Martin F. Beckmann

Multispectral photoacoustic coded excitation imaging using unipolar orthogonal Golay codes.

We present a method to speed up the acquisition of multispectral photoacoustic data sets by using unipolar orthogonal Golay codes as excitation sequences for the irradiation system. Multispectral photoacoustic coded excitation (MS-PACE) allows acquiring photoacoustic data sets for two irradiation wavelengths simultaneously and separating them afterwards, thus improving the SNR or speeding up the measurement. We derive an analytical estimation of the SNR improvement using MS-PACE compared to time equivalent averaging. We demonstrate the feasibility of the method by successfully imaging a phantom composed of two dyes using unipolar orthogonal Golay codes as excitation sequence for two high power laser diodes operating at two different wavelengths. The experimental results show very good agreement with the theoretical predictions.

Size-dependent multispectral photoacoustic response of solid and hollow gold nanoparticles

Photoacoustic (PA) imaging attracts a great deal of attention as an innovative modality for longitudinal, non-invasive, functional and molecular imaging in oncology. Gold nanoparticles (AuNPs) are identified as superior, NIR-absorbing PA contrast agents for biomedical applications. Until now, no systematic comparison of the optical extinction and PA efficiency of water-soluble AuNPs of various geometries and small sizes has been performed. Here spherical AuNPs with core diameters of 1.0, 1.4 and 11.2 nm, nanorods with longitudinal/transversal elongation of 38/9 and 44/12 nm and hollow nanospheres with outer/inner diameters of 33/19, 57/30, 68/45 and 85/56 nm were synthesized. The diode laser set-up with excitations at 650, 808, 850 and 905 nm allowed us to correlate the molar PA signal intensity with the molar extinction of the respective AuNPs. Deviations were explained by differences in heat transfer from the particle to the medium and, for larger particles, by the scattering of light. The molar PA intensity of 1.0 nm AuNPs was comparable to the commonly used organic dye methylene blue, and rapidly increased with the lateral size of AuNPs.

Multispectral photoacoustic coded excitation using pseudorandom codes

Photoacoustic imaging (PAI) combines high ultrasound resolution with optical contrast. Laser-generated ultrasound is potentially beneficial for cancer detection, blood oxygenation imaging, and molecular imaging. PAI is generally performed using solid state Nd:YAG lasers in combination with optical parametric oscillators. An alternative approach uses laser diodes with higher pulse repetition rates but lower power. Thus, improvement in signal-to-noise ratio (SNR) is a key step towards applying laser diodes in PAI. To receive equivalent image quality using laser diodes as with Nd:YAG lasers, the lower power must be compensated by averaging, which can be enhanced through coded excitation. In principle, perfect binary sequences such as orthogonal Golay codes can be used for this purpose when acquiring data at multiple wavelengths. On the other hand it was shown for a single wavelength that sidelobes can remain invisible even if imperfect sequences are used. Moreover, SNR can be further improved by using an imperfect sequence compared to Golay codes. Here, we show that pseudorandom sequences are a good choice for multispectral photoacoustic coded excitation (MSPACE). Pseudorandom sequences based upon maximal length shift register sequences (m-sequences) are introduced and analyzed for the purpose of use in MSPACE. Their gain in SNR exceeds that of orthogonal Golay codes for finite code lengths. Artefacts are introduced, but may remain invisible depending on SNR and code length.

Photoacoustic clutter reduction by inversion of a linear scatter model using plane wave ultrasound measurements.

Photoacoustic imaging aims to visualize light absorption properties of biological tissue by receiving a sound wave that is generated inside the observed object as a result of the photoacoustic effect. In clinical applications, the strong light absorption in human skin is a major problem. When high amplitude photoacoustic waves that originate from skin absorption propagate into the tissue, they are reflected back by acoustical scatterers and the reflections contribute to the received signal. The artifacts associated with these reflected waves are referred to as clutter or skin echo and limit the applicability of photoacoustic imaging for medical applications severely. This study seeks to exploit the acoustic tissue information gained by plane wave ultrasound measurements with a linear array in order to correct for reflections in the photoacoustic image. By deriving a theory for clutter waves in k-space and a matching inversion approach, photoacoustic measurements compensated for clutter are shown to be recovered.

Optimized SNR simultaneous multispectral photoacoustic imaging with laser diodes

Multispectral photoacoustic laser diode systems have multiple wavelengths available simultaneously. In addition to multispectral imaging, this can be exploited to increase the signal to noise ratio (SNR) by combining these wavelengths to form a combined image, but at the loss of spectral information. Here, a novel signal processing concept is introduced, which optimizes the SNR in the reconstructions of single wavelength data from combined acquisitions while simultaneously permitting to obtain a higher SNR fused image from the same data. The concept is derived for an arbitrary number of wavelengths; it is also applicable at low pulse repetition frequencies. The concept is applied in an experiment using two wavelengths, verifying the theoretical results.

Enhanced photoacoustic signal from DNA assembled gold nanoparticle networks

We report an experimental finding of photoacoustic signal enhancement from finite sized DNA–gold nanoparticle networks. We synthesized DNA-functionalized hollow and solid gold nanospheres (AuNS) to form finite sized networks, which were characterized by means of optical extinction spectroscopy, dynamic light scattering, and scanning electron microscopy in transmission mode. It is shown that the signal amplification scales with network size for networks comprising either hollow or solid AuNS as well as networks consisting of both types of nanoparticles. The laser intensities applied in our multispectral setup (λ = 650 nm, 850 nm, 905 nm) were low enough to maintain the structural integrity of the networks. This reflects that the binding and recognition properties of the temperature-sensitive cross-linking DNA-molecules are retained.

Photoacoustic coded excitation using periodically perfect sequences

Photoacoustic imaging is based on the generation of ultrasound using laser irradiation. Nd:YAG laser systems are commonly employed for this purpose, but cheap and handy pulsed laser diodes can be an attractive alternative. They emit significantly lower pulse energies, but fast averaging is feasible due to high repetition rates. Averaging is limited by the time-of-flight of the acoustic signal, but coded excitation can be used to overcome this limit. Here, we examine the performance of difference set based sequences with perfect correlation properties (periodically perfect sequences, PPS). PPS can be used for continuous, artifact free acquisition, the acquisition scheme is simpler than for all previously reported coding strategies. The coding gain reached for periodic imaging is higher than for Golay codes and Legendre sequences.

Iterative photoacoustic reconstruction in heterogeneous media using the Kaczmarz method

The aim of a photoacoustic image reconstruction is to locate acoustic sources that originate from laser irradiation. Many conventional one-step methods suffer from artifacts caused by simplifications in their derivation. For example, the limited view of most detection systems is usually not considered. Recently, several iterative reconstruction algorithms have been proposed that are able to approximate the original distribution more accurately. Some of them have been shown to reduce the influence of a limited view. However, the efficiency of these algorithms is usually only assessed on a circular detection setup. Also, acoustic medium heterogeneities are often neglected. We propose a new iterative reconstruction algorithm that can be applied to linear array acquisitions and considers known medium heterogeneities. The approach is based on the Kaczmarz method as commonly used in Computed Tomography.

Multispectral photoacoustic coded excitation with low PRF high power laser diodes

Photoacoustic imaging is based on the generation of ultrasound using laser irradiation. Solid state laser systems are commonly employed for this purpose, but pulsed laser diodes can be an attractive alternative. Currently, a photoacoustic handheld probe is developed within the European Union project FULLPHASE and will include multiple wavelengths. In this probe, multiple wavelengths will be available simultaneously. This can be exploited for increasing the signal to noise ratio of acquired images by generating images containing all wavelengths (“fused” images) at the loss of spectral information. Here we show that applying a multispectral code instead of a simple fusion of the wavelengths, allows to generate both a high SNR fused image and separate wavelength images simultaneously. The concept is applied in experiments using two wavelengths.

Photoacoustic clutter reduction using plane wave ultrasound and a linear scatter estimation approach

In clinical photoacoustic imaging, strong light absorption in the skin and the resulting acoustic backscatter from inside the tissue are a major challenge. In the reconstructed image, these reflections, referred to as clutter, can hardly be distinguished from actual photoacoustic sources. It has been shown that especially in setups, where the strong skin absorption is within the field of view, clutter amplitudes are very strong. In this contribution, we focus on such setups with all absorbers and scatterers inside the imaging plane. Our approach uses additional plane wave ultrasound measurements from various angles that are processed according to linear scatter theory in order to approximate the reflections in the photoacoustic measurement. Using this information, the corresponding artifacts can be reduced.