*These invasive imaging procedures will not be discussed

systems provide a high imaging performance with an increasing number of modalities allowing a comprehensive assessment of cardiac structure function and hemodynamics with unprecedented versatility (table 1).
The method is now being used to image the heart at any age from the fetus to the very old and in any situation from mobile services to intraoperative. A definitive diagnosis and quantitative assessment of most cardiac conditions can now be made and minimizing the need for further work-up by invasive testing in many patients. Consequently, cardiac ultrasound has revolutionized the practice of cardiology.
Recently, microprocessor techniques and miniaturization have led to revolutionary advances in cardiac ultrasound which will have a further impact on both its diagnostic potential and its use in clinical practice. Portable lightweight battery-powered systems represent the one end of the spectrum of these developments and these devices can be used just like a standard stethoscope as part of the physical examination at the place-of-care. Real-time three-dimensional echocardiography represents the other end and allows capturing a series of three-dimensional data sets within one cardiac beat.
In this paper we will briefly review the innovative revolutionary developments and their future impact on clinical cardiology.

Multidimensional anatomy is mentally reassembled from sequential tomographic views. This is a difficult process for complex cardiac structures and pathology of unknown morphology. Computer technology has resulted in presenting three-dimensional depictions of cardiac anatomy as well as physiology by reorganisation of multiple spatially related cross-sectional images into a volumetric dataset. Over the years, several directions have been followed in three-dimensional echocardiography.1

Undoubtedly, the most challenging is real-time three-dimensional echocardiography.This method, developed by Von Ramm et al of Duke University, is based on novel matrix phased-array transducer technology and using parallel processing techniques allows to scan a pyramidal volume at once. Real-time three-dimensional echocardiography will find a major application for global ventricular and regional wall function in stress echocardiography and for myocardial perfusion studies using echo contrast agents. With improving image quality, real-time three-dimensional echocardiography will eventually become the standard echocardiographic examination procedure.

At present, most experience with three-dimensional echocardiography is with software controlled mechanical acquisition of a consecutive series of cardiac cross-sectional images using currently available standard transducers and computer assisted “off-line” three-dimensional reconstruction. This approach necessitates the simultaneous registration of the accurate spatial position and timing of the cross-sectional images. Positional information can be obtained with acoustic (spark gap) or magnetic location systems allowing unrestricted (free-hand) scanning from any available precordial acoustic window. Surface rendered or wire-frame reconstructions of selected structures are generated from manually or automatically derived contours in the cross-sectional images. This approach allows improved quantification of left ventricular volumes and mass but provides limited structure information.

Scanning techniques using a predetermined geometric acquisition pattern (linear, fan-like and rotational which is the most commonly used) allow for the recording of closely and evenly spaced cross-sectional images. The rotational acquisition has practical advantages and is performed with either multiplane transesophageal probes or precordial probe assemblies accommodating standard transducers interfaced to standard echocardiographic equipment.3,4 Image acquisition is controlled by a software-based steering logic which considers both cardiac and respiratory cycle variation. Volume rendering algorithms are applied and provide grey-scale tissue information in the reconstructions representing a significant advance over surface rendered reconstructions.

Recently, an ultrafast continuously rotating phased-array transducer which allows the acquisition of 16 volumetric data sets per second has been developed at the Thoraxcentre.5 Standard imaging processing equipment is used for reconstruction and analysis. This “near real-time” or ultrafast approach may become an alternative to real-time volumetric imaging systems in specific clinical conditions and for global and regional myocardial function analysis.

Figure 1

Figure 1 . Three-dimensional images of aortic valves viewed from above. The upper panels show an electronically extracted image of a normal valve in systole and diastole. The tricuspid valve and the coaptation lines are clearly seen in the diastolic image. The middle panels show a stenotic aortic valve. The valve cusps are thickened and there is incomplete opening with a small stenotic orifice in systole. The lower panels show three-dimensional volume rendered images of a bicuspid aortic valve (surgical view). The mouth-like orifice can be appreciated in a single projection and the raphe is seen both in systole and diastole (arrow).

With currently available technology, details of cardiac anatomy and pathology are already well appreciated. “En face” views of cardiac structures are unique for three-dimensional echocardiography. They have been proven valuable in valvular 6,7 and congenital heart disease8-10 by providing a dynamic view of pathology and better insights into what will be found during the surgical procedure (figures 1-2). This information is already of particular help in reconstructive surgery of mitral valve, for repair of congenital heart defects and device closure of an atrial septal defect11 (table 2).

Table 3: Three-dimensional echocardiography

The ultrasound stethoscope

Table 4: THE ULTRASOUND STETHOSCOPE

Miniaturisation and digital techniques have resulted in the development of high resolution battery-powered personal imaging devices with excellent grey-scale and colour blood flow imaging capabilities (figure 5). These devices are practical to use and allows visualization of the heart and its pathology during the physical examination and to address specific clinical problems anywhere at the point of care.19,20

Figure 4

Figure 4. Left ventricular volume measurement using a three-dimensional dataset from which a series of eight long-axis views are used. The left panel is a computer-generated short-axis view and serves as a reference view. The surface area of each cross-section is measured and the volume calculated.

They are approximately named “ultrasound stethoscopes” since they allow to look inside the chest (stethos = chest and skopein = see). These small personal ultrasound devices should not be confused with the portable desk-top systems which are full featured systems. The ultrasound stethoscope extends the perception of a physical examination by direct “visualising the invisible pathology” and provides information beyond what we can perceive with palpation and auscultation. Murmurs and abnormal precordial movements can be directly related to cardiac structural, functional and flow abnormalities (figures 6 and 7). A cardiac abnormality (pericardial effusion, dilated heart, valvular disease, mass lesion) is rapidly confirmed during a routine physical examination (table 4). Often a specific diagnosis is made and incidental and unexpected findings are regularly recognised.

Table 4: THE ULTRASOUND STETHOSCOPE

A major strength is that a limited echo/color Doppler examination may allow exclusion of cardiac abnormality with great certainty after limited training. Overall, the use of these devices will strengthen our physical diagnostic accuracy and will add quantitative information.

Figure 6

Figure 6. Apical four chamber view of a 25-years-old-female with systemic lupus erythematosus and shortness of breath. The referral diagnosis was: pericarditis? The patient has regurgitant jets of aortic regurgitant (A) and mitral regurgitation (B), but no pericarditis (OptiGoTM).

Figure 7

Figure 7. Apical four chamber view of a 45-years-old male with dilated cardiomyopathy. (A) A mitral regurgitant jet is visualised (SonoHeartTM). (B) The imaging quality of the hand-held device can be appreciated against that of a standard echocardiographic system (HP, Sonos 5000TM).

A major strength is that a limited echo/color Doppler examination may allow exclusion of cardiac abnormality with great certainty after limited training. Overall, the use of these devices will strengthen our physical diagnostic accuracy and will add quantitative information.

The diagnosis and follow-up of many cardiac conditions requires only a fraction of the potential of the high-end ultrasound systems and a specific clinical question can often be answered within little time and with little examination protocols. The ultrasound stethoscope is suitable for such a limited “goal-oriented examination” (e.g. resolution of pericardial effusion after pericardicentesis, left ventricular function, left ventricular hypertrophy). These devices can effectively assist in the initial evaluation and rapid diagnosis of potentially life threatening conditions or in situations where quick decision-making is essential (emergent tamponade, low output states, acute valvular pathology, right ventricular involvement and mechanical complications of acute myocardial infarction).21,22

Better indications and more targeted referral for expensive imaging technologies may lead to significant cost savings. Detection or exclusion of regional wall motion abnormalities is a potential that can be utilised in patients with acute chest pain and a non-diagnostic electrocardiogram. Right ventricular involvement in acute myocardial infarction and the mechanical complications are readily diagnosed in the intensive care unit.

The ultrasound stethoscope allows rapid screening for a dilated aorta or occult aortic abdominal aneurysm in patients at risk,23,24 for left ventricular hypertrophy in patients with hypertension,25 early heart failure (asymptomatic LV dysfunction, less than 50% collapse of the inferior vena cava) (figure 8),26 mitral valve prolapse 27 and dilated aorta (Marfan’s disease).

Obviously, a small ultrasound imager cannot substitute for the high-end ultrasound systems but there is no doubt, however, that these devices will revolutionize the physical cardiac examination (table 5). Their use involves compromises some of which are still unknown and will be learned when applications are expanding. Training may become an important issue and should focus on criteria of normalcy and the identification of specific and major cardiac disorders. In the future, advances in communications and software will allow for diagnostic support from experienced laboratories and help to solve training issues.

CONCLUSION

Table 5: The ultrasound stethoscope

Three-dimensional echocardiography provides the clinician with more confidence for the diagnosis of cardiac disease and adds insights to the understanding of complex pathology. The availability of a three-dimensional data-set allowsthe cardiologist to retrieve an infinite number of different views after the examination procedure providing accurate quantitative data together with new functional parameters. Additional information will allow to address new clinical questions. For these reasons, three-dimensional echocardiography will become an essential part of the practice of cardiology. 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 other imaging methods.

Figure 8

Figure 8. Imaging of the inferior vena cava (IVC) through the liver during expiration (A) and inspiration (B). The caliper function allows measurement of the IVC dimension during expiration (2.6 cm) and during inspiration (1.9 cm). A collapse of less than 50% indicates an elevated right-sided filling pressure37 (OptiGoTM).

Table 5: The ultrasound stethoscope

References