Journal of Interventional Cardiac Electrophysiology 8, 27–36, 2003C© 2003 Kluwer Academic Publishers. Manufactured in The Netherlands.

Real-Time Cardiac Catheter Navigationon Three-Dimensional CT Images

Stephen B. Solomon,1 Timm Dickfeld,2 andHugh Calkins2

1Department of Radiology and 2Cardiology,Johns Hopkins School of Medicine, Baltimore, MD

Abstract. Introduction: Targets for ablation of atrialfibrillation, atrial flutter, and non-idiopathic ventricu-lar tachycardia are increasingly being selected basedon anatomic considerations. Because fluoroscopy pro-vides only limited information about the relationshipbetween catheter positions and cardiac structures, andis associated with radiation risk, other approaches tomapping may be beneficial.

Methods: The spatial and temporal information ofan electromagnetic catheter tip position sensing system(Magellan, Biosense Inc.) was superimposed on a three-dimensional (3D) CT of the chest in swine using fidu-cial markers for image registration. Position and orien-tation of a 6 French catheter with an electromagneticsensor was displayed in real-time on a corresponding3D-CT. Catheter navigation within the heart and thegreat vessels was guided by detailed knowledge aboutcatheter location in relation to cardiac anatomy.

Results: Anatomic structures including the atrialseptum, pulmonary veins, and valvular apparatus wereeasily identified and used to direct catheter navigation.During the right heart examination, the catheter wasnavigated through the superior and inferior vena cavato predetermined anatomic locations in right atrium,right ventricle and pulmonary artery. The ablationcatheter was also navigated successfully from the aortathrough the aortic valve in the left ventricle. No com-plication was encountered during the experiments. Theaccuracy and precision of this novel approach to map-ping was 4.69 ± 1.70 mm and 2.22 ± 0.69 mm, respectively.

Conclusions: Real-time display of catheter positionand orientation on 3D-CT scans allows accurate andprecise catheter navigation in the heart. The detailedanatomic information may improve anatomically basedprocedures like pulmonary vein ablation and has thepotential to decrease radiation times.

Key Words. catheters, heart diseases, electrophysiol-ogy, CT, magnetic resonance imaging

Introduction

Catheter ablation has become first line therapyfor many cardiac arrhythmias including atrioven-tricular nodal reentrant tachycardia, atrial flutter,idiopathic ventricular tachycardia, and accessorypathway mediated arrhythmias. Attempts to ab-

late more complex arrhythmias, such as atrial fib-rillation, non-idiopathic ventricular tachycardia,and reentrant atrial arrhythmias in the setting ofprior cardiac surgery or corrected congenital heartdisease have proved more challenging.

It has recently become appreciated that abla-tion strategies that are based more on anatomicalconsiderations rather than mapping are neededto improve the efficacy of the catheter ablationof these more complex arrhythmias. For example,anatomic knowledge of the pulmonary veins, es-pecially the atriopulmonary junction and the trib-utaries and branching pattern of the pulmonaryveins has become a critical part of pulmonary veinablation for paroxysmal atrial fibrillation [1–4].This type of three-dimensional information is dif-ficult to obtain from fluoroscopic images alone.In order to overcome the limitations on conven-tional fluoroscopic imaging a pre-procedure radi-ological study such as CT and MR imaging is of-ten obtained [5]. However, once in the proceduresuite, electrophysiologists rely almost exclusivelyon two-dimensional fluoroscopic images to per-form the intervention. Fluoroscopy times exceed-ing one hour are not uncommon for these complexprocedures and expose the patient and the staff[6] to significant ionizing radiation.

Recently, electroanatomical mapping systemshave played an increasingly important role in fa-cilitating catheter ablation procedures in the set-ting of complex arrhythmia substrates [1,7–10].These systems allow catheter tip real-time posi-tion to be displayed within an electroanatomicalmap of the heart. However, these “cartoon” imagescreated using the current position sensing technol-ogy lack detailed anatomic information [11].

Address for correspondence: Stephen B. Solomon, MD, JohnsHopkins Hospital, Jefferson Building 173, 600 North WolfeStreet, Baltimore, MD 21287, USA. E-mail: ssolomon@post.harvard.edu

Received 24 June 2002; accepted 16 October 2002

27

28 Solomon, Dickfeld and Calkins

The purpose of this study was to test the fea-sibility of using position sensing technology to su-perimpose the real-time ablation catheter tip posi-tion on previously acquired three-dimensional CTimages of the heart and to perform catheter nav-igation through the great vessels and chambersof the heart solely guided by this non-fluoroscopicimaging system.

Methods

Animal PreparationThis study was performed in a domestic swine(weight 50 kg) under a protocol approved by theinstitutional animal care and use committee. Theswine was sedated with acepromazine 50 mg IMand ketamine 75 mg IM. Thiopental 75 mg IV wereadministered prior to intubation. The animal wasmaintained on inhaled isoflurane 2% in air dur-ing the catheter procedure. During transportation

Fig. 1. Surface markers and reference catheter. Nine nipple markers were placed on the animals chest from right lateral-posteriorto left lateral-posterior position (#3 to #8 shown), which were used for surface registration. The reference catheter is taped to thesternum to adjust the frame of reference with respiratory movement.

to the CT scanner and during scanning the swinewas given pentobarbital IV to maintain anesthe-sia. At the end of the procedure the animal waseuthanized using an overdose of IV pentobarbital.

CT Scanning and Investigational ProtocolPrior to scanning nine 1.0 mm metallic nipplemarker stickers were placed across the chest ofthe pig to allow for later registration of the im-ages (Fig. 1). The swine was imaged with a spiralCT (Somatom Plus 4, Siemens, Iselin, NJ) usingparameters of 2 mm thick slices, 4 mm/sec tablespeed, and approximate exam time of 40 seconds.Intravenous iohexol contrast (Omnipaque 350,Nycomed, Buckinghamshire, United Kingdom)100 ml at a rate of 2 cc/sec was administered.Imaging began approximately 20 seconds after thecontrast injection began. End expiration breathhold was simulated by turning off the ventila-tor for approximately 45 seconds during the scan

Cardiac Catheter Navigation on Three-Dimensional CT 29

while the pig was paralyzed with pancuronium(0.5 mg/kg IV).

The images were then electronically transmit-ted to the navigation computer in the fluoroscopysuite.

Navigation SystemThe navigation system (Magellan, BiosenseWebster Inc., New Brunswick, NJ) consists of acomputer containing the three-dimensional CT orMR images. Additionally there is an electromag-netic locator pad, which is placed under the pa-tient. This pad generates ultralow magnetic fields(5 × 10−5 to 5 × 10−6 T) that code both tem-porally and spatially the mapping space aroundthe animal’s chest. The locator pad, specifically,consists of three electromagnetic field generatingcoils. These fields decay with distance allowing theposition sensor antenna at the tip of the catheter toidentify position in space. Orientation is providedby the presence of three orthogonal antennae ineach catheter tip sensor. Previous studies haveshown accuracy for in vitro work to be approxi-mately 1 mm [8].

The navigation system relies on two positionsensor catheters, the reference catheter and theactive procedural catheter. The reference catheterwith a position sensor at its tip was taped to thechest of the swine. This supplied additional infor-mation about respiratory, positional changes andhelped maintain the registered frame of reference.The procedural catheter with a similar positionsensor at its tip for tracking its position and ori-entation was used to navigate within the heart andvascular tree.

Image RegistrationThe CT images were transmitted to the naviga-tion system computer (Magellan, Biosense Inc.) lo-cated in the fluoroscopy suite. Three-dimensionalreconstructions were made using the relative dif-ferences in CT Hounsfield units of the variousstructures.

The reference catheter was taped to the swine’schest. The procedural catheter was then used totouch each of the nine metallic stickers placedacross the animal’s chest prior to CT. With eachsticker the computer cursor was placed over thecorresponding marker on the CT image. This al-lowed the “registration” of the image with the livepig.

Accuracy and Precision AssessmentRepeated measurements, as described below, ofthe nine surface markers were performed at thebeginning and end of the study and served as asurrogate to estimate accuracy and precision ofintracardiac manipulation. To test accuracy theprocedural catheter was moved to each of the nine

markers on the chest. At each marker the distancebetween the location that the navigation systembelieved was the location of the marker (M) andthe actual location of the marker (T) was deter-mined. The position error was calculated using thefollowing equation:

√(Mx − Tx)2 + (My − Ty)2 + (Mz − Tz)2 (1)

where (Mx, My, Mz) and (Tx, Ty, Tz) are the coor-dinates of points M and T, respectively. Five in-dependent attempts at touching each of the ninemarkers were performed. Data was averaged anderror ranges noted for the nine marker points.

To test the precision of the system an averagepoint was obtained from the average coordinatesof the five independent measurements per markerin three-dimensional space. Distance from eachof the five measured points to this virtual pointwas then measured. Data was averaged and errorranges noted for the nine marker points.

Catheterization and Image CorrelationRight femoral 8F sheaths were placed in bothfemoral vein and artery. The procedural catheterwith the position sensor at its tip was inserted intothe femoral vein and then into the femoral artery.Real-time movement of the catheter was observedon the CT images as noted by a cross-hair display.Correlation with biplane fluoroscopic images wasobserved after positioning the catheter in the rightatrium, right/left ventricle and pulmonary artery.

Results

Accuracy and Precision AssessmentAccuracy measurements were repeated five timesper actual marker in three-dimensional space. Thedistance between the actual marker on the skinand where the computer indicated the tip waslocated was measured. These measurements forthe individual nine skin markers are shown inTable 1. The average accuracy was determined tobe 4.69 ± 1.70 mm. However, in the current studythe reference catheter primarily accounted forantero-posterior motion of the chest wall duringrespiration. This accounts for an increased error inthe lateral points, for which lateral chest wall mo-tion is the main source of movement. In neglectingthe most lateral two points the accuracy measuredin this study improved to 3.98 ± 1.04 mm.

Precision measurements were made by measur-ing the distance between a virtual point repre-senting the three-dimensional average of the fiveregistrations and each of the five registrations.These data are displayed in Table 2 for each ofthe nine markers separately. The precision was

30 Solomon, Dickfeld and Calkins

Table 1. Accuracy: For each marker point the average distance from 5 individual measurements was obtained with the real-timepositioning sensor technique to the actual surface marker as visualized by CT scan. Results are shown as distance in each of thethree dimensional planes and as total distance in mm

x-axis y-axis z-axis Distance Location of marker

Marker 1 0.650 −6.266 4.088 7.829 Right posteriorMarker 2 −0.624 1.212 0.512 2.458 Right lateralMarker 3 −2.282 2.208 −1.710 4.056 Right mid-axillaryMarker 4 −3.277 1.871 −2.713 5.187 Right parasternalMarker 5 0.573 −0.647 −1.483 4.038 Mid sternalMarker 6 0.477 1.813 −4.193 4.703 High sternalMarker 7 3.678 2.010 −1.152 4.743 Left parasternalMarker 8 1.020 1.764 0.840 2.725 Left axillaryMarker 9 4.550 −1.236 3.900 6.489 Left lateral-posterior

Mean 4.692SD 1.696

Table 2. Precision: For each marker the 5 measurements with the real-time positioning sensor were averaged and thus a virtualcentral point obtained. Distance for each of the 5 measurements from this virtual point was obtained in each of the threedimensional planes. Results here show the standard deviation in each of the three dimensions and the absolute distance from thispoint for each marker separately in mm

x-axis y-axis z-axis Distance Location of marker

Marker 1 1.167 1.160 1.897 2.082 Right posteriorMarker 2 1.622 1.726 0.854 2.142 Right lateralMarker 3 1.706 1.537 0.723 2.021 Right mid-axillaryMarker 4 1.838 2.292 0.678 2.522 Right parasternalMarker 5 2.091 3.963 1.887 3.769 Mid sternalMarker 6 1.184 0.774 0.738 1.299 High sternalMarker 7 1.442 0.616 1.685 1.885 Left parasternalMarker 8 0.603 1.129 1.624 1.799 Left axillaryMarker 9 1.846 1.683 1.388 2.468 Left lateral-posterior

Mean 1.500 1.653 1.275 2.221SD 0.456 1.008 0.523 0.685

determined to be 2.22 ± 0.69 mm. In neglectingthe most lateral two points the precision measuredwas 2.21 ± 0.78 mm.

Catheterization and Image CorrelationAs the catheter moved from the femoral arteryand the femoral vein to the heart, the movementwas observed on the computer monitor’s three-dimensional CT display in real-time. Catheter po-sition was recorded in the aorta, IVC, right atrium,right and left ventricle (Fig. 2). No fluoroscopicimaging was needed to navigate to these struc-tures. In one example, the catheter was navigatedthrough the heart into a branch of the left infe-rior pulmonary artery, which was confirmed byfluoroscopy (Fig. 3a and b). Other examples of thecatheter going from the aorta into the left carotidand into the left renal artery were observed onthe CT data and then confirmed on fluoroscopy.Confirmation of the catheter at the left ventricleapex and the aortic root were also confirmed byfluoroscopy.

Discussion

Main FindingsThe results of this study demonstrate, for the firsttime, the feasibility of precise intravascular andintracardiac catheter navigation in real-time onCT images without the use of fluoroscopy. This wasdone by combining the position and orientation in-formation of a magnetic field catheter tip sensingsystem with the anatomic information of previ-ously acquired CT images. Our findings indicatethat (1) CT images of the chest can be successfullycombined with a low magnetic field tip sensingsystem to yield an anatomically based real-timepositioning system, (2) intravascular navigationincluding the aorta, inferior and superior venacava, pulmonary arteries, as well as, intracar-diac navigation including right atrium, right andleft ventricle with directed passage through tri-cuspid, pulmonary, and aortic valves can be reli-ably performed without fluoroscopic guidance, and(3) indicated catheter tip position is accurate andprecise.

Cardiac Catheter Navigation on Three-Dimensional CT 31

Fig. 2. Catheter navigation through the RA. The real-time catheter position is indicated by the cross hair on coronal, sagittal, andaxial views moving along the lateral wall of the RA. On the cardiac 3D-reconstruction in the right lower quadrant, the catheterposition is indicated by the tip of the rod and updated in real time.

Stereotactic GuidanceThe concept of stereotactic guidance is not newand has been extensively used in neurosurgeryand otorhinolaryngology, where since the 1950’sstereotactic frames and more recently framelessstereotactic systems have allowed true anatomi-cal catheter guidance combining CT or MR imageswith magnetic field, infrared light, laser beam,

and ultrasound positioning systems [12,13]. In on-cology stereotactic guidance was used for radio-surgery of liver, lung, and brain tumors [14,15] andsuccessfully directed radiofrequency ablation in avariety of abdominal neoplasms [16,17].

As the importance of anatomically correctcatheter positioning for electrophysiological pro-cedures became evident intracardiac endoscopy

32 Solomon, Dickfeld and Calkins

Fig. 3a. Coronal, sagittal, and axial images of the chest showing the catheter tip at the postero-lateral wall in the distal leftinferior pulmonary artery.

[18,19] as well as echocardiography [20–22] wereused for catheter guidance. More recently, newtechnologies using anatomical “cartoon images”became available for stereotactic catheter guid-ance and mapping.

The CARTO system (Biosense Webster, Dia-mond Bar, CA) uses the same electromagnetic po-sitioning system as used in this study, but displaysthe catheter position on a reconstructed figurederived from multiple catheter recordings alongthe endocardium [8,23]. A non-contact multielec-trode mapping system (Ensite, Endocardial Solu-

tions, Inc., St. Paul, MN, USA) can reconstructthe cardiac anatomy from the electrically codedcatheter position while moving along the cham-ber wall [24]. Using external electrical fields, an-other commercially available system, LocaLisa,calculates the spatial catheter position by thevoltage drop along the internal organs [25]. Theultrasound-based tracking system from CardiacPathways uses a triangulation algorithm to deter-mine the ablation catheter position in comparisonto two intracardiac reference catheters [26]. Thepracticability of these positioning techniques has

Cardiac Catheter Navigation on Three-Dimensional CT 33

Fig. 3b. The radiograph confirms the catheter position in a distal branch of the left inferior pulmonary artery.

been validated in a variety of ablation procedures[8,11,23,26].

However, all these systems rely on mathemati-cally reconstructed the cardiac chambers and can-not accurately describe the true 3D-morphology ofe.g. the pulmonary vein ostium and proximally orseparately branching pulmonary veins [11,26].

Applications in CardiologyThis is the first report extending the use of CTimages with frameless stereotactic catheter track-ing to the field of cardiology and demonstrating itsrole in facilitated, anatomic catheter guidance inthe heart.

The combination of CT images with this elec-tromagnetic tracking system (Magellan, BiosenseInc.) has already been validated for thoracic andintravascular procedures. Bronchoscopy as well asplacement of vena cava filters and transjugular in-trahepatic portosystemic shunts was successfullyfacilitated in human and animal series [27]. Pro-viding the true 3D-morphology may provide excit-ing advantages over the existing technology, whichuses mathematic reconstruction to describe car-diac anatomy.

In this study, fluoroscopy was only used to con-firm catheter position within anatomic structuresand showed an excellent correlation with the nav-igation system. The accuracy and precision were4.69 ± 1.70 mm and 2.22 ± 0.69 mm, respectively.This is within the range, which is considered ac-ceptable in e.g. neurosurgery [28]. Although theelectromagnetical catheter tracking system itselfshowed a slightly better accuracy and precisionwhen tested in vivo and in vitro (<1 mm) [8],our results compared favorably with other stud-ies where CT images were superimposed over thetracking system. When used to guide neurosur-gical procedures, transjugular intrahepatic porto-systemic shunt (TIPS) placement or bronchoscopyaccuracies ranged between 2.8 mm, 3 mm and5.6 mm (with 3.6 mm due to respiratory motion),respectively [28].

Several modifications can improve accuracyand precision. The reference catheter on the ster-num accounts mostly for antero-posterior mo-tion of the thoracic wall. Neglecting the lateralpoints with higher position error lead to an im-proved accuracy and precision of 3.98 ± 1.04 mmand 2.21 ± 0.78 mm, respectively. Placement of

34 Solomon, Dickfeld and Calkins

the reference catheter in the coronary sinus os-tium could, therefore, account more accuratelyfor the respiratory and, to some extent, cardiacmotion.

The current study superimposed the CT imagesusing fiducial markers. An alternative form of reg-istration (“surfit registration”) involves definingan intrathoracic structure like the IVC throughmultiple catheter recordings and using a computeralgorithm to align the defined structure with theCT image. This method improved the position er-ror in patients undergoing bronchoscopy [27].

To account better for cardiac motion the staticCT images could be replaced by a dynamic datasetof the “beating heart”. This would be linked to thepatient’s EKG to display the exact systolic or di-astolic phase and provide additional informationabout wall motion abnormalities.

LimitationsThis study has several important limitations,which need to be considered. Firstly, it was de-signed as a feasibility study to examine the pos-sibility of stereotactic catheter guidance based on3D-CT images. Thus, the number of experimentswas limited to obtain the proof of concept. Sec-ondly, accuracy and precision in this study wereassessed by the use of surrogate markers on thechest surface. Although this method has been val-idated by several other studies [13,27,28] it maydiffer from the actual intracardiac position error.The surface markers are at the edges of a fieldof highest resolution of the catheter tip trackingsystem (20 cm × 20 cm × 20 cm), which is placedover the heart. Therefore, accuracy and precisionfor intracardiac manipulation, although different,will rather be improved compared to our measure-ments. Thirdly, previously acquired 3D-CT imageswere used to anatomically guide the catheter nav-igation. Therefore, respiratory and cardiac motionduring the procedure is not visualized on the CTimages and may increase the position error. Simi-larly, interval changes between the scan time andthe electrophysiological procedure, which can bekept as short as 30 minutes, are not accounted for.However, the concept of acquiring a static ‘map’ toguide the further interventional procedures hasbeen successfully validated with the CARTO sys-tem [8,10]. In our experience accuracy and preci-sion were acceptable even if images were acquiredup to 6 hours prior to the study.

Clinical ImplicationsThe importance of anatomically based catheter ab-lation procedures is now recognized. Catheter ab-lation of isthmus dependent atrial flutter, whichis strictly based on anatomical criteria, is consid-ered as first-line therapy in atrial flutter. Simi-larly, the best results of catheter ablation of parox-

ysmal atrial fibrillation have been achieved asprocedures have become increasingly based onanatomical considerations [11,29]. The disap-pointing results of catheter ablation of chronicatrial fibrillation are generally ascribed to theinability to create continuous transmural linearlesions in critical anatomic areas [30]. We an-ticipate that improvements in mapping technol-ogy that provide more specific anatomic informa-tion, such as the one described in this report willultimately result in catheter ablation becomingfirst line therapy for complex arrhythmias such asatrial fibrillation and certain types of ventriculartachycardia. Further studies are needed to evalu-ate clinical utility and advantages of this systemfor anatomically guided ablation procedures likepulmonary vein isolation and continuous linearlesions.

Stereotactic catheter guidance can significantlydecrease the amount of ionizing radiation [10]. Re-placing the CT with MR images can help to re-duce the radiation exposure further and improveanatomic guidance through superior soft tissueresolution. MRI also offers the ability to image re-cent radiofrequency ablation lesions, which mayprovide additional guidance for repeat RF abla-tions [31]. Combination of the described CT or MR“road mapping” with recent work demonstratingguide wire navigation using external magnets [32]may enable advanced anatomically guided wiremanipulation.

Superimposing the electrical mapping capabili-ties of the CARTO system on the CT or MR imageswould greatly enhance the versatility of the sys-tem and result in the first real electro-anatomicalmapping system.

Conclusions

The current study has, for the first time, demon-strated the feasibility of facilitated intracar-diac catheter navigation by combining an elec-tromagnetic navigation system with the true3D-morphology of CT images. Considering the in-creasing importance of anatomically based treat-ment strategies this may provide exciting ad-vantages over other available catheter guidingtechniques.

Acknowledgments

This work was supported by an award from the American HeartAssociation.

Biosense is a wholly-owned subsidiary of Johnson &Johnson. Dr. Solomon owns Johnson & Johnson stock, which issubject to certain restrictions under Johns Hopkins Universitypolicy.

Cardiac Catheter Navigation on Three-Dimensional CT 35

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