This protocol describes a novel method for the 3D comparison of whole-heart myocardial tissue with MRI. This is designed for the accurate assessment of intramyocardial injections in the infarct border zone of a chronic porcine model of myocardial infarction.
Narrator}The overall goal of this protocol is to facilitate a structured and reproducible method for 3D whole-heart tissue processing that can be used to assess the targeting accuracy of intramyocardial injections into the infarct border zone. This method can help to answer key questions in the cardioregenerative medicine field, such as practice development and optimization of injection techniques. The main advantage of this technique is that it offers a standardized and reproducible method for 3D whole-heart tissue processing.
The implications of this technique extend towards development of cardiac regenerative therapies and validating injection accuracy of normal delivery techniques. The aim of this cardiac stem cell therapy is to inject cells into the infarct border zone for stimulation of local vasculogenesis and cardiomyocytes protection. Capturing the heart in end diastolic phase requires practice and patience.
A visual demonstration of these steps is critical. Begin by separating the pericardium from the microbead injected heart, keeping the atria and ventricles intact. When all of the tissue has been removed, use Klinkenberg scissors to dissect the ascending aorta about one centimeter above the aortic valve, and cut the inferior caval and pulmonary veins about one centimeter from the atrium.
After fixing the apex of the heart to the bottom of the plastic embedding container to prevent flotation, place 2-0 sutures through the remaining part of the aorta to the rims of the container, and fixate the heart. Take care that the heart is centered and is not touching the walls of the container. When the organ has been secured in an end diastolic like geometry, use mosquito clamps to grip the inferior caval vein and use a 50 milliliter syringe to slowly inject freshly prepared 50 to 60 degree Celsius agar into the right atrium via the superior caval vein until both the right atrium and ventricle are completely filled.
To obtain an embedded heart resembling the end diastolic geometry as closely as possible, fix the left ventricle apex to the bottom of the container and fill the right ventricle with agar while the pulmonary veins are clamped. Next, place a mosquito clamp on the pulmonary veins and gently pass a new agar filled 50 milliliter syringe in the retrograde direction through the aortic valves. Slowly inject the solution into the left ventricle until the ventricle and left atrium are completely full, clamping the aorta once the left ventricle has been filled.
Now, pour the remaining agar into the container until the heart is fully covered and place two rigid plastic tubes within the embedding container to serve as reference structures during the image registration. Then place the heart at two to seven degrees Celsius for at least four hours to solidify the agar. After magnetic resonance imaging, turn the container upside down and allow air between the agar and the sides of the container to remove the solid agar solution and the heart from the container.
Remove the plastic rods. Then use a meat slicer to section the agar block and the heart in five millimeter slices from the apex to the base of the heart, cutting parallel to the bottom of the agar block to keep the angulation of the slices the same as in the acquired magnetic resonance images. When all of the slices have been obtained, stain the tissue samples in TTC.
After 15 minutes, photograph the slices on both sides at a perpendicular angle, then carefully rinse the sections in 0.9%saline. To image the samples by fluorescence microscopy, select fluorescence mode imaging on the variable mode scanner and set the pixel size to 100 by 100 microns. For the first filter block, select a band pass filter that overlaps with the emission wavelength of the bead fluorescence in channel one, then to band pass filter for the second filter block outside the emission wavelength in channel two.
Next, select the appropriate filter block, set the photomultiplier tube to the appropriate voltage, and the excitation laser to the excitation wavelength closest to that of the injected microbeads. After all of the images have been obtained, in the appropriate post processing software, segment the myocardium and endo-and epicardial left ventricle borders in the magnetic resonance images followed by a similar segmentation of the myocardium and injection sites in the fluorescence images. Then linearly interpolate the image registered segments from both sides of the slices to reconstruct the original sample thickness and to create a 3D model of the data.
With the heart secured in an end diastolic like geometry as just demonstrated, in this representative experiment, the agar successfully adhered to the heart tissue enabling the tissue to be sliced at the desired angulation with equal slice thicknesses. In both the 2D fluorescence and magnetic resonance images, the scar and injection sites are clearly distinct. TTC stained tissue and late gadolinium enhanced magnetic resonance images provide a control for scar assessment in fluorescence imaging.
Post processing imaging enables reconstruction of the 3D geometry of the ex vivo heart based on the segmentations and fluorescence images, allowing assessment of the 3D injection accuracy. In this study, the injection depositions and the infarct border zone were protected onto the endocardial wall, and the distances between the projections on the endocardial surface were measured. Following this procedure, other methods like a surgical assessment, infarct size quantification, or validation of other three dimensional imaging modalities can be performed.
We have used this technique to assess the accuracy of injection into the infarct border zone. This research was conducted to improve cardioregenerative therapy for heart failure. After watching this video, you should have a good understanding of how to prepare the heart, how to embed and process cardiac tissue, and how to perform fluorescence imaging of cardiac tissue samples.
