Multimodal Ultrasound-Photoacoustic Imaging of Tissue Engineering Scaffolds and Blood Oxygen Saturation In and Around the Scaffolds

Abstract

Preclinical, noninvasive imaging of tissue engineering polymeric scaffold structure and/or the physiological processes such as blood oxygenation remains a challenge. In vitro or ex vivo, the widely used scaffold characterization modalities such as porosimetry, electron or optical microscopy, and X-ray microcomputed tomography have limitations or disadvantages-some are invasive or destructive, others have limited tissue penetration (few hundred micrometers) and/or show poor contrast under physiological conditions. Postmortem histological analysis, the most robust technique for the evaluation of neovascularization is obviously not appropriate for acquiring physiological or longitudinal data. In this study, we have explored the potential of ultrasound (US)-coregistered photoacoustic (PA) imaging as a noninvasive multimodal imaging modality to overcome some of the above challenges and/or provide complementary information. US-PA imaging was employed to characterize poly(lactic-co-glycolic acid) (PLGA) polymer scaffolds or single-walled carbon nanotube (SWCNT)-incorporated PLGA (SWCNT-PLGA) polymer scaffolds as well as blood oxygen saturation within and around the scaffolds. Ex vivo, PLGA and SWCNT-PLGA scaffolds were placed at 0.5, 2, and 6 mm depths in chicken breast tissues. PLGA scaffolds could be localized with US imaging, but generate no PA signal (excitation wavelengths 680 and 780 nm). SWCNT-PLGA scaffolds generated strong PA signals at both wavelengths due to the presence of the SWCNTs and could be localized with both US and PA imaging depths between 0.5-6 mm (lateral resolution = 90 μm, axial resolution = 40 μm). In vivo, PLGA and SWCNT-PLGA scaffolds were implanted in subcutaneous pockets at 2 mm depth in rats, and imaged at 7 and 14 days postsurgery. The anatomical position of both the scaffolds could be determined from the US images. Only SWCNT-PLGA scaffolds could be easily detected in the US-PA images. SWCNT-PLGA scaffolds had significant four times higher PA signal intensity compared with the surrounding tissue and PLGA scaffolds. In vivo blood oxygen saturation maps around and within the PLGA scaffolds could be obtained by PA imaging. There was no significant difference in oxygen saturation for the PLGA scaffolds at the two time points. The blood oxygen saturation maps complemented the histological analysis of neovascularization of the PLGA scaffolds.

Imaging Strategies for Tissue Engineering Applications

“…115,116 Linear ultrasound array-based PA imaging is capable of detecting a target at deeper regions than PA microscopy and thus can be applied for a rat model, having been commonly used for tissue regeneration studies. 117,118 For example, it was demonstrated that mesenchymal stem cells labeled with gold nanospheres and delivered within 96 NAM ET AL.…”

“…106,131 Therefore, fusion of ultrasonically acquired structural data and photoacoustically obtained functional and molecular data can provide synergetic benefits for a variety of applications that require noninvasive monitoring of implanted tissue-engineered constructs. 117,118 Novel dual contrast agents for combined ultrasound and PA imaging have recently been developed by indocyanine green loading to perfluorocarbon nanodroplets. 132 Also, hybrid PA imaging is possible with other optical imaging modalities, such as OCT, by sharing a photon delivery system to complement structural information.…”

Label-Free Photoacoustic Cell-Tracking In Real-Time

“…Combined ultrasound and photoacoustic images were created by overlaying photoacoustic intensities higher than a user-de ned threshold on the grayscale ultrasound images. The system supports real-time B-mode imaging with spatial resolution down to 40 µm [17].…”

1984 : on monitoring cell fate in three-dimensional polymeric scaffolds for tissue engineering applications

“…For in vitro applications, however, to date nuclear-based methods are not yet commonly applied. The lateral resolution of nuclear imaging modalities is limited, since most available devices are mainly optimized for human-scale use with a resolution of 1-2 mm [130]. To be able to achieve contrast in nuclear based imaging methods such as single-photon emission computed tomography (SPECT) and positron emission tomography (PET), radioactive agents are required.…”

“…By modulating the US detection frequency after laser irradiation, PAT enables high-resolution imaging of biological structures with strong optical absorption contrasts [212]. Although PAT and PAM are promising imaging methods for in vivo applications, to date just a few researchers have investigated the potential of these methods for the field of tissue engineering [130]. Recently, PAM has been applied to retrieve structural information on tissue engineering constructs in vitro [213,214] and in vivo [122,211].…”