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Signal and Image Processing in Biomedical Photoacoustic Imaging: a Review

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Signal and Image Processing in Biomedical Photoacoustic Imaging: a Review Review Signal and Image Processing in Biomedical Photoacoustic Imaging: A Review Rayyan Manwar 1,*,†, Mohsin Zafar 2,† and Qiuyun Xu 2 1 Richard and Loan Hill Department of Bioengineering, University of Illinois at Chicago, Chicago, IL 60607, USA 2 Department of Biomedical Engineering, Wayne State University, Detroit, MI 48201, USA; [email protected] (M.Z.); [email protected] (Q.X.) * Correspondence: [email protected] † These authors have equal contributions. Abstract: Photoacoustic imaging (PAI) is a powerful imaging modality that relies on the PA effect. PAI works on the principle of electromagnetic energy absorption by the exogenous contrast agents and/or endogenous molecules present in the biological tissue, consequently generating ultrasound waves. PAI combines a high optical contrast with a high acoustic spatiotemporal resolution, al- lowing the non-invasive visualization of absorbers in deep structures. However, due to the optical diffusion and ultrasound attenuation in heterogeneous turbid biological tissue, the quality of the PA images deteriorates. Therefore, signal and image-processing techniques are imperative in PAI to provide high-quality images with detailed structural and functional information in deep tissues. Here, we review various signal and image processing techniques that have been developed/implemented in PAI. Our goal is to highlight the importance of image computing in photoacoustic imaging. Keywords: photoacoustic; signal enhancement; image processing; SNR; deep learning 1. Introduction Citation: Manwar, R.; Zafar, M.; Xu, Photoacoustic imaging (PAI) is a non-ionizing and non-invasive hybrid imaging Q. Signal and Image Processing in modality that has made significant progress in recent years, up to a point where clinical Biomedical Photoacoustic Imaging: studies are becoming a real possibility [1–6]. Due to the hybrid nature of PAI, i.e., optical ex- A Review. Optics 2021, 2, 1–24. citation and acoustic detection, this modality benefits from both rich and versatile optical https://doi.org/10.3390/opt2010001 contrast and high (diffraction-limited) spatial resolution associated with low-scattering Received: 11 December 2020 nature of ultrasonic wave propagation [7–11]. Photoacoustic imaging breaks through Accepted: 28 December 2020 the diffusion limit of high-resolution optical imaging (~1 mm) by using electromagnetic Published: 31 December 2020 energy induced ultrasonic waves as a carrier to obtain optical absorption information of tissue [12,13]. PAI, being a relatively new imaging modality, can effectively realize the Publisher’s Note: MDPI stays neu- structural and functional information of the biological tissue, providing a powerful imaging tral with regard to jurisdictional clai- tool for studying the morphological structure, physiological, pathological characteristics, ms in published maps and institutio- and metabolic functions in biological tissues [14–16]. nal affiliations. The PA effect initiates when optically absorbing targets (absorbers/chromophores) within the tissue are irradiated by a short (~nanosecond) pulse laser [17]. The pulse energy is absorbed by the target and converted into heat, generating a local transient temperature rise, followed by a local acoustic pressure rise through thermo-elastic expansion [18–21]. Copyright: © 2020 by the authors. Li- The pressure waves propagating as ultrasonic waves, are detected by ultrasonic censee MDPI, Basel, Switzerland. transducers present outside the tissue, termed as raw data (Figure1). These data carry This article is an open access article information of inherent acoustic and optical properties (as presented in [22]) of the ab- distributed under the terms and con- ditions of the Creative Commons At- sorbers in combination with noisy data originating from electromagnetic interferences. tribution (CC BY) license (https:// The acquired data are further processed (known as signal processing) to extract the de- creativecommons.org/licenses/by/ sired PA signal from the noisy background and utilized to reconstruct a PA image [23,24]. 4.0/). These images represent internal structures and corresponding functionality of the tissue Optics 2021, 2, 1–24. https://doi.org/10.3390/opt2010001 https://www.mdpi.com/journal/optics Optics 2021, 2, FOR PEER REVIEW 2 Optics 2021, 2 2 The acquired data are further processed(known as signal processing) to extract the desired PA signal from the noisy background and utilized to reconstruct a PA image [23,24]. These images represent internal structures and corresponding functionality of the tissue target regiontarget region[25–31]. [25 Several–31]. Several image imagereconstruction reconstruction algorithms algorithms have been have studied been studied for PA forimag- PA ingimaging [30,32–34] [30,32 where–34] where the reconstruction the reconstruction algorith algorithmsms can be can interpreted be interpreted as an as acoustic an acoustic in- verseinverse source source problem problem [35]. [35 ].Conventional Conventional PA PA im imageage reconstruction reconstruction algorithms algorithms assume assume that the object object of of interest interest possesses possesses homogeneous homogeneous ac acousticoustic properties [36]. [36]. However, the tissue medium in reality is heterogeneous with sp spatiallyatially variant sound of speed and density distribution [[37].37]. This introducesintroduces varyingvarying effectseffects known known as as acoustic acoustic aberration aberration (i.e., (i.e., ampli- am- plitudetude attenuation, attenuation, signal signal broadening, broadening, mode mode conversion) conversion) that that eventually eventually amplifies amplifies the lowthe lowfrequency frequency signals signals and and affects affects the smallthe small wavelengths, wavelengths, corresponding corresponding to the to microstructuresthe microstruc- turesand sharp and sharp edges. edges. Consequently, Consequently, image image resolution; resolution; one ofone the of main the main contributions contributions of PAI, of PAI,is forsaken. is forsaken. Moreover, Moreover, due due to variableto variable acoustic acoustic aberration, aberration, significant significant distortionsdistortions and artifacts are also introduced [38–40]. [38–40]. There There have have been been advancements advancements of of PA PA image image recon- recon- struction algorithmsalgorithms thatthat cancan compensate compensate for for variations variations in in the the acoustical acoustical properties properties [41 –[41–46], 46],however, however, further further image image enhancement enhancement in terms in terms of post-processing of post-processing is essential. is essential . Figure 1. A conceptual flow flow of the photoacousticphotoacoustic imaging working principle. To accurately obtain the morphological and functional information of the the tissue tissue chro- chro- mophores, the initial goal is to retrieve the initial pressure distribution inside the object due to thethe absorbedabsorbed laser laser energy energy [47 [47].]. Therefore, Therefore, knowledge knowledge of theof the local local optical optical fluence fluence (op- (opticaltical energy energy per per unit unit area) area) in biological in biological tissue tissue is of fundamentalis of fundamental importance importance for biomedical for bio- medicalPA imaging PA [imaging48]. However, [48]. However, initial pressure initial distribution pressure distribution is a function is ofa function depth (lateral of depth and (lateralaxial), wavelength, and axial), wavelength, thermal properties thermal (i.e., proper specificties heat,(i.e., Gruneisenspecific heat, parameter) Gruneisen and parame- optical ter)properties and optical (i.e., absorption properties and (i.e., scattering absorption coefficient, and scattering anisotropy coefficien factor,t, and anisotropy refractive factor, index) andof the refractive medium index) including of the the medium tissue target. including To simplifythe tissue the target. initial To pressure simplify distribution the initial pressureretrieval process,distribution the amountretrieval of process, optical fluencethe amount reaching of optical the region-of-interest fluence reaching (ROI) the region- neces- of-interestsitates to consider (ROI) necessitates homogenous to consider distribution homo ofgenous light [ 49distribution]. However, of inlight reality, [49]. theHowever, strong inoptical reality, absorption the strong by optical heterogeneous absorption turbid by superficialheterogeneous tissue turbid structure superficial pose major tissue obstruc- struc- turetion inpose irradiating major obstruction the actual in target irradiating located the deep actual inside target the tissuelocated medium deep inside with the sufficient tissue mediumoptical energy with sufficient [7,10,16]. optical This limits energy accurate [7,10,16 quantification]. This limits measurementaccurate quantification such as oxygen meas- urementsaturation, such blood as oxygen volume saturation, calculations blood etc. volu [50].me There calculations are several etc. [50]. methods There [36 are,51 several] have methodsbeen proposed [36,51] to have optimize been proposed the fluence to decay, optimize however, the fluence these decay, models however, are based these on optical mod- elsproperties are based of differenton optical tissue properties types availableof different in literature.tissue types Unfortunately, available in literature. the exact opticalUnfor- tunately,properties the of biologicalexact optical tissues properties are unknown, of biolog theical medium
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