IMAGING TECHNOLOGY

For guidance of intracardiac beating-heart repair, the operator relies greatly on visual feedback from imaging modalities while manipulating rapidly moving delicate anatomic structures in the presence of blood. Ideally, the surgeon receives the visual information as close as possible to the direct vision picture of the intracardiac lesion the surgeon is trained to interpret. Thus, in hybrid beating-heart surgery, imaging should provide real-time three-dimensional (3D) information regarding the target lesion, neighboring structures, and position of the instruments and devices.

Until very recently, x-ray fluoroscopy, echocardiography, magnetic resonance imaging (MRI), and computed tomography (CT) were the predominant preoperative diagnostic tools, at times used for surgical planning. Parallel developments in video-assisted endoscopy and robotic surgery in the past decade, together with advances in optics and digital electronics, allowed precise visualization of the intracardiac structures. Although each of these imaging modalities has advantages and drawbacks and may be used for guidance of intracardiac repairs, fluoroscopy and echocardiography are the most commonly used for guidance of beating-heart interventions. Video-assisted endoscopy or cardioscopy has been successfully used as an additional imaging tool for both beating-heart and open-heart hybrid procedures.

Echocardiography

Echocardiography is a viable imaging tool for guiding beating-heart interventions. The nonionizing nature of ultrasound, the ease of data acquisition, the ability to focus on a specific anatomic structure, and a variety of additional quantification tools have enabled virtually routine application of 2D and 3D echocardiography in the operating room.¹^–³ Real-time 3D echocardiography (RT3DE) is a unique imaging and guiding tool, providing ample intraoperative assessment of intracardiac anatomy and enabling navigation of instruments and devices toward the target inside the beating heart (Fig. 133–1).⁴^–⁹ Although intraoperative transesophageal RT3DE imaging was recently introduced, epicardial imaging remains a significant alternative for visualization of complex lesions in younger patients. Obstacles to application of 3D echocardiography as the sole guiding tool for beating-heart procedures, such as acoustic interference from metallic instruments causing shadowing and side-lobe artifacts in the field of view, have been reported. These can be minimized with instrument modifications (e.g., by applying a surface coating) and by varying the angle of instrument navigation with respect to the ultrasound probe.^10,¹¹

Figure 133–1 A, Representation of 3D echocardiography images of an Amplatzer device (AGA Medical Corp., Plymouth, MN) as it is being deployed live. View from the left atrium. B, Isolated A1 segment prolapse of the mitral valve, surgical view from the left atrium. C, Mitral valve quantification (MVQ) analysis of the mitral valve apparatus using QLab software (Philips Ultrasound, Andover, MA) shows specific location of the P2 segment prolapse of the mitral valve. Ao, aorta; AL, anterolateral commissure; PM, posteromedial commissure.

(A modified from Salgo IS. Semin Thorac Cardiovasc Surg 2007;19:325-9; B and C modified from Fischer GW, Salgo IS, Adams DH. J Cardiothorac Vasc Anesth 2008;22:904-12.)

X-Ray Fluoroscopy

Fluoroscopy is still in most common use in the catheterization laboratory. It provides great resolution at fast frame rate (>30 Hz). However, it has significant limitations, such as absence of 3D information and lack of comprehensive representation of soft tissue. In addition, the potential harmfulness of x-rays has been widely discussed.¹²^–¹⁴ Nevertheless, this imaging modality is still used with intraoperative angiography for additional diagnostic measurements of residual septal defects or aortic arch repairs, but it is crude and limited for other purposes (e.g., pulmonary artery plasty, unifocalization, aortopulmonary shunts).

Video-Assisted Cardioscopy

Alternative imaging techniques for visualization inside the beating heart in real time include video-assisted cardioscopy (VAC) using visible wavelength light. However, the real limitation to endoscopic intracardiac imaging has been inability to see through the blood. Although VAC offers detailed, high-magnification pictures of the target and provides greater confidence for fine manipulations of instruments, depth of field is extremely limited and the scope window must be pressed directly against the target structures for visualization.¹⁵^–¹⁷ VAC is successfully used as an additional imaging tool in open-heart procedures to assess distal lesions and may assist with hybrid procedures such as pulmonary artery (PA) stent placement or apical VSD device closure.¹⁸^–²¹

Recently, fiberoptic infrared endoscopy was introduced to overcome the depth-of-field problem, because the wavelength used permits transmission through blood for a few millimeters.²² The depth of field, however, is still less than 1 to 2 cm, making navigation through the cardiac structures difficult and requiring the use of other imaging techniques such as fluoroscopy. An additional limitation of current infrared systems is that they have a relatively low frame rate, which requires significant computer processing for real-time imaging.

Multimodality Imaging and Surgical Displays

The purpose of multimodality imaging is to provide the surgeon with additional anatomic information that can be obtained with only one modality but when combined gives a more complete road map for navigation. Examples of such imaging include overlay of real-time x-ray or echocardiography information with preoperative CT or MRI patient data.^23,²⁴ Images are aligned based on anatomic features or on man-made objects, known as fiducials, that are introduced into the image. This approach is successfully implemented in arrhythmia ablation and other procedures.²⁵^–²⁷ With advancements in computer graphics, novel surgical displays can simultaneously represent these complex 2D and 3D anatomic data from various imaging modalities, real-time instrument positioning, and patient functional information (Fig. 133–2). This combined visual information is extremely important for surgical decision making during intracardiac image-guided procedures, with the absence of direct visualization and haptic feedback.