Article Text

Real-time three dimensional transoesophageal echocardiography in imaging key anatomical structures of the left atrium: potential role during atrial fibrillation ablation
  1. Francesco Fulvio Faletra1,
  2. Siew Yen Ho2,
  3. François Regoli1,
  4. Marta Acena1,
  5. Angelo Auricchio1
  1. 1Division of Cardiology, Fondazione Cardiocentro Ticino, Lugano, Switzerland
  2. 2Cardiac Morphology, Royal Brompton Hospital and Imperial College London, London, UK
  1. Correspondence to Professor Francesco Fulvio Faletra, Division of Cardiology, Fondazione Cardiocentro Ticino, Via Tesserete 48, Lugano CH-6900, Switzerland; francesco.faletra{at}

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Atrial fibrillation (AF) is the most prevalent arrhythmia in western countries and it is estimated that its prevalence will increase further over the coming decades. AF affects 1% of the general population and up to 17% of patients aged >84 years.1 Until recently, pharmacological therapy has been the only treatment, but its efficacy is limited in terms of maintenance of sinus rhythm and control of symptoms.2 Over the last decade catheter based ablation of AF has proved to be more efficacious in comparison to medical treatment,3 especially in the paroxysmal form. Moreover, the catheter ablation strategy has evolved considerably, leading to significant improvements in efficacy and reductions in the procedural complication rate. Initially, linear lesions were performed in the atria mimicking the surgical Maze procedure4 which, however, resulted in limited success and significant complications. Haissaguerre et al5 and Chen et al6 found that AF is most commonly initiated by a premature beat from the orifices of the pulmonary veins (PVs). As a result, the procedure initially targeted elimination of foci inside the triggering vein (the earliest PV electrogram that initiates tachycardia), but this strategy was not effective enough to prevent recurrences which originated from the same or other veins. Therefore, electrical disconnection of all four PVs from the atrium became the proposed treatment option. The antral (also called circumferential) isolation of the veins is the approach that is most frequently used nowadays,7 and in many cases cures the disease.

As the atrium becomes more dilated and fibrotic, other structures in addition to PVs are increasingly being recognised as a possible source of AF initiation and/or are key in the perpetuation of AF. Typical locations of triggers and the micro-reentry circuit are one or more of the following atrial structures: left lateral ridge (LLR), left atrial isthmus (LAI), coronary sinus, left atrial appendage (LAA), and posterior wall. The ablation of AF is therefore a complex procedure that requires a profound knowledge of the anatomy of the left atrium.

Real-time three dimensional transoesophageal echocardiography (RT 3D TOE) is one of the most significant advances of the last decade in ultrasound imaging of the heart. This technique provides real-time high quality images of the internal surface of the heart.8 Because the atria are close to the transducer, specific internal atrial anatomical structures can be visualised accurately. RT 3D TOE descriptions of the normal appearance and anatomical variants of several right atrial structures—potential targets of electrophysiological procedures—have been recently published.9 In this review we specifically describe RT 3D TOE images of the four left atrial structures mostly involved in left atrial ablation procedures, namely LLR, LAI, LAA, and PVs, and the potential usefulness of this technique during ablation.

Technical aspects of RT 3D TOE image acquisition

Currently, 3D TOE is capable of two modalities of acquisition: RT 3D modality and ECG triggered multi-beat acquisition. RT 3D acquisition refers to acquisition of a pyramidal dataset per second in a single heartbeat. RT images are obtained either as ‘live’ 3D narrow volume or as live 3D ‘zoom modality’. Live 3D is a button on/off mode used to switch the system from 2D to real 3D images. In the ‘zoom modality’ a truncated pyramidal dataset is generated. In both modalities imaging volumes can be adjusted in size in order to include structures of interest, while at the same time maintaining a high resolution (ie, high number of scans lines) and an acceptable frame rate (ie, up to 20–25 Hz). Both modalities are amenable to further online/offline processing. Indeed, the volumetric dataset permits cropping of the 3D image in order to visualise the inside of the volumetric pyramid. There are two cropping modules: the X–Y–Z box, and the single free-plane. Both modules are useful to remove tissues covering the target structures. Other controls such as gain, compression, brightness, smoothing, and 3D vision control are available. The use of these controls depends on the specific anatomical structures targeted.9 3D TOE images of left atrial structures presented in this review have been acquired by using 3D vision control H (which gives the perception of depth using shades of colours from brown to blue), low compression (up to 20 on a scale 1 to 100), high smoothing (up to 8–9 on a scale of 1 to 10), and medium-high brightness (up to 70 on a scale of 1 to 100). Finally, the gain was set at the medium range in order to remove noise, thus yielding a clear image of the desired anatomic structures.

Left atrial anatomical structures

Left lateral ridge

The LLR is the most prominent ridge protruding into the left atrium, and potentially a challenging ablation area for catheter stability and proper wall contact. Notably, the LLR is not a muscular crest but an enfolding of the atrial wall. Within the fatty tissue of the epicardial site of the enfolding, atrial arteries, nerve bundles and, not infrequently, the oblique vein of Marshal or its remnant that descends into the coronary sinus are located.10 Morphological studies have shown large anatomic variations of the LLR.11 In the vast majority (about 75%) of heart specimens, it has been described as a rounded structure, and in the few remaining cases either as a flat (15%) or pointed structure (10%).10 The length and width of the ridge may vary greatly as well. Those hearts presenting with a more inferior location of the LAA orifice also show a tendency to have a shorter LLR.11 Structural heart disease does not impact morphology and the length of the LLR.

3D imaging of the LLR is derived from 2D imaging of the LAA using a zoom modality focused on the LAA orifice. From this view, the LLR is shown to protrude into the left atrial cavity in between the orifices of the LAA and the left upper PV (LUPV), and to extend along the left atrial lateral wall from antero-superior to postero-inferior. Because of its favourable position, imaging of the LLR is easily obtainable when exploring LAA and does not require dedicated perspectives, transducer manipulations or additional time.

3D TOE nicely demonstrates in vivo the morphological variability (figure 1A,B,C,D) and the different lengths and width (figure 1E,F) of the LLR.

Figure 1

Different shape and size of left lateral ridge (arrow). (A) Round, (B) flat, (C) pointed, (D) irregular, (E) short and thick, (F) long and thin. LAA, left atrial appendage; LUPV, left upper pulmonary vein.

The relationship between a more inferior location of the LAA orifice (ie, a short distance between the LAA and the mitral valve) and a tendency to have a shorter LLR is shown in figure 2.

Figure 2

Real-time three dimensional transoesophageal echocardiography imaging of the relationship between the distance from the left atrial appendage and the mitral valve (double head dotted red arrows) and the length of the left lateral ridge (LLR) (double head with arrow)—the shorter the distance, the shorter the LLR. LAA, left atrial appendage; LUPV, left upper pulmonary vein; MV, mitral valve.

When viewed from a lateral perspective, the ridge has a concave profile (figure 3) and the truly anatomical structure (enfolded wall) is easily appreciated by cropping the ridge perpendicularly (figure 4). RT TOE imaging of the LLR may enable a better understanding of the variable morphology of this atrial structure in individuals undergoing ablation procedures. Moreover, preliminary results have shown that the PV isolation procedure guided by RT 3D TOE allows continuous assessment of the catheter tip stability in this challenging area, possibly leading to tailored energy delivery.12

Figure 3

The ‘concave’ profile (arrows) of the left lateral ridge (LLR) can be appreciated by rotating left-to-right (curved arrow) the pyramidal dataset from panel A to panel B. LAA, left atrial appendage; LUPV, left upper pulmonary vein.

Figure 4

(A) By cropping the left lateral ridge according to the dotted line and rotating the pyramidal dataset in down-to-up direction (curved arrow), the enfolding of the atrial wall that forms the ridge can be visualised (arrow) (B) corresponding to the cut specimen (C). LAA, left atrial appendage; LUPV, left upper pulmonary vein.

Left atrial isthmus

PV isolation alone is effective for treating paroxysmal AF, but it is usually not enough in the case of persistent AF or longstanding persistent AF.13 Adding an ablation line which connects the inferior margin of the ostium of the left lower PV (LLPV) to the mitral annulus appears to increase the success rate in treating chronic AF.14 Another approach is to encircle both the right and left pairs of PV ostia followed by a linear lesion connecting both areas and another linear lesion from the inferior margin of the left-sided encircling lesion to the mitral annulus.15 Both techniques thus include a linear lesion in the posterior-inferior wall of the left atrium through an area described by electrophysiologists as the LAI (or mitral isthmus). The atrial myocardium in this area is relatively thin with an average width of only 1.2–2.2 mm16; notably, considerable differences in the thickness of the atrial myocardium at different sites of the LAI may be found.17 Although the internal surface of the isthmus has been described as being relatively smooth, some areas close to the base of the LAA may occasionally present trabeculae presumed to be an extension of pectinate muscles from the LAA.16 The LAI is anatomically bounded by the mitral annulus and LLPV and superiorly by the LAA. 3D imaging of the LAI is derived from a 2D four chamber view in ‘zoom modality’ using a sector large enough to include the entire left atrial cavity. By cropping the medial part of the left atrium and rotating from the right to left direction the volumetric dataset, the smooth surface of the LAI is revealed ‘en face’. Alternatively, imaging of the LAI can be obtained with a narrower volume dataset focused on the lateral wall of the left atrium. The endocardial surface of the LAI is generally smooth; however, irregularities such as remnants of pectinate muscles around the orifice of the LAA (figure 5) or diverticulae can be visualised.16 Both of these perspectives are easy to obtain and do not require additional time or probe manipulations.

Figure 5

The surface of the left atrial isthmus (LAI) seen by cropping the medial half of the left atrial cavity. The surface is smooth with the exception of the area lateral to the left atrial appendage where some trabeculae (presumably remnants of pectinate muscles) can be appreciated (arrows). Ao, aorta; LAA, left atrial appendage; LLPV, left lower pulmonary vein; LPA, left pulmonary artery; LUPV, left upper pulmonary vein; MV, mitral valve.

Left atrial appendage

While it is well known that thrombi in AF are located in nearly 90% of cases in the LAA,18 it has only recently been determined that the LAA has been implicated as a site of non-PV foci for AF. It has been hypothesised that the LAA may be responsible for arrhythmias in about 27% of patients presenting with AF recurrence and undergoing a repeated ablation procedure.19 From an embryological standpoint, the absorption of the primordial PVs and their branches into the left atrial myocardium during cardiac development has resulted in limiting the sinus venous component to a small area encircling the orifice of the LAA.20 This arrangement may explain why the LAA may initiate AF, like the PVs. However, this hypothesis has been verified only in a single study, and it was not clear whether the arrhythmia's origin was truly from the LAA or adjacent structures, such as the vein of Marshall, or from unablated portions of the ostium of the adjacent PV. Thus, the proposal that recurrence of AF may originate from the LAA is not commonly accepted.

The acquisition of RT 3D TOE images of the LAA have been previously described.21 Briefly, RT 3D TOE imaging was performed by acquiring a pyramidal dataset large enough to include the entire LAA, using the zoom mode. The orifice of the LAA can be seen in an ‘en-face’ view, or by cropping and removing half of the LAA, in a long axis view. From the ‘en-face’ view the orifice of the LAA appears roughly elliptical in patients with sinus rhythm. However, in patients with increasing frequency of AF episodes (ie, from paroxysmal to permanent form), the dimensions of the LAA increase and the shape of the orifice becomes less elliptical and more rounded.22 When viewed from the ‘en face’ perspective, lobes and pectinate muscles can be easily recognised. The dual colour blue/bronze algorithm produces shades of colours from bronze to blue which enhance the perception of depth. The lobes appear as small cavities and pectinate muscles as fine ridges lining the lumen of the appendage (figure 6A). Proper cropping and an oblique perspective may help to distinguish structures in the LAA cavity (figure 6B). Cropping along the long axis produces imaging of the LAA similar to that of 2D TOE (figure 6C,D).

Figure 6

Left atrial appendage (LAA) seen (A) ‘en face’ perspective. The orifice in individuals with sinus rhythm is usually elliptical. (B) Cropping and rotation allow better appreciation of lobes (arrows) and pectinate muscles. (C) Longitudinal cut similar to 2D transoesophageal echocardiography plane (panel D); however, while in 2D plane, lobes and pectinate muscles are missing (as in the heart they lie deeper with respect to the 2D plane), they can be easily visualised with the same 3D cut (arrows). MV, mitral valve.

Pulmonary veins

The most common pattern of the entry of PVs into the left atrium is two veins from the hilum of each lung. It is common to find the presence of a short or long common venous trunk on the left side and supernumerary veins on the right side.23 The transition between the atrium and vein is smooth. Musculature of the atrial wall extends into the veins for variable depths. Overall, the myocardial sleeves are thickest in the inferior walls of the superior veins and the superior walls of the inferior veins.23 PVs focal firing by abnormal automaticity may trigger AF or act as a rapid driver to maintain AF. Also, the PV–left atrial junction has heterogeneous electrophysiological proprieties capable of sustaining reentry.24 Thus PVs play a critical role not only in triggering but also in maintaining AF.

The roof of the left atrium lies very close to the oesophagus and to the intra-oesophageal transducer. At this distance the pyramidal beam is too narrow to include the orifices of all four PVs in one volumetric dataset. Thus, the left and right PVs need to be acquired using different angles.

Left pulmonary veins

An easy way to visualise the LUPV is by using the zoom mode directly towards the LAA. Once the LAA is visualised, a slight counter clockwise rotation of the probe allows the ostium of the LUPV to be imaged ‘en face’. The LLPV is more difficult to visualise in the same image because it enters into the atrial cavity at a different angle. Thus, a perfect side-by-side ‘en face’ view of both ostia may be difficult to obtain from a single pyramidal dataset. However, when both ostia are inside the pyramidal dataset, a gentle angulation of the 3D pyramidal dataset may visualise both veins (figure 7A). A longitudinal cut of both PVs can be obtained by cropping the pyramidal dataset. From this perspective, when both veins are imaged in their longitudinal view, their oblique directions appear evident (figure 7B,C).

Figure 7

(A) Real-time three dimensional transoesophageal echocardiography imaging of both orifices of left upper  and lower  pulmonary veins—an ‘en face’ perspective. After cropping and removing the left third of the pyramidal dataset (B), left-to-right rotation (curved arrow) reveals the long axis view of both veins (C). LLPV, left lower pulmonary vein; LUPV, left upper pulmonary vein.

Figure 8

The inter-venous ridge (arrow) separating the left upper from the lower vein. Different perspectives (obtained by rotating and angulating the pyramidal dataset according to the curved arrows) reveal how the inter-venous ridge may lay at a different level (double head arrow) relative to the left lateral ridge. LLPV, left lower pulmonary vein; LLR, left lateral ridge; LUPV, left upper pulmonary vein.

The left inter-venous ridge divides the LUPV from the LLPV. Because the LLR may protrude more or less deeply into the left atrium, the inter-venous ridge may be located at a different height in relation to the LLR (figure 8). The PV–left atrial junction can be determined from the epicardial aspect as the part where the vein enters the atrium. When seen from the endocardial side, however, the junction has more indistinct boundaries, since they are funnel-shaped and have a gradual entrance to the atrium. However, by rotating slightly the volume dataset in order to make shadowing more favourable, the junction may be recognised (figure 9).

Figure 9

The junction between the left upper pulmonary vein and atrial wall. Usually the borders are somewhat indistinct because of gradual entry of the vessel into the left atrium. However, a slight angulation (curved arrow) may reveal the junction because of more favourable shadowing (arrows). LAA, left atrial appendage; LUPV, left upper pulmonary vein.

Right pulmonary veins

The best approach to visualise the right PVs is similar to that used to visualise the atrial septum.25 Once the whole atrial septum has been visualised in ‘en face’ view and rotated in such a way that the mitral valve is towards the left lower corner of the image, a perspective similar to a long axis view may be obtained. From this perspective the right PVs are situated adjacent to the plane of the atrial septum, yet they are not visible because of overlap by surrounding structures. A left-to-right rotation reveals both orifices in an ‘en face’ perspective (figure 10). The inter-venous ridge can be visualised either from ‘en face’ or long axis view (figure 11).

Figure 10

(A) Real-time three dimensional transoesophageal echocardiography imaging of the left side of the atrial septum. The orifices of the right pulmonary veins are indicated by arrows, but they cannot be seen because they are covered by surrounding structures. Rotation left-to-right of the pyramidal dataset reveals both the right upper and the lower orifices in ‘en face’ perspective. Ao, aorta; CS, coronary sinus; FO, foramen ovale; RLPV, right lower pulmonary vein; RUPV, right upper pulmonary vein.

Figure 11

(A) Real-time three dimensional transoesophageal echocardiography imaging of the right upper and lower orifices in ‘en face’ perspective. Cropping according to the dotted line, removing the right half of the pyramidal dataset and rotating right-to-left (curved arrow) shows the veins in their long axis orientation. The arrow points to the inter-venous ridge. RLPV, right lower pulmonary vein; RUPV, right upper pulmonary vein.

Variations in anatomy

There are many variants from the usual pattern of four PVs. These include the presence of short or long left (more frequent) or right common trunk, or right supra-numerary vein when the upper and middle lobes drain independently.26 Most of these variants can be recognised by RT 3D TOE (figure 12).

Figure 12

(A) Common left (CLPV) and (B) common right (CRPV) pulmonary veins. (C) Right upper (RUPV), accessory (RAPV), and lower (RLPV) pulmonary veins. LAA, left atrial appendage.

Limitations in imaging PVs

While the entire contour of the upper PVs’ ostia can be visualised in almost 100% of cases, this is not the case for the lower PVs that in our experience can be visualised in about 40–50% of patients. The difficulty of consistently imaging the lower PVs may depend on the different angulations with which they drain into the left atrium. The variable distance of the transducer placed into the oesophagus and the PVs may also affect imaging. We have noted that when the oesophagus (and transducer inside) is close to the left PVs, imaging of both right PVs can be easily obtained, while imaging of the lower left PV is troublesome. Conversely, when the oesophagus is close to the right PVs, imaging of the left PVs is easily acquired and then imaging the right PVs becomes difficult. Thus, a certain distance between the transducer and the PVs is necessary for optimal imaging.

Implications for PV ablation

RT 3D TOE has the potential to be used as a complementary guide during the procedure, allowing imaging of the catheter position relative to the anatomic structures to which ablation energy is applied, as well as continuous assessment of catheter–tissue contact. Catheter–pulmonary ostium contact is established when RT 3D TOE imaging does not show any gap between the catheter tip and the PVs’ ostium in multiple perspectives (figure 13). Once the contact is established, the act of burning during radiofrequency (RF) delivery may be appreciated as an emerging stream of 3D microbubbles flowing away from the ablation catheter (AC) tip (figure 14). These microbubbles are caused by a cavitation phenomenon due to generation of volatile substances produced by tissue heating. Coagulum/thrombus formation on the catheter during ablation can also be detected if they reach the size within the resolution power of the system. Accidental air bubble formation can also be easily detected, as during cardiac surgery. As with RT 3D TOE, intracardiac ultrasound imaging (ICE) can be used for real-time monitoring of ablation procedures. Indeed, it allows for the direct visualisation of the PVs, location of the atrial–venal junction, assurance of the AC tip location within the PV antrum, and continuous monitoring of RF energy delivery. However, ICE is costly because the probe is assembled as a part of a single-use disposable catheter, and, as far as we know, ICE RT 3D imaging technology is still not available.

Figure 13

(A–D) Real-time three dimensional transoesophageal echocardiography imaging of ablation of right upper pulmonary vein from different perspectives. The tip of the ablation catheter (AC) is always in contact with the antrum of the vein (arrow). MC, mapping catheter; RUPV, right upper pulmonary vein.

Figure 14

(A,B) Real-time three dimensional transoesophageal echocardiography imaging and (C,D) the equivalent 2D imaging derived from 3D pyramidal dataset before (A,C) and during (B,D) ablation of the left upper pulmonary vein. Microbubbles appear during heating (red arrows). The white arrow points to the tip of the ablation catheter (AC). Please note the lack of visible changes in the atrial wall during the ablation (asterisk). LAA, left atrial appendage; LUPV, left upper pulmonary vein.

Current limitations of the use of RT 3D TOE during ablation for AF

The first limitation of RT 3D TOE is the need for general anaesthesia, which adds additional costs to the procedure and carries small but possible additional risks. Moreover, it increases patient discomfort. The temperature of the probe, and therefore also the contact tissue temperature, during visualisation of PVs is about 38°C; however, the prolonged and continuous use of 3D imaging may cause an increase in the temperature of the probe to as high as 40°C. Whether this temperature interferes with or adds to the heating produced by RF ablation remains an open question. Moreover, RT 3D TOE does not allow evaluation of the immediate effect of heating upon atrial tissues (ie, oedema). Oedema causes an increase in tissue thickening and variation of texture, both of which cannot be assessed by the current technology, even when 2D imaging derived from 3D dataset is used (figure 14). Finally, although both mapping and ablation catheters can be easily recognised, artefacts created by their metallic structure may produce disturbing reverberations and artificial dropout beyond the catheter (figure 15). Whenever the catheter is parallel to the ultrasound beam the reverberation appears to prolong the catheter and can be misinterpreted as the catheter tip. The new cryoballoon catheter, by covering the PV's ostium, may limit artefacts (which will take place beyond the balloon) while maintaining interpretable images. Finally, the pronounced displacement of the oesophagus needed in order to visualise PVs may deform the left atrial cavity and interfere with catheter manipulation.

Figure 15

Artefacts due to reverberations (white arrows) arising from the tip of the catheter (asterisk) and dropout beyond the catheter (red arrows). RUPV, right upper pulmonary vein.


Preliminary experience shows that the RT 3D TOE guided PV ablation procedure is feasible, allowing fluoroscopy-free navigation and precise delineation of the site of RF energy delivery. However, given the current limitations of the technique in providing consistent imaging of all four PVs, an experienced cardiac imaging physician is required during PV ablation, imposing an additional logistic burden to an already complex procedure.

Potential role of RT 3D TOE during atrial fibrillation ablation: key points

  • The pulmonary veins (PVs) play a critical role both in triggering and in maintaining atrial fibrillation (AF). Nowadays electrical disconnection of all four PVs from the atrium is an effective option for curing AF.

  • In chronic AF, as the atrium becomes more dilated and fibrotic, other structures such as the left lateral ridge, left atrial isthmus (LAI), coronary sinus, and left atrial appendage (LAA) are recognised as a possible source of initiation or perpetuation of AF.

  • From an anatomic point of view, the lateral ridge is not a muscular crest but an enfolding of the atrial wall; located within the fatty tissue of the epicardial site of the enfolding are atrial arteries, nerve bundle and the oblique vein of Marshall. This structure is a potentially challenging ablation area for catheter stability and proper wall contact.

  • The LAI is an area bounded by the mitral annulus and the left lower PV, and superiorly by the LAA of the left atrial wall. In this area an additional linear lesion may increase the success rate in treating chronic AF.

  • The LAA may be responsible from arrhythmias in about 27% of patients undergoing repeated ablation procedures.

  • Real-time three dimensional transoesophageal echocardiography (RT 3D TOE) can accurately visualise the normal appearance and anatomical variants of specific internal left atrial anatomical structures—potential targets of electrophysiological procedures.

  • RT 3D TOE may be used as a complementary guide during ablation of AF, allowing imaging of the catheter position relative to anatomic structures to which ablation energy is applied, as well as continuous assessment of catheter–tissue contact.

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  1. This study shows that, among selected patients with a long history of paroxysmal AF, an ablation strategy is more effective than antiarrhythmic drug therapy in suppressing AF with fewer adverse events.
  2. This was the first study showing the superb quality of RT 3D TOE imaging, suggesting this modality as guidance for percutaneous interventions.
  3. This study showed for the first time the capability of RT 3D TOE in imaging right atrial structures which can be targets for ablation procedures.
  4. A description of the macroscopic and microscopic anatomy of atrial structures which any electrophysiologist should know.
  5. This was the first study showing RT 3D TOE imaging of the left atrial appendage and modality of acquisition.
  6. This study describes the normal appearance of the atrial septum using RT 3D TOE. 3D TOE images are matched side-by-side with equivalent anatomical specimens. The role of 3D TOE during transseptal puncture is discussed.
View Abstract


  • Contributors All authors have contributed to the scientific content and revision of the manuscript.

  • Competing interests In compliance with EBAC/EACCME guidelines, all authors participating in Education in Heart have disclosed potential conflicts of interest that might cause a bias in the article. FF and AA are consultants for Philips.

  • Provenance and peer review Commissioned; externally peer reviewed.

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