Laser ultrasound wave pattern analysis for efficient defect detection in samples with curved surfaces.

Many production processes involve curved sample surfaces, such as welding or additive manufacturing. These pose new challenges to characterization methods for quality inspection, which are usually optimized for flat extended sample geometries. In this paper, we present a laser ultrasound (LUS) method that can be used to efficiently detect defects (e.

Efficient testing of weld seam models with radii of curvature in the millimeter range using laser ultrasound.

Laser ultrasound is a widely used tool for industrial quality assurance when a contactless and fast method is required. In this work, we used a laboratory setup based on a confocal Fabry-Perot interferometer to examine weld seam models. The focus was placed on small samples with curved surfaces (small in the sense that the radius of curvature is comparable to the largest ultrasonic wavelength) and on efficient ways to detect the presence and volume of process pores, with the goal to transfer this method to industrial applications.

Photoacoustic tomography reveals structural and functional cardiac images of animal models.

Three-dimensional photoacoustic tomography synchronized with an electrocardiogram provides highly resolved images of a beating heart with optical absorption contrast and enables investigation of cardiovascular diseases in animal models.

Photoacoustic computational ghost imaging.

Photoacoustic imaging with optical resolution usually requires a single-pixel raster scan. An alternative approach based on illumination with patterns obtained from a Hadamard matrix, measurement of the generated ultrasound wave with a single detector, followed by a reconstruction known from computational ghost imaging is demonstrated here. Since many pixels on the object are illuminated at the same time, thereby contributing to the recorded signal, this approach gives a better contrast-to-noise ratio compared to the raster scan, as demonstrated in a phantom experiment.

Conical ring array detector for large depth of field photoacoustic macroscopy.

Photoacoustic microscopy and macroscopy (PAM) using focused detector scanning are emerging imaging methods for biological tissue, providing high resolution and high sensitivity for structures with optical absorption contrast. However, achieving a constant lateral resolution over a large depth of field for deeply penetrating photoacoustic macroscopy is still a challenge. In this work, a detector design for scanning photoacoustic macroscopy is presented.

Modeling photoacoustic imaging with a scanning focused detector using Monte Carlo simulation of energy deposition.

Photoacoustic imaging using a focused, scanning detector in combination with a pulsed light source is a common technique to visualize light-absorbing structures in biological tissue. In the acoustic resolution mode, where the imaging resolution is given by the properties of the transducer, there are various challenges related to the choice of sensors and the optimization of the illumination. These are addressed by linking a Monte Carlo simulation of energy deposition to a time-domain model of acoustic propagation and detection.

Piezoelectric line detector array for photoacoustic tomography.

Photoacoustic tomography relies on a dense coverage of the surface surrounding the imaged object with ultrasound sensors in order to enable an accurate reconstruction. A curved arrangement of integrating line sensors is proposed that is able to acquire data for a linear projection image of the absorbed energy density distribution in the object. Upon rotation of the object relative to the array, three-dimensional (3D) images can be obtained.

Combined photoacoustic, pulse-echo laser ultrasound, and speed-of-sound imaging using integrating optical detection.

A purely optical setup for the coregistration of photoacoustic (PA), ultrasound (US), and speed-of-sound (SOS) section images is presented. It extends a previously developed method for simultaneous PA and laser-US (LUS) pulse-echo imaging with a LUS transmission imaging setup providing two-dimensional (2-D) SOS maps. For transmission imaging, the sound waves traversing the investigated object are generated instantaneously by illuminating optically absorbing targets that are arranged at various distances in front of the sample.

High resolution three-dimensional photoacoutic tomography with CCD-camera based ultrasound detection.

A photoacoustic tomograph based on optical ultrasound detection is demonstrated, which is capable of high resolution real-time projection imaging and fast three-dimensional (3D) imaging. Snapshots of the pressure field outside the imaged object are taken at defined delay times after photoacoustic excitation by use of a charge coupled device (CCD) camera in combination with an optical phase contrast method. From the obtained wave patterns photoacoustic projection images are reconstructed using a back propagation Fourier domain reconstruction algorithm.

Deblurring algorithms accounting for the finite detector size in photoacoustic tomography.

Most reconstruction algorithms for photoacoustic tomography, like back projection or time reversal, work ideally for point-like detectors. For real detectors, which integrate the pressure over their finite size, images reconstructed by these algorithms show some blurring. Iterative reconstruction algorithms using an imaging matrix can take the finite size of real detectors directly into account, but the numerical effort is significantly higher compared to the use of direct algorithms.

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.

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.

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.

Photoacoustic section imaging with an integrating cylindrical detector.

A piezoelectric detector with a cylindrical shape is investigated for photoacoustic section imaging. Images are acquired by rotating a sample in front of the cylindrical detector. With its length exceeding the size of the imaging object, it works as an integrating sensor and therefore allows reconstructing section images with the inverse Radon transform.

Downstream Fabry-Perot interferometer for acoustic wave monitoring in photoacoustic tomography.

An optical detection setup consisting of a focused laser beam fed into a downstream Fabry-Perot interferometer (FPI) for demodulation of acoustically generated optical phase variations is investigated for its applicability in photoacoustic tomography. The device measures the time derivative of acoustic signals integrated along the beam. Compared to a setup where the detection beam is part of a Mach-Zehnder interferometer, the signal-to-noise ratio of the FPI is lower, but the image quality of the two devices is similar.

Photoacoustic tomography using a Mach-Zehnder interferometer as an acoustic line detector.

A three-dimensional photoacoustic imaging method is presented that uses a Mach-Zehnder interferometer for measurement of acoustic waves generated in an object by irradiation with short laser pulses. The signals acquired with the interferometer correspond to line integrals over the acoustic wave field. An algorithm for reconstruction of a three-dimensional image from such signals measured at multiple positions around the object is shown that is a combination of a frequency-domain technique and the inverse Radon transform.

Clinical testing of a photoacoustic probe for port wine stain depth determination.

Background And Objective: Successful laser treatment of port wine stain (PWS) birthmarks requires knowledge of lesion geometry. Laser parameters, such as pulse duration, wavelength, and radiant exposure, and other treatment parameters, such as cryogen spurt duration, need to be optimized according to epidermal melanin content and lesion depth. We designed, constructed, and clinically tested a photoacoustic probe for PWS depth determination.