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Abstract



Using echocardiography, left ventricular function was evaluated in accordance with the diverse ultrasound methods. M-Mode and two-dimensional methods have some limitations due to the geometric assumptions to calculate the different parameters, which may cause important errors. 3-D imaging can be used for direct calculation of intracavitary volumes and global/regional ejection fraction. In the present review, the first and second scanner’s generation of 3D echocardiography are illustrated. The image acquisition and reconstruction were exposed and the advantages and limitations of this technique are also reported. “Live” 3D echocardiography, which directly and more rapidly provides a free quantification of global and regional LV function, appears to be superior to other versions of real-time 3D imaging. Finally, rapid three-dimensional echocardiography allows the immediate collection of data within a few seconds, making this technique feasible in most clinical scenarios. Heart Views 2008; 9(2) 71-79. © Gulf Heart Association 2008

Key words:¨ Left ventricular function ¨ ejection fraction ¨ conventional echocardiography ¨ RT3D ¨ live 3D ¨ rapid 3D echocardiography.

Introduction


      Over the last decades, Echocardiography has evolved from single-beam imaging to 3-D techniques that enables us to study cardiac structures, function and hemodynamics in detail1. At present, echocardiography is the most commonly used tool to evaluate left ventricular function. Today, two-dimensional echocardiography is the most prevalently used mode among all ultrasound methods to define the structural and functional cardiac status. But this mode has some limitations dependent on geometric assumptions that may introduce important errors. In addition, in 2D echocardiography, an intraobserver variability exists because individual observers interpolate the data in different ways.

   In the early 1990s, von Ramm and coworkers developed the first 3-D echocardiographic scanner2 to acquire multiple slices of the left ventricle. This generation of scanners however offered limited image quality and data processing was slow. The second generation scanners and transducers strongly reduced these limitations and offered an improved image quality and better spatial resolution. These transducers are capable of near real-time acquisition of lager pyramidal 3D datasets from an apical acoustic window. As a result, the measurement of cardiac function was improved because they provide more precise and rapid identification of cardiac abnormalities and ventricular function3,4. The potential applications of 3D-echo can be categorized into some major areas:

    ¨ Congenital heart disease;
    ¨ anatomy and pathology of the heart and the great vessels;
    ¨ Mitral valve disease/repair and aortic dissection;
    ¨ Global and regional left ventricular volumes, function and mass;
    ¨ Visualization of complex anatomic features;
    ¨ Catheter visualization.

In the present review, we describe the validity of 3-D echocardiography in comparison to other conventional echocardiographic methods and to define left ventricular function through global and regional definition of ejection fraction.

Measurement of ejection fraction


      With M-Mode echocardiography, ejection fraction is estimated as a percentage derived from the mid-left ventricular diameters measured in end-diastole and end-systole and is expressed as fractional shortening (% LV shortening) (Fig.1).

% Fractional shortening = LVEDD - LVESD X 100
LVEDD

Fig.1: M-Mode echocardiography of the left ventricle showing end-systolic and end-diastolic diameters from which LV fractional shortening is evaluated.

Measurements of left ventricular diameters are also obtained with 2-D echocardiography. In spite of some limitations due to the cardiac morphology, this technique considerably improved the accuracy of left ventricular volume measurement. Of the different mathematical models, modified biplane Simpson’s (based on disc summation) provided more accurate data (Fig. 2).

Fig.2: Two-dimensional echocardiography of left ventricle recorded from the apical four-chamber approach. Measurement of left ventricular volumes in diastole and systole from which Simpson’s biplane disc method (to obtain E. F.) is shown.

The calculation of each disc is automatically performed by an ultrasound machine software and biplane data acquisition is obtained in apical four and two chambers views and is averaged. Simpson’s biplane method underestimates left ventricular volumes when compared with MRI and radionuclide ventriculography which are considered as the gold standard5. To obtain E. F.% with this method, the systolic ventricular volume (in ml) is subtracted from the diastolic ventricular volume (in ml). The difference is divided by the diastolic ventricular volume and the result is %, in accordance with the formula :

EF% = diastolic ventricular volume - systolic ventricular volume %
diastolic ventricular volume

Another 2-dimensional method employed for the calculation of ventricular volumes in diastole and systole and E. F. is the area-length method. Measurements of LV can be performed from the apical two and four chamber views (Fig.3).

Fig.3: Evaluation of left ventricular volumes by the apical 4-chamber window and ejection fraction measurement performed using the area-length method.

   The volumes are derived from the areas (measured in diastole and systole) of LV squared, divided by length and multiplied by 0.85 (automatically calculated with a software dedicated). This method is appropriate in presence of symmetrical LV cavities alone.

    Three-dimensional echocardiography has been shown to be a more accurate assessment of ventricular volume and ejection fraction than its 2D echocardiography counterpart. This has been shown in multiple studies in comparisons to the left ventricular angiography and MRI6-12. Most approaches towards 3D echocardiography were based on the sequential rotational scanning and acquisition of multiple cross-sectional images. To minimize reconstruction artifacts, images are gated to both electrocardiography and respiration. The quality of 3D reconstructions from 2D depends on numerous factors, such as the quality of ultrasound images, their number and the ability to limit motion artifact.

   Once the 2D images have been obtained, they are processed with an available software. Fundamental steps in the performance of 3D echocardiography include image acquisition, image processing and analysis, reconstruction, digital storage and archiving. Acquisition of a complete data set typically depends on respiration and heart rates. In addition, the quality of 3D reconstruction from 2D images depends on numer of factors, including the quality of the ultrasound images, the number of the 2D images used to reconstruct the 3D image, the ability to limit motion artifact, and adeguate ECG and respiratory gating. Usually, a variable number from 4 to 6 serial images is adeguate for volume reconstruction of the left ventricle. Once the 2D images have been obtained, they are processed offline with commercially available software.

   The earliest devices employed have been developed by Von Ramm at Duke University2. This system (Volumetric Medical Imaging) makes use of a sparse-array matrix transducer consisting of 256 elements to scan 60° x 60° pyramidal tissue volume using parallel processing technology. Left ventricular volumes are calculated with a dedicated analytic software from either a series of parallel-C scans (short-axis view) or a series of rotated apical long-axis views. The main obstacles of this 3D include difficult image acquisition, limited image quality and laborious data manual analysis. Images are recorded during end-expiratory apnea and the recording is completed in less than five minutes.

   RT3D system is a real-time 3D ultrasound that instantaneously acquires the image container in a pyramidal volume. It uses a massive matrix array transducer with more than 3000 elements compared with the 256 elements present in the sparse array transducer. To minimize reconstruction artifacts, data should be acquired during suspended respiration. Images of the ventricles are obtained from various orthogonal planes: the sagittal plane which corresponds to a vertical long-axis view of the heart; the coronal plane which corresponds to a 4-chamber view and the transverse plane which corresponds to a short axis plane (Fig. 4).

Fig.4: Left ventricular volumes acquired respectively from the coronal plane, sagittal plane and transverse plane, and the left ventricular volume in 3D image seen according to a Cartesian coordinate system.

RT3D system generally has 3 acquisitions: real time (narrow), zoom (magnified), and wide-angle. The real-time mode displays a pyramidal data set 50° x 30°. The zoom mode displays a pyramidal data set of 30° x 30°. The wide-angle mode provides a pyramidal data 90° x 90°, which allows inclusion af a lager cardiac volume. The choice of narrow-angle or wide-angle imaging acquisition modes depends on the cardiac structure to be examined. For imaging of the ventricles, it is best to use a wide-angle acquisition in the apical window (4-chamber) so as to include the entire ventricle obtained during 4 consecutive heart beats. Ejection fraction obtained is referred to entire ventricle (Fig. 5).

Fig.5: Global ejection fraction of the left ventricle (upper) and report of its global function (lower).

To better define the ventricular volumes, the delineation of endocardial border is requested. Manual endocardial tracing is both laborious and prone for subjective errors. Development of various automatic or semiautomatic border detection algorithms should be able to avoid the need of manual border tracing and facilitate volume measurement.

Once a 3 data set is acquired, if the small cardiac structures are to be evaluated, the data must be sliced to visualize them. Instead, ventricular volumes can be calculated with the centroid-algorithm when the LV volumes are appropriately aligned. LV volumes can be segmented, which allows for regional LV function assessment (Fig.6).

Fig.6: Regional ejection fraction of the left ventricle referred to 17 zones evaluated.

To obtain the regional LVEF, volumes obtained can be segmented by dividing the LV into 16 segments13 but into seventeen zones. In fact, 3D echocardiography also evaluates the LV apex.
In addition, the function of any ventricular wall can be objectively assessed by measuring a variety of wall motion parameters14 (Fig. 7).

Fig.7: Timing and excursion parametric imaging report.

   In clinical scenarios, the commonly measured parameters of regional function are wall thickening15,16 and wall motion17,18. Wall-thickening analysis measures absolute wall thickening and fractional wall thickening. Wall motion analysis measures the displacement of the left ventricular endocardial wall at two instant in the cardiac cycle. Real-time 3D imaging has also been used during dobutamine stress testing and found to be feasible and useful for the detection of stress-induced wall motion abnormalities19.

     Recently, a more advanced version of real-time 3D imaging was introduced by Philips Medical System and is called “live 3D”. The major advantage of this latest version of real-time 3D imaging is the improved image quality due to a fully sampled or dense array configuration of the transducer. This dense array transducer, called as “matrix array” consists of 3000 elements. The elements are divided into subgroups and each subgroup is connected to one of the 128 channels of the ultrasound system. Live 3D imaging is based on the premise that a 3D image can be created using ultrasound beam through a 3D volume. A semiautomated detection of LV borders from 3D images is then applied to perform a quantitative assessment of left ventricular volume and ejection fraction with accurate results and then reconstruct them into an entire cardiac volume. Each cardiac cycle provides one fourth of the volume data set.

    Using live 3D echocardiography, significant advantages were obtained in patients who have heart failure despite optimal medical therapy. In these, delayed activation of LV segments from conduction delays leads to ventricular dyssynchrony which compromises ventricular function. Resynchronization can be obtained by simutaneously pacing both ventricles. This, in turn, leads to improved functional class, exercise tolerance, quality of life, and LV function. In this arena, live 3D Echo has been used by some investigators to measure the dyssynchronous motion of the heart (Fig. 8).

Fig. 8: Regional ejection fractions respectively recorded in external zones, mid zones and internal zones in a patient with dilated cardiomyopathy.

    These indexes will be useful not only in determining which patients will benefit from the resynchronization cardiac therapy, but to guide physicians in finding the optimal position of the pacing lead20.

    Others have proposed a rapid (6 seconds) acquisition technique that collects apical tomograms using an internal continuously rotating transthoracic transducer21. This approach represents a solution to a clinically feasible acquisition of 3D data and provide precise and accurate diastolic and systolic volumes for functional assessment of the left ventricle.

    In summary, 3D echocardiography is significantly better than 2D echocardiography in defining ventricular volumes and ejection fraction and there are some studies which show very good correlation between MRI and live 3D echocardiography22-24. Nevertheless, further technological improvements and additional clinical studies will broaden the list of appropriate applications for this exciting new ultrasound modality.¨

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THE WASP AND THE PRINCE

A wasp named Pin Tail was long in quest of some deed that would make him forever famous. So one day he entered the king's palace and stung the little prince, who was in bed. The prince awoke with loud cries. The king and his courtiers rushed in to see what had happened. The prince was yelling as the wasp stung him again and again. The courtiers tried to catch the wasp, and each in turn was stung. The whole royal household rushed in, the news soon spread, and people flocked to the palace. The city was in an uproar, all business suspended. Said the wasp to itself, before it expired from its efforts, "A name without fame is like fire without flame. There is nothing like attracting notice at any cost."
INDIAN FABLE