Name: Vacuuming and Pouring of the Phantom Material Samples into the Mold
Uploaded: 2023-06-16T12:20:00
Duration: 1 min 38 s
Description: Watch this Scientific Journal Video about Vacuuming and Pouring of the Phantom Material Samples into the Mold at JoVE.com

vacuuming and pouring of the phantom material samples into the mold

Fabrication of a Stable, Biologically Relevant Phantom Material

Establishing the precision and accuracy of novel optical imaging biomarkers through technical validation1,2 is paramount to ensuring their successful implementation in clinical practice. To achieve this, technical validation studies frequently employ durable physical phantoms, which facilitate inter-instrument performance assessment and routine quality control. For widespread use of a phantom material in research and clinical translation, a simple, highly reproducible fabrication protocol is required. An ideal biophotonic phantom material should include the following properties3: (1) independently tunable properties within biologically relevant ranges; (2) mechanical robustness; (3) long-term stability; (4) flexibility in geometry and architecture; (5) safe handling; (6) widely available ingredients that can be purchased from standard scientific suppliers; and (7) low cost. At present, biophotonic applications lack a standardized protocol for a widely accepted phantom material that fulfils the outlined requirements and also includes tunable acoustic properties for hybrid applications, such as photoacoustic imaging (PAI).
Biologically relevant phantom materials targeted for combined optical and acoustic applications include hydrogels4,5, polyvinyl alcohol (PVA)6,7,8,9, and polyvinyl chloride plastisol (PVCP)10,11,12,13,14,15,16. However, these materials are characterized by certain limitations that restrict their application as a stable phantom material. Hydrogels, for example, are prone to dehydration, mechanical damage, and bacterial ingrowth, limiting their shelf life17,18,19. The addition of chemicals can increase the longevity, but common preservatives, such as formaldehyde20 or benzalkonium chloride21, are hazardous and require cautionary measures during handling. Additionally, targets containing water-soluble dyes can diffuse within the base material if not encapsulated. PVA cryogels are characterized by a higher longevity and structural robustness, but their preparation process involves long freeze-thaw cycles22. This can limit the independent tunability of acoustic and optical parameters23 and-if slightly varied-can lead to inhomogeneities6, thereby compromising reproducibility. Moreover, the diffusion of dyes from inclusions has been observed after 1 year13. PVCP has a complex fabrication process that includes high temperatures of up to 180-220 &#176;C13,14,24,25. PVCP also suffers from a lack of a supply chain with scientific suppliers26 and can contain plasticizers based on phthalates, which may cause reproductive and developmental harm27, making them controlled substances in some countries.
Copolymer-in-oil compositions, such as gel wax28,29,30,31 or blends based on thermoplastic styrenic elastomers32,33,34,35,36, exhibit good longitudinal stability and feature tissue-like acoustic and optical properties31,35,36,37, thereby having high potential as a durable phantom candidate in multimodal applications. Additionally, this class of material is cost-effective, non-water absorbing, non-toxic, and biologically inert35,38. The speed of sound c(f) and acoustic attenuation coefficient &#945;(f) can be tuned in a biologically relevant range (Table 1) by variation of the polymer concentration33,35,39, whilst optical absorption &#181;a(&#955;) and reduced scattering &#181;s&#39;(&#955;) coefficients can be primarily varied by the addition of oil-soluble dyes or titanium dioxide (TiO2)39, respectively.
Here, a simple, easy-to-follow protocol is presented for the creation of durable copolymer-in-oil phantoms suitable for use in optical, ultrasound, or photoacoustic device calibration. All ingredients have defined Chemical Abstract Service (CAS) numbers and are readily available from standard scientific suppliers. Potential difficulties in the fabrication procedure are highlighted and ways to overcome them are presented. Whilst the protocol allows the fabrication of materials with a range of acoustic and optical properties, the presented protocol yields a material with a speed of sound of ~1,481 m&#183;s-1, aligning with the speed of sound of water at room temperature (20 &#176;C)40. This value was selected as a neutral standard for representing the wide range of existing tissue properties (Table 1), allowing the establishment of a consistent and reliable reference point for comparison. By providing this detailed protocol, we aim to broaden the uptake and manufacturing reproducibility of this promising phantom material type, thereby facilitating biophotonic, acoustic, and photoacoustic validation studies and supporting routine quality control in preclinical and clinical imaging applications.

Author Spotlight: A Stable Phantom Material for Optical and Acoustic Imaging  

A Stable Phantom Material for Optical and Acoustic Imaging

Our research is focused on developing new imaging modalities for early cancer detection. In this study in particular, we focused on developing a stable test object, or also called phantom, to validate imaging modalities based on light and/or sound. Different resources have been proposed to develop tissue mimicking materials in the acoustic optical regimen.For example, PVA, hydrogels, polyurethane, or PVCP. This study focuses on a promising new material type based on core polyman oil composition, which overcomes many of the tunability and stability challenges. The field of biomedical optics covers a wide range of imaging modalities that can be applied to some of the key challenges facing medicine today.Many of the all optical imaging modalities are relatively depth limited, but we can also couple light with sound through the photoacoustic effect to take advantage of some of the depth penetration benefits of ultrasound. Photoacoustic imaging has shown promise in a wide range of clinical trials, from imaging inflammation to cancer diagnosis. However, quantitative performance assessment remains challenging due to a lack of available phantom materials that can accurately mimic the optical and acoustic properties of tissue and remain stable over time.Many new optical and photoacoustic imaging systems are being developed, but we lack a standardized reference phantom to validate those systems and to compare their performance. Our material is a promising candidate to fill this gap and support further development and translation of these exciting new techniques into the clinic. In the future, we would like to focus on creating more anatomically realistic phantom designs and architectures that are suitable to evaluate the performance of different system configurations.For example, microscopic, mesoscopic and macroscopic systems, which have different geometries and spatial resolutions.

Watch this Scientific Journal Video about Vacuuming and Pouring of the Phantom Material Samples into the Mold at JoVE.com

Vacuuming and Pouring of the Phantom Material Samples into the Mold  

To begin, place the hot beaker filled with the uniformly mixed, smooth, and homogenous phantom material solution into the vacuum chamber. Vacuum the samples on the lowest vacuum setting for two to three minutes. Remove any air bubbles accumulated on the surface using a metal spatula.If air bubbles persist, reheat the mixture and repeat the vacuuming step until complete removal of the air bubbles. In the case of complex shaped molds, coat the mold with a thin layer of oil before pouring. Wear heat resistant gloves, use adequate protective equipment, and carefully pour the solution into a suitable mold without introducing any air bubbles.Once poured, quickly remove any air bubbles from the top of the samples using a metal spatula. Allow the solution to be set overnight at room temperature. Three representative phantom material designs were created for photoacoustic imaging targeted for different system designs with different optical illumination and acoustic detection geometries.The successful procedure resulted in smooth and homogenous phantom material preparation without trapped air bubbles, impurities, or artifacts. Insufficient removal of air bubbles and inhomogenous mixing of base components resulted in inhomogeneities in the resulting photoacoustic image.

vacuuming-and-pouring-of-the-phantom-material-samples-into-the-mold

Research

JoVE Journal

Medicine

Establishing tissue-mimicking biophotonic phantom materials that provide long-term stability are imperative to enable the comparison of biomedical imaging devices across vendors and institutions, support the development of internationally recognized standards, and assist the clinical translation of novel technologies. Here, a manufacturing process is presented that results in a stable, low-cost, tissue-mimicking copolymer-in-oil material for use in photoacoustic, optical, and ultrasound standardization efforts.
The base material consists of mineral oil and a copolymer with defined Chemical Abstract Service (CAS) numbers. The protocol presented here yields a representative material with a speed of sound c(f) = 1,481 &#177; 0.4 m&#183;s-1 at 5 MHz (corresponds to the speed of sound of water at 20 &#176;C), acoustic attenuation &#945;(f) = 6.1 &#177; 0.06 dB&#183;cm-1 at 5 MHz, optical absorption &#181;a(&#955;) = 0.05 &#177; 0.005 mm-1 at 800 nm, and optical scattering &#181;s&#39;(&#955;) = 1 &#177; 0.1 mm-1 at 800 nm. The material allows independent tuning of the acoustic and optical properties by respectively varying the polymer concentration or light scattering (titanium dioxide) and absorbing agents (oil-soluble dye). The fabrication of different phantom designs is displayed and the homogeneity of the resulting test objects is confirmed using photoacoustic imaging.
Due to its facile, repeatable fabrication process and durability, as well as its biologically relevant properties, the material recipe has high promise in multimodal acoustic-optical standardization initiatives.

This protocol describes the fabrication of a stable, biologically relevant phantom material for optical and acoustic biomedical imaging applications, featuring independently tunable acoustic and optical properties.

Establishing tissue-mimicking biophotonic phantom materials that provide long-term stability are imperative to enable the comparison of biomedical imaging ...

Preparation and Heating of the Phantom Material Mixture 

Begin the sonication of 0.15 grams of titanium dioxide and one milliliter of the absorber dye stock solution in 100 milliliters of mineral oil at 90 degrees Celsius for 60 minutes. Let all the components dissolve completely. Use suitable glassware and silicone oil to create an oil bath and carefully secure it on the hot plate.For uniform heat distribution, place a magnetic stir bar inside the oil bath. Turn on the hot plate, set the heating temperature to 160 degrees Celsius and adjust the stir to 50 RPM. Next, weigh 25.14 grams of SEBS and 6.70 grams of LDPE and transfer them into the glass beaker containing the sonicated mineral oil.Place a stir bar in the beaker and transfer it to the center of the oil bath to heat the measured components. When the added polymers float over the mineral oil, manually stir the mineral oil solution using a metal spatula and distribute the floating polymer inside the mineral oil. Leave the mixture at 160 degrees Celsius for 1.5 hours until all the polymer is dissolved and the solution appears uniformly mixed, smooth, and homogenous.