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Over the past decade, three dimensional transoesophageal echocardiography (3D TOE) has emerged as the primary imaging modality used for guidance in catheter based cardiac interventions. This is because real-time (RT) 3D TOE images have high spatial and temporal resolution, which allows unique detailed views of cardiac structures. This additional structural information helps guide interventional procedures leading to safer and shorter procedures with higher technical success, decreased radiation exposure, and improved patient outcomes. 3D TOE is also highly mobile and can therefore be performed at the site of care, including the cardiac catheterisation laboratory and surgical suite. In this article we will introduce the technique of RT 3D TOE, describe general views, and then focus discussion on the contemporary use of RT 3D TOE in percutaneous procedures such as mitral balloon valvuloplasty, mitral valve repair with clips, left atrial appendage occlusion, atrial and ventricular septal defect closure, aortic valve replacement, and paravalvular leak occlusion.
3D TOE images are currently obtained using matrix array transducers with more than 3000 crystals, which combined with new processors allows the acquisition of real-time images without the need for offline reconstruction. With this transducer, three imaging modes are available: real-time full volumes imaging, live 3D, and multi-plane imaging. The choice of imaging mode used is determined by the structure that is being examined and the objectives of the study. Ideally images of the left ventricle, which would be used to determine size and function, are obtained using full volume mode. Images of smaller and more mobile structures such as heart valves, left atrial appendage, and fossa ovalis are best acquired using live 3D or live 3D zoom modes. However, colour acquisition, used to examine blood flows, is only available in the full volume acquisition mode, which limits its utility in real-time imaging as the 4–6 beat full volume, colour acquisition must first take place before review of the colour images. Multiplane imaging is useful when visualisation of the longitudinal and transverse plane of a structure is needed.
The differences between the three imaging modes are related to acquisition (table 1). Full volume mode captures data in 4–7 narrower wedges of volume over 4–7 consecutive heart beats to form a complete pyramidal volume of 65°×56° up to 102°×105°. This mode requires ECG gating. With live 3D imaging, the 3D image is seen in real-time rather than after 4–7 heart beats and does not require ECG gating. However, live 3D imaging displays a narrower pyramid of data (30°×60°). When using the live 3D zoom mode, a smaller section of the live 3D window is viewed in closer detail. The pyramid size in the zoom mode is 30°×30°. In contrast to full volume and live 3D, multiplane imaging does not display 3D images but shows two different plane-cut images.
Our standard approach to acquiring 3D images is first to identify the structure being imaged by two dimensional (2D) TOE. Then gain settings should be optimised using live 3D (narrow angled acquisition mode). After the image quality is optimised, 3D zoom or full volume mode should be used to acquire the images needed. Once the 3D datasets are acquired, they can be cropped to optimally visualise cardiac structures. For percutaneous procedures a few standard views are used. These views will be discussed in detail for each of the procedures described (table 2). Operators should also inform themselves of the proper way to store the 3D echo data on the particular machine they are using. In order to do offline analysis and multiplanar reconstruction of the images, it is essential that the actual 3D dataset be stored. In addition, it is also possible to store 2D DICOM clips showing the 3D rendered images that may be useful for later review on standard echo viewers.
Percutaneous balloon mitral valvuloplasty (PBMV) is performed for patients with mitral stenosis and suitable anatomy. It has been found to have good short and long term outcomes.w1–4 Traditionally, it is performed with fluoroscopic guidance alone. However, orientation using radiographic anatomic landmarks is often challenging, even for experienced cardiac interventionalists. RT 3D TOE allows excellent visualisation of the valve as well as the catheters involved in balloon valvuloplasty and so is a method that offers much to this procedure.
After optimisation of the 3D image, using live 3D, the interatrial septum is viewed from the right atrium with the superior vena cava and the inferior vena cava at the 12 and 6 o'clock positions. This view will show the catheter as it progresses into the right atrium and assists with positioning the catheter at the site of interatrial septal puncture. With 3D imaging, damage to structures such as the aorta can be avoided as relationships between various structures and the catheter are better appreciated.
After the interatrial septum is punctured, live 3D can be used to assess the mitral valve. Once again, the most useful view is the en face view where the mitral valve is viewed from the left atrial perspective with the valve in the centre of the image, and the aortic valve is rotated to the 12 o'clock position and the left atrial appendage is at the 9 o'clock position. From this view, identification of the mitral valve orifice and assessment of the commissures is performed. Live 3D imaging in this view also helps in optimising the position of the balloon between the mitral leaflet tips (figure 1), as the key to successful PBMV is to inflate the balloon such that the result is a controlled commissural tear. Localisation of the balloon in the mitral valve orifice can be accurately achieved using the 3D zoom mode and by examining the balloon position from several angles by rotating the 3D image. This is advantageous over 2D imaging as there is no need to change the probe angle or position.
When the en face image is rotated so that the mitral valve is viewed from the left ventricular perspective, thickening of the papillary muscles and leaflet tips can be appreciated as well as progression of the balloon catheter through the mitral valve orifice. Post-procedure these en face views from the left atrial and left ventricular perspective allow close inspection of the commissures to determine if splitting has occurred.
Pre-and post-procedural 2D TOE can be useful to assess the presence of an atrial clot and severity of mitral regurgitation. The addition of RT 3D TOE views allows the assessment of possible complications such as leaflet tears and more accurate quantification of the location and severity of post-procedure mitral regurgitation using full volume colour acquisition.1–3
Catheter based mitral valve repair for mitral regurgitation
This procedure is a recent advancement in the non-surgical repair of mitral regurgitation (MR) in selected patients. Using a catheter based system, a clip can be delivered percutaneously to grab the tips of the mitral leaflets, creating an edge-to-edge repair.4 This results in two mitral orifices, with significant reduction in the total regurgitant orifice and improvement in the patient's symptoms and functional capacity. While this technology is still considered experimental, it has been used to treat hundreds of patients with encouraging clinical results.5 Real-time imaging is crucial to the procedure's success.
Initially, conventional 2D TOE was used, but its utility was limited by poor spatial resolution. The newer RT 3D TOE provides good spatial resolution,6 allowing thorough assessment of the valve anatomy, including all scallops of both valve leaflets. Using 3D zoom mode allows the visualisation of the mitral valve from angles that are not available by other imaging modalities, including the surgical view of the mitral valve en face from the left atrium. RT 3D TOE also permits imaging of the entire intracardiac portion of wires and catheters, including their tips, as well as the clip7 (figure 2).
Before the procedure, RT 3D TOE is used, in combination with the conventional 2D TOE, to define the mitral valve anatomy, pathology, mechanism of MR, and suitability for the clipping procedure. Requirements include: severe MR, a regurgitant jet which is emanating mainly between scallops A2 and P2, a coaptation length of 2 mm or more, and a flail segment <10mm.7 8
During the procedure, live 3D TOE is helpful in the accurate selection of the transseptal puncture site (as described above), and is essential for guiding the catheter and the clip as they are advanced across the atrial septum into the left atrium and positioned just proximal to the centre of the mitral orifice. From a left atrial en face view, the clip arms are carefully aligned in a position perpendicular to the mitral closure line. The catheter with the clip is passed across the valve, and the leaflets are grasped at the ventricular side of the valve. 2D and 3D TOE images confirm the position of the clip and the degree of residual regurgitation, if any. Occasionally, based on the echocardiographic assessment of the result of the procedure, placement of a second clip may be required.9 RT 3D images identify the precise site of the residual defect and guide the operator to reach this site. Insertion of a second clip results in three mitral orifices.
After the procedure, echocardiography can identify complications of the procedure such as pericardial effusion or a larger than expected atrial septal defect. In several cases clip detachment resulting in mitral regurgitation has been diagnosed and an interventional or surgical procedure recommended. Clip(s) induced symptomatic mitral stenosis has not been observed.
Left atrial appendage device occlusion
Left atrial appendage (LAA) occlusion is an experimental procedure recommended to reduce the risk of emboli in patients who are unable to receive anticoagulation treatment for atrial fibrillation.w5–10 The LAA is a common site for thrombus formation in patients with atrial fibrillation and mitral valve diseases. It is believed that by isolating the LAA, the opportunity for thrombus formation is reduced, thereby reducing the risk of embolic events. This procedure must be performed carefully as residual communications between the LAA and the left atrium increase the risk of embolic events, due to creation of a low flow chamber with stasis where thrombus can form and then embolise to the left atrium.w11 LAA occlusion is performed either surgically by oversewing the LAA orifice or percutaneously with placement of an occluder device. Currently used percutaneous devices include the Watchman Left Atrial Appendage System and the Amplatzer Cardiac Plug III.
To determine the eligibility of patients and the device required, pre-procedural assessment includes measurement of the LAA orifice area and length. RT 3D TOE can perform these measurements in a single en face view compared to the multiple view required with 2D TOE. RT 3D TOE is performed using live 3D zoom mode with LAA in the centre of the image, the mitral valve at the 6 o'clock position, and the left upper pulmonary vein at the 12 o'clock position. Shah et al have shown that RT 3D TOE allowed good visualisation of the LAA orifice area and quantitation from the 3D images correlated well with 64-slice cardiac CT.10 In contrast, 2D TOE was found to underestimate LAA orifice area compared to 3D TOE and cardiac CT.
During the percutaneous procedure, live 3D TOE can visualise the catheter tips and guide the interatrial septal puncture. Importantly, it optimises device placement as 3D zoom mode allows en face visualisation of the LAA orifice, which provides confirmation of adequate device placement (figure 3).
Post-LAA occluder placement 3D TOE imaging is advantageous over 2D TOE in that off-angle device placement is more easily appreciated. Recognition of off-angle device placement by 2D TOE imaging requires multiple imaging angles to demonstrate the spatial relationship between the device and the LAA, as well as colour Doppler to prove residual communication with the left atrium. In addition, as with other percutaneous devices, RT 3D TTE is useful for visualisation under circumstances where the device needs to be recaptured or redeployed.
Device closure of atrial septal defects and patent foramen ovale
The development of modern, catheter based technologies of septal defect closure has created an alternative to the conventional surgical repair.w12 w13 In many centres worldwide, closure of atrial and ventricular septal defects is performed using transcutaneous, catheter based systems which are capable of carrying and deploying a closure device that totally occludes the defect without the trauma and complications of open heart surgery. These procedures have been shown to have excellent short and long term outcomes.w14–18
RT 3D TOE has become the imaging modality of choice, and in many cases it serves as the ‘eyes of the operator’ to evaluate, guide, and assess the results of the closing procedure.11 Importantly, the success of these procedures is predicated upon careful case selection based on anatomic considerations.12 13
2D transthoracic echocardiography (TTE) is useful in demonstrating the type and location of the septal defect. It is also helpful in the evaluation of the shunt magnitude and direction. 2D TOE and intracardiac echocardiography are frequently used during the procedure to guide device placement. These modalities, however, suffer from poor spatial resolution.
RT 3D has excellent spatial resolution and has been shown to provide additional insight in atrial septal defect (ASD) and patent foramen ovale (PFO) morphology.
Pre-procedural assessment by live 3D TOE provides information on the adequacy of tissue rims around the defects in the area of the aorta, as well as incremental information not obtainable with 2D TOE through the use of non-traditional en face views (figure 4). These en face views allow the true size and morphology of the defect to be assessed and frequently confirm that the atrial defects are not perfectly round. The use of RT 3D TOE increases the pre-procedural identification of complicated defects such as fenestrated ASD or those with a partial membrane in the defect, which is useful for case planning. Also, it permits accurate measurement of defect size as well as allowing better estimation of shunt flow using spectral Doppler imaging.
Beyond case selection and planning, RT 3D TOE imaging assists by guiding the procedure, ensuring excellent outcomes. During the procedure the size of the device is chosen based on the results of a balloon sizing manoeuvre, where the balloon is inflated across the defect and colour Doppler is used to confirm that the balloon has completely obliterated the defect. Once the device size has been determined, live 3D TOE assists by allowing continuous visualisation of the tip of the guiding catheter as well as guiding the deployment of the pre-expanded device, allowing verification of their location before, during, and after deployment, minimising the opportunity for complications. One of the advantages of live 3D TOE is that it allows visualisation of the guiding catheter in its entirety.
Post-procedure, full volume and live 3D TOE can visualise the left and right atrial sides of the occluder device as well as the tissue rim nested between the two device plates (figure 2). Colour Doppler imaging can be effectively used to verify the absence of flow across the device. If the device is misaligned, RT 3D TOE can be used to reposition or remove the device. If a fenestrated ASD is present, RT 3D TOE can be used to guide the catheters into the correct location to close both defects successfully.
Transcatheter closure of ventricular septal defect
Recently, RT 3D echocardiography has proven useful in the diagnosis and catheter based treatment of ventricular septal defects (VSDs). 3D TTE can provide en face images of the defect from both right ventricular and left ventricular perspectives. The shape and the area of the defect can be accurately assessed, leading (in combination with the calculation of shunt velocity time integral by spectral Doppler) to better estimation of shunt volume.14 RT 3D TOE has also been used to guide the procedure and to evaluate its results (figure 5).15
Before the procedure, the accurate estimation of VSD dimensions helps in the selection of the closure device size. The ability to demonstrate the entire intracardiac portion of the catheters allows easy navigation of the occluder device into the VSD. The presence of residual shunting can easily be assessed by colour Doppler. The procedure appears to be quite safe for congenital VSDs. In patients with acquired, post-myocardial infarction VSDs, the percutaneous approach may be safer than surgery since these patients are frequently haemodynamically unstable, and because surgery has a high mortality due to its inability to suture into the necrotic tissue surrounding the VSD.w19
Post-procedure, RT 3D TOE can visualise the left and right ventricular sides of the device as well as the tissue nested between both plates of the device. Colour Doppler imaging can be used to verify the absence of flow across the device. If the device is misaligned, RT 3D TOE can be used to reposition or remove the device.
Percutaneous aortic valve replacement
Untreated, severe symptomatic aortic stenosis carries a very poor prognosis.w20 The only effective treatment is aortic valve replacement. However, due to advanced age, frailty, and comorbidity, surgery is not a practical option in up to one third of these patients.w21
The recent development of systems that can deliver and deploy a prosthetic valve in the aortic position has been shown to be effective and life saving in selected patients. Currently, two systems have been tried clinically: the Edwards SAPIEN, and the Medtronic CoreValve. The first uses a balloon to expand the prosthetic valve for deployment in the aortic position, while the second uses a nitinol self-expanding prosthesis. The native aortic valve cusps are compressed with the deployment between the prosthetic valve struts and the aortic wall.w22
In addition to fluoroscopy, RT 2D and 3D TOE are the essential imaging modalities before, during, and immediately after this procedure16 (figure 6). Before the procedure TOE allows accurate assessment of the left ventricular outflow tract and aortic annulus dimensions. 3D permits the additional assessment of the outflow tract and annular shape. These measurements are essential for the selection of prosthesis size: an undersized device may result in paravalvular aortic insufficiency or even detachment and embolisation of the prosthetic valve. An oversized device may result in damage or rupture of the aortic ring, which may be complicated by bleeding and cardiac tamponade. TOE should also be used to evaluate the aortic arch and the descending aorta for the presence and size of atherosclerotic plaques. Large (≥4 mm), ulcerated, and mobile plaques present an increased risk of embolic events and stroke during the procedure.17
During the procedure, RT 2D and 3D TOE help in guiding the catheter and the prosthetic valve into an optimal position. The exact spatial orientation of the device is crucial, as the valve and the catheter should be aligned coaxially in the left ventricular outflow tract. Advancing the device too far into the aorta may result in occlusion of the coronary ostia, while retraction towards the LVOT may interfere with the motion of the anterior mitral leaflet resulting in mitral regurgitation.18
After the procedure, RT 2D and 3D TOE are useful in the evaluation of the result and potential complications. These include paravalvular and transvalvular aortic regurgitation, new wall motion abnormalities, mitral regurgitation, damage to the aortic ring, aortic dissection, pericardial effusion, and cardiac tamponade.
Transcatheter occlusion of paravalvular leaks
Paravalvular leaks (PVLs) are not an uncommon complication of surgical valve replacement. It is estimated that up to 10% of prosthetic aortic valves and up to 15% of prosthetic mitral valves have some degree of PVL.w23 24 Most PVLs are small and asymptomatic; however, in 1–2% of all operated patients there are symptoms of heart failure or of severe haemolysis, which require closure of the PVL. Until recently, the treatment of symptomatic PVL was open heart surgery.w23
Unfortunately, the surgical approach has often been disappointing, with significant morbidity and mortality rates that increase with the number of reoperations. Mortality was particularly high in patients with comorbidities such as renal insufficiency, coronary artery disease, diabetes, hypertension, and chronic lung disease.w25
Transcatheter closure of PVL is therefore an attractive, albeit challenging, alternative. Hourian et al first reported it almost 20 years ago.19 Until recently, however, many practitioners have avoided it because of the technical challenges and the lack of an imaging technique to pinpoint the PVL location accurately.
Both TTE and TOE 2D echocardiography provide multiple acoustic windows to describe the valvular anatomic structures. Continuous, pulsed, and colour Doppler flow imaging are used to evaluate the degree as well as to characterise the nature and origin of regurgitant jets. However, these techniques are limited in their spatial resolution, and therefore cannot be used to guide catheters accurately within the heart.
Together with 2D TOE, RT 3D TOE is now used to evaluate patients who present with PVL.20 The 3D zoom modality can provide en face views of both the mitral and aortic valves. Using the clock system, the mitral valve image is rotated into a position, which shows the aortic valve at 12 o'clock and the left atrial appendage at 9 o'clock. Dehiscence sites can be identified, with special attention to their location, shape, size, and area. Full volume acquisition can provide wider angle images with higher time resolution. All acquisitions can be rotated, manipulated, and cropped to obtain optimal exposure of the PVL and its blood flow. The size and shape of the dehiscence site dictates the route of approach as well as the selection of the closure device.20 Usually, smaller PVLs may be occluded with an Amplatzer patent ductus arteriosus (PDA) device while larger PVLs require the larger Amplatzer ventricular septal defect (VSD) device21 (figure 7).
The approach to the dehiscence can be antegrade (eg, approaching the mitral PVL from the left atrium after transseptal puncture) or retrograde (eg, approaching the aortic valve PVL via the systemic artery or approaching the mitral valve PVL from the left ventricle via the left ventricular apical puncture).21
During this procedure the RT 3D TOE images of the cardiac structures, catheters, and occlusion devices are displayed together with the fluoroscopic images, and are used to guide the operator during the various stages of the procedure.
This includes the selection of the site for the atrial transseptal puncture, and the guidance of the catheter and the device into the PVL while avoiding the valve orifice (figure 7). Special attention is paid to the prosthetic valve function before and after device insertion. New valvular stenosis or severe transvalvular insufficiency mandates device repositioning or withdrawal.
At the end of the procedure 2D and 3D RT evaluation of all cardiac structures is mandatory. Special attention is paid to the location and function of the occlusion device, and to the possible creation of a new site of PVL (due to stretching of the suture line by deployment of the device). Finally, possible complications of the procedure, such as pericardial effusion or iatrogenic atrial septal defect, are assessed.7
There are now several reports of relatively small series of catheter PVL closure demonstrating relatively high technical success rates (63–92%) with relatively low morbidity and mortality. Equipment and imaging quality, as well as catheter operator experience and echocardiographer skills—and the communication between these two individuals—are crucial to the success of the procedure.
Limitations of RT 3D TOE imaging include the need for additional training, reduced temporal resolution, lack of standardised views, and occasional tissue dropout (table 3). Poor temporal resolution is due to frame rates of less than 10 Hz while imaging in 3D zoom mode. This limitation has been recently overcome with the introduction of new software that allows imaging at higher rates approaching 30 Hz. Also, single beat, real-time, full volume data acquisition, currently available in transthoracic probes, has been recently introduced into clinical practice. Single beat acquisition will minimise artefacts and facilitate the evaluation of patients with arrhythmias.
Currently there is no standardisation of views for assessment or presentation of images. The multitude of viewing angles, orientation and cut planes present abundant data and acquisition options. Standardisation of views for interventional procedures has yet to be established but will be advantageous in improving the communication between the echocardiographer and the interventionalist, decreasing manoeuvring error, and increasing procedural efficiency.
Tissue dropout may occasionally be interpreted as an anatomic defect and increasing the gain may result in blurry images. However, experience as well as combining information of several imaging planes and using colour and Doppler information will help differentiate between a true defect and tissue dropout.
Current technology is still hampered by relatively low temporal and spatial resolution. This is most evident during the acquisition of 3D colour Doppler. In the future, with improved computer capabilities it would be of great benefit to be able to acquire colour Doppler datasets in a single beat. With this advancement, we will see the development of software to better quantitate regurgitant lesions. In addition, we will see further miniaturisation of transducer matrix technology, which will lead to an increase in its use in paediatric TOE probes and intracardiac echocardiographic catheters. Lastly, further automatisation and online quantitative software will need to be developed for this technology to be used in daily clinical practice.
Currently there are no association or societal guidelines regarding the training required to perform or guide 3D TOE. At a minimum, as per North American and European guidelines, practitioners should have achieved level 2 training (300–350 studies) for TTE and 125 TTE examinations to have a strong basis in 2D imaging before attempting 3D imaging.w26 27
RT 3D echocardiography is an excellent imaging tool that adds to the available information provided by traditional imaging modalities. The main advantage of this technique is the ability to visualise the entire length of intracardiac catheters as well as the balloons or devices attached to the catheters, and their position in relation to important cardiac structures. These images only require a single probe and transducer angle minimising the need to manipulate the probe. As well, with RT 3D echocardiography, an en face view of certain structures is now possible. This view allows precise evaluation of anatomic structures during procedures. RT 3D imaging also provides information not available by 2D TOE, which may prove crucial in completing the intervention. Overall, RT 3D TOE improves procedural imaging and is becoming the technique of choice for percutaneous procedures.
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Study showing the accuracy of 3D echocardiographic findings in native and prosthetic valve diseases compared to surgical findings.
This article reviews the standardised echocardiography imaging protocol used in the EVEREST I trial.
Comparison study demonstrating the accuracy of 3D TOE assessment of left atrial appendage dimensions over 2D echocardiography.
Echocardiographic 3D study demonstrating the value of 3D echocardiography in assessing the anatomy and physiology of ASDs.
Study on the use of TOE in transcatheter heart valve implantation.
Study showing the benefit of transcatheter umbrella closure of valvular and paravalvular leaks in poor operative candidates.
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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. Itzhak Kronzon has undertaken research contracts with GE and received honoraria from Philips for lectures.
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