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 long-term community efforts required to overcome them, both within and beyond the IPASC group.

Feasibility of correlation mapping optical coherence tomography angiographic technique using a 200 kHz vertical-cavity surface-emitting laser source for in vivo microcirculation imaging applications.

Optical coherence tomography (OCT) angiography is a well-established in vivo imaging technique to assess the overall vascular morphology of tissues and is an emerging field of research for the assessment of blood flow dynamics and functional parameters such as oxygen saturation. In this study, we present a modified scanning-based correlation mapping OCT using a 200 kHz high-speed swept-source OCT system operating at 1300 nm and demonstrate its wide field-imaging capability in ocular angiographic studies.

Application of cmOCT and continuous wavelet transform analysis to the assessment of skin microcirculation dynamics.

Correlation mapping optical coherence tomography (cmOCT) is a powerful technique for the imaging of skin microvessels structure, based on the discrimination of the static and dynamic regions of the tissue. Although the suitability of cmOCT to visualize the microcirculation has been proved in humans and animal models, less evidence has been provided about its application to examine functional dynamics. Therefore, the goal of this research was validating the cmOCT method for the investigation into microvascular function and vasomotion. A spectral domain optical coherence tomography (SD-OCT) device was employed to image 90 sequential three-dimensional (3-D) OCT volumes from the forearm of 12 volunteers during a 25-min postocclusive reactive hyperemia (PORH) test. The volumes were processed using cmOCT to generate blood flow maps at selected cutaneous depths. The maps clearly trace flow variations during the PORH response for both capillaries and arterioles/venules microvascular layers. Continuous blood flow signals were reconstructed from cmOCT maps to study vasomotion by applying wavelet transform spectral analysis, which revealed fluctuations of flow during PORH, reflecting the regulation of microvascular tone mediated by endothelial cells and sympathetic nerves. The results clearly demonstrate that cmOCT allows the generation of functional information that may be used for diagnostic applications.

Preoperative measurement of cutaneous melanoma and nevi thickness with photoacoustic imaging.

Photoacoustic imaging (PAI) is an emerging biomedical imaging technology, which can potentially be used in the clinic to preoperatively measure melanoma thickness and guide biopsy depth and sample location. We recruited 27 patients with pigmented cutaneous lesions suspicious for melanoma to test the feasibility of a handheld linear-array photoacoustic probe in imaging lesion architecture and measuring tumor depth. The probe was assessed in terms of measurement accuracy, image quality, and ease of application. Photoacoustic scans included single wavelength, spectral unmixing, and three-dimensional (3-D) scans. The photoacoustically measured lesion thickness gave a high correlation with the histological thickness measured from resected surgical samples ([Formula: see text], [Formula: see text] for melanomas, [Formula: see text], [Formula: see text] for nevi). Thickness measurements were possible for 23 of 26 cases for nevi and all (6) cases for melanoma. Our results show that handheld, linear-array PAI is highly reliable in measuring cutaneous lesion thickness in vivo, and can potentially be used to inform biopsy procedure and improve patient management.

Simultaneous en-face imaging of multiple layers with multiple reference optical coherence tomography.

A technique based on multiple reference optical coherence tomography (MR-OCT) is proposed for simultaneous imaging at multiple depths. The technique has been validated by imaging a reference sample and a fingerprint in-vivo. The principle of scanning multiple selected layers is shown by imaging a partial fingerprint with 200×200×200 voxels of 3×3×0.5 mm size and obtaining an arbitrary amount of layers merely by digital processing. The spacing among the layers can be adjusted arbitrarily, and the SNR roll-off is shown for three different spacings. At a mirror scan frequency of 1 kHz and an A-line rate of 2 kHz, the acquisition time was 20 s for one volume. The results show the feasibility of the application of layer scanning MR-OCT that uses a partial mirror in the reference arm of the Michelson interferometer. The reduced scan range required for layer scanning allows even higher scan rates that are limited only by the voice coil design and the mass-spring system, e.g., mirror mass, spring constant, and damping.

Development of a first-generation miniature multiple reference optical coherence tomography imaging device.

Multiple reference optical coherence tomography (MR-OCT) is a technology ideally suited to low-cost, compact OCT imaging. This modality is an extension of time-domain OCT with the addition of a partial mirror in front of the reference mirror. This enables extended, simultaneous depth scanning with the relatively short scan range of a miniature voice coil motor on which the scanning mirror is mounted. This work details early stage development of the first iteration of a miniature MR-OCT device. This iteration utilizes a fiber-coupled input from an off-board superluminescent diode. The dimensions of the module are 40 × 57 ?? mm . Off-the-shelf miniature optical components, voice coil motors, and photodetectors are used, with the complexity of design depending on the specific application. The photonic module can be configured as either polarized or nonpolarized and can include balanced detection. The results shown in this work are from the nonpolarized device. The system was characterized through measurement of the input spectrum, axial resolution, and signal-to-noise ratio. Typical B-scans of static and in vivo samples are shown, which illustrate the potential applications for such a technology.

An Updated Review of Methods and Advancements in Microvascular Blood Flow Imaging.

There has been a consistent growth in research involving imaging of microvasculature over the past few decades. By 2008, publications mentioning the microcirculation had grown more than 2000 per annum. Many techniques have been demonstrated for the measurement of the microcirculation ranging from the earliest invasive techniques to the present high-speed, high-resolution noninvasive imaging techniques. Understanding the microvasculature is vital in tackling fundamental research questions as well as to understand the effects of disease progression on the physiological well-being of an individual. We have previously provided a wide-ranging review covering most of the available techniques and their applications. In this review, we discuss the recent advances made and applications in the field of microcirculation imaging.

Linear-array-based photoacoustic imaging of human microcirculation with a range of high frequency transducer probes.

Photoacoustic imaging (PAI) with a linear-array-based probe can provide a convenient means of imaging the human microcirculation within its native structural context and adds functional information. PAI using a multielement linear transducer array combined with multichannel collecting system was used for in vivo volumetric imaging of the blood microcirculation, the total concentration of hemoglobin (HbT), and the hemoglobin oxygen saturation (sO2) within human tissue. Three-dimensional (3-D) PA and ultrasound (US) volumetric scans were acquired from the forearm skin by linearly translating the transducer with a stepper motor over a region of interest, while capturing two-dimensional images using 15, 21, and 40 MHz frequency transducer probes. For the microvasculature imaging, PA images were acquired at 800- and 1064-nm wavelengths. For the HbT and sO2 estimates, PA images were collected at 750- and 850-nm wavelengths. 3-D microcirculation, HbT, and sO2 maps of the forearm skin were obtained from normal subjects. The linear-array-based PAI has been found promising in terms of resolution, imaging depth, and imaging speed for in vivo microcirculation imaging within human skin. We believe that a reflection type probe, similar to existing clinical US probes, is most likely to succeed in real clinical applications. Its advantages include ease of use, speed, and familiarity for radiographers and clinicians.

Feasibility of intracoronary frequency domain optical coherence tomography derived fractional flow reserve for the assessment of coronary artery stenosis.

Frequency domain optical coherence tomography (FD-OCT) provides cross-sectional images of coronary arteries and deployed stents with micron resolution and measures lumen dimensions with excellent reproducibility. FD-OCT combined with a blood flow resistances model can overcome many limitations of conventional measures of stenosis severity based on quantitative coronary angiography (QCA) and intravascular ultrasound (IVUS). The aim of this feasibility study was to investigate the relationship between pressure derived fractional flow reserve (FFR) and FD-OCT derived FFR, a new method for quantitative measure of stenosis severity that estimates the blood flow resistance and microvascular resistance of the vessel segments imaged by FD-OCT. A total of 26 coronary stenoses in 20 patients were studied consecutively with QCA, pressure derived FFR, and FD-OCT. There was a moderate but significant correlation between pressure derived FFR and FD-OCT derived FFR (r = 0.69, P < 0.001). Bland-Altman analysis showed that the mean differences between pressure derived FFR and FD-OCT derived FFR were 0.05 ± 0.14 (limits of agreement: -0.09 to 0.19). The root mean square error (RMSE) between FD-OCT derived FFR and pressure derived FFR was found to be ± 0.087 FFR units. FD-OCT derived FFR has the potential to become a valuable tool for the assessment of coronary artery stenosis.

Evaluation of hemodynamically severe coronary stenosis as determined by fractional flow reserve with frequency domain optical coherence tomography measured anatomical parameters.

OBJECTIVES: The main objective of this study is to determine the correlation between fractional flow reserve (FFR)- and frequency domain optical coherence tomography (FD-OCT)-measured lumen parameters, and to determine the diagnostic competence of FD-OCT concerning the identification of severe coronary stenosis. METHODS: A total of 41 coronary stenoses in 30 patients were assessed consecutively by quantitative coronary angiography (QCA), FFR, and FD-OCT. Stenoses were labeled severe if FFR ≤ 0.80. The minimal lumen area (MLA), minimal lumen diameter (MLD), and percent lumen area stenosis (%AS) were measured using FD-OCT. RESULTS: FFR was ≤ 0.80 in 10 stenoses (24.4%). A poor but significant correlation between FFR and FD-OCT-measured MLA (r(2) = 0.4, p < 0.001), MLD (r(2) = 0.28, p < 0.001), and %AS (r(2) = 0.13, p = 0.02) was found. In the overall group, the diagnostic efficiency of MLA and MLD in identifying significant stenosis was moderate. The area under the curve (AUC) was 0.80 [95% confidence interval (CI): 0.64-0.91] for MLA and 0.76 (95% CI: 0.60-0.88) for MLD. The best cut-off values of FD-OCT-measured lumen parameters to identify stenosis with FFR ≤ 0.80 were 1.62 mm(2) [specificity 97%, sensitivity 70%, positive predictive value (PPV) 89% and negative predictive value (NPV) 91%] for MLA and 1.23 mm (specificity 87%, sensitivity 70%, PPV 64% and NPV 90%) for MLD. The diagnostic efficiency of MLA in identifying significant stenosis in vessels having reference diameter < 3 mm was high. The AUC was 0.96 (95% CI: 0.83-1.0). CONCLUSIONS: The FFR values and FD-OCT anatomical parameters MLA, MLD were found to be significantly correlated. In the overall group, the FD-OCT-measured MLA and MLD have shown moderate diagnostic efficiency in the functional evaluation of significant stenosis. FD-OCT-measured MLA has high diagnostic efficiency in identifying severe coronary stenosis in vessels having reference diameter < 3 mm.

Microcirculation imaging based on full-range high-speed spectral domain correlation mapping optical coherence tomography.

Microcirculation imaging is a key parameter for studying the pathophysiological processes of various disease conditions, in both clinical and fundamental research. A full-range spectral-domain correlation mapping optical coherence tomography (cm-OCT) method to obtain a complex-conjugate-free, full-range depth-resolved microcirculation map is presented. The proposed system is based on a high-speed spectrometer at 91 kHz with a modified scanning protocol to achieve higher acquisition speed to render cm-OCT images with high-speed and wide scan range. The mirror image elimination is based on linear phase modulation of B-frames by introducing a slight off-set of the probe beam with respect to the lateral scanning fast mirror's pivot axis. An algorithm that exploits the Hilbert transform to obtain a complex-conjugate-free image in conjunction with the cm-OCT algorithm is used to obtain full-range imaging of microcirculation within tissue beds in vivo. The estimated sensitivity of the system was around 105 dB near the zero-delay line with ∼20 dB roll-off from ±0.5 to ±3 mm imaging-depth position. The estimated axial and lateral resolutions are ∼12 and ∼30 µm, respectively. A direct consequence of this complex conjugate artifact elimination is the enhanced flow imaging sensitivity for deep tissue imaging application by imaging through the most sensitive zero-delay line and doubling the imaging range.

Measurement of the blood flow rate and velocity in coronary artery stenosis using intracoronary frequency domain optical coherence tomography: Validation against fractional flow reserve.

OBJECTIVES: The main objective of this study was to assess the blood flow rate and velocity in coronary artery stenosis using intracoronary frequency domain optical coherence tomography (FD-OCT). A correlation between fractional flow reserve (FFR) and FD-OCT derived blood flow velocity is also included in this study. METHODS & RESULTS: A total of 20 coronary stenoses in 15 patients were assessed consecutively by quantitative coronary angiography (QCA), FFR and FD-OCT. A percutaneous coronary intervention (PCI) optimization system was used in this study which combines wireless FFR measurement and FD-OCT imaging in one platform. Stenoses were labelled severe if FFR ≤ 0.8. Blood flow rate and velocity in each stenosis segment were derived from the volumetric analysis of the FD-OCT pull back images. The FFR value was ≤ 0.80 in 5 stenoses (25%). The mean blood flow rate in severe coronary stenosis (n = 5) was 2.54 ± 0.55 ml/s as compared to 4.81 ± 1.95 ml/s in stenosis with FFR > 0.8 (n = 15). A good and significant correlation between FFR and FD-OCT blood flow velocity in coronary artery stenosis (r = 0.74, p < 0.001) was found. CONCLUSION: The assessment of stenosis severity using FD-OCT derived blood flow rate and velocity has the ability to overcome many limitations of QCA and intravascular ultrasound (IVUS).

In vivo full-field en face correlation mapping optical coherence tomography.

A full-field optical coherence tomography (OCT) system has been developed for the purpose of performing nonscanning en face flow imaging. The light source is centered at 840 nm with a bandwidth of 50 nm resulting in an axial resolution of 8 µm in air. Microscope objectives with a numerical aperture of 0.1 were incorporated giving a transverse resolution of 5 µm. A magnification of 5.65 was measured, resulting in a field of view of 1260×945 µm. Pairs of interference fringe images are captured with opposing phase and a two-step phase image reconstruction method is applied to reconstruct each en face image. The OCT frame rate is 10 Hz. A two-dimensional cross-correlation technique is applied to pairs of consecutive en face images in order to distinguish dynamic from static light-scatterers. The feasibility of the method was examined by simulating blood flow by creating a phantom with 5% intralipid solution. In vivo imaging of a Xenopus laevis tadpole was also performed in order to investigate the feasibility of imaging the vascular system. We present for what we believe to be the first time, the application of correlation mapping optical coherence tomography to full-field OCT to provide in vivo functional imaging of blood vessels.

Optimization and extraction of functional information from in vitro flow models using dual-beam spectral-domain optical coherence tomography cross-correlation analysis.

As in vivo flow behavior can be pulsatile, intermittent, and/or otherwise changeable with time, the ability to provide clinicians with a means of real-time visualization and functional assessment of structures is of particular importance. The discernment of pulsatile flow behavior using a dual-beam spectral domain optical coherence tomography system (db-SdOCT) by quasi-simultaneous measurement by two planes of illumination is demonstrated. By cross-correlation analysis, it is possible to compute velocity metrics pertaining to flowing particle motion, without a priori angular knowledge. This is the first application of cross-correlation-based dynamic assessment for the extraction of pulsatile behavior in an in vitro environment using an optimized db-SdOCT system. The experimental results outlined have shown the db-SdOCT system and its associated algorithms to be successful in the discernment of intermittent pulsatile flow behavior in in vitro models, concurrent to yielding velocity values in good agreement with that of the applied flow rate.

Feasibility of correlation mapping optical coherence tomography (cmOCT) for anti-spoof sub-surface fingerprinting.

We propose the use of correlation mapping optical coherence tomography (cmOCT) to deliver additional biometrics associated with the finger that could complement existing fingerprint technology for law enforcement applications. The current study extends the existing fingerprint paradigm by measuring additional biometrics associated with sub-surface finger tissue such as sub-surface fingerprints, sweat glands, and the pattern of the capillary bed to yield a user-friendly cost effective and anti-spoof multi-mode biometric solution associated with the finger. To our knowledge no other method has been able to capture sub-surface fingerprint, papillary pattern and horizontal vessel pattern in a single scan or to show the correspondence between these patterns in live adult human fingertip. Unlike many current technologies this approach incorporates 'liveness' testing by default. The ultimate output is a biometric module which is difficult to defeat and complements fingerprint scanners that currently are used in border control and law enforcement applications.

Blood optical clearing studied by optical coherence tomography.

The main limitation of optical imaging techniques for studying biological tissues is light scattering leading to decreasing of transmittance, which lowers the imaging quality. In this case, an immersion method for optical clearing of biological tissues can provide a possible solution to this problem, because the application of biocompatible clearing agents can reduce light scattering. Optical clearing represents a promising approach to increasing the imaging depth for various techniques, for example, various spectroscopy and fluorescent methods, and optical coherence tomography (OCT). We investigate the improvement of light penetration depth in blood after application of polyethylene glycol, polypropylene glycol, propylene glycol, and hemoglobin solutions using an OCT system. Influence of clearing agents on light transport in tissues and blood was also investigated in the mouse tail vein.

Feasibility of capillary velocity assessment by statistical means using dual-beam spectral-domain Optical Coherence Tomography: a preliminary study.

The assessment of vascular dynamics has been shown to yield both qualitative and quantitative metrics and thus play a pivotal role in the diagnosis and prognosis of various diseases, which may manifest as microcirculatory irregularities. Optical Coherence Tomography (OCT) is an established imaging modality which utilises the principle of optical interferometry to distinguish between spatial changes in refractive index and thus formulate a multi-dimensional representation of a specimen in vivo. Nonetheless, difficulties remain in obtaining accurate data (morphological and/or transient) in an environment which is subject to such large biological variability. In an effort to address the issue of angular dependence as with Doppler based analysis, a dual-beam Spectral-domain OCT system for quasi-simultaneous specimen scanning is described. A statistical based method of phase correlation is outlined which is capable of quantifying velocity values in addition to the ability to discern bidirectionality, without the necessity of angular computation.

'Go with the flow ': a review of methods and advancements in blood flow imaging.

Physics has delivered extraordinary developments in almost every facet of modern life. From the humble thermometer and stethoscope to X-Ray, CT, MRI, ultrasound, PET and radiotherapy, our health has been transformed by these advances yielding both morphological and functional metrics. Recently high resolution label-free imaging of the microcirculation at clinically relevant depths has become available in the research domain. In this paper, we present a comprehensive review on current imaging techniques, state-of-the-art advancements and applications, and general perspectives on the prospects for these modalities in the clinical realm.

Rapid quantification of histamine in human psoriatic plaques using microdialysis and ultra high performance liquid chromatography with fluorescence detection.

Psoriasis is a chronic skin disease resulting from abnormal immune function and is characterized by the presence of scaly psoriatic plaques which are areas of inflammation and excessive skin production. The psoriatic plaques contain mast cells which are increased in number in the uppermost dermis of the psoriatic lesion and which may play a role in the initiation and maintenance of the lesion. These processes are thought to be mediated via the local release of histamine along with other mediators from the mast cells; however their precise role still remains a mystery. Our study involved the development of a rapid and ultra-sensitive liquid chromatographic method for the separation and detection of histamine. To this end a state-of-the-art ultra high pressure liquid chromatography (UHPLC) system incorporating the latest technology in fluorescence detection system was employed which allowed for the rapid and reliable trace level detection of histamine in human derived microdialysate samples. This new reverse phase method utilized a sub-two-micron packed C(18) stationary phase (50 mm × 4.6 mm, 1.8 µm particle size) and a polar mobile phase of ACN:H(2)O:acetic acid (70:30:0.05) (v/v). The column temperature was maintained at (30±2°C), the injection volume was (8 µl), with a flow rate of (1.1 ml/min). Dermal microdialysis was used to collect (20 µl) samples from healthy, peri-lesional and lesional skin regions, in the forearms of a small cohort of subjects (n=6), and the ultra sensitive liquid chromatographic method allowed for nanomolar quantitation of histamine in 6.7 min. To date this represents one of the fastest reported separations of histamine using fluorescence detection with very high chromatographic efficiency (258,000/m) and peak symmetry of (0.88). Prior to sample analysis being performed method linearity, precision and limit of detection (LOD) were investigated. The results showed that intracutaneous histamine measured at 70 min after catheter implantation was (3.44±.52 nmol) (mean±SEM) in non-lesional (control) skin and was not dissimilar to that observed in either lesional (3.10±.76 nmol) or peri-lesional skin (2.24±.20 nmol). A second fraction collected 190 min after implantation also revealed similar levels with no difference in intracutaneous histamine observed between control (2.41±.56 nmol), lesional (2.69±.54 nmol), or peri-lesional skin (2.25±.50 nmol).

Correlation mapping method for generating microcirculation morphology from optical coherence tomography (OCT) intensity images.

Standard optical coherence tomography (OCT) in combination with software tools can be harnessed to generate vascular maps in vivo. In this study we have successfully combined a software algorithm based on correlation statistic to reveal microcirculation morphology on OCT intensity images of a mouse brain in vivo captured trans-cranially and through a cranial window. We were able to estimate vessel geometry at bifurcation as well as along vessel segments down-to mean diameters of about 24 µm. Our technique has potential applications in cardiovascular-related parameter measurements such as volumetric flow as well as in assessing vascular density of normal and diseased tissue.