Photoacoustic clinical imaging

Abstract

Photoacoustic is an emerging biomedical imaging modality, which allows imaging optical absorbers in the tissue by acoustic detectors (light in - sound out). Such a technique has an immense potential for clinical translation since it allows high resolution, sufficient imaging depth, with diverse endogenous and exogenous contrast, and is free from ionizing radiation. In recent years, tremendous developments in both the instrumentation and imaging agents have been achieved. These opened avenues for clinical imaging of various sites allowed applications such as brain functional imaging, breast cancer screening, diagnosis of psoriasis and skin lesions, biopsy and surgery guidance, the guidance of tumor therapies at the reproductive and urological systems, as well as imaging tumor metastases at the sentinel lymph nodes. Here we survey the various clinical and pre-clinical literature and discuss the potential applications and hurdles that still need to be overcome.

Fig. 1

various configurations for PAI. (a) Optical resolution photoacoustic microscopy. Here the optical excitation is focused and co-aligned with the acoustic transducer. While many configurations exist to maintain alignment, a simple but effective one was devised by Wang et al. [1]. Here two prisms are joined together with a thin layer of silicone oil between them. This layer transmits light interruptedly but reflects the acoustic waves and thus acts as an optical/acoustic splitter. A rhomboid prism is required for the acoustic side to ascertain that a longitudinal wave (rather than sheer) reaches the transducer. Penetration depth is very shallow in order to maintain the optical focusing. The optical focal region (mm scale) is much smaller than the acoustic one and thus determines the (lateral) resolution which can be sub-cellular. (b) Acoustic resolution photoacoustic microscopy. Here the light is delivered around the transducer to create uniform illumination. The unfocused light allows for greater penetration. Both the lateral and axial resolutions are determined by the acoustic focal region. See for example Kim et al. [3] (c) Acoustic resolution photoacoustic tomography/macroscopy. Here neither the light nor the sound are focused which allows several cm of imaging depth. Imaging is based on multiple transducers which views each point in the region of interest (ROI) from a different angle and thus able to resolve it. See for example Witte et al [8]. US- Ultrasound, MO - Microscope objective, UST - Ultrasound transducer

Fig. 2

Multispectral photoacoustic imaging of glioblastoma and liposomal ICG distribution in brain. (a) Schematic representation of the MSOT system. A curved array of wideband and cylindrically focused ultrasound transducers enables parallel data acquisition. Optical fibers are used to homogeneously illuminate the object. A special animal holder with a transparent plastic membrane is used for animal positioning. (b) A picture of the mouse during a scan, showing the position of the mouse and illumination with respect to the array of focused wideband ultrasound transducers. (c) The spectrally unmixed Hb pseudo color overlay on an 800 nm single wavelength MSOT image from an animal 34 day following implantation with U87 glioblastoma cells (d) The corresponding ex vivo cryosection. (e) A Hb image 16 day following implantation following a 10% carbon dioxide challenge. (f) The corresponding ex vivo fluorescence image showing tumor size and location. (g) A 900 nm single wavelength MSOT image (grayscale) with an overlay (green) of the spectrally-resolved liposome-ICG signal, from a mouse injected intraventricularly with ICG encapsulated into liposomes. (h) The equivalent cryoslice with an overlay of the fluorescence from the injected particles. Panels A, B were adapted from: Adrian Taruttis, Eva Herzog, Daniel Razansky, and Vasilis Ntziachristos, "Real-time imaging of cardiovascular dynamics and circulating gold nanorods with multispectral optoacoustic tomography," Opt. Express 18, 19592–19602 (2010); https://doi.org/10.1364/OE.18.019592 Panels C-H were from NeuroImage, Vol 65, Neal C. Burton, Manishkumar Patel, Stefan Morscher, Wouter H.P. Driessen, Jing Claussen, Nicolas Beziere, Thomas Jetzfellner, Adrian Taruttis, Daniel Razansky, Bohumil Bednar, Vasilis Ntziachristos, Multispectral Opto-acoustic Tomography (MSOT) of the Brain and Glioblastoma Characterization, p.522–528, Copyright (2013), with permission from Elsevier.

Fig. 3

photoacoustic and ultrasound Doppler thyroid imaging using a hand-held probe. (a) The curved, hand-held photoacoustic device with wide optical illumination for in vivo thyroid imaging.. (b) Anatomy of thyroid gland including cardio-vascular and respiratory system; the cross-sectional 2D imaging plane is highlighted in green. (c) PAI cross-section of the left thyroid lobe from a healthy volunteer. The image, leveled and normalized from 0 to 1, shows with high sensitivity vascular features of skin, muscles and within the thyroid lobe. (d) The corresponding ultrasound image in grayscale also depicts Directional Power Doppler signals superimposed in color (red/blue colormaps indicating opposite flow directions). C: Carotid, T: Thyroid, Tr: Trachea, s: sternocleidomastoid muscle, m: infrahyoid muscle; axes in mm. From: Alexander Dima, Vasilis Ntziachristos, "In-vivo handheld optoacoustic tomography of the human thyroid," Photoacoustics Volume 4, Issue 2, Copyright June 2016, Pages 65–69; https://doi.org/10.1016/j.pacs.2016.05.003; https://creativecommons.org/licenses/by/4.0/

Fig. 4

An example of PA mass appearance seen in a 63-year-old patient (P55) with infiltrating ductal carcinoma (IDC). The breast lesion was highly suspicious on XRM (not shown) by the presence of an irregularly shaped, unsharply delineated, 20-mm mass. (a) The average intensity projection (AIP) PA image is shown tilted due to the breast being tilted during the PA measurement to position the lesion favorably in front of the detector. In the PA image, the lesion is clearly visible as an irregular, high-contrast, 29-mm mass. The lesion colocalized perfectly with the lesion on XRM (not shown). The lesion also colocalized well with (b) the AIP MR image after tilting the PA image. The dashed box in the MR image indicates the area from which the PA image is acquired. The MR appearance is described as an irregularly shaped mass. (c) A histopathological assessment of the tissue specimen post-surgery revealed the presence of a 34-mm IDC, grade 2. (d) The CD31-stained tumor section shows the microvascularity spread over the entire lesion supporting the mass appearance observed in PA and MR images. It is intriguing that the patterns in ‘a’–‘d’ appear roughly similar in appearance. © 2015 IEEE. Reprinted, with permission, from Michelle Heijblom, Wiendelt Steenbergen, Srirang Manohar, "Clinical Photoacoustic Breast Imaging: The Twente experience," IEEE Pulse Volume 6, Issue 3, Pages 42-26; DOI: 10.1109/MPUL.2015.2409102.

Fig. 5

Comparison of visibilities of blood vessels between PAI and MRI using maximum intensity projection (MIP) images on the healthy breast. Case 1(a–c): (a) PA image, (b) MR image deformed to correspond to the PA image, and (c) fusion image of PA (cyan) and MR (red). Case 2 (d–f): (d) PA image, (e) MR image deformed to correspond to the PA image, and (f) fusion image of PA (cyan) and MR (red). All images are coronal views. We colored the signals according to the depth using the color chart shown in (g). Images were taken at 755 nm and 795 nm. From: Toi, M. et al. Visualization of tumor-related blood vessels in human breast by photoacoustic imaging system with a hemispherical detector array. Sci. Rep. 7, 41970; doi: 10.1038/srep41970 (2017). http://creativecommons.org/licenses/by/4.0/

Fig. 6

PA Imaging of the psoriasis biomarkers in the skin. (a), Schematic of the operation of UB-RSOM. The transducer is raster-scanned parallel to the skin surface, acquiring acoustic signals (A lines) that are generated by laser illumination from two fixed fiber bundles. The focal point of the transducer is kept above the skin surface. (b), Photograph of the scanning head and the articulated arm. (c), Photograph of the scanning head in position, ready for data acquisition. (d,e), The collected optoacoustic signals are filtered in two frequency bands and reconstructed into two images shown in red and green, representing low and high spatial frequencies, respectively. (f), UB-RSOM image of healthy skin that combines low and high spatial frequencies in (D,E) in a single color-coded image. Different biological structures appear in red or green depending on their spatial frequency. The epidermis (EP) appeared mostly in red, and could be clearly distinguished from the dermis (DR). In the dermal vasculature, the capillary loops (CL) could be distinguished from the epidermis and from the vascular plexus (VP) in the deep dermis. The larger vessels appeared mainly in red, whereas the smaller vessels appeared mainly in green. (g–i), Maximum intensity projections in the coronal direction of the epidermis, showing the indentations of the skin (g), the capillary loop layer (h) and the dermis (i). All scale bars, 500 μm. Reprinted by permission from RightsLink: Springer Nature; Nature Biomedical Engineering; "Precision assessment of label-free psoriasis biomarkers with ultra-broadband optoacoustic mesoscopy," Juan Aguirre, Mathias Schwarz, Natalie Garzorz, Murad Omar, Andreas Buehler et al. Copyright 2017.

Fig. 7

Photoacoustic imaging with corresponding fluorescence images of resected pancreatic cancer. (A) Bright field of primary pancreatic tumor in breadloaf section, (B) fluorescence overlay (C) heat-map fluorescent. (D) Corresponding B-mode ultrasound image of hypoechoic tumor, surrounded by white dotted line, (E) and corresponding photoacoustic image. (F). B-mode ultrasound (left) and Photoacoustic images (right) of tumor-positive lymph node surrounded by white dotted line. (G) Corresponding bright field (H), fluorescence overlay, (I), and heat-map fluorescent images of tumor-positive lymph node. Corresponding Scale bar represents 1 cm. T = tumor, P = normal pancreatic tissue. Reprinted by permission from RightsLink: Springer Nature; Annals of Surgical Oncology; "Intraoperative Pancreatic Cancer Detection using Tumor-Specific Multimodality Molecular Imaging," Willemieke S. Tummers, Sarah E. Miller, Nutte T. Teraphongphom et al. Copyright 2018.

Fig. 8

Photoacoustic-US imaging approach for minimally invasive SLN staging. (a) Anatomy showing the relevant structures. (b) Periareolar injection of methylene blue dye, which is collected by lymphatic vessels and drained to the sentinel lymph node. (c) PAT-US imaging pinpoints the SLN. (d) PAT-US imaging guides a FNAB of the SLN with high contrast. (e) The proposed probe features a flat surface for light delivery, and the fiber bundles are integrated into the ultrasonic transducer housing for ergonomic handling. (f) In vivo PAT image of the SLN and needle. (g) PAT image acquired at 650 nm showing both methylene blue dye in lymphatic vessels and hemoglobin in blood vessels. (h) PAT image acquired at 1064 nm showing primarily hemoglobin in blood vessels while suppressing the signals from methylene blue dye. (i) Image displaying the fractional change (650 nm signal relative to 1064 nm signal) convincingly highlights the locations of methylene blue dye. PA650 and PA1064: photoacoustic amplitudes measured at 650 nm and 1064 nm, respectively. From: Garcia-Uribe, A. et al. Dual-Modality Photoacoustic and Ultrasound Imaging System for Noninvasive Sentinel Lymph Node Detection in Patients with Breast Cancer. Sci. Rep. 5, Article number: 15748 (2015); https://doi.org/10.1038/srep15748. http://creativecommons.org/licenses/by/4.0/

Fig. 9

Dual transrectal photoacoustic and ultrasonic imaging of prostate cancer. (a) Schematic representation of the home built TRUSPA device imaging the human prostate. The devices uses a linear capacitive micro-machined ultrasound array integrated with fiber optic bundles and encapsulated with polydimethylsiloxane (PDMS) in the lens shape. The RF cable connects the CMUT array to the external hardware. (b) T1-weighted MRI overlaid of a 53-year-old male patient was diagnosed with prostate cancer in the left base of the prostate. Bladder (Bl), rectum (R), and prostate (P) in green contour and Tumor region (T) in red contour in (b,c) and yellow contour in (d,e) are marked. (c) Diffusion-weighted-MRI showing the malignancy (red contour) in the left peripheral zone of the prostate. (d) Co-registered ultrasound (US) on gray-scale and photoacoustic (PA) image on red color scale with intravenous ICG injection (25 mg; 10 ml at 2.5 mg/ml). Image was acquired 6-minute post-ICG injection. Green contour shows prostatic region and yellow contour surrounds the tumor region in the left base of the prostate. (e) Spectrally unmixed ICG image, obtained from multi-wavelength PA data of the post-ICG injection, shows ICG accumulation in the tumor region. Scale bars are 1 cm in length.

Fig. 10

Photoacoustic imaging of blood constituents. (a) Schematic of selected components of the experimental system. DM, dichroic mirror; MEMS, micro-electro-mechanical-system scanning mirror; OAC, optical-acoustic combiner; PBS, polarizing beamsplitter; UT, ultrasonic transducer. The 1064 nm and 532 nm imaging lasers are employed to image CTCs and vasculature, respectively. (b–e) Scheme of real-time detection and laser killing of CTCs. The CTC detector compares the earlier 1064 nm laser-induced CTC-specific PA signal against an optimized threshold level (purple dashed line in ‘b’ and ‘c’) above the Hb signal, and thus can reliably distinguish CTCs and trigger the therapy laser ‘d’. Within ˜10 μs, the therapy laser is fired and focused to the detected CTC location to photomechanically kill the CTC’ e’. (f) 532 nm laser-induced (Top), 1064 nm laser-induced (Middle), and fused (Bottom) flow cytography images. In the 1064 nm laser-induced image, the white arrow and yellow square indicate the detected CTC; the red and blue dashed lines delineate the artery and vein boundaries, respectively. From: He, Y. et al. in vivo label-free photoacoustic flow cytography and on-the-spot laser killing of single circulating melanoma cells. Scientific Reports 6, Article number: 39616 (2016); https://doi.org/10.1038/srep39616. http://creativecommons.org/licenses/by/4.0/

Fig. 11

Opportunities and challenges of PAI. (a) Photo of a physical PA phantom contains a round and a semi-triangular insertions with similar absorption that were embedded inside. (b) Reconstructed PAI of the phantom. Only the boundary of the round insertion is visible in the image. (c) Calculated fluence map (d) Calculated absorption coefficient after fluence compensation. Notice the differences between ‘B’ and ‘D’. (e) Absorption spectra of major endogenous contrast agents in biological tissue. Oxy-hemoglobin, red line (150 g/L in blood); Deoxy-hemoglobin, blue line (150 g/L in blood);Lipid, brown line (20% by volume in tissue); Water, green line (80% by volume in tissue);DNA, magenta line (1 g/L in cell nuclei); RNA, orange line (1 g/L in cell nuclei); Melanin, black line (14.3 g/L in medium human skin); Glucose, purple line (720 mg/L in blood). Panels A–D from © 2009 IEEE. Reprinted, with permission, from Amir Rosenthal, Daniel Razansky, Vasilis Ntziachristos, "Quantitative Optoacoustic Signal Extraction Using Sparse Signal Representation," IEEE Transactions on Medical Imaging Volume 28, Issue 12, Pages 1997–2006; DOI: 10.1109/TMI.2009.2027116. Panel E from Yao, J. et al. Photoacoustic microscopy. Laser Photonics Rev. 7, No.5, 758–778 (2013). DOI 10.1002/lpor.201200060. Published by John Wiley and Sons. © 2012 by WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.