Table 2 Ground truth definitions of each cardiac structure and their optional zones.

LV/RV

The LV and RV are differentiated by considering chamber shape, surface smoothness, placement of the atrioventricular valves, and the radiology report. Note that it is possible for the anatomic LV to be on the right side of the body and the anatomic RV on the left side of the body. There are several single ventricle subtypes. In Double Inlet Left Ventricle (DILV) cases, there is a large LV, and the small outflow chamber is labeled as RV. In Double Inlet/Double Outlet Right Single Ventricle cases, the RV is large and there is a very small labeled LV. In Holmes cases, there is a single LV and the RV label is empty. For all other single ventricle hearts, the single ventricle is labeled as LV and the RV label is empty (we desired a consistent definition to increase the feasibility of automated segmentation via machine learning, as in our experience, trained networks often output a mixture of LV and RV labels within single ventricles).

LV

Typically bordered by the mitral valve, aortic valve, and/or VSD (if present). Includes the outflow portion. As is standard clinical practice at Boston Children’s Hospital and as advised by cardiologists, the papillary muscles are not included in order to create realistic 3D heart models for surgical planning.

RV

Typically bordered by the tricuspid valve, pulmonary valve, and/or VSD (if present). Includes the outflow portion. As is standard clinical practice at Boston Children’s Hospital and as advised by cardiologists, the trabeculations are not included in order to create realistic 3D heart models for surgical planning. For patients who have undergone a Glenn or Fontan procedure, the residual PA stump is labeled as RV. In the setting of significant RV hypertrophy, insufficient image resolution and noise may cause manual segmentations to be undersegmented.

LA/RA

We choose to label the chamber with attached pulmonary veins (PVs) as the LA, and the other chamber (if present) as the RA. If there is a common atrium, it is labeled as LA (RA is empty). However, like the LV and RV, the atria are clinically defined by anatomy, and not by pulmonary veins coming in. In particular, heterotaxy patients do not have a LA or a RA, but rather a “left-sided atrium” and a “right-sided atrium” based purely on their position in the thorax. However, we desired a more consistent definition that would be the same for both heterotaxy and non-heterotaxy patients, again based on our experience that trained networks can assign a mixture of LA and RA labels to a common atrium. In clinical practice, the assignment of left vs. right for heterotaxy patients may need to be manually edited following inference.

LA

Typically bordered by the mitral valve and/or ASD (if present). The ground truth includes the PVs attached to the LA until they branch. Any confluence between the PVs and the LA is also segmented as LA. The PVs can be optionally shorter. This was defined by manually cutting each PV to require its stump only.

RA

Typically bordered by the tricuspid valve, SVC insertion, IVC insertion, and/or ASD (if present).

AO

From the aortic valve through the ascending and descending aorta, until the most inferior level of the LV/RV/LA/RA/PA. In AO-PA anastomosis cases, the AO label includes the segment of the attached original PA. Includes the aortic ductus diverticulum if present. Can optionally continue to the bottom of the image.

PA

Typically includes the main PA trunk, from the pulmonary valve, and the left and right PA branches. The ground truth segmentations of the left and right branches have equal length, which is defined by the distance from the bifurcation point to behind the right upper lobe segmental branch. For Glenn/Fontan patients, the PA often consists of only the two branches. Effort was made to track through CMR inhomogeneity artifacts; if this was impossible then any disconnected segments were labeled as optional. The distal ~25% of each ground truth branch segmentation is optional. The left and right branches can optionally continue beyond the ground truth stopping point, until the point at which they split into their lower lobe segmental branches.

SVC

From the most superior axial slice at the level of its bifurcation into the brachiocephalic veins, down to its insertion into the attached atrium (angled according to atrium curvature) or the PA branches (Glenn/Fontan patients). A second SVC may be present (bilateral SVC). The superior ~25% in the ground truth is optional. Each SVC can optionally continue higher, through the brachiocephalic vein and internal jugular veins.

IVC

From its insertion into the attached atrium (angled according to atrium curvature) or the PA branches (Fontan patients: baffle included), down through the hepatic segment and subsequent branches of the hepatic veins. Rarely, has two connected components, depending on the insertion of the hepatics. The ground truth length was defined by identifying the level of the first bifurcation, counting down by 5% of the image height, and then dilating the pre-bifurcation segment to this level (so that branches are cut at an angle). The required segment was defined by repeating this using the lowest axial slice in which the IVC appeared round (i.e., above any branching) and counting by 2/3·5% of the image height. The IVC can optionally continue branching to the bottom of the image.