Photoacoustic section imaging with an integrating cylindrical detector

Approaching closed spherical, full-view detection for photoacoustic tomography

SIGNIFICANCE

Photoacoustic tomography (PAT) is a widely explored imaging modality and has excellent potential for clinical applications. On the acoustic detection side, limited-view angle and limited-bandwidth are common key issues in PAT systems that result in unwanted artifacts. While analytical and simulation studies of limited-view artifacts are fairly extensive, experimental setups capable of comparing limited-view to an ideal full-view case are lacking.

AIMS

A custom ring-shaped detector array was assembled and mounted to a 6-axis robot, then rotated and translated to achieve up to 3.8π steradian view angle coverage of an imaged object.

APPROACH

Minimization of negativity artifacts and phantom imaging were used to optimize the system, followed by demonstrative imaging of a star contrast phantom, a synthetic breast tumor specimen phantom, and a vascular phantom.

RESULTS

Optimization of the angular/rotation scans found ≈212 effective detectors were needed for high-quality images, while 15-mm steps were used to increase the field of view as required depending on the size of the imaged object. Example phantoms were clearly imaged with all discerning features visible and minimal artifacts.

CONCLUSIONS

A near full-view closed spherical system has been developed, paving the way for future work demonstrating experimentally the significant advantages of using a full-view PAT setup.

Ultrasound Detection Using Acoustic Apertures

Ultrasound detection is commonly performed by piezoelectric transducers that are optimized for a specific application. Since the piezoelectric technology is not configurable, transducers designed for one application may not be compatible with other applications. In addition, some designs of ultrasound transducers may be difficult to implement owing to production constraints. In this paper, we propose a simple, low-cost method to reconfigure the geometry of ultrasound transducers. The technique is based on using apertures in thin sheets of acoustic blockers. We experimentally demonstrate this method for an ultrasound transducer with a central frequency of 1 MHz and show that it can emulate detectors of various sizes. An added advantage of this technique is its capability to achieve semi-isotropic detection sensitivity due to diffraction when the aperture size is comparable to the acoustic wavelength even when the angular sensitivity of the transducer is inherently limited.

Parametric modeling of wideband piezoelectric polymer sensors: Design for optoacoustic applications

In this work, we present a three-dimensional model for the design of wideband piezoelectric polymer sensors which includes the geometry and the properties of the transducer materials. The model uses FFT and numerical integration techniques in an explicit, semi-analytical approach. To validate the model, we made electrical and mechanical measurements on homemade sensors for optoacoustic applications. Each device was implemented using a polyvinylidene fluoride thin film piezoelectric polymer with a thickness of 25 μm. The sensors had detection areas in the range between 0.5 mm² and 35 mm² and were excited by acoustic pressure pulses of 5 ns (FWHM) from a source with a diameter around 10 μm. The experimental data obtained from the measurements agree well with the model results. We discuss the relative importance of the sensor design parameters for optoacoustic applications and we provide guidelines for the optimization of devices.

Advanced optoacoustic methods for multiscale imaging of in vivo dynamics

Visualization of dynamic functional and molecular events in an unperturbed in vivo environment is essential for understanding the complex biology of living organisms and of disease state and progression. To this end, optoacoustic (photoacoustic) sensing and imaging have demonstrated the exclusive capacity to maintain excellent optical contrast and high resolution in deep-tissue observations, far beyond the penetration limits of modern microscopy. Yet, the time domain is paramount for the observation and study of complex biological interactions that may be invisible in single snapshots of living systems. This review focuses on the recent advances in optoacoustic imaging assisted by smart molecular labeling and dynamic contrast enhancement approaches that enable new types of multiscale dynamic observations not attainable with other bio-imaging modalities. A wealth of investigated new research topics and clinical applications is further discussed, including imaging of large-scale brain activity patterns, volumetric visualization of moving organs and contrast agent kinetics, molecular imaging using targeted and genetically expressed labels, as well as three-dimensional handheld diagnostics of human subjects.

Photoacoustics and speed-of-sound dual mode imaging with a long depth-of-field by using annular ultrasound array

Speed-of-sound and optical absorption reflect the structure and function of tissues from different aspects. A dual-mode microscopy system based on a concentric annular ultrasound array is proposed to simultaneously acquire the long depth-of-field images of speed-of-sound and optical absorption of inhomogeneous samples. First, speed-of-sound is decoded from the signal delay between each element of the annular array. The measured speed-of-sound could not only be used as an image contrast, but also improve the resolution and accuracy of spatial location of photoacoustic image in inhomogeneous acoustic media. Secondly, benefitting from dynamic focusing of annular array and the measured speed-of-sound, it is achieved an advanced acoustic-resolution photoacoustic microscopy with a precise position and a long depth-of-field. The performance of the dual-mode imaging system has been experimentally examined by using a custom-made annular array. The proposed dual-mode microscopy might have the significances in monitoring the biological physiological and pathological processes.

Analysis of Negatively Focused Ultrasound Detectors in Optoacoustic Tomography

In optoacoustic tomography, negatively focused transducers may be used for improving the tangential image resolution while preserving a high signal-to-noise ratio. Commonly, image reconstruction in such scenarios is facilitated by the use of the virtual-detector approach. Although the validity of this approach has been experimentally verified, it is based on an approximation whose effect on optoacoustic image reconstruction has not yet been studied. In this paper, we analyze the response of negatively focused acoustic detectors in 2D in both time and frequency domains. Based on this analysis, tradeoffs between the detector size, curvature, and sensitivity are formulated. In addition, our analysis reveals the geometrical underpinning of the virtual-detector approximation and quantifies its deviation from the exact solution. The error involved in the virtual-detector approximation is studied in image reconstruction simulations and its effect on image quality is shown. The theoretical tools developed in this work may be used in the design of new optoacoustic detection geometries as well as for improved image reconstruction.

Optimal self-calibration of tomographic reconstruction parameters in whole-body small animal optoacoustic imaging

In tomographic optoacoustic imaging, multiple parameters related to both light and ultrasound propagation characteristics of the medium need to be adequately selected in order to accurately recover maps of local optical absorbance. Speed of sound in the imaged object and surrounding medium is a key parameter conventionally assumed to be uniform. Mismatch between the actual and predicted speed of sound values may lead to image distortions but can be mitigated by manual or automatic optimization based on metrics of image sharpness. Although some simple approaches based on metrics of image sharpness may readily mitigate distortions in the presence of highly contrasting and sharp image features, they may not provide an adequate performance for smooth signal variations as commonly present in realistic whole-body optoacoustic images from small animals. Thus, three new hybrid methods are suggested in this work, which are shown to outperform well-established autofocusing algorithms in mouse experiments in vivo.

Artifact removal in photoacoustic section imaging by combining an integrating cylindrical detector with model-based reconstruction

Photoacoustic section imaging reveals optically absorbing structures within a thin slice of an object. It requires measuring acoustic waves excited by absorption of short laser pulses with a cylindrical acoustic lens detector rotating around the object. Owing to the finite detector size and its limited depth of focus, various artifacts arise, seen as distortions within the imaging slice and cross-talk from neighboring areas of the object. The presented solution aims at avoiding these artifacts by a special design of the sensor and by use of a model-based reconstruction algorithm that improves section images by incorporating information from neighboring sections. The integrating property of the cylindrical detector, which exceeds in direction of the cylinder axis the size of the imaged object, avoids the lateral blurring that normally results from the finite width of a small detector. Applying a maximum likelihood reconstruction method for the inversion of the imaging system matrix to the temporal pressure signals yields line projections of the initial energy distribution, from which section images are obtained by applying the inverse Radon transform. By using data from few sections, a significant reduction of artifacts related to the imperfections of the sensor is demonstrated both in simulations and in phantom experiments.

Three-dimensional optoacoustic tomography using a conventional ultrasound linear detector array: whole-body tomographic system for small animals

PURPOSE

Optoacoustic imaging relies on the detection of ultrasonic waves induced by laser pulse excitations to map optical absorption in biological tissue. A tomographic geometry employing a conventional ultrasound linear detector array for volumetric optoacoustic imaging is reported. The geometry is based on a translate-rotate scanning motion of the detector array, and capitalizes on the geometrical characteristics of the transducer assembly to provide a large solid angular detection aperture. A system for three-dimensional whole-body optoacoustic tomography of small animals is implemented.

METHODS

The detection geometry was tested using a 128-element linear array (5.0∕7.0 MHz, Acuson L7, Siemens), moved by steps with a rotation∕translation stage assembly. Translation and rotation range of 13.5 mm and 180°, respectively, were implemented. Optoacoustic emissions were induced in tissue-mimicking phantoms and ex vivo mice using a pulsed laser operating in the near-IR spectral range at 760 nm. Volumetric images were formed using a filtered backprojection algorithm.

RESULTS

The resolution of the optoacoustic tomography system was measured to be better than 130 μm in-plane and 330 μm in elevation (full width half maximum), and to be homogenous along a 15 mm diameter cross section due to the translate-rotate scanning geometry. Whole-body volumetric optoacoustic images of mice were performed ex vivo, and imaged organs and blood vessels through the intact abdominal and head regions were correlated to the mouse anatomy.

CONCLUSIONS

Overall, the feasibility of three-dimensional and high-resolution whole-body optoacoustic imaging of small animal using a conventional linear array was demonstrated. Furthermore, the scanning geometry may be used for other linear arrays and is therefore expected to be of great interest for optoacoustic tomography at macroscopic and mesoscopic scale. Specifically, conventional detector arrays with higher central frequencies may be investigated.

Optoacoustic imaging and tomography: reconstruction approaches and outstanding challenges in image performance and quantification

This paper comprehensively reviews the emerging topic of optoacoustic imaging from the image reconstruction and quantification perspective. Optoacoustic imaging combines highly attractive features, including rich contrast and high versatility in sensing diverse biological targets, excellent spatial resolution not compromised by light scattering, and relatively low cost of implementation. Yet, living objects present a complex target for optoacoustic imaging due to the presence of a highly heterogeneous tissue background in the form of strong spatial variations of scattering and absorption. Extracting quantified information on the actual distribution of tissue chromophores and other biomarkers constitutes therefore a challenging problem. Image quantification is further compromised by some frequently-used approximated inversion formulae. In this review, the currently available optoacoustic image reconstruction and quantification approaches are assessed, including back-projection and model-based inversion algorithms, sparse signal representation, wavelet-based approaches, methods for reduction of acoustic artifacts as well as multi-spectral methods for visualization of tissue bio-markers. Applicability of the different methodologies is further analyzed in the context of real-life performance in small animal and clinical in-vivo imaging scenarios.

Hybrid photoacoustic and ultrasound section imaging with optical ultrasound detection

A setup is proposed that provides perfectly co-registered photoacoustic (PA) and ultrasound (US) section images. Photoacoustic and ultrasound backscatter signals are generated by laser pulses coming from the same laser system, the latter by absorption of some of the laser energy on an optically absorbing target near the imaged object. By measuring both signals with the same optical detector, which is focused into the selected section by use of a cylindrical acoustic mirror, the information for both images is acquired simultaneously. Co-registered PA and US images are obtained after applying the inverse Radon transform to the data, which are gathered while rotating the object relative to the detector. Phantom experiments demonstrate a resolution of 1.1 mm between the sections of both imaging modalities and a in-plane resolution of about 60 µm and 120 µm for the US and PA modes, respectively. The complementary contrast mechanisms of the two modalities are shown by images of a zebrafish.

Non-invasive whole-body imaging of adult zebrafish with optoacoustic tomography

Zebrafish has emerged as an excellent vertebrate model organism for studies of evolution, development and disease. Due to its external development and optical transparency in embryonic stages, zebrafish offers a major advantage over other vertebrate model organisms by being amenable for microscopic studies of biological processes within their natural environment directly in the living organism. However, commonly used zebrafish strains lose their transparency within their first two weeks of development and thus are no longer accessible for optical imaging approaches at juvenile or adult stages. In this study we successfully apply optoacoustic imaging for non-invasive three-dimensional imaging of adult zebrafish. Since optoacoustics does not necessarily require labeling, but can instead rely on the intrinsic tissue contrast, this imaging method has the potential to become a versatile tool for developmental studies from juvenile to adult stages in the intact zebrafish.

Photoacoustic section imaging using an elliptical acoustic mirror and optical detection

A method is proposed that utilizes the advantages of optical ultrasound detection in two-dimensional photoacoustic section imaging, combining an optical interferometer with an acoustic mirror. The concave mirror has the shape of an elliptical cylinder and concentrates the acoustic wave generated around one focal line in the other one, where an optical beam probes the temporal evolution of acoustic pressure. This yields line projections of the acoustic sources at distances corresponding to the time of flight, which, after rotating the sample about an axis perpendicular to the optical detector, allows reconstruction of a section using the inverse Radon transform. A resolution of 120 [micro sign]m within and 1.5 mm between the sections can be obtained with the setup. Compared to a bare optical probe beam, the signal-to-noise ratio (SNR) is seven times higher with the mirror. Furthermore, the imaging system is tested on a biological sample.

Label-free imaging of zebrafish larvae in vivo by photoacoustic microscopy

Zebrafish play an important role in biological and biomedical research. Traditional in vivo imaging methods for studying zebrafish larvae primarily require fluorescence labeling. In this work, relying on tissue intrinsic optical absorption contrast, we acquired high resolution label-free 3D images of zebrafish larvae by using photoacoustic microscopy (PAM) in vivo. The spatial resolution reaches several microns, allowing the study of microstructures in various living organs. We demonstrated that our method has the potential to be a powerful non-invasive imaging method for studying various small animal models, including zebrafish larvae, Caenorhabditis elegans, frogs and drosophila larvae.