3D-Printed Tissue-Mimicking Phantoms for Medical Imaging and Computational Validation Applications

Metamaterial design for aortic aneurysm simulation using 3D printing

INTRODUCTION

The use of three-dimensional (3D) printed anatomic models is steadily increasing in research and as a tool for clinical decision-making. The mechanical properties of polymers and metamaterials were investigated to evaluate their application in mimicking the biomechanics of the aortic vessel wall.

METHODOLOGY

Uniaxial tensile tests were performed to determine the elastic modulus, mechanical stress, and strain of 3D printed samples. We used a combination of materials, designed to mimic biological tissues' properties, the rigid VeroTM family, and the flexible Agilus30™. Metamaterials were designed by tessellating unit cells that were used as lattice-reinforcement to tune their mechanical properties. The lattice-reinforcements were based on two groups of patterns, mainly responding to the movement between links/threads (chain and knitted) or to deformation (origami and diamond crystal). The mechanical properties of the printed materials were compared with the characteristics of healthy and aneurysmal aortas.

RESULTS

Uniaxial tensile tests showed that the use of a lattice-reinforcement increased rigidity and may increase the maximum stress generated. The pattern and material of the lattice-reinforcement may increase or reduce the strain at maximum stress, which is also affected by the base material used. Printed samples showed max stress ranging from 0.39 ± 0.01 MPa to 0.88 ± 0.02 MPa, and strain at max stress ranging from 70.44 ± 0.86% to 158.21 ± 8.99%. An example of an application was created by inserting a metamaterial designed as a lattice-reinforcement on a model of the aorta to simulate an abdominal aortic aneurysm.

CONCLUSION

The maximum stresses obtained with the printed models were similar to those of aortic tissue reported in the literature, despite the fact that the models did not perfectly reproduce the biological tissue behavior.

Left atrial appendage occlusion: On the need of a numerical model to simulate the implant procedure

Left atrial appendage occlusion (LAAO) is a percutaneous procedure to prevent thromboembolism in patients affected by atrial fibrillation. Despite its demonstrated efficacy, the LAA morphological complexity hinders the procedure, resulting in postprocedural drawbacks (device-related thrombus and peri-device leakage). Local anatomical features may cause difficulties in the device's positioning and affect the effectiveness of the device's implant. The current work proposes a detailed FE model of the LAAO useful to investigate implant scenarios and derive clinical indications. A high-fidelity model of the Watchman FLX device and simplified parametric conduits mimicking the zone of the LAA where the device is deployed were developed. Device-conduit interactions were evaluated by looking at clinical indicators such as device-wall gap, possible cause of leakage, and device protrusion. As expected, the positioning of the crimped device before the deployment was found to significantly affect the implant outcomes: clinician's choices can be improved if FE models are used to optimize the pre-operative planning. Remarkably, also the wall mechanical stiffness plays an important role. However, this parameter value is unknown for a specific LAA, a crucial point that must be correctly defined for developing an accurate FE model. Finally, numerical simulations outlined how the device's configuration on which the clinician relies to assess the implant success (i.e., the deployed configuration with the device still attached to the catheter) may differ from the actual final device's configuration, relevant for achieving a safe intervention.

Fabrication of Soft Transparent Patient-Specific Vascular Models with Stereolithographic 3D printing and Thiol-Based Photopolymerizable Coatings

An ideal vascular phantom should be anatomically accurate, have mechanical properties as close as possible to the tissue, and be sufficiently transparent for ease of visualization. However, materials that enable the convergence of these characteristics have remained elusive. The fabrication of patient-specific vascular phantoms with high anatomical fidelity, optical transparency, and mechanical properties close to those of vascular tissue is reported. These final properties are achieved by 3D printing patient-specific vascular models with commercial elastomeric acrylic-based resins before coating them with thiol-based photopolymerizable resins. Ternary thiol-ene-acrylate chemistry is found optimal. A PETMP/allyl glycerol ether (AGE)/polyethylene glycol diacrylate (PEGDA) coating with a 30/70% AGE/PEGDA ratio applied on a flexible resin yielded elastic modulus, UTS, and elongation of 3.41 MPa, 1.76 MPa, and 63.2%, respectively, in range with the human aortic wall. The PETMP/AGE/PEGDA coating doubled the optical transmission from 40% to 80%, approaching 88% of the benchmark silicone-based elastomer. Higher transparency correlates with a decrease in surface roughness from 2000 to 90 nm after coating. Coated 3D-printed anatomical replicas are showcased for pre-procedural planning and medical training with good radio-opacity and echogenicity. Thiol-click chemistry coatings, as a surface treatment for elastomeric stereolithographic 3D-printed objects, address inherent limitations of photopolymer-based additive manufacturing.

Clinical situations for which 3D Printing is considered an appropriate representation or extension of data contained in a medical imaging examination: vascular conditions

BACKGROUND

Medical three-dimensional (3D) printing has demonstrated utility and value in anatomic models for vascular conditions. A writing group composed of the Radiological Society of North America (RSNA) Special Interest Group on 3D Printing (3DPSIG) provides appropriateness recommendations for vascular 3D printing indications.

METHODS

A structured literature search was conducted to identify all relevant articles using 3D printing technology associated with vascular indications. Each study was vetted by the authors and strength of evidence was assessed according to published appropriateness ratings.

RESULTS

Evidence-based recommendations for when 3D printing is appropriate are provided for the following areas: aneurysm, dissection, extremity vascular disease, other arterial diseases, acute venous thromboembolic disease, venous disorders, lymphedema, congenital vascular malformations, vascular trauma, vascular tumors, visceral vasculature for surgical planning, dialysis access, vascular research/development and modeling, and other vasculopathy. Recommendations are provided in accordance with strength of evidence of publications corresponding to each vascular condition combined with expert opinion from members of the 3DPSIG.

CONCLUSION

This consensus appropriateness ratings document, created by the members of the 3DPSIG, provides an updated reference for clinical standards of 3D printing for the care of patients with vascular conditions.

Individualized Ultrasound-Guided Intervention Phantom Development, Fabrication, and Proof of Concept

Commercial ultrasound vascular phantoms lack the anatomic diversity required for robust pre-clinical interventional device testing. We fabricated individualized phantoms to test an artificial intelligence enabled ultrasound-guided surgical robotic system (AI-GUIDE) which allows novices to cannulate deep vessels. After segmenting vessels on computed tomography scans, vessel cores, bony anatomy, and a mold tailored to the skin contour were 3D-printed. Vessel cores were coated in silicone, surrounded in tissue-mimicking gel tailored for ultrasound and needle insertion, and dissolved with water. One upper arm and four inguinal phantoms were constructed. Operators used AI-GUIDE to deploy needles into phantom vessels. Two groin phantoms were tested due to imaging artifacts in the other two phantoms. Six operators (medical experience: none, 3; 1-5 years, 2; 5+ years, 1) inserted 27 inguinal needles with 81% (22/27) success in a median of 48 seconds. Seven operators performed 24 arm injections, without tuning the AI for arm anatomy, with 71% (17/24) success. After excluding failures due to motor malfunction and a defective needle, success rate was 100% (22/22) in the groin and 85% (17/20) in the arm. Individualized 3D-printed phantoms permit testing of surgical robotics across a large number of operators and different anatomic sites. AI-GUIDE operators rapidly and reliably inserted a needle into target vessels in the upper arm and groin, even without prior medical training. Virtual device trials in individualized 3-D printed phantoms may improve rigor of results and expedite translation.Clinical Relevance- Individualized phantoms enable rigorous and efficient evaluation of interventional devices and reduce the need for animal and human subject testing.

Ultrasonic bandgaps in viscoelastic 1D-periodic media: Mechanical modeling and experimental validation

Multi-material additive manufacturing is receiving increasing attention in the field of acoustics, in particular towards the design of micro-architectured periodic media used to achieve programmable ultrasonic responses. To unravel the effect of the material properties and spatial arrangement of the printed constituents, there is an unmet need in developing wave propagation models for prediction and optimization purposes. In this study, we propose to investigate the transmission of longitudinal ultrasound waves through 1D-periodic biphasic media, whose constituent materials are viscoelastic. To this end, Bloch-Floquet analysis is applied in the frame of viscoelasticity, with the aim of disentangling the relative contributions of viscoelasticity and periodicity on ultrasound signatures, such as dispersion, attenuation, and bandgaps localization. The impact of the finite size nature of these structures is then assessed by using a modeling approach based on the transfer matrix formalism. Finally, the modeling outcomes, i.e., frequency-dependent phase velocity and attenuation, are confronted with experiments conducted on 3D-printed samples, which exhibit a 1D periodicity at length-scales of a few hundreds of micrometers. Altogether, the obtained results shed light on the modeling characteristics to be considered when predicting the complex acoustic behavior of periodic media in the ultrasonic regime.

Use of 3D anatomical models in mock circulatory loops for cardiac medical device testing

INTRODUCTION

Mock circulatory loops (MCLs) are mechanical representations of the cardiovascular system largely used to test the hemodynamic performance of cardiovascular medical devices (MD). Thanks to 3 dimensional (3D) printing technologies, MCLs can nowadays also incorporate anatomical models so to offer enhanced testing capabilities. The aim of this review is to provide an overview on MCLs and to discuss the recent developments of 3D anatomical models for cardiovascular MD testing.

METHODS

The review first analyses the different techniques to develop 3D anatomical models, in both rigid and compliant materials. In the second section, the state of the art of MCLs with 3D models is discussed, along with the testing of different MDs: implantable blood pumps, heart valves, and imaging techniques. For each class of MD, the MCL is analyzed in terms of: the cardiovascular model embedded, the 3D model implemented (the anatomy represented, the material used, and the activation method), and the testing applications.

DISCUSSIONS AND CONCLUSIONS

MCLs serve the purpose of testing cardiovascular MDs in different (patho-)physiological scenarios. The addition of 3D anatomical models enables more realistic connections of the MD with the implantation site and enhances the testing capabilities of the MCL. Current attempts focus on the development of personalized MCLs to test MDs in patient-specific hemodynamic and anatomical scenarios. The main limitation of MCLs is the impossibility to assess the impact of a MD in the long-term and at a biological level, for which animal experiments are still needed.

Fabrication of Compliant and Transparent Hollow Cerebral Vascular Phantoms for In Vitro Studies Using 3D Printing and Spin-Dip Coating

Endovascular surgery through flow diverters and coils is increasingly used for the minimally invasive treatment of intracranial aneurysms. To study the effectiveness of these devices, in vitro tests are performed in which synthetic vascular phantoms are typically used to reproduce in vivo conditions. In this paper, we propose a manufacturing process to obtain compliant and transparent hollow vessel replicas to assess the mechanical behaviour of endovascular devices and perform flow measurements. The vessel models were obtained in three main steps. First, a mould was 3D-printed in a water-soluble material; two techniques, fusion deposition modelling and stereolithography, were compared for this purpose. Then, the mould was covered with a thin layer of silicone through spin-dip coating, and finally, when the silicone layer solidified, it was dissolved in a hot water bath. The final models were tested in terms of the quality of the final results, the mechanical properties of the silicone, thickness uniformity, and transparency properties. The proposed approach makes it possible to produce models of different sizes and complexity whose transparency and mechanical properties are suitable for in vitro experiments. Its applicability is demonstrated through idealised and patient-specific cases.

A Micro-Scale Non-Linear Finite Element Model to Optimize the Mechanical Behavior of Bioprinted Constructs

Extrusion-based bioprinting is an enabling biofabrication technique that is used to create heterogeneous tissue constructs according to patient-specific geometries and compositions. The optimization of bioinks as per requirements for specific tissue applications is an essential exercise in ensuring clinical translation of the bioprinting technologies. Most notably, optimum hydrogel polymer concentrations are required to ensure adequate mechanical properties of bioprinted constructs without causing significant shear stresses on cells. However, experimental iterations are often tedious for optimizing the bioink properties. In this work, a nonlinear finite element modeling approach has been undertaken to determine the effect of different bioink parameters such as composition, concentration on the range of stresses being experienced by the cells in the bioprinted construct. The stress distribution of the cells at different parts of the constructs has also been modeled. It is found that both bioink chemical compositions and concentrations can substantially alter the stress effects experienced by the cells. Concentrated regions of softer cells near pore regions were found to increase stress concentrations by almost three times compared with stress generated in cells away from the pores. The study provides a method for rapid optimization of bioinks, design of bioprinted constructs, as well as toolpath plans for fabricating constructs with homogenous properties.

Ultrasound characterization of the viscoelastic properties of additively manufactured photopolymer materials

Photopolymer-based additive manufacturing has received increasing attention in the field of acoustics over the past decade, specifically towards the design of tissue-mimicking phantoms and passive components for ultrasound imaging and therapy. While these applications rely on an accurate characterization of the longitudinal bulk properties of the materials, emerging applications involving periodic micro-architectured media also require the knowledge of the transverse bulk properties to achieve the desired acoustic behavior. However, a robust knowledge of these properties is still lacking for such attenuating materials. Here, we report on the longitudinal and transverse bulk properties, i.e., frequency-dependent phase velocities and attenuations, of photopolymer materials, which were characterized in the MHz regime using a double through-transmission method in oblique incidence. Samples were fabricated using two different printing technologies (stereolithography and polyjet) to assess the impact of two important factors of the manufacturing process: curing and material mixing. Overall, the experimentally observed dispersion and attenuation could be satisfactorily modeled using a power law attenuation to identify a reduced number of intrinsic ultrasound parameters. As a result, these parameters, and especially those reflecting transverse bulk properties, were shown to be very sensitive to slight variations of the manufacturing process.

Thermochromic Tissue-Mimicking Phantoms for Thermal Ablation Based on Polyacrylamide Gel

In recent years, thermal ablation has played an increasingly important role in treating various tumors in the clinic. A practical thermochromic phantom model can provide a favorable platform for clinical thermotherapy training of young physicians or calibration and optimization of thermal devices without risk to animals or human participants. To date, many tissue-mimicking thermal phantoms have been developed and are well liked, especially the polyacrylamide gel (PAG)-based phantoms. This review summarizes the PAG-based phantoms in the field of thermotherapy, details their advantages and disadvantages and provides a direction for further optimization. The relevant physical parameters (such as electrical, acoustic, and thermal properties) of these phantoms are also presented in this review, which can assist operators in a deeper understanding of these phantoms and selection of the proper recipes for phantom fabrication.

Additively manufactured test phantoms for mimicking soft tissue radiation attenuation in CBCT using Polyjet technology

OBJECTIVES

To develop and validate a simple approach for building cost-effective imaging phantoms for Cone Beam Computed Tomography (CBCT) using a modified Polyjet additive manufacturing technology where a single material can mimic a range of human soft-tissue radiation attenuation.

MATERIALS AND METHODS

Single material test phantoms using a cubic lattice were designed in 3-Matic 15.0 software . Keeping the individual cubic lattice volume constant, eight different percentage ratio (R) of air: material from 0% to 70% with a 10% increment were assigned to each sample. The phantoms were printed in three materials, namely Vero PureWhite, VeroClear and TangoPlus using Polyjet technology. The CT value analysis, non-contact profile measurement and microCT-based volumetric analysis was performed for all the samples.

RESULTS

The printed test phantoms produced a grey value spectrum equivalent to the radiation attenuation of human soft tissues in the range of -757 to +286 HU on CT. The results from dimensional comparison analysis of the printed phantoms with the digital test phantoms using non-contact profile measurement showed a mean accuracy of 99.07 % and that of micro-CT volumetric analysis showed mean volumetric accuracy of 84.80-94.91%. The material and printing costs of developing 24 test phantoms was 83.00 Euro.

CONCLUSIONS

The study shows that additive manufacturing-guided macrostructure manipulation modifies successfully the radiographic visibility of a material in CBCT imaging with 1 mm³ resolution, helping customization of imaging phantoms.

3D-printed tissue-simulating phantoms for near-infrared fluorescence imaging of rheumatoid diseases

SIGNIFICANCE

Fluorescence imaging of rheumatoid diseases with indocyanine green (ICG) is an emerging technique with unique potential for diagnosis and therapy. Device characterization, monitoring of the performance, and further developments of the technique require tissue-equivalent fluorescent phantoms of high stability with appropriate anatomical shapes.

AIM

Our investigations aim at the development of a three-dimensional (3D) printing technique to fabricate hand and finger models with appropriate optical properties in the near-infrared spectral range. These phantoms should have fluorescence properties similar to ICG, and excellent photostability and durability over years.

APPROACH

We modified a 3D printing methacrylate photopolymer by adding the fluorescent dye Lumogen IR 765 to the raw material. Reduced scattering and absorption coefficients were adjusted to values representative of the human hand by incorporating titanium dioxide powder and black ink. The properties of printed phantoms of various compositions were characterized using UV/Vis and fluorescence spectroscopy, and time-resolved measurements. Photostability and bleaching were investigated with a hand imager. For comparison, several phantoms with ICG as fluorescent dye were printed and characterized as well.

RESULTS

The spectral properties of Lumogen IR 765 are very similar to those of ICG. By optimizing the concentrations of Lumogen, titanium dioxide, and ink, anatomically shaped hand and vessel models with properties equivalent to in vivo investigations with a fluorescence hand imager could be printed. Phantoms with Lumogen IR 765 had an excellent photostability over up to 4 years. In contrast, phantoms printed with ICG showed significant bleaching and degradation of fluorescence over time.

CONCLUSIONS

3D printing of phantoms with Lumogen IR 765 is a promising method for fabricating anatomically shaped fluorescent tissue models of excellent stability with spectral properties similar to ICG. The phantoms are well-suited to monitor the performance of hand imagers. Concepts can easily be transferred to other fluorescence imaging applications of ICG.

Development of 3D Printed Mitral Valve Constructs for Transcatheter Device Modeling of Tissue and Device Deformation

Transcatheter mitral valve repair (TMVR) therapies offer a minimally invasive alternative to surgical mitral valve (MV) repair for patients with prohibitive surgical risks. Pre-procedural planning and associated medical device modeling is primarily performed in silico, which does not account for the physical interactions between the implanted TMVR device and surrounding tissue and may result in poor outcomes. We developed 3D printed tissue mimics for modeling TMVR therapies. Structural properties of the mitral annuli, leaflets, and chordae were replicated from multi-material blends. Uniaxial tensile testing was performed on the resulting composites and their mechanical properties were compared to those of their target native components. Mimics of the MV annulus printed in homogeneous strips approximated the tangent moduli of the native mitral annulus at 2% and 6% strain. Mimics of the valve leaflets printed in layers of different stiffnesses approximated the force–strain and stress–strain behavior of native MV leaflets. Finally, mimics of the chordae printed as reinforced cylinders approximated the force–strain and stress–strain behavior of native chordae. We demonstrated that multi-material 3D printing is a viable approach to the development of tissue phantoms, and that printed patient-specific geometries can approximate the local deformation force which may act upon devices used for TMVR therapies.

Projection-based stereolithography for direct 3D printing of heterogeneous ultrasound phantoms

Modern ultrasound (US) imaging is increasing its clinical impact, particularly with the introduction of US-based quantitative imaging biomarkers. Continued development and validation of such novel imaging approaches requires imaging phantoms that recapitulate the underlying anatomy and pathology of interest. However, current US phantom designs are generally too simplistic to emulate the structure and variability of the human body. Therefore, there is a need to create a platform that is capable of generating well-characterized phantoms that can mimic the basic anatomical, functional, and mechanical properties of native tissues and pathologies. Using a 3D-printing technique based on stereolithography, we fabricated US phantoms using soft materials in a single fabrication session, without the need for material casting or back-filling. With this technique, we induced variable levels of stable US backscatter in our printed materials in anatomically relevant 3D patterns. Additionally, we controlled phantom stiffness from 7 to >120 kPa at the voxel level to generate isotropic and anisotropic phantoms for elasticity imaging. Lastly, we demonstrated the fabrication of channels with diameters as small as 60 micrometers and with complex geometry (e.g., tortuosity) capable of supporting blood-mimicking fluid flow. Collectively, these results show that projection-based stereolithography allows for customizable fabrication of complex US phantoms.

Use of patient specific 3D printed neurovascular phantoms to simulate mechanical thrombectomy

BACKGROUND

The ability of the patient specific 3D printed neurovascular phantoms to accurately replicate the anatomy and hemodynamics of the chronic neurovascular diseases has been demonstrated by many studies. Acute occurrences, however, may still require further development and investigation and therefore we studied acute ischemic stroke (AIS). The efficacy of endovascular procedures such as mechanical thrombectomy (MT) for the treatment of large vessel occlusion (LVO), can be improved by testing the performance of thrombectomy devices and techniques using patient specific 3D printed neurovascular models.

METHODS

3D printed phantoms were connected to a flow loop with physiologically relevant flow conditions, including input flow rate and fluid temperature. A simulated blood clot was introduced into the model and placed in the proximal Middle Cerebral Artery (MCA) region. Clot location, composition, length, and arterial angulation were varied and MTs were simulated using stent retrievers. Device placement relative to the clot and the outcome of the thrombectomy were recorded for each situation. Digital subtraction angiograms (DSA) were captured before and after LVO simulation. Recanalization outcome was evaluated using DSA as either 'no recanalization' or 'recanalization'. Forty-two 3DP neurovascular phantom benchtop experiments were performed.

RESULTS

Clot angulation within the MCA region had the most significant impact on the MT outcome, with a p-value of 0.016. Other factors such as clot location, clot composition, and clot length correlated weakly with the MT outcome.

CONCLUSIONS

This project allowed us to gain knowledge of how such characteristics influence thrombectomy success and can be used in making clinical decisions when planning the procedure and selecting specific thrombectomy tools and approaches.

Mechanical characterization of 3D printed mimic of human artery affected by atherosclerotic plaque through numerical and experimental methods

This paper proposes a novel experimental investigation based on 3D printing to validate numerical models for biomechanics simulations. Soft elastomeric materials have been used in Polyjet multi-material 3D printer to mimicking arteries affected by atherosclerotic plaque. The nonlinear mechanical properties of five digital materials are characterized and used as an input for finite element (FE) modeling. Pressurized air is applied to the internal cavity of the printed model to reproduce the internal blood pressure in the artery. Digital Imaging Correlation is adopted to measure the displacement and deformation. A 1D linear higher-order FE model based on the Carrera Unified Formulation is compared to 3D nonlinear FE solutions.

3D Printing for Cardiovascular Applications: From End-to-End Processes to Emerging Developments

3D printing as a means of fabrication has seen increasing applications in medicine in the last decade, becoming invaluable for cardiovascular applications. This rapidly developing technology has had a significant impact on cardiovascular research, its clinical translation and education. It has expanded our understanding of the cardiovascular system resulting in better devices, tools and consequently improved patient outcomes. This review discusses the latest developments and future directions of generating medical replicas ('phantoms') for use in the cardiovascular field, detailing the end-to-end process from medical imaging to capture structures of interest, to production and use of 3D printed models. We provide comparisons of available imaging modalities and overview of segmentation and post-processing techniques to process images for printing, detailed exploration of latest 3D printing methods and materials, and a comprehensive, up-to-date review of milestone applications and their impact within the cardiovascular domain across research, clinical use and education. We then provide an in-depth exploration of future technologies and innovations around these methods, capturing opportunities and emerging directions across increasingly realistic representations, bioprinting and tissue engineering, and complementary virtual and mixed reality solutions. The next generation of 3D printing techniques allow patient-specific models that are increasingly realistic, replicating properties, anatomy and function.

Development of Custom Wall-Less Cardiovascular Flow Phantoms with Tissue-Mimicking Gel

PURPOSE

Flow phantoms are used in experimental settings to aid in the simulation of blood flow. Custom geometries are available, but current phantom materials present issues with degradability and/or mimicking the mechanical properties of human tissue. In this study, a method of fabricating custom wall-less flow phantoms from a tissue-mimicking gel using 3D printed inserts is developed.

METHODS

A 3D blood vessel geometry example of a bifurcated artery model was 3D printed in polyvinyl alcohol, embedded in tissue-mimicking gel, and subsequently dissolved to create a phantom. Uniaxial compression testing was performed to determine the Young's moduli of the five gel types. Angle-independent, ultrasound-based imaging modalities, Vector Flow Imaging (VFI) and Blood Speckle Imaging (BSI), were utilized for flow visualization of a straight channel phantom.

RESULTS

A wall-less phantom of the bifurcated artery was fabricated with minimal bubbles and continuous flow demonstrated. Additionally, flow was visualized through a straight channel phantom by VFI and BSI. The available gel types are suitable for mimicking a variety of tissue types, including cardiac tissue and blood vessels.

CONCLUSION

Custom, tissue-mimicking flow phantoms can be fabricated using the developed methodology and have potential for use in a variety of applications, including ultrasound-based imaging methods. This is the first reported use of BSI with an in vitro flow phantom.

Utility of 3D printed abdominal aortic aneurysm phantoms: a systematic review

BACKGROUND

3D printed (3DP) abdominal aortic aneurysm (AAA) phantoms are emerging in the literature as an adjunct for the visualization of complex anatomy, particularly for presurgical device selection and simulation. This is the first systematic review to provide a comprehensive overview of 3DP for endovascular aneurysm repair (EVAR) planning and intervention, evaluating the readiness of current levels of technology for mainstream implementation.

METHODS

A systematic literature search of PubMed and MEDLINE was performed as per PRISMA guidelines using the terms '3D Printing', 'AAA' OR 'EVAR' and related index terms, and further relevant articles were appraised via a snowballing approach. Our last search was conducted on 14 November 2020.

RESULTS

Twenty-five articles were identified for critical analysis, with 14 cases or technical reports. Nineteen publications utilized 3DP AAA phantoms to aid presurgical decision making, device selection and design. Four publications explored the utility of 3DP phantoms as EVAR trainers, and one publication examined the technology as a tool for patient education. Flexible, transparent phantoms were deemed most useful; however, the cost and availability of higher end machines limited accessibility.

CONCLUSION

3DP phantoms have been used in EVAR to facilitate visualization of complex patient anatomy, appropriate device selection, in predicting navigational difficulties and the shape and position of endograft after deployment. These phantoms show promise in reducing known complications such as endoleak, stent graft occlusion and migration; however, larger scale prospective studies are required to validate its impacts on patient outcomes and cost savings to the healthcare system.

Assessment of transfer of morphological characteristics of Anomalous Aortic Origin of a Coronary Artery from imaging to patient specific 3D Printed models: A feasibility study

BACKGROUND AND OBJECTIVE

This study aims to determine the accuracy of patient specific 3D printed models in capturing pathological anatomical characteristics derived from CT angiography (CTA) in children with anomalous aortic origin of a coronary artery (AAOCA).

METHODS & MATERIALS

Following institutional regulatory approval, a standardized protocol for CTA of AAOCA was utilized for imaging. Blood volume of the aorta and coronaries were segmented from the DICOM images. A total of 10 models from 8 AAOCA patients were created, including 2 post-operative models. Mechanical properties of Agilus30 a flexible photopolymer coated with a thin layer of parylene, polyurethane (PU) and silicone and native aortic tissue from a postmortem specimen were compared. AAOCA models with wall thicknesses of 2mm aorta and 1.5mm coronaries were 3D printed in Agilus30 and coated with PU. CT of the printed models was performed, and 3D virtual models were generated. Transfer of anatomical characteristics and geometric accuracy were compared between the patient model virtual models.

RESULTS

Dynamic modulus of Agilus30 at 2mm thickness was found to be close to native aortic tissue. Structured reporting of anatomical characteristics by imaging experts showed good concordance between patient and model CTA Comparative patient and virtual model measurements showed Pearson's correlation (r) of 0.9959 for aorta (n=70) and 0.9538 for coronaries (n=60) linear, and 0.9949 for aorta (n=30) and 0.9538 for coronaries (n=30) cross-sectional, dimensions. Surface contour map mean difference was 0.08 ± 0.29mm.

CONCLUSIONS

Geometrically accurate AAOCA models preserving morphological characteristics, essential for risk stratification and decision-making, can be 3D printed from a patient's CTA.

Additive Manufacturing Processes in Medical Applications

Additive manufacturing (AM, 3D printing) is used in many fields and different industries. In the medical and dental field, every patient is unique and, therefore, AM has significant potential in personalized and customized solutions. This review explores what additive manufacturing processes and materials are utilized in medical and dental applications, especially focusing on processes that are less commonly used. The processes are categorized in ISO/ASTM process classes: powder bed fusion, material extrusion, VAT photopolymerization, material jetting, binder jetting, sheet lamination and directed energy deposition combined with classification of medical applications of AM. Based on the findings, it seems that directed energy deposition is utilized rarely only in implants and sheet lamination rarely for medical models or phantoms. Powder bed fusion, material extrusion and VAT photopolymerization are utilized in all categories. Material jetting is not used for implants and biomanufacturing, and binder jetting is not utilized for tools, instruments and parts for medical devices. The most common materials are thermoplastics, photopolymers and metals such as titanium alloys. If standard terminology of AM would be followed, this would allow a more systematic review of the utilization of different AM processes. Current development in binder jetting would allow more possibilities in the future.

USE OF ANTHROPOMORPHIC HETEROGENEOUS PHYSICAL PHANTOMS FOR VALIDATION OF COMPUTATIONAL DOSIMETRY OF MEDICAL PERSONNEL AND PATIENTS

In the dosimetry of ionizing radiation, the phantoms of the human body, which are used as a replacement for thehuman body in physical measurements and calculations, play an important, but sometimes underestimated, role.There are physical phantoms used directly for measurements, and mathematical phantoms for computationaldosimetry. Their complexity varies from simple geometry applied for calibration purposes up to very complex, whichsimulates in detail the shapes of organs and tissues of the human body. The use of physical anthropomorphic phantoms makes it possible to effectively optimize radiation doses by adjusting the parameters of CT-scanning (computed tomography) in accordance with the characteristics of the patient without compromising image quality. The useof phantoms is an indispensable approach to estimate the actual doses to the organs or to determine the effectivedose of workers - values that are regulated, but cannot be directly measured.The article contains an overview of types, designs and the fields of application of anthropomorphic heterogeneousphysical phantoms of a human with special emphasis on their use for validation of models and methods of computational dosimetry.

Advanced three-dimensionally engineered simulation model for aortic valve and proximal aorta procedures

OBJECTIVES

A 3-dimensionally (3D) engineered model for simulation of aortic valve and proximal aortic procedures is a reliable tool both for training young surgeons and for simulating complex cases. To achieve a realistic simulation, the artificial model should reproduce the angles and orientations of the cardiac structures based on the patient's anatomical condition, reproduce tissue mechanical characteristics and be easy to obtain and easy to use. The goal of the study was the production and validation of realistic training models, based on the patient's actual anatomical characteristics, to provide training for aortic valve procedures.

METHODS

An anatomical model was manufactured using 3D printing and silicone casting. The digital anatomical model was obtained by segmenting computed tomography imaging. The segmented geometrical images were processed and a casting mould was designed. The mould was manufactured on a 3D printer. Silicone was cast into the mould; after curing, the finished model was ready. The realistic reproduction was evaluated by mechanical hardness tests and a survey by cardiac surgeons.

RESULTS

Six 3D silicone models were produced that represented the patient's anatomy including aortic valve leaflets, aortic root with coronary ostia, ascending aorta and proximal arch. Aortic valve replacement was performed, and 100% of the participants evaluated the model in a survey as perfectly reproducing anatomy and surgical handling.

CONCLUSIONS

We produced a realistic, cost-effective simulator for training purposes and for simulation of complex surgical cases. The model reproduced the real angulation and orientation of the aortic structures inside the mediastinum, permitting a real-life simulation of the desired procedure. This model offers opportunities to simulate various surgical procedures.

A Brush-Spin-Coating Method for Fabricating In Vitro Patient-Specific Vascular Models by Coupling 3D-Printing

PURPOSE

In vitro patient-specific flexible vascular models are helpful for understanding the haemodynamic changes before and after endovascular treatment and for effective training of neuroendovascular interventionalists. However, it is difficult to fabricate models of overall unified or controllable thickness using existing manufacturing methods. In this study, we developed an improved and easily implemented method by combining 3D printing and brush-spin-coating processes to produce a transparent silicone model of uniform or varied thickness.

METHODS

First, a water-soluble inner-skeleton model, based on clinical data, was printed on a 3D printer. The skeleton model was subsequently fixed in a single-axis-rotation machine to enable continuous coating of silicone, the thickness of which was manually controlled by adsorption and removal of excess silicone in a brush-spinning operation. After the silicone layer was solidified, the inner skeleton was further dissolved in a hot water bath, affording a transparent vascular model with real geometry. To verify the controllability of the coating thickness by using this method, a straight tube, an idealised aneurysm model, a patient-specific aortic arch model, and an abdominal aortic aneurysm model were manufactured.

RESULTS

The different thicknesses of the manufactured tubes could be well controlled, with the relative standard deviations being 5.6 and 8.1% for the straight and aneurysm tubes, respectively. Despite of the diameter changing from 33 to 20 mm in the patient-specific aorta, the thickness of the fabricated aortic model remains almost the same along the longitudinal direction with a lower standard deviation of 3.1%. In the more complex patient-specific abdominal aneurysm model, varied thicknesses were realized to mimic the measured data from the CT images, where the middle of the aneurysm was with 2 mm and abdominal aorta as well as the iliac arteries had the normal thickness of 2.3 mm.

CONCLUSION

Through the brush-spin-coating method, models of different sizes and complexity with prescribed thickness can be manufactured, which will be helpful for developing surgical treatment strategies or training neuroendovascular interventionalists.

Mimicking the Mechanical Properties of Aortic Tissue with Pattern-Embedded 3D Printing for a Realistic Phantom

3D printing technology has been extensively applied in the medical field, but the ability to replicate tissues that experience significant loads and undergo substantial deformation, such as the aorta, remains elusive. Therefore, this study proposed a method to imitate the mechanical characteristics of the aortic wall by 3D printing embedded patterns and combining two materials with different physical properties. First, we determined the mechanical properties of the selected base materials (Agilus and Dragonskin 30) and pattern materials (VeroCyan and TPU 95A) and performed tensile testing. Three patterns were designed and embedded in printed Agilus-VeroCyan and Dragonskin 30-TPU 95A specimens. Tensile tests were then performed on the printed specimens, and the stress-strain curves were evaluated. The samples with one of the two tested orthotropic patterns exceeded the tensile strength and strain properties of a human aorta. Specifically, a tensile strength of 2.15 ± 0.15 MPa and strain at breaking of 3.18 ± 0.05 mm/mm were measured in the study; the human aorta is considered to have tensile strength and strain at breaking of 2.0-3.0 MPa and 2.0-2.3 mm/mm, respectively. These findings indicate the potential for developing more representative aortic phantoms based on the approach in this study.

Multilayer fabrication of durable catheter-deployable soft robotic sensor arrays for efficient left atrial mapping

Devices that perform cardiac mapping and ablation to treat atrial fibrillation provide an effective means of treatment. Current devices, however, have limitations that either require tedious point-by-point mapping of a cardiac chamber or have limited ability to conform to the complex anatomy of a patient's cardiac chamber. In this work, a detailed, scalable, and manufacturable technique is reported for fabrication of a multielectrode, soft robotic sensor array. These devices exhibit high conformability (~85 to 90%) and are equipped with an array of stretchable electronic sensors for voltage mapping. The form factor of the device is intended to match that of the entire left atrium and has a hydraulically actuated soft robotic structure whose profile facilitates deployment from a 13.5-Fr catheter. We anticipate that the methods described in this paper will serve a new generation of conformable medical devices that leverage the unique characteristics of stretchable electronics and soft robotics.

Method to simulate distal flow resistance in coronary arteries in 3D printed patient specific coronary models

BACKGROUND

Three-dimensional printing (3DP) offers a unique opportunity to build flexible vascular patient-specific coronary models for device testing, treatment planning, and physiological simulations. By optimizing the 3DP design to replicate the geometrical and mechanical properties of healthy and diseased arteries, we may improve the relevance of using such models to simulate the hemodynamics of coronary disease. We developed a method to build 3DP patient specific coronary phantoms, which maintain a significant part of the coronary tree, while preserving geometrical accuracy of the atherosclerotic plaques and allows for an adjustable hydraulic resistance.

METHODS

Coronary computed tomography angiography (CCTA) data was used within Vitrea (Vital Images, Minnetonka, MN) cardiac analysis application for automatic segmentation of the aortic root, Left Anterior Descending (LAD), Left Circumflex (LCX), Right Coronary Artery (RCA), and calcifications. Stereolithographic (STL) files of the vasculature and calcium were imported into Autodesk Meshmixer for 3D model optimization. A base with three chambers was built and interfaced with the phantom to allow fluid collection and independent distal resistance adjustment of the RCA, LAD and LCX and branching arteries. For the 3DP we used Agilus for the arterial wall, VeroClear for the base and a Vero blend for the calcifications, respectively. Each chamber outlet allowed interface with catheters of varying lengths and diameters for simulation of hydraulic resistance of both normal and hyperemic coronary flow conditions. To demonstrate the manufacturing approach appropriateness, models were tested in flow experiments.

RESULTS

Models were used successfully in flow experiments to simulate normal and hyperemic flow conditions. The inherent mean resistance of the chamber for the LAD, LCX, and RCA, were 1671, 1820, and 591 (dynes ∙ sec/ cm5), respectively. This was negligible when compared with estimates in humans, with the chamber resistance equating to 0.65-5.86%, 1.23-6.86%, and 0.05-1.67% of the coronary resistance for the LAD, LCX, and RCA, respectively at varying flow rates and activity states. Therefore, the chamber served as a means to simulate the compliance of the distal coronary trees and to allow facile coupling with a set of known resistance catheters to simulate various physical activity levels.

CONCLUSIONS

We have developed a method to create complex 3D printed patient specific coronary models derived from CCTA, which allow adjustable distal capillary bed resistances. This manufacturing approach permits comprehensive coronary model development which may be used for physiologically relevant flow simulations.

Biomechanical performance design of joint prosthesis for medical rehabilitation via generative structure optimization

This paper proposes a biomechanical performance design method of joint prosthesis for medical rehabilitation via Generative Structure Optimization (GSO). Firstly, the 3D reconstruction of manifold structure involving hard bone and cartilage is sequentially and progressively implemented from heterogeneous medical images such as Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) via iteration. On the basis of reconstructed mesh structure, the finite element method (FEM) is hereby employed to verify the structure by evaluating the mechanical force distribution. The biomechanical performance design model for 3 D printing (3DP) is then built using multi-objective optimization (MOO) by considering adaptive layer thickness, infill patterns and infill trajectories, etc. The GSO outlets a generative data-driven system which covers various stages such as personalized CT, subsequent 3 D reconstruction, further finite element analysis (FEA) and even structural parameter optimization. The physical experiment of Additive manufacturing (AM) proves that, the relative density, surface topography and wear-resisting performance of joint prosthesis can be improved by GSO which helps to improve biomechanical performance, including kinematics and dynamics. The proposed method may arouse the huge attention in the prosthesis applications to promote patients' high-end customization well-being.

Development of a CT imaging phantom of anthromorphic lung using fused deposition modeling 3D printing

Development of patient-specific CT imaging phantoms with randomly incorporated lesions of various shapes and sizes for calibrating image intensity and validating quantitative measurement software is very challenging. In this investigation, a physical phantom that accurately represents a patient's specific anatomy and the intensity of lung CT images at the voxel level will be fabricated using fused deposition modeling (FDM) 3D printing. Segmentation and modeling of a patient's CT data were performed by an expert and the results were confirmed by a thoracic radiologist with more than 20 years of experience. This facilitated the extraction of the details of the patient's anatomy; various kinds of nodules with different shapes and sizes were randomly added to the modeled lung for evaluating the size-accuracy of the quantification software. To achieve these Hounsfield Units (HU) ranges for the corresponding voxels in acquired CT scans, the infill ratios of FDM 3D printing were controlled. Based on CT scans of the 3D printed phantoms, the measured HU for normal pulmonary parenchyma, ground glass opacity (GGO), and solid nodules were determined to be within target HU ranges. The accuracy of the mean absolute difference and the mean relative difference of nodules were less than 0.55 ± 0.30 mm and 3.72 ± 1.64% (mean difference ± 95 CI), respectively. Patient-specific CT imaging phantoms were designed and manufactured using an FDM printer, which could be applied for the precise calibration of CT intensity and the validation of image quantification software.

Evaluation of a Desktop 3D Printed Rigid Refractive-Indexed-Matched Flow Phantom for PIV Measurements on Cerebral Aneurysms

PURPOSE

Fabrication of a suitable flow model or phantom is critical to the study of biomedical fluid dynamics using optical flow visualization and measurement methods. The main difficulties arise from the optical properties of the model material, accuracy of the geometry and ease of fabrication.

METHODS

Conventionally an investment casting method has been used, but recently advancements in additive manufacturing techniques such as 3D printing have allowed the flow model to be printed directly with minimal post-processing steps. This study presents results of an investigation into the feasibility of fabrication of such models suitable for particle image velocimetry (PIV) using a common 3D printing Stereolithography process and photopolymer resin.

RESULTS

An idealised geometry of a cerebral aneurysm was printed to demonstrate its applicability for PIV experimentation. The material was shown to have a refractive index of 1.51, which can be refractive matched with a mixture of de-ionised water with ammonium thiocyanate (NH₄SCN). The images were of a quality that after applying common PIV pre-processing techniques and a PIV cross-correlation algorithm, the results produced were consistent within the aneurysm when compared to previous studies.

CONCLUSIONS

This study presents an alternative low-cost option for 3D printing of a flow phantom suitable for flow visualization simulations. The use of 3D printed flow phantoms reduces the complexity, time and effort required compared to conventional investment casting methods by removing the necessity of a multi-part process required with investment casting techniques.

Mechanically Powered Motion Imaging Phantoms: Proof of Concept

Motion imaging phantoms are expensive, bulky and difficult to transport and set-up. The purpose of this paper is to demonstrate a simple approach to the design of multi-modality motion imaging phantoms that use mechanically stored energy to produce motion.We propose two phantom designs that use mainsprings and elastic bands to store energy. A rectangular piece was attached to an axle at the end of the transmission chain of each phantom, and underwent a rotary motion upon release of the mechanical motor. The phantoms were imaged with MRI and US, and the image sequences were embedded in a 1D non linear manifold (Laplacian Eigenmap) and the spectrogram of the embedding was used to derive the angular velocity over time. The derived velocities were consistent and reproducible within a small error. The proposed motion phantom concept showed great potential for the construction of simple and affordable motion phantoms.

Measurement of Frictional Properties of Aortic Stent Grafts and Their Delivery Systems

Stent grafts are medical devices used to treat abdominal aortic aneurysms (AAAs) in endovascular aneurysm repair (EVAR). Computational and experimental models have been developed to study stent graft delivery and deployment during EVAR; however, frictional properties have not been taken into account in most previous studies. The objective of this study was to determine the coefficients of friction of three commercially available stent grafts (Cook Zenith, Medtronic Endurant, and Vascutek Anaconda), their delivery sheaths, a porcine aorta, and two mock arterial materials. Stent grafts were obtained and separated into stents, graft fabric, and sheaths. Using a custom-made friction measurement apparatus, the coefficients of friction were measured between five material pairs: (i) the stents and inner surface of the sheath, (ii) the graft fabric and inner surface of the sheath, (iii) the outer surface of the sheath and a porcine aorta, (iv) the outer surface of the sheath and three different polyvinyl alcohol (PVA) cryogels, and (v) the outer surface of the sheath and a polydimethylsiloxane (PDMS) sheet. The results show that the coefficients of friction between the graft fabric and the sheath were higher than those between the stents and the sheath. The PVA cryogels showed more comparable frictional properties to the porcine aorta than did the PDMS sheet, suggesting that PVA cryogels provide a more accurate approximation for the in vivo frictional properties. These results can be used to improve the accuracy of computational models for stent graft delivery and deployment and to select appropriate materials for vascular phantoms.

3D printing of surgical hernia meshes impregnated with contrast agents: in vitro proof of concept with imaging characteristics on computed tomography

BACKGROUND

Selected medical implants and other 3D printed constructs could potentially benefit from the ability to incorporate contrast agents into their structure. The purpose of the present study is to create 3D printed surgical meshes impregnated with iodinated, gadolinium, and barium contrast agents and characterize their computed tomography (CT) imaging characteristics. Commercial fused deposition layering 3D printing was used to construct surgical meshes impregnated with imaging contrast agents in an in vitro model. Polycaprolactone (PCL) meshes were printed containing iodinated, gadolinium, or barium contrast; control PCL meshes without contrast were also fabricated. The three different contrast agents were mixed with PCL powder and directly loaded into the 3D printer. CT images of the three contrast-containing meshes and the control meshes were acquired and analyzed using small elliptical regions of interest to record the Hounsfield units (HU) of each mesh. Subsequently, to test their solubility and sustainability, the contrast-containing meshes were placed in a 37 °C agar solution for 7 days and imaged by CT at days 1, 3 and 7.

RESULTS

All 3D printed meshes were visible on CT. Iodinated contrast meshes had the highest attenuation (2528 mean HU), significantly higher than both and gadolinium (1178 mean HU) and barium (592 mean HU) containing meshes. Only barium meshes sustained their visibility in the agar solution; the iodine and gadolinium meshes were poorly perceptible and had significantly lower mean HU compared to their pre-agar solution imaging, with iodine and gadolinium present in the adjacent agar at day 7 CT.

CONCLUSION

3D prints embedded with contrast materials through this method displayed excellent visibility on CT; however, only barium mesh maintained visibility after 7 days incubation on agar at human body temperature. This method of 3D printing with barium may have potential applications in a variety of highly personalized and CT visible medical devices.

Manufacturing Abdominal Aorta Hydrogel Tissue-Mimicking Phantoms for Ultrasound Elastography Validation

Ultrasound (US) elastography, or elasticity imaging, is an adjunct imaging technique that utilizes sequential US images of soft tissues to measure the tissue motion and infer or quantify the underlying biomechanical characteristics. For abdominal aortic aneurysms (AAA), biomechanical properties such as changes in the tissue's elastic modulus and estimates of the tissue stress may be essential for assessing the need for the surgical intervention. Abdominal aortic aneurysms US elastography could be a useful tool to monitor AAA progression and identify changes in biomechanical properties characteristic of high-risk patients. A preliminary goal in the development of an AAA US elastography technique is the validation of the method using a physically relevant model with known material properties. Here we present a process for the production of AAA tissue-mimicking phantoms with physically relevant geometries and spatially modulated material properties. These tissue phantoms aim to mimic the US properties, material modulus, and geometry of the abdominal aortic aneurysms. Tissue phantoms are made using a polyvinyl alcohol cryogel (PVA-c) and molded using 3D printed parts created using computer aided design (CAD) software. The modulus of the phantoms is controlled by altering the concentration of PVA-c and by changing the number of freeze-thaw cycles used to polymerize the cryogel. The AAA phantoms are connected to a hemodynamic pump, designed to deform the phantoms with the physiologic cyclic pressure and flows. Ultra sound image sequences of the deforming phantoms allowed for the spatial calculation of the pressure normalized strain and the identification of mechanical properties of the vessel wall. Representative results of the pressure normalized strain are presented.

A compliant aortic model for in vitro simulations: Design and manufacturing process

We design and manufacture a silicone model of the human aorta, able to mimic both the geometrical and the mechanical properties of physiological individuals, with a specific focus on reproducing the compliance. In fact, while the models available in the literature exhibit an unrealistic compliant behavior, though they are detailed from the geometrical viewpoint, here the goal is to provide an accurate compliant tool for in vitro testing the devices that interface with the vascular system. A parametric design of the aortic model is obtained based on the available literature data, and the model is manufactured with a specific silicone mixture using rapid prototyping and molding techniques. The manufactured prototype has been tested by means of computed tomography scans for evaluating the matching of the mechanical properties with the desired ones. Results show a high degree of adherence between the imposed and the measured compliance values for each main aortic section. Thus, our work proves the feasibility of the approach, and the possibility to manufacture compliant models that reproduce the mechanical behavior of the aorta for in vitro studies.

Role of CT and MRI in the design and development of orthopaedic model using additive manufacturing

OBJECTIVE

To study the role of Computed tomography (CT) and Magnetic resonance imaging (MRI) for design and development of orthopaedic model using additive manufacturing (AM) technologies.

METHODS

A significant number of research papers in this area are studied to provide the direction of development along with the future scope.

RESULTS

Briefly discussed various steps used to create a 3D model by Additive Manufacturing using CT and MRI scan. These scanning technologies are used to produce medical as well as orthopaedic implants by using AM technologies. The images so produced are exported in different software like OsiriX Imaging Software, 3D slicer, Mimics, Magics, 3D doctor and InVesalius to produce a 3D digital model. Various criteria's achieved by CT and MRI scan for design and development of orthopaedic implant using additive manufacturing are also discussed briefly. AM model created by this process show exact shape, size, dimensions, textures, colour and features.

CONCLUSION

AM technologies help to convert the digital model into a 3D physical object, thereby improving the understanding of patient anatomy for treatment as well as for educational purpose. These scanning technologies have various applications to enhance the AM in the field of orthopaedic. In orthopaedic every patient model is a customised unit, sourced from the individual patient. 3D CAD data captured by these scanning technologies are directly exported in standard triangulate language (STL) format for printing by AM technologies. Crossestion of the physical model fabricated by this process shows a patient's anatomy if the model prepared by using the bone-like material.

3-D-Printed Phantom Fabricated by Photopolymer Jetting Technology for High-Frequency Ultrasound Imaging

In the field of high-frequency ultrasound imaging ( MHz), tools for characterizing the performance of imaging systems are lacking. Indeed, commercial phantoms are often inadequate for this frequency range. The development of homemade phantoms on the laboratory scale is often required but is hindered by the difficulty in making very small structures that must be distributed with high accuracy in 3-D space. We propose investigating the use of 3-D photopolymer printing to create resolution and calibration phantoms designed for high-frequency ultrasound imaging. The quality and importance of these phantoms are discussed from the point of view of ultrasound parameters and imaging. First, the compressional wave group velocity, acoustic impedance, and attenuation of six photopolymerized materials were measured using temporal and spectral methods in a substitution experimental setup. Measurements were performed on printed samples using a broadband-focused single-element transducer covering a large frequency range (15-55 MHz). Two 3-D phantoms incorporating different shapes and dimensions were designed and printed. Finally, 3-D acoustic images were obtained using either a mechanically driven single-element transducer or a high-frequency commercial imaging system. Three-dimensional printing enabled us to generate phantoms suitable for high-frequency imaging with complex geometry inclusions and with a surrounding material having acoustic properties close to those of human skin. The calculated SNR between the inclusion and surrounding media is approximately 50 dB. In conclusion, 3-D printing is a useful tool for directly, easily, and rapidly manufacturing ultrasound phantoms for ultrasound imaging system assessments and computational calibration or validation.

Design of a portable phantom device to simulate tissue oxygenation and blood perfusion

We propose a portable phantom system for calibration and validation of medical optical devices in a clinical setting. The phantom system comprises a perfusion module and an exchangeable tissue-simulating phantom that simulates tissue oxygenation and blood perfusion. The perfusion module consists of a peristaltic pump, two liquid storage units, and two pressure suppressors. The tissue-simulating phantom is fabricated by a three-dimensional (3D) printing process with microchannels embedded to simulate blood vessels. Optical scattering and absorption properties of biologic tissue are simulated by mixing graphite powder and titanium dioxide powder with clear photoreactive resin at specific ratios. Tissue oxygen saturation (StO₂) and blood perfusion are simulated by circulating the mixture of blood and intralipid at different oxygenation levels and flow rates. A house-made multimodal imaging system that combines multispectral imaging and laser speckle imaging are used for non-invasive detection of phantom oxygenation and perfusion, and the measurements are compared with those of a commercial Moor device as well as numerical simulation. By acquiring multimodal imaging data from one phantom and applying the calibration factors in different settings, we demonstrate the technical feasibility to calibrate optical devices for consistent measurements. By simulating retina tissue vasculature and acquiring functional images at different tissue oxygenation and blood perfusion levels, we demonstrate the clinical potential to simulate tissue anomalies. Our experiments imply the clinical potential of a portable, low-cost, and traceable phantom standard to calibrate and validate medical optical devices for improved performance.

On the optimization of low-cost FDM 3D printers for accurate replication of patient-specific abdominal aortic aneurysm geometry

BACKGROUND

There is a potential for direct model manufacturing of abdominal aortic aneurysm (AAA) using 3D printing technique for generating flexible semi-transparent prototypes. A patient-specific AAA model was manufactured using fused deposition modelling (FDM) 3D printing technology. A flexible, semi-transparent thermoplastic polyurethane (TPU), called Cheetah Water (produced by Ninjatek, USA), was used as the flexible, transparent material for model manufacture with a hydrophilic support structure 3D printed with polyvinyl alcohol (PVA). Printing parameters were investigated to evaluate their effect on 3D-printing precision and transparency of the final model. ISO standard tear resistance tests were carried out on Ninjatek Cheetah specimens for a comparison of tear strength with silicone rubbers.

RESULTS

It was found that an increase in printing speed decreased printing accuracy, whilst using an infill percentage of 100% and printing nozzle temperature of 255 °C produced the most transparent results. The model had fair transparency, allowing external inspection of model inserts such as stent grafts, and good flexibility with an overall discrepancy between CAD and physical model average wall thicknesses of 0.05 mm (2.5% thicker than the CAD model). The tear resistance test found Ninjatek Cheetah TPU to have an average tear resistance of 83 kN/m, higher than any of the silicone rubbers used in previous AAA model manufacture. The model had lower cost (4.50 GBP per model), shorter manufacturing time (25 h 3 min) and an acceptable level of accuracy (2.61% error) compared to other methods.

CONCLUSIONS

It was concluded that the model would be of use in endovascular aneurysm repair planning and education, particularly for practicing placement of hooked or barbed stents, due to the model's balance of flexibility, transparency, robustness and cost-effectiveness.

Ultrasound Phantom Using Sodium Alginate as a Gelling Agent

For medical workers, ultrasound phantoms for human soft tissue are used not only for accuracy management of ultrasound diagnosis but also to aid ultrasound-guided needle and blind catheter insertion training without risk to real patients. For the phantoms, ultrasound characteristics and a texture are required to mimic the human soft tissue. The proposed phantom was composed of sodium alginate, calcium sulfate dihydrate, trisodium phosphate 12-hydrate, glycerol, and water. The propagation speed, attenuation coefficient, acoustic impedance, and texture of the proposed phantom were almost the same as those of human soft tissue. Expensive chemicals and special equipment are not required.

3D printing materials and their use in medical education: a review of current technology and trends for the future

3D printing is a new technology in constant evolution. It has rapidly expanded and is now being used in health education. Patient-specific models with anatomical fidelity created from imaging dataset have the potential to significantly improve the knowledge and skills of a new generation of surgeons. This review outlines five technical steps required to complete a printed model: They include (1) selecting the anatomical area of interest, (2) the creation of the 3D geometry, (3) the optimisation of the file for the printing and the appropriate selection of (4) the 3D printer and (5) materials. All of these steps require time, expertise and money. A thorough understanding of educational needs is therefore essential in order to optimise educational value. At present, most of the available printing materials are rigid and therefore not optimum for flexibility and elasticity unlike biological tissue. We believe that the manipuation and tuning of material properties through the creation of composites and/or blending materials will eventually allow for the creation of patient-specific models which have both anatomical and tissue fidelity.

Detecting Regional Stiffness Changes in Aortic Aneurysmal Geometries Using Pressure-Normalized Strain

Transabdominal ultrasound elasticity imaging could improve the assessment of rupture risk for abdominal aortic aneurysms by providing information on the mechanical properties and stress or strain states of vessel walls. We implemented a non-rigid image registration method to visualize the pressure-normalized strain within vascular tissues and adapted it to measure total strain over an entire cardiac cycle. We validated the algorithm's performance with both simulated ultrasound images with known principal strains and anatomically accurate heterogeneous polyvinyl alcohol cryogel vessel phantoms. Patient images of abdominal aortic aneurysm were also used to illustrate the clinical feasibility of our imaging algorithm and the potential value of pressure-normalized strain as a clinical metric. Our results indicated that pressure-normalized strain could be used to identify spatial variations in vessel tissue stiffness. The results of this investigation were sufficiently encouraging to warrant a clinical study measuring abdominal aortic pressure-normalized strain in a patient population with aneurysmal disease.

From medical imaging data to 3D printed anatomical models

Anatomical models are important training and teaching tools in the clinical environment and are routinely used in medical imaging research. Advances in segmentation algorithms and increased availability of three-dimensional (3D) printers have made it possible to create cost-efficient patient-specific models without expert knowledge. We introduce a general workflow that can be used to convert volumetric medical imaging data (as generated by Computer Tomography (CT)) to 3D printed physical models. This process is broken up into three steps: image segmentation, mesh refinement and 3D printing. To lower the barrier to entry and provide the best options when aiming to 3D print an anatomical model from medical images, we provide an overview of relevant free and open-source image segmentation tools as well as 3D printing technologies. We demonstrate the utility of this streamlined workflow by creating models of ribs, liver, and lung using a Fused Deposition Modelling 3D printer.