TECHNOLOGY


FOCUS: Echocardiography

Three-Dimensional Echocardiography: State-of-the-art-1999

Jos R.T.C. Roelandt, MD, PhD, FACC, FESC*
The Thoraxcentre, Erasmus University Medical Centre Rotterdam,
Rotterdam, The Netherlands
 

ABSTRACT
   
     Three-dimensional echocardiography depicts the heart and its structures in their realistic forms. This capability decreases variability both in the quality and the interpretation of complex pathology, among investigators. Therefore, it is likely that the method will become the standard echocardiography examination in the future. The availability and versatility of the volumetric data set obtained allows retrieval of an infinite number of cardiac cross-sections. The technique, by obviating geometric assumptions, is more accurate and reproducible for measurements of valve areas, masses and cavity volumes. In the future, additional information will be provided by new physiologic parameters, which may provide answers to new clinical questions.

    Three-dimensional imaging to objectively view the heart and its structures is already “state-of-the-art.” However, progress in three-dimensional echocardiography has been rather slow since the first attempts were made in the early 1970s. In its early stage, three-dimensional echocardiography was applied mainly in volume measurement of the ventricles using multiple cross-sectional images that employ laborious manual tracing of the cardiac borders.1-8 Reconstruction of those tracings into static wire-frame pictures demonstrated the shape of the ventricular cavity, but did not provide tissue-depicting information.9-11 Recently, along with the rapid evolution in computer technology, three-dimensional echocardiography has grown into a well-developed imaging modality, able to display dynamic images of cardiac structures in their realistic forms that depict depth and contain tissue information.12-16 Many clinical and experimental studies have demonstrated that the method is now ready for clinical application.17-22 (Heart Views. 1999;1:102-115)


APPROACHES TO THREE -DIMENSIONAL ECHOCARDIOGRAPHY (table 1)


A. Real-time three-dimensional echocardiography

     The ideal, but technically the most challenging direction, is real-time three-dimensional imaging when the ultrasound examination is performed. A real-time volumetric ultrasound imaging system has been developed by Von Ramm et al. of Duke University,16,23 and is based on novel matrix phased-array transducer technology. The elements are arranged in a two-dimensional grid. Using parallel processing techniques, the matrix array offers steering in both the beam’s azimuth and elevation, which compensates for the fundamental limitation of speed of ultrasound in tissue. This permits scan of a pyramidal volume at the same time.

    This mode of three-dimensional echocardiography has great potential in improving anatomical visualization, fast diagnosis of morphological cardiac disorders, left ventricular function assessment, and regional or global wall motion analysis in various situations, including stress or exercise tests. Due to a short image acquisition time, this method is optimal for myocardial perfusion study in combination with myocardial contrast agents. So far, this method has been applied in surface examination only. For transesophageal use, the size of the probe needs to be further minimized. Though initial results with real-time three-dimensional echocardiography are promising, improvement in image quality is needed for its widespread clinical application.
 

B. Off-line three-dimensional reconstruction

    The commonly used term “three-dimensional echocardiography”, usually means “off-line” computer assisted reconstruction of three-dimensional displays. The image is reconstructed from a series of cross-sectional echocardiographic images, using currently available transesophageal and precordial transducers. This approach necessitates the use of a dedicated computer for the simultaneous registration of accurate spatial position, timing of the cross-sectional images, and reconstruction and analysis of three-dimensional data.

   Systems are now available for random and sequential data collection from multiple cross-sectional images controlled by computers, as well as for on-line data acquisition with multiplane transducers directly controlled by the ultrasound machine itself. Devices for intravascular, intracardiac, and peripheral vascular three-dimensional imaging have been developed.24-28 Some authors suggest the term “four-dimensional echocardiography”13 for dynamic display of three-dimensional data, the fourth dimension being time.

    Though various methods have been employed, the essential steps of three-dimensional reconstruction are as follows: 1) data acquisition which can be random or sequential, 2) data processing, 3) three-dimensional image rendering and display.

1. Random data acquisition

    Positional information can be obtained with a mechanical articulated arm scanner1,5,8,29-31, acoustic spark gap 2,9-11,32 , or a magnetic location system3,33 allowing unrestricted (free hand) scanning from any available precordial acoustic window (table 1). Therefore, the limitations by restricted or suboptimal acoustic windows are minimized. However, care must be taken to avoid big gaps between imaging planes. The volumetric data set can be used to extract static wire-frame objects or surfaces (surface rendering) of selected structures, which are converted to geometric rather than anatomic representations for projection onto a two-dimensional screen. These representations are usually generated from manually derived contours in the cross-sectional images, which is tedious and time consuming. These approaches allow for assessment of structure and surface shapes, and for improved quantification of left ventricular volumes.

2. Sequential data acquisition

     There are three modes of sequential data acquisition currently available using predetermined steps for sequential image collection (table 1). The steering logic using ECG and respiration gating for controlled temporal and spatial position image registration was developed by Tomtec GmbH, Munich, Germany and is applied in most commercially available reconstruction systems.

I. Linear scanning

     Parallel equidistantly spaced images can be collected by computer controlled movement of the ultrasound probe or transducer in a linear direction. A surface probe can be moved in predefined steps by a computer controlled sliding device adapted externally to it. A special transesophageal probe was developed for this mode of data acquisition, the distal part of which consists of multiple semicircular plastic segments that can be mechanically straightened after the probe is advanced into the esophagus. A sliding carriage, on which the transducer is mounted, is housed in the distal part of the probe and can be moved in equal steps under the control of computer considering cardiac and respiratory cycles.34-38 But this probe did not succeed in routine clinical use due to difficult intra-esophagus introduction, and poor patient acceptance of its size. Three-dimensional data set from parallel acquisition is prism in shape, and the “stepping resolution” at each depth depends on the distance between the parallel images, with better resolution in data acquired with smaller acquisition steps.

II. Fan-like scanning

A pyramidal data set can be obtained by moving the ultrasound transducer in a fan-like arc at prescribed angles. This is accomplished by computer controlled motors adapted to the transducer or probe in surface or transesophageal approaches.39-42 Distances between imaging planes vary with depth with the largest gaps in the far field, thus, this region contains less structural information and has less resolution.

III. Rotational scanning

• Stepwise acquisition

   In this approach, the transducer is rotated in a semicircle of 180 degrees around the central axis of the imaging plane that results in a conical volume data set. Computer-controlled rotation of the transducer can be realized with a rotational device adapted either to a multiplane transesophageal probe14 or a regular surface probe.43-46 We were the first, among several pioneer centres, to use rotational data acquisition which made this technique practical and widely applied. This mode of data acquisition with a commercially available ultrasound unit alone (Hewlett Packard, Sonos 2500/5500, Andover, MA, USA) has been brought to clinical use with both transthoracic and transesophageal multiplane transducers.47,48 Rotation of the transducer and collection of images at every step is controlled by the ultrasound unit itself with ECG and respiration gating obviating the need of interfacing an additional computer or an externally mounted motoring device for transducer rotation and data storage. This fashion of data acquisition needs a relatively smaller acoustic window comparing to the above two methods and is the most commonly employed mode by now. Distances between individual scanning planes obtained with rotational technique vary in both axial and lateral fields. This results in different structural information and resolution in any given point for each cutting plane.

• Continuous acquisition

   Recently, an ultrafast continuously rotating phased-array transducer has been developed at the Thoraxcentre, which allows the acquisition of 16 volumetric datasets per second. This makes ECG and respiration gating less critical. Standard ultrasound systems are used and the image quality in the data-set is that of state-of-the-art two-dimensional echocardiography. The transducer assembly is currently interfaced to a Vingmed System 5. Basic two-dimensional images are digitally stored in the instrument memory and transferred either directly or off-line to a reconstruction and analysis PC. Initial experience indicates that this near real-time approach may become an alternative to real-time volumetric imaging systems in specific clinical conditions.

DATA PROCESSING

    In the real-time volumetric scanning device, the echo data are parallel-processed to produce focused image points in pre-selected image planes. At present, 5 cross-sectional images are displayed on-line: 2 orthogonal and 3 cross-sections at a selected depth or C-scan. A volumetric display is produced off-line by additional processing.

   The sequentially collected two-dimensional images for off-line three-dimensional reconstruction are digitized and realigned according to their spatial and temporal sequences. Geometric transformation is necessary for images acquired in rotational or fan-like scanning manners to convert the data points into an isotropic cubic data set. Gaps between individual images are filled by the computer with different interpolation algorithms for different acquisition modes, such as “trilinear cylindrical interpolation” for data from rotational scanning. Then the pixels in two-dimensional images are transformed into voxels in a volumetric three-dimensional data set. Small motion artifacts resulting from movement of the patient, respiration-related movement of the heart, or movement of the probe, as well as artefacts caused by ultrasound noises can be minimised using various image processing filters. At this stage, the volumetric data set can be sliced to derive cross-sectional images in any desired cutting planes or be rendered into various forms of three-dimensional images.14,15,46

     With voxel-based volumetric three-dimensional data sets, cross-sectional cutting planes can be derived arbitrarily using various algorithms:

Anyplane method is the basic algorithm in generating cross-sectional images. Three perpendicular axes in the three-dimensional data set referred to the Cartesian co-ordinate system are used for guiding cutting plane manipulation. Innumerable cross-sectional views of the heart, which is difficult or physically impossible to obtain from conventional precordial or transesophageal acoustic windows can be computed from three-dimensional data set and displayed dynamically in cine-loop format.

Paraplane method is used to derive multiple parallel equidistant cross-sectional views through a region of the heart at selected intervals, based on definition of one anyplane image (figure 1). Long-axis and short-axis methods are used to produce multiple long or short axis images of a ventricle, or an object in the heart at equally distributed intervals (distances or angles) (figure 2).

Mainplane method creates three orthogonal cutting planes perpendicular to each other through a region of interest in the heart, by first defining one anyplane. These secondary derived cross-sectional images form three-dimensional data set aids in systematic review of the cardiac structures, selection of optimal cutting planes, and quantification of regional volumes of a selected territory.
 

Fig. 1. Paraplane echocardiography using the three-dimensional dataset of a normal heart. The parasternal long axis view is shown in the upper left panel and the lines indicate eight equidistant computer generated parallel short axis views of the left ventricle from base to apex. The endocardial contours are manually traced. Analytic software is presently available for automated contour detection which considerably shortens analysis time.

 

Fig. 2. Anyplane echocardiography of the left ventricle of a normal heart using the three-dimensional data set. Eight long axis views are reconstructed by computer and their orientation is shown in the upper left panel. This approach allows a comprehensive and standardised analysis of the shape, size and wall motion of the left ventricle.

RENDERING AND DISPLAY

     To view the heart in three-dimensions, reconstruction and display of three-dimensional images from the processed three-dimensional data set is essential. Several rendering techniques have been developed for this purpose.

    Wire-frame formation is used to generate three-dimensional images of subsets of the entire dataset in a cage-like picture. This algorithm is mainly used in randomly collected data (though it can also be used for sequentially collected images), and for chamber volume quantification.10 Manual tracing of the acquired cardiac images is required, and the reconstructed image is often displayed in static mode. Although a surface calculated by the computer can be applied to the wire-frame image, this form of reconstruction can not provide details of the cardiac structure or texture of the cardiac tissue.

Surface rendering technique extracts the contour of the structure from three-dimensional data set and displays in a solid appearance the surface of the object that faces the observer. Shadowing algorithms can be used to create a three-dimensional perspective. Information of the tissue beneath the surface is missing.15

Volume rendering technique engages all the information of the cardiac structures within the volumetric data set to create three-dimensional images that closely resemble the true anatomy of the heart. Depending on the level of opacification, shading, and lighting of a volume-rendered image, the structure may either appear solid (similar to the effect of surface-rendered images)or transparent, the latter allowing one to see through the “surface”.

   Three kinds of shading techniques (distance shading, gray level gradient coding, and texture shading) are usually used and mixed with different weighting factors to generate a three-dimensional display of the depths and textures of the cardiac structures.49 The three-dimensional effect can be further enhanced by creating rotational sequences of the image upon display.
 

   Volume-rendered three-dimensional data set can be electronically segmented and sectioned. To display intracardiac structures, the heart can be opened by choosing a cutting plane and reconstruct the image beyond this plane as if the heart is cut open in surgery. Three-dimensional projections of the heart in conventional orientations employed in two-dimensional echocardiography are easily perceived, and enthusiasm is evoked by projections of unconventional views and those similar to surgeon’s views of the heart not accessible with any other techniques.50 These can be achieved by manipulation of the cutting planes and rotation of the three-dimensional image to obtain ideal projections.

   Mitral and tricuspid valve can be viewed either from above (electronically simulating atriotomy), or from below (as with ventriculotomy). Likewise, the aortic valve can be visualised from above with electronic aortotomy (figures 3 and 4) and from below, looking through the left ventricular outflow tract. In dynamic mode display, the opening and closing of the cardiac valves can be observed readily. Atrial and ventricular septa can be examined en face with better perception of their spatial relationship with adjacent structures (figure 5). Longitudinal views are useful to display chamber size and ventricular function, as well as valvular movements and intracardiac flow jets. Special structures can be revealed by various display projections including unconventional views, especially in patients with complex congenital heart diseases.

Unrestricted Cross-Sectional Cutting Planes

   Limitations of acoustic access to discretionary cutting planes and spatial registration of individual images with conventional two-dimensional echocardiography can potentially be overcome by three-dimensional echocardiography. It is possible to select unique cutting planes of the heart, which are difficult or impossible to obtain with transducer manipulation from standard acoustic windows from a volumetric three-dimensional data set, and to display the corresponding cross-sectional images in cine-loop format. In other words, it is now possible to scan the heart from the dataset after the patient has left the laboratory. Slicing of a given region with parallel equidistant cutting planes can be performed accurately in a fashion similar to computed tomography or magnetic resonance imaging. A structure or a cavity can be cut in true longitudinal or transverse planes referred to a common “long-axis”, independent to the transducer position during image collection (figure 2). Optimal cross-sectional planes of the heart can be obtained for accurate measurement of various dimensions (of cavity or defect) and areas (figure 6) (of stenotic valve or regurgitant orifice), and for better evaluation of morphology and function of a given structure with more objectivity and less operator dependency. Regions of interest can be electronically extracted from the dataset and structures of interest removed from their surroundings for detailed analysis (figure 3). “En face” views uniquely allow understanding surface areas (figure 5).
 

   The strength of offering unlimited number of cross-sectional views and three-dimensional projections could also be too intimidating. Guidelines to identify clinically useful cutting planes are being formulated for application in various categories of disease.15

   In the future three-dimensional displays by volumetric holography may be considered.51,52. Three-dimensional echocardiography is the gateway to virtual reality offering great potential for teaching and training, aiding in complex diagnostic situations and to assist in planning surgical procedures.53

Three-Dimensional Display Projections

    A major advantage of three-dimensional echocardiography over any two-dimensional approaches is that it can reproduce numerous novel cross-sectional cutting planes and three-dimensional display projections of the cardiac structures. Dynamic volume-rendered three-dimensional reconstructions provide accurate spatial and temporal information valuable for comprehensive evaluation of cardiac function and morphologic abnormalities. The reconstructed images can be displayed in dynamic (cine-loop), static or frame-by-frame fashion.
 

Clinical Applications And Current Experience

   Three-dimensional echocardiography has produced promising results from both experimental and clinical studies in the past two and half decades. It has been applied in various clinical scenarios with different settings including echocardiographic laboratory, various in-patient care units, operating room, and emergency room. Favourable experience has been gained in its clinical applications with both transthoracic and transesophageal image data acquisition. A volumetric data set of the heart can be achieved in a few minutes, from which multiple two-dimensional and three-dimensional images can be reconstructed and displayed off-line.

   Echocardiographic examinations can now, and will increasingly, be performed with computer controlled transducer systems standardised for specific cardiac conditions. This shortens the examination time of imaging plane manipulation for morphologic study, makes the procedure less dependent on the skill of the operator, and decreases the discomfort of the patient.
 

    Congenital Heart Disease

   Three-dimensional echocardiography has been proven valuable in congenital heart disease for better evaluation of morphologic abnormalities and understanding of complex spatial relationships.18,19,34,42, 54-59 Three-dimensional en face views of atrial (figure 5) or ventricular septal defect, from right or left side, not only provide surgeon’s view of the defect before the heart is open but also enable accurate measurement of the dimensions of the defect directly on three-dimensional images and, most importantly, measurement of the tissues surrounding the defect, the later being crucial for planning interventional procedures, especially for close-chest closure of the defect using transcatheter closing device. Various three-dimensional projections help discern congenital malformations of the heart such as bicuspid aortic valve (figure 4), subaortic membrane (figure 7), and in differentiating mitral valve from tricuspid valve in patients with transposition of great arteries. Other congenital anomalies such as cleft or parachute mitral valve and tricuspid valve atresia, can be demonstrated by three-dimensional reconstruction and direct measurement such as extent of mitral valve cleft derived.
 

    Valvular Heart Disease

  Evaluation of valvular heart disease can also be improved by three-dimensional echocardiography since valvular abnormalities can be delineated more precisely and in greater detail than by conventional imaging. 9,17,60-68 Diastolic doming and restricted motion of a stenotic mitral valve, thickness and pliability of the mitral valve leaflets, and involvement of the subvalvular structures can be displayed with longitudinal and transverse views from different directions. Anyplane and paraplane analysis of the stenotic valve helps to find the cutplane through the smallest orifice area for accurate planimetry.68

   Three-dimensional echocardiography can provide a dynamic view of the surgical anatomy of the stenotic valve, giving the surgeon an advance look on what one may encounter intraoperatively. Mitral or tricuspid valve prolapse is shown on three-dimensional images as a bulging or protrusion on the atrial side of the mitral valve (figure 10). The exact location and extension of prolapse can be visualised, which is important information for the surgeons when planning surgical procedures. Three-dimensional echocardiography has been shown highly accurate for diagnosing ruptured chordae tendineae and identifying the flail scallop (figure 11). 65

   Similarly, aortic (figure 4) and pulmonary valve abnormalities can be observed in multiple projections as well. Prosthetic valves can be reconstructed and their sitting and function evaluated. When combined with color Doppler flow imaging, intracardiac flow jets with relatively high velocities, such as regurgitant jets or blood flow through a stenotic valve, can be reconstructed and displayed in three dimensions.69-72 The site of origin, direction of trajectory, geometric distribution, and morphology of the jet is better appreciated. Multiple jets, unusual path of jet travelling, and interaction between compound jets can be better understood with dynamic volumetric display. Three-dimensional echocardiography might also provide better access for quantitative evaluation of valvular abnormalities, using proximal flow convergence or vena contracta of the flows.
 

   Coronary Artery Disease

   Coronary heart disease is one of the most commonly encountered diseases for cardiologists. Three-dimensional echocardiography has shown its potential in accurate evaluation of volumes and function of the ventricles, in analysis and quantitative measurements of regional wall motion abnormalities, and myocardial perfusion territories applied with myocardial contrast agents.73-76 Initial experience with direct three-dimensional visualization of coronary arteries also exists (figure 12).
 

   Other Cardiac Diseases

   Three-dimensional echocardiography has been used in almost all kinds of cardiac disorders and various benefits have been achieved. Evaluation of intracardiac or intravascular masses including vegetations, tumours, thrombi or plaques is facilitated both qualitatively (by three-dimensional display of their site, size, attachment and mobility), and quantitatively (by accurate measurement of their dimensions and volumes) (figures 13 and 14).77 We have also examined aortic diseases such as dilatation, aneurysm, dissection or coarctation with three-dimensional echocardiography and incremental information was obtained78,79.

   Volume Quantification

   Three-dimensional echocardiography is superior to two-dimensional echocardiography for quantitative volume measurements of the left and right ventricles because no assumptions are necessary about left ventricular geometry. Good correlations with angiography, magnetic resonance imaging and anatomical measurements (in vitro) have been reported (table 2).7,10,16,80-96

   At present, ventricular volumes are calculated by manual endocardial tracing of sequential short axis views, derived by parallel slicing through three-dimensional data set at prescribed thickness intervals at either end-systole or end-diastole. The method is time consuming. Several investigators attempted to limit the time requirements by reducing the number of component cross-sections. Volume quantification is achieved by summation of the voxels included in the traced area, with subsequent summing of the subvolumes of each slice with known slice thickness (figures 1 and 15).
 
   The same approach has proven effective for the quantification of myocardial mass (table 2). Technology for automated border detection is now available to calculate surface areas. This permits accurate and reproducible measurements of volumes, which is important for clinical decision-making and serial follow-up studies. Stroke volume and ejection fraction of a given chamber can be derived from its end-systolic and end-diastolic volumes.

   Unlike any two-dimensional method, volume measurement with three-dimensional echocardiography is accurate and reproducible, since it obviates any geometric assumptions of the measured object. Hence, the method can be applied to any structure, intracardiac mass lesions, or abnormal blood flow jets.

 Limitations

   The currently used volume-rendered three-dimensional reconstruction techniques, though already accepted for clinical application in major institutions, have some limitations for practical use and application in specific conditions. Expertise and experience is needed for performance of image collection and reconstruction and the prescribed data acquisition with ECG and respiratory gating requires several minutes (usually 2-5 min., even as long as 10 min. depending on incremental step, ECG and respiration rate and rhythm and gating range). This might prevent it from being used for stress or perfusion contrast echocardiography, in which the time window for data acquisition is very much limited.

   Also, during data collection, both the patient and the probe or carriage device have to be still, otherwise motion artifacts will be produced. New transducer assemblies such as the ultrafast rotating phased-array transducer will bring the acquisition time down to seconds for specific quantitative studies.

   The time needed for data processing and three-dimensional reconstruction, though being steadily shortened, ranges from minutes to hours depending on the perspective and amount of images and type of analysis performed. Thus, on-line diagnosis needed in situations like emergency room or operation room is restricted.

   Accuracy of three-dimensional displays and computer generated any-/paraplane cross-sections from the data sets remains a problem with three-dimensional reconstruction techniques. Ultrasound is a reflection technique and sampling the structures to build up the dataset is often incomplete. Structure resolution deteriorates with depth and in the lateral parts of the basic images. Electronic interpolation is necessary to fill gaps between cross-sections in the far field and may be inaccurate. Overall threshold settings for segmentation create artificial structures and or tissue overgrowth. The heart is a moving target relative to the stationary transducer causing blur. Non-uniform rotation speed during acquisition and rotational axis deviation (patient movement) may also cause significant artifacts. The solution for this problem is real-time or ultrafast acquisition.


CONCLUSION


   Three-dimensional echocardiography provides the clinician with more confidence for the diagnosis of cardiac disease and adds insights to the understanding of complex pathology. It decreases variability both in quality and interpretation among operators. The availability and versatility in using a three-dimensional data set allows the cardiologist to retrieve an infinite number of different views after the examination procedure, and to re-examine the patient after he has left the laboratory. More accurate and reproducible measurements together with new physiologic parameters such as wall motion phase analysis, LV curvature analysis for regional wall stress, flow jets, and myocardial perfusion will provide additional information, allowing new clinical questions to be addressed, which are uniquely three-dimensional (table 3). Indeed, progress in cardiology has often followed in the wake of new technologies, especially when they render better insights in pathology, and thus, providing answers to new questions. For these reasons, three-dimensional echocardiography will be an essential part of the clinical practice of cardiology in the future.

   Further developments and improvement for its widespread routine application include faster and / or real-time acquisition, processing and reconstruction, and easier and versatile approaches to quantitative analysis. Clinical research must be directed towards identifying those cardiac conditions in which the diagnostic potential of three-dimensional echocardiography is superior or more cost-effective than competing imaging methods such as magnetic resonance. Obviously, one needs a critical number of echocardiographers to give the technique a chance and robust evidence to convince cardiologists to use this new tool. At present, its additional information in surgical decision-making and the increasing number of clinical questions that can be addressed and answered, can already justify the clinical use of the technique99,100.



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