1.1.

Epicardial Ablation

Current treatment methods for ventricular tachycardia include antiarrhythmic drugs, implantable defibrillators, and catheter ablation.⁵ Implantable defibrillators and antiarrhytmic drugs palliate arrhythmia symptoms, whereas ablation therapy is a curative method. Because of the large percentage of epicardial circuits for VT, percutaneous epicardial ablation is an attractive method. Pericardial access is obtained through a subxyphoid approach, where a 17-gauge needle is introduced under fluroscopic guidance. A small amount of contrast medium is injected, and the needle is advanced into the pericardial space. Next, a guide wire is passed over the needle, and a sheath is advanced in which the ablation catheter can be introduced and maneuvered within the pericardial space for mapping and ablation.⁶ Acute success of epicardial ablation for VT is between 60 and 75% using standard rf catheters.⁶ In a study of 112 patients with Chagas’ VT, the $3\text{-year}$ recurrence rate of patients that received simultaneous epicardial ablation and endocardial ablation was 30%.⁷ One factor limiting success rates is attributed to rf energy delivery in the presence of epicardial fat. Fat attenuates rf energy, resulting in limiting lesion depth⁴ and may mimic infarction during electroanatomical mapping.⁸ In addition, complications can arise, requiring early termination of the procedure. A major complication that has been encountered in epicardial RFA is vascular injury and thrombosis due to energy delivery in close proximity to a coronary vessel. A previous study by d’Avila showed that the susceptibility of damage of coronary vessels is inversely proportional to vessel diameter, where it is hypothesized that larger vessels are protected due to increased blood flow.⁹ For coronaries within $1\mathrm{mm}$ of an ablation lesion, small vessels showed severe injury, the presence of thrombosis, and intimal hyperplasia.¹⁰ One of the most important criteria for a successful RFA lesion is good and continuous catheter-tissue contact. However, this is difficult to assess during rf delivery.

1.2.

Preprocedural Imaging

Currently, procedures are guided by real-time fluoroscopy, electrograms, electrode temperature, and tissue-electrode impedance. However, a real-time view of the epicardial surface is not available for procedural guidance and identification of critical structures. The use of preprocedural imaging has been demonstrated to be valuable for planning complex RFA procedures, obtaining a detailed anatomy of the heart, identifying areas of infarction using MRI^{11, 12} and echocardiography,¹³ and epicardial fat using computed tomography.^{14, 15} Three-dimensional volumes of the heart structure and segmented volumes of fat and infarction can be integrated with 3-D voltage mapping for improved procedural planning. These volumes are obtained hours to a day before the procedure, are low resolution, and static. This results in displacement errors once correlated with 3-D voltage maps⁵ or real-time fluoroscopy, limiting the utility of merged images. Directly before the start of ablation therapy, coronary angiography can be conducted to provide a visual of coronary vessels. If the site of arrhythmia focus is near a coronary, a risk stratification analysis is conducted before the start of rf energy delivery. There is a clinical need to assess epicardial substrates with high resolution and in real time, to reduce procedural complications by avoiding coronary vessels and increasing efficacy, by titrating rf energy dosage in the presence of epicardial fat, validating lesion formation, and assessing rf catheter-tissue contact.

1.3.

Optical Coherence Tomography

Optical coherence tomography (OCT) is a real-time, high-resolution imaging tool that provides cross sectional images of tissue microstructure¹⁶ with $10\text{-}\mu \mathrm{m}$ resolution and $1\text{to}2\text{-}\mathrm{mm}$ imaging depth into cardiac tissue.¹⁷ OCT can address unmet clinical needs of epicardial radiofrequency ablation therapy by assessing the contact and contact angle of the rf catheter with the tissue, confirming that a lesion has been formed after energy delivery, detecting early damage and identifying structures for procedural guidance. We have previously demonstrated the ability to monitor thermal ablation of the myocardium by radiofrequency ablation (RFA) using OCT.¹⁸ Viable tissue is characterized by a polarization artifact dark band within conventional OCT images¹⁸ due to the birefringence property of the highly organized myocardial tissue. Previous studies have also been conducted to evaluate laser ablation of porcine myocardium¹⁹ and thermal heating of porcine skin²⁰ with polarization-sensitive OCT (PSOCT), observing changes in birefringence properties. OCT has been demonstrated to visualize critical structures related to electrical conduction, including the Purkinje network,¹⁷ and imaging the fast and slow pathways in the atrial-ventricular (AV) node,^{21, 22} myofiber organization^{22, 23} in animal models, and in vitro preparations of human tissue.²⁴ In addition, OCT has been used extensively for intravascular applications,^{25, 26} analyzing blood flow and detecting small blood vessels in ophthalmology,²⁷ and characterizing adipose tissue in breast cancer.^{28, 29}

We hypothesize that OCT can be used for monitoring and guidance of epicardial ablation procedures. Providing a cross sectional view of coronary vessels will allow evaluation of vessel lumen size. With this information, the clinician will have more information to assess if energy can be delivered safely. Assessing tissue contact, validating lesion formation, and visualization of epicardial fat will reduce ambiguity in interpreting electrograms and enable titration of rf energy dosage to ensure effective energy delivery. Here we report the results of OCT imaging of normal untreated myocardium, ablation lesions, epicardial fat, and coronary vessels, and describe the distinguishing characteristics, including differences in optical properties and tissue architecture, and assessment of tissue contact with catheter-based imaging. This sets the foundation for developing OCT as a guidance tool for epicardial RFA. Real-time imaging of epicardial RFA therapy with OCT can potentially provide a direct visual of the epicardial surface, assess contact, provide visual confirmation of energy delivery and feedback to reduce complications, and titrate dosage by visualizing critical structures of interest.

2.1.

Sample Preparation

Freshly excised ventricular wedges from swine hearts were used for characterization of epicardial structures and ablation lesions with OCT imaging. Following the onset of general anesthesia, a lateral thoracotomy was performed, and the heart was rapidly excised and placed in ice-cold phosphate-buffered saline (PBS). Eight hearts were used for this study. The right ventricular free wall and left ventricular free wall were dissected and placed in ice-cold PBS, up to half an hour, until the start of imaging. All animal studies were conducted according to protocols approved by the Institutional Animal Care and Use Committee of Case Western Reserve University (Cleveland, Ohio).

For characterization of epicardial rf lesions, dissected ventricular wedges were placed in a bath with PBS maintained at $37°\mathrm{C}$ with superperfusion flow [Fig. 1 ]. A series of ablative lesions were created with a temperature-controlled $(60°\mathrm{C})$ protocol with a maximum delivered power of $50\mathrm{W}$ and duration of $60\sec$ , using the Maestro 3000 RFA generator (Boston Scientific, Natick, Massachusetts). Epicardial lesions were created using a $7\text{-}\mathrm{Fr}$ , $5\text{-}\mathrm{mm}$ tip Blazer II catheter (Boston Scientific). A total of eight lesions were created using this protocol.

Fig. 1

Experimental samples. Gross view of epicardial surface of a wedge of (a) right ventricle and (b) left ventricle. Yellow arrows indicate subepicardial fat, black arrows indicate coronary vessels, and white arrows indicate radiofrequency ablation lesions. (Color online only.)

2.2.

Imaging Protocol

Three-dimensional image sets of swine ventricular epicardium were acquired with a microscope-integrated spectral domain OCT (SDOCT) system with a light source centered at $1310\mathrm{nm}$ with $70\text{-}\mathrm{nm}$ bandwidth (InPhenix, Livermore California). Spectral interferograms were acquired with a linear-in-wave number $(k=2\pi ∕\lambda )$ spectrometer³⁰ onto a $1024\text{-pixel}$ line scan camera (Goodrich, Princeton New Jersey) spectrometer, acquired at a $40\text{-}\mathrm{kHz}$ line scan rate. The system has a $4.3\text{-}\mathrm{mm}$ imaging range, $2\text{-}\mathrm{mm}$ $-6\text{-}\mathrm{dB}$ fall-off range, and $110\text{-}\mathrm{dB}$ sensitivity. The measured axial and lateral resolution of the system was 16 and $12\mu \mathrm{m}$ (in air) respectively.³¹ Each image was $4\mathrm{mm}$ in transverse length, 1000 lines per image, and $512\text{pixels}$ per line. Each volume consisted of 400 images. Assuming an index of refraction of 1.38 for ventricular tissue,³² the dimensions of the volumes were $4\times 4\times 3.11\mathrm{mm}$ (L, W, H), with a corresponding pixel resolution of 4, 10, and $6\mu \mathrm{m}$ , respectively. Summed voxel projection was used for rapid visualization of the 3-D image sets, and planes parallel to the sample surface were obtained by detecting the surface with an intensity threshold and digitally flattening the tissue surface.²²

Imaging of epicardial substrates with a flexible catheter probe was conducted to show the feasibility of OCT imaging of epicardial substrates using a probe suitable for in vivo imaging. The catheter probe has been described elsewhere.³³ Briefly, the probe is a contact, forward-imaging probe that provides cone scanning. The probe uses a gradient-index (GRIN) lens for focusing, a Risely prism to deflect the beam off axis, and applying torque on the fiber rotates the entire optical assembly to provide cone scanning (Fig. 2 ). An optical window isolates the probe optics from the environment. The probe sheath and end cap are polymers that are biocompatible and can operate in temperatures experienced in rf therapy, up to $95°\mathrm{C}$ . The rigid end length is $18\mathrm{mm}$ and the outer diameter is $2.5\mathrm{mm}$ . The scan diameter is $2\mathrm{mm}$ , which results in a $6.28\mathrm{mm}$ lateral scanning range. The probe maintains a full-width at half maximum (FWHM) spot size of less than $30\mu \mathrm{m}$ over an entire $1\text{-}\mathrm{mm}$ working range from the optical window. Images with the forward imaging catheter were acquired at $40\mathrm{kHz}$ line scan rate, 2000 lines per image, and $512\text{pixels}$ per line. This corresponds to a 6- and $3.1\text{-}\mu \mathrm{m}$ pixel size in the axial and lateral dimensions, respectively. A correlation-based method³⁴ was used to correct for nonuniform scanning rates, removing highly correlated axial scans.

Fig. 2

Optical schematic of forward imaging catheter probe. A GRIN lens is used for focusing, a Risley prism deflects the beam off axis, and optical glass isolates optics from the environment. The beam is focused $0.5\mathrm{mm}$ from the optical glass. Fiber is glued to the GRIN lens-Risley prism unit. Application of torque on fiber creates cone scanning.

2.3.

Validation

For all volumetric images, a microscopic image of the surface of the sample was acquired and registered with the OCT volume. Staining with triphenyltetrazolium chloride (TTC) was used to identify necrotic and viable tissue for samples with ablation lesions. TTC is a vital staining where viable tissue stains red and necrotic tissue stains white. TTC vital staining is a standard procedure for assessing acute necrosis. Immediately after the imaging protocol was completed, the tissue was sliced through the center of the ablation lesion and incubated in 1.0% TTC in PBS for $30\min$ at $37°\mathrm{C}$ .

4.1.

Substrate Characterization

Previous studies have characterized the optical properties of myocardium,^{35, 36} blood,³⁵ epicardial fat,³⁷ vessel wall of different compositions,³⁵ and myocardial radiofrequency ablation lesions³⁶ and myocardial laser ablation lesions.³⁷ It was shown that epicardial fat has significantly lower absorption than myocardium,³⁷ blood has high absorption,³⁵ and myocardial RFA lesions have an increased scattering coefficient.^{36, 37} Histological analysis of myocardium exposed to thermal damage showed decrease in birefringence within ablation lesions.³⁸ These varied optical properties are recapitulated at $1310\text{-}\mathrm{nm}$ wavelength and provide intrinsic contrast within OCT images of epicardial substrates. In addition to bulk optical properties, each tissue type has a characteristic microstructure that provides contrast within OCT images.

The normal myocardium was characterized by a strong birefringence artifact and bright epicardial layer. High contrast between the epicardium and the myocardium was observed within all images of the untreated myocardium. In all cases, there was an absence of a birefringence-dependent artifact within RFA lesions, and a decrease in contrast between the epicardium and myocardium. These result are consistent with our previous results, imaging lesions created on the endocardial surface, where we demonstrated a sensitivity and specificity of 94.5 and 86.7% respectively, for distinguishing ablation lesions from untreated tissue.¹⁸ The location in depth of the birefringence-dependent artifact within untreated myocardium varies, and is affected by multiple factors including the orientation of the myofibers and bending of the probe. This variability in the location of the band within images of the untreated myocardium can be seen in Figs. 3 and 3, and in a few cases the band may be difficult to discern [e.g., Fig. 3]. Although the polarization-dependent artifact is strong, future work will explore the use of polarization-sensitive OCT for quantitative assessment of tissue birefringence to more accurately distinguish ablation lesions from untreated tissue. The loss of contrast between the epicardium and myocardium within an ablation lesion may be due to the increase in scattering coefficient of the necrotic myocardium, reducing the scattering mismatch between the epicardium and myocardium.

Our experimental samples were freshly excised swine ventricular wedges, without coronary blood flow. We were able to identify vessels as signal-poor regions, correlating to the lumen of the vessel. OCT has been demonstrated to visualize blood vessels in vivo in ophthalmology with the use of the Doppler effect and to observe shadowing created by light absorption by blood.^{27, 39} In vivo, coronary vessels will obviously be filled with blood, so the lumens will not appear as voids. However, shadows will be apparent due to light absorption and scattering by the blood. In addition, the use of Doppler OCT³⁹ and/or speckle modulation⁴⁰ can be used to identify blood vessels using blood flow as the contrast mechanism. Therefore, we believe OCT will be able to identify coronary vessels of different sizes by identifying tissue regions with a shadow and exhibiting flow. Epicardial fat has a clear, characteristic heterogeneous appearance within OCT images. These results are consistent with previous applications of low coherence interferometery of adipose tissue from the breast, showing a heterogeneous axial scan profile.²⁸

4.2.

Real-Time Guidance

Because of the severe adverse complications that can arise due to epicardial ablation, preliminary studies have been conducted to evaluate the use of a flexible, steerable endoscope, $5.3\mathrm{mm}$ in diameter, to guide epicardial procedures by identifying coronaries.⁴¹ Air was used to expand the pericardial space and provided visualization of critical structures, rf catheter, and ablation lesions. However, image quality was diminished in the presence of saline flushing. As compared to the use of an endoscope for guidance, OCT provides subsurface imaging and can be implemented in small diameter catheter probes. As demonstrated here, even with $30\text{-}\mu \mathrm{m}$ lateral resolution, catheter-based OCT can be used to identify critical structures. By imaging with a catheter probe, the same heterogeneous pattern was observed with images of epicardial fat, identification of vessel lumens, epicardial layer, and polarization artifact.

Due to the fact that there is no convective cooling of the tissue surface during epicardial ablation, as there is during endocardial ablation due to blood flow, the use of saline irrigated rf catheters are becoming increasingly common. Saline irrigated catheters keep the surface of the tissue cool, allowing increased power delivery, resulting in deeper lesions. Real-time viewing with the prototype flexible forward imaging OCT catheter provided a visualization of the epicardial surface (Fig. 8). Maintenance of adequate tissue contact is one of the main parameters ensuring efficient energy delivery. Using the forward imaging probe, tissue contact can be assessed in the presence or absence of saline flushing. OCT has been demonstrated to image in the presence of saline flushes for intravascular imaging²⁵ and within a saline bath with superperfusion flow (Fig. 8). Although the field of view of OCT is limited, real-time imaging with OCT, integrated with rf ablation, can potentially inform the operator when the ablation catheter is in the vicinity of epicardial fat or a coronary vessel.

4.3.

Future Work

All of the current work was conducted using freshly excised swine hearts. We demonstrated that we can observe similar image features in OCT images of epicardial fat, normal myocardium, and coronary vessels using bench-top and catheter-based imaging. Future work is needed to evaluate the ability for OCT to observe these features in large animals in vivo. Due to the millimeter-scale field of view of OCT, for monitoring and guidance of epicardial RFA through a percutaneous approach, the OCT catheter should be integrated with an rf ablation catheter. An integrated catheter will allow OCT imaging of epicardial substrates directly in contact with the RFA catheter. The diameter of the integrated catheter should be small enough to allow percutaneous access to the pericardium through a standard introducer sheath used in the RFA procedure, typically $10\mathrm{Fr}$ . In vivo experiments are needed to determine whether the texture and architectural features observed during our ex vivo experiments can be observed in the presence of the motion of a beating heart and blood flow within coronary vessels. Other epicardial substrates of interest are myocardial infarction⁴² for the ablation of ventricular tachycardia, and the pulmonary veins⁴³ during epicardial ablation of atrial fibrillation.