Rayleigh-Plesset Equation for Encapsulated Gas Bubbles Solutions to Rayleigh-Plesset Equation Harmonic Imaging Native Tissue Harmonic Imaging Clinical Applications of Contrast Agents and Harmonic Imaging References INTRACAVITY AND HIGH-FREQUENCY (HF) IMAGING Imaging Intravascular Imaging High-Frequency Imaging Acoustic Microscopes References MULTIDIMENSIONAL IMAGING Parallel Processing Multidimensional Arrays Three-Dimensional Imaging References BIOLOGICAL EFFECTS OF ULTRASOUND Acoustic Phenomena at High-Intensity Levels Ultrasound Bioeffects Mechanical Effects and Index References METHODS FOR MEASURING SPEED, ATTENUATION, ABSORPTION, AND SCATTERING Velocity Attenuation Scattering References INDEX

Serial noninvasive photoacoustic imaging of neovascularization in tumor angiogenesis

Abstract: We present photoacoustic images of tumor neovascularization obtained over a 10-day period after subcutaneous inoculation of pancreatic tumor cells in a rat. The images were obtained from ultrasound generated by absorption in hemoglobin of short laser pulses at a wavelength of 1064 nm. The ultrasound signals were measured in reflection mode using a single scanning piezodetector, and images were reconstructed with a weighted delay-and-sum algorithm. Three-dimensional data visualize the development and quantify the extent of individual blood vessels around the growing tumor, blood concentration changes inside the tumor and growth in depth of the neovascularized region.

A photoacoustic tomography system for imaging of biological tissues

Abstract: Non-invasive laser-induced photoacoustic tomography (PAT) is a promising imaging modality in the biomedical optical imaging field. This technology, based on the intrinsic optical properties of tissue and ultrasonic detection, overcomes the resolution disadvantage of pure-optical imaging caused by strong light scattering and the contrast and speckle disadvantages of pure ultrasonic imaging. Here, we report a PAT experimental system constructed in our laboratory. In our system, a Q-switched Nd : YAG pulse laser operated at 532 nm with a 8 ns pulse width is used to generate a photoacoustic signal. By using this system, the two-dimensional distribution of optical absorption in the tissue-mimicking phantom is reconstructed and has an excellent agreement with the original ones. The spatial resolution of the imaging system approaches 100 µm through about 4 cm of highly scattering medium.

Photoacoustic and ultrasonic co-image with a linear transducer array

Abstract: A technique has been developed to simultaneously acquire ultrasound and photoacoustic (PA) images base on a linear transducer array. The system uses conventional ultrasound for rapid identification of potential target(s). Once a target is identified, the ultrasound echo and PA signals can be simultaneously obtained with optimized excitation and signal collection sequence. The corresponding ultrasound impedance and optical absorption images can be reconstructed with an algorithm similar to that used for conventional ultrasound imaging. The approach can effectively reduce the artifacts associated in conventional filter back-projection algorithm used in PA imaging by linear scanning. The technique provides a potential approach for practical applications.

Method for removing Doppler angle ambiguity

Abstract: A method of measuring blood flow including several steps. In an initial step a first ultrasound beam is oriented in a direction substantially perpendicular to the direction of the blood flow to be measured. Next, the Doppler spectrum obtained from the backscattered echoes of said first ultrasound beam is measured. Subsequently, the ultrasound beam is reoriented so that the Doppler spectrum of the backscattered echoes of the ultrasound beam is substantially symmetrical around the zero frequency. The Doppler frequency of the backscattered echoes of a second ultrasound beam oriented at a fixed angle to the first ultrasound beam is then measured. Finally, the rate of blood flow is calculated based on the angle between the ultrasound beams and the measured Doppler frequency of the backscattered echoes of the second ultrasound beam.

Pulsed photoacoustic signal characterization incorporating near- and far-field diffraction effects

Abstract: For the interpretation of photoacoustic waveforms, a diffraction correction method is presented based on an exact Lommel diffraction formulation. It is already known that tissue optical absorption coefficients may be determined by using a time resolved analysis. Normally, corresponding experiments measure signals close (a few mm) to a photoacoustic source. However, it is sometimes desirable to perform signal detection at distances extending towards the far field. Changes in pulse shape occur as a photoacoustic wave propagates into the far field, due to diffraction effects from the finite source/detector geometry. The paper presents a new method to recover initial photoacoustic signal profiles. Photoacoustic pressure signals in the frequency range of 100 kHz to 12.5 MHz were considered in this paper. Experimental evidence validates results predicted for the case of a 1.0 mm hydrophone.

High-resolution photoacoustic vascular imaging in vivo using a large-aperture acoustic lens

Abstract: Reflection-mode photoacoustic microscopy with dark-field laser pulse illumination and high frequency ultrasonic detection is used to non-invasively image blood vessels in the skin in vivo. Dark-field illumination minimizes the interference caused by strong photoacoustic signals from superficial structures. A high numerical-aperture acoustic lens provides high lateral resolution, 45-120 micrometers in this system while a broadband ultrasonic detection system provides high axial resolution, estimated to be ~15-20 micrometers. The optical illumination and ultrasonic detection are in a coaxial confocal configuration for optimal image quality. The system is capable of imaging optical-absorption contrast at up to 3 mm depth in biological tissue.

Deep penetrating photoacoustic tomography in biological tissues

Abstract: Photoacoustic tomography (PAT) in a circular scanning configuration was developed to image the deeply embedded optical heterogeneity in biological tissues. Based on the intrinsic contrast between blood and chicken breast muscle, an embedded blood object that was 5 cm deep in the tissue was detected using pulsed laser light at a wavelength of 1064 nm. Compared with detectors for flat active surfaces, cylindrically focused ultrasonic transducers can reduce the interference generated from the off-plane photoacoustic sources and make the image in the scanning plane clearer. While the optical penetration was optimized with near-infrared laser pulses of 800 nm in wavelength, the optical contrast was enhanced by indocyanine green (ICG) whose absorption peak matched the laser wavelength. This optimized PAT was able to image fine objects embedded at a depth of up to 5.2-cm, which is 6.2 times the 1/e optical penetration depth, in chicken breast muscle, at a resolution of < ~750 microns with a sensitivity of <7 pmol of ICG in blood. The resolution was found to deteriorate slowly with increasing imaging depth.

Photoacoustic imaging with attenuation rectification of different frequent components of photoacoustic signal

Abstract: Photoacoustic tomography is a potential and noninvasive medical imaging technology. It combines the advantages of pure optic imaging and pure ultrasound imaging. Photoacoustic signals induced by a short pulse laser cover a wide spectral range. We have explored the influences of attenuation of photoacoustic signals, which vary according to frequencies, to the quality of reconstructed photoacoustic images. It reveals that the attenuation of low frequent components are less than that of high frequencies, and the latter is more important for photoacoustic imaging with high resolution. Based on the ultrasonic attenuation theory, the photoacoustic imaging with rectification of the attenuation of different frequent component was performed. The experiments results show that this method improves the resolution of reconstructed images, which improves from 0.3mm to 0.2mm. A Q-switched Nd:YAG laser operating at 1064nm was used as light source. The laser had a pulse width of 6ns and a repetition frequency of 20Hz. A needle PVDF hydrophone with diameter of 1mm was used to detect photoacoustic signals.

Analysis of multiple scattering effects in optical Doppler tomography

Abstract: We present results of a theoretical analysis of optical Doppler tomography where multiple scattering effects are included. This analysis explains previous measurements of depth-resolved retinal flow profiles where the influence of multiple scattering was observed.

Photoacoustic tomography imaging of biological tissues

Abstract: Non-invasive laser-induced photoacoustic tomography is attracting more and more attentions in the biomedical optical imaging field. This imaging modality takes the advantages in that the tomography image has the optical contrast similar to the optical techniques while enjoying the high spatial resolution comparable to the ultrasound. Currently, its biomedical applications are mainly focused on breast cancer diagnosis and small animal imaging. In this paper, we report in detail a photoacoustic tomography experiment system constructed in our laboratory. In our system, a Q-switched ND:YAG pulse laser operated at 532nm with a 10ns pulse width is employed to generate photoacoustic signal. A tissue-mimicking phantom was built to test the system. When imaged, the phantom and detectors were immersed in a water tank to facilitate the acoustic detection. Based on filtered back-projection process of photoacoustic imaging, the two-dimension distribution of optical absorption in tissue phantom was reconstructed.

Laser Doppler velocimetry based on the photoacoustic effect in a CO2 laser

Abstract: We report a simple laser Doppler velocimeter in which the photoacoustic effect was used to measure the rotation wheel speed. A Doppler signal, caused by mixing a returning wave with an originally existing wave inside the CO2 laser cavity, was detected using a microphone in the laser tube. Frequency of the microphone output was in proportion to the rotation speed of a wheel and is dependent on the cosine of the angle between the direction of the laser beam and tangent of wheel velocity. A Doppler-shifted frequency as high as 34kHz was detected using this method. A frequency response of a few megahertz is expected from the laser Doppler velocimeter based on the photoacoustic effect in a CO2 laser by using a wider bandwidth microphone.

Synthetic Aperture Photoacoustic Imaging on Small Vessel Phantom Study

Abstract: Photoacoustic imaging has been shown to have higher spatial resolution and optical contrast than conventional ultrasonic imaging. In this study, we investigate the photoacoustuc imaging characteristic on small vessel phantoms of 1mm TYGON and 100μm microdialysis tubes. A 1064 nm Nd:YAG laser was used to excite the tubes, which were filled with hemoglobin and were immersed in tissue like scattering media. The photoacoustic images were obtained by using 3.5MHz and 20MHz transducers. We also used synthetic aperture technique to reconstruct photoacoustic image and improve the image contrast and resolution. This study demonstrates the improvement of photoacoustic image by synthetic aperture technique.

Photoacoustic imaging: consideration of bandwidth of ultrasonic transducer

Abstract: Photoacoustic tomography is a potential and noninvasive medical imaging technology. It combines the advantages of pure optic imaging and pure ultrasound imaging. Photoacoustic signals induced by a short pulse laser cover a wide spectral range. We have explored the frequency spectrum of absorbers with different sizes and the influence of photoacoustic signals with different spectral components on photoacoustic imaging. The simulations and experiments demonstrated that the major frequency ranges of photoacoustic pressures of absorbers with diameters of ~cm, ~mm and hundreds of mm are about 20kHz~300kHz, 70kHz~2.5MHz and 400kHz~20MHz, respectively. The low spectral components of photoacoustic signals contribute to the non-boundary region of absorbers, and the high spectral components contribute to small structures, especially, to boundaries. It suggests that the ultrasonic transducers used to detect photoacoustic pressures should be designed and selected according to the frequency ranges of absorbers.