Multidimensional Analysis of the Adult Human Heart in Health and Disease Using Hierarchical Phase-Contrast Tomography

Abstract

Background Current clinical imaging modalities such as CT and MRI provide resolution adequate to diagnose cardiovascular diseases but cannot depict detailed structural features in the heart across length scales. Hierarchical phase-contrast tomography (HiP-CT) uses fourth-generation synchrotron sources with improved x-ray brilliance and high energies to provide micron-resolution imaging of intact adult organs with unprecedented detail. Purpose To evaluate the capability of HiP-CT to depict the macro- to microanatomy of structurally normal and abnormal adult human hearts ex vivo. Materials and Methods Between February 2021 and September 2023, two adult human donor hearts were obtained, fixed in formalin, and prepared using a mixture of crushed agar in a 70% ethanol solution. One heart was from a 63-year-old White male without known cardiac disease, and the other was from an 87-year-old White female with a history of multiple known cardiovascular pathologies including ischemic heart disease, hypertension, and atrial fibrillation. Nondestructive ex vivo imaging of these hearts without exogenous contrast agent was performed using HiP-CT at the European Synchrotron Radiation Facility. Results HiP-CT demonstrated the capacity for high-spatial-resolution, multiscale cardiac imaging ex vivo, revealing histologic-level detail of the myocardium, valves, coronary arteries, and cardiac conduction system across length scales. Virtual sectioning of the cardiac conduction system provided information on fatty infiltration, vascular supply, and pathways between the cardiac nodes and adjacent structures. HiP-CT achieved resolutions ranging from gross (isotropic voxels of approximately 20 µm) to microscopic (approximately 6.4-µm voxel size) to cellular (approximately 2.3-µm voxel size) in scale. The potential for quantitative assessment of features in health and disease was demonstrated. Conclusion HiP-CT provided high-spatial-resolution, three-dimensional images of structurally normal and diseased ex vivo adult human hearts. Whole-heart image volumes were obtained with isotropic voxels of approximately 20 µm, and local regions of interest were obtained with resolution down to 2.3-6.4 µm without the need for sectioning, destructive techniques, or exogenous contrast agents. Published under a CC BY 4.0 license Supplemental material is available for this article. See also the editorial by Bluemke and Pourmorteza in this issue.

Graphical abstract

Figure 1:

Three-dimensional cinematic renderings of the (A) control and (B) diseased hearts in anatomic orientation. Epicardial fat has been removed digitally to show the course of the major coronary arteries plus details of smaller arteries penetrating into the myocardium that are not typically seen on clinical CT scans. In the control heart, the coronary arteries remain close to the epicardial surface, while in the diseased heart, they are lifted away by epicardial fat, increasing the perfusion distance between the major coronary arteries and the myocardium. Segmentations and high-spatial-resolution details of coronary arteries in the diseased heart are also shown in Figure 5.

Figure 2:

(A) Three-dimensional (3D) cinematic rendering from hierarchical phase-contrast tomography (HiP-CT) of the control heart, viewed from the left posterior aspect, with epicardial fat digitally removed using thresholding, shows the left atrium and the left atrial appendage (LAA), which can be seen running over the circumflex coronary artery (CX). This anatomic relationship is a key landmark for interventional closure of the left atrial appendage with devices. Ao = aorta, CS = coronary sinus, LAD = left anterior descending coronary artery, LPV = left pulmonary vein, LV = left ventricle, PT = pulmonary trunk. (B) High-spatial-resolution local HiP-CT scan of the wall of the left atrial appendage in the control heart (view plane is given by the dashed line in A) shows histologic spatial resolution of endocardial, myocardial, and epicardial layers, allowing measurement of thickness (lines). (C) Box and whiskers plots show the thickness of the endocardium (Endo), myocardium (Myo), and epicardium (Epi) in the left and right atria in the control heart and the diseased heart. Circles indicate individual measurements, the boundaries of boxes indicate the lower and upper quartiles, the red lines indicate medians, and the whiskers indicate ranges. Measurements were obtained manually at 15 distinct locations along the left and right atrial walls. Thickness measurements in the control and diseased hearts, and in right versus left atrium, were compared using unpaired multiple t tests with multiple comparison correction applied via the Holm-Sidak method and a set α threshold of .05. Statistical analysis revealed significant differences (P < .001) in the thickness of the left and right atrial endocardium within the control heart and within the diseased heart and in the thickness of the left and right atrial epicardium between the control heart and the diseased heart.

Figure 3:

Mid-ventricular short-axis section of the ventricles in the control heart shows the feasibility of myomapping the orientation of myocyte aggregates on hierarchical phase-contrast tomography images at low spatial resolution. Myocyte orientation was calculated by means of structure tensor analysis with an in-house code based on the Python library structure-tensor (5,6). Colors represent the helical angle of aggregates of myocytes, which show a gradual transition in the thicker left ventricle from epicardium to endocardium.

Figure 4:

Hierarchical phase-contrast tomography images show the feasibility of identifying and tracing the conduction system with virtual multiplanar sectioning in (A–C) control and (D–F) diseased heart. In the control heart, the sinus and atrioventricular nodes are tightly adjacent to the surrounding atrial myocardium, paranodal area (PN), and terminal crest (TC), with multiple potential sites of connection. Multiplanar sectioning allows vascular connections from the atrioventricular node to be followed (red arrowheads in B, C, E, F). These connections can be seen breaking up the bright fibrous tissue between atrial and ventricular myocardium and represent potential sites of normal nodoventricular connection. In the diseased heart, there is fatty infiltration in the region of the sinus and atrioventricular nodes, separating the respective nodal tissue from surrounding right atrial (RA) myocardium (yellow arrowheads in B, C, E, F) and from the paranodal area. This fatty infiltration elongates the connections between nodal tissue and surrounding atrial myocardium (* in D). In particular, the connection between the atrioventricular node and the atrial septum (★ in B, C, E, F), a potential fast pathway, is diminished in the diseased heart compared with the control heart. An attenuated connection with the vestibule of the right atrium (potential slow pathway) can also be seen in a more inferior section of the diseased heart, made across the inferior pyramidal space (* in F). ISR = inferoseptal recess (within the left ventricle), LA = left atrium.

Figure 5:

(A, B) Three-dimensional cinematic renderings and (C, D) images from hierarchical phase-contrast tomography (HiP-CT) show segmentation of the coronary arterial and venous tree in the diseased heart. (A) The heart is viewed on its apex to show the course of the major coronary arteries and veins as well as smaller vessels such as the sinoatrial nodal artery (SNA). The coronary vasculature is lifted away from the epicardial surface by lipomatous hypertrophy, lengthening the penetrating arteries (see also Fig 1). Box indicates the location of the stent shown in B; line indicates the cross-section shown in C. (B) Cinematic rendering of one of the coronary stents, along with surrounding calcification (purple), shows feasibility of imaging both soft and hard tissue with minimal artifacts. (C) High-spatial-resolution local tomography of the left descending coronary artery shows atherosclerotic plaque including calcification (box). (D) Digital zoom scan of box in C shows details of the atherosclerotic plaque and calcification within the intima. Adip = adipose tissue, Adv = tunica adventitia, Ao = aorta, Ath = atherosclerosis, C = calcification, LAD = left anterior descending coronary artery, Lum = lumen, Med = tunica media, RCA = right coronary artery, VV = vasa vasorum.

Figure 6:

Hierarchical phase-contrast tomography scans of ventricular myocardium in (A–C) control and (D–F) diseased heart. (A, D) Long-axis views show the extent of myocardial infarction (MI) in the diseased heart, together with lipomatous hypertrophy, bright areas of calcification in the aortic valve and mitral annulus (yellow arrowheads in D), and stent in the right coronary artery (red arrowhead in D). (B, E) Zoom scans of boxes in A and D show the high spatial resolution obtained even at this voxel size and the extensive nature of the myocardial infarction that lies immediately beneath the trabecular myocardium (T). (C, F) High-spatial-resolution scans of the boxed areas in B and E show differences in myocardial aggregates, which are separated by replacement fibrosis in the diseased heart (yellow arrowheads in F) but tightly packed in the control heart. Ao = aorta, C = compact myocardium, LA = left atrium, LV = left ventricle, RV = right ventricle.