Intracardiac echocardiography (ICE) is an intravascular ultrasound (IVUS) modality that provides diagnostic imaging of cardiac structures from within the heart and has become widely used for guiding noncoronary interventions in the catheterization and electrophysiology laboratories. The first IVUS catheters used high-frequency transducers (20-40 MHz) containing a single ultrasound crystal that rapidly rotated at the end of the catheter, producing a radial 2-dimensional image.¹ This type of high-frequency IVUS transducer provides excellent spatial resolution in the near field, making it uniquely suited for imaging the coronary arteries and other small vessels. The main limitation of IVUS in this frequency domain, however, is the short imaging depth (several millimeters).^1,2 To accomplish ICE imaging from atria to apex, lower frequency transducers (5-12 MHz) have been miniaturized and mounted onto catheters capable of percutaneous insertion and manipulation within the heart.^1,3-7 These lower frequency transducers are capable of greater tissue penetration and imaging depth, permitting high-resolution two-dimensional imaging of the whole heart.^2,8-11 The earliest experiences with such low-frequency ICE catheters were described in the late 1970s and early 1980s.^3,4 More recently, with the introduction of the newest phased array transducers, full Doppler flow data can be obtained.

Two different types of ICE catheters are currently available for clinical use. A mechanical transducer, similar to that used with IVUS, has a rotating ultrasound transducer driven by a motor unit at the opposite end of the drive shaft, which results in a 360° “radial” view perpendicular to the axis of the catheter. The second type is a fixed or phased array catheter-mounted transducer that uses electronically controlled transducers mounted on one side of the catheter shaft, which results in a wedge-shaped 90° image sector similar to that of transthoracic or transesophageal echo probes.

Both types of catheters provide high-resolution, real-time images of cardiac morphology and devices or catheters in the heart. Current ultrasound catheters, with a diameter size between 6- and 10- Fr, are typically introduced via a sheath in the femoral vein. Phased array catheters offer a large depth of field and add Doppler imaging capabilities, whereas mechanical catheters offer superior near-field resolution.

ICE offers imaging that is comparable or, in some cases, superior in quality to transesophageal echocardiography (TEE). ICE adds substantial anatomic information to x-ray fluoroscopy for electrophysiologic ablation procedures and transcatheter atrial septal defect closure. ICE has been shown to provide procedural benefits in the context of radiofrequency ablation procedures for atrial fibrillation and transcatheter atrial septal closure procedures,^12-26 and as such, ICE has become the “reference standard” for imaging during these procedures. The major advantage over TEE is that no general anesthesia is needed for ICE. Patients who have contraindications to TEE, eg, those with significant esophageal disease, can also avoid TEE while maintaining adequate imaging using ICE. In addition, if sufficiently skilled and knowledgeable, the operator performing the percutaneous closure can also manipulate the ICE catheter and interpret the images. In this setting, ICE has been shown to improve patient comfort and shorten procedure and fluoroscopy time, at comparable cost, compared to TEE-guided interventions under general anesthesia.^21,27,28 ICE is currently used as the primary imaging modality during percutaneous transcatheter closure (PTC) without need for supplemental transthoracic echocardiography (TTE) or TEE. ICE has also been shown to provide incremental diagnostic benefit over TTE and TEE. In a series of 94 patients undergoing PTC,²⁸ ICE revealed additional diagnoses in 32% of patients, despite preprocedural performance of TTE and TEE in the vast majority of patients. These diagnoses included additional atrial septal defects, atrial septal aneurysms, atrial septum redundancy, anomalous pulmonary venous return, pulmonary arteriovenous malformations, and pulmonary vein stenoses.

Additional interventional applications of ICE include guidance of transseptal catheterization, left atrial appendage occlusion, placement of ventricular assist device cannulas, percutaneous mitral balloon valvuloplasty, and many others (Table 13-1).^29-43 Intracardiac imaging may serve as a diagnostic alternative in patients with contraindications for TEE (eg, esophageal pathology) or to assess anatomic regions that are inadequately visualized by TEE (eg, tracheal shadowing of the aorta).^44-46 Similarly, ICE can readily evaluate native and prosthetic valves that are not invariably well visualized by TEE.

In the electrophysiology laboratory, ICE provides guidance for pulmonary vein isolation in patients with atrial fibrillation, sinus node modification, ablation procedures for idiopathic ventricular tachycardia (right ventricular outflow tract and aortic sinus locations), ablation procedures in the left ventricle, and extraction of pacemaker or ICD leads.^13,47-50 In the electrophysiology laboratory, shorter procedure times, reduced arrhythmia recurrence, and reduced pulmonary vein stenosis occurrence have all been attributed to ICE guidance.^{13-16,18,19,51}

Currently there are 3 commercially available ICE systems, each with unique features and advantages.⁵² The Boston Scientific UltraICE Plus system uses a mechanical radial ICE imaging transducer, is not steerable, and does not provide Doppler-derived information. This 8.5-Fr system requires a short amount of time for catheter preparation: the housing must be flushed with saline and then physically spun prior to use. The transducer rotates at 1800 rpm and has a fixed frequency of 9 MHz. Both Siemens AcuNav and the St. Jude (formerly EP Medsystems) ViewFlex Xtra use phased array transducers. These transducers are steerable and deflectable. They provide 2-dimensional sector imaging with color and spectral Doppler capabilities. The ViewFlex Xtra and AcuNav systems have 4 directions of steering (anterior, posterior, right, and left). All 3 systems use a single-use ICE imaging catheter, require 8- to 11-Fr venous access, and are relatively expensive, with an approximate price range of $800 to $2800. The 8-Fr transducer catheters provide somewhat less depth of imaging when compared to larger French systems.

Phased array ICE systems are generally bulkier than rotational ICE, with a handle and 2 dials to aid in maneuvering the catheter to a desired image while providing a 90° field of imaging. They also provide color Doppler and do not require flushing prior to use. Real-time 3-dimensional imaging is under investigation in the United States for both phased array systems. A housed rotational ICE system provides single 360° planar imaging but does not provide color Doppler. It is also less bulky and less maneuverable. No 3-dimensional imaging is currently available with rotational ICE.

Limitations of ICE

Current limitations of ICE include the relatively high cost of the single-use catheters and their large shaft diameters (less of an issue with the release of 8-Fr catheters). They provide only single-plane imaging with a relatively narrow field of view. Complications from the use of ICE are rare, but include all of the traditional vascular access–related complications, as well as perforation of the caval vein or cardiac chambers, which could result in cardiac tamponade (Table 13-2).

Principles of Imaging

Images acquired with the newer phased-array steerable transducers such as the AcuNav catheter produce images that are similar to TEE images, except that they originate from within the heart and accordingly require that the interpreter become familiar with a different set of image orientations. The AcuNav transducer has lockable steering controls, so that a particular imaging plane may be set and held in a stable position. Typically, AcuNav images are 90° sector images originating from the transducer location within a cardiac cavity. The AcuNav catheter is usually inserted via a femoral vein. Advancing and withdrawing the catheter shifts the images sideways (in-plane) on screen. A small marker placed outside the imaging sector indicates the proximal boundaries of the image (“operator end”). By emerging convention the image is displayed with the (marked) proximal end of the cross-section on the left side of the screen. Similar to the handling of a TEE probe, subtle rotation of the catheter will result in different angular cross-sections. While the transducer is typically positioned in the right atrium, images can be obtained from any cardiac and vascular structure into which the transducer tip of the catheter can be maneuvered. By systemic venous vascular access, the right atrium, the venae cavae, and the right ventricle can be reached, as well as the left atrium through the atrial septum. In contrast, with radial ICE imaging, the catheter is typically placed in the right atrium or venae cavae, with a more limited range of motion, as these catheters are neither steerable nor deflectable.

The imaging sequence with AcuNav usually begins from the right atrium, with the catheter in a neutral position and the locking mechanism disengaged. In the default position, the ultrasound array is sagittally oriented, parallel to the catheter, and upward in correspondence with the indicator line on the handle. All subsequent maneuvers are described relative to this starting position. The catheter is gently rotated approximately 15° to 30° clockwise from the default position to image the “home view” (Fig. 13-1). This view provides excellent imaging of the mid-right atrium, tricuspid valve, and right ventricle, and typically provides an oblique or short-axis view of the aortic valve.

Further clockwise rotation up to approximately 30° will display the right ventricular outflow tract and pulmonic valve. The aortic valve appears in a near short-axis view (Fig. 13-2). With clockwise rotation up to approximately 45°, the left ventricle can be seen in an oblique long-axis view with the apex toward the outer edge of the imaging sector. Also the left ventricular outflow tract and aortic valve will be visible (Fig. 13-3). Color-flow Doppler interrogation in this orientation may reveal aortic regurgitation (Fig. 13-4).

Further clockwise rotation to approximately 60° will provide a long-axis view of the left ventricle (LV) and the mitral valve (Fig. 13-5). Minimal anterior tilting toward the septum may improve visualization of the LV. At 70° to 80° clockwise rotation, the interatrial septum (IAS) is visualized, with the left atrial appendage in the far field (Fig. 13-6). At 90° to 100° clockwise rotation and with the ICE transducer positioned along the IAS, the left inferior and left superior pulmonary veins in the far field are seen (Fig. 13-7). Color-flow Doppler and pulse-wave Doppler flow patterns help differentiate the left atrial appendage (LAA) from the left pulmonary veins. From this position, the beam is aligned parallel to the pulmonary vein flow direction for optimal Doppler interrogation.