Introduction
The heart is a dynamic organ and places special demands on three-dimensional (3-D) techniques. To understand its physiology and pathophysiology, not only the spatial distribution of its structures is important but also their movement during the cardiac cycles. Previous approaches to 3-D echocardiography (3-DE) were offline and based on sequential rotational scanning and acquisition of multiple cross-sectional images together with their spatial position using internal co-ordinate reference systems. These methods were hampered by long acquisition and analysis times in combination with limited image quality. Realtime 3-D echocardiography (RT 3-DE) now allows visualisation of the heart and its structure dynamics in a realistic fashion with instantaneous online volume-rendered reconstruction.1-5
The availability of volumetric data sets allows retrieval of an infinite number of cardiac cross-sections. This capability provides an improved understanding of unpredictable morphology and decreases the variability in interpretation of complex pathology among investigators.5-7 In particular, typical 3-DE parameters, such as ejection fraction, segmental wall function, left ventricle (LV) shape and flow jets, become important diagnostic parameters based on 3-DE.
RT 3-DE
RT 3-DE allows online acquisition of a 3-D data set without the need for echocardiogram (ECG) and respiratory gating, avoiding spatial motion artefacts. RT 3-DE is first based on novel matrix phased-array transducer technology in which the elements are arranged in a two-dimensional (2-D) grid.8 The first system consisted of a sparse matrix phased array transducer of 512 elements to scan a 60˚ x 60˚ pyramidal volume using parallel processing technology. More recently, Philips Medical Systems introduced the live 3-D system using a matrix phased-array transducer with 3,000 transmit-receive elements. In this transducer, multiple recordings are automatically performed, based on ECG gating, to cover the LV in a full volume data set. This is particularly useful in dilated ventricles because of the limited sector angle. The multidirectional beam steering capability enables visualisation of two views of the heart simultaneously. Although experience remains limited, promising results have been reported.
Image Rendering and Analysis
To display the heart in three dimensions, reconstruction and display of 3-D images from the processed 3-D data sets is essential. The term rendering indicates the procedure whereby structures are reconstructed in the computed memory.7 The volume-rendered 3-D data set can be electronically segmented and sectioned. To display intracardiac structures, the heart can be opened by choosing a cutting plane and the image can be reconstructed beyond this plane as if the heart is cut open in surgery.9 The display and analysis of size, shape and motion of cardiac structures from any desired perspective becomes possible and allows one to address any clinical question offline without re-examination of the patient. By manipulation of the cutting planes and rotation of the 3-D image the ideal projection can be obtained.10 The mitral and tricuspid valves can be viewed from above (electronically simulating atriotomy) or from below (as with ventriculotomy). Likewise, the aortic valve can be visualised from above with electronic aortotomy and from below looking through the left ventricular outflow tract. In the dynamic mode display, the opening and closing of the cardiac valves can be observed. Interatrial and -ventricular septa can be examined en face with more accurate perception of their spatial relationship with adjacent structures. Most notably in patients with congenital heart disease, special structures can be identified by various display projections including unconventional views.
Wire-frame or surface-rendered reconstructions of selected structures are obtained from manually or electronically derived contours in cross-sectional images generated from the data set. This approach allows for the assessment of characteristics such as structure and shape and for improved quantification of left ventricular volume and function.11
Clinical Application and Perspectives
LV Analysis
The ability to accurately assess LV function is essential for patient management, prognosis and follow-up. A fast, practical and accurate method is a pre-requisite for obtaining this important information. Although two-dimensional echocardiography (2-DE) is routinely used in clinical practice to obtain information on LV dimension, wall thickness and function, this technique is limited because it relies heavily on geometrical assumptions to provide quantitative parameters of LV function. In order to avoid these geometrical constraints, 3-D reconstructions of the LV cavity have been performed using a variety of methods (see Figure 1).
Currently, data analysis and quantitative assessment is mostly performed on a computer with dedicated software. Semi-automatic border detection algorithms can be used to track endocardial borders throughout the cardiac cycle to obtain a dynamic display of the LV, as well as instantaneous global and regional LV volumes compared with time curves (see Figure 2). Several studies have shown that 3-DE allows accurate calculation of LV volume and ejection fraction without geometric assumptions of its shape (see Table 1).12-15 An alternative method of calculating ventricular volumes from a realtime 3-D cardiac volume data set is using the disc summation method, which has been well validated in the past (see Figure 3). The acquisition can be combined with the infusion of a contrast agent, particularly in patients with a difficult acoustic window in whom it might be of benefit to improve the delineation of the endocardial contour.
Stress ECG
Stress ECG is a clinical procedure to detect regional wall dysfunction induced by ischaemia by comparing wall motion information in pre-stress and post-stress ultrasound images. Good endocardial border delineation is essential and has been improved using harmonic imaging and contrast agents, but the time-consuming serial acquisition of identical 2-D imaging during different stress levels remains a significant limitation of conventional stress echo. Complete LV acquisition by 3-DE ensures detection of all wall motion abnormalities. The possibility of scanning a full volume containing all LV wall segments in less than eight seconds during breath hold has made this new technique suitable for stress ECG.16,17 Sensitivity and specificity will further improve by increasing image resolution and the use of contrast agents. RT-3DE will become a major application for global ventricular and regional wall function analysis in stress ECG and for myocardial perfusion studies using echo contrast agents.
LV Dyssynchrony and Biventricular Pacing
ECG has an important role in the evaluation of patients with mechanical dyssynchrony before biventricular pacemaker implantation. Despite the promising results of cardiac resynchronisation therapy (CRT) on acute haemodynamic performance and long-term functional status, the selection of suitable patients is still ill defined. Since RT 3-DE allows rapid and accurate evaluation of LV volumes and their changes in these patients, it has great potential for assessment of LV dyssynchrony before and during CRT.18 RT 3-DE with appropriate software for segmental wall motion analysis allows the determination of dyssynchrony between all segments.19
Phase analysis of segmental volume-time curves based on 3-D data demonstrates changes of regional myocardial motion and LV contraction pattern in a quantitative way.20 Defining and expressing the delay remains an important issue and there is a need for standardisation.
Right Ventricle Analysis
The complex geometry of the right ventricle (RV) limits accurate assessment of right ventricular volume and function on conventional 2-DE.21 The asymmetric shape of the RV, the limited number of well-defined landmarks and the position in relation to the usual acoustic windows limit the possibility for conventional 2-D assessments. RT 3-DE permits depiction of both the complete tricuspid valve and the complex shape of the RV.22 Contrast enhancement is needed to improve the detection of the anterior RV wall since poor near-field resolution significantly interferes with endocardial visualisation in unenhanced 3-DE in many patients.23 Assessment of RV volumes and function based on 3-DE show improved agreements with magnetic resonance imaging (MRI) considered now as the reference standard compared with 2-DE;24-28 however, in daily practice 3-DE of the RV is limited by the sector angle. The asymmetric shape of the RV makes it difficult to obtain the entire RV, inflow and outflow tract of the RV, into one full volume data set.
Congenital Heart Disease
Conventional 2-DE is the most commonly used non-invasive diagnostic method for delineation of the presence and nature of congenital heart defects. Although the presence and anatomy of many congenital heart defects can be diagnosed readily by 2-DE, the anatomy is rarely displayed in views similar to those encountered during surgery. The 3-D relationship of the cardiac structures in these complex hearts is particularly hard to understand referring to conventional 2-D images.29 The clinician must mentally integrate the sequentially-obtained 2-D images to build up a 3-D perception of the morphology, which is difficult in the presence of complex congenital defects.30,31 The display of images in a format closely resembling 3-D reality clearly facilitates image interpretation and reduces interobserver variability of interpretation.
The development of new techniques of atrial septal defect closure, including minimal-access surgery and percutaneous catheter closure, has increased the need for accurate assessment of not only defect size but also defect morphology and its spatial relations.32 Studies in patients with atrioventricular septal defects have shown an incremental diagnostic value of 3-DE in the assessment of dynamic morphology of such defects and in the accurate description of the mechanism of atrioventricular (AV) valve insufficiency after defect repair (see Figure 4).33,34 These findings are clinically important despite the improvement in surgical repair techniques. Early mortality and re-operation after repair of complex congenital heart malformations could be further reduced by more detailed preoperative patho-morphological information.
Valve Morphology and Function
3-DE has already had an impact on both the diagnosis and treatment of mitral valve disorders.5 Dynamic 3-D reconstruction of mitral valve structure en face from either a left atrial or LV perspective, including detailed visualisation of the commissures, leaflet scallops and fibrous trigones, are helpful in a comprehensive analysis of the underlying valve anomaly and pre-operative assessment.
The quantitative analysis of 3-DE can be utilised for volume and area quantification of the valve morphology and function, particularly in mitral and aortic valve disease (see Figure 5).35,36 In the assessment of valve incompetence, 3-D reconstruction of colour Doppler flow jets adds information to that available from 3-D reconstructions of heart structures.37
Methods for quantification of 3-DE colour flow mapping have been described since the mid-1990s. The development of 3-DE greyscale volume-rendered reconstructions raised the question of the possibility of reconstructing and quantifying colour flow intracardiac jets and has recently been described; however, reliable quantification of 3-DE colour flow jets in vivo remains investigational.37-41
Intracardiac 3-DE
A catheter device with integrated ultrasound imaging array has been recently introduced. With 3-D reconstruction, intracardiac ECG can provide a valuable tool for online evaluation of the intracardiac anatomy, mainly for electrophysiological and catheter-based interventions.42 By using 3-DE the most relevant right atrial structures, interatrial septum and its relationship with adjacent structures are readily recognised.
In particular, the location of the foramen ovale or atrial septal defect is known to be unpredictable. Visualisation of the exact location and relation to the other cardiac landmarks therefore has advantages during device implantation procedures. Intracardiac ECG with 3-D reconstruction software is currently used in a limited number of centres; however, it requires additional training, experience and the extra cost of the technique is considerable.
Virtual Reality in Cardiac Ultrasound
3-DE offers great potential for teaching and training, aiding in complex diagnostic situations and assisting in the planning of surgical procedures. At present, 3-D reconstructions are often presented in 2-D format. These reconstructions can cause interpretation difficulties, mainly in understanding the orientation of the views. Virtual reality models assist in the interpretation of 3-D presentations of the heart and can be useful as a permanent reference environment for diagnosis and assistant in planning the surgical or catheter-interventional procedures.
Summary
RT 3-DE provides the clinician with additional knowledge of cardiac disease and adds insight to the understanding of complex pathology. The availability and versatility in use of a 3-DE 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/she 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 and allow addressing of new clinical questions that are uniquely 3-DE (see Table 2). Further developments and improvements for widespread routine applications include faster acquisition, processing and reconstruction, improved image quality and easier approaches to quantitative analysis, e.g., by using more intelligent automated border detection algorithms. The recent perspective is that RT 3-DE may eventually become the standard echocardiographic examination procedure.