Enhancing photoacoustic visualization of medical devices with elastomeric nanocomposite coatings

Ultrasound (US) imaging is widely used for guiding minimally invasive procedures. However, with this modality, there can be poor visibility of interventional medical devices such as catheters and needles due to back-reflections outside the imaging aperture and low echogenicity. Photoacoustic (PA) imaging has shown promise with visualising bare metallic needles. In this study, we demonstrate the feasibility of a light emitting diode (LED)-based PA and US dual-modality imaging system for imaging metallic needles and polymeric medical catheters in biological tissue. Four medical devices were imaged with the system: two 20-gauge spinal needles with and without a multi-walled carbon nanotube / polydimethylsiloxane (MWCNT/PDMS) composite coating, and two 18-gauge epidural catheters with and without the MWCNT/PDMS composite coating. These devices were sequentially inserted into layers of chicken breast tissue within the US imaging plane. Interleaved PA and US imaging was performed during insertions of the needle and catheter. With US imaging, the uncoated needle had very poor visibility at an insertion angle of 45°. With PA imaging, the uncoated needle was not visible, but its coated counterpart was clearly visualised up to depths of 35 mm. Likewise, both catheters were not visible with US imaging. The uncoated catheter was not visible on PA images, but its coated counterpart was clearly visualised up to depths of 35 mm. We conclude that the highly absorbing CNT/PDMS composite coating conferred excellent visibility for medical devices with the LED-based PA imaging system and that it is promising for translation in minimally invasive procedures

Imaging of human peripheral blood vessels during cuff occlusion with a compact LED-based photoacoustic and ultrasound system

Non-invasive imaging plays an important role in diagnosing and monitoring peripheral artery disease (PAD). Doppler ultrasound imaging can be used for measuring blood flow in this context. However, this technique frequently provides low contrast for flow in small vessels. Photoacoustic imaging can allow for the visualization of blood in small vessels, with direct contrast from optical absorption of hemoglobin. In this work, we investigate the potential applications of a compact LED-based photoacoustic (850 nm) and ultrasound imaging system for visualizing human peripheral blood vessels during cuff occlusion. Each measurement comprised three stages. First, a baseline measurement of a digital artery of a human finger from a volunteer without a diagnosis of PAD was performed for several seconds. Second, arterial blood flow was stopped using an occlusion cuff, with a rapid increase of pressure up to 220 mm Hg. Third, the occlusion cuff was released rapidly. Raw photoacoustic and ultrasound image data (frame rate: 70 Hz) were recorded for the entire duration of the measurement (20 s). The average photoacoustic image amplitude over an image region that enclosed the digital artery was calculated. With this value, pulsations of image amplitudes from the arteries was clearly visualized. The average photoacoustic image amplitude decreased during the increase in cuff pressure and it was followed by a rapid recovery during cuff release. With real-time non-invasive measurements of peripheral blood vessel dynamics in vivo, the compact LED-based system could be valuable for point-of-care imaging to guide treatment of PAD

Photoacoustic imaging of the human placental vasculature

Minimally invasive fetal interventions require accurate imaging from inside the uterine cavity. Twin‐to‐twin transfusion syndrome (TTTS), a condition considered in this study, occurs from abnormal vascular anastomoses in the placenta that allow blood to flow unevenly between the fetuses. Currently, TTTS is treated fetoscopically by identifying the anastomosing vessels, and then performing laser photocoagulation. However, white light fetoscopy provides limited visibility of placental vasculature, which can lead to missed anastomoses or incomplete photocoagulation. Photoacoustic (PA) imaging is an alternative imaging method that provides contrast for hemoglobin, and in this study, two PA systems were used to visualize chorionic (fetal) superficial and subsurface vasculature in human placentas. The first system comprised an optical parametric oscillator for PA excitation and a 2D Fabry‐Pérot cavity ultrasound sensor; the second, light emitting diode arrays and a 1D clinical linear‐array ultrasound imaging probe. Volumetric photoacoustic images were acquired from ex vivo normal term and TTTS‐treated placentas. It was shown that superficial and subsurface branching blood vessels could be visualized to depths of approximately 7 mm, and that ablated tissue yielded negative image contrast. This study demonstrated the strong potential of PA imaging to guide minimally invasive fetal therapies

A review of a strategic roadmapping exercise to advance clinical translation of photoacoustic imaging: From current barriers to future adoption

Photoacoustic imaging (PAI), also referred to as optoacoustic imaging, has shown promise in early-stage clinical trials in a range of applications from inflammatory diseases to cancer. While the first PAI systems have recently received regulatory approvals, successful adoption of PAI technology into healthcare systems for clinical decision making must still overcome a range of barriers, from education and training to data acquisition and interpretation. The International Photoacoustic Standardisation Consortium (IPASC) undertook an community exercise in 2022 to identify and understand these barriers, then develop a roadmap of strategic plans to address them. Here, we outline the nature and scope of the barriers that were identified, along with short-, medium- and longterm community efforts required to overcome them, both within and beyond the IPASC group

Identification and elimination of reflection artifacts in biomedical photoacoustic imaging

Photoacoustic (PA) or optoacoustic imaging is a hybrid imaging modality that acoustically detects optical absorption contrast via the PA effect, a physical phenomenon that converts absorbed optical energy into acoustic energy. PA imaging is one of the fastest growing fields in biomedical optics, and has emerged as a powerful biomedical imaging modality in the last decade. Influence of acoustic inhomogeneities and resulting reflection artifacts is an important problem in PA imaging. These artifacts cause problems in image interpretation and significantly impact the contrast and imaging depth in any clinical applications of epi-mode or interstitial illumination PA imaging. The research presented in this thesis proposes a novel method called PAFUSion (photoacoustic-guided focused ultrasound) for identification and elimination of reflection artifacts in biomedical PA imaging. Potential of PAFUSion in epi-mode and interstitial illumination PA imaging is demonstrated in various chapters using simulations, phantom experiments, ex vivo studies, and in vivo measurements on human volunteers

PhotoAcoustic-guided Focused UltraSound imaging (PAFUSion) for reducing reflection artifacts in photoacoustic imaging

Reflection artifacts caused by acoustic reflectors is an important problem in reflection-mode photoacoustic imaging. The light absorbed by skin and superficial optical absorbers may produce high photoacoustic signals, which traverse into the tissue and get reflected from structures having different acoustic impedance. These reflected photoacoustic signals, when reconstructed may appear in the region of interest, which causes complications in interpreting the images. We propose a novel method to identify and reduce reflection artifacts in photoacoustic images by making use of PhotoAcoustic-guided Focused UltraSound [PAFUSion]. Our method ultrasonically mimics the photoacoustic image formation process and thus delivers a clinically feasible way to reduce reflection artifacts. Simulation and phantom measurement results are presented to demonstrate the validity and impact of this method. Results show that PAFUSion technique can identify and differentiate reflection signals from the signals of interest and thus foresees good potential for improving photoacoustic imaging of deep tissue

In vivo demonstration of reflection artifact reduction in photoacoustic imaging using synthetic aperture photoacoustic-guided focused ultrasound (PAFUSion)

Reflection artifacts caused by acoustic inhomogeneities are a critical problem in epi-mode biomedical photoacoustic imaging. High light fluence beneath the probe results in photoacoustic transients, which propagate into the tissue and reflect back from echogenic structures. These reflection artifacts cause problems in image interpretation and significantly impact the contrast and imaging depth. We recently proposed a method called PAFUSion (Photoacoustic-guided focused ultrasound) to identify such reflection artifacts in photoacoustic imaging. In its initial version, PAFUSion mimics the inward-travelling wavefield from small blood vessel-like PA sources by applying ultrasound pulses focused towards these sources, and thus provides a way to identify the resulting reflection artifacts. In this work, we demonstrate reduction of reflection artifacts in phantoms and in vivo measurements on human volunteers. In view of the spatially distributed PA sources that are found in clinical applications, we implemented an improved version of PAFUSion where photoacoustic signals are backpropagated to imitate the inward travelling wavefield and thus the reflection artifacts. The backpropagation is performed in a synthetic way based on the pulse-echo acquisitions after transmission on each single element of the transducer array. The results provide a direct confirmation that reflection artifacts are prominent in clinical epi-photoacoustic imaging, and that PAFUSion can strongly reduce these artifacts to improve deep-tissue photoacoustic imaging

Reflection-artifact-free photoacoustic imaging using PAFUSion (photoacoustic-guided focused ultrasound)

Reflection artifacts caused by acoustic inhomogeneities are a main challenge to deep-tissue photoacoustic imaging. Photoacoustic transients generated by the skin surface and superficial vasculature will propagate into the tissue and reflect back from echogenic structures to generate reflection artifacts. These artifacts can cause problems in image interpretation and limit imaging depth. In its basic version, PAFUSion mimics the inward travelling wave-field from blood vessel-like PA sources by applying focused ultrasound pulses, and thus provides a way to identify reflection artifacts. In this work, we demonstrate reflection artifact correction in addition to identification, towards obtaining an artifact-free photoacoustic image. In view of clinical applications, we implemented an improved version of PAFUSion in which photoacoustic data is backpropagated to imitate the inward travelling wave-field and thus the reflection artifacts of a more arbitrary distribution of PA sources that also includes the skin melanin layer. The backpropagation is performed in a synthetic way based on the pulse-echo acquisitions after transmission on each single element of the transducer array. We present a phantom experiment and initial in vivo measurements on human volunteers where we demonstrate significant reflection artifact reduction using our technique. The results provide a direct confirmation that reflection artifacts are prominent in clinical epi-photoacoustic imaging, and that PAFUSion can reduce these artifacts significantly to improve the deep-tissue photoacoustic imaging