Methods to Prepare Perfusion Fixed Cardiac Specimens for Multimodal Imaging: The Use of Formalin and Agar Gels

Recent advances in cardiac imaging have resulted in a growing understanding of both the form and function of the heart in vivo. Currently, the primary modalities for cardiac imaging are (1) two-dimensional cardiac ultrasound or echocardiography, (2) computed tomography (CT), and (3) magnetic resonance imaging (MRI). Yet, high resolution imaging with these modalities can be complicated by motion artifacts and long acquisition times resulting in most of the high resolution anatomical cardiac imaging protocols being reserved for ex vivo studies. Our laboratory has had the privilege to obtain fresh human heart specimens for educational and research purposes. These specimens have been perfusion fixed in 10% buffered formalin, by attaching cannulas to the great vessels, so to create a pressure head of approximately 50 mm Hg. The hearts were then suspended in containers and positioned in anatomically correct orientations before being embedded in 0.7% agar gel, at approximately $45°C$⁠. The cooled specimens were then scanned using the aforementioned clinical imaging modalities (2D and 3D echocardiography, CT, and 3T MRI). The stability of the embedded specimen, the physical properties of the gel, and the lack of motion artifacts allows for the acquisition of extremely high resolution images. These images have subsequently been used in the analysis of cardiac anatomies for a variety of pathologic investigations, not possible with current clinical imaging protocols, and/or for high resolution diffusion tensor MR imaging studies (e.g., of fiber orientations in heart failure in swine ventricles). Future work will include investigations as to whether this gelling approach could be used to prepare other organ specimens for such imaging.