Ultrasound detection is one of the major components of photoacoustic imaging systems. Advancement in ultrasound transducer technology has a significant impact on the translation of photoacoustic imaging to the clinic. Here, we present an overview on various ultrasound transducer technologies including conventional piezoelectric and micromachined transducers, as well as optical ultrasound detection technology. We explain the core components of each technology, their working principle, and describe their manufacturing process. We then quantitatively compare their performance when they are used in the receive mode of a photoacoustic imaging system.

Overview of Ultrasound Detection Technologies for Photoacoustic Imaging

Photoacoustic imaging (PAI) is an emerging functional and molecular imaging technology that has attracted much attention in the past decade. Recently, many researchers have used the vantage system from Verasonics for simultaneous ultrasound (US) and photoacoustic (PA) imaging. This was the motivation to write on the details of US/PA imaging system implementation and characterization using Verasonics platform. We have discussed the experimental considerations for linear array based PAI due to its popularity, simple setup, and high potential for clinical translatability. Specifically, we describe the strategies of US/PA imaging system setup, signal generation, amplification, data processing and study the system performance.

Technical considerations in the Verasonics research ultrasound platform for developing a photoacoustic imaging system

One of the key challenges in linear array transducer‐based photoacoustic computed tomography is to image structures embedded deep within the biological tissue with limited optical energy. Here, we utilized a manually controlled multi‐angle illumination technique to allow the incident photons to interact with the imaging targets for longer periods of time and diffuse further in all directions. We have developed and optimized a compact probe that enables manual changes to the angle of illumination while acquiring photoacoustic signals. The performance has been demonstrated and evaluated by imaging complex blood vessel mimicking phantoms in‐vitro and sheep brain samples ex‐vivo. For effective image reconstruction from the data acquired by multi‐angle illumination method, we have utilized a method based on the extraction of maximum intensity. In both cases, multi‐angle illumination has out‐performed the conventional fixed angle illumination technique to improve the overall image quality. Specifically, extraction of the imaging targets located at greater axial depths was possible using this multi‐angle illumination technique.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/jbio.202200016

Randomized multi‐angle illumination for improved linear array photoacoustic computed tomography in brain

An ultrasound sparse array consists of a sparse distribution of elements over a 2-D aperture. Such an array is typically characterized by a limited number of elements, which in most cases is compatible with the channel number of the available scanners. Sparse arrays represent an attractive alternative to full 2-D arrays that may require the control of thousands of elements through expensive application-specific integrated circuits (ASICs). However, their massive use is hindered by two main drawbacks: the possible beam profile deterioration, which may worsen the image contrast, and the limited signal-to-noise ratio (SNR), which may result too low for some applications. This article reviews the work done for three decades on 2-D ultrasound sparse arrays for medical applications. First, random, optimized, and deterministic design methods are reviewed together with their main influencing factors. Then, experimental 2-D sparse array implementations based on piezoelectric and capacitive micromachined ultrasonic transducer (CMUT) technologies are presented. Sample applications to 3-D (Doppler) imaging, super-resolution imaging, photo-acoustic imaging, and therapy are reported. The final sections discuss the main shortcomings associated with the use of sparse arrays, the related countermeasures, and the next steps envisaged in the development of innovative arrays. 

/pdf/design-implementation-and-medical-applications-of-2-d-1kxgp3vd.pdf

Design, Implementation, and Medical Applications of 2-D Ultrasound Sparse Arrays

Abstract The capability of photoacoustic (PA) imaging to measure oxygen saturation through a fontanelle has been demonstrated in large animals in-vivo . We called this method, transfontanelle photoacoustic imaging (TFPAI). A surgically induced 2.5 cm diameter cranial window was created in an adult sheep skull to model the human anterior fontanelle. The performance of the TFPAI has been evaluated by comparing the PA-based predicted results against the gold standard of blood gas analyzer measurements. 

/pdf/transfontanelle-photoacoustic-imaging-for-in-vivo-cerebral-2t0z91nw.pdf

Transfontanelle photoacoustic imaging for in-vivo cerebral oxygenation measurement

Transfontanelle ultrasound imaging (TFUI) is the conventional approach for diagnosing brain injury in neonates. Despite being the first stage imaging modality, TFUI lacks accuracy in determining the injury at an early stage due to degraded sensitivity and specificity. Therefore, a modality like photoacoustic imaging that combines the advantages of both acoustic and optical imaging can overcome the existing TFUI limitations. Even though a variety of transducers have been used in TFUI, it is essential to identify the transducer specification that is optimal for transfontanelle imaging using the photoacoustic technique. In this study, we evaluated the performance of 6 commercially available ultrasound transducer arrays to identify the optimal characteristics for transfontanelle photoacoustic imaging. We focused on commercially available linear and phased array transducer probes with center frequencies ranging from 2.5MHz to 8.5MHz which covers the entire spectrum of the transducer arrays used for brain imaging. The probes were tested on both in vitro and ex vivo brain tissue, and their performance in terms of transducer resolution, size, penetration depth, sensitivity, signal to noise ratio, signal amplification and reconstructed image quality were evaluated. The analysis of selected transducers in these areas allowed us to determine the optimal transducer for transfontanelle imaging, based on vasculature depth and blood density in tissue using ex vivo sheep brain. The outcome of this evaluation identified the two most suitable ultrasound transducer probes for transfontanelle photoacoustic imaging.

Transfontanelle photoacoustic imaging: ultrasound transducer selection analysis.

Photoacoustic imaging (PAI) is an emerging label-free and non-invasive modality for imaging biological tissues. PAI has been implemented in different configurations, one of which is photoacoustic computed tomography (PACT) with a potential wide range of applications, including brain and breast imaging. Hemispherical Array PACT (HA-PACT) is a variation of PACT that has solved the limited detection-view problem. Here, we designed an HA-PACT system consisting of 50 single element transducers. For implementation, we initially performed a simulation study, with parameters close to those in practice, to determine the relationship between the number of transducers and the quality of the reconstructed image. We then used the greatest number of transducers possible on the hemisphere and imaged copper wire phantoms coated with a light absorbing material to evaluate the performance of the system. Several practical issues such as light illumination, arrangement of the transducers, and an image reconstruction algorithm have been comprehensively studied.

/pdf/development-of-a-stationary-3d-photoacoustic-imaging-system-36wyxq1a93.pdf

Development of a stationary 3D photoacoustic imaging system using sparse single-element transducers: Phantom study

This study introduces a novel and powerful algorithm for optoacoustic tomography (OAT) in heterogeneous optical scattering media surrounded by optical fibers and ultrasonic transducers in a ring arrangement. The proposed algorithm uses the extended formulation of full-waveform inversion (FWI) based on the alternating direction method of multipliers (ADMM) and fluence compensation (FC). The speed of sound (SOS) map is required in this algorithm as prior information to perform classification and find different regions in the SOS map with equal optical absorption coefficients. The SOS map can be extracted by another FWI procedure applied to the acoustic data generated and recorded by transducers. The fluence map was calculated by Zemax inside the medium at near-infrared (NIR) optical wavelength. Finally, the proposed algorithm iteratively updates the optical absorption coefficients until the data computed with the estimated coefficients match the recorded photoacoustic signals at ultrasonic transducers. This approach shows extremely better results than classical time-reversal algorithms.

2D-FC-ADMM reconstruction algorithm for quantitative optoacoustic tomography in a highly scattering medium: simulation study

A synthetic phantom model is typically utilized to evaluate the initial performance of a photoacoustic image reconstruction algorithm. The characteristics of the phantom model (structural, optical, and acoustic) are required to be very similar to those of the biological tissue. Typically, generic two-dimensional shapes are used as imaging targets to calibrate reconstruction algorithms. However, these structures are not representative of complex biological tissue, and therefore the artifacts that exist in reconstructed images of biological tissue vasculature are ignored. Real data from 3D MRI/CT volumes can be extrapolated to create high-quality phantom models; however, these sometimes involve complicated pre-processing and mostly are challenging, due to the inaccessibility of these datasets or the requirement for approval to utilize the data. Therefore, it is necessary to develop a 3D tissue-mimicking phantom model consisting of different compartments with characteristics that can be easily modified. In this tutorial, we present an optimized development process of a generic 3D complex digital vasculature phantom model in Blender. The proposed workflow is such that an accurate and easily editable digital phantom can be developed. Other workflows for creating the same phantom will take much longer to set up and require more time to edit. We have made a few examples of editable 3D phantom models, which are publicly available to test and modify.

/pdf/tutorial-on-development-of-3d-vasculature-digital-phantoms-1367jwwd.pdf

Seyed Mohsen Ranjbaran

Papers

Transfontanelle photoacoustic imaging: ultrasound transducer selection analysis.

Development of a stationary 3D photoacoustic imaging system using sparse single-element transducers: Phantom study

Randomized multi‐angle illumination for improved linear array photoacoustic computed tomography in brain

2D-FC-ADMM reconstruction algorithm for quantitative optoacoustic tomography in a highly scattering medium: simulation study

Tutorial on Development of 3D Vasculature Digital Phantoms for Evaluation of Photoacoustic Image Reconstruction Algorithms