Optical and acoustic properties at 1064 nm of polyvinyl chloride-plastisol for use as a tissue phantom in biomedical optoacoustics

Development, evaluation, and overview of standardized training phantoms for abdominal ultrasound-guided interventions

PURPOSE

Ultrasound (US) represents the primary approach for abdominal diagnosis and is regularly used to guide diagnostic and therapeutic interventions (INVUS). Due to possible serious INVUS complications, structured training concepts are required. Phantoms can facilitate teaching, but their use is currently restricted by complex manufacturing and short durability of the materials. Hence, the aim of this study was the development and evaluation of an optimized abdominal INVUS phantom.

MATERIALS AND METHODS

Phantom requirements were defined in a structured research process: Skin-like surface texture, homogeneous matrix with realistic tissue properties, implementation of lesions and abscess cavities in different sizes and depths as well as a modular production process allowing for customized layouts. The phantom prototypes were evaluated in certified ultrasound courses.

RESULTS

In accordance with the defined specifications, a new type of matrix was developed and cast in multiple layers including different target materials. The phantom structure is based on features of liver anatomy and includes solid focal lesions, vessels, and abscess formations. For a realistic biopsy procedure, ultrasound-proof material was additionally included to imitate bone. The evaluation was performed by US novices (n=40) and experienced participants (n=41). The majority (73/81) confirmed realistic visualization of the lesions. The 3D impression was rated as "very good" in 64% of cases (52/81) and good in 31% (25/81). Overall, 86% (70/81) of the participants certified high clinical relevance of the phantom.

CONCLUSION

The presented INVUS phantom concept allows standardized and realistic training for interventions.

Adaptation of a Clinical High-Frequency Transrectal Ultrasound System for Prostate Photoacoustic Imaging: Implementation and Pre-clinical Demonstration

OBJECTIVE

High-frequency, high-resolution transrectal micro-ultrasound (micro-US: ≥15 MHz) imaging of the prostate is emerging as a beneficial tool for scoring disease risk and accurately targeting biopsies. Adding photoacoustic (PA) imaging to visualize abnormal vascularization and accumulation of contrast agents in tumors has potential for guiding focal therapies. In this work, we describe a new imaging platform that combines a transrectal micro-US system with transurethral light delivery for PA imaging.

METHODS

A clinical transrectal micro-US system was adapted to acquire PA images synchronous to a tunable laser pulse. A transurethral side-firing optical fiber was developed for light delivery. A polyvinyl chloride (PVC)-plastisol phantom was developed and characterized to image PA contrast agents in wall-less channels. After resolution measurement in water, PA imaging was demonstrated in phantom channels with dyes and biodegradable nanoparticle contrast agents called porphysomes. In vivo imaging of a tumor model was performed, with porphysomes administered intravenously.

RESULTS

Photoacoustic imaging data were acquired at 5 Hz, and image reconstruction was performed offline. PA image resolution at a 14-mm depth was 74 and 261 μm in the axial and lateral directions, respectively. The speed of sound in PVC-plastisol was 1383 m/s, and the attenuation was 4 dB/mm at 20 MHz. PA signal from porphysomes was spectrally unmixed from blood signals in the tumor, and a signal increase was observed 3 h after porphysome injection.

CONCLUSION

A combined transrectal micro-US and PA imaging system was developed and characterized, and in vivo imaging demonstrated. High-resolution PA imaging may provide valuable additional information for diagnostic and therapeutic applications in the prostate.

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.

Evaluation of physical properties and image of polyvinyl chloride as breast tissue equivalence for dual-modality (mammography and ultrasound)

TMP is gradually becoming a fundamental element for quality assurance and control in ionizing and non-ionizing radiation imaging modalities as well as in the development of different techniques. This study aimed to evaluate and obtain polyvinyl chloride tissue mimicking material for dual-modality breast phantoms in mammography and ultrasound. Breast tissue equivalence was evaluated based on X-ray attenuation properties, speed of sound, attenuation, and acoustic impedance. There are six samples of PVC-plasticizer material with variations of PVC concentration and additives. The evaluation of X-ray attenuation was carried out using mammography from 23 to 35 kV, while the acoustic properties were assessed with mode A ultrasound and a transducer frequency of 5 MHz. A breast phantom was created from TMP material with tissue equivalence and was then evaluated using mammography as well as ultrasound to analyze its image quality. The results showed that samples A (PVC 5%, DOP 95%), B (PVC 7%, DOP 93%), C (PVC 10%, DOP 90%), E (PVC 7%, DOP 90%, graphite 3%), and F (PVC 7%, DOP 90%, silicone oil 3%) have the closest equivalent to the ACR breast phantom material with a different range of 0.01-1.39 in the 23-35 kV range. Based on the evaluation of the acoustic properties of ultrasound, A had high similarity to fat tissue with a difference of 0.03 (dB cm- 1 MHz- 1) and 0.07 (106 kg m- 2 s- 1), while B was close to the glandular tissue with a difference of 9.2 m s- 1. Multilayer breast phantom images' results showed gray levels in mammography and ultrasound modalities. Therefore, this study succeeded in establishing TMP material for mammography and ultrasound. It can also be used for simple quality assurance and control programs.

Flexible array transducer for photoacoustic-guided interventions: phantom and ex vivo demonstrations

Photoacoustic imaging has demonstrated recent promise for surgical guidance, enabling visualization of tool tips during surgical and non-surgical interventions. To receive photoacoustic signals, most conventional transducers are rigid, while a flexible array is able to deform and provide complete contact on surfaces with different geometries. In this work, we present photoacoustic images acquired with a flexible array transducer in multiple concave shapes in phantom and ex vivo bovine liver experiments targeted toward interventional photoacoustic applications. We validate our image reconstruction equations for known sensor geometries with simulated data, and we provide empirical elevation field-of-view, target position, and image quality measurements. The elevation field-of-view was 6.08 mm at a depth of 4 cm and greater than 13 mm at a depth of 5 cm. The target depth agreement with ground truth ranged 98.35-99.69%. The mean lateral and axial target sizes when imaging 600 μm-core-diameter optical fibers inserted within the phantoms ranged 0.98-2.14 mm and 1.61-2.24 mm, respectively. The mean ± one standard deviation of lateral and axial target sizes when surrounded by liver tissue were 1.80±0.48 mm and 2.17±0.24 mm, respectively. Contrast, signal-to-noise, and generalized contrast-to-noise ratios ranged 6.92-24.42 dB, 46.50-67.51 dB, and 0.76-1, respectively, within the elevational field-of-view. Results establish the feasibility of implementing photoacoustic-guided surgery with a flexible array transducer.

Panoramic volumetric clinical handheld photoacoustic and ultrasound imaging

Photoacoustic (PA) imaging has gained much attention, providing structural and functional information in combination with clinical ultrasound (US) imaging systems. 2D PA and US imaging is easily implemented, but its heavy dependence on operator skills makes 3D imaging preferable. In this study, we propose a panoramic volumetric clinical PA and US imaging system equipping a handheld imaging scanner weighing 600 g and measuring 70 × 62 × 110 mm³. Multiple PA/US scans were performed to cover a large field-of-view (FOV), and the acquired PA/US volumes were mosaic-stitched after manually correcting the positions and rotations in a total of 6 degrees of freedom. PA and US maximum amplitude projection images were visualized online, while spectral unmixed data was quantified offline. The performance of the system was tested via tissue-mimicking phantom experiments. The system's potential was confirmed in vivo by panoramically imaging vascular networks in human arms and necks, with FOVs of 331 × 38 and 129 × 120 mm², respectively. Further, we quantified hemoglobin oxygen saturation levels in the radial artery, brachial artery, carotid artery, and jugular vein. We hope that this system can be applied for various clinical fields such as cardiovascular imaging, dermatology, vascular surgery, internal medicine, and oncology.

Wide-field three-dimensional photoacoustic/ultrasound scanner using a two-dimensional matrix transducer array

Two-dimensional matrix transducer arrays are the most appropriate imaging probes for acquiring dual-modal 3D photoacoustic (PA)/ultrasound (US) images. However, they have small footprints which limit the field-of-view (FOV) to less than 10 mm × 10 mm and degrade the spatial resolution. In this study, we demonstrate a dual-modal PA and US imaging system (using a 2D matrix transducer array and a motorized 2D scanning system) to enlarge the FOV of volumetric images. Multiple PA volumes were merged to form a wide-field image of approximately 45 mm × 45 mm. In vivo imaging was demonstrated using rat sentinel lymph nodes (SLNs) and bladders stained with methylene blue. We believe that this volumetric PA/US imaging technique with a 2D matrix transducer array can be a useful tool for narrow-field real-time monitoring and wide-field imaging of various preclinical and clinical studies.

Investigation of silk as a phantom material for ultrasound and photoacoustic imaging

Comprehensive characterization of biomedical imaging systems require phantoms that are easy to fabricate and can mimic human tissue. Additionally, with the arrival of engineered tissues, it is key to develop phantoms that can mimic bioengineered samples. In ultrasound and photoacoustic imaging, water-soluble phantom materials such as gelatin undergo rapid degradation while polymer-based materials such as polyvinyl alcohol are not conducive for generating bioengineered tissues that can incorporate cells. Here we propose silk protein-based hydrogels as an ultrasound and photoacoustic phantom material that has potential to provide a 3D environment for long-term sustainable cell growth. Common acoustic, optical, and biomechanical properties such as ultrasound attenuation, reduced scattering coefficient, and Young's modulus were measured. The results indicate that silk acoustically mimics many tissue types while exhibiting similar reduced optical scattering in the wavelength range of 400-1200 nm. Furthermore, silk-based materials can be stored long-term with no change in acoustic and optical properties, and hence can be utilized to assess the performance of ultrasound and photoacoustic systems.

Photoacoustic imaging phantoms for assessment of object detectability and boundary buildup artifacts

Standardized phantoms and test methods are needed to accelerate clinical translation of emerging photoacoustic imaging (PAI) devices. Evaluating object detectability in PAI is challenging due to variations in target morphology and artifacts including boundary buildup. Here we introduce breast fat and parenchyma tissue-mimicking materials based on emulsions of silicone oil and ethylene glycol in polyacrylamide hydrogel. 3D-printed molds were used to fabricate solid target inclusions that produced more filled-in appearance than traditional PAI phantoms. Phantoms were used to assess understudied image quality characteristics (IQCs) of three PAI systems. Object detectability was characterized vs. target diameter, absorption coefficient, and depth. Boundary buildup was quantified by target core/boundary ratio, which was higher in transducers with lower center frequency. Target diameter measurement accuracy was also size-dependent and improved with increasing transducer frequency. These phantoms enable evaluation of multiple key IQCs and may support development of comprehensive standardized test methods for PAI devices.

In Vitro Monitoring of Magnesium-Based Implants Degradation by Surface Analysis and Optical Spectroscopy

Magnesium (Mg)-based degradable alloys have attracted substantial attention for tissue engineering applications due to their biodegradability and potential for avoiding secondary removal surgeries. However, insufficient data in the existing literature regarding Mg’s corrosion and gas formation after implantation have delayed its wide clinical application. Since the surface properties of degradable materials constantly change after contact with body fluid, monitoring the behaviour of Mg in phantoms or buffer solutions could provide some information about its physicochemical surface changes over time. Through surface analysis and spectroscopic analysis, we aimed to investigate the structural and functional properties of degradable disks. Since bubble formation may lead to inflammation and change pH, monitoring components related to acidosis near the cells is essential. To study the bubble formation in cell culture media, we used a newly developed Mg alloy (based on Mg, zinc, and calcium), pure Mg, and commercially available grade 2 Titanium (Ti) disks in Dulbecco’s Modified Eagle Medium (DMEM) solution to observe their behaviour over ten days of immersion. Using surface analysis and the information from near-infrared spectroscopy (NIRS), we concluded on the conditions associated with the medical risks of Mg alloy disintegration. NIRS is used to investigate the degradation behaviour of Mg-based disks in the cell culture media, which is correlated with the surface analysis where possible.

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

A lack of accepted standards and standardized phantoms suitable for the technical validation of biophotonic instrumentation hinders the reliability and reproducibility of its experimental outputs. In this Perspective, we discuss general criteria for the design of tissue-mimicking biophotonic phantoms, and use these criteria and state-of-the-art developments to critically review the literature on phantom materials and on the fabrication of phantoms. By focusing on representative examples of standardization in diffuse optical imaging and spectroscopy, fluorescence-guided surgery and photoacoustic imaging, we identify unmet needs in the development of phantoms and a set of criteria (leveraging characterization, collaboration, communication and commitment) for the standardization of biophotonic instrumentation.

Accuracy of peak-power compensation in fiber-guided and free-space acoustic-resolution photoacoustic microscopy

Acoustic resolution photoacoustic microscopy (AR-PAM) has gained much attention in the past two decades due to its high contrast, scalable resolution, and relatively higher imaging depth. Multimode optical fibers (MMF) are extensively used to transfer light to AR-PAM imaging scan-head from the laser source. Typically, peak-power-compensation (PPC) is used to reduce the effect of pulse-to-pulse peak-power variation in generated photoacoustic (PA) signals. In MMF, the output intensity profile fluctuates due to the coherent nature of light and mode exchange caused by variations in the bending of the fibers during scanning. Therefore, using a photodiode (PD) to capture a portion of the total power of pulses as a measure of illuminated light on the sample may not be appropriate for accurate PPC. In this study, we have investigated the accuracy of PPC in fiber-guided and free-space AR-PAM systems. Experiments were conducted in the transparent and highly scattering medium. Based on obtained results for the MMF-based system, to apply PPC to the generated PA signals, tightly focused light confocal with the acoustic focus in a transparent medium must be used. In the clear medium and highly focused illumination, enhancement of about 45% was obtained in the homogeneity of an optically homogeneous sample image. In addition, it is shown that, as an alternative, free-space propagation of the laser pulses results in more accurate PPC in both transparent and highly scattering mediums. In free-space light transmission, enhancement of 25-75% was obtained in the homogeneity of the optically homogeneous sample image.

A Copolymer-in-Oil Tissue-Mimicking Material With Tuneable Acoustic and Optical Characteristics for Photoacoustic Imaging Phantoms

Photoacoustic imaging (PAI) standardisation demands a stable, highly reproducible physical phantom to enable routine quality control and robust performance evaluation. To address this need, we have optimised a low-cost copolymer-in-oil tissue-mimickingmaterial formulation. The base material consists of mineral oil, copolymer and stabiliser with defined Chemical Abstract Service numbers. Speed of sound c(f) and acoustic attenuation coefficient α (f) were characterised over 2-10 MHz; optical absorption μa ( λ ) and reduced scattering μs '( λ ) coefficients over 450-900 nm. Acoustic properties were optimised by modifying base component ratios and optical properties were adjusted using additives. The temporal, thermomechanical and photo-stabilitywere studied, alongwith intra-laboratory fabrication and field-testing. c(f) could be tuned up to (1516±0.6) [Formula: see text] and α (f) to (17.4±0.3)dB · cm -1 at 5 MHz. The base material exhibited negligible μa ( λ ) and μs '( λ ), which could be independently tuned by addition of Nigrosin or TiO2 respectively. These properties were stable over almost a year and were minimally affected by recasting. The material showed high intra-laboratory reproducibility (coefficient of variation <4% for c ( f ), α ( f ), optical transmittance and reflectance), and good photo- and mechanical-stability in the relevant working range (20-40°C). The optimised copolymer-in-oil material represents an excellent candidate for widespread application in PAI phantoms, with properties suitable for broader use in biophotonics and ultrasound imaging standardisation efforts.

In Situ Visualization for 3D Ultrasound-Guided Interventions with Augmented Reality Headset

Augmented Reality (AR) headsets have become the most ergonomic and efficient visualization devices to support complex manual tasks performed under direct vision. Their ability to provide hands-free interaction with the augmented scene makes them perfect for manual procedures such as surgery. This study demonstrates the reliability of an AR head-mounted display (HMD), conceived for surgical guidance, in navigating in-depth high-precision manual tasks guided by a 3D ultrasound imaging system. The integration between the AR visualization system and the ultrasound imaging system provides the surgeon with real-time intra-operative information on unexposed soft tissues that are spatially registered with the surrounding anatomic structures. The efficacy of the AR guiding system was quantitatively assessed with an in vitro study simulating a biopsy intervention aimed at determining the level of accuracy achievable. In the experiments, 10 subjects were asked to perform the biopsy on four spherical lesions of decreasing sizes (10, 7, 5, and 3 mm). The experimental results showed that 80% of the subjects were able to successfully perform the biopsy on the 5 mm lesion, with a 2.5 mm system accuracy. The results confirmed that the proposed integrated system can be used for navigation during in-depth high-precision manual tasks.

Glycerol-in-SEBS gel as a material to manufacture stable wall-less vascular phantom for ultrasound and photoacoustic imaging

Styrene-ethylene/butylene-styrene (SEBS) copolymer-in-mineral oil gel is an appropriate tissue-mimicking material to manufacture stable phantoms for ultrasound and photoacoustic imaging. Glycerol dispersion has been proposed to further tune the acoustic properties and to incorporate hydrophilic additives into SEBS gel. However, this type of material has not been investigated to produce wall-less vascular flow phantom for these imaging modalities. In this paper, the development of a wall-less vascular phantom for ultrasound and photoacoustic imaging is reported. Mixtures of glycerol/TiO₂-in-SEBS gel samples were manufactured at different proportions of glycerol (10%, 15%, and 20%) and TiO₂(0% to 0.5%) to characterize their optical and acoustic properties. Optical absorption in the 500-950 nm range was independent of the amount of glycerol and TiO₂, while optical scattering increased linearly with the concentration of TiO₂. Acoustic attenuation and speed of sound were not influenced by the presence of TiO₂. The sample manufactured using weight percentages of 10% SEBS, 15% glycerol, and 0.2% TiO₂was selected to make the vascular phantom. The phantom proved to be stable during the pulsatile blood-mimicking fluid (BMF) flow, without any observed damage to its structure or leaks. Ultrasound color Doppler images showed a typical laminar flow, while the B-mode images showed a homogeneous speckled pattern due to the presence of the glycerol droplets in the gel. The photoacoustic images of the phantom showed a well-defined signal coming from the surface of the phantom and from the vessels where BMF was flowing. The Spearman's correlations between the photoacoustic and tabulated spectra calculated from the regions containing BMF, in this case a mixture of salt solutions (NiCl₂and CuSO₄), were higher than 0.95. Our results demonstrated that glycerol-in-SEBS gel was an adequate material to make a stable vascular flow phantom for ultrasound photoacoustic imaging.

Model-based optical and acoustical compensation for photoacoustic tomography of heterogeneous mediums

Photoacoustic tomography (PAT) is a non-invasive, high-resolution imaging modality, capable of providing functional and molecular information of various pathologies, such as cancer. One limitation of PAT is the depth and wavelength dependent optical fluence, which results in reduced PA signal amplitude from deeper tissue regions. These factors can therefore introduce errors into quantitative measurements such as oxygen saturation (sO₂) or the localization and concentration of various chromophores. The variation in the speed-of-sound between different tissues can also lead to distortions in object location and shape. Compensating for these effects allows PAT to be used more quantitatively. We have developed a proof-of-concept algorithm capable of compensating for the heterogeneity in speed-of-sound and depth dependent optical fluence. Speed-of-sound correction was done by using a straight ray-based algorithm for calculating the family of iso-time-of-flight contours between the transducers and every pixel in the imaging grid, while fluence compensation was done by utilizing the graphics processing unit (GPU) accelerated software MCXCL for Monte Carlo modeling of optical fluence variation. This algorithm was tested on a polyvinyl chloride plastisol (PVCP) phantom, which contained cyst mimics and blood inclusions to test the algorithm under relatively heterogeneous conditions. Our results indicate that our PAT algorithm can compensate for the speed-of-sound variation and depth dependent fluence effects within a heterogeneous phantom. The results of this study will pave the way for further development and evaluation of the proposed method in more complex in-vitro and ex-vivo phantoms, as well as compensating for the wavelength-dependent optical fluence in spectroscopic PAT.

A novel matrix-array-based MR-conditional ultrasound system for local hyperthermia of small animals

OBJECTIVE

The goal of this work was to develop a novel modular focused ultrasound hyperthermia (FUS-HT) system for preclinical applications with the following characteristics: MR-compatible, compact probe for integration into a PET/MR small animal scanner, 3D-beam steering capabilities, high resolution focusing for generation of spatially confined FUS-HT effects.

METHODS

For 3D-beam steering capabilities, a matrix array approach with 11 11 elements was chosen. For reaching the required level of integration, the array was mounted with a conductive backing directly on the interconnection PCB. The array is driven by a modified version of our 128 channel ultrasound research platform DiPhAS. The system was characterized using sound field measurements and validated using tissue-mimicking phantoms. Preliminary MR-compatibility tests were performed using a 7T Bruker MRI scanner.

RESULTS

Four 11 11 arrays between 0.5 and 2 MHz were developed and characterized with respect to sound field properties and HT generation. Focus sizes between 1 and 4 mm were reached depending on depth and frequency. We showed heating by 4C within 60 s in phantoms. The integration concept allows a probe thickness of less than 12 mm.

CONCLUSION

We demonstrated FUS-HT capabilities of our modular system based on matrix arrays and a 128 channel electronics system within a 3D-steering range of up to 30. The suitability for integration into a small animal MR could be demonstrated in basic MR-compatibility tests.

SIGNIFICANCE

The developed system presents a new generation of FUS-HT for preclinical and translational work providing safe, reversible, localized, and controlled HT.

Feasibility of Reference Material Certification for Speed of Sound and Attenuation Coefficient Based on Standard Tissue-Mimicking Material

Speed of sound and attenuation are essential for characterizing reference materials such as biological tissue-mimicking materials (TMMs) used in ultrasonic applications. There are many publications on the manufacture of TMMs and the measurement of their properties. However, no studies in the literature have applied the metrological approach of International Organization for Standardization (ISO) Guide 35 to certify biological ultrasound TMMs as candidates for reference materials (RMs). The work described here was aimed at studying the process for manufacturing fat, muscle and aorta artery TMMs, including the study of the homogeneity, stability, trend and characterization of TMMs. The properties of interest were the speed of sound (SoS) and attenuation coefficient (AttC) at 7.5 MHz, with target expanded uncertainty of 40 m/s and 0.3 dB/cm, respectively. The short-term stability study was 2 mo at 4°C and 40°C (simulating possible transportation conditions). The long-term stability study lasted an additional 4 mo with the TMM at 22°C (simulating possible storage conditions). Homogeneity was evaluated before the stability study. Uncertainties associated with homogeneity, stability, characterization and trend were duly calculated. No trend was observed in this study, but the AttC spread widely during the stability test, substantially enlarging the final uncertainty. Therefore, this property could not be used to certify TMM candidates as RMs. However, the SoSs for most TMMs lay within the target uncertainty, disclosing viability to certify TMMs as RMs for this property. Assigned values for SoS were 1560 m/s for aorta TMM with an average expanded uncertainty for certificate validity of 12 mo (U_e;12=20 m/s), 1552 m/s for muscle TMM (U_e;12=20 m/s) and 1494 m/s for fat TMM (U_e;12=11 m/s). Thus, TMMs were proved suitable to be certified as RMs for SoS.

Polyacrylamide hydrogel phantoms for performance evaluation of multispectral photoacoustic imaging systems

As photoacoustic imaging (PAI) begins to mature and undergo clinical translation, there is a need for well-validated, standardized performance test methods to support device development, quality control, and regulatory evaluation. Despite recent progress, current PAI phantoms may not adequately replicate tissue light and sound transport over the full range of optical wavelengths and acoustic frequencies employed by reported PAI devices. Here we introduce polyacrylamide (PAA) hydrogel as a candidate material for fabricating stable phantoms with well-characterized optical and acoustic properties that are biologically relevant over a broad range of system design parameters. We evaluated suitability of PAA phantoms for conducting image quality assessment of three PAI systems with substantially different operating parameters including two commercial systems and a custom system. Imaging results indicated that appropriately tuned PAA phantoms are useful tools for assessing and comparing PAI system image quality. These phantoms may also facilitate future standardization of performance test methodology.

Development and Testing of an Ultrasound-Compatible Cardiac Phantom for Interventional Procedure Simulation Using Direct Three-Dimensional Printing

Organ phantoms are widely used for evaluating medical technologies, training clinical practitioners, as well as surgical planning. In the context of cardiovascular disease, a patient-specific cardiac phantom can play an important role for interventional cardiology procedures. However, phantoms with complicated structures are difficult to fabricate by conventional manufacturing methods. The emergence of three-dimensional (3D) printing with soft materials provides the opportunity to produce phantoms with complex geometries and realistic properties. In this work, the aim was to explore the use of a direct 3D printing technique to produce multimodal imaging cardiac phantoms and to test the physical properties of the new materials used, namely the Poro-Lay series and TangoPlus. The cardiac phantoms were first modeled using real data segmented from a patient chest computer tomography (CT) scan and then printed with the novel materials. They were then tested quantitatively in terms of stiffness and ultrasound (US) acoustic values and qualitatively with US, CT, and magnetic resonance imaging systems. From the stiffness measurements, Lay-fomm 40 had the closest Young's modulus to real myocardium with an error of 29-54%, while TangoPlus had the largest difference. From the US acoustics measurements, Lay-fomm 40 also demonstrated the closest soft tissue-mimicking properties with both the smallest attenuation and impedance differences. Furthermore, the imaging results show that the phantoms are compatible with multiple imaging modalities and thus have potential to be used for interventional procedure simulation and testing of novel interventional devices. In conclusion, direct 3D printing with Poro-Lay and TangoPlus is a promising method for manufacture of multimodal imaging phantoms with complicated structures, especially for soft patient-specific phantoms.

Development of oral cancer tissue-mimicking phantom based on polyvinyl chloride plastisol and graphite for terahertz frequencies

SIGNIFICANCE

A new concept of a biotissue phantom for terahertz (THz) biomedical applications is needed for reliable and long-term usage.

AIM

We aimed to develop a new type of biotissue phantom without water content and with controllable THz optical properties by applying graphite powders into a polyvinyl chloride plastisol (PVCP) matrix and to give a numerical description to the THz optical properties of the phantoms using the Bruggeman model (BM) of the effective medium theory (EMT).

APPROACH

The THz optical properties of graphite and the PVCP matrix were measured using THz time-domain spectroscopy, which works in the frequency range from 0.1 to 1 THz. Two phantoms with 10% and 12.5% graphite were fabricated to evaluate the feasibility of describing phantoms using the EMT. The EMT then was used to determine the concentration of graphite required to mimic the THz optical properties of human cancerous and healthy oral tissue.

RESULTS

The phantom with 16.7% of graphite has the similar THz optical properties as human cancerous oral tissue in the frequency range of 0.2 to 0.7 THz. The THz optical properties of the phantom with 21.9% of graphite are close to those of human healthy oral tissue in the bandwidth from 0.6 to 0.8 THz. Both the refractive index and absorption coefficient of the samples increase with an increase of graphite concentration. The BM of the EMT was used as the numerical model to describe the THz optical properties of the phantoms. The relative error of the BM for the refractive index estimation and the absorption coefficient is up to 4% and 8%, respectively.

CONCLUSIONS

A water-free biotissue phantom that mimics the THz optical properties of human cancerous oral tissue was developed. With 21.9% of graphite, the phantom also mimics human healthy oral tissue in a narrow frequency range. The BM proved to be a suitable numerical model of the phantom.

Cardiac Tissue-Mimicking Ballistic Gel Phantom for Ultrasound Imaging in Clinical and Research Applications

Ballistic gel was investigated as a tissue-mimicking material in an anthropomorphic cardiac phantom for ultrasound imaging. The gel was tested for its acoustic properties and its compatibility with conventional plastics molding techniques. Speed of sound and attenuation were evaluated in the range 2-12 MHz. The speed of sound was 1537 ± 39 m/s, close to typical values for cardiac tissue (∼1576 m/s). The attenuation coefficient was 1.07 dB/cm·MHz, within the range of values previously reported for cardiac tissue (0.81-1.81 dB/cm·MHz). A cardiac model based on human anatomy was developed using established image segmentation processes and conventional plastic molding techniques. Key anatomic features were observed, captured and identified in the model using an intracardiac ultrasound imaging system. These favorable results along with the material's durability and processes that allow for repetitive production of detailed whole-heart models at low cost are promising. There are numerous applications for geometrically complex phantoms in research, training, device development and clinical use.

The Effect of Curing Temperature and Time on the Acoustic and Optical Properties of PVCP

Polyvinyl chloride plastisol (PVCP) has been increasingly used as a phantom material for photoacoustic and ultrasound imaging. As one of the most useful polymeric materials for industrial applications, its mechanical properties and behavior are well-known. Although the acoustic and optical properties of several formulations have previously been investigated, it is still unknown how these are affected by varying the fabrication method. Here, an improved and straightforward fabrication method is presented, and the effect of curing temperature and curing time on the PVCP acoustic and optical properties, as well as their stability over time, is investigated. The speed of sound and attenuation were determined over a frequency range from 2 to 15 MHz, while the optical attenuation spectra of samples were measured over a wavelength range from 500 to 2200 nm. The results indicate that the optimum properties are achieved at curing temperatures between 160 °C and 180 °C, while the required curing time decreases with increasing temperature. The properties of the fabricated phantoms were highly repeatable, meaning that the phantoms are not sensitive to the manufacturing conditions provided that the curing temperature and time are within the range of complete gelation-fusion (samples are optically clear) and below the limit of thermal degradation (indicated by the yellowish appearance of the sample). The samples' long-term stability was assessed over 16 weeks, and no significant change was observed in the measured acoustic and optical properties.

Opto-acoustic imaging of relative blood oxygen saturation and total hemoglobin for breast cancer diagnosis

Opto-acoustic imaging involves using light to produce sound waves for visualizing blood in biological tissue. By using multiple optical wavelengths, diagnostic images of blood oxygen saturation and total hemoglobin are generated using endogenous optical contrast, without injection of any external contrast agent and without using any ionizing radiation. The technology has been used in recent clinical studies for diagnosis of breast cancer to help distinguish benign from malignant lesions, potentially reducing the need for biopsy through improved diagnostic imaging accuracy. To enable this application, techniques for mapping oxygen saturation differences within tissue are necessary. Using biologically relevant opto-acoustic phantoms, we analyze the ability of an opto-acoustic imaging system to display colorized parametric maps that are generated using a statistical mapping approach. To mimic breast tissue, a material with closely matching properties for optical absorption, optical scattering, acoustic attenuation, and speed of sound is used. The phantoms include two vessels filled with whole blood at oxygen saturation levels determined using a sensor-based approach. A flow system with gas-mixer and membrane oxygenator adjusts the oxygen saturation of each vessel independently. Datasets are collected with an investigational Imagio^® breast imaging system. We examine the ability to distinguish vessels as the oxygen saturation level and imaging depth are varied. At depth of 15 mm and hematocrit of 42%, a sufficient level of contrast to distinguish between two 1.6-mm diameter vessels was measured for an oxygen saturation difference of ∼4.6  %  . In addition, an oxygenated vessel was visible at a depth of 48 mm using an optical wavelength of 1064 nm, and a deoxygenated vessel was visible to a depth of 42 mm with 757 nm. The results provide insight toward using color mapped opto-acoustic images for diagnosing breast cancer.

Semi-anthropomorphic photoacoustic breast phantom

Imaging parameters of photoacoustic breast imaging systems such as the spatial resolution and imaging depth are often characterized with phantoms. These objects usually contain simple structures in homogeneous media such as absorbing wires or spherical objects in scattering gels. While these kinds of basic phantoms are uncluttered and useful, they do not challenge the system as much as a breast does, and can thereby overestimate the system's performance. The female breast is a complex collection of tissue types, and the acoustic and optical attenuation of these tissues limit the imaging depth, the resolution and the ability to extract quantitative information. For testing and challenging photoacoustic breast imaging systems to the full extent before moving to in vivo studies, a complex breast phantom which simulates the breast's most prevalent tissues is required. In this work we present the first three dimensional multi-layered semi-anthropomorphic photoacoustic breast phantom. The phantom aims to simulate skin, fat, fibroglandular tissue and blood vessels. The latter three are made from custom polyvinyl chloride plastisol (PVCP) formulations and are appropriately doped with additives to obtain tissue realistic acoustic and optical properties. Two tumors are embedded, which are modeled as clusters of small blood vessels. The PVCP materials are surrounded by a silicon layer mimicking the skin. The tissue mimicking materials were cast into the shapes and sizes expected in the breast using 3D-printed moulds developed from a magnetic resonance imaging segmented numerical breast model. The various structures and layers were assembled to obtain a realistic breast morphology. We demonstrate the phantom's appearance in both ultrasound imaging as photoacoustic tomography and make a comparison with a photoacoustic image of a real breast. A good correspondence is observed, which confirms the phantom's usefulness.

Hybrid organosilicon/polyol phantom for photoacoustic imaging

The rapid development of hardware and software for photoacoustic technologies is urging the establishment of dedicated tools for standardization and performance assessment. In particular, the fabrication of anatomical phantoms for photoacoustic imaging remains an open question, as current solutions have not yet gained unanimous support. Here, we propose that a hybrid material made of a water-in-oil emulsion of glycerol and polydimethylsiloxane may represent a versatile platform to host a broad taxonomy of hydrophobic and hydrophilic dyes and recapitulate the optical and acoustic features of bio tissue. For a full optical parameterization, we refer to Wróbel, et al. [ Biomed. Opt. Express7, 2088 (2016)], where this material was first presented for optical imaging. Instead, here, we complete the picture and find that its speed of sound and acoustic attenuation resemble those of pure polydimethylsiloxane, i.e. respectively 1150 ± 30 m/s and 3.5 ± 0.4 dB/(MHz·cm). We demonstrate its use under a commercial B-mode scanner and a home-made A-mode stage for photoacoustic analysis to retrieve the ground-truth encoded in a multilayer architecture containing indocyanine green, plasmonic particles and red blood cells. Finally, we verify the stability of its acoustic, optical and geometric features over a time span of three months.

Characterizing mechanical and medical imaging properties of polyvinyl chloride-based tissue-mimicking materials

Polyvinyl chloride (PVC) is a commonly used tissue‐mimicking material (TMM) for phantom construction using 3D printing technology. PVC‐based TMMs consist of a mixture of PVC powder and dioctyl terephthalate as a softener. In order to allow the clinical use of a PVC‐based phantom use across CT and magnetic resonance imaging (MRI) imaging platforms, we evaluated the mechanical and physical imaging characteristics of ten PVC samples. The samples were made with different PVC‐softener ratios to optimize phantom bioequivalence with physiologic human tissue. Phantom imaging characteristics, including computed tomography (CT) number, MRI relaxation time, and mechanical properties (e.g., Poisson’s ratio and elastic modulus) were quantified. CT number varied over a range of approximately −10 to 110 HU. The relaxation times of the T1‐weighted and T2‐weighted images were 206.81 ± 17.50 and 20.22 ± 5.74 ms, respectively. Tensile testing was performed to evaluate mechanical properties on the three PVC samples that were closest to human tissue. The elastic moduli for these samples ranged 7.000–12.376 MPa, and Poisson’s ratios were 0.604–0.644. After physical and imaging characterization of the various PVC‐based phantoms, we successfully produced a bioequivalent phantom compatible with multimodal imaging platforms for machine calibration and image optimization/benchmarking. By combining PVC with 3D printing technologies, it is possible to construct imaging phantoms simulating human anatomies with tissue equivalency.

The Impact of Dust and Confinement on Fragmentation of Kidney Stones by Shockwave Lithotripsy in Tissue Phantoms

Objective: The goal was to test whether stone composition and kidney phantom configuration affected comminution in extracorporeal shockwave lithotripsy (SWL) laboratory tests. Confinement may enhance the accumulation of dust and associated cavitation bubbles in the fluid surrounding the stone. It is known that high shockwave delivery rates in SWL are less effective because bubbles generated by one shockwave do not have sufficient time to dissolve, thereby shielding the next shockwave. Materials and Methods: Experiments were conducted with a lithotripter coupled to a water bath. The rate of comminution was measured by weighing fragments over 2 mm at 5-minute time points. First, plaster and crystal stones were broken in four phantoms: a nylon wire mesh, an open polyvinyl chloride (PVC) cup, a closed PVC cup, and an anatomical kidney model-the phantoms have decreasing fluid volumes around the stone. Second, the fluid volume in the kidney model was flushed with water at different rates (0, 7, and 86 mL/min) to remove dust. Results: The efficiency of breakage of stones decreases for the dust emitting plaster stones (percentage of breakage in 5 minutes decreased from 92% ± 2% [n = 3] in wire mesh to 19% ± 3% [n = 3] in model calix) with increasing confinement, but not for the calcite crystal stones that produced little dust (percentage of breakage changed from 87% ± 3% [n = 3] in wire mesh to 81% ± 3% [n = 3] in kidney model). Flushing the kidney phantom at the fastest rate improved comminution of smaller plaster stones by 27%. Conclusions: Phantoms restricting dispersion of dust were found to affect stone breakage in SWL and in vitro experiments should replicate kidney environments. The dust around the stone and potential cavitation may shield the stone from shockwaves and reduce efficacy of SWL. Understanding of stone composition and degree of hydronephrosis could be used to adapt patient-specific protocols.

3D-printed breast phantom for multi-purpose and multi-modality imaging

Background

Breast imaging technology plays an important role in breast cancer planning and treatment. Recently, three-dimensional (3D) printing technology has become a trending issue in phantom constructions for medical applications, with its advantages of being customizable and cost-efficient. However, there is no current practice in the field of multi-purpose breast phantom for quality control (QC) in multi-modalities imaging. The purpose of this study was to fabricate a multi-purpose breast phantom with tissue-equivalent materials via a 3D printing technique for QC in multi-modalities imaging.

Methods

We used polyvinyl chloride (PVC) based materials and a 3D printing technique to construct a breast phantom. The phantom incorporates structures imaged in the female breast such as microcalcifications, fiber lesions, and tumors with different sizes. Moreover, the phantom was used to assess the sensitivity of lesion detection, depth resolution, and detectability thresholds with different imaging modalities. Phantom tissue equivalent properties were determined using computed tomography (CT) attenuation [Hounsfield unit (HU)] and magnetic resonance imaging (MRI) relaxation times.

Results

The 3D-printed breast phantom had an average background value of 36.2 HU, which is close to that of glandular breast tissue (40 HU). T1 and T2 relaxation times had an average relaxation time of 206.81±17.50 and 20.22±5.74 ms, respectively. Mammographic imaging had improved detection of microcalcification compared with ultrasound and MRI with multiple sequences [T1WI, T2WI and short inversion time inversion recovery (STIR)]. Soft-tissue lesion detection and cylindrical tumor contrast were superior with mammography and MRI compared to ultrasound. Hemispherical tumor detection was similar regardless of the imaging modality used.

Conclusions

We developed a multi-purpose breast phantom using a 3D printing technique and determined its value for multi-modal breast imaging studies.

Clinical optoacoustic imaging combined with ultrasound for coregistered functional and anatomical mapping of breast tumors

Optoacoustic imaging, based on the differences in optical contrast of blood hemoglobin and oxyhemoglobin, is uniquely suited for the detection of breast vasculature and tumor microvasculature with the inherent capability to differentiate hypoxic from the normally oxygenated tissue. We describe technological details of the clinical ultrasound (US) system with optoacoustic (OA) imaging capabilities developed specifically for diagnostic imaging of breast cancer. The combined OA/US system provides co-registered and fused images of breast morphology based upon gray scale US with the functional parameters of total hemoglobin and blood oxygen saturation in the tumor angiogenesis related microvasculature based upon OA images. The system component that enabled clinical utility of functional OA imaging is the hand-held probe that utilizes a linear array of ultrasonic transducers sensitive within an ultrawide-band of acoustic frequencies from 0.1 MHz to 12 MHz when loaded to the high-impedance input of the low-noise analog preamplifier. The fiberoptic light delivery system integrated into a dual modality probe through a patented design allowed acquisition of OA images while minimizing typical artefacts associated with pulsed laser illumination of skin and the probe components in the US detection path. We report technical advances of the OA/US imaging system that enabled its demonstrated clinical viability. The prototype system performance was validated in well-defined tissue phantoms. Then a commercial prototype system named Imagio™ was produced and tested in a multicenter clinical trial termed PIONEER. We present examples of clinical images which demonstrate that the spatio-temporal co-registration of functional and anatomical images permit radiological assessment of the vascular pattern around tumors, microvascular density of tumors as well as the relative values of the total hemoglobin [tHb] and blood oxygen saturation [sO2] in tumors relative to adjacent normal breast tissues. The co-registration technology enables increased accuracy of radiologist assessment of malignancy by confirming, upgrading and/or downgrading US categorization of breast tumors according to Breast Imaging Reporting And Data System (BI-RADS). Microscopic histologic examinations on the biopsied tissue of the imaged tumors served as a gold standard in verifying the functional and anatomic interpretations of the OA/US image feature analysis.

PVCP-based anthropomorphic breast phantoms containing structures similar to lactiferous ducts for ultrasound imaging: A comparison with human breasts

The purpose of this work was to obtain an anthropomorphic phantom with acoustic properties similar to those of breast tissue, possessing lactiferous duct-like structures, which would be a first for this type of phantom. Breast lesions usually grow in glandular tissues or lactiferous ducts. Shape variations in these structures are detectable by using ultrasound imaging. To increase early diagnosis, it is important to develop computer-aided diagnosis (CAD) systems and improve medical training. Using tissue-like materials that mimic known internal structures can help achieve both of these goals. However, most breast ultrasound phantoms described in the literature emulate only fat tissues and lesion-like masses. In addition, commercially available phantoms claim to be realistic, but do not contain lactiferous duct structures. In this work, we collected reference images from both breasts of ten healthy female volunteers aged between 20 and 30 years using a 10 MHz linear transducer of a B-mode medical ultrasound system. Histograms of the grey scale distribution of each tissue component of interest, the grey level means, and standard deviations of the regions of interest were obtained. Phantoms were produced using polyvinyl chloride plastisol (PVCP) suspensions. The lactiferous duct-like structures were prepared using pure PVCP. Solid scatterers, such as alumina (mesh #100) and graphite powders (mesh #140) were added to the phantom matrix to mimic glandular and fat tissue, respectively. The phantom duct-like structure diameters observed on B-mode images (1.92 mm ± 0.44) were similar to real measures obtained with a micrometer (2.08 mm ± 0.23). The phantom ducts are easy to produce and are largely stable for at least one year. This phantom allows the researchers to elaborate the structure at their will and may be used in training and as a reference for development of CAD systems.

Dynamic blood flow phantom with negative and positive photoacoustic contrasts

In vivo photoacoustic (PA) flow cytometry (PAFC) has great clinical potential for early, noninvasive diagnosis of cancer, infections (e.g., malaria and bacteremia), sickle anemia, and cardiovascular disorders, including stroke prevention through detection of circulating white clots with negative PA contrast. For clinical applications, this diagnostic platform still requires optimization and calibration. We have already demonstrated that this need can be partially addressed by in vivo examination of large mouse blood vessels, which are similar to human vessels used. Here, we present an alternative method for PAFC optimization that utilizes novel, clinically relevant phantoms resembling pigmented skin, tissue, vessels, and flowing blood. This phantom consists of a scattering-absorbing medium with a melanin layer and plastic tube with flowing beads to model light-absorbing red blood cells (RBCs) and circulating tumor cells (CTCs), as well as transparent beads to model white blood cells and clots. Using a laser diode, we demonstrated the extraordinary ability of PAFC to dynamically detect fast-moving mimic CTCs with positive PA contrast and white clots with negative PA contrast in an RBC background. Time-resolved detection of the delayed PA signals from blood vessels demonstrated complete suppression of the PA background from the modeled pigmented skin. This novel, medically relevant, dynamic blood flow phantom can be used to calibrate and maintain PAFC parameters for routine clinical applications.

Tissue mimicking materials in image-guided needle-based interventions: A review

Image-guided interventions are widely employed in clinical medicine, which brings significant revolution in healthcare in recent years. However, it is impossible for medical trainees to experience the image-guided interventions physically in patients due to the lack of certificated skills. Therefore, training phantoms, which are normally tissue mimicking materials, are widely used in medical research, training, and quality assurance. This review focuses on the tissue mimicking materials used in image-guided needle-based interventions. In this case, we need to investigate the microstructure characteristics and mechanical properties (for needle intervention), optical properties and acoustical properties (for imaging) of these training phantoms to compare with the related properties of human real tissues. The widely used base materials, additives and the corresponding concentrations of the training phantoms are summarized from the literatures in recent ten years. The microstructure characteristics, mechanical behavior, optical properties and acoustical properties of the tissue mimicking materials are investigated, accompanied with the common experimental methods, apparatus and theoretical algorithm. The influence of the concentrations of the base materials and additives on these characteristics are compared and classified. In this review, we assess a comprehensive overview of the existing techniques with the main accomplishments, and limitations as well as recommendations for tissue mimicking materials used in image-guided needle-based interventions.

Combined Burst Wave Lithotripsy and Ultrasonic Propulsion for Improved Urinary Stone Fragmentation

PURPOSE

Burst wave lithotripsy (BWL) is a new technology in development to fragment urinary stones. Ultrasonic propulsion (UP) is a separate technology under investigation for displacing stones. We measure the effect of propulsion pulses on stone fragmentation from BWL.

MATERIALS AND METHODS

Two artificial stone models (crystalline calcite, BegoStone plaster) and human calcium oxalate monohydrate (COM) stones measuring 5 to 8 mm were subjected to ultrasound exposures in a polyvinyl chloride tissue phantom within a water bath. Stones were exposed to BWL with and without propulsion pulses interleaved for set time intervals depending on stone type. Fragmentation was measured as a fraction of the initial stone mass fragmented to pieces smaller than 2 mm.

RESULTS

BegoStone model comminution improved from 6% to 35% (p < 0.001) between BWL and BWL with interleaved propulsion in a 10-minute exposure. Propulsion alone did not fragment stones, whereas addition of propulsion after BWL slightly improved BegoStone model comminution from 6% to 11% (p < 0.001). BegoStone model fragmentation increased with rate of propulsion pulses. Calcite stone fragmentation improved from 24% to 39% in 5 minutes (p = 0.047) and COM stones improved from 17% to 36% (p = 0.01) with interleaved propulsion.

CONCLUSIONS

BWL with UP improved stone fragmentation compared with BWL alone in vitro. The improvement was greatest when propulsion pulses are interleaved with BWL treatment and when propulsion pulses are applied at a higher rate. Thus, UP may be a useful adjunct to enhance fragmentation in lithotripsy in vivo.

Gel wax-based tissue-mimicking phantoms for multispectral photoacoustic imaging

Tissue-mimicking phantoms are widely used for the calibration, evaluation and standardisation of medical imaging systems, and for clinical training. For photoacoustic imaging, tissue-mimicking materials (TMMs) that have tuneable optical and acoustic properties, high stability, and mechanical robustness are highly desired. In this study, gel wax is introduced as a TMM that satisfies these criteria for developing photoacoustic imaging phantoms. The reduced scattering and optical absorption coefficients were independently tuned with the addition of TiO₂ and oil-based inks. The frequency-dependent acoustic attenuation obeyed a power law; for native gel wax, it varied from 0.71 dB/cm at 3 MHz to 9.93 dB/cm at 12 MHz. The chosen oil-based inks, which have different optical absorption spectra in the range of 400 to 900 nm, were found to have good photostability under pulsed illumination with photoacoustic excitation light. Optically heterogeneous phantoms that comprised of inclusions with different concentrations of carbon black and coloured inks were fabricated, and multispectral photoacoustic imaging was performed with an optical parametric oscillator and a planar Fabry-Pérot sensor. We conclude that gel wax is well suited as a TMM for multispectral photoacoustic imaging.

Laser Doppler sensing for blood vessel detection with a biologically inspired steerable needle

Puncturing blood vessels during percutaneous intervention in minimally invasive brain surgery can be a life threatening complication. Embedding a forward looking sensor in a rigid needle has been proposed to tackle this problem but, when using a rigid needle, the procedure needs to be interrupted and the needle extracted if a vessel is detected. As an alternative, we propose a novel optical method to detect a vessel in front of a steerable needle. The needle itself is based on a biomimetic, multi-segment design featuring four hollow working channels. Initially, a laser Doppler flowmetry probe is characterized in a tissue phantom with optical properties mimicking those of human gray matter. Experiments are performed to show that the probe has a 2.1 mm penetration depth and a 1 mm off-axis detection range for a blood vessel phantom with 5 mm s^-1 flow velocity. This outcome demonstrates that the probe fulfills the minimum requirements for it to be used in conjunction with our needle. A pair of Doppler probes is then embedded in two of the four working channels of the needle and vessel reconstruction is performed using successive measurements to determine the depth and the off-axis position of the vessel from each laser Doppler probe. The off-axis position from each Doppler probe is then used to generate a 'detection circle' per probe, and vessel orientation is predicted using tangent lines between the two. The vessel reconstruction has a depth root mean square error (RMSE) of 0.3 mm and an RMSE of 15° in the angular prediction, showing real promise for a future clinical application of this detection system.

Phantom-based image quality test methods for photoacoustic imaging systems

As photoacoustic imaging (PAI) technologies advance and applications arise, there is increasing need for standardized approaches to provide objective, quantitative performance assessment at various stages of the product development and clinical translation process. We have developed a set of performance test methods for PAI systems based on breast-mimicking tissue phantoms containing embedded inclusions. Performance standards for mature imaging modalities [magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound] were used to guide selection of critical PAI image quality characteristics and experimental methods. Specifically, the tests were designed to address axial, lateral, and elevational spatial resolution, signal uniformity, penetration depth, sensitivity, spatial measurement accuracy, and PAI-ultrasound coregistration. As an initial demonstration of the utility of these test methods, we characterized the performance of a modular, bimodal PAI-ultrasound system using four clinical ultrasound transducers with varying design specifications. Results helped to inform optimization of acquisition and data processing procedures while providing quantitative elucidation of transducer-dependent differences in image quality. Comparison of solid, tissue-mimicking polymer phantoms with those based on Intralipid indicated the superiority of the former approach in simulating real-world conditions for PAI. This work provides a critical foundation for the establishment of well-validated test methods that will facilitate the maturation of PAI as a medical imaging technology.

Stable phantom materials for ultrasound and optical imaging

Phantoms mimicking the specific properties of biological tissues are essential to fully characterize medical devices. Water-based materials are commonly used to manufacture phantoms for ultrasound and optical imaging techniques. However, these materials have disadvantages, such as easy degradation and low temporal stability. In this study, we propose an oil-based new tissue-mimicking material for ultrasound and optical imaging, with the advantage of presenting low temporal degradation. A styrene-ethylene/butylene-styrene (SEBS) copolymer in mineral oil samples was made varying the SEBS concentration between 5%-15%, and low-density polyethylene (LDPE) between 0%-9%. Acoustic properties, such as the speed of sound and the attenuation coefficient, were obtained using frequencies ranging from 1-10 MHz, and were consistent with that of soft tissues. These properties were controlled varying SEBS and LDPE concentration. To characterize the optical properties of the samples, the diffuse reflectance and transmittance were measured. Scattering and absorption coefficients ranging from 400 nm-1200 nm were calculated for each compound. SEBS gels are a translucent material presenting low optical absorption and scattering coefficients in the visible region of the spectrum, but the presence of LDPE increased the turbidity. Adding LDPE increased the absorption and scattering of the phantom materials. Ultrasound and photoacoustic images of a heterogeneous phantom made of LDPE/SEBS containing a spherical inclusion were obtained. Annatto dye was added to the inclusion to enhance the optical absorbance. The results suggest that copolymer gels are promising for ultrasound and optical imaging, making them also potentially useful for photoacoustic imaging.

Imaging technique for real-time temperature monitoring during cryotherapy of lesions

Noninvasive real-time temperature imaging during thermal therapies is able to significantly improve clinical outcomes. An optoacoustic (OA) temperature monitoring method is proposed for noninvasive real-time thermometry of vascularized tissue during cryotherapy. The universal temperature-dependent optoacoustic response (ThOR) of red blood cells (RBCs) is employed to convert reconstructed OA images to temperature maps. To obtain the temperature calibration curve for intensity-normalized OA images, we measured ThOR of 10 porcine blood samples in the range of temperatures from 40°C to ?16°C and analyzed the data for single measurement variations. The nonlinearity (?Tmax) and the temperature of zero OA response (T0) of the calibration curve were found equal to 11.4±0.1°C and ?13.8±0.1°C, respectively. The morphology of RBCs was examined before and after the data collection confirming cellular integrity and intracellular compartmentalization of hemoglobin. For temperatures below 0°C, which are of particular interest for cryotherapy, the accuracy of a single temperature measurement was ±1°C, which is consistent with the clinical requirements. Validation of the proposed OA temperature imaging technique was performed for slow and fast cooling of blood samples embedded in tissue-mimicking phantoms.

Biologically relevant photoacoustic imaging phantoms with tunable optical and acoustic properties

Established medical imaging technologies such as magnetic resonance imaging and computed tomography rely on well-validated tissue-simulating phantoms for standardized testing of device image quality. The availability of high-quality phantoms for optical-acoustic diagnostics such as photoacoustic tomography (PAT) will facilitate standardization and clinical translation of these emerging approaches. Materials used in prior PAT phantoms do not provide a suitable combination of long-term stability and realistic acoustic and optical properties. Therefore, we have investigated the use of custom polyvinyl chloride plastisol (PVCP) formulations for imaging phantoms and identified a dual-plasticizer approach that provides biologically relevant ranges of relevant properties. Speed of sound and acoustic attenuation were determined over a frequency range of 4 to 9 MHz and optical absorption and scattering over a wavelength range of 400 to 1100 nm. We present characterization of several PVCP formulations, including one designed to mimic breast tissue. This material is used to construct a phantom comprised of an array of cylindrical, hemoglobin-filled inclusions for evaluation of penetration depth. Measurements with a custom near-infrared PAT imager provide quantitative and qualitative comparisons of phantom and tissue images. Results indicate that our PVCP material is uniquely suitable for PAT system image quality evaluation and may provide a practical tool for device validation and intercomparison.

Polyvinyl chloride plastisol breast phantoms for ultrasound imaging

Ultrasonic phantoms are objects that mimic some features of biological tissues, allowing the study of their interactions with ultrasound (US). In the diagnostic-imaging field, breast phantoms are an important tool for testing performance and optimizing US systems, as well as for training medical professionals. This paper describes the design and manufacture of breast lesions by using polyvinyl chloride plastisol (PVCP) as the base material. Among the materials available for this study, PVCP was shown to be stable, durable, and easy to handle. Furthermore, it is a nontoxic, nonpolluting, and low-cost material. The breast's glandular tissue (image background) was simulated by adding graphite powder with a concentration of 1% to the base material. Mixing PVCP and graphite powder in differing concentrations allows one to simulate lesions with different echogenicity patterns (anechoic, hypoechoic, and hyperechoic). From this mixture, phantom materials were obtained with speed of sound varying from 1379.3 to 1397.9ms(-1) and an attenuation coefficient having values between 0.29 and 0.94dBcm(-1) for a frequency of 1MHz at 24°C. A single layer of carnauba wax was added to the lesion surface in order to evaluate its applicability for imaging. The images of the phantoms were acquired using commercial ultrasound equipment; a specialist rated the images, elaborating diagnoses representative of both benign and malignant lesions. The results indicated that it was possible to easily create a phantom by using low-cost materials, readily available in the market and stable at room temperature, as the basis of ultrasonic phantoms that reproduce the image characteristics of fatty breast tissue and typical lesions of the breast.

Characterisation of a phantom for multiwavelength quantitative photoacoustic imaging

Quantitative photoacoustic imaging (qPAI) has the potential to provide high- resolution in vivo images of chromophore concentration, which may be indicative of tissue function and pathology. Many strategies have been proposed recently for extracting quantitative information, but many have not been experimentally verified. Experimental phantom-based validation studies can be used to test the robustness and accuracy of such algorithms in order to ensure reliable in vivo application is possible. The phantoms used in such studies must have well-characterised optical and acoustic properties similar to tissue, and be versatile and stable. Polyvinyl chloride plastisol (PVCP) has been suggested as a phantom for quality control and system evaluation. By characterising its multiwavelength optical properties, broadband acoustic properties and thermoelastic behaviour, this paper examines its potential as a phantom for qPAI studies too. PVCP's acoustic properties were assessed for various formulations, as well as its intrinsic optical absorption, and scattering with added TiO2, over a range of wavelengths from 400-2000 nm. To change the absorption coefficient, pigment-based chromophores that are stable during the phantom fabrication process, were used. These yielded unique spectra analogous to tissue chromophores and linear with concentration. At the high peak powers typically used in photoacoustic imaging, nonlinear optical absorption was observed. The Grüneisen parameter was measured to be [Formula: see text]  =  1.01  ±  0.05, larger than typically found in tissue, though useful for increased PA signal. Single and multiwavelength 3D PA imaging of various fabricated PVCP phantoms were demonstrated.

Designed Er(3+)-singly doped NaYF4 with double excitation bands for simultaneous deep macroscopic and microscopic upconverting bioimaging

Simultaneous deep macroscopic imaging and microscopic imaging is in urgent demand, but is challenging to achieve experimentally due to the lack of proper fluorescent probes. Herein, we have designed and successfully synthesized simplex Er(3+)-doped upconversion nanoparticles (UCNPs) with double excitation bands for simultaneous deep macroscopic and microscopic imaging. The material structure and the excitation wavelength of Er(3+)-singly doped UCNPs were further optimized to enhance the upconversion emission efficiency. After optimization, we found that NaYF4:30%Er(3+)@NaYF4:2%Er(3+) could simultaneously achieve efficient two-photon excitation (2PE) macroscopic tissue imaging and three-photon excitation (3PE) deep microscopic when excited by 808 nm continuous wave (CW) and 1480 nm CW lasers, respectively. In vitro cell imaging and in vivo imaging have also been implemented to demonstrate the feasibility and potential of the proposed simplex Er(3+)-doped UCNPs as bioprobe.

Construction of 3-Dimensional Printed Ultrasound Phantoms With Wall-less Vessels

Ultrasound phantoms are invaluable as training tools for vascular access procedures. We developed ultrasound phantoms with wall-less vessels using 3-dimensional printed chambers. Agar was used as a soft tissue-mimicking material, and the wall-less vessels were created with rods that were retracted after the agar was set. The chambers had integrated luer connectors to allow for fluid injections with clinical syringes. Several variations on this design are presented, which include branched and stenotic vessels. The results show that 3-dimensional printing can be well suited to the construction of wall-less ultrasound phantoms, with designs that can be readily customized and shared electronically.

Estimation and imaging of breast lesions using a two-layer tissue structure by ultrasound-guided optical tomography

A new two-step estimation and imaging method is developed for a two-layer breast tissue structure consisting of a breast tissue layer and a chest wall underneath. First, a smaller probe with shorter distance source-detector pairs was used to collect the reflected light mainly from the breast tissue layer. Then, a larger probe with 9×14 source-detector pairs and a centrally located ultrasound transducer was used to collect reflected light from the two-layer tissue structure. The data collected from the smaller probe were used to estimate breast tissue optical properties. With more accurate estimation of the average breast tissue properties, the second layer properties can be assessed from data obtained from the larger probe. Using this approach, the unknown variables have been reduced from four to two and the estimated bulk tissue optical properties are more accurate and robust. In addition, a two-step reconstruction using a genetic algorithm and conjugate gradient method is implemented to simultaneously reconstruct the absorption and reduced scattering maps of targets inside a two-layer tissue structure. Simulations and phantom experiments have been performed to validate the new reconstruction method, and a clinical example is given to demonstrate the feasibility of this approach.

Deep, high contrast microscopic cell imaging using three-photon luminescence of β-(NaYF4:Er(3+)/NaYF4) nanoprobe excited by 1480-nm CW laser of only 1.5-mW

It is challenging to achieve deep microscopic imaging for the strong scattering in biotissue. An efficient three-photon luminescence can effectively increase the penetration depth. Here we report that β-NaYF4: Er(3+)/NaYF4 UCNPs were excited by a 1480-nm CW-laser and emitted 543/653-nm light through a three-photon process. With the merit of the hexagonal crystal phase, sub-milliwatt laser power was utilized to excite the UCNP-probed cells to minimize the heating effect. The polymer-coated UCNPs were shown to be harmless to cells. The deep, high contrast in vitro microscopic imaging was implemented through an artificial phantom. Imaging depth of 800 μm was achieved using only 1.5 mW excitation and a 0.7 NA objective. The green/red emission intensities ratio after penetrating the phantom was studied, indicating that longer emission wavelength is preferred for deep multiphoton microscopy. The proposed and demonstrated β-UCNPs would have great potential in three-photon microscopy.

Tissue phantoms to compare spatial and temporal offset modes of deep Raman spectroscopy

Tissue phantoms were created with embedded biomineral-simulating inclusions of varying size and depth, and formed of different mixtures of CaCO₃ and hydroxyapatite, for comparison of deep Raman spectroscopy techniques.