Elsevier

Biomaterials

Volume 34, Issue 28, September 2013, Pages 6615-6630
Biomaterials

Review
Imaging challenges in biomaterials and tissue engineering

https://doi.org/10.1016/j.biomaterials.2013.05.033Get rights and content

Abstract

Biomaterials are employed in the fields of tissue engineering and regenerative medicine (TERM) in order to enhance the regeneration or replacement of tissue function and/or structure. The unique environments resulting from the presence of biomaterials, cells, and tissues result in distinct challenges in regards to monitoring and assessing the results of these interventions. Imaging technologies for three-dimensional (3D) analysis have been identified as a strategic priority in TERM research. Traditionally, histological and immunohistochemical techniques have been used to evaluate engineered tissues. However, these methods do not allow for an accurate volume assessment, are invasive, and do not provide information on functional status. Imaging techniques are needed that enable non-destructive, longitudinal, quantitative, and three-dimensional analysis of TERM strategies. This review focuses on evaluating the application of available imaging modalities for assessment of biomaterials and tissue in TERM applications. Included is a discussion of limitations of these techniques and identification of areas for further development.

Introduction

Researchers in the fields of tissue engineering and regenerative medicine (TERM) are investigating new techniques for the regeneration, replacement and repair of lost or damaged tissues. These approaches are designed to restore tissue function and/or structure. While the specifics of a given strategy may vary, an approach typically involves some combination of biomaterials, cells, and inducible factors that are expected to generate tissues in bioreactors and/or following implantation in vivo. The unique environments resulting from interfaces between biomaterials, cells, and tissues found in TERM applications result in distinct challenges in regards to monitoring and assessing outcomes.

Imaging technologies for three-dimensional (3D) analysis have been identified as a strategic priority in TERM research and are required for acceleration of progress in the field [1]. Traditionally, histological and immunohistochemical techniques have been used to evaluate engineered tissues. However, these methods do not allow for an accurate volume assessment, are destructive, and do not provide information on functional status. There is a great need for the development and evaluation of 3D imaging tools that enable quantitative analysis of engineered tissues. Many imaging techniques exist, but when applied using standard methods may not provide the information required for assessment. The need for improved methods is well-known in the field, and researchers have begun to address some of the challenges. However, there has been little presentation in the literature of neither the unique aspects of this challenge nor discussion of the advantages/disadvantages of existing methods and those under development. In this paper we seek to present these issues as a means of guidance and for promoting discussion amongst researchers in the fields of biomaterials and TERM.

The primary imaging challenges in TERM depend, in part, on the therapeutic approach under investigation. While there is significant variability within a given approach, they can be placed in four generic categories as related to imaging needs (Fig. 1): 1) Regeneration that is based exclusively on the transplantation or injection of cells. The cells can be isolated from a variety of sources, can be a combination of multiple cell types, and may be modified in some way (e.g. gene transfer). Regardless, the cells are expected to stimulate growth and functional improvement via release of soluble signals, production of extracellular matrix (ECM), and/or differentiation and incorporation into new tissues. 2) Cell-free approaches where biomaterials are implanted and induce cell recruitment, proliferation, or healing either directly or through the release of therapeutic factors. 3) Combined techniques where cells and biomaterials are organized in a defined way and then implanted as a single unit. 4) Bioreactor-based approaches where tissue formation is initiated and optimized in controlled in vitro settings prior to implantation in the body.

Generic imaging needs can be identified that are applicable to one or more of the approaches. For techniques involving biomaterials, the ability to quantitatively evaluate the 3D structure of scaffolds used is important prior to application, in bioreactors and within tissue. While imaging materials for characterization prior to application is relatively well-developed, it is often difficult to visualize the 3D structure of a degradable scaffold as it interacts with cells and tissue in TERM applications. For methods based on cell delivery, it may be important to track the location, differentiation and function of these cells within the engineered tissues and potentially in ectopic regions. Both tracking of cells and biomaterials is important, but the ultimate imaging goal of any application is examination of the structure and function of the tissue response generated following application of these therapies. There are several imaging modalities that have been investigated for specific TERM applications. This review will focus on available imaging modalities summarizing how they have been employed to address imaging challenges in TERM as well as discussing their limitations and potential for further development.

All forms of imaging require interactions of electromagnetic or mechanical energy with an object. Images are generated by measuring changes in the energy due to absorption, refraction, or scatter resulting from these interactions. The imaging depth, contrast, and spatial resolution achieved by a given imaging modality are largely based on the type and frequency of energy employed. Imaging depths range from less than a hundred microns to the entire body, while spatial resolution ranges from submicron to a few millimeters (Fig. 2B). This review will focus on six wavelength, or equivalently, frequency, ranges of electromagnetic or acoustic radiation and the imaging modalities that use them, namely: Ultrasound (US), Photoacoustic Microscopy (PAM), Magnetic Resonance Imaging (MRI), Optical Imaging, X-ray Imaging, and Nuclear Imaging.

Section snippets

Ultrasound

Conventional ultrasound (US) imaging (1–50 MHz) utilizes acoustic waves produced by a transducer that travel through the medium to a specific focusing depth. The transducer not only generates energy but also acts as a receiver of the returning signal. Contrast results from differences in ultrasonic reflectivity, and an image is generated based on the time required for the wave to echo back as well as the strength of the signal received. This process can be repeated at several depths in order to

Optical imaging

Optical imaging is a general term used to describe systems that measure the interaction of infrared (300 GHz–430 THz, 700 nm–1 mm), visible (430–790 THz, 380–700 nm), or ultraviolet light (790 THz–30 PHz, 10–300 nm) with matter. Depending on the imaging system, the techniques measure scatter, absorption, or luminescence of light that is either transmitted through, or reflected out, of the sample. Properties of the light source, both wavelength and intensity, control the depth of penetration and

Photoacoustic microscopy

Photoacoustic microscopy (PAM) is a rapidly emerging hybrid modality that combines optical image contrast with US detection principles [76]. The goal of PAM is to estimate an object's spatially variant absorbed optical energy density from measurements of pressure wavefields that are induced via the photoacoustic effect. Because the optical absorption characteristics of tissue vary strongly with hemoglobin content, knowledge of the absorbed optical energy distribution can yield both structural

Magnetic resonance imaging

Image contrast in magnetic resonance imaging (MRI) is typically related to differences in the proton density in a sample. A large magnetic field, ranging from 1.5 to 11.7 T, is applied to the sample aligning the majority of the nuclei in the direction of the field. Radiofrequency pulses are applied to alter the magnetization systematically generating rotating magnetic fields that can be measured. Protons in different materials and conditions realign at different rates generating image contrast.

X-ray imaging

X-ray imaging exploits variations in the X-ray absorption, refraction, and/or scattering properties of an object to form image contrast. Most diagnostic X-ray imaging methods utilize higher energy photons (>15 KeV) that have the capability to penetrate through the entire body. Imaging in computed tomography (CT) mode allows for creation of 3D images of the sample. Spatial resolution of X-ray imaging is a function of spot size of the X-ray tube source, or more generally the X-ray beam coherence

Nuclear imaging

Nuclear Imaging is based on techniques that detect gamma rays emitted from radioactive substances. Radiopharmaceuticals are introduced into the body or tissue and areas of high uptake are detected as they decay and emit radiation. Specific radiotracers can be used to identify targeted activity. This technique has been useful in the diagnosis of disease including many cancers and hyperthyroidism as well as in detecting localized inflammation through the mapping of leukocyte distribution [64].

Conclusions

The ability to monitor and assess TERM therapies is a critical need. The imaging techniques described here have all been successfully applied to evaluate specific aspects of TERM strategies. However, the ideal technique depends on specifics of the application and research. Optical imaging and PAM/US appear better suited for in vitro monitoring with potential for certain in vivo studies. MRI, X-ray, and nuclear imaging show more promise for in vivo applications due to the spatial resolution and

Acknowledgements

Research described here has been supported in part by the Veterans Administration, the National Science Foundation (DIIS 1125412 and CBET 0854430), and the National Institute of Health (R01EB009715). The authors would like to thank Dr. Bin Jiang, Dr. Lihong Wang, and Dr. Laura Suggs for providing images for the figures.

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