Ultrasound Image and Augmented Reality Guidance forOff-pump, Closed, Beating, Intracardiac Surgery

*,†,¶Daniel Bainbridge, *,†,§Douglas L. Jones, *,†,‡,**Gerard M. Guiraudon,and *,†,‡,††Terrence M. Peters

*Canadian Surgical Technologies and Advance Robotics; †Lawson Health Research Institute, Imaging Group; ‡RobartsResearch Institute; §Departments of Physiology & Pharmacology, Medicine; ¶Department of Anaesthesia; **Department of

Surgery; and ††Medical Biophysics, University of Western Ontario, and London Health Science Center, London,Ontario, Canada

Abstract: Our project is the reintroduction of off-pumpintracardiac surgery using the Universal Cardiac Intro-ducer (UCI) for safe intracardiac access. The purpose ofthis study was to evaluate multimodality visualization usingthree ultrasound modalities and ultrasound augmentedwith virtual reality. Image guidance was tested on implant-ing a mitral valve prosthesis via the UCI in 12 pigs. Initially,two-dimensional (2-D) transesophageal echocardiography(TEE) ultrasound, intravascular ultrasound (intracardiacechocardiography [ICE]), and three-dimensional (3-D)epicardial ultrasound were utilized. Ultrasound augmentedwith virtual reality was used in the last three experiments.A2-D TEE assisted navigating the prosthesis into the orifice.Positioning was not intuitive and required trial and error

method. A 3-D epicardial ultrasound allowed positioningof the valve into the orifice. Positioning of the clip wasdifficult because of artifacts with multiple reflections andshadowing. Augmented reality displayed the entire pros-thesis and the tools without artifacts; provided intuitiveinformation on navigation, positioning, and orientation oftools; and improved significantly image guidance and sur-gical skill. Augmented virtual reality, with tracked 2-D or3-D ultrasound imaging, provides guidance that can effec-tively substitute for direct vision during beating heartintracardiac surgery. Key Words: Image guidance—Virtual reality—Ultrasound image guidance—Off-pump—Minimally invasive—Mitral valve surgery—Trans-esophageal echocardiography.

Off-pump, closed, beating intracardiac surgery,developed in the 1950s (1,2), was abandoned after theadvent of the heart-lung machine, which providedintracardiac access on the arrested heart, and directvision of intracardiac targets.The heart-lung machinepermitted cardiac surgery to grow until it becameclear that severe side effects denied the higher-riskpatient group from benefiting from surgery (3). The

objective of our work was to reintroduce off-pump,closed, beating, intracardiac surgery as an alternativeto the open heart approach, with the goal of dupli-cating the open heart intervention with fewer sideeffects. The two major initial challenges were accessto and visualization of the target organ, that is, off-pump, intracardiac access and substitution for directvision. We addressed the issue of access through theuse of a new device (the Universal Cardiac Intro-ducer [UCI]) to introduce instruments into thecardiac chamber (4–9). This report presents results,in a porcine mitral valve replacement model, on theuse of three ultrasound modalities (two-dimensional[2-D] transesophageal echocardiography [TEE],three-dimensional [3-D] epicardial, 2-D intravascu-lar) and an integrated approach employing 2-D TEEand virtual reality for visualization and guidance,during closed beating intracardiac surgery.

doi:10.1111/j.1525-1594.2008.00639.x

Received May 2008.Address correspondence and reprint requests to Dr. Daniel

Bainbridge, Associate Professor, Department of Anesthesiology,University of Western Ontario, LHSC, University Campus, 339Windermere Road, London ON, Canada N6A 5A5. E-mail: daniel.bainbridge@lhsc.on.ca

Presented in part at the 16th World Congress of the WorldSociety of Cardio-Thoracic Surgeons held on August 17–20, 2006 inOttawa, Canada.

Artificial Organs32(11):840–845, Wiley Periodicals, Inc.© 2008, Copyright the AuthorsJournal compilation © 2008, International Center for Artificial Organs and Transplantation and Wiley Periodicals, Inc.

840

MATERIALS AND METHODS

The UCIThe UCI (4,10) is an access device adaptable for all

heart chambers,and is designed to introduce tools anddevices into the heart in a safe and reliable manner sothat open heart interventions can be duplicated on theclosed beating heart. The final UCI design has twoparts: an attachment cuff and an airlock-introductorychamber with sleeves for tools and device holders(Fig. 1),to allow easy access of tools and to manipulatethem without collision. The attachment cuff for con-trolling port access (i) has a secure attachment to thetargeted heart chamber; (ii) is made of a soft, collaps-ible material that is easily and quickly occluded by aconventional vascular clamp, and (iii) has a secureconnection with the main chamber of the UCI. Theintroductory airlock chamber is sufficiently large toaccommodate bulky tools and devices but can beclosed tightly to prevent air embolism. The sleeves fitsnugly around the tool holders to avoid blood leaks orair suction. This chamber also acts as a bubble trap.The two-part design of the UCI permits the introduc-tion of tools and devices in a retrograde fashion,accommodating larger tools or devices, but keepingholder diameters as small as possible. The UCI iscustom made from Dacron vascular graft material andbefore use is pre-clotted in the usual fashion.The totaltime to apply the UCI ranges from 10–20 min. Ouranimal laboratory experiments have demonstratedthat this approach can duplicate endocardial surgeryfor atrial fibrillation (11); can provide access to themitral valve and position a prosthetic valve or a ringon the annulus with attempted anchoring (12–14);andcan create an atrial septal defect and application of apatch (13,15,16) as well as access to the left ventriclefor insertion of a ventricular assist device.

Virtual realityAt the beginning of our project, we used ultra-

sound imaging as the sole substitute for direct vision.This experience identified the advantages and limita-tions of ultrasound image guidance for navigatingand positioning tools or devices (14) onto the thera-peutic targets, and provided an incentive to augmentthe ultrasound image with virtual representationsof preoperative images and surgical instruments tocreate a comprehensive navigation system. Thisdevice (Fig. 2) comprises a number of components.The tracked ultrasound transducer provides real-time images to which other imaging modalities maybe synchronized and registered, as well as a coordi-nate system within which additional image informa-tion may be integrated. Tracking is provided by the

Aurora magnetic tracking system (Northern DigitalInc., Waterloo, Ontario) that determines the spatialposition and orientation of miniature magneticsensors attached to the instruments. When the instru-ments and ultrasound beam are tracked using theAurora, the movements of the tracked objects may bereplicated as virtual representations on the screen.Because the TEE probe is tracked, the position of theTEE image may be displayed within this platformrelative to the instruments. Within this framework,other imaging modalities such as preoperative 3-Dmagnetic resonance imaging (MRI) and computedtomography (CT) scans, as well as intraoperativeelectrophysiological maps can be visualized.Thus, theultrasound image provides real-time informationabout cardiac structures while the tracked tools arevirtually represented within the same volume. Thecomplete navigation system, which is known as the“Atamai Viewer” (http://www.atamai.com), can inte-grate all the images in real time in a user-friendlyvisualization environment. Dynamic cardiac modelscan be registered to the patient in the targetedregions of interest and be rapidly synchronized withthe preoperative 3-D MRI or CT images (9,17–19).

For this experiment we chose a mitral valve replace-ment procedure in which a modified bioprosthetic ormechanical mitral valve prosthesis was directed ontothe mitral valve annulus. The protocol was approvedby the Animal Care Committee of the University ofWestern Ontario and followed the Guidelines of theCanadian Council on Animal Care. A total of 12 pigswere tranquilized with Telazol (Wyeth, Madison, NJ,USA) and Rompun (Bayer, Pittsburgh, PA, USA)before transportation to the laboratory.A TEE probewas inserted in all animals prior to insertion of theendotracheal tube. Following intubation, femoralarterial and venous lines were inserted.The heart wasexposed via a left anterior thoracotomy, the pericar-dium opened and the left atrial appendage exposed.After the appendage was excluded using a large Sat-inski vascular clamp to obtain as much exposure of theappendage, it was opened and the UCI was attached(12).A size 26 or 28 mitral valve was placed within theUCI, and under TEE image guidance, combined withintravascular ultrasound (intracardiac echocardio-graphy [ICE]) and or 3-D epicardial ultrasound, themitral valve was advanced into the left atrium, navi-gated to and positioned onto the mitral valve annulus.In the final six experiments, a stapling device (EndoUniversal 65°AutoSuture,United States Surgical Cor-poration, Norwalk, CT, USA) was advanced onto themitral valve cuff to determine whether ultrasoundguidance could assist in positioning and orienting thestapler (Fig. 3). After completion of the intervention,

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the animals were observed for 30 min and euthanized.The heart and lungs were excised en bloc for grossexamination.

Imaging

TEEIn all pigs a 4–7 Mhz Minimulti TEE probe (Philips

Medical Systems, Bothell, WA, USA) was introducedprior to tracheal intubation.The TEE probe was usedto assess the diameter of the mitral valve annulus forprosthetic valve selection and for navigation andpositioning.

Intravascular ultrasoundAn 8 Mhz Siemens Accunav intravascular ultra-

sound probe (Siemens Medical Solutions USA Inc.,Malvern, PA, USA) was placed in the atria of fourpigs, through a right internal jugular cut down. Twopigs had the intravascular ultrasound placed directlyinto the left atrium via the UCI.

FIG. 1. Schematic depiction of the Universal Cardiac Introducer.The drawing shows the two parts: the cuff that controls the heartport access and the introductory airlock chamber that can accom-modate multiple and bulky tools or devices.

Atamai Viewer

Integrated Virtual Image

Pre-op Imaging

Patho/electro Physiology

Tools and Devices

Magnetic Tracking System

Real time 2D & 3D ultrasound

Real-Space

Synchronous Image Beating

Atamai Viewer

Dynamic Recalibration/ Reregistration

FIG. 2. Schematic representation of the “functional anatomy” ofthe virtual reality guiding system: The foundation is the 2-Dtracked ultrasound image that provides a real-time real-spaceimage for registration of other image modalities or virtual repre-sentations of tracked tools. Preoperative scan(s) can be regis-tered, as well as electrophysiological data, virtual representationof tools or devices, or any other image-based information. Adynamic, augmented virtual reality image can be obtained,beating synchronously to the heartbeat and recalibrated if intra-operative displacements occur. The virtual realty is displayed ona single screen, the Atamai viewer.

FIG. 3. Schematic representation ofimplantation of a mitral valve prosthesisvia the Universal Cardiac Introducer: #1shows the attachment of the cuff to the leftatrial appendage; #2 the introductoryairlock chamber is shown with the mitralvalve prosthesis introduced in a retrogradefashion; #3 the Universal Cardiac Intro-ducer is connected with the left atrial cavityand the valve has been introduced andpositioned; #4 a 3-D epicardial view of theclip applier over the prosthetic valve rim;#5 a postmortem examination showing thevalve well seated with two clips on its rim;#6 an operative view with the surgeonusing Head Mounted Display for aug-mented virtual reality image guidance.

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Epicardial ultrasoundA Philips 3-D ¥4 probe (Philips Medical Systems)

was used in all of the cases for epicardial imaging.Thetransducer imaged the heart through a sterile plasticsheath filled with 10 cc of water and placed on theepicardial or UCI surface. The ¥4 probe allows eitherlive small field of view (FOV) 3-D imaging or anexpanded FOV gated acquisition mode. We used thesmall FOV live mode to allow real-time guidance.

Virtual realityTo create our virtual environment, the ultrasound

probe, along with the valve holder and the clipapplier, were tracked using the Aurora trackingsystem. The tracked ultrasound probe and theinstruments were calibrated with respect to thesubject using standard techniques (20,21). Prior tocommencing the porcine experiments, trials wereconducted to assess the reliability and accuracy of theintegrated virtual reality platform using a cardiacphantom (22).

RESULTS

The 2-D TEE proved effective for guidance of theprosthetic mitral valve onto the mitral valve annulus.It was also effective in centering of the prostheticvalve within the mitral valve annulus. However, mul-tiple imaging planes were required to determineaccurate centering of the prosthetic valve within theannulus, which was time consuming. In addition,information on the direction required to repositionthe valve was not intuitive and required trial anderror (Fig. 4). The same applied to positioning theclip applier onto the mitral valve annulus, and deter-mining the location of the tip of the clip applier andits orientation. In general, 2-D imaging proved inad-equate for the task; for example, when the imaging

plane of the TEE is not aligned with the imagingplane of the applier, it is difficult to appreciate theposition or angulation of the device. In addition, mul-tiple reflections and shadowing from the valve andvalve holder resulted in multiple artifacts within theTEE image. However, when using the Doppler modewith the TEE probe,TEE did allow assessment of theprosthesis valve function and its snug positioning byevaluating peri-prosthetic leaks.

Intravascular ultrasoundIntravascular ultrasound was attempted in four

pigs. Images produced were of excellent quality withexcellent imaging resolution. However, obtaining2-D pictures of the desired structures in the desired

FIG. 5. This is an illustration of the value of 3-D epicardial ultra-sound image for positioning the valve, after it has been navigatedon target using a trial and error method. The view of the target isgood; however, the field of view is limited, making guidancedifficult.

FIG. 4. Schematic description of the limitations of the 2-D trans-esophageal ultrasound image. As shown on the right drawing, the2-D cross section cannot provide any 3-D context of the cardiacchamber. Therefore, it cannot display the entire relationship ofthe valve to the target.

FIG. 6. Virtual reality image guidance on a heart phantom. Thebox is the CT scan of the plastic box of the phantom. Theesophageal ultrasound probe, as well as its field of view, aremapped on their virtual representation. The valve and its holderand the clip applier are displayed. All images are viewed in realtime and real space.

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plane was difficult as the catheter floated withincardiac chambers, making appropriate positioning ofthe probe virtually impossible. In addition, the field ofview was limited because of the close proximity tothe objects being imaged.

The 3-D epicardial ultrasound imagingThe 3-D epicardial ultrasound imaging was used in

all pigs. The S4 ultrasound probe (Philips MedicalSystems) was placed on the epicardial surface ordirectly on the UCI. The 3-D images allowed place-ment of the prosthetic mitral valve within the annulusand was especially useful in ensuring that the valvewas centered within the orifice. Corrections wereintuitive and easy to appreciate on the screen (Fig. 5).Confirmation of valve motion and comprehensivecolor Doppler assessment of perivalvular leak wasalso possible. The limited field of view, however,made guidance of the clip applier onto the mitralvalve annulus difficult. However, once placed onthe annulus, small intuitive adjustments to the clipapplier could easily be made and appreciated on 3-Dechocardiography. Once placed on the annulus, clipscould be identified on 3-D epicardial images. Thebiggest limitations of the 3-D probe was that its largesize interfered with surgical manipulation of the toolsand devices, and it was necessary to place it over theepicardial surface of the heart or the outer surface ofthe UCI.

Augmented reality guidancePerhaps the most important contribution of the

virtual environment was to display the entire mitralvalve in 3-D and to guide positioning of tool and itsorientation with respect to the plane of the prostheticvalve cuff—information that is not evident in theultrasound only approach. Indeed, positioning thatseemed appropriate using 2-D ultrasound alone wasoften several millimeters off target, both in transla-tion and angulation (22). Virtual representation oftools and the ultrasound field of view eliminated theneed to track tools using ultrasound. Therefore, arti-facts and noise that had prevented accurate visualiza-tion of tool and target were minimized enhancingnavigation and positioning.

DISCUSSION

Ultrasound image guidanceThese experiments demonstrated the benefits and

limitations of various applications of ultrasoundmodalities for guidance of tools during off-pumpintracardiac beating heart surgery. It was apparentfrom our results that ICE was not superior to 2-D

TEE or 3-D epicardial echocardiography. 2-D TEE,in displaying a single imaging plane, made fine posi-tioning and centering difficult, was time consuming,and provided no intuitive guidance to the surgeon.3-D epicardial echocardiography solved many ofthese problems, but had a narrower imaging field(maximum distance imaged between two objectsheld far apart) and interfered with the surgicalprocedure.All ultrasound imaging platforms sufferedfrom artifacts and interference caused by tools ordevices.

Virtual reality platform augmented the ultrasoundimage by providing a large field of view and permit-ting intuitive manipulation of tools. Once positionedwithin the ultrasound field of view, ultrasound alonecould be used for fine positioning of the tool.

Other investigators have evaluated the use of 3-Decho to guide intracardiac surgery in the laboratorysetting. Suematsu et al. (23) studied the ability tovisualize a needle and accurately deliver a stitch.They were able to show good visualization using the3-D platform, and the ability to visualize movements.The use of 3-D ultrasound has also been used in aporcine model to guide the placement of coronarysinus cannulae.

In the clinical setting, ultrasound is commonly usedfor the guidance of catheters during cardiac surgery.Percutaneous coronary sinus catheters are oftenplaced under ultrasound guidance as are percu-taneous femoral or jugular venous and/or arterialcatheters. X-rays, especially fluoroscopy, have alsobeen used to guide the insertion of cannulae.However, radiation exposure is always a concern, andcontrast dye is often required which has deleteriouseffects on the kidneys. X-rays are not ideal to visual-ize soft tissue, and may also suffer from artifacts pro-duced by tools and devices.

The ability to track, in real time, the position of theheart within the virtual 3-D environment lends itselfto the integration of other imaging modalities includ-ing CT scans and MRIs. By identifying landmarks ona 3-D reconstruction of a cardiac CT or MRI andmatching it to the same landmarks on the real-timeultrasound, it is possible to import and accuratelyplace the CT or MRI scan within the virtual worldwith greater accuracy (20). This procedure was donefor the Phantom heart we used to test integration ofultrasound and tracked tools (Fig. 6)(22). This proce-dure may also be applied to imaging for port locationduring robotic cardiac surgery, to ensure adequateaccess to the left anterior descending and left inter-nal mammary arteries, which theoretically shouldimprove surgical times and reduce conversions.Finally, the ability to visualize a tool or device con-

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tinuously and overlay real-time patient data via anultrasound probe should improve the safety of manypercutaneous procedures currently performed blindor with the aid of 2-D ultrasound alone.

CONCLUSION

The limitation of ultrasound image guidance canbe corrected and augmented by building virtualreality on tracked 2-D or 3-D ultrasound image. Thisprovides a virtual reality image guidance that sub-stitutes the absence of direct vision that has thepotential to prove a better, more informative obser-vational platform.

Our objective is to offer less-invasive (lower-risk)surgery for a larger patient population currentlybeing treated with open heart techniques, to providea less invasive surgical option to both high- and low-risk patients, and to offer a safe backup for catheter-based interventions.

Acknowledgments: This work has been supportedin part by grants from the Department of Surgery,University of Western Ontario and CIHR; CFI,ORDCF; and NSERC.

We want to thank Cristian Linte, John Moore,Andrew Wiles, and Chris Wedlake for their expertassistance.

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