Atrial septal defect assessment: challenges of 4D cardiac computed tomography

Introduction

In the January 2026 issue of AME Case Reports, Nishimoto et al. reported the clinical use of four-dimensional computed tomography (4D CT) for dynamic assessment of atrial septal defects (ASDs) in patients undergoing robot-assisted ASD closure (1). Accurate measurement of ASD size is necessary for procedural planning, particularly in minimally invasive robotic surgery, where precise patch selection and limited operative exposure demand reliable preoperative imaging (2). Although echocardiography remains the standard diagnostic modality, its assessment may be influenced by acoustic window limitations and operator dependency (2). In contrast, CT offers superior spatial resolution, and 4D CT enables phase-specific visualization of cardiac structures throughout the cardiac cycle. Among 18 patients who underwent robotic ASD closure, 4D CT-based measurements were performed in seven cases and demonstrated that ASD morphology varied between atrial systole (As) and atrial diastole (Ad). This work addresses an important methodological question in structural heart CT imaging and merits further investigation to better define its clinical utility, technical feasibility, and potential role in optimizing preoperative assessment and surgical planning.

Strengths of the study

A strength of this case series is that it shows how CT can support echocardiography in the evaluation of ASD. CT enables detailed multiplanar and en face assessment of ASD morphology, accurate measurement of defect size and shape, evaluation of surrounding septal rims, and delineation of adjacent structures such as the pulmonary veins, aortic root, and coronary arteries (3). This study raises an important question regarding adequate robotic closure of ASD since 4D CT systolic and diastolic sizing as well as intraprocedural sizing varied. They highlighted the limitations of all three measurements to assist the surgeons in making the best possible decision intraoperatively regarding direct or patch closure; and choosing the size of the patch. By examining phase-specific changes in defect area, the authors introduce a dynamic framework for imaging assessment that moves beyond static measurements. This concept is particularly intriguing in the context of tailoring CT acquisition protocols according to defect size and contrast flow dynamics.

Key findings and interpretation

In this case series, the authors highlight the dynamic variation in ASD size between As and Ad, underscoring the limitations of static single-phase three-dimensional (3D) CT measurements. On 4D CT, Ad corresponds to the phase of maximal ASD dimension, whereas As represents the smallest defect size, occurring at approximately 45% and 0–10% of the RR interval, respectively. The authors suggest that a standard retrospective full-cardiac-cycle CT acquisition with predominantly thicker-slice reconstructions may be adequate for ASD evaluation when combined with targeted thin-slice imaging at ~40% (Ad), which coincides with ventricular systole and maximal defect size, and ~75% for ventricular assessment. This approach is particularly practical given that ASD repair is often performed in conjunction with other cardiac surgical procedures, where comprehensive ventricular evaluation is also required. Such targeted phase reconstruction may therefore provide reliable ASD characterization while potentially reducing overall radiation exposure. Using 4D CT, the number, morphology, and size of ASD defects can be evaluated dynamically throughout the cardiac cycle. This comprehensive assessment may help inform surgical strategy and patch sizing, potentially reducing cardiac arrest and operative times, and highlights the potential value of 4D CT for preoperative planning in ASD repair.

Intraoperative measurements of ASD size were consistently larger than those obtained by CT during Ad. However, CT-based measurement was feasible in only four patients because septal thinning during Ad made it difficult to distinguish the atrial septum from the atrial cavity, resulting in unrealistically large defect appearances. Dynamic assessment of ASD across the cardiac cycle permits characterization of defect behavior and elasticity, information that is directly relevant to surgical sizing. Intraoperative measurement, obtained under cardioplegic arrest, reflects a single static dimension of the defect in the absence of active hemodynamic loading. While cardioplegic arrest produces atrial relaxation analogous to the atrial diastolic state, the atria are devoid of blood volume at that time point, limiting the validity of direct comparison of the intraoperative and CT-derived diastolic measurements.

Limitations and critique

This timely and conceptually compelling case series examines the utility of 4D CT for phase-specific morphologic assessment of ASDs, contributing to the growing literature supporting non-invasive advanced imaging in preoperative structural heart disease planning. However, as a retrospective single-center series with only 7 analyzable cases from an initial cohort of 18, inferential validity is constrained, and the findings are best regarded as hypothesis-generating. Several methodological limitations warrant careful consideration when interpreting these results. Most critically, the imaging protocol introduces an asymmetric spatial resolution between the two primary measurement time points: As measurements are derived from lower-resolution acquisitions (128–160 detector rows at the 0% RR phase), while Ad measurements are derived from high-resolution thin-slice acquisitions (256–320 detector rows at the 40% phase). This differential spatial resolution introduces a systematic bias that may underestimate defect dimensions during As relative to Ad (2). The authors’ phase identification methodology lacks the temporal precision required for a reproducible, institutionally transferable protocol. Defining As and Ad as 10-percentage-point RR-interval bins rather than fixed millisecond delays from defined ECG landmarks may be insufficient, as the same RR percentage represents a substantially different absolute time window depending on the heart rate. Future protocol iterations should anchor acquisition to fixed post-trigger delays relative to the P-wave onset for As and the QRS onset for Ad, consistent with established prospective cardiac CT gating conventions (4). Complementary validation via direct assessment of mitral valve motion on 4D CT, leveraging valve excursion as an independent anatomic phase landmark, could further strengthen phase verification. As this was not a dedicated study for ASD evaluation, the methods relied on a fixed CT protocol for each patient; however, in practice, variation in protocol, contrast volume, and timing is needed to account for differences in ASD size and morphology. Additionally, defect area was calculated using the elliptical approximation formula (π × major radius × minor radius), which assumes a geometrically regular orifice. This assumption is particularly problematic for fenestrated secundum defects, where multiple communicating orifices render single-ellipse area calculations unreliable. ASD morphology frequently departs from this idealization, and published data demonstrate that 3D planimetric measurements can exceed two-dimensional (2D) elliptical estimates by up to 27% in morphologically complex defects (5). Details regarding CT measurement, orthogonal vs. 3D planimetry, or curved multiplanar measurements for the ASD were not provided. Similarly, details regarding rims and associated structures were not discussed. Inter- and intra-observer variability in the measurement of ASD was not presented. Finally, although echocardiographic measurements were performed, a comparison in sizing with CT was not reported. Taken together, the retrospective design and highly selected, analyzable cohort significantly limit generalizability. Patients with atrial fibrillation and multi-hole defect morphology were excluded entirely, sinus venosus defects were unrepresented, and even within the final cohort of 7 cases, defect dimensions could not be measured in at least one cardiac phase in 2 patients. These constraints underscore that the current protocol remains insufficiently robust for application to broader clinical populations.

Clinical implications

These reported findings carry meaningful clinical implications for the preoperative planning of surgical ASD closure. Robotic and minimally invasive procedures demand detailed anatomic characterization prior to intervention, and 4D CT is uniquely positioned to fulfill this role by providing phase-specific morphologic data that static single-phase imaging cannot. By characterizing defect dimensions at both their maximum and minimum across the cardiac cycle, 4D CT enables quantification of the dynamic change in ASD area between Ad and As. This level of detail in preoperative imaging directly informs the choice between direct vs. patch closure and facilitates advanced patch preparation, potentially reducing cardiopulmonary bypass and operative time. Beyond defect sizing, 4D CT enables systematic assessment of rim lengths at all relevant anatomical landmarks with accuracy comparable to transesophageal echocardiography (TEE) (6,7). At this stage, 4D CT is best understood as a tool to more robustly support and supplement conventional echocardiographic assessment rather than replace it, with further prospective validation required before it can be adopted as a standalone preoperative standard.

Future directions

The present case series establishes a compelling conceptual framework for phase-specific 4D CT assessment of ASD morphology, but several unresolved challenges must first be addressed before broader investigation is feasible. Most immediately, the exclusion of patients with atrial fibrillation and multi-hole defect morphology reflects a fundamental limitation of current electrocardiogram (ECG)-gated CT acquisition, in which irregular RR intervals and complex contrast flow dynamics preclude reliable phase-specific defect visualization. Development of arrhythmia-tolerant acquisition protocols, potentially leveraging prospective beat-selection algorithms or motion-correction reconstruction techniques, will be essential to broaden the applicability of this approach across the full spectrum of patients encountered in clinical practice. Once these technical limitations are overcome, rigorous comparison of 4D CT-derived defect measurements against an accepted reference standard will be required to establish measurement validity. Real-time 3D-TEE, with its superior temporal resolution (8), represents the most appropriate comparator given its established role in intraoperative ASD assessment. With measurement validity established, the field will be positioned to address the most clinically meaningful question: whether 4D CT-guided preoperative surgical planning, including advance patch preparation based on CT-predicted defect morphology, translates into measurable clinical benefit relative to the current standard of care. Prospective multicenter comparative studies with pre-specified endpoints, including surgical outcomes, cardiopulmonary bypass time, operative time, procedural resource utilization, and cost-effectiveness, will ultimately be necessary to justify broader adoption of 4D CT as a routine component of the preoperative workup for surgical ASD closure.

Conclusions

This case series provides a valuable preliminary demonstration that 4D CT can characterize phase-specific ASD morphology in the preoperative setting, with meaningful implications for surgical planning in robotic ASD closure. While methodological limitations preclude definitive conclusions at this stage, the conceptual framework established here warrants rigorous prospective investigation. Standardization of imaging protocols, validation against established reference standards, and ultimately, prospective multicenter trials demonstrating measurable clinical benefit will be necessary to define the role of 4D CT as a routine adjunct to conventional echocardiography in the preoperative assessment of structural atrial septal disease.

Acknowledgments

None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, AME Case Reports. The article did not undergo external peer review.

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://acr.amegroups.com/article/view/10.21037/acr-2026-0073/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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