The physics of ICE are the same that are used for all applications of ultrasound. These include: 1) Mechanical waves with frequencies greater than 20,000 Hz; 2) Laws of sound wave reflection and refraction while crossing borders between materials of different densities; and 3) The application of miniaturized transducers and techniques that create images.
These images can then be displayed as M and B modes, with Doppler Effect (pulsed wave, continuous wave, or color flow imaging), and as three-dimensional reconstruction.
Two-dimensional ICE is the primary echo modality used in today s electrophysiology laboratory because it provides meaningful, real-time anatomic information occurring within the structures of the heart. The catheter-tipped miniaturized echo transducer uses either a series of crystals (phased arrays: EPMedSystems, Siemens, JoMed), or a single crystal in which the beam is moved by mechanical means around a circle (Boston Scientific). The phased array systems consist of either linear phased arrays (sector shaped images with side-firing arrays: EPMedSystems and Siemens), or circular phased arrays (radially arranged crystals around the tip of the catheter with a circular image format: JoMed).

Applications for ICE in EP Diagnosis and Intervention
Applications for ICE utilization in the EP laboratory environment are as varied as the procedures performed within those labs. With ablation procedures for atrial fibrillation, ICE imaging allows for the direct visualization of the pulmonary veins, location of the atrial-venal junction and assurance of the ablation catheter tip location within the pulmonary vein antrum. ICE also permits continuous monitoring for radiofrequency (RF) energy delivery during ablation, hemodynamic performance of the myocardium and pericardial space monitoring. For transseptal puncture procedures, right atrial ICE catheter positioning provides clear visualization of the fossa ovalis, tenting by the transseptal catheter and presence of saline bubbles in the left atrium once penetrated by the Brockenbrough needle. ICE has also recently contributed to the understanding of AVNRT mechanisms by confirming the association of a dilated coronary sinus ostium with the slow pathway.³
Phased array ICE catheters have been placed into the right ventricle to assess left ventricular septal/left free-wall strain rates and aortic flow/velocity for the evaluation of dyssynchrony and optimizing cardiac resynchronization therapy. ICE can therefore be utilized during the implant of coronary sinus placed, left ventricular pacing leads to assess the effects of pacing on ventricular wall motion and resynchrony efforts.⁴ ICE can also be used to accurately place a right atrial pacing lead above the fossa ovalis to shorten the p-wave duration. This cannot reliably be accomplished using fluoroscopy alone.⁵

Basic ICE Catheter Insertion Techniques
The typical approach for the insertion of an ICE catheter has been from the inferior vena cava and positioning the catheter within the right atrium. This approach offers easy accessibility from either femoral vein and affords relatively free movement of the catheter. Imaging deep into the left atrium and its appendage are excellent using this approach. The downside of this approach is that there can be increased reflection from other catheters sharing the entrance of the IVC, and the possibility of entanglement with those catheters below the level of the IVC, which can limit maneuverability.
An alternative approach, gaining some popularity, is placing the ICE catheter into the right atrium from the superior vena cava. Access for this approach can be achieved either from the right internal jugular vein or the left subclavian vein. Visualizing images from this approach does require some re-orientation to viewing structures "upside down," but offers either the same or better quality images than is seen with the IVC approach. At our institution, we prefer access from the left subclavian vein because views can often be achieved that are less prone to interference and reflection from other catheters, especially in the setting of ablation for atrial fibrillation.

Cardiac Structure Recognition
One of the biggest hurdles to overcome in the use of intracardiac echo is recognition of cardiac structures, especially for the allied professional. For most of us, ICE images look as foreign as intracardiac signals did when we first came into the lab setting. The following short segments will attempt to cover the basic structures imaged during EP procedures. The goal of this article is not to make experts in intracardiac echo imaging, but to provide a foundation that can hopefully be built on with your own lab experiences.
For the purposes of this article, the images used will be from a phased array intracardiac echo system.

The Right Atrium
The right atrium is the chamber most often imaged in the EP lab setting, and it s certainly the starting point for procedures in which ICE is utilized. From the "home view" (ICE catheter placed either via the IVC or SVC to the level of the mid-septum), most right atrial structures can be visualized with very little catheter manipulation.
As seen in Figure 1A, the fossa ovalis is clearly visible. This image was obtained from the SVC approach with the transducer at the level of the superior portion of the tricuspid valve, slight anteflexion placed on the catheter tip and rotated toward the septum. By relaxing the flexion on the catheter and rotating slightly clockwise, the coronary sinus ostium and Thebesian valve can be viewed en face (Figure 1B). If the catheter is placed from the IVC, a sagittal plane view of the coronary sinus can be imaged (Figure 1C).

For evaluation of and monitoring during ablation for AVNRT, the membranous septum (structural location of the HIS bundle) can be observed (Figure 1D). This view could conceivably be used for attempting to place a permanent pacing lead directly into the HIS bundle.

The Left Atrium
Using fluoroscopy alone, differentiation of the left superior pulmonary vein (LSPV) and the mouth of the left atrial appendage (LAA) can be difficult to say the least. By positioning the ICE transducer high in the right atrium, minute catheter tip rotation from anterior to posterior can delineate the LAA from the LSPV (Figure 2A). Figure 2B is a LAO fluoroscopic image of the ICE catheter placement in the right atrium for imaging of the left atrium. Often, the left inferior pulmonary vein can be seen in the same image as the superior vein (Figure 2C). This view is crucial if using a circular catheter alone for pulmonary vein ostial identification and ablation. As seen in Figure 2D, by shortening the imaging depth, anteflexing the catheter tip and rotating the catheter directly posterior, the right pulmonary veins can be visualized. During ablation, continuous ICE monitoring of the left atrium is important for determining the amount of RF energy that can safely be used.⁶ The presence of micro-bubbles and the formation of clefts or craters can only be seen using ICE, and are indicative of the need to either decrease the RF wattage (few bubbles) or terminate the RF application completely (bubble shower or crater formation). This method has been popularized by Dr. Andrea Natale, and has become the standard for circular catheter-guided ablation for atrial fibrillation.

The Right and Left Ventricles
ICE imaging of the right ventricle and its surrounding structures can be useful during ablative therapy for right ventricular outflow tract tachycardias. The tricuspid valve, right ventricular apex and outflow tract can be imaged by placing the transducer in the low right atrium and at the level of the tricuspid valve (Figure 3A). The right and left ventricles, as well as the intraventricular septum, can be imaged by advancing the ICE catheter into the right ventricle (SVC approach), and anteflexing the catheter tip slightly (Figure 3B). This would be the easiest view to use for assessment of septal wall motion with biventricular pacing. The mitral valve, left ventricle and the left ventricular outflow tract can be viewed by placing slight counter-clockwise torque on the catheter (Figure 3C). Figure 3D is a RAO fluoroscopic view of the catheter position for left and right ventricular imaging.

Conclusion
Intracardiac echo imaging can potentially be one of the most useful tools in your arsenal for diagnosis and treatment of multiple disorders corrected in the electrophysiology laboratory. No other system currently available can offer real-time, direct visualization of cardiac structures during procedures. With new technology emerging for three- and four-dimensional ICE, and the possibility for registration of ICE with three-dimensional mapping systems, this modality will only become more important in the electrophysiology laboratory of tomorrow.