Criteria for the design of tissue-mimicking phantoms for the standardization of biophotonic instrumentation

Trackable Liposomes for In Vivo Delivery Tracing toward Personalized Medicine Care under NIR Light on Skin Tumor

We report an near-infrared (NIR)-trackable and therapeutic liposome with skin tumor specificity. Liposomes with a hydrodynamic diameter of ∼20 nm are tracked under the vein visualization imaging system in the presence of loaded paclitaxel and NIR-active agents. The ability to track liposome nanocarriers is recorded on the tissue-mimicking phantom model and in vivo mouse veins after intravenous administration. The trackable liposome delivery provides in vitro and in vivo photothermal heat (∼40 °C) for NIR-light-triggered area-specific chemotherapeutic release. This approach can be linked with a real-time vein-imaging system to track and apply area-specific local heat, which hitchhikes liposomes from the vein and finally releases them at the tumor site. We conducted studies on mice skin tumors that indicated the disappearance of tumors visibly and histologically (H&E stains). The ability of nanocarriers to monitor after administration is crucial for improving the effectiveness and specificity of cancer therapy, which could be achieved in the trackable delivery system.

Mechanical properties of human hepatic tissues to develop liver-mimicking phantoms for medical applications

Using liver phantoms for mimicking human tissue in clinical training, disease diagnosis, and treatment planning is a common practice. The fabrication material of the liver phantom should exhibit mechanical properties similar to those of the real liver organ in the human body. This tissue-equivalent material is essential for qualitative and quantitative investigation of the liver mechanisms in producing nutrients, excretion of waste metabolites, and tissue deformity at mechanical stimulus. This paper reviews the mechanical properties of human hepatic tissues to develop liver-mimicking phantoms. These properties include viscosity, elasticity, acoustic impedance, sound speed, and attenuation. The advantages and disadvantages of the most common fabrication materials for developing liver tissue-mimicking phantoms are also highlighted. Such phantoms will give a better insight into the real tissue damage during the disease progression and preservation for transplantation. The liver tissue-mimicking phantom will raise the quality assurance of patient diagnostic and treatment precision and offer a definitive clinical trial data collection.

Development of a glass-based imaging phantom to model the optical properties of human tissue

The fabrication of a stable, reproducible optical imaging phantom is critical to the assessment and optimization of optical imaging systems. We demonstrate the use of an alternative material, glass, for the development of tissue-mimicking phantoms. The glass matrix was doped with nickel ions to approximate the absorption of hemoglobin. Scattering levels representative of human tissue were induced in the glass matrix through controlled crystallization at elevated temperatures. We show that this type of glass is a viable material for creating tissue-mimicking optical phantoms by providing controlled levels of scattering and absorption with excellent optical homogeneity, long-term stability and reproducibility.

Noninvasive Visualization of Amyloid-Beta Deposits in Alzheimer's Amyloidosis Mice via Fluorescence Molecular Tomography Using Contrast Agent

Alzheimer's disease is pathologically featured by the accumulation of amyloid-beta (Aβ) plaque and neurofibrillary tangles. Compared to small animal positron emission tomography, optical imaging features nonionizing radiation, low cost, and logistic convenience. Optical detection of Aβ deposits is typically implemented by 2D macroscopic imaging and various microscopic techniques assisted with Aβ-targeted contrast agents. Here, we introduce fluorescence molecular tomography (FMT), a macroscopic 3D fluorescence imaging technique, convenient for in vivo longitudinal monitoring of the animal brain without the involvement of cranial window opening operation. This chapter aims to provide the protocols for FMT in vivo imaging of Aβ deposits in the brain of rodent model of Alzheimer's disease. The materials, stepwise method, notes, limitations of FMT, and emerging opportunities for FMT techniques are presented.

Musa acuminata as electroporation model

Electrochemotherapy (ECT) and Irreversible electroporation (IRE) are cancer treatments based on electric field distribution in tissues. Solanum tuberosum (potato tissue) phantom is known to mimic changes in the electrical conductivity that occur in animal tissues during electroporation (EP). Electric field distribution is assessed through enzymatic staining. However, the 24-h wait for this assessment could slow agile response scenarios. We developed and validated the Musa acuminata (cavendish banana) conductivity model, which quickly evaluates EP by tissue staining. We investigated the frequency response of the tissue using impedance spectroscopy analysis, conductivity changes, and enzymatic staining. We optimized three usual EP models: adapted Gompertz, smoothed Heaviside, and the sigmoid or logistic function. We found dielectric parameters in banana tissue similar to those in potato (electrical conductivity of 0.035 S/m and relative permittivity of 4.1×104). The coefficients of determination R2 were 99.94% (Gompertz), 99.85% (Heaviside), and 99.58% (sigmoid). The sigmoid and Heaviside functions described the calibration and validation electric currents with 95% confidence. We observed the electroporated areas in bananas 3h30m after EP. Staining was significant after 450 V/cm. The conductivity model of Musa acuminata suits treatment planning, hardware development, and training scenarios. Banana phantom supports the 3Rs practice and is a reliable alternative for potato in EP studies.

Gentle label-free nonlinear optical imaging relaxes linear-absorption-mediated triplet

Sample health is critical for live-cell fluorescence microscopy and has promoted light-sheet microscopy that restricts its ultraviolet-visible excitation to one plane inside a three-dimensional sample. It is thus intriguing that laser-scanning nonlinear optical microscopy, which similarly restricts its near-infrared excitation, has not broadly enabled gentle label-free molecular imaging. We hypothesize that intense near-infrared excitation induces phototoxicity via linear absorption of intrinsic biomolecules with subsequent triplet buildup, rather than the commonly assumed mechanism of nonlinear absorption. Using a reproducible phototoxicity assay based on the time-lapse elevation of auto-fluorescence (hyper-fluorescence) from a homogeneous tissue model (chicken breast), we provide strong evidence supporting this hypothesis. Our study justifies a simple imaging technique, e.g., rapidly scanned sub-80-fs excitation with full triplet-relaxation, to mitigate this ubiquitous linear-absorption-mediated phototoxicity independent of sample types. The corresponding label-free imaging can track freely moving C. elegans in real-time at an irradiance up to one-half of water optical breakdown.

Fabrication and characterization of multi-biomarker optimized tissue-mimicking phantoms for multi-modal optical spectroscopy

Rapid advancement of novel optical spectroscopy and imaging systems relies on the availability of well-characterised and reproducible protocols for phantoms as a standard for the validation of the technique. The tissue-mimicking phantoms are also used to investigate photon transport in biological samples before clinical trials that require well-characterized phantoms with known optical properties (reduced scattering (μ's) and absorption (μa) coefficients). However, at present, there is limited literature available providing well-characterized phantom recipes considering various biomarkers and tested over a wide range of optical properties covering most of the human organs and applicable to multimodal optical spectroscopy. In this study, gelatin-based phantoms were designed to simulate tissue optical properties where India ink and Intralipid were used as absorbing and scattering agents, respectively. Multiple biomarkers were simulated by varying the gelatin concentration to mimic the change in tissue hydration and hydroxyapatite concentration to mimic bone signature. The recipe along with biomarkers were optimized and characterised over a wide range of optical properties (μa from 0.1 to 0.5 cm-1; μ's from 5 to 15 cm-1) relevant to human tissue using a broadband time-domain diffuse optical spectrometer. The data collected showed a linear relationship between the concentration of ink/lipids and μa/μ's values with negligible coupling between μa and μ's values. While being stored in a refrigerator post-fabrication, the μa and μ's did not change significantly (<4% coefficient of variation, 'CV') over three weeks. The reproducibility in three different sets was validated experimentally and found to be strong with a variation of ≤6% CV in μa and ≤9% CV in μ's. From the 3 × 3 data of μa and μ's matrices, one can deduce the recipe for any target absorption or reduced scattering coefficient. The applicability of the phantoms was tested using diffuse reflectance and Raman spectrometers. A use case application was demonstrated for Raman spectroscopy where hydration and hydroxyapatite phantoms were designed to characterize the Raman instrument. The Raman instrument could detect the change in 1% of HA and 5% of hydration. This study presents a first-of-its-kind robust, well-characterized, multi-biomarker phantom recipe for calibration and benchmarking of multimodal spectroscopy devices assisting in their clinical translation.

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.

Community Survey Results Show that Standardisation of Preclinical Imaging Techniques Remains a Challenge

PURPOSE

To support acquisition of accurate, reproducible and high-quality preclinical imaging data, various standardisation resources have been developed over the years. However, it is unclear the impact of those efforts in current preclinical imaging practices. To better understand the status quo in the field of preclinical imaging standardisation, the STANDARD group of the European Society of Molecular Imaging (ESMI) put together a community survey and a forum for discussion at the European Molecular Imaging Meeting (EMIM) 2022. This paper reports on the results from the STANDARD survey and the forum discussions that took place at EMIM2022.

PROCEDURES

The survey was delivered to the community by the ESMI office and was promoted through the Society channels, email lists and webpages. The survey contained seven sections organised as generic questions and imaging modality-specific questions. The generic questions focused on issues regarding data acquisition, data processing, data storage, publishing and community awareness of international guidelines for animal research. Specific questions on practices in optical imaging, PET, CT, SPECT, MRI and ultrasound were further included.

RESULTS

Data from the STANDARD survey showed that 47% of survey participants do not have or do not know if they have QC/QA guidelines at their institutes. Additionally, a large variability exists in the ways data are acquired, processed and reported regarding general aspects as well as modality-specific aspects. Moreover, there is limited awareness of the existence of international guidelines on preclinical (imaging) research practices.

CONCLUSIONS

Standardisation of preclinical imaging techniques remains a challenge and hinders the transformative potential of preclinical imaging to augment biomedical research pipelines by serving as an easy vehicle for translation of research findings to the clinic. Data collected in this project show that there is a need to promote and disseminate already available tools to standardise preclinical imaging practices.

Stable tissue-mimicking phantoms for longitudinal multimodality imaging studies that incorporate optical, CT, and MRI contrast

Significance

Tissue phantoms that mimic the optical and radiologic properties of human or animal tissue play an important role in the development, characterization, and evaluation of imaging systems. Phantoms that are easily produced and stable for longitudinal studies are highly desirable.

Aim

A new type of long-lasting phantom was developed with commercially available materials and was assessed for fabrication ease, stability, and optical property control. Magnetic resonance imaging (MRI) and x-ray computed tomography (CT) contrast properties were also evaluated.

Approach

A systematic investigation of relationships between concentrations of skin-like pigments and composite optical properties was conducted to realize optical property phantoms in the red and near-infrared (NIR) wavelength range that also offered contrast for CT and MRI.

Results

Phantom fabrication time was < 1    h and did not involve any heating or cooling processes. Changes in optical properties were < 2 % over a 12-month period. Phantom optical and spectral features were similar to human soft tissue over the red to NIR wavelength ranges. Pigments used in the study also had CT and MRI contrasts for multimodality imaging studies.

Conclusions

The phantoms described here mimic optical properties of soft tissue and are suitable for multimodality imaging studies involving CT or MRI without adding secondary contrast agents.

Solid optical tissue phantom tools based on upconverting nanoparticles for biomedical applications

SIGNIFICANCE

Phantoms play a critical role in the development of biophotonics techniques. There is a lack of novel phantom tools in the emerging field of upconverting nanoparticles (UCNPs) for biophotonics application. This work provides a range of UCNP-based phantom tools and a manufacturing recipe to bridge the gap and accelerate the development of UCNP-based biophotonics applications.

AIM

The study aims to provide a well-characterized UCNP-based solid phantom recipe and set of phantom tools to address a wide range of UCNP-based biophotonics applications.

APPROACH

A solid phantom recipe based on silicone matrix was developed to manufacture UCNP-based phantoms. A lab built UCNP imaging system was used to characterize upconverted fluorescence emission of phantoms for linearity, homogeneity, and long-term stability. A photon time-of-flight spectroscopy technique was used to characterize the optical properties of the phantoms.

RESULTS

In total, 24 phantoms classified into 4 types, namely homogeneous, multilayer, inclusion, and base phantoms, were manufactured. The phantoms exhibit linear behavior over the dosage range of UCNPs. The phantoms were found to be stable over a limited observed period of 4 months with a coefficient of variation of < 4 % . The deep tissue imaging case showed that increasing the thickness of tissue reduced the UCNP emission.

CONCLUSIONS

A first-of-its-kind UCNP-based solid phantom recipe was developed, and four types of UCNP phantom tools to explore biophotonics applications were presented. The UCNP phantoms exhibited a linear behavior with dosage and were stable over time. An example case showed the potential use of the phantom for deep tissue imaging applications. With recent advance in the use of UCNPs for biophotonics, we believe our recipe and tools will play a pivotal role in the growth of the UCNPs for biophotonics applications.

3D-Printed Tumor Phantoms for Assessment of In Vivo Fluorescence Imaging Analysis Methods

PURPOSE

Interventional fluorescence imaging is increasingly being utilized to quantify cancer biomarkers in both clinical and preclinical models, yet absolute quantification is complicated by many factors. The use of optical phantoms has been suggested by multiple professional organizations for quantitative performance assessment of fluorescence guidance imaging systems. This concept can be further extended to provide standardized tools to compare and assess image analysis metrics.

PROCEDURES

3D-printed fluorescence phantoms based on solid tumor models were developed with representative bio-mimicking optical properties. Phantoms were produced with discrete tumors embedded with an NIR fluorophore of fixed concentration and either zero or 3% non-specific fluorophore in the surrounding material. These phantoms were first imaged by two fluorescence imaging systems using two methods of image segmentation, and four assessment metrics were calculated to demonstrate variability in the quantitative assessment of system performance. The same analysis techniques were then applied to one tumor model with decreasing tumor fluorophore concentrations.

RESULTS

These anatomical phantom models demonstrate the ability to use 3D printing to manufacture anthropomorphic shapes with a wide range of reduced scattering (μ_s': 0.24-1.06 mm^-1) and absorption (μ_a: 0.005-0.14 mm^-1) properties. The phantom imaging and analysis highlight variability in the measured sensitivity metrics associated with tumor visualization.

CONCLUSIONS

3D printing techniques provide a platform for demonstrating complex biological models that introduce real-world complexities for quantifying fluorescence image data. Controlled iterative development of these phantom designs can be used as a tool to advance the field and provide context for consensus-building beyond performance assessment of fluorescence imaging platforms, and extend support for standardizing how quantitative metrics are extracted from imaging data and reported in literature.

Optical Tissue Phantoms for Quantitative Evaluation of Surgical Imaging Devices

Optical tissue phantoms (OTPs) have been extensively applied to the evaluation of imaging systems and surgical training. Due to their human tissue-mimicking characteristics, OTPs can provide accurate optical feedback on the performance of image-guided surgical instruments, simulating the biological sizes and shapes of human organs, and preserving similar haptic responses of original tissues. This review summarizes the essential components of OTPs (i.e., matrix, scattering and absorbing agents, and fluorophores) and the various manufacturing methods currently used to create suitable tissue-mimicking phantoms. As photobleaching is a major challenge in OTP fabrication and its feedback accuracy, phantom photostability and how the photobleaching phenomenon can affect their optical properties are discussed. Consequently, the need for novel photostable OTPs for the quantitative evaluation of surgical imaging devices is emphasized.

BIAN: A Multilayer Microfluidic-Based Tissue-Mimicking Phantom for Near-Infrared Imaging

Near-infrared spectroscopy (NIRS) is a non-invasive optical method for monitoring cerebral oxygenation. Changes in regional blood flow and oxygenation due to neurovascular coupling are important biomarkers of neuronal activation. So far, there has been little research on multilayer tissue phantoms with tuneable blood flow, blood volume, and optical properties to simulate local changes in oxygenation at different depths. The aim of this study is to design, fabricate and characterize a complex dynamic phantom based on multilayer microfluidics with controllable blood flow, blood volume, and optical properties for testing NIRS instruments. We developed a phantom prototype with two microfluidic chips embedded at two depths inside a solid silicone phantom to mimic the vessels in the scalp and in the cortex. To simulate the oxygenation and perfusion of tissue, a solution with blood-like optical properties was sent into the microchannels by a pump with a programmable pressure controller. The pressure adjusted the volume of the microfluidic chips representing a distension of blood vessels. The optical changes in the superficial and deep layers were measured by a commercially available frequency domain NIRS instrument. The NIRS successfully detected the changes in light intensity elicited by the changes in the pressure input to the two layers. In conclusion, the microfluidics-based imaging phantom was successfully designed and fabricated and mimics brain functional activity. This technique has great potential for testing other optical devices, e.g., diffuse correlation spectroscopy, pulse oximetry, and optical coherence tomography.

NIR-II Dyad-Doped Ratiometric Nanosensor with Enhanced Spectral Fidelity in Biological Media for In Vivo Biosensing

Ratiometric fluorescence nanosensors provide quantitative biological information. However, spectral shift and distortion of ratiometric nanosensors in biological media often compromise sensing accuracy, limiting in vivo applications. Here, we develop a fluorescent dyad (aBOP-IR1110) in the second near-infrared (NIR-II) window by covalently linking an asymmetric aza-BODIPY with a ONOO^--responsive meso-thiocyanine. The dyad encapsulated in the PEGylated nanomicelle largely improves spectral fidelity in serum culture by >9.4 times compared to that of its noncovalent counterpart. The increased molecular weights (>1480 Da) and hydrophobicity (LogP of 7.87-12.36) lock dyads inside the micelles, which act as the shield against the external environment. ONOO^--altered intramolecular Förster resonance energy transfer (FRET) generates linear ratiometric response with better serum tolerance, enabling us to monitor the dynamics of oxidative stress in traumatic brain injury and evaluate therapeutic efficiency. The results show high correlation with in vitro triphenyltetrazolium chloride staining, suggesting the potential of NIR-II dyad-doped nanosensor for in vivo high-fidelity sensing applications.