4 | DISCUSSION
TEE is the primary image guidance modality used for trans-catheter structural heart intervention technologies because of its better real-time visualization of cardiac anatomical structures. However, its limited sector width and complex probe orientation prolong the learning curve to comprehend the spatial configuration of procedural TEE imaging. Although CT has been widely used as a preoperative planning tool for trans-catheter structural heart interventions, the created 2D CT images are usually taken from different visual perspectives of intraoperative TEE images. This creates issues in positioning, docking and taking other specific key steps of these image-guided interventions, because the procedural TEE imaging may fail to reproduce the same prerequisite patterns as the CT based preoperative planning. Compared to the high degree of freedom in the CT MPR slices, the motion of TEE probe is rather limited. Therefore, the methodology which we propose here is capable of carrying out a preoperative plan by CT MPR to simulate the intraoperative TEE rather than applying the TEE to acquire images conforming to what is showed on CT slices. In this article, we demonstrate a stepwise CT MPR manipulating method to simulate the TEE probe motion and preview the procedural TEE images on a different imaging modality. This reduces somewhat the differences in deciding the size of devices from these two imaging modalities. Besides validating this method in different TEE views, we also applied it on dynamic CT to mimic TEE videos. This concept has not previously been introduced in a systematic fashion.
On the other hand, such a learning curve is also steep for interventional echocardiographers to precisely offer the procedure-specific imaging needs in every patient. 13Although basic acquisition protocols of TEE have been introduced by Hahn and colleagues, 13–16 many patient-specific anatomical conditions, such as clockwise/counterclockwise rotation of the heart, trajectory of esophageal tract, geometry of gastric space and its relative position with cardiac structures all lead to significant differences in the spatial configuration of procedural TEE imaging. As a result, the TEE imaging is of large variability and highly technique dependent. Sometimes, it is even impossible to obtain the ideal procedure-specific views in patients of challenging anatomy. In the application of a traditional method, interventional echocardiographers discern not the feasibility of those procedure-specific views until TEE probe is inserted. Fortunately, in our simulation, the CT datasets in nature contain all the anatomical information, and thus broaden the vision of interventional echocardiographers beyond the scanning sector, offer the spatial orientation between esophagus and the interested cardiac structure, and thus enable us to foresee whether the desired procedure-specific imaging is feasible or not since each step in our method can be translated back into a patient-specific TEE acquisition protocol. If the procedure-specific imaging is not feasible, the heart-team can promptly use our method to seek surrogate TEE views at other esophageal or transgastric levels. This assists interventional echocardiographers and operators reach to a consensus in the preoperative rehearsal and may even shorten the procedure time and fluoroscopic time.
In this era of echocardiography-fluoroscopic and CT-fluoroscopic fusion imaging, one might underestimate the importance in comprehending the 3D orientation of 2D TEE images. Nevertheless, it is important for transcatheter interventionists to extend their proprioception all the way to the catheter tip and nearby structures. Proprioception needs visual assistance to become precise perception; however, unlike the electroanatomical mapping systems of the ablation catheters, current devices for structural heart diseases are still unable to work on precise targeting or tracking in the 3D space but only projecting to the fluoroscopy where essential spatial resolution is lacking. In fact, simply putting all modalities onto a cine fluoroscopy produces nothing more than confusions because there are either just static segmentations or mere artificial intelligence based modeling rather than imaging. Although fusion imaging with high-resolution TEE images is feasible nowadays, anatomical details are still discernible only when the image intensifier is at the correct projection angles to spread the 2D images. Our method allows patient-specific fluoroscopic viewing angles to be generated simultaneously during the MPR manipulation. In combination with the concept of the optimal projection curve and the projection-slice relationship, 9,10 CT MPR can enhance a better understanding of the spatial orientation of echocardiography-fluoroscopic fusion imaging.