UCSS is a rare special type of atrial septal defect, which refers to the partial or complete defect of CS septum, causing CS to connect to LA. The clinical presentation of UCSS is not characteristic, and when it is combined with other cyanotic cardiac malformations, an accurate early diagnosis is even more difficult . Therefore, the preoperative diagnosis of UCSS relies heavily on imaging, such as TTE, CCT, MRI, etc.
With high temporal resolution and advantages of 3D visualization of cardiac anatomy, CCT technology has gradually become the preferred modality for cardiovascular disease in most medical centers [12, 13]. CCT can reconstruct the image in any plane without losing spatial resolution, and minimize the partial volume effect . MPR images based on the short axis of the heart are called "CS views", which show the entire course of CS (Figs. 1A, 2, 3, 4, 5A) , while reconstructions based on the long axis of the heart are more conducive to display the CS defect and are used to diagnose and classify UCSS clearly (Figs. 1B, 2D, 4B). MIP can provide a good "roadmap" to vascularity (Figs. 1C, 3C, 5B, 5C) . The interactive evaluation of MPR images and MIP images can better understand the course of CS, PLSVC and their adjacent tissue. VR can provide density and spatial information, and be customized in real time to display anatomical details from different perspectives . For the display of UCSS, VR is not only able to present the entire spatial relationship of RSVC-RA-CS-LA-PLSVC-Inn.v. (Figs. 1D, 3D) in its entirety, but also to reproduce complex vascular connections in 3D, especially for UCSS type V, VR can directly diagnose and classify it (Fig. 5D). CR technology is a new postprocessing technology. Compared with VR, CR technology can obtain highly realistic 3D images with photo level authenticity, further enhance the evaluation of spatial relationship and improve the perception of depth and shape . Tomographic imaging of CR can isolate or remove specific anatomical features such as coronary trees, calcification and bone, as well as lungs and airways. Clip planes and crop boxes can be applied to cut the rendered volume . The application of CR technology allows for a near-realistic view of the spatial relationship between CS defects and combined malformations (Figs. 1F, 2G, 3F, 4F). In this study, CCT applied multiple postprocessing methods as described above to clearly diagnose all 9 patients and further typed the UCSS. In addition, our patients also underwent TTE, but only 4 UCSS patients (44.4%) were detected, which was roughly consistent with the results of Raghib, G (46%) . And it is worth noting that TTE missed types III-V patients completely. The reason may be that compared with types I-II, types III-V have smaller CS defects, which are difficult to show directly with the limited acoustic window. Thus, in the UCSS diagnosis, TTE still has a non-negligible disadvantage compared to CCT.
UCSS is often associated with other malformations, such as atrial septal defect, ventricular septal defect, uniatrial heart, completely abnormal pulmonary venous drainage and tetralogy of Fallot . The excellent temporal (millisecond) and spatial (sub-millimetre) resolution of the CCT undoubtedly ensures the accuracy of the diagnosis of cardiac malformations and the description of the overall intracardiac-vascular-thoracic structures [22, 23]. In this cohort, 12 extra malformations were diagnosed by CCT comparied to 5 diagnosed by TTE (41.7%). TTE showed only ASD and cor triatrium but neglected PLSVC, pulmonary vein structural anomalies, which certainly demonstrated the superiority of CCT for the diagnosis of extracardiac malformations in UCSS. Moreover, CCT identified 2 rare cases of UCSS. The one was a type III patient with combined ectopic drainage of the great cardiac vein into the left atrium (Fig. 3D), according to literature review, this is the first report in patients with UCSS. Previously, only 4 cases of simple ectopic drainage of left atrial great cardiac vein were reported [10, 24]. The other type II patient was accompanied by cor triatriatum and total anomalous pulmonary vena cava (TAPVC) (Fig. 2), the possibly similar case was also reported by Kwak, J .
The surgical option for UCSS depends on the presence of PLSVC and the presence of a vascular bridge between RSVC and PLSVC . When not combined with PLSVC, it is enough to use a patch to repair the gap between CS and LA, or ligate coronary sinus ostium of RA . Preoperative observation of the CS defect size and morphology by CCT to assess the ease of patch placement is important. In this study, with CCT suggestions, 2 type II patients rescheduled the surgical strategy from patch repairment to close coronary sinus ostium, due to large CS defects were prone to loosening. If PLSVC is combined, central venous pressure (CVP) needs to be measured to determine if the RSVC and LSVC are obstructed or connected. And if there is no Inn.V., the LSVC should not be ligated . The CCT reproductive coronal views allow visual assessment of PLSVC course, diameter, and the presence or absence of the Inn.V., which can assist in surgical planning and postoperative follow-up.