Nonlinear pth root spectral magnitude scaling beamforming for clinical photoacoustic and ultrasound imaging

Nonlinear beamforming for intracardiac echocardiography: a comparative study

Intracardiac echocardiography (ICE) enables cardiac imaging with a wide field of view, deep imaging depth, and high frame rate during surgery. However, strong sidelobe and grating lobe artifacts created by the ultra-compact transducer degrade its image quality, making diagnosis and monitoring of treatment difficult. Conventionally, aperture apodization algorithms are often used to suppress sidelobe and grating lobe artifacts at the expense of lateral resolution, which is undesirable in ICE. In this study, we present comparative results of the beamforming methods specifically in ICE application. We demonstrate and compare five nonlinear beamforming algorithms in ICE: nonlinear pth root delay and sum (NL-p-DAS), nonlinear pth root spectral magnitude scaling (NL-p-SMS), delay-and-sum with coherence factors (DAS + SCF), delay and sum with apodization (DAS + apodization) and delay and sum (DAS). Phantom and ex-vivo experiment compare the performance of each algorithm in static and dynamic conditions. DAS + SCF shows the best lateral resolution, and all four algorithms improve the image contrast and sidelobe suppression over conventional DAS. NL-p-SMS stands out for the best axial resolution and suppression of grating lobe artifacts. For motion tracking, NL-p-SMS shows better temporal resolution than other methods. Overall, all the beamforming algorithms other than DAS showed improved image quality. Among them, NL-p-SMS, which has a high temporal resolution, showed the potential for providing more accurate information regards movement tracking.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13534-024-00352-9.

Functional photoacoustic imaging: from nano- and micro- to macro-scale

Functional photoacoustic imaging is a promising biological imaging technique that offers such unique benefits as scalable resolution and imaging depth, as well as the ability to provide functional information. At nanoscale, photoacoustic imaging has provided super-resolution images of the surface light absorption characteristics of materials and of single organelles in cells. At the microscopic and macroscopic scales. photoacoustic imaging techniques have precisely measured and quantified various physiological parameters, such as oxygen saturation, vessel morphology, blood flow, and the metabolic rate of oxygen, in both human and animal subjects. This comprehensive review provides an overview of functional photoacoustic imaging across multiple scales, from nano to macro, and highlights recent advances in technology developments and applications. Finally, the review surveys the future prospects of functional photoacoustic imaging in the biomedical field.

Motion Compensation for 3D Multispectral Handheld Photoacoustic Imaging

Three-dimensional (3D) handheld photoacoustic (PA) and ultrasound (US) imaging performed using mechanical scanning are more useful than conventional 2D PA/US imaging for obtaining local volumetric information and reducing operator dependence. In particular, 3D multispectral PA imaging can capture vital functional information, such as hemoglobin concentrations and hemoglobin oxygen saturation (sO₂), of epidermal, hemorrhagic, ischemic, and cancerous diseases. However, the accuracy of PA morphology and physiological parameters is hampered by motion artifacts during image acquisition. The aim of this paper is to apply appropriate correction to remove the effect of such motion artifacts. We propose a new motion compensation method that corrects PA images in both axial and lateral directions based on structural US information. 3D PA/US imaging experiments are performed on a tissue-mimicking phantom and a human wrist to verify the effects of the proposed motion compensation mechanism and the consequent spectral unmixing results. The structural motions and sO₂ values are confirmed to be successfully corrected by comparing the motion-compensated images with the original images. The proposed method is expected to be useful in various clinical PA imaging applications (e.g., breast cancer, thyroid cancer, and carotid artery disease) that are susceptible to motion contamination during multispectral PA image analysis.

Achieving depth-independent lateral resolution in AR-PAM using the synthetic-aperture focusing technique

Acoustic-resolution photoacoustic microscopy (AR-PAM) is a promising imaging modality that renders images with ultrasound resolution and extends the imaging depth beyond the optical ballistic regime. To achieve a high lateral resolution, a large numerical aperture (NA) of a focused transducer is usually applied for AR-PAM. However, AR-PAM fails to hold its performance in the out-of-focus region. The lateral resolution and signal-to-noise ratio (SNR) degrade substantially, thereby leading to a significantly deteriorated image quality outside the focal area. Based on the concept of the synthetic-aperture focusing technique (SAFT), various strategies have been developed to address this challenge. These include 1D-SAFT, 2D-SAFT, adaptive-SAFT, spatial impulse response (SIR)-based schemes, and delay-multiply-and-sum (DMAS) strategies. These techniques have shown progress in achieving depth-independent lateral resolution, while several challenges remain. This review aims to introduce these developments in SAFT-based approaches, highlight their fundamental mechanisms, underline the advantages and limitations of each approach, and discuss the outlook of the remaining challenges for future advances.

Photoacoustic imaging systems based on clinical ultrasound platform

Photoacoustic imaging has drawn a significant amount of attention due to its unique capacity for functional, metabolic, and molecular imaging, which is achieved by the combination of optical excitation and acoustic detection. With both strengths of light and ultrasound, photoacoustic images can provide strong optical contrast at high ultrasound resolution in deep tissue. As photoacoustic imaging can be used to visualize complementary information to ultrasound imaging using the same data acquisition process, several studies have been conducted on combining photoacoustic imaging with existing clinical ultrasound systems. This review highlights our development of a photoacoustic/ultrasound dual-modal imaging system, various features and functionalities implemented for clinical translation, and preclinical/clinical studies performed by using the systems.

Three-dimensional Multistructural Quantitative Photoacoustic and US Imaging of Human Feet in Vivo

Background Monitoring the microcirculation in human feet is crucial in assessing peripheral vascular diseases, such as diabetic foot. However, conventional imaging modalities are more focused on diagnosis in major arteries, and there are limited methods to provide microvascular information in early stages of the disease. Purpose To investigate a three-dimensional (3D) noncontrast bimodal photoacoustic (PA)/US imaging system that visualizes the human foot morphologically and also reliably quantifies podiatric vascular parameters noninvasively. Materials and Methods A clinically relevant PA/US imaging system was combined with a foot scanner to obtain 3D PA and US images of the human foot in vivo. Healthy participants were recruited from September 2020 to June 2021. The collected 3D PA and US images were postprocessed to present structural information about the foot. The quantitative reliability was evaluated in five repeated scans of 10 healthy feet by calculating the intraclass correlation coefficient and minimal detectable change, and the detectability of microvascular changes was tested by imaging 10 healthy feet intentionally occluded with use of a pressure cuff (160 mm Hg). Statistically significant difference is indicated with P values. Results Ten feet from six healthy male volunteers (mean age ± standard deviation, 27 years ± 3) were included. The foot images clearly visualized the structure of the vasculature, bones, and skin and provided such functional information as the total hemoglobin concentration (HbT), hemoglobin oxygen saturation (SO₂), vessel density, and vessel depth. Functional information from five independent measurements of 10 healthy feet was moderately reliable (intraclass correlation coefficient, 0.51-0.74). Significant improvements in HbT (P = .006) and vessel density (P = .046) as well as the retention of SO₂ were observed, which accurately described the microvascular change due to venous occlusion. Conclusion Three-dimensional photoacoustic and US imaging was able to visualize morphologic and physiologic features of the human foot, including the peripheral microvasculature, in healthy volunteers. © RSNA, 2022 Online supplemental material is available for this article. See also the editorial by Mezrich in this issue.

A photoacoustic finder fully integrated with a solid-state dye laser and transparent ultrasound transducer

The standard-of-care for evaluating lymph node status in breast cancers and melanoma metastasis is sentinel lymph node (SLN) assessment performed with a handheld gamma probe and radioisotopes. However, this method inevitably exposes patients and physicians to radiation, and the special facilities required limit its accessibility. Here, we demonstrate a non-ionizing, cost-effective, handheld photoacoustic finder (PAF) fully integrated with a solid-state dye laser and transparent ultrasound transducer (TUT). The solid-state dye laser handpiece is coaxially aligned with the spherically focused TUT. The integrated finder readily detected photoacoustic signals from a tube filled with methylene blue (MB) beneath a 22 mm thick layer of chicken tissue. In live animals, we also photoacoustically detected both SLNs injected with MB and subcutaneously injected melanomas. We believe that our radiation-free and inexpensive PAF can play a vital role in SLN assessment.

Nonlinear mechanisms in photoacoustics-Powerful tools in photoacoustic imaging

Many nonlinear effects have been discovered and developed in photoacoustic imaging. These nonlinear mechanisms have been explored for different utilizations, such as enhancing imaging contrast, measuring tissue temperature, achieving super-resolution imaging, enabling functional imaging, and extracting important physical parameters. This review aims to introduce different nonlinear mechanisms in photoacoustics, underline the fundamental principles, highlight their representative applications, and outline the occurrence conditions and applicable range of each nonlinear mechanism. Furthermore, this review thoroughly discusses the nonlinearity rule concerning how the mathematical structure of the nonlinear dependence is correlated to its practical applications. This summarization is useful for identifying and guiding the potential applications of nonlinearity based on their mathematical expressions, and is helpful for new nonlinear mechanism discovery or implementation in the future, which facilitates further breakthroughs in nonlinear photoacoustics.

Photoacoustic tomography reconstruction using lag-based delay multiply and sum with a coherence factor improves in vivo ovarian cancer diagnosis

Ovarian cancer is the fifth most common cause of death due to cancer, and it is the deadliest of all gynecological cancers. Diagnosing ovarian cancer via conventional photoacoustic delay-and-sum beamforming (DAS) presents several challenges, such as poor image resolution and low lesion to background tissue contrast. To address these concerns, we propose an improved beamformer named lag-based delay multiply and sum combined with coherence factor (DMAS-LAG-CF). Simulations and phantom experiments demonstrate that compared with the conventional DAS, the proposed algorithm can provide 1.39 times better resolution and 10.77 dB higher contrast. For patient data, similar performance on contrast ratios has been observed. However, since the diagnostic accuracy between cancer and benign/normal groups is a significant measure, we have extracted photoacoustic histogram features of mean, kurtosis and skewness. DMAS-LAG-CF can improve cancer diagnosis with an AUC of 0.91 for distinguishing malignant vs. benign ovarian lesions when mean and skewness are used as features.