Current concepts and future perspectives on surgical optical imaging in cancer

Probe fusion all-optic OCT-PAM dual-mode imaging system for biomedical imaging

We proposed a non-contact photoacoustic (PA) detection method using spectral domain optical coherence tomography (SDOCT). Two interference spectrums (A-lines) were acquired before and after the PA excitation with SDOCT. PA signal propagated within the sample causing the vibration. The vibration inner the sample introduced phase change between the acquired two A-lines. Thus, the PA signal can be detected by evaluating the difference in phase between the two A-lines. Based on the method, an OCT-PAM dual-mode imaging system was constructed. In the system, SDOCT served as the detection unit for PAM. Thus, the combination of the two imaging modalities was simplified. Another advantage of the system is that it realizes non-contact all-optic detection, which is attractive for biomedical imaging. Using the system, we imaged phantoms of carbon fibers, asparagus leaves and human hairs. Furthermore, the cortical vasculature of rat was imaged in vivo and the flow status was evaluated quantitatively.

Spectral interference contrast based non-contact photoacoustic microscopy realized by SDOCT

We introduce a method to extract the photoacoustic (PA) signal from a contrast reduction of the interference spectrum acquired by spectral domain optical coherence tomography (SDOCT). This all-optical detection is achieved in a noncontact manner directly on the water surface covered on the sample by using its specular reflection. During SDOCT exposure, the phase of the interference spectrum keeps shaking according to the water surface vibration induced by PA excitation. This results in an interference contrast reduction which is quantified by a fast Fourier transform (FFT) for PA imaging. A tungsten filament, asparagus fern leaf, and mouse auricle are imaged to demonstrate the method.

All Optical Speckle Contrast-Based Vibration Sensor for Photoacoustic Signal Detection

Remote detection of photoacoustic signals is a well desired ability, enabling to perform advanced imaging in scenarios where contact is not possible. Various unique solutions have been suggested, including a camera-based speckle contrast photoacoustic detection. In this manuscript, a significant upgrade to the camera-based speckle contrast approach is presented and experimentally demonstrated. This solution is based on all-optical vibration sensing setup. The technique is based on spectral estimation of speckle pattern contrast and relies on several pre-developed works. First, it relies on the suggested application of speckle contrast to vibration sensing, and then on the realization of intensity pattern spectral manipulation, using a shearing interferometer. The method is evaluated and compared to traditional contrast estimation, and demonstrated in several applications in various vibration frequency band such as photoacoustic signal analysis and phonocardiographic heart sounds. The method is also applicable to measuring contrast changes due to a general speckle changing behavior, rather than surface vibration alone.

Cerebrovascular imaging in vivo by non-contact photoacoustic microscopy based on photoacoustic remote sensing with a laser diode for interrogation

Photoacoustic microscopy (PAM) is a unique tool for biomedical applications because it can visualize optical absorption contrast in vivo. Recently, non-contact PAM based on non-interferometric photoacoustic remote sensing (PARS), termed PARS microscopy, has shown promise for selected imaging applications. A variety of superluminescent diodes (SLDs) have been employed in the PARS microscopy system as the interrogation light source. Here, we investigate the use of a low-cost laser diode (LD) as the interrogation light source in PARS microscopy, termed PARS-LD. A side-by-side comparison of PARS-LD and a PARS microscopy system using an SLD was conducted that showed comparable performance in terms of resolution and signal-to-noise ratio. More importantly, for the first time to our knowledge, in vivo PAM imaging of mouse brain vessels was conducted in a non-contact manner, and the results show that PARS-LD provides great performance.

Comprehensive review of surgical microscopes: technology development and medical applications

SIGNIFICANCE

Surgical microscopes provide adjustable magnification, bright illumination, and clear visualization of the surgical field and have been increasingly used in operating rooms. State-of-the-art surgical microscopes are integrated with various imaging modalities, such as optical coherence tomography (OCT), fluorescence imaging, and augmented reality (AR) for image-guided surgery.

AIM

This comprehensive review is based on the literature of over 500 papers that cover the technology development and applications of surgical microscopy over the past century. The aim of this review is threefold: (i) providing a comprehensive technical overview of surgical microscopes, (ii) providing critical references for microscope selection and system development, and (iii) providing an overview of various medical applications.

APPROACH

More than 500 references were collected and reviewed. A timeline of important milestones during the evolution of surgical microscope is provided in this study. An in-depth technical overview of the optical system, mechanical system, illumination, visualization, and integration with advanced imaging modalities is provided. Various medical applications of surgical microscopes in neurosurgery and spine surgery, ophthalmic surgery, ear-nose-throat (ENT) surgery, endodontics, and plastic and reconstructive surgery are described.

RESULTS

Surgical microscopy has been significantly advanced in the technical aspects of high-end optics, bright and shadow-free illumination, stable and flexible mechanical design, and versatile visualization. New imaging modalities, such as hyperspectral imaging, OCT, fluorescence imaging, photoacoustic microscopy, and laser speckle contrast imaging, are being integrated with surgical microscopes. Advanced visualization and AR are being added to surgical microscopes as new features that are changing clinical practices in the operating room.

CONCLUSIONS

The combination of new imaging technologies and surgical microscopy will enable surgeons to perform challenging procedures and improve surgical outcomes. With advanced visualization and improved ergonomics, the surgical microscope has become a powerful tool in neurosurgery, spinal, ENT, ophthalmic, plastic and reconstructive surgeries.

Towards non-contact photoacoustic imaging [review]

Photoacoustic imaging (PAI) takes advantage of both optical and ultrasound imaging properties to visualize optical absorption with high resolution and contrast. Photoacoustic microscopy (PAM) is usually categorized with all-optical microscopy techniques such as optical coherence tomography or confocal microscopes. Despite offering high sensitivity, novel imaging contrast, and high resolution, PAM is not generally an all-optical imaging method unlike the other microscopy techniques. One of the significant limitations of photoacoustic microscopes arises from their need to be in physical contact with the sample through a coupling media. This physical contact, coupling, or immersion of the sample is undesirable or impractical for many clinical and pre-clinical applications. This also limits the flexibility of photoacoustic techniques to be integrated with other all-optical imaging microscopes for providing complementary imaging contrast. To overcome these limitations, several non-contact photoacoustic signal detection approaches have been proposed. This paper presents a brief overview of current non-contact photoacoustic detection techniques with an emphasis on all-optical detection methods and their associated physical mechanisms.

All-optical, an ultra-thin endoscopic photoacoustic sensor using multi-mode fiber

Photoacoustic endoscopy (PAE) is a method of in-vivo imaging that uses tissue absorption properties. In PAE, the main tools used to detect the acoustic signal are mechanical ultrasound transducers, which require direct contact and which are difficult to miniaturize. All-optic photoacoustic sensors can challenge this issue as they can provide contact-free sensing. Here, we demonstrate sensing of photo-acoustic signals through a multimode fiber (MMF) which can provide an ultra-thin endoscopic photoacoustic sensor. Furthermore, we show the advantage of using the optical-flow method for speckle sensing and extract the photoacoustic signal despite the mode-mixing along the MMF. Moreover, it is demonstrated for the first time that the speckle reconstruction method can be used without the need for imaging of the speckles as this enables the use of multimode fibers for the speckle method.

A background correction method to compensate illumination variation in hyperspectral imaging

Hyperspectral imaging (HSI) can measure both spatial (morphological) and spectral (biochemical) information from biological tissues. While HSI appears promising for biomedical applications, interpretation of hyperspectral images can be challenging when data is acquired in complex biological environments. Variations in surface topology or optical power distribution at the sample, encountered for example during endoscopy, can lead to errors in post-processing of the HSI data, compromising disease diagnostic capabilities. Here, we propose a background correction method to compensate for such variations, which estimates the optical properties of illumination at the target based on the normalised spectral profile of the light source and the measured HSI intensity values at a fixed wavelength where the absorption characteristics of the sample are relatively low (in this case, 800 nm). We demonstrate the feasibility of the proposed method by imaging blood samples, tissue-mimicking phantoms, and ex vivo chicken tissue. Moreover, using synthetic HSI data composed from experimentally measured spectra, we show the proposed method would improve statistical analysis of HSI data. The proposed method could help the implementation of HSI techniques in practical clinical applications, where controlling the illumination pattern and power is difficult.

Self-assembling as regular nanoparticles dramatically minimizes photobleaching of tumour-targeted GFP

Fluorescent proteins are useful imaging and theranostic agents, but their potential superiority over alternative dyes is weakened by substantial photobleaching under irradiation. Enhancing protein photostability has been attempted through diverse strategies, with irregular results and limited applicability. In this context, we wondered if the controlled oligomerization of Green Fluorescent Protein (GFP) as nanoscale supramolecular complexes could stabilize the fluorophore through the newly formed protein-protein contacts, and thus, enhance its global photostability. For that, we have here analyzed the photobleaching profile of several GFP versions, engineered to self-assemble as tumour-homing nanoparticles with different targeting, size and structural stability. This has been done under prolonged irradiation in confocal laser scanning microscopy and by small-angle X-ray scattering. The results show that the oligomerization of GFP at the nanoscale enhances, by more than seven-fold, the stability of fluorescence emission. Interestingly, GFP nanoparticles are much more resistant to X-ray damage than the building block counterparts, indicating that the gained photostability is linked to enhanced structural resistance to radiation. Therefore, the controlled oligomerization of self-assembling fluorescent proteins as protein nanoparticles is a simple, versatile and powerful method to enhance their photostability for uses in precision imaging and therapy. STATEMENT OF SIGNIFICANCE: Fluorescent protein assembly into regular and highly symmetric nanoscale structures has been identified to confer enhanced structural stability against radiation stresses dramatically reducing their photobleaching. Being this the main bottleneck in the use of fluorescent proteins for imaging and theranostics, this protein architecture engineering principle appears as a powerful method to enhance their photostability for a broad applicability in precision imaging, drug delivery and theranostics.

Review on practical photoacoustic microscopy

Photoacoustic imaging (PAI) has many interesting advantages, such as deep imaging depth, high image resolution, and high contrast to intrinsic and extrinsic chromophores, enabling morphological, functional, and molecular imaging of living subjects. Photoacoustic microscopy (PAM) is one form of the PAI inheriting its characteristics and is useful in both preclinical and clinical research. Over the years, PAM systems have been evolved in several forms and each form has its relative advantages and disadvantages. Thus, to maximize the benefits of PAM for a specific application, it is important to configure the PAM system optimally by targeting a specific application. In this review, we provide practical methods for implementing a PAM system to improve the resolution, signal-to-noise ratio (SNR), and imaging speed. In addition, we review the preclinical and the clinical applications of PAM and discuss the current challenges and the scope for future developments.

Non-contact photoacoustic imaging using laser speckle contrast analysis

A novel method for non-contact and continuous detection of photoacoustic signals is presented and experimentally demonstrated. The approach is based on analysis of the contrast of time-varying speckle patterns, and suggests a more robust alternative in respect to interferometric and refractometric available solutions.

Noncontact photoacoustic imaging based on optical quadrature detection with a multiport interferometer

A noncontact photoacoustic imaging method based on optical quadrature detection is proposed. The photo-induced acoustic signal is detected by an optical method without contacting the specimen. By utilizing the intrinsic phase difference of a multiport optical interferometer, the quadrature signal of a conventional interferometric signal could be obtained. With this quadratic signal pair, we could reconstruct the photoacoustic signal without suffering from the initial phase drift that usually occurs in a conventional interferometric system. The performance of the proposed system is verified by imaging human hairs embedded in a polydimethylsiloxane resin block. The system's lateral and axial resolutions are measured to be 84 and 86 μm at a 1.5 mm depth of a PDMS resin block, respectively. The experimental result is good enough to distinguish the hairs staggered in depth.

Coherence-gated photoacoustic remote sensing microscopy

Photoacoustic remote sensing microscopy (PARS) represents a new paradigm within the optical imaging community by providing high sensitivity (>50 dB in vivo) non-contact optical absorption contrast in scattering media with a reflection-mode configuration. Unlike contact-based photoacoustic modalities which can acquire complete A-scans with a single excitation pulse due to slow acoustic propagation facilitating the use of time-gated collection of returning acoustic signals, PARS provides depth resolution only through optical sectioning. Here we introduce a new approach for providing coherence-gated depth-resolved PARS imaging using a difference between pulsed-interrogation optical coherence tomography scan-lines with and without excitation pulses. Proposed methods are validated using simulations which account for pulsed-laser induced initial-pressures and accompanying refractive index changes. The changes in refractive index are shown to be proportional to optical absorption. It is demonstrated that to achieve optimal image quality, several key parameters must be selected including interrogation pulse duration and delay. The proposed approach offers the promise of non-contact depth-resolved optical absorption contrast at optical-resolution scales and may complement the scattering contrast offered by optical coherence tomography.

Temporal evolution of low-coherence reflectrometry signals in photoacoustic remote sensing microscopy

Recently, a new noncontact reflection-mode imaging modality called photoacoustic remote sensing (PARS) microscopy was introduced providing optical absorption contrast. Unlike previous modalities, which rely on interferometric detection of a probe beam to measure surface oscillations, the PARS technique detects photoacoustic initial pressures induced by a pulsed laser at their origin by monitoring intensity modulations of a reflected probe beam. In this paper, a model describing the temporal evolution from a finite excitation pulse is developed with consideration given to the coherence length of the interrogation beam. Analytical models are compared with approximations, finite-difference time-domain (FDTD) simulations, and experiments with good agreement.

Non-interferometric photoacoustic remote sensing microscopy

Elasto-optical refractive index modulation due to photoacoustic initial pressure transients produced significant reflection of a probe beam when the absorbing interface had an appreciable refractive index difference. This effect was harnessed in a new form of non-contact optical resolution photoacoustic microscopy called photoacoustic remote sensing microscopy. A non-interferometric system architecture with a low-coherence probe beam precludes detection of surface oscillations and other phase-modulation phenomenon. The probe beam was confocal with a scanned excitation beam to ensure detection of initial pressure-induced intensity reflections at the subsurface origin where pressures are largest. Phantom studies confirmed signal dependence on optical absorption, index contrast and excitation fluence. In vivo imaging of superficial microvasculature and melanoma tumors was demonstrated with ~2.7±0.5 μm lateral resolution.

Fluorescent probes and materials for detecting formaldehyde: from laboratory to indoor for environmental and health monitoring

Formaldehyde (FA), as a vital industrial chemical, is widely used in building materials and numerous living products.

Optical technologies for intraoperative neurosurgical guidance

Biomedical optics is a broadly interdisciplinary field at the interface of optical engineering, biophysics, computer science, medicine, biology, and chemistry, helping us understand light-tissue interactions to create applications with diagnostic and therapeutic value in medicine. Implementation of biomedical optics tools and principles has had a notable scientific and clinical resurgence in recent years in the neurosurgical community. This is in great part due to work in fluorescence-guided surgery of brain tumors leading to reports of significant improvement in maximizing the rates of gross-total resection. Multiple additional optical technologies have been implemented clinically, including diffuse reflectance spectroscopy and imaging, optical coherence tomography, Raman spectroscopy and imaging, and advanced quantitative methods, including quantitative fluorescence and lifetime imaging. Here we present a clinically relevant and technologically informed overview and discussion of some of the major clinical implementations of optical technologies as intraoperative guidance tools in neurosurgery.

In vivo tumor identification of colorectal liver metastases with diffuse reflectance and fluorescence spectroscopy

BACKGROUND AND OBJECTIVE

Over the last decade, an increasing effort has been put towards the implementation of optical guidance techniques to aid surgeons during cancer surgery. Diffuse reflectance spectroscopy (DRS) and fluorescence spectroscopy (FS) are two of these new techniques. The objective of this study is to investigate whether in vivo optical spectroscopy is able to accurately discriminate colorectal liver metastases (CRLM) from normal liver tissue in vivo.

MATERIALS AND METHODS

DRS and FS were incorporated at the tip of a needle and were used for in vivo tissue differentiation during resection of CRLM. Measurements were taken in and around the tumor lesions and measurement sites were marked and correlated to histology (i.e., normal liver tissue or tumor tissue). Patients with and without neoadjuvant systemic chemotherapy were included into the study.

RESULTS

Four hundred and eighty-four measurements were taken in and near 19 liver lesions prior to resection. Overall sensitivity and specificity for DRS was 95% and 92%, respectively. Bile was the most discriminative parameter. The addition of FS did not improve the overall accuracy. Sensitivity and specificity was not hampered by neo-adjuvant chemotherapy; sensitivity and specificity after neo-adjuvant chemotherapy were 92% and 100%, respectively.

CONCLUSION

We have successfully integrated spectroscopy technology into a disposable 15 Gauge optical needle and we have shown that DRS and FS can accurately discriminate CRLM from normal liver tissue in the in vivo setting regardless of whether the patient was pre-treated with systemic therapy. This technique makes in vivo guidance accessible for common surgical practice. Lasers Surg. Med. 48:820-827, 2016. © 2016 Wiley Periodicals, Inc.

Dual Modality Noncontact Photoacoustic and Spectral Domain OCT Imaging

We developed a multimodal imaging system, combining noncontact photoacoustic imaging and optical coherence tomography (OCT). Photoacoustic signals are recorded without contact to the specimens' surface by using an interferometric technique. The interferometer is realized within a fiber-optic network using a fiber laser at 1550 nm as source. The fiber-optic network allows the integration of a fiber-based OCT system operating at a wavelength region around 1310 nm. Light from the fiber laser and the OCT source are multiplexed into one fiber using wavelength-division multiplexing. The same focusing optics is used for both modalities. Back-reflected light from the sample is demultiplexed and guided to the respective imaging systems. As the same optical components are used for OCT and photoacoustic imaging, the obtained images are co-registered intrinsically in lateral direction. Three-dimensional imaging is implemented by hybrid galvanometer and mechanical scanning. To allow fast B-scan measurements, scanning of the interrogation beam along one dimension is executed by a galvanometer scanner. Slow-axis scanning, perpendicular to the fast axis, is performed utilizing a linear translational stage. We demonstrate two-dimensional and three-dimensional imaging on agarose phantoms.

Intraoperative mapping of sentinel lymph node metastases using a clinically translated ultrasmall silica nanoparticle

The management of regional lymph nodes in patients with melanoma has undergone a significant paradigm shift over the past several decades, transitioning from the use of more aggressive surgical approaches, such as lymph node basin dissection, to the application of minimally invasive sentinel lymph node (SLN) biopsy methods to detect the presence of nodal micrometastases. SLN biopsy has enabled reliable, highly accurate, and low-morbidity staging of regional lymph nodes in early stage melanoma as a means of guiding treatment decisions and improving patient outcomes. The accurate identification and staging of lymph nodes is an important prognostic factor, identifying those patients for whom the expected benefits of nodal resection outweigh attendant surgical risks. However, currently used standard-of-care technologies for SLN detection are associated with significant limitations. This has fueled the development of clinically promising platforms that can serve as intraoperative visualization tools to aid accurate and specific determination of tumor-bearing lymph nodes, map cancer-promoting biological properties at the cellular/molecular levels, and delineate nodes from adjacent critical structures. Among a number of promising cancer-imaging probes that might facilitate achievement of these ends is a first-in-kind ultrasmall tumor-targeting inorganic (silica) nanoparticle, designed to overcome translational challenges. The rationale driving these considerations and the application of this platform as an intraoperative treatment tool for guiding resection of cancerous lymph nodes is discussed and presented within the context of alternative imaging technologies. WIREs Nanomed Nanobiotechnol 2016, 8:535-553. doi: 10.1002/wnan.1380 For further resources related to this article, please visit the WIREs website.

Polymeric Micelles as Carriers for Nerve-Highlighting Fluorescent Probe Delivery

Nerve damage during surgery is a common morbidity experienced by patients that leaves them with chronic pain and/or loss of function. Currently, no clinically approved imaging technique exists to enhance nerve visualization in the operating room. Fluorescence image-guided surgery has gained in popularity and clinical acceptance over the past decade with a handful of imaging systems approved for clinical use. However, contrast agent development to complement these fluorescence-imaging systems has lagged behind with all currently approved fluorescent agents providing untargeted blood pool information. Nerve-specific fluorophores are known, however translations of these agents to the clinic has been complicated by their lipophilic nature, which necessitates specialized formulation strategies for successful systemic administration. To date the known nerve-specific fluorophores have only been demonstrated preclinically due to the necessity of a dimethyl sulfoxide containing formulation for solubilization. In the current study, a polymeric micellar (PM) formulation strategy was developed for a representative nerve-specific fluorophore from the distyrylbenzene family, BMB. The PM formulation strategy was able to solubilize BMB and demonstrated improved nerve-specific accumulation and fluorescence intensity when the same fluorophore dose was administered to mice utilizing the previous formulation strategy. The success of the PM formulation strategy will be important for moving toward clinical translation of these novel nerve-specific probes as it is nontoxic and biodegradable and has the potential to decrease the necessary dose for imaging while also improving the safety profile.

Delta-Opioid Receptor (δOR) Targeted Near-Infrared Fluorescent Agent for Imaging of Lung Cancer: Synthesis and Evaluation In Vitro and In Vivo

In the United States, lung cancer is the leading cause of cancer death and ranks second in the number of new cases annually among all types of cancers. Better methods or tools for diagnosing and treating this disease are needed to improve patient outcomes. The delta-opioid receptor (δOR) is reported to be overexpressed in lung cancers and not expressed in normal lung. Thus, we decided to develop a lung cancer-specific imaging agent targeting this receptor. We have previously developed a δOR-targeted fluorescent imaging agent based on a synthetic peptide antagonist (Dmt-Tic) conjugated to a Cy5 fluorescent dye. In this work, we describe the synthesis of Dmt-Tic conjugated to a longer wavelength near-infrared fluorescent (NIRF) dye, Li-cor IR800CW. Binding affinity of Dmt-Tic-IR800 for the δOR was studied using lanthanide time-resolved fluorescence (LTRF) competitive binding assays in cells engineered to overexpress the δOR. In addition, we identified lung cancer cell lines with high and low endogenous expression of the δOR. We confirmed protein expression in these cell lines using confocal fluorescence microscopy imaging and used this technique to estimate the cell-surface receptor number in the endogenously expressing lung cancer cell lines. The selectivity of Dmt-Tic-IR800 for imaging of the δOR in vivo was shown using both engineered cell lines and endogenously expressing lung cancer cells in subcutaneous xenograft models in mice. In conclusion, the δOR-specific fluorescent probe developed in this study displays excellent potential for imaging of lung cancer.

Noncontact photoacoustic tomography of in vivo chicken chorioallantoic membrane based on all-fiber heterodyne interferometry

We present three-dimensional (3-D) in vivo photoacoustic (PA) images of the blood vasculature of a chicken chorioallantoic membrane (CAM) obtained by using a fiber-based noncontact PA tomography system. With a fiber-optic heterodyne interferometer, the system measures the surface displacement of a sample, induced by the PA wave, which overcomes the disadvantage of physical-contact of ultrasonic transducer in a conventional system. The performance of an implemented system is analyzed and its capability of in vivo 3-D bioimaging is presented. At a depth of 2.5 mm in a phantom experiment, the lateral and axial resolutions were measured as 100 and 30  μm, respectively. The lateral resolution became doubled at a depth of 7.0 mm; however, interestingly, the axial resolution was not noticeably deteriorated with the depth. With the CAM experiment, performed under the American National Standards Institute laser safety standard condition, blood vessel structures placed as deep as 3.5 mm were clearly recognized.

Augmented microscopy: real-time overlay of bright-field and near-infrared fluorescence images

Intraoperative applications of near-infrared (NIR) fluorescent contrast agents can be aided by instrumentation capable of merging the view of surgical field with that of NIR fluorescence. We demonstrate augmented microscopy, an intraoperative imaging technique in which bright-field (real) and electronically processed NIR fluorescence (synthetic) images are merged within the optical path of a stereomicroscope. Under luminance of 100,000 lx, representing typical illumination of the surgical field, the augmented microscope detects 189 nM concentration of indocyanine green and produces a composite of the real and synthetic images within the eyepiece of the microscope at 20 fps. Augmentation described here can be implemented as an add-on module to visualize NIR contrast agents, laser beams, or various types of electronic data within the surgical microscopes commonly used in neurosurgical, cerebrovascular, otolaryngological, and ophthalmic procedures.

Tailoring Adjuvant Radiation Therapy by Intraoperative Imaging to Detect Residual Cancer

For many solid cancers, radiation therapy is offered as an adjuvant to surgical resection to lower rates of local recurrence and improve survival. However, a subset of patients treated with surgery alone will not have a local recurrence. Currently, there is no way to accurately determine which patients have microscopic residual disease in the tumor bed after surgery and therefore are most likely to benefit from adjuvant radiation therapy. To address this problem, a number of technologies have been developed to try to improve margin assessment of resected tissue and to detect residual cancer in the tumor bed. Moreover, some of these approaches have been translated from the preclinical arena into clinical trials. Here, we review different types of intraoperative molecular imaging systems for cancer. Optical imaging techniques like epi-illumination, fluorescence molecular tomography and optoacoustic imaging can be coupled with exogenous fluorescent imaging probes that accumulate in tumors passively via the enhanced permeability and retention effect or are targeted to tumor tissues based on affinity or enzyme activity. In these approaches, detection of fluorescence in the tumor bed may indicate residual disease. Protease activated probes have generated great interest because of their potential for leading to high tumor to normal contrast. Recently, the first Phase I clinical trial to assess the safety and activation of a protease activated probe was conducted. Spectroscopic methods like radiofrequency spectroscopy and Raman spectroscopy, which are based on energy absorption and scattering, respectively, have also been tested in humans and are able to distinguish between normal and tumors tissues intraoperatively. Most recently, multimodal contrast agents have been developed that target tumors and contain both fluorescent dyes and magnetic resonance imaging contrast agents, allowing for preoperative planning and intraoperative margin assessment with a single contrast agent. Further clinical testing of these various intraoperative imaging approaches may lead to more accurate methods for margin assessment and the intraoperative detection of microscopic residual disease, which could guide further resection and the use of adjuvant radiation therapy.

On the horizon: Optical imaging for cutaneous squamous cell carcinoma

BACKGROUND

Surgical resection with negative margins remains the standard of care for high-risk cutaneous squamous cell carcinoma (SCC). However, surgical management is often limited by poor intraoperative tumor visualization and inability to detect occult nodal metastasis. The inability to intraoperatively detect microscopic disease can lead to additional surgery, tumor recurrence, and decreased survival.

METHODS

A comprehensive literature review was conducted to identify studies incorporating optical imaging technology in the management of cutaneous SCC (January 1, 2000-December 1, 2014).

RESULTS

Several innovative optical imaging techniques, Raman spectroscopy, confocal microscopy, and fluorescence imaging, have been developed for intraoperative surgical guidance. Fifty-seven studies review the ability of these techniques to improve cutaneous SCC localization at the gross and microscopic level.

CONCLUSION

Significant advances have been achieved with real-time optical imaging strategies for intraoperative cutaneous SCC margin assessment and tumor detection. Optical imaging holds promise in improving the percentage of negative surgical margins and in the early detection of micrometastatic disease. © 2015 Wiley Periodicals, Inc. Head Neck 38: E2204-E2213, 2016.

Improved Intraoperative Visualization of Nerves through a Myelin-Binding Fluorophore and Dual-Mode Laparoscopic Imaging

The ability to visualize and spare nerves during surgery is critical for avoiding chronic morbidity, pain, and loss of function. Visualization of such critical anatomic structures is even more challenging during minimal access procedures because the small incisions limit visibility. In this study, we focus on improving imaging of nerves through the use of a new small molecule fluorophore, GE3126, used in conjunction with our dual-mode (color and fluorescence) laparoscopic imaging instrument. GE3126 has higher aqueous solubility, improved pharmacokinetics, and reduced non-specific adipose tissue fluorescence compared to previous myelin-binding fluorophores. Dosing and kinetics were initially optimized in mice. A non-clinical modified Irwin study in rats, performed to assess the potential of GE3126 to induce nervous system injuries, showed the absence of major adverse reactions. Real-time intraoperative imaging was performed in a porcine model. Compared to white light imaging, nerve visibility was enhanced under fluorescence guidance, especially for small diameter nerves obscured by fascia, blood vessels, or adipose tissue. In the porcine model, nerve visualization was observed rapidly, within 5 to 10 minutes post-intravenous injection and the nerve fluorescence signal was maintained for up to 80 minutes. The use of GE3126, coupled with practical implementation of an imaging instrument may be an important step forward in preventing nerve damage in the operating room.

Bioresponsive probes for molecular imaging: concepts and in vivo applications

Molecular imaging is a powerful tool to visualize and characterize biological processes at the cellular and molecular level in vivo. In most molecular imaging approaches, probes are used to bind to disease-specific biomarkers highlighting disease target sites. In recent years, a new subset of molecular imaging probes, known as bioresponsive molecular probes, has been developed. These probes generally benefit from signal enhancement at the site of interaction with its target. There are mainly two classes of bioresponsive imaging probes. The first class consists of probes that show direct activation of the imaging label (from "off" to "on" state) and have been applied in optical imaging and magnetic resonance imaging (MRI). The other class consists of probes that show specific retention of the imaging label at the site of target interaction and these probes have found application in all different imaging modalities, including photoacoustic imaging and nuclear imaging. In this review, we present a comprehensive overview of bioresponsive imaging probes in order to discuss the various molecular imaging strategies. The focus of the present article is the rationale behind the design of bioresponsive molecular imaging probes and their potential in vivo application for the detection of endogenous molecular targets in pathologies such as cancer and cardiovascular disease.

Multimodal noncontact photoacoustic and optical coherence tomography imaging using wavelength-division multiplexing

We present multimodal noncontact photoacoustic (PA) and optical coherence tomography (OCT) imaging. PA signals are acquired remotely on the surface of a specimen with a Mach-Zehnder interferometer. The interferometer is realized in a fiber-optic network using a fiber laser at 1550 nm as the source. In the same fiber-optic network, a spectral-domain OCT system is implemented. The OCT system utilizes a supercontinuum light source at 1310 nm and a spectrometer with an InGaAs line array detector. Light from the fiber laser and the OCT source is multiplexed into one fiber using a wavelength-division multiplexer; the same objective is used for both imaging modalities. Reflected light is spectrally demultiplexed and guided to the respective imaging systems. We demonstrate two-dimensional and three-dimensional imaging on a tissue-mimicking sample and a chicken skin phantom. The same fiber network and same optical components are used for PA and OCT imaging, and the obtained images are intrinsically coregistered.

An integrated widefield imaging and spectroscopy system for contrast-enhanced, image-guided resection of tumors

Tumor recurrence following surgery is a common and unresolved medical problem of great importance since surgery is the most widely used treatment for solid-mass tumors worldwide. A contributing factor to tumor recurrence is the presence of residual tumor remaining at or near the surgical site following surgery.

GOAL

The primary objective of this study was to develop and evaluate an image-guided surgery system based on a near-infrared, handheld excitation source and spectrograph in combination with a widefield video imaging system.

METHODS

This system was designed to detect the fluorescence of near-infrared contrast agents and, in particular, indocyanine green (ICG). The imaging system was evaluated for its optical performance and ability to detect the presence of ICG in tumors in an ectopic murine tumor model as well as in spontaneous tumors arising in canines.

RESULTS

In both settings, an intravenous ICG infusion provided tumor contrast. In both the murine models and surgical specimens from canines, ICG preferentially accumulated in tumor tissue compared to surrounding normal tissue. The resulting contrast was sufficient to distinguish neoplasia from normal tissue; in the canine surgical specimens, the contrast was sufficient to permit identification of neoplasia on the marginal surface of the specimen.

CONCLUSION

These results demonstrate a unique concept in image-guided surgery by combining local excitation and spectroscopy with widefield imaging.

SIGNIFICANCE

The ability to readily detect ICG in canines with spontaneous tumors in a clinical setting exemplifies the potential for further clinical translation; the promising results of detecting neoplasia on the marginal specimen surface underscore the clinical utility.

A standardized light-emitting diode device for photoimmunotherapy

Antibody-based photodynamic therapy-photoimmunotherapy (PIT)-is an ideal modality to improve cancer treatment because of its selective and tumor-specific mode of therapy. Because the use of PIT for cancer treatment is continuing to be described, there is great need to characterize a standardized light source for PIT application. In this work, we designed and manufactured a light-emitting diode (LED)/PIT device and validated the technical feasibility, applicability, safety, and consistency of the system for cancer treatment.

METHODS

To outline the characteristics and photobiologic safety of the LED device, multiple optical measurements were performed in accordance with a photobiologic safety standard. A luciferase-transfected breast cancer cell line (2LMP-Luc) in combination with panitumumab-IRDye 700DX (pan-IR700) was used to validate the in vitro and in vivo performance of our LED device.

RESULTS

Testing revealed the light source to be safe, easy to use, and independent of illumination and power output (mW cm(-2)) variations over time. For in vitro studies, an LED dose (2, 4, 6 J cm(-2))-dependent cytotoxicity was observed using propidium iodide exclusion and annexin V staining. Dose-dependent blebbing was also observed during microscopic analysis. Bioluminescence signals of tumors treated with 0.3 mg of pan-IR700 and 50 J cm(-2) decreased significantly (>80%) compared with signals of contralateral nontreated sites at 4 h and at 1 d after PIT.

CONCLUSION

To our knowledge, a normalized and standardized LED device has not been explicitly described or developed. In this article, we introduce a standardized light source and validate its usability for PIT applications.

Near-infrared triple-helical peptide with quenched fluorophores for optical imaging of MMP-2 and MMP-9 proteolytic activity in vivo

The gelatinase members of the MMP family have consistently been associated with tumor invasiveness, which make them an attractive target for molecular imaging. We report new activatable proteolytic optical imaging agents that consist of triple-helical peptide (THP) conjugates, with high specificity to the gelatinases, bearing quenched cypate dyes. With quenching efficiencies up to 51%, the amplified fluorescence signal upon cypate3-THP hydrolysis by the gelatinases (kcat/KM values of 6.4×10(3) M(-1) s(-1) to 9.1×10(3) M(-1) s(-1) for MMP-2 and MMP-9, respectively) in mice bearing human fibrosarcoma xenografted tumors was monitored with fluorescence molecular tomography. There was significant fluorescence enhancement within the tumor and this enhancement was reduced by treatment with pan-MMP inhibitor, Ilomastat. These data, combined with the gelatinase substrate specificity observed in vitro, indicated the observed fluorescence at the site of the tumor was due to gelatinase mediated hydrolysis of cypate3-THP.

Towards a successful clinical implementation of fluorescence-guided surgery

During the European Molecular Imaging Meeting (EMIM) 2013, the fluorescence-guided surgery study group held its inaugural session to discuss the clinical implementation of fluorescence-guided surgery. The general aim of this study group is to discuss and identify the steps required to successfully and safely bring intraoperative fluorescence imaging to the clinics. The focus group intends to use synergies between interested groups as a tool to address regulatory and implementation hurdles in Europe and operates within the intraoperative focus group of the World Molecular Imaging Society (WMIS) that promotes the same interests at the WMIS level. The major topics on the critical path of implementation identified within the study group were quality controls and standards for ensuring accurate imaging and the ability to compare results from different studies, regulatory affairs, and strategies to increase awareness among physicians, regulators, insurance companies, and a broader audience. These hurdles, and the possible actions discussed to overcome them, are summarized in this report. Furthermore, a number of recommendations for the future shape of the fluorescence-guided study group are discussed. A main driving conclusion remains that intraoperative imaging has great clinical potential and that many of the solutions required are best addressed with the community working together to optimally promote and accelerate the clinical implementation of fluorescence imaging towards improving surgical procedures.

Enriching the interventional vision of cancer with fluorescence and optoacoustic imaging

Among several techniques considered for surgical and endoscopic imaging, novel optical methods are evolving as a promising approach for interventional guidance. Pilot clinical applications of fluorescence molecular imaging have demonstrated the benefits of using targeted fluorescent agents in cancer surgery. This premise can be extended broadly to interventional guidance through an increasing number of targeted agents and detection techniques. Beyond epi-illumination fluorescence imaging, optoacoustic (photoacoustic) methods are emerging to offer high-resolution cross-sectional optical imaging through several millimeters to centimeters of depth. We present an overview of key recent developments in optical interventional imaging and outline the potential for a paradigm shift in surgical and endoscopic visualization.

Intraoperative fluorescent imaging of intracranial tumors: a review

A review of fluorescent imaging for intracranial neoplasms is presented. Complete resection of brain cancer is seldom possible because of the goal to preserve brain tissue and the inability to visualize individual infiltrative tumor cells. Verification of histology and identification of tumor invasion in macroscopically normal-appearing brain tissue determine prognosis after resection of malignant gliomas. Therefore, imaging modalities aim to facilitate intraoperative decision-making. Intraoperative fluorescent imaging techniques have the potential to enable precise histopathologic diagnosis and to detect tumor remnants in the operative field. Macroscopic fluorescence imaging is effective for gross tumor detection. Microscopic imaging techniques enhance the sensitivity of the macroscopic observations and provide real-time histological information. Further development of clinical grade fluorescent agents specifically targeting tumor cells could improve the diagnostic and prognostic yield of intraoperative imaging.

Non-contact photoacoustic imaging using a fiber based interferometer with optical amplification

In photoacoustic imaging the ultrasonic signals are usually detected by contacting transducers. For some applications contact with the tissue should be avoided. As alternatives to contacting transducers interferometric means can be used to acquire photoacoustic signals remotely. In this paper we report on non-contact three and two dimensional photoacoustic imaging using an optical fiber-based Mach-Zehnder interferometer. A detection beam is transmitted through an optical fiber network onto the surface of the specimen. Back reflected light is collected and coupled into the same optical fiber. To achieve a high signal/noise ratio the reflected light is amplified by means of optical amplification with an erbium doped fiber amplifier before demodulation. After data acquisition the initial pressure distribution is reconstructed by a Fourier domain reconstruction algorithm. We present remote photoacoustic imaging of a tissue mimicking phantom and on chicken skin.

Quantitative, spectrally-resolved intraoperative fluorescence imaging

Intraoperative visual fluorescence imaging (vFI) has emerged as a promising aid to surgical guidance, but does not fully exploit the potential of the fluorescent agents that are currently available. Here, we introduce a quantitative fluorescence imaging (qFI) approach that converts spectrally-resolved data into images of absolute fluorophore concentration pixel-by-pixel across the surgical field of view (FOV). The resulting estimates are linear, accurate, and precise relative to true values, and spectral decomposition of multiple fluorophores is also achieved. Experiments with protoporphyrin IX in a glioma rodent model demonstrate in vivo quantitative and spectrally-resolved fluorescence imaging of infiltrating tumor margins for the first time. Moreover, we present images from human surgery which detect residual tumor not evident with state-of-the-art vFI. The wide-field qFI technique has broad implications for intraoperative surgical guidance because it provides near real-time quantitative assessment of multiple fluorescent biomarkers across the operative field.

Novel toll-like receptor 2 ligands for targeted pancreatic cancer imaging and immunotherapy

Toll-like receptor 2 (TLR2) is a target for immune system stimulation during cancer immunotherapy and a cell-surface marker for pancreatic cancer. To develop targeted agents for cancer imaging and therapy, we designed, synthesized, and characterized 13 novel, fully synthetic high affinity TLR2 agonists. Analogue 10 had the highest agonist activity (NF-κB functional assay, EC(50) = 20 nM) and binding affinity (competitive binding assay, K(i) = 25 nM). As an immune adjuvant, compound 10 stimulated the immune system in vivo by generation and persistence of antigen-specific CD8+ T cells indicating its potential use in cancer immunotherapy. After conjugation of near-infrared dye to 10, agonist activity (EC(50) = 34 nM) and binding affinity (K(i) = 11 nM) were retained in 13. Fluorescence signal was present in TLR2 expressing pancreatic tumor xenografts 24 h after injection of 13, while an excess of unlabeled ligand blocked 13 from binding to the tumor, resulting in significantly decreased signal (p < 0.001) demonstrating in vivo selectivity.

Dual-mode laparoscopic fluorescence image-guided surgery using a single camera

Iatrogenic nerve damage is a leading cause of morbidity associated with many common surgical procedures. Complications arising from these injuries may result in loss of function and/or sensation, muscle atrophy, and chronic neuropathy. Fluorescence image-guided surgery offers a potential solution for avoiding intraoperative nerve damage by highlighting nerves that are otherwise difficult to visualize. In this work we present the development of a single camera, dual-mode laparoscope that provides near simultaneous display of white-light and fluorescence images of nerves. The capability of the instrumentation is demonstrated through imaging several types of in situ rat nerves via a nerve specific contrast agent. Full color white light and high brightness fluorescence images and video of nerves as small as 100 µm in diameter are presented.

Non-contact biomedical photoacoustic and ultrasound imaging

The detection of ultrasound in photoacoustic tomography (PAT) usually relies on ultrasonic transducers in contact with the biological tissue through a coupling medium. This is a major drawback for important potential applications such as surgery. Here we report the use of a remote optical method, derived from industrial laser-ultrasonics, to detect ultrasound in tissues. This approach enables non-contact PAT (NCPAT) without exceeding laser exposure safety limits. The sensitivity of the method is based on the use of suitably shaped detection laser pulses and a confocal Fabry-Perot interferometer in differential configuration. Reliable image reconstruction is obtained by measuring remotely the surface profile of the tissue with an optical coherence tomography system. The proposed method also allows non-contact ultrasound imaging (US) by applying a second reconstruction algorithm to the data acquired for NCPAT. Endogenous and exogenous inclusions exhibiting optical and acoustic contrasts were detected ex vivo in chicken breast and calf brain specimens. Inclusions down to 0.3 mm in size were detected at depths exceeding 1 cm. The method could expand the scope of photoacoustic and US to in-vivo biomedical applications where contact is impractical.

Quantitative measurement of optical attenuation coefficients of cell lines CNE1, CNE2, and NP69 using optical coherence tomography

The radiotherapy-related types of nasopharyngeal carcinoma (NPC) have been established, which give the most effective treatment for NPC patients using the individual therapy. To diagnose the types of NPC, we assess the general NPC cell lines CNE1, CNE2 and normal nasopharyngeal cell line NP69 using optical coherence tomography (OCT) in two steps: firstly, the OCT images of the three different types of cell pellets are captured. Secondly, by fitting Beer's law to the averaged A-scans in these OCT datasets, the attenuation coefficients (μ t ) of the cells can be extracted. The median attenuation coefficients (interquartile range) of CNE1, CNE2, and NP69 are 5.58 mm(-1) (IQR 5.55 to 5.65 mm(-1)), 5.91 mm(-1) (IQR 5.82 to 5.88 mm(-1)), and 8.96 mm(-1) (IQR 8.80 to 9.47 mm(-1)), respectively. The distinguishable quantitative OCT analysis (by μ t ) shows that the types of NPC could potentially be differentiated in real time and noninvasive.

Compact Fluorescence and White Light Imaging System for Intraoperative Visualization of Nerves

Fluorescence image guided surgery (FIGS) allows intraoperative visualization of critical structures, with applications spanning neurology, cardiology and oncology. An unmet clinical need is prevention of iatrogenic nerve damage, a major cause of post-surgical morbidity. Here we describe the advancement of FIGS imaging hardware, coupled with a custom nerve-labeling fluorophore (GE3082), to bring FIGS nerve imaging closer to clinical translation. The instrument is comprised of a 405 nm laser and a white light LED source for excitation and illumination. A single 90 gram color CCD camera is coupled to a 10 mm surgical laparoscope for image acquisition. Synchronization of the light source and camera allows for simultaneous visualization of reflected white light and fluorescence using only a single camera. The imaging hardware and contrast agent were evaluated in rats during in situ surgical procedures.

Non-contact photoacoustic tomography and ultrasonography for tissue imaging

The detection of ultrasound in photoacoustic tomography (PAT) and ultrasonography (US) usually relies on ultrasonic transducers in contact with the biological tissue. This is a major drawback for important potential applications such as surgery and small animal imaging. Here we report the use of remote optical detection, as used in industrial laser-ultrasonics, to detect ultrasound in biological tissues. This strategy enables non-contact implementation of PAT and US without exceeding laser exposure safety limits. The method uses suitably shaped laser pulses and a confocal Fabry-Perot interferometer in differential configuration to reach quantum-limited sensitivity. Endogenous and exogenous inclusions exhibiting optical and acoustic contrasts were detected ex vivo in chicken breast and calf brain specimens. Inclusions down to 0.5 mm in size were detected at depths well exceeding 1 cm. The method could significantly expand the scope of applications of PAT and US in biomedical imaging.