Role of Cross-Sectional Imaging in Pediatric Interventional Cardiac Catheterization

Management of congenital heart disease (CHD) has recently increased utilization of cross-sectional imaging to plan percutaneous interventions. Cardiac computed tomography (CT) and cardiac magnetic resonance (CMR) imaging have become indispensable tools for pre-procedural planning prior to intervention in the pediatric cardiac catheterization lab. In this article, we review several common indications for referral and the impact of cross-sectional imaging on procedural planning, success, and patient surveillance.


Background
Historically, echocardiography has been the primary imaging tool used to help guide percutaneous interventional procedures in the congenital heart disease (CHD) population. Cross-sectional imaging is now emerging as an essential complement to patient selection and monitoring. With improvement in pre-procedural imaging, catheterization procedures are becoming more directed toward interventions and less diagnostic.
Cardiac computed tomography (CT) and cardiac magnetic resonance imaging (CMR) have made substantial improvements in sequencing, spatial resolution, and reduction in radiation exposure to the point where they are now routinely being employed to assist with pre-procedural planning. Meticulous pre-procedural planning has now led to significantly improved outcomes in the cardiac catheterization laboratory.
This review article will focus on how cross-sectional imaging aids interventional cardiologists in the catheterization lab. Echocardiographic-fluoroscopic fusion imaging and interventions play a substantial role; however, this topic will be omitted for the purposes of this review article. Case examples will be presented in the general format of pre-procedural, intra-procedural, and post-procedural considerations.

Case Examples
We have selected specific case examples to illustrate the uses of cross-sectional imaging to aid with percutaneous congenital cardiology interventions.   Revised from ref [4].
Children 2022, 9, x FOR PEER REVIEW 2 of 21

Sinus Venosus Atrial Septal Defect (SVASD)
SVASD are characterized by an embryological defect between the wall of the superior vena cava (SVC) and right upper pulmonary vein (RUPV), leading to a significant left to right shunt [1][2][3]. Cross-sectional imaging has played a vital role in the diagnosis and interventional planning for percutaneous closure. Not all patients are suitable candidates for transcatheter intervention as many factors must be considered pre-operatively including pulmonary vein insertion, length of SVC, and sites of possible obstruction from the covered stent placement.
The following Figures 1-6 describe in detail the pre-procedural, intra-procedural, and post-procedural considerations [4]. Figure 7 demonstrates how the "virtual reality" environment can provide a critical look at the cross-sectional imaging in an interactive platform to deliver a reliable understanding of the patient's anatomical limitations [5].  Series of images depicting pre-procedural planning with CT (A), three-dimensional (3D) printed models (B,C), and 3D virtual modeling (D). The defect is represented by an asterisk (*) in (A). After review with 3D imaging, each patient was either referred for covered stent placement (B, D) or surgical repair (C), if deemed inappropriate for percutaneous closure. Revised from ref [4].

Figure 2.
Series of images depicting pre-procedural planning with CT (A), three-dimensional (3D) printed models (B,C), and 3D virtual modeling (D). The defect is represented by an asterisk (*) in (A). After review with 3D imaging, each patient was either referred for covered stent placement (B, D) or surgical repair (C), if deemed inappropriate for percutaneous closure. Revised from ref [4]. Children 2022, 9, x FOR PEER REVIEW 3 of 21 Figure 3. Series of fluoroscopy images depicting balloon test occlusion prior to covered stent placement. The patient was deemed to be an unsuitable candidate for covered stent placement as both RUPVs demonstrated an obstructive pattern returning to the LA (A,B). Venous return from the RMPV (C) was normal. A catheter is seen at the asterisk (*). Revised from ref [4].

Patent Ductus Arteriosus (PDA) Stenting for Ductal-Dependent Pulmonary Blood Flow
PDA stenting is emerging as an alternative to surgical shunts for PDA-dependent pulmonary blood flow in newborns with CHD. Every PDA is not created equal, and there are many considerations that must be evaluated before undertaking PDA stenting.

Pre-Procedural
Prior to catheterization, many centers routinely obtain cross-sectional imaging with contrast CT scan to better assess the take-off, tortuosity, and size of the PDA (Figures 8  and 9) [6][7][8]. A single-center study has shown a statistically significant reduction in number of access sites, contrast exposure, as well as fluoroscopic and procedural time without significantly increasing the cumulative radiation burden. . Series of fluoroscopy images depicting balloon test occlusion prior to covered stent placement. The patient was deemed to be an unsuitable candidate for covered stent placement as both RUPVs demonstrated an obstructive pattern returning to the LA (A,B). Venous return from the RMPV (C) was normal. A catheter is seen at the asterisk (*). Revised from ref [4].
Children 2022, 9, x FOR PEER REVIEW 3 of 21 Figure 3. Series of fluoroscopy images depicting balloon test occlusion prior to covered stent placement. The patient was deemed to be an unsuitable candidate for covered stent placement as both RUPVs demonstrated an obstructive pattern returning to the LA (A,B). Venous return from the RMPV (C) was normal. A catheter is seen at the asterisk (*). Revised from ref [4].

Patent Ductus Arteriosus (PDA) Stenting for Ductal-Dependent Pulmonary Blood Flow
PDA stenting is emerging as an alternative to surgical shunts for PDA-dependent pulmonary blood flow in newborns with CHD. Every PDA is not created equal, and there are many considerations that must be evaluated before undertaking PDA stenting.

Pre-Procedural
Prior to catheterization, many centers routinely obtain cross-sectional imaging with contrast CT scan to better assess the take-off, tortuosity, and size of the PDA (Figures 8  and 9) [6][7][8]. A single-center study has shown a statistically significant reduction in number of access sites, contrast exposure, as well as fluoroscopic and procedural time without significantly increasing the cumulative radiation burden.

Post-Procedural
Following the procedure, it is important to continue to monitor saturations to ensure the patient maintains adequate pulmonary blood flow. Figure 10 demonstrates an example of PDA stenting where the aortic end was not fully covered. The patient returned to the catheterization lab after this lesion was identified by CT scan and the stenotic aortic end was fully stented. The patient did well afterwards, saturations stabilized, and the patient was discharged home with close follow-up.

Patent Ductus Arteriosus (PDA) Stenting for Ductal-Dependent Pulmonary Blood Flow
PDA stenting is emerging as an alternative to surgical shunts for PDA-dependent pulmonary blood flow in newborns with CHD. Every PDA is not created equal, and there are many considerations that must be evaluated before undertaking PDA stenting.

Pre-Procedural
Prior to catheterization, many centers routinely obtain cross-sectional imaging with contrast CT scan to better assess the take-off, tortuosity, and size of the PDA (Figures 8 and 9) [6][7][8].
A single-center study has shown a statistically significant reduction in number of access sites, contrast exposure, as well as fluoroscopic and procedural time without significantly increasing the cumulative radiation burden.

Cross-Sectional Overlay Fusion in the Catheterization Lab
Cross-sectional overlay enabled registration of previously acquired CT and/or MRI images is being used to rapidly fuse with X-ray fluoroscopic imaging in single and biplane systems [10,11]. Several single-plane overlay examples (VesselNavigator system (Philips Healthcare, Best, The Netherlands)) are outlined below (Figures 14-18) [12]. A biplane overlay example (Siemens bi-plane system (Siemens Healthineers, Munich, Germany) is depicted in Figure 19 [13]. Overlay fusion has demonstrated a reduction in overall radiation burden [8,9].

Coronary Artery Assessment in CHD
Congenital coronary interventions are rare, and the anatomy for each case is quite

Cross-Sectional Overlay Fusion in the Catheterization Lab
Cross-sectional overlay enabled registration of previously acquired CT and/or MRI images is being used to rapidly fuse with X-ray fluoroscopic imaging in single and biplane systems [10,11]. Several single-plane overlay examples (VesselNavigator system (Philips Healthcare, Best, The Netherlands)) are outlined below (Figures 14-18) [12]. A biplane overlay example (Siemens bi-plane system (Siemens Healthineers, Munich, Germany) is depicted in Figure 19 [13]. Overlay fusion has demonstrated a reduction in overall radia-

Post-Procedural
Following the procedure, it is important to continue to monitor saturations to ensure the patient maintains adequate pulmonary blood flow. Figure 10 demonstrates an example of PDA stenting where the aortic end was not fully covered. The patient returned to the catheterization lab after this lesion was identified by CT scan and the stenotic aortic end was fully stented. The patient did well afterwards, saturations stabilized, and the patient was discharged home with close follow-up.  [14,15] to compliment traditional assessment by X-ray fluoroscopic catheter angiography. Figure 10. Patient is status post PDA stenting with concern for cyanosis. ECHO/CT scan demonstrated that the aortic end was uncovered (A,B). Patient returned to the catheterization lab for an additional stent placement (C-E). Follow up PDA stenting showed full coverage (F). Figure 10. Patient is status post PDA stenting with concern for cyanosis. ECHO/CT scan demonstrated that the aortic end was uncovered (A,B). Patient returned to the catheterization lab for an additional stent placement (C-E). Follow up PDA stenting showed full coverage (F).

Percutaneous Pulmonary Valve Placement
Pre-Procedural Cross-sectional imaging is an indispensable tool for patient counseling and planning prior to percutaneous pulmonary valve placement. The three leading       Overlay planning using the VesselNavigator system (Philips Healthcare, Best, NL) for a patient with a discrete coarctation of the aorta (CoA) prior to stenting using CT images. The area of interest is outlined in blue. Revised from ref [9]. A 26-French DrySeal sheath was advanced from the femoral venous access to deliver the Harmony valve. Additional access was obtained from the right internal jugular vein for angiography during valve placement.

Cross-Sectional Overlay Fusion in the Catheterization Lab
Cross-sectional overlay enabled registration of previously acquired CT and/or MRI images is being used to rapidly fuse with X-ray fluoroscopic imaging in single and biplane systems [10,11]. Several single-plane overlay examples (VesselNavigator system (Philips Healthcare, Best, The Netherlands)) are outlined below (Figures 14-18) [12]. A biplane overlay example (Siemens bi-plane system (Siemens Healthineers, Munich, Germany) is depicted in Figure 19 [13]. Overlay fusion has demonstrated a reduction in overall radiation burden [8,9]. A 26-French DrySeal sheath was advanced from the femoral venous access to deliver the Harmony valve. Additional access was obtained from the right internal jugular vein for angiography during valve placement.

Figure 14.
Overlay planning using the VesselNavigator system (Philips Healthcare, Best, NL) for a patient with a discrete coarctation of the aorta (CoA) prior to stenting using CT images. The area of interest is outlined in blue. Revised from ref [9].

Figure 14.
Overlay planning using the VesselNavigator system (Philips Healthcare, Best, The Netherlands) for a patient with a discrete coarctation of the aorta (CoA) prior to stenting using CT images. The area of interest is outlined in blue. Revised from ref [9].

Coronary Artery Assessment in CHD
Congenital coronary interventions are rare, and the anatomy for each case is quite varied. Therefore, cross-sectional imaging can be extremely helpful when planning such interventions. An example of an anomalous left coronary artery from the pulmonary artery (ALCAPA) repair with residual left main coronary artery (LMCA) narrowing is outlined in Figures 20 and 21. In addition, ongoing studies aim to assess pediatric coronary allograft vasculopathy with CT angiography and CMR adenosine stress perfusion [14,15] to compliment traditional assessment by X-ray fluoroscopic catheter angiography.

Future Directions
Invasive CMR (iCMR) is an attempt at incorporating the best of both modali 20]. There is concern that radiation exposure during childhood may predispose t ulation to increased risk of cancer later in life [21]. By pursuing catheter-based pro under MR guidance instead of X-ray fluoroscopy, patients will benefit from redu overall procedural radiation exposure. iCMR utilizes a dilute gadolinium-filled tip catheter in combination with an MR-conditional guidewire to obtain cardiac h namics with real-time CMR guidance. Examples of iCMR equipment and proced outlined in Figures 22-25.

Future Directions
Invasive CMR (iCMR) is an attempt at incorporating the best of both modalities [16][17][18][19][20]. There is concern that radiation exposure during childhood may predispose this population to increased risk of cancer later in life [21]. By pursuing catheter-based procedures under MR guidance instead of X-ray fluoroscopy, patients will benefit from reduction in overall procedural radiation exposure. iCMR utilizes a dilute gadolinium-filled balloon-tip catheter in combination with an MR-conditional guidewire to obtain cardiac hemodynamics with real-time CMR guidance. Examples of iCMR equipment and procedures are outlined in

Discussion
CT and CMR continue to advance the field of congenital cardiology by allowing for new and unique procedures not previously imagined. Enabling the interventionalist to enter the catheterization lab with a plan of attack expedites the procedure and has been shown to reduce procedural times and improve outcomes. Substantial advances in cross-sectional image resolution with a significant decrease in radiation exposure for cardiac CTs have led to more mainstream utilization. CMR has a unique ability to deliver real-time functional imaging in several views without exposing the patient to the detrimental effects of ionizing radiation. This can reduce procedural times in the interventional fluoroscopic suite to allow for more directed procedures. iCMR evaluations are especially beneficial for the pulmonary hypertension [22] and single-ventricle patient populations as assessment by cardiac catheterization alone may not be adequate to assess important variables such as cardiac flows [23,24] and lymphatic physiology [25].

Conclusions
A significant shift toward pre-procedural planning utilizing cross-sectional imaging and 3D reconstruction is now being embraced by congenital heart centers to better serve their patients. With continual advancements, we anticipate new interventions and improved outcomes for the future of congenital cardiology.

Discussion
CT and CMR continue to advance the field of congenital cardiology by allowing for new and unique procedures not previously imagined. Enabling the interventionalist to enter the catheterization lab with a plan of attack expedites the procedure and has been shown to reduce procedural times and improve outcomes. Substantial advances in cross-sectional image resolution with a significant decrease in radiation exposure for cardiac CTs have led to more mainstream utilization. CMR has a unique ability to deliver real-time functional imaging in several views without exposing the patient to the detrimental effects of ionizing radiation. This can reduce procedural times in the interventional fluoroscopic suite to allow for more directed procedures. iCMR evaluations are especially beneficial for the pulmonary hypertension [22] and single-ventricle patient populations as assessment by cardiac catheterization alone may not be adequate to assess important variables such as cardiac flows [23,24] and lymphatic physiology [25].

Conclusions
A significant shift toward pre-procedural planning utilizing cross-sectional imaging and 3D reconstruction is now being embraced by congenital heart centers to better serve their patients. With continual advancements, we anticipate new interventions and improved outcomes for the future of congenital cardiology.  Acknowledgments: The authors would like to thank the entire cardiac catheterization and advanced imaging staff at Children's Medical Center Dallas.

Conflicts of Interest:
The authors declare no conflict of interest.