Hydrogel-based cardiac tissue engineering offers great promise for myocardial infarction repair. The ability to visualize engineered systems in vivo in animal models is desired to monitor the performance of cardiac constructs. However, due to the low density and weak X-ray attenuation of hydrogels, conventional radiography and micro-computed tomography are unable to visualize the hydrogel cardiac constructs upon their implantation, thus limiting their use in animal systems. This paper presents a study on the optimization of synchrotron X-ray propagation-based phase-contrast imaging computed tomography (PCI-CT) for three-dimensional (3D) visualization and assessment of the hydrogel cardiac patches. First, alginate hydrogel was 3D-printed into cardiac patches, with the pores filled by fibrin. The hydrogel patches were then surgically implanted on rat hearts. A week after surgery, the hearts including patches were excised and embedded in a soft-tissue-mimicking gel for imaging by using PCI-CT at an X-ray energy of 25 keV. During imaging, the sample-to-detector distances, CT-scan time and the region of interest (ROI) were varied and examined for their effects on both imaging quality and radiation dose. The results showed that phase-retrieved PCI-CT images provided edge-enhancement fringes at a sample-to-detector distance of 147 cm that enabled visualization of anatomical and microstructural features of the myocardium and the implanted patch in the tissue-mimicking gel. For visualization of these features, PCI-CT offered a significantly higher performance than the dual absorption-phase and clinical magnetic resonance (3 T) imaging techniques. Furthermore, by reducing the total CT-scan time and ROI, PCI-CT was examined for lowering the effective dose, meanwhile without much loss of imaging quality. In effect, the higher soft tissue contrast and low-dose potential of PCI-CT has been used along with an acceptable overall animal dose to achieve the high spatial resolution needed for cardiac implant visualization. As a result, PCI-CT at the identified imaging parameters offers great potential for 3D assessment of microstructural features of hydrogel cardiac patches.