Imaging of Injectable Hydrogels Delivered into Myocardium with SPECT/CT

Abstract

Injectable hydrogels are being widely explored for treatment after myocardial infarction (MI) through mechanical bulking or the delivery of therapeutics. Despite this interest, there have been few approaches to image hydrogels upon injection to identify their location, volume, and pattern of delivery, features that are important to understand toward clinical translation. Using a hyaluronic acid (HA) hydrogel as an example, the aim of this study is to introduce radiopacity to hydrogels by encapsulating a clinically used contrast agent (Omnipaque Iohexol, GE Healthcare) for imaging upon placement in the myocardium. Specifically, iohexol is encapsulated into shear-thinning and self-healing hydrogels formed through the mixing of HA-hydrazide and HA-aldehyde. Upon examination of a range of iohexol concentrations, a concentration of 100 mg mL^-1 iohexol is deemed optimal based on the greatest contrast, while maintaining hydrogel mechanical properties and acceptable injection forces. In an acute porcine model of MI, hybrid single-photon emission computed tomography/computed tomography (SPECT/CT) perfusion imaging is performed immediately and 3-4 days after hydrogel delivery to assess radiopacity and verify the hydrogel location within the perfusion defect. Hybrid SPECT/CT imaging demonstrates excellent radiopacity of the hydrogel within the perfusion defect immediately after intramyocardial hydrogel injection, demonstrating the feasibility of this method for short-term noninvasive hydrogel monitoring.

Keywords: SPECT/CT; hydrogel; image-guided delivery; multimodal imaging; myocardial infarction; radiopacity.

FIGURE 1.. Hydrogel formulation for encapsulation of iohexol.

A) Modification of hyaluronic acid (HA) with either hydrazides (HA-HYD) or aldehydes (HA-ALD). B) Incorporation of iohexol confers radiopacity to hydrogels formed from the mixing of HA-ALD and HA-HYD and the hydrogel properties allow shear-thinning for direct injection into the myocardium.

FIGURE 2.. Shear-thinning and self-healing properties allow injectability.

A) Shear-thinning of hydrogels on shear oscillatory rheometry, demonstrating decreasing viscosity with increasing shear-rates (Inset: ejection of hydrogel through 27G x 1/2” syringe). B) Self-healing of dyed hydrogel discs placed together and allowed to heal for 10 minutes in air after cutting; manual stretching of healed hydrogel discs after 10 minutes demonstrates self-healing behavior. C) Application of cyclic low (0.5%) and high (500%) strains on shear-oscillatory rheometry demonstrating a rapid decrease in modulus in response to high strain and rapid recovery upon cessation of shear, applicable over multiple cycles of loading.

FIGURE 3.. Encapsulation of iohexol up to 100 mg/mL does not affect mechanical properties.

A) (left) Representative profiles of the gelation (storage modulus: G’, loss modulus: G’’) behavior of HA-HYD and HA-ALD hydrogels either without (0 mg/mL) or with (100 mg/mL) iohexol; (right) quantification of G’ and G’’ across various iohexol concentrations (*p<0.001 compared to hydrogels without iohexol, n=3, each column represents mean±SEM). B) (left) Representative ejection force profiles of hydrogels either without (0 mg/mL) or with (100 mg/mL) iohexol; (right) quantification of ejection forces across various iohexol concentrations (*p<0.001 compared to hydrogels without iohexol, n=3, each column represents mean±SEM).

FIGURE 4.. Radiopacity of hydrogels can be monitored across different CT intensities and over time.

A) CT images of hydrogels with various iohexol concentrations in the presence and absence of PBS acquired on a clinical CT scanner using standard acquisition parameters (80 keV, 250 mA). Hydrogels with iohexol concentrations above 35 mg/mL were visible. B) CT intensity profiles (expressed as Hounsfield Units (HU)) across a hydrogel with 100 mg/mL iohexol and PBS (shown from bottom of tube on left through the PBS above the hydrogel on the right) over the range of CT energy levels used clinically. C) CT images (acquired at 80 keV, 250 mA) and associated quantitative intensities (HU, mean ± SD) for a hydrogel containing 100 mg/mL iohexol in a tube with PBS above the hydrogel imaged daily over four days. PBS was changed daily (inset: visualization of tube over time). (*p<0.05 for CT intensity between the hydrogel and PBS, n=3, each column represents mean±SEM)

FIGURE 5.. Intramyocardial injection of radiopaque hydrogels.

A) For hydrogel delivery, a flexible plastic patch (1-2 mm thick) with 9 poles (each 7 mm tall) distributed in a 3×3 grid 1 cm apart was sown on the surface of heart over the infarct area. B) Insulin syringes loaded with hydrogel were passed through the poles on the patch in order to reproducibly deliver the hydrogel into the middle of the myocardial wall in the infarct area (MI) in a 3×3 array (white arrow). LV indicates left ventricle; RV, right ventricle.

FIGURE 6.. Radiopaque hydrogels can be guided into the area of perfusion defect via hybrid SPECT/CT imaging.

A) In vivo contrast CT long axis 2D image (120 keV, 450 mA) acquired on the day of delivery, demonstrating the placement of the hydrogel in the anterior-lateral wall of the LV. B) 3D rendering of the in vivo contrast CT angiogram in which multiple hydrogel injections can be clearly visualized over contrast in LV cavity. C) In vivo ^99mTc-Tetrofosmin hybrid SPECT/CT image. The perfusion defect can be visualized on the antero-lateral wall (blue-green) versus the normal perfusion area (yellow-orange) and the hydrogel can be clearly visualized (yellow arrow) in the hypo-perfused region.

FIGURE 7.. Radiopaque hydrogels are visible in vivo immediately after delivery.

Serial in vivo CT imaging demonstrating radiopacity of hydrogel (A) immediately following intramyocardial delivery (yellow arrows) and (B) 3 days post-delivery.

FIGURE 8.. Radiopaque hydrogels are only visible via ex vivo CT four days after delivery.

(A) High resolution ex vivo CT image of the heart following casting of the ventricle cavities with alginate to maintain the heart shape. A hypo-enhanced area can be visualized (yellow arrow) that corresponds to the hydrogel visualized on (B) a corresponding ex vivo slice of the left ventricle within the infarct area (red arrow).