Complex intra-cardiac anatomy and spatial relationships are inherent to congenital heart defects (CHD). Until recently, our ability to image the heart by echocardiography has been limited to two-dimensional (2-D) techniques. Advances in transducer technology, beamforming, and computer processing power and speed have led to significant improvements in spatial and temporal resolution using 2-D echocardiography (2DE). However, 2DE has fundamental limitations. The nature of a 2-D slice, which has no thickness, necessitates the use of multiple orthogonal 'sweeps'. The echocardiographer then mentally reconstructs the anatomy, and uses the structure of the report to express this vision. This means that the only 3-D image of the heart is the 'virtual image' seen by the echocardiographer alone. It is not easy for an untrained, nevertheless interested, observer to understand the images obtained in the course of a sweep: expert translation is required. As a corollary to this, 2DE techniques do not lend themselves to quantitation of cardiac structures of irregular shapes; it is impossible to quantitate a virtual image.
Recognition of these limitations of 2DE led to burgeoning research and clinical interest in the modality of 3-D echocardiography (3DE). Early reconstructive approaches were based on 2DE acquisitions and subsequent stacking of 2DE images to recreate a 3-D dataset. However, the need for offline processing imposed fundamental limitations on the practicality of these approaches. More recently, the focus has shifted toward the acquisition of a 3-D 'wedge' or trapezoid in realtime. New transducer technology has enabled this, evolving from the familiar phased array of 2DE (with 128 elements) to the matrix array that is designed for 3DE (with 2,500-3,000 elements). 3DE matrix array transducers have been commercially available since the spring of 2003. Given the early phase of technology development at the time, it is not surprising that these transducers were heavy, with a large footprint, low frame rates, and limited features. The inherently low frequency of these transducers translated into poor spatial resolution. As a result, the pediatric community continued to view 3DE with skepticism. The gap between existing imaging technologies and the ideal imaging modality has remained large. But what is ideal?
The Ideal Modality
The ideal modality for non-invasive imaging of the heart would have the following characteristics:
For the echocardiographer:
- the modality should be able to image the entire region of interest in three dimensions in realtime;
- the modality should be integrated into current imaging equipment;
- the modality should be portable and easily performed and repeated;
- spatial and temporal resolution should be of diagnostic quality;
- it should be adaptable to patients of varying sizes and heart rates;
- the modality should overcome the limitations imposed by the 2-D display; and
- images should be easily viewed, manipulated, and quantified at any time, and should be accessible from any PC.
For the hospital:
- the modality should be financially viable;
- it should lead to improvements in work flow; and
- it should be integrated into existing image management systems.
For the operator:
- the modality should have a realistic learning curve;
- it should be fun to use and easy to interpret; and
- the ergonomics of the modality should be reasonable.
For the surgeon:
- images should be amenable to post-processing. This provides the potential for viewing perspectives that may be critical for surgery, but not necessarily obvious to others.
For the patient:
- the modality should provide valuable additional information;
- it should provide the potential for making a real difference to the patient's care and outcome; and
- it should be quick, risk-free and easy.
For the researcher:
- the researcher should be able to rely on the industry's commitment (in its on-going research and development) to the new technology; and
- the modality should yield new information that is important enough to make the researcher's proposals competitive for grant funding.
While the ideal modality as detailed above does not exist currently, recent advances hold great promise for the future. This review focuses on the new pediatric matrix transthoracic X7-2 3DE transducer (Philips Medical Systems, Andover, MA), which our laboratory has helped to develop, optimize, and validate.
Advances in Computer Technology
In 1965, Gordon Moore suggested that the number of transistors per integrated circuit would grow exponentially, doubling every couple of years. In the subsequent 40 years, the number of components actually doubled every 18 months or so—and thus the power of central processing unit (CPU) chips also doubled at that rate. Concomitantly, other components of computers including switching, buses, memory, and disk density have all increased exponentially in both capacity and speed. As a result, the availability of high-powered, miniaturized computing power has increased dramatically. This has been an important contributor to the advances in pediatric echocardiographic imaging.
Advances Specific to the X7-2 Transducer
The piezoelectric material in an ultrasound transducer is a fundamental determinant of system image quality. Piezoelectric transducer elements are responsible for delivery of ultrasound energy into the scanned tissue and for converting returning ultrasound echoes into electric signals. Their coupling efficiency in converting electrical energy to mechanical energy or vice versa is a key determinant of image quality, Doppler sensitivity and penetration. To create an overall piezoelectric effect, these elements must be subject to the application of an external electric field to align dipoles within polycrystalline materials. For almost 40 years, a ceramic polycrystalline material, PZT (lead-zirconate-titanate) or PZT composites, has been the standard piezoelectric material used in medical imaging. This material is a uniform powder that is mixed with an organic binder and baked into a dense polycrystalline structure. At its best, it achieves ~70% alignment of dipoles due to imperfect alignment of the individual dipoles. This leads to a corresponding constraint in the electromechanical coupling efficiency of the material.
The X7-2 transducer utilizes PureWave crystal technology, which represents a completely new type of piezoelectric material. This technology involves growing crystals from a molten ceramic material, resulting in a homogenous crystal with fewer defects, lower losses, and no grain boundaries. When these crystals are poled at the preferred orientation(s), near perfect alignment of dipoles (~100%) can be achieved, resulting in dramatically enhanced electromechanical properties. The efficiency of converting electric-to-mechanical energy improves by as much as 68-85% when compared with PZT ceramics currently used in ultrasound transducers. These efficiencies translate into the ability to increase miniaturization, as exemplified by the X7-2 transducer, which contains 2,500 active elements.
3DE in CHD—Our Laboratory's Role
Over the past four years, our laboratory has been exploring the applicability of 3DE techniques to CHD. Our prior work on 3DE, albeit using earlier-generation, low-frequency transducers, has shown the promise of this modality. We initially performed descriptive studies that evaluated the role of 3DE in patients with atrioventricular septal defects and aortic arch anomalies and in guiding endomyocardial biopsies in children. More recently, we have critically evaluated tools that are available for 3DE quantification of (LV) volumes and ejection fraction. We have demonstrated that these tools provide the ability to rapidly quantitate LV function in a reproducible manner, and that there is a negotiable learning curve with these tools. However, the transducers that have been available to date have had limited acceptance in pediatrics due to their weight, large footprint, low imaging frequency, and low frame rates. Since 2005, we have been involved in the development, optimization, evaluation, and validation of the X7-2 MHz miniaturized matrix 3DE transducer.This transducer has been optimized for pediatric applications in terms of size, footprint, imaging frequency, near-field resolution, and frame rates.
Imaging with the X7-2 Transducer
The X7-2 transducer weighs 65g (approximately 2oz) and has a footprint that measures 1.7x1.3cm.We have been able to obtain high-quality 3-D images on patients ranging from neonates to small adults (see Figure 1). Image resolution in the near field has been outstanding. The quality of 3-D color flow imaging, presented with the black and white image suppressed so as to present an 'echocardiographic angiogram', has been excellent (see Figure 2). Frame rates for live 3DE have ranged from 24 to 46 frames per second, and frame rates for full volume 3DE have ranged from 23 to 83 frames per second. Frames rates for 3DE color flow have ranged from 19 to 33 frames per second. We have found 3DE to be particularly useful in evaluating complex intra-cardiac anatomy to guide surgical options. The X7-2 transducer has also provided excellent quality 2DE images, including gray-scale, color flow, and pulsed wave Doppler.
Future Directions
The X7-2 transducer provides pediatric echocardiographers with a new array of imaging capabilities. Advances in software tools must keep pace with these advances. Existing tools allow easy quantification of LV systolic function. However, now that we have access to a digital dataset that contains the right ventricle,we would like to similarly quantify RV volume and systolic function. Similarly, the ability to capture a 3-D dataset that includes all of the color flow across a valve leads to questions regarding reliable quantification of flows. The continuing evolution of such techniques will probably result in a paradigm shift in our ability to obtain quantitative information on our patients.
Challenges
The introduction of new technologies can be intimidating for the busy echocardiographer and laboratory. New techniques must be evaluated, old paradigms re-examined, and inertia overcome. Over the short term, the path of least resistance is that of the skeptic. What happens over the long term? Our community has now been empowered with a revolutionary transducer optimized for pediatric 3-D applications (interestingly, there is not yet a 3-D PureWave transducer optimized for imaging adults). Will we adopt it?
Conclusions
The new 2-7MHz 3DE transducer provided excellent image quality, and high temporal and spatial resolution in patients spanning a wide range of diagnoses and body sizes.The new availability of high frame rates should potentially enable the wider application of 3DE quantitative methods to infants and children.