EAE/ASE Recommendations for Image Acquisition and Display Using Three-Dimensional Echocardiography

Abstract

Attention ASE: Members:ASE has gone green! Visit www.aseuniversity.org to earn free continuing medical education credit through an online activity related to this article. Certificates are available for immediate access upon successful completion of the activity. Nonmembers will need to join ASE to access this great member benefit! ASE has gone green! Visit www.aseuniversity.org to earn free continuing medical education credit through an online activity related to this article. Certificates are available for immediate access upon successful completion of the activity. Nonmembers will need to join ASE to access this great member benefit! Three-dimensional (3D) echocardiographic (3DE) imaging represents a major innovation in cardiovascular ultrasound. Advancements in computer and transducer technologies permit real-time 3DE acquisition and presentation of cardiac structures from any spatial point of view. The usefulness of 3D echocardiography has been demonstrated in (1) the evaluation of cardiac chamber volumes and mass, which avoids geometric assumptions; (2) the assessment of regional left ventricular (LV) wall motion and quantification of systolic dyssynchrony; (3) presentation of realistic views of heart valves; (4) volumetric evaluation of regurgitant lesions and shunts with 3DE color Doppler imaging; and (5) 3DE stress imaging. However, for 3D echocardiography to be implemented in routine clinical practice, a full understanding of its technical principles and a systematic approach to image acquisition and analysis are required. The main goal of this document is to provide a practical guide on how to acquire, analyze, and display the various cardiac structures using 3D echocardiography, as well as limitations of the technique. In addition, this document describes the current and potential clinical applications of 3D echocardiography along with their strengths and weaknesses. An important milestone in the history of real-time 3D echocardiography was reached shortly after the year 2000, with the development of fully sampled matrix-array transducers. These transducers provided excellent real-time imaging of the beating heart in three dimensions and required significant technological developments in both hardware and software, including transducer design, microelectronic techniques, and computing. Currently, 3DE matrix-array transducers are composed of nearly 3,000 piezoelectric elements with operating frequencies ranging from 2 to 4 MHz and from 5 to 7 MHz for transthoracic echocardiographic (TTE) and transesophageal echocardiographic (TEE) imaging, respectively. These piezoelectric elements are arranged in a matrix configuration within the transducer and require a large number of digital channels for these fully sampled elements to be connected. To reduce both power consumption and the size of the connecting cable, several miniaturized circuit boards are incorporated into the transducer, allowing partial beam-forming to be performed in the probe. Additionally, developments in transducer technology have resulted in a reduced transthoracic transducer footprint, improved side-lobe suppression, increased sensitivity and penetration, and the implementation of harmonic capabilities that can be used for both grayscale and contrast imaging. The most recent generation of matrix transducers are significantly smaller than the previous ones, and the quality of two-dimensional (2D) and 3D imaging has improved significantly, allowing a single transducer to acquire both 2D and 3DE studies. Currently, there are two different methods for 3DE data acquisition: real-time or live 3DE imaging and electrocardiographically triggered multiple-beat 3DE imaging. Real-time or live 3DE refers to the acquisition of multiple pyramidal data sets per second in a single heartbeat. Most ultrasound systems have real-time 3DE volume imaging available in the following modes: live 3D narrow volume, live 3D zoomed, live 3D wide angled (full volume), and live 3D color Doppler. Although this methodology overcomes the limitations imposed by rhythm disturbances or respiratory motion (Figure 1), it is limited by poor temporal and spatial resolution. In contrast, multiple-beat 3D echocardiography provides images of higher temporal resolution. This is achieved through multiple acquisitions of narrow volumes of data over several heartbeats (ranging from two to seven cardiac cycles) that are subsequently stitched together to create a single volumetric data set (Figure 2). However, gated imaging of the heart is inherently prone to imaging artifacts created by patient or respiratory motion or irregular cardiac rhythms. Simultaneous multi-plane imaging is unique to the matrix array transducer and permits the use of a dual screen to simultaneously display two real-time images. The first image is typically a reference view of a particular structure, while the second image or “lateral plane” represents a plane rotated 30 to 1500 from the reference plane. Multiplane imaging in the elevation plane is also available. Color flow Doppler imaging can also be superimposed onto the 2D images. Live 3D using the matrix array transducer permits a real-time display of a 300 x 600 pyramidal volume. While the size of the sector is usually insufficient to visualize the entirety of a single structure in any one imaging plane, the superior spatial and temporal resolution permits accurate diagnoses of complex pathologies while preserving optimal temporal resolution. The “ZOOM” mode permits a focused, wide sector view of cardiac structures. It must be noted that enlarging the region of interest excessively will result in a further detrimental decrease of the spatial and temporal resolution relative to real-time 3DE. The full volume mode has the largest acquisition sector possible, which is ideal when imaging specific structures such as the mitral valve or aortic root. This mode also has optimal spatial resolution, which permits detailed diagnosis of complex pathologies. As well, it has high temporal resolution (>30 Hz). Similar to the real-time 3D and the focused wide sector—“ZOOM” modalities, the gated full volume can also be rotated to orient structures such as valves in unique en face views. Furthermore, the full volume data set can be cropped or multiplane transected to remove tissue planes in order to identify components of valvular structures within the volume or to visualize 2D cross-sectional x, y, and orthogonal planes using off-line analysis software. When 3DE color flow Doppler imaging was first introduced using a matrix array transducer, it could only be displayed using a full volume, gated reconstruction technique. This required the incorporation/“stitching“ of 7-14 individual pyramidal volume slabs gated to the ECG, to create a 3D composite volume, in the upper-end range of a 400 x 400 sector at a frame rate of 15-25 Hz depending upon the selected line density. However, currently 3D color full volume can be acquired with less than the 7-14 individual gated volumes and the most recently developed software allows acquisition of as low as 2 beats, albeit at the cost of temporal resolution. The main trade-off in 3DE imaging is between volume rate (i.e., temporal resolution) and spatial resolution. To improve spatial resolution, an increased number of scan lines per volume (scan line density) is required, which takes longer to acquire and process and thereby limits the overall volume rate. Fortunately, imaging volumes can be adjusted in size (i.e., made smaller) to increase volume rate while maintaining spatial resolution. Because of the frequent artifacts associated with gating, ultrasound companies are developing real-time technology associated with methods for improving the ultrasound system processing power needed to provide full volume (90° × 90°) real-time 3DE data sets with adequate spatial and temporal resolution. Gated data sets are most challenging in patients with arrhythmias and/or respiratory difficulties. Figure 3 is an example of a 2D depiction of an artifact caused by gated 3DE acquisition. Note that the data set shown in the left panel of Figure 3 appears to be free of artifacts, whereas the image in the right panel has distinct stitching artifacts. If the gated acquisition acquires sector slices in a sweeping motion parallel to the reference image, then every image parallel to the reference image will appear normal. Gating artifacts are most prominent when the volumetric data set is viewed from a cut plane perpendicular to the sweep plane. Methods to minimize the effects of gating artifacts are described in Table 1. As well, the ECG tracing needs to be optimized to obtain a distinct R wave. Because the most frequent artifacts of gated acquisitions are stitching artifacts, the number of acquisition beats should be tailored to the clinical question to be addressed, taking into account that with more beats, the volume will be wider and the temporal resolution higher. To improve spatial resolution (i.e., the number of scan lines per volume), the pyramidal volume should be optimized to acquire the smaller volume able to encompass the cardiac structure of interest. Before 3DE acquisition, the 2D image should be optimized: “suboptimal 2D images result in suboptimal 3DE data sets.”1Hung J. Lang R. Flachskampf F. Shernan S.K. McCulloch M.L. Adams D.B. et al.3D echocardiography: a review of the current status and future directions.J Am Soc Echocardiogr. 2007; 20: 213-233Abstract Full Text Full Text PDF PubMed Scopus (139) Google ScholarTable 1Methods to avoid gating artifacts and improve 3D data set qualityView Large Image Figure ViewerDownload Hi-res image Download (PPT)TGC, Time gain control. Open table in a new tab TGC, Time gain control. Low gain settings result in echo dropout, with the potential of artificially eliminating anatomic structures that cannot be recovered during postprocessing. Alternatively, with excess gain, there is a decrease in resolution and a loss of the 3D perspective or depth within the data set. As a general rule, both gain and compression settings should be set in the midrange (50 units) and optimized with slightly higher time gain controls (time gain compensation) to enable the greatest flexibility with postprocessing gain and compression. Table 1 illustrates the issue of overgaining as well as undergaining. Therefore, it is recommended to slightly overcompensate the brightness of the image with time gain compensation rather than using the power-output gain. Using the postprocessing controls allows adjustments between high and low gain settings. However, it is important to note the even distribution of gain using the time gain compensation controls, as uneven areas of brightness cannot be compensated or corrected using postprocessing controls. As with 2D echocardiography, optimizing lateral and axial resolution remains equally important during 3DE acquisition. The concept of cropping is inherent to 3D echocardiography. In contrast to cross-sectional (i.e., tomographic) modalities, 3D echocardiography requires that the “viewing perspective” be in the chamber that is in immediate continuity with the region of interest. For example, to view the atrioventricular junctions “en face,” the operator must crop off the base and the apex of the heart, so that the operator may visualize the junctions looking up from below, or looking down from above. Similarly, to view the ventricular septum en face, the echocardiographer must crop off the free walls of both ventricles to view the right ventricular (RV) aspect of the septum from right to left or the LV aspect of the septum from left to right. The paradigm for the echocardiographer, therefore, is to change from the cross-sectional approach to that of the anatomist or surgeon, who can only view intracardiac structures after exposing them, by cropping the walls of the different chambers. Three-dimensional cropping can be performed either before (during) or after data acquisition. Cropping that is performed before the acquisition has the advantages of providing better temporal and spatial resolution, while also providing immediate availability of the cropped image. However, if a cropped image is stored, that image may not be amenable to “uncropping” later. In contrast, if a wide data set is acquired and cropped after acquisition, it provides the advantage of retaining more diagnostic information, but at the expense of loss of spatial and temporal resolution. Once a 3DE data set is acquired, it can be viewed interactively using a number of 3D visualization and rendering software packages. Display of 3DE images can be divided into three broad categories: (1) volume rendering (Figure 4A), (2) surface rendering (including wireframe display; Figures 4B and C), and (3) 2D tomographic slices (Figure 4D). The choice of the display technique is generally determined by the clinical application. Volume rendering is a technique that uses different types of algorithms (e.g., ray casting, shear warp, and others) to preserve all 3DE information and project it, after processing, onto a 2D plane for viewing.2Fenster A. Downey D.B. Cardinal H.N. Three-dimensional ultrasound imaging.Phys Med Biol. 2001; 46: R67-R99Crossref PubMed Scopus (321) Google Scholar Essentially, these algorithms cast a light beam through the collected voxels. Then, all voxels along each light beam are weighted to obtain a voxel gradient intensity that integrated with different levels of opacification, shading and lighting allows an individual structure to appear solid (i.e., tissue) or transparent (i.e., blood pool).3Cao Q.L. Pandian N.G. Azevedo J. Schwartz S.L. Vogel M. Fulton D. et al.Enhanced comprehension of dynamic cardiovascular anatomy by three-dimensional echocardiography with the use of mixed shading techniques.Echocardiography. 1994; 11: 627-633Crossref PubMed Google Scholar, 4Rankin R.N. Fenster A. Downey D.B. Munk P.L. Levin M.F. Vellet A.D. Three-dimensional sonographic reconstruction: techniques and diagnostic applications.AJR Am J Roentgenol. 1993; 161: 695-702Crossref PubMed Google Scholar Finally, a variety of shading techniques (distance shading, gray-level gradient coding, and texture shading) are used to generate a 3D display of the depths and textures of cardiac structures.3Cao Q.L. Pandian N.G. Azevedo J. Schwartz S.L. Vogel M. Fulton D. et al.Enhanced comprehension of dynamic cardiovascular anatomy by three-dimensional echocardiography with the use of mixed shading techniques.Echocardiography. 1994; 11: 627-633Crossref PubMed Google Scholar, 4Rankin R.N. Fenster A. Downey D.B. Munk P.L. Levin M.F. Vellet A.D. Three-dimensional sonographic reconstruction: techniques and diagnostic applications.AJR Am J Roentgenol. 1993; 161: 695-702Crossref PubMed Google Scholar Volume-rendered 3DE data sets can be electronically segmented and sectioned. To obtain ideal cut planes, the 3D data set can be manipulated, cropped, and rotated. Volume rendering provides complex spatial relationships in a 3D display that is particularly useful for evaluating valves and adjacent anatomic structures. Surface rendering is a visualization technique that shows the surfaces of structures or organs in a solid appearance. To use this technique, segmentation of the data set can be applied to identify the structure of interest.2Fenster A. Downey D.B. Cardinal H.N. Three-dimensional ultrasound imaging.Phys Med Biol. 2001; 46: R67-R99Crossref PubMed Scopus (321) Google Scholar Surface rendering of selected structures is obtained by manual tracing or using semiautomatic border detection algorithms to trace the endocardium in cross-sectional images generated from the 3D data set segmentation. These contours can be combined together to generate a 3D shape that can be visualized as either a solid or a wireframe object used to create a 3D perspective.5Pandian N.G. Roelandt J. Nanda N.C. Sugeng L. Cao Q.L. Azevedo J. et al.Dynamic three-dimensional echocardiography: methods and clinical potential.Echocardiography. 1994; 11: 237-259Crossref PubMed Google Scholar Wireframe reconstruction is used to generate 3D images of subsets of the entire data set in a cagelike picture. Stereoscopic presentation of the left ventricle has been used to improve the visual assessment of ventricular shape as well as the appraisal of ventricular structures and the quantification of cardiac chamber volumes and function. However, surface rendering frequently fails to provide details of cardiac structures or textures. Solid and wireframe surface-rendering techniques can be combined to allow appreciation of the extent of cardiac structure motion (i.e., cardiac chamber volume changes during the cardiac cycle). The volumetric data set can be sliced or cropped to obtain multiple simultaneous 2D views of the same 3D structure. In this manner, the limitations of acoustic imaging with conventional 2D echocardiography can be overcome by 3D echocardiography, which allows the acquisition of different cutting planes from virtually any acoustic window. Indeed, it is possible to select unique 2D cutting planes (which may be difficult or virtually impossible to obtain with 2D transducer manipulation from standard windows) from a volumetric 3D data set and to display the corresponding 2D tomographic images in a cine loop format. For example, a cardiac chamber can be cut in true longitudinal or transverse planes, referred to as common long-axis or short-axis views. Multiple slicing methods are available, such as arbitrary plane, simultaneous orthogonal (or arbitrary angle) slices, and parallel slice planes.6Yang H.S. Bansal R.C. Mookadam F. Khandheria B.K. Tajik A.J. Chandrasekaran K. Practical guide for three-dimensional transthoracic echocardiography using a fully sampled matrix array transducer.J Am Soc Echocardiogr. 2008; 21: 979-989Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar The arbitrary plane cut allows the operator to orient the cutting plane in any direction, for optimal cropping of the cardiac structures of interest. The simultaneous orthogonal 2D slice mode consists of two or three 2D planes (coronal, sagittal, and transverse) displayed simultaneously. Finally, it is possible to obtain multiple 2D parallel tomographic slices with uniformly spaced 2D parallel slices. These optimized cross-sectional planes of the heart allow accurate measurements of chamber dimensions and valve or septal defect areas as well as improved evaluation of the morphology and function of different structures with more objectivity and less operator dependency.7Zamorano J. Cordeiro P. Sugeng L. Perez de Isla L. Weinert L. Macaya C. et al.Real-time three-dimensional echocardiography for rheumatic mitral valve stenosis evaluation: an accurate and novel approach.J Am Coll Cardiol. 2004; 43: 2091-2096Abstract Full Text Full Text PDF PubMed Scopus (118) Google Scholar, 8Matsumura Y. Fukuda S. Tran H. Greenberg N.L. Agler D.A. Wada N. et al.Geometry of the proximal isovelocity surface area in mitral regurgitation by 3-dimensional color Doppler echocardiography: difference between functional mitral regurgitation and prolapse regurgitation.Am Heart J. 2008; 155: 231-238Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar, 9Cheng T.O. Xie M.X. Wang X.F. Wang Y. Lu Q. Real-time 3-dimensional echocardiography in assessing atrial and ventricular septal defects: an echocardiographic-surgical correlative study.Am Heart J. 2004; 148: 1091-1095Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar The simultaneous orthogonal 2D slice mode provides multiple visualization of the same segment within a single cardiac cycle, which can be useful for ventricular function analysis as well as for wall motion assessment during stress echocardiography.10Yoshitani H. Takeuchi M. Mor-Avi V. Otsuji Y. Hozumi T. Yoshiyama M. Comparative diagnostic accuracy of multiplane and multislice three-dimensional dobutamine stress echocardiography in the diagnosis of coronary artery disease.J Am Soc Echocardiogr. 2009; 22: 437-442Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar, 11Badano L.P. Muraru D. Rigo F. Del Mestre L. Ermacora D. Gianfagna P. et al.High volume-rate three-dimensional stress echocardiography to assess inducible myocardial ischemia: a feasibility study.J Am Soc Echocardiogr. 2010; 23: 628-635Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar Until 3D echocardiography is fully incorporated into daily clinical practice, protocols and techniques will remain focus oriented and vary according to disease process as well as institutional use. Currently, many laboratories perform full 2DE exams followed by focused 3DE studies.12Mor-Avi V. Sugeng L. Lang R.M. Real-time 3-dimensional echocardiography: an integral component of the routine echocardiographic examination in adult patients?.Circulation. 2009; 119: 314-329Crossref PubMed Scopus (73) Google Scholar The reason for this inconvenient work flow was that the 2D image quality obtained with the 3D TTE probe was inferior to that of dedicated 2D TTE probes. Thus, the success of using 3DE in clinical practice depends on a practical work flow, which requires (1) a single transducer solution capable of 2D and 3D imaging, (2) accurate automated chamber quantification, and (3) automated display of standard 3DE or 2DE cut planes views from each window acquisition. With the latest generation of 3D TTE and TEE probes, the first requirement has been achieved, because the 2DE images obtained with these new transducers are comparable in quality with those obtained with dedicated 2D transducers. As well, multiple imaging ultrasound companies have developed or are in the process of developing software with automated chamber quantification and automated display of cut planes, which addresses the second and third requirements. Beyond acquisition work flow, data management, which refers to the manner in which 3DE data are stored and recalled for analysis, also needs to be optimized. Currently, a 2DE exam requires on average 300 to 500 MB of storage space, whereas a combined 3DE and 2DE exam may require up to 1.5 GB of storage. These large data sets place a strain on the digital systems of laboratories not only with regard to transmission but also in terms of overall storage capacity. A Digital Imaging and Communications in Medicine standard for 3D echocardiography was approved in 2008, which called for the storage of Cartesian data sets without compression, which requires a large amount of digital storage space. Because of the storage requirements, this standard has not been widely adopted. Greater use of this standard and perhaps adoption of a standard with compression will ease 3DE data storage concerns. Similar to conventional 2D echocardiography, color Doppler superimposes flow information onto 3DE morphology. Three-dimensional color Doppler acquisition is performed using live 3D or multiple-beat full-volume acquisition. Although larger data volumes are achieved with multiple-beat full-volume color Doppler acquisition, it is limited by stitching artifacts. In contrast, live 3D color Doppler acquisition is not affected by stitching artifacts but is limited by smaller color Doppler volumes and lower frame rates. Although 3D color Doppler data acquisition is feasible with TTE and TEE examinations, 3D TEE acquisition currently provides significantly better color Doppler image quality and therefore is recommended for detailed color flow analysis. Similar to what occurs during non–color Doppler 3D data set acquisition, the size and location of the 3D color Doppler volumes should be carefully defined according to the flow region to be analyzed. Color flow analysis includes (1) distal jets, (2) the proximal flow field of valvular flow regurgitation, and (3) flow through heart defects such as ventricular or atrial septal defects. Cropping of 3D color Doppler data sets follows the same principles as non–color Doppler data set cropping and is determined mainly by the analysis intended. For regurgitant jets, it is recommended to crop the 3D color Doppler data set to show two long-axis views of the jet: one with the narrowest and one with the broadest width of the jet. This display should also include a short-axis view of the jet at the level of the vena contracta (Figures 513Kahlert P. Plicht B. Schenk I.M. Janosi R.A. Erbel R. Buck T. Direct assessment of size and shape of noncircular vena contracta area in functional versus organic mitral regurgitation using real-time three-dimensional echocardiography.J Am Soc Echocardiogr. 2008; 21: 912-921Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar and 6). Alternatively, color Doppler flow can be displayed using a multiple slice representation extracted from the 3D color Doppler data set, as shown in Figure 7.Figure 6Example of 3D assessment of functional mitral regurgitation by 3D transesophageal echocardiography and 3D color Doppler. Two-dimensional cross-sectional views demonstrating mitral leaflet tethering and tenting (top left) causing significant eccentric mitral regurgitation (top middle). Three-dimensional echocardiographic view to the mitral valve shows only moderate focal nodular degeneration of mitral leaflet (top right). Cropping of a 3D color Doppler data set reveals a vena contracta area, which is narrow in the five-chamber view (bottom left), broad in the two-chamber view (bottom middle), and asymmetric along the commissural line in an en face view to the mitral valve (bottom right). Ao, Aorta; LA, left atrium; LAA, left atrial appendage; LV, left ventricle.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 7Measurement of vena contracta dimensions from a 3D TEE color Doppler data set (bottom right) using 3D analysis software. The data set is cropped to create a four-chamber (top left) and a two-chamber view (top right). Note that the vena contracta width is smaller when measured on the four-chamber view compared with the two-chamber view. The cropping plane can be adjusted to present an en face view of the vena contracta from which a planar area of 1.11 cm2 (bottom right) can be measured. R, Proximal isovelocity surface area radius.View Large Image Figure ViewerDownload Hi-res image Download (PPT) Understanding the orientation of color Doppler flow within the displayed views is clinically important. To help with the orientation, it is recommended to display the 3D color Doppler data in at least two different views with known orientation to each other as indicated by different colored cutting planes (Figure 5, Figure 7). It is also recommended to display 3D color Doppler data together with characteristic anatomic 3D information using standard views. The limitations of 3DE color Doppler acquisition include poor spatial and temporal resolution, both expected to improve with the advancement of 3DE technology. Currently, live 3DE color Doppler acquisition is limited to small color Doppler volumes, usually with limited temporal resolution of 10 to 15 voxels/sec. Alternatively, multiple-beam full-volume acquisition of color Doppler providing larger color Doppler volumes and volume rates (up to 40 voxels/sec) are limited by stitching artifacts, resulting in significant displacement between different subvolumes (Figure 3, bottom). Three-dimensional TTE full-volume acquisition mode can accommodate most of the entire heart structures within a single 3D data set. However, with existing technology, the decreases in both spatial-temporal resolution and penetration that would result from enlarging the volume angle to acquire the entire heart from a single acoustic window makes this impractical. To overcome these limitations, 3DE data sets “should be” acquired from multiple transthoracic transducer positions. In clinical practice, two protocols have been used: (1) focused examination and (2) complete examination.1Hung J. Lang R. Flachskampf F. Shernan S.K. McCulloch M.L. Adams D.B. et al.3D echocardiography: a review of the current status and future directions.J Am Soc Echocardiogr. 2007; 20: 213-233Abstract Full Text Full Text PDF PubMed Scopus (139) Google Scholar, 6Yang H.S. Bansal R.C. Mookadam F. Khandheria B.K. Tajik A.J. Chandrasekaran K. Practical guide for three-dimensional transthoracic echocardiography using a fully sampled matrix array transducer.J Am Soc Echocardiogr. 2008; 21: 979-989Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar A focused 3DE examination usually consists of relatively few 3DE data sets acquired to complement a complete 2D study. Some examples of focused 3DE examinations are (1) acquisition of a gated 3DE full-volume data set from the apical window to quantify LV volumes, LV ejection fraction, and LV shape and to evaluate for LV dyssynchrony in patients with heart failure; (2) data sets acquired from both the parasternal and apical approaches to visualize the mitral valve apparatus with the aim of measuring orifice area in a patient with mitral stenosis; and (3) 3D zoom mode acquisition, with high density from the parasternal window to visualize the aortic valve in a patient with suspected bicuspid valve. For a focused exam, start with 2D imaging to localize the structure of interest, then switch to live 3DE imaging to check if the structure of interest is en