Medical imaging of tissue engineering and regenerative medicine constructs

Abstract

Advancement of tissue engineering and regenerative medicine (TERM) strategies to replicate tissue structure and function has led to the need for noninvasive assessment of key outcome measures of a construct's state, biocompatibility, and function. Histology based approaches are traditionally used in pre-clinical animal experiments, but are not always feasible or practical if a TERM construct is going to be tested for human use. In order to transition these therapies from benchtop to bedside, rigorously validated imaging techniques must be utilized that are sensitive to key outcome measures that fulfill the FDA standards for TERM construct evaluation. This review discusses key outcome measures for TERM constructs and various clinical- and research-based imaging techniques that can be used to assess them. Potential applications and limitations of these techniques are discussed, as well as resources for the processing, analysis, and interpretation of biomedical images.

Fig. 1

Schematic depicting key outcome measures for assessing tissue engineering and regenerative medicine constructs as well as the specific imaging techniques that can be used to assess them (italic). GAGs – glycosaminoglycans. RBCs – Red blood cells.

Fig. 2

Ultrasound: B-mode structural imaging (left) of a tissue engineered vascular graft can be used to measure the size of the lumen and assess patency of the graft at 3 months (top) and 6 months (bottom) after implantation. Doppler shift imaging can be used to measure blood flow through the tissue engineered vascular graft (middle column). Figure reproduced with permission from Springer Nature (2017). Strain maps assessed with elastography (right) of a subcutaneously implanted tissue engineered scaffold before (top) and after (bottom) in vivo degradation, demonstrating that ultrasound elastography can be used to assess changes in mechanical properties of a scaffold associated with degradation. The dashed orange boxes represent the boundaries of the scaffold, the color overlay represents the in vivo strain map. Figure reproduced with permission from Elsevier (2008).

Fig. 3

Computed tomography: Macroscopic view (top row) of TERM treated (left, middle) and untreated (right) osteochondral defects. μCT was used to render the osteochondral defect and subchondral bone to assess state of the TERM constructs and de novo tissue production. Figure reproduced with permission from Springer Nature (2018). (Right) A cationic contrast agent that is sensitive to glycosaminoglycan distribution in degenerated and normal cartilage. The contrast agent is attracted to the strong negative charge of glycosaminoglycans and increases radiopacity regions with high glycosaminoglycan concentration. This demonstrates how contrast agents can be used to assess the presence of biomaterials. Figure reproduced with permission from Elsevier (2018).

Fig. 4

Structural MRI: T1-weighted (A) and T2-weighted (B) images of saline filled breast implants. Arrows highlight a thick, low signal fibrous capsule around the implant has formed. These images demonstrate how structural MRI can be used to visualize different features of TERM constructs (i.e. water rich regions) and its interaction with nearby tissues. Figure reproduced with permission from Springer Nature (2016). UTE: axial MRIs of a Achilles tendon repair. The internal structures of the Achilles tendon are not visible in a proton density weighted MRI (C; arrow), but are visible in a UTE pulse sequence (D; arrows). This demonstrates how UTE can be used to visualize fibrous structures with short echo times. Figure reproduced with permission from John Wiley and Sons (2015). CEST: Time course of a hydrogel injected into a mouse brain striatum over the course of 42 days (E). Serial T2-weighted imaging was used to identify the hydrogel (top row; arrow) which is easily identified due to its high water content. Relatively little change in the hydrogel size is observed over 42 days, even though the hydrogel is degrading, due to the large amount of unbound water in the hydrogel. Using CEST MRI, a continuous decrease in hydrogel signal was observed, consistent with hydrogel degeneration in vivo. This demonstrates how CEST can be used to monitor the presence of biomaterials and degradation of the construct better than routine structural imaging. Figure reproduced with permission from John Wiley and Sons (2019).

Fig. 5

Diffusion tensor MRI: Axial (A) and coronal (B) fractional anisotropy maps overlaid on structural images of the lumbar paraspinal muscles, demonstrating the variance in tissue microstructural properties throughout a normal muscle. These maps can be used to assess microstructural organization of a TERM construct. In a coronal structural MRI scan (C) it is difficult to assess the 3D orientation of the paraspinal muscle fibers or assess fiber length. However, using tractography the orientation and length of the paraspinal muscles fibers can be measured. This technique can be used to assess how well a TERM construct is aligned and integrating with local tissue. Figure reproduced with permission from John Wiley and Sons (2020).

Fig. 6

3D printing can be used to validate imaging techniques (Top). Histology of normal muscle (a) was used to inform the design of a phantom (b) with known geometric properties, which could be 3D printed (c) and scanned using MRI. This approach was used to relate measurements made using diffusion tensor MRI to known microstructural properties of the phantom. Biomedical imaging can also be used to inform the design of TERM constructs. A light based 3D printer (d) was used to print a scaffold (e) with x–y geometry informed by the axial distribution of white matter and grey matter in the spinal chord. Structural MRI of a complete spinal cord injury (f) can be used to inform the 3D geometry of the TERM scaffold (g), which can be precisely printed (h) to the precise dimensions of a patient’s lesion.