Damian Sendler: New ways to fabricating structures capable of duplicating complex tissue structure and function have been spurred by rapid advancements in tissue engineering and regenerative medicine (TERM). In order to heal and/or regenerate, and restore function in injured tissues, 1 TERM structures are intended to integrate with the local host environment. 2 Acellular scaffolds, cell-only, scaffold-free designs, and hybrid cellularized scaffolds are the three basic types of TERM models, however the particular method differs from technique to technique.
Damian Jacob Sendler: These novel 3D bioprinting fabrication methods, which enable for precise creation of tissue informed structures including diverse cell types, biomaterials,6,7 and growth factors, have fueled advancements in tissue engineering and regeneration (TERM). 8 Numerous preclinical uses of the term TERM use animal studies to illustrate possible human translation and clinical viability. A phase I FDA-approved clinical study cannot begin unless repeated testing in one animal rather than different cohorts has been adopted to progress these pre-clinical research from bench to bedside. To ensure that TERM constructs work as intended in humans, rigorous analytic methods must be created and put to use in conjunction with pre-clinical investigations. These methods and approaches are fundamentally different.
Damian Sendler
Validation of TERM constructions’ biological performance and compatibility is presently done using histology as the gold standard. Histology, on the other hand, is a highly invasive, semi-quantitative, tissue-destructive procedure that does not allow for longitudinal analysis, necessitates the use of multiple animals, and has the potential to alter a construct’s structure during the processing stage. It also does not represent the entire tissue volume and does not directly test function. As a result, quantitative imaging tools are needed to monitor how TERM constructions interact with and influence tissue physiology in vivo, over time, and in 3D. Understanding the medical imaging modalities and procedures that may be utilized to assess the efficacy and function of implantable constructions is essential for the fast progress and translation of TERM structures. Tissue engineering, physiology, and radiology must continue to collaborate in order to produce both TERM structures and the noninvasive imaging tools needed to assess them in vivo.
Reviewing current advances in TERM imaging is a primary goal for this article. It identifies and discusses particular outcome measures of relevance that are crucial in the assessment of these constructs, as well as imaging methods and procedures that may be employed to do so. An important goal of this study is facilitating communication between tissue engineers and the imaging community by providing an overview of imaging techniques that may be used to evaluate TERM constructions in vivo for common outcomes of interest. There are a variety of tools and approaches that may be used with ultrasound, X-ray-based imaging, and magnetic resonance imaging (MRI) that will be discussed in this paper (MRI).
For quantitative imaging of TERM structures, see: TERM imaging considerations
The first decisions for selecting an imaging modality are based on certain outcome metrics that meet FDA guidelines for TERM construct assessment. The FDA is more likely to approve outcome measures from phase I trials if they fulfill FDA standards in pre-clinical investigations. When it comes to the TERM construct, the main outcomes of interest can be categorized as: (1) the state of the construct (i.e. size, degradation, mechanical properties, presence of key cells or materials), (2) the biocompatibility and biointegration of the construct (i.e. perfusion, inflammation, fibrous encapsulation, cell viability, etc.), and (3) its function (i.e. stimulation of de novo tissue production, microstructural organization, mechanical functionality, biological functionality)
For this reason, it is critical to keep track of these outcomes on a regular basis, since they might determine which imaging modality is most appropriate. Immediately after implantation, biocompatibility is a critical success indicator, while stimulation of de novo tissue creation is necessary at longer timepoints. To reduce the need for animals and people in early stage research, as well as to increase statistical power in a study, serial assessment is used. It is possible that no one kind of imaging modality will be able to capture all relevant information, even though most imaging modalities are multi-parametric.
Damian Jacob Markiewicz Sendler: In order to compare the advantages and disadvantages of different imaging techniques, it is necessary to first identify the outcome metrics that matter. Clinically relevant approaches are available for each imaging modality, often providing simple structural information about a TERM construct that can be used to conduct volumetric or shape-based evaluations. Because they are more sensitive to the outcomes described above, research-oriented imaging methods often are not standard on every machine or need acquisition improvement. During a typical imaging session (minutes to an hour), this optimization procedure aims to increase signal at a physiologically appropriate voxel (volumetric pixel) size.
Damian Jacob Sendler
It is based on the creation and reception of sound waves that penetrate a material and are partly reflected at interfaces between tissues of varying densities in order to generate and receive images (i.e., acoustic impedance). Using a probe that can transmit and receive sound waves, ultrasound imaging employs a TERM device near a tissue (usually skin) and needs a qualified operator to see the underlying anatomy. Gray-scale contrast in the reconstructed picture is caused by tissues with varied impedance. Ultrasound’s spatial resolution may be as low as 0.1 mm on average, and it becomes better as the frequency of the ultrasound rises, but the depth of penetration is reduced.
A further advantage of ultrasound imaging is its near-real-time capability (24–120 Hz), although this technology has trouble penetrating strong materials like bone. Transvaginal, transesophageal, and intravenous ultrasonography are all examples of more invasive ultrasound probe designs that may be used in addition to the more common non-invasive ultrasound imaging. Imaging using ultrasound may be done for as little as $2000, making it the most affordable and portable imaging modality we have examined so far. 9 Ultrasound, in comparison to X-ray and MRI, has weak contrast and makes it difficult to distinguish between neighboring tissues because of the identical acoustic impedance across tissue types. Ng et al.10 provide a thorough introduction to ultrasonic imaging theory and practice.
In the case of a Doppler shift, the frequency of a sound wave changes as it is reflected off of a moving object. Macrovascular blood flow measurements may also be made using Doppler shifts. Using ultrasonic imaging to monitor the performance of tissue-engineered vascular grafts has increased in popularity. 18,19 In contrast to normal ultrasound imaging, which can only determine the graft’s lumen and wall dimensions, Doppler imaging can measure the flow rate in cm s1. Quantitative measures of blood flow may be used to evaluate the patency of a tissue-engineered graft,20–22 identify the collateral vasculature surrounding tissue-engineered vessels,11 detect possible mechanical flaws in grafts,23 and indicate graft failure.20 a dozen and a thirteen (Fig. 2).
Doppler imaging, on the other hand, is impossible to quantify blood flow directly in microvasculature (less than 100 m in diameter) because of resolution limits. Some investigations use microbubbles as an exogenous contrast agent to augment the blood flow signal in order to improve sensitivity to blood flow in the microvasculature. 24,25 In TERM constructions, neovascularization and perfusion may be seen and quantified using microbubble-enhanced US imaging, even if individual capillaries’ blood flow cannot be determined. The incorporation of biomaterials into the microbubble structure allows for therapeutic applications such as the administration of growth factors to specific regions of the body. 26
Damien Sendler: ultrasonic imaging is used to obtain mechanical characteristics from tissues or TERM constructs in response to an external mechanical stress. Both strain-based elastography and shear wave ultrasonic elastography employ force from the ultrasound probe to create waves perpendicular to the ultrasound beam, resulting in transitory displacements, and these are the two most common forms of elastography on commercial ultrasound equipment. 27,28 Degradation is directly linked to mechanical properties, which are an important outcome metric for TERM constructions, hence ultrasonic elastography may be a useful method for swiftly monitoring the status of soft materials (i.e. hydrogels).
The mechanical characteristics of biodegradable scaffolds may be monitored using either strain-based or shear-wave ultrasonic elastography29–32 (Fig. 2). As part of the evaluation of the performance of an in-vivo TERT, elastography may be utilized to determine whether or not it is at danger of failure. 33 Elastography has also been shown to be sensitive to TERM construct modification owing to tissue ingrowth, according to certain research.
Dr. Damian Jacob Sendler and his media team provided the content for this article.