Congenital heart disease affects approximately 0.8% of the population.1 It is also estimated that there are approximately one million adults in the US living with some form of congenital heart disease.2 In daily practice, echocardiography is the most widely used imaging modality for diagnosis and evaluation of these patients. As medical technology has developed, this modality has continuously evolved to provide better qualitative and quantitative assessment of cardiac anatomy and function.
One of the more recent developments in echocardiography is realtime three-dimensional echocardiography (3-DE). Recent technological advances, including the matrix array transducer and increased computer processing speed, have transformed 3-DE from a complex, time-consuming process into a practical bedside tool. 3-DE now provides the echocardiographer with a virtual window into the three-dimensional complexity of congenital heart disease. Using this unique and powerful tool, normal and pathological entities can be analyzed in completely new ways. One of the more important contributions of 3-DE is the ability to accurately and directly assess entire cardiac chambers. Using 3-DE, one can dynamically evaluate global and segmental left ventricular volume at the patient’s bedside, which has led to significant advances in our ability to objectively quantify left ventricular function.
Quantitative 3-DE—Acquiring the Data Set
Quantitative assessment of ventricular function plays a vital role in the management of patients with congenital heart disease. This has traditionally been accomplished with two-dimensional (2-D) echocardiography; however, 2-D quantitative methods are limited by their dependence on geometric assumptions of ventricular shape. 3-DE achieves freedom from geometric assumptions with 3-D volumetric data in the form of voxels. Voxels are the 3-D correlate of a 2-D pixel, and offer a more robust representation of ventricular volume. Earlier approaches to 3-D reconstruction interpolated 2-D pixel data to build 3-D images. In contrast, with 3-D voxels, there is no need for reconstructions or assumptions. In order to capture entire cardiac structures and chambers, full volume 3-DE (FV-3-DE) is used. FV-3-DE is a technology that rapidly acquires large pyramidal sections of 3-D data called FV-data sets. These FV-data sets are acquired as separate pieces over four to seven consecutive cardiac cycles. The information from each cycle is matched with the other cardiac cycles through ECG gating; the data sets are stitched together to create a single large pyramid-shaped wedge of 3-D data. Once acquired, these FV-data sets can be analyzed immediately at the bedside or stored for later analysis. With this technology, we now have the ability to rapidly acquire the left ventricle, through the entire cardiac cycle, in a single FV-data set. A wide variety of analyses can be performed on the FV-data sets with various software tools. These range from virtual dissection using cut-plane cropping, to complex quantitative analysis of myocardial wall deformation and dyssynchrony.
3-D Global and Segmental Ventricular Analysis
One important and useful analysis that can be performed on FV-data sets is left ventricular volume analysis (LVVA). There are multiple software platforms available for LVVA. Most of these involve the echocardiographer aligning the data set along its axes and then marking a series of anatomical landmarks. This process allows a semi-automated border detection algorithm to trace the LV endocardium throughout the cardiac cycle and create a dynamic cast of the LV chamber. Numerous studies have validated the diagnostic potential of 3-D LVVA and its superior accuracy to 2-D methods in adults and children.3,4 While LVVA has the potential to become part of the routine echocardiogram, the integration of any new technology into mainstream clinical medicine is, in large part, dependent on its reproducibility in the clinical setting and the resource utilization it requires. The authors, in their echocardiography lab, recently performed a study to evaluate the time resource utilization of this new modality using a commercially available software application in patients with congenital heart disease. This demonstrated that global LVVA could be completed in approximately two to three minutes by users with varying experience levels in a highly reproducible manner.5
In addition to global LVVA, a more sophisticated segmental analysis can be performed by dividing the left ventricle into 16 segments (American Society of Echocardiography model).6 The volume of each ventricular segment can be graphed separately as a function of time throughout the cardiac cycle (see Figure 1). Segmental ejection fractions can be calculated and compared, and regional differences in myocardial wall deformation can be illustrated. In addition, one can use the dispersion of volume change over time to evaluate dyssynchrony.
3-D Evaluation of Dyssynchrony
The relationship between left ventricular dyssynchrony and heart failure has been well demonstrated in recent medical literature.7 Dyssynchrony contributes to heart failure by causing blood to undulate between early-and late-contracting regions of the left ventricle rather than contributing to cardiac output, which decreases ventricular efficiency and performance. Cardiac resynchronization therapy targets ventricular dyssynchrony and has been shown to improve symptoms and quality of life.8 A recent multicenter study found that congenital heart disease patients undergoing resynchronization exhibited a significant increase in mean ejection fraction; however, long-term results regarding percentage of responders and degree of benefit are not yet available.9
Echocardiography has emerged as the modality of choice for assessment of dyssynchrony. Using FV-3-DE data sets, dyssynchrony can be assessed at the bedside with sophisticated 3-D software, which allows for temporal analysis of dispersion in segmental ventricular volumes. The process involves calculating the time from end-diastole to the minimal systolic volume for each ventricular segment. The dispersion of these times can be used to create an index of dyssynchrony.10 A recent study found that adult patients with dilated cardiomyopathy had significantly higher 3-DE indices of dyssynchrony compared with healthy controls and that 3-DE dyssynchrony indices had a strong negative correlation with ejection fraction.11
3-D Color Doppler
Another promising aspect of 3-DE is 3-D color Doppler. As with 3-DE imaging of cardiac structures, the ability to acquire 3-D color Doppler information has the potential to significantly enhance quantitative assessment of blood flow through cardiac structures. Jets of valvular insufficiency can be captured in their entirety, rather than in sequential 2-D sweeps. Their direction, orifice size, and extension can be easily appreciated. Moreover, the regurgitant orifice area can be measured directly and the flow through the orifice can be calculated, providing the regurgitant volume.12 With this same technology, the flow through the aortic or pulmonary valve can be measured providing a direct, non-invasive measurement of cardiac output.13 A portable, rapid, non-invasive method for quantifying cardiac output could have a profound effect on the care of critically ill patients and improve our ability to follow patients with cardiac failure.
There is no doubt that 3-DE has become an important part of the echocardiographer’s imaging armamentarium and is quickly finding mainstream clinical acceptance. It is likely that the rate of technological advancement will play an important role in determining the timeline over which 3-DE is completely integrated into daily practice. Current quantitative chamber analysis is somewhat limited for abnormal ventricular morphology, which is not uncommon in congenital heart disease. This represents a hurdle for widespread application of quantitative 3-DE to our patient population.
The advent of a reasonably sized, affordable holographic display will also be an exciting addition to 3-DE technology. This would allow the cardiac surgeon and invasive cardiologist to navigate the heart using a realtime 3-D holographic display. In the catheterization laboratory, this could lead to a significant decrease in fluoroscopy use and improved visualization of device implantation. There is also the potential to guide intracardiac surgery and lessen the need for cardiopulmonary bypass. These and other exciting developments will allow us to fully exploit the power of this unique modality and improve patient care.