Technical Note: Development of a 3D printed subresolution sandwich phantom for validation of brain SPECT analysis

Comparisons of 3D printed materials for biomedical imaging applications

In biomedical imaging, it is desirable that custom-made accessories for restraint, anesthesia, and monitoring can be easily cleaned and not interfere with the imaging quality or analyses. With the rise of 3D printing as a form of rapid prototyping or manufacturing for imaging tools and accessories, it is important to understand which printable materials are durable and not likely to interfere with imaging applications. Here, 15 3D printable materials were evaluated for radiodensity, optical properties, simulated wear, and capacity for repeated cleaning and disinfection. Materials that were durable, easily cleaned, and not expected to interfere with CT, PET, or optical imaging applications were identified.

A customized anthropomorphic 3D-printed phantom to reproducibility assessment in computed tomography: an oncological case study

Introduction

Studies on computed tomography (CT) reproducibility at different acquisition parameters have to take into account radiation dose administered and related ethical issues. 3D-printed phantoms provide the possibility to investigate these features deeply and to foster CT research, also taking advantage by outperforming new generation scanners. The aim of this study is to propose a new anthropomorphic 3D-printed phantom for chest lesions, tailored on a real patient CT scan, to investigate the variability of volume and Hounsfield Unit (HU) measurements at different CT acquisition parameters.

Methods

The chest CT of a 75-year-old patient with a paramediastinal lung lesion was segmented based on an eight-compartment approach related to HU ranges (air lung, lung interstitium, fat, muscle, vascular, skin, bone, and lesion). From each mask produced, the 3D.stl model was exported and linked to a different printing infill value, based on a preliminary test and HU ratios derived from the patient scan. Fused deposition modeling (FDM) technology printing was chosen with filament materials in polylactic acid (PLA). Phantom was acquired at 50 mAs and three different tube voltages of 80, 100, and 120 kVp on two different scanners, namely, Siemens Somatom Force (Siemens Healthineers, Erlangen, Germany; same setting of real patient for 80 kVp acquisition) and GE 750 HD CT (GE Healthcare, Chicago, IL). The same segmentation workflow was then applied on each phantom acquisition after coregistration pipeline, and Dice Similarity Coefficient (DSC) and HU averages were extracted and compared for each compartment.

Results

DSC comparison among real patient versus phantom scans at different kVp, and on both CT scanners, demonstrated a good overlap of different compartments and lesion vascularization with a higher similarity for lung and lesion masks for each setting (about 0.9 and 0.8, respectively). Although mean HU was not comparable with real data, due to the PLA material, the proportion of intensity values for each compartment remains respected.

Discussion

The proposed approach demonstrated the reliability of 3D-printed technology for personalized approaches in CT research, opening to the application of the same workflow to other oncological fields.

Technical note: 3D-printed phantom for dedicated cardiac protocols and geometries in nuclear medicine

PURPOSE

The purpose of this study was to create and validate a 3D-printed nuclear cardiac phantom for low cost, user-friendly design and easy implementation with modern cardiac SPECT systems. This new phantom design aims to address common problems with commercial phantoms such as lengthy setup, prohibitive cost, and overly large size, while improving the overall functionality of the phantom.

METHODS

The phantom was developed using computer aided design software and fabricated with a 3D printer using optimized watertight printing protocols. The phantom design includes six low perfusion lesions within a stylized myocardium of the left ventricle that are placed in the common quantitation sectors for polar maps. The validation of this phantom was completed with two dedicated cardiac SPECT systems; a dual head gamma camera and a multi-pinhole CZT system. Multiple SPECT acquisitions were used to demonstrate the functionality of the phantom. Polar maps were reconstructed and used to score the contrast detectability based on the number of visible low contrast objects representing "lesions."

RESULTS

The images reconstructed from the various acquisitions on both SPECT systems closely resemble a clinical examination. Lesion visibility followed the expected relationships between protocol changes affecting contrast and spatial resolution. Lesion visibility improved with iterative reconstruction against filtered back projection.

CONCLUSION

A phantom of a stylized left ventricle with fillable myocardium was developed, 3D printed, and implemented for cardiac nuclear medicine. The phantom simulates the task of perfusion imaging and successfully demonstrates differences in image quality depending on imaging protocol. This study validates the 3D-printed design as a low cost and user-friendly phantom that can be easily scanned and scored using various systems, in particular those implementing a nontraditional cardio-centric geometry.

Absolute Quantification in Diagnostic SPECT/CT: The Phantom Premise

The application of absolute quantification in SPECT/CT has seen increased interest in the context of radionuclide therapies where patient-specific dosimetry is a requirement within the European Union (EU) legislation. However, the translation of this technique to diagnostic nuclear medicine outside this setting is rather slow. Clinical research has, in some examples, already shown an association between imaging metrics and clinical diagnosis, but the applications, in general, lack proper validation because of the absence of a ground truth measurement. Meanwhile, additive manufacturing or 3D printing has seen rapid improvements, increasing its uptake in medical imaging. Three-dimensional printed phantoms have already made a significant impact on quantitative imaging, a trend that is likely to increase in the future. In this review, we summarize the data of recent literature to underpin our premise that the validation of diagnostic applications in nuclear medicine using application-specific phantoms is within reach given the current state-of-the-art in additive manufacturing or 3D printing.

Manufacturing and evaluation of a multi-purpose Iranian head and neck anthropomorphic phantom called MIHAN

A new multi-purpose Iranian head and neck (MIHAN) anthropomorphic phantom was designed and manufactured to be used in diagnostic and therapeutic applications. Geometry of MIHAN phantom was determined based on the average dimensions acquired by CT scans of twenty patients without any medical problems in their head and neck site. Because the phantom was expected to be used with different modalities with a wide range of photon energies, attenuation coefficients of some selected materials were determined using Monte Carlo simulation. Based on analytical and simulation results, acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) were found suitable choices for soft and bony tissues, respectively. They were used in the 3D printer to build the phantom. The suitability of the materials was checked by CT number value comparison between the organs included in the phantom and the corresponding body tissues and also film dosimetry of a typical intensity-modulated radiation therapy (IMRT) plan.. Hounsfield Unit agreement and 95% ± 2% pass rate for the IMRT plan verification proved the suitability of material selection. Also, the film dosimetry showed feasibility of using MIHAN in radiotherapy plan verification workflow. In addition, PLA was introduced as a spongy bone tissue substitute for the first time.

Radioactive 3D printing for the production of molecular imaging phantoms

Quality control tests of molecular imaging systems are hampered by the complexity of phantom preparation. It is proposed that radioisotopes can be directly incorporated into photo-polymer resins. Use of the radio-polymer in a 3D printer allows phantoms with more complex and reliable activity distributions to be produced whilst simplifying source preparation. Initial tests have been performed to determine the practicality of integrating Tc-99m into a photo-polymer and example phantoms produced to test suitability for quality control. Samples of build and support resins were extracted from the print cartridges of an Objet30Pro Polyjet 3D printer. The response of the resin to external factors including ionising radiation, light and dilution with Tc-99m pertechnetate were explored. After success of the initial tests the radio-polymer was used in the production of different phantoms. Radionuclide dose calibrator and gamma camera acquisitions of the phantoms were used to test accuracy of activity concentration, print consistency, uniformity and heterogeneous reproducibility. Tomographic phantoms were also produced including a uniform hot sphere, a complex configuration of spheres and interlacing torus's and a hot rod phantom. The coefficient of variation between repeat prints of a 12 g disk phantom was 0.08%. Measured activity within the disks agreed to within 98 ± 2% of the expected activity based on initial resin concentration. Gamma camera integral uniformity measured across a 3D printed flood field phantom was 5.2% compared to 6.0% measured with a commercial Co-57 flood source. Heterogeneous distributions of activity were successfully reproduced for both 2D and 3D imaging phantoms. Count concentration across regions of heterogeneity agreed with the planned activity assigned to those regions on the phantom design. 3D printing of radioactive phantoms has been successfully demonstrated and is a promising application for quality control of Positron Emission Tomography and Single Photon Emission Computed Tomography systems.

Assessment of the attenuation of metal-infused filaments for 3D printing a gamma camera calibration phantom

The capability to produce customizable 3D printed imaging phantoms out of a growing number of materials has contributed to the increased use of such phantoms in clinical practice and research. Many of these materials have not been characterized at clinically relevant parameters for nuclear medicine imaging. In this work, we measured the attenuation of several 3D printing metal-infused filament materials with the potential for blocking 122 keV gamma photons using a scintillation detector. Understanding the ability of each material to attenuate gamma rays would allow modelling of varying levels of attenuation required for different body tissues at this energy. In addition, it allows for the determination of the thickness of the material needed to attenuate 122 keV photons, in order to build high-contrast spatial resolution phantoms. To achieve this goal, we performed attenuation experiments for three different 3D printing materials in this work (Tungsten infused Acrylonitrile Butadiene Styrene (ABS), Iron infused Polylactic Acid (PLA), and Stainless Steel (SS) infused PLA), by observing gamma transmission through blocks made of each material as a function of thickness. A Cobalt-57 (Co-57; 122 keV) source was chosen because of its relevance to nuclear medicine. Tungsten infused ABS showed the highest measured attenuation coefficient out of the three candidates at this energy (1.452 cm²/g). We further designed and 3D printed an imaging phantom to aid in characterizing the spatial resolution of novel gamma imaging systems at the above energy. The phantom design contains multiple line pairs (lp) located in four quadrants ranging from 1.2 mm - 8 mm. The 3D printed phantom was imaged by a molecular breast tomosynthesis (MBT) gamma camera. Assessment of the intrinsic spatial resolution of the MBT camera showed that our 3D printed phantom might be a viable option for routine spatial resolution quality control (QC) of gamma cameras used in clinical practice.

3D printing of radioactive phantoms for nuclear medicine imaging

Background

For multicenter clinical studies, PET/CT and SPECT/CT scanners need to be validated to ensure comparability between various scanner types and brands. This validation is usually performed using hollow phantoms filled with radioactive liquids. In recent years, 3D printing technology has gained increasing popularity for manufacturing of phantoms, as it is cost-efficient and allows preparation of phantoms of almost any shape. So far, however, direct 3D printing with radioactive building materials has not yet been reported. The aim of this work was to develop a procedure for preparation of ^99mTc-containing building materials and demonstrate successful application of this material for 3D printing of several test objects.

Method

The desired activity of a [^99mTc]pertechnetate solution eluted from a ⁹⁹Mo/^99mTc-generator was added to the liquid 3D building material, followed by a minute amount of trioctylphosphine. The resulting two-phase mixture was thoroughly mixed. Following separation of the phases and chemical removal of traces of water, the radioactive building material was diluted with the required volume of non-radioactive building material and directly used for 3D printing.

Results

Using our optimized extraction protocol with trioctylphosphine as complex-forming phase transfer agent, technetium-99m was efficiently transferred from the aqueous ⁹⁹Mo/^99mTc-generator eluate into the organic liquid resin monomer. The observed radioactivity concentration ratio between the organic phase and the water phase was > 2000:1. The radioactivity was homogeneously distributed in the liquid resin monomer. We did not note differences in the 3D printing behavior of the radiolabeled and the unlabeled organic liquid resin monomers. Radio-TLC and SPECT studies showed homogenous 2D and 3D distribution of radioactivity throughout the printed phantoms. The radioactivity was stably bound in the resin, apart from a small amount of surface-extractable radioactivity under harsh conditions (ethanol at 50 °C).

Conclusions

3D printing of radioactive phantoms using ^99mTc-containing building materials is feasible. Compared to the classical fillable phantoms, 3D printing with radioactive building materials allows manufacturing of phantoms without cold walls and in almost any shape. Related procedures with longer-lived radionuclides will enable production of phantoms for scanner validation and quality control.

Physical imaging phantoms for simulation of tumor heterogeneity in PET, CT, and MRI: An overview of existing designs

BACKGROUND

In oncology, lesion characterization is essential for tumor grading, treatment planning, and follow-up of cancer patients. Hybrid imaging systems, such as Single Photon Emission Computed Tomography (SPECT)/CT, Positron Emission Tomography (PET)/CT, or PET/magnetic resonance imaging (MRI), play an essential role for the noninvasive quantification of tumor characteristics. However, most of the existing approaches are challenged by intra- and intertumor heterogeneity. Novel quantitative imaging parameters that can be derived from textural feature analysis (as part of radiomics) are promising complements for improved characterization of tumor heterogeneity, thus, supporting clinically relevant implementations of personalized medicine concepts. Nevertheless, establishing new quantitative parameters for tumor characterization requires the use of standardized imaging objects to test the reliability of results prior to their implementation in patient studies.

METHODS

In this review, we summarize existing reports on heterogeneous phantoms with a focus on simulating tumor heterogeneity. We discuss the techniques, materials, advantages, and limitations of the existing phantoms for PET, CT, and MR imaging modalities.

CONCLUSIONS

Finally, we outline the future directions and requirements for the design of cross modality imaging phantoms.

A Systematic Review on 3D-Printed Imaging and Dosimetry Phantoms in Radiation Therapy

INTRODUCTION

Additive manufacturing or 3-dimensional printing has become a widespread technology with many applications in medicine. We have conducted a systematic review of its application in radiation oncology with a particular emphasis on the creation of phantoms for image quality assessment and radiation dosimetry. Traditionally used phantoms for quality assurance in radiotherapy are often constraint by simplified geometry and homogenous nature to perform imaging analysis or pretreatment dosimetric verification. Such phantoms are limited due to their ability in only representing the average human body, not only in proportion and radiation properties but also do not accommodate pathological features. These limiting factors restrict the patient-specific quality assurance process to verify image-guided positioning accuracy and/or dose accuracy in "water-like" condition.

METHODS AND RESULTS

English speaking manuscripts published since 2008 were searched in 5 databases (Google Scholar, Scopus, PubMed, IEEE Xplore, and Web of Science). A significant increase in publications over the 10 years was observed with imaging and dosimetry phantoms about the same total number (52 vs 50). Key features of additive manufacturing are the customization with creation of realistic pathology as well as the ability to vary density and as such contrast. Commonly used printing materials, such as polylactic acid, acrylonitrile butadiene styrene, high-impact polystyrene and many more, are utilized to achieve a wide range of achievable X-ray attenuation values from -1000 HU to 500 HU and higher. Not surprisingly, multimaterial printing using the polymer jetting technology is emerging as an important printing process with its ability to create heterogeneous phantoms for dosimetry in radiotherapy.

CONCLUSION

Given the flexibility and increasing availability and low cost of additive manufacturing, it can be expected that its applications for radiation medicine will continue to increase.

3D printed testing aids for radiographic quality control

Quality control testing of radiographic and fluoroscopic imaging systems requires positioning of test objects in the x‐ray beam in a precise and repeatable fashion. In this work we present several three‐dimensional (3D) printed testing aids that improve efficiency, accuracy, and repeatability of quality control testing. We also present a new device for determining the location of the perpendicular ray in radiographic systems. These devices were designed in an open source software program (OpenScad, http://www.openscad.org) and 3D models were saved in .stl format for printing. The models were printed on either a MakerBot Replicator 2 or Replicator Z18 printer (MakerBot Industries, LLC, Brooklyn, NY). The testing aids were printed using polylactic acid (PLA) filament. To investigate the radiographic characteristics of the PLA used, test articles were printed and used to measure the half‐value layer (HVL) thicknesses in mm of PLA and half‐value densities (HVD) in g/cm² of PLA for two different colors and over a wide range of radiographic beam qualities, using a portable fluoroscopic c‐arm system. HVL thicknesses of clear PLA ranged from approximately 20 mm at 50 kV nominal tube voltage to 27 mm at 120 kV nominal tube voltage. For green PLA, the HVL thickness was 19 mm at 50 kV tube voltage and 25.7 mm at 120 kV tube voltage. The HVD of clear PLA ranged from 2.37 g/cm² at 50 kV nominal tube voltage to 3.19 g/cm² at 120 kV nominal tube voltage. For green PLA, the HVD was 2.35 g/cm² at 50 kV tube voltage and 3.17 g/cm² at 120 kV tube voltage. The cost of the devices range from under $2 to approximately $20 in materials. The files used to create the models are freely available at https://github.com/Upstate3DLab/3D-Printed-Radiographic-Test-Tools.

Applications of 3D printing in small animal magnetic resonance imaging

Three-dimensional (3D) printing has significantly impacted the quality, efficiency, and reproducibility of preclinical magnetic resonance imaging. It has vastly expanded the ability to produce MR-compatible parts that readily permit customization of animal handling, achieve consistent positioning of anatomy and RF coils promptly, and accelerate throughput. It permits the rapid and cost-effective creation of parts customized to a specific imaging study, animal species, animal weight, or even one unique animal, not routinely used in preclinical research. We illustrate the power of this technology by describing five preclinical studies and specific solutions enabled by different 3D printing processes and materials. We describe fixtures, assemblies, and devices that were created to ensure the safety of anesthetized lemurs during an MR examination of their brain or to facilitate localized, contrast-enhanced measurements of white blood cell concentration in a mouse model of pancreatitis. We illustrate expansive use of 3D printing to build a customized birdcage coil and components of a ventilator to enable imaging of pulmonary gas exchange in rats using hyperpolarizedXe 129 . Finally, we present applications of 3D printing to create high-quality, dual RF coils to accelerate brain connectivity mapping in mouse brain specimens and to increase the throughput of brain tumor examinations in a mouse model of pituitary adenoma.

The subresolution DaTSCAN phantom: a cost-effective, flexible alternative to traditional phantom technology

Supplemental Digital Content is available in the text.

The Alderson striatal phantom is frequently used to assess ¹²³I-FP-CIT (Ioflupane) image quality and to test semi-quantification software. However, its design is associated with a number of limitations, in particular: unrealistic image appearances and inflexibility. A new physical phantom approach is proposed on the basis of subresolution phantom technology. The design incorporates thin slabs of attenuating material generated through additive manufacturing, and paper sheets with radioactive ink patterns printed on their surface, created with a conventional inkjet printer. The paper sheets and attenuating slabs are interleaved before scanning. Use of thin layers ensures that they cannot be individually resolved on reconstructed images. An investigation was carried out to demonstrate the performance of such a phantom in producing simplified ¹²³I-FP-CIT uptake patterns. Single photon emission computed tomography imaging was carried out on an assembled phantom designed to mimic a healthy patient. Striatal binding ratio results and linear striatal dimensions were calculated from the reconstructed data and compared with that of 22 clinical patients without evidence of Parkinsonian syndrome, determined from clinical follow-up. Striatal binding ratio results for the fully assembled phantom were: 3.1, 3.3, 2.9 and 2.6 for the right caudate, left caudate, right putamen and right caudate, respectively. All were within two SDs of results derived from a cohort of clinical patients. Medial–lateral and anterior–posterior dimensions of the simulated striata were also within the range of values seen in clinical data. This work provides the foundation for the generation of a range of more clinically realistic, physical phantoms.

Recent advances on the development of phantoms using 3D printing for imaging with CT, MRI, PET, SPECT, and ultrasound

PURPOSE

Printing technology, capable of producing three-dimensional (3D) objects, has evolved in recent years and provides potential for developing reproducible and sophisticated physical phantoms. 3D printing technology can help rapidly develop relatively low cost phantoms with appropriate complexities, which are useful in imaging or dosimetry measurements. The need for more realistic phantoms is emerging since imaging systems are now capable of acquiring multimodal and multiparametric data. This review addresses three main questions about the 3D printers currently in use, and their produced materials. The first question investigates whether the resolution of 3D printers is sufficient for existing imaging technologies. The second question explores if the materials of 3D-printed phantoms can produce realistic images representing various tissues and organs as taken by different imaging modalities such as computer tomography (CT), positron emission tomography (PET), single-photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), ultrasound (US), and mammography. The emergence of multimodal imaging increases the need for phantoms that can be scanned using different imaging modalities. The third question probes the feasibility and easiness of "printing" radioactive or nonradioactive solutions during the printing process.

METHODS

A systematic review of medical imaging studies published after January 2013 is performed using strict inclusion criteria. The databases used were Scopus and Web of Knowledge with specific search terms. In total, 139 papers were identified; however, only 50 were classified as relevant for this paper. In this review, following an appropriate introduction and literature research strategy, all 50 articles are presented in detail. A summary of tables and example figures of the most recent advances in 3D printing for the purposes of phantoms across different imaging modalities are provided.

RESULTS

All 50 studies printed and scanned phantoms in either CT, PET, SPECT, mammography, MRI, and US-or a combination of those modalities. According to the literature, different parameters were evaluated depending on the imaging modality used. Almost all papers evaluated more than two parameters, with the most common being Hounsfield units, density, attenuation and speed of sound.

CONCLUSIONS

The development of this field is rapidly evolving and becoming more refined. There is potential to reach the ultimate goal of using 3D phantoms to get feedback on imaging scanners and reconstruction algorithms more regularly. Although the development of imaging phantoms is evident, there are still some limitations to address: One of which is printing accuracy, due to the printer properties. Another limitation is the materials available to print: There are not enough materials to mimic all the tissue properties. For example, one material can mimic one property-such as the density of real tissue-but not any other property, like speed of sound or attenuation.