Intraoperative 3-D Echocardiography

Published on 06/02/2015 by admin

Filed under Anesthesiology

Last modified 22/04/2025

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Intraoperative 3-D Echocardiography

Gian Paparcuri

Transesophageal echocardiography is probably the most frequently used imaging technique in cardiac surgery. This powerful technology provides timely information, about cardiac structures (anatomy) and function (hemodynamics) without disrupting the surgical workflow.

Developed decades ago, far from being a dead technology, echocardiography is a constantly evolving technique, requiring new skills and expertise. Over the years echocardiography has evolved from 2-D into 3-D. Just like the movies. Some authors say that “3-D represents the natural evolution of 2-D echocardiography”. But why do we need 3-D echocardiography?

The answer is simple:

3-D Echo:

With 3-D echocardiography, people who aren’t echocardiographers can appreciate valve anatomy and physiology in three dimensions. Surgeons tend to very much appreciate 3-D images because the echo image looks exactly like what they see when the heart is open. Finally, for those of you taking the well-known echocardiography course in San Diego, expect to see a lot of 3-D. It seems like the speakers are not allowed to talk about echocardiography if they don’t complement their presentation by adding some 3-D images.

Believe it or not, there are 3-D detractors out there. People are afraid of change. But it is human to feel anxious when a new technology is trying to be introduced. It is easier to welcome this new technology when limitations of existing technology are appreciated.

2-D echocardiography is the representation of a 3-D structure (the heart) in 2 planes. The echocardiographer has to mentally visualize (cognitive 3-D reconstruction) a complex 3-D structure based on a series of 2-D views in order to obtain a 3-D object (the heart). Another limitation is that 2-D echocardiography is based on a lot of assumptions. We assume specific geometric shapes of cardiac structures for ejection fraction quantification every day. Even worse, we assume the whole ventricle behaves like the one displayed on the screen. Many times, these assumptions fail because of the presence of RWMA.

To be honest, 3-D echocardiography is nothing new under the sun—it’s been here for more than 15 years. But because of early limitations it was not universally applied. In its beginnings 3-D echocardiography was a tedious process: slow acquisition process, images acquired in 2-D had to be reconstructed in 3-D using a program. It used to take a long time, simply because it was not really 3-D. Displaying 3-D images requires a huge amount of data to process. More than 60 heart beats—images, acquired over several hundred consecutive beats had to be gathered with the ECG or respiratory rate for subsequent volume rendering and cropping. And after this time-consuming process the computer was able to reconstruct (offline) a 3-D image which very often was of poor quality with frequent artifacts. In its early years 3-D echocardiography was very impractical in the operative theater. New technology overcomes some of these limitations.

With real time 3-D gone is the need to sum information for 61 beats. What is new about it these days is that instead of images that are acquired in 2-D and then reconstructed into a 3-D image using software, the images can be acquired in 3-D volumes and displayed in real time. The new technology uses a special scanner (probe). The scanner used a unique matrix array to scan a volume without physically moving the transducer. The transducer gathers information from a single beat, resulting in a very low-latency display.

TEE Probes

Non-3-D echocardiography probes use a limited number (64–128) of piezoelectric elements (crystals) to scan tissue. Sequential (phased) activation of individual crystals generates an ultrasound beam that is steered back and forth over a 90° angle to sweep a flat, “pie-shaped” scanning plane or sector. 3-D echocardiographic probes have on their tips more than 2500 active elements, incorporated onto the probe, conforming a rectangular grid of 50 rows and 50 columns for a total of 2500 independent piezoelectric crystals, generating a matrix array.

Individual piezoelectric crystals are activated and generate a US beam that can be steered in the azimuthal (x-y) and the elevational plane (x-z) over 90° to cover a pyramidal scanning volume.

How to create a 3-D display? How does it work? It’s a simple 4-step process. 3-D echocardiography is based on volumetric techniques. Ultrasonic data about a volume of tissue are obtained with the probe scanner. This is the first step (data acquisition). The fancy scanner with thousands of crystals (matrix array probe) is able to obtain all the information required from the tissue in order to create this pyramid-shaped volume. All in real time, in just one beat. Simpler and faster.

Step 2 involves temporary data storage within the computer random access memory (RAM) of the US machine.

Step 3 is data processing, requiring the transformation of the scanned raw data (volume of tissue) into a code (3-D dataset) necessary to generate 3-D objects on the display. This is actually 2 sequential, integrated processes: conversion and interpolation.

Volume scanning generates a data stream using the computer RAM and creates voxels while scanning, with near simultaneous conversion and interpolation. The matrix array probe scans over the elevational axis resulting in a pyramid-shaped volume with a curved base.

The surfaces and volumes are then rendered for display. Objects and their relative positions in this 3-D space or coordinate system can be quantified.

Finally, step 4 or 3-D image display: makes 3-D dataset visible.

3-D graphic reproduction is the product of a computer graphics rendering technique of the 3-D echo dataset (2 steps):

image 1st step is segmentation: separates the object to be rendered from surrounding structures (differentiates between cardiac tissue, blood, pericardial fluid, blood, air; given their diverse physical properties and different ability to reflect US; this requires setting a threshold of echo intensity). The program excludes from further processing any point with echo intensity equal to or lower than blood, and delineates the 3-D surfaces of cardiac tissue.

image 2nd step: The 3-D dataset undergoes 1 of 3 increasingly complex rendering techniques to create a visible 3-D object: wireframe rendering, surface rendering or volume rendering.

Wireframe rendering: The simplest technique. Defines and connects equidistant points on the surface of a 3-D object with lines (wires) to create a mesh of small polygonal tiles. Smoothing algorithms refine the narrow angles making rudimental object appear more real. This technique is used for relatively flat surfaces such as the LV and the atrial cavities. It cannot display objects/structures with complex shapes, such as valves (requires greater anatomic detail for meaningful analysis). Processes small amount of data (fast and efficiently performed on basic computers).

Surface rendering: similar to wireframe but defines more points on the surface of a 3-D object making the lines joining them visible. Displays details of a 3-D surface and makes morphologic assessment of the corresponding anatomic structure feasible. Generates 3-D objects with rendered surfaces and a hollow core.

Volume rendering: displays 3-D objects with a rendered surface and details of its inner structure. Enables the potential display of every voxel of the 3-D object permitting a “virtual dissection”.

Although composed of voxels, 3-D objects are seen in the screen as pixels of a 2-D image. Perspective, light casting and depth color coding are used to give a visual sense of depth and reality. Stereoscopic displays and holograms may display a 3-D rendered object more realistically but are currently used only for research purposes. Any volume-rendered 3-D object can be freely rotated on the display screen to be viewed in any orientation either as a static or a moving object.

Mitral Valve

3-D provides a “surgical view”—the same image that the surgeon will see in the field. This is very important when repairs are being planned. It can also accurately measure the size of the orifice. MV position within the heart and its relationship to the esophagus allows perpendicular alignment of the TEE US scanning plane making the MV an easily imaged structure!!! It is important for the ecocardiographer to provide the surgeon with detailed information of MV pathology, etiology, and a clear understanding of individual patient (surgical planning).

The use of 3-DE has significantly contributed to a better understanding of normal MV anatomy and the pathology of MV dysfunction:

The MV can be easily imaged using all the 3-D imaging modes described:

Stored MV 3-D images can be cropped on any plane to further delineate leaflet morphology.

Typically, the MV 3-D image is presented “en face” in the surgeon’s orientation as viewed from the LA with the AV at the top of the image and the LAA to the left.

The role of 3-D transesophageal echocardiography is expanding to become a powerful tool guiding surgical repair (treatment of choice for MR). In mitral stenosis, 3-D echo can consistently identify MV commissural fusion and predict the success of MV balloon valvuloplasty. Some authors have proposed planimetry by real-time 3-D echocardiography as a “gold standard” in the assessment of MS.

LV Volume

image 3-D-guided biplanes: by simultaneously displaying 2 perpendicular 2-D planes (ME 4C and ME 2C combination minimizes LV foreshortening) cutting the LV along its long axis at the true apex. Allows calculation (by applying modified Simpson biplane methods of disks to ES and ED frames) of LV volume, EF, mass. Limitations: still relies on geometric assumptions.

image Direct volumetric analysis: rendering a cast of the LV cavity and measure its volume throughout the cardiac cycle. Required identification of 4 LV walls and apex derived from full-volume 3-D dataset. Semiautomatic endocardial border detection creates a dynamic cast of the LV endocardial cavity. EDV and ESV are measured and SV and EF calculated. It is a more accurate method for LV volume in patients with abnormal ventricular shape or regional wall motion abnormalities due to a better alignment through the cardiac apex—inclusion of more endocardial surface during analysis and the lack of geometric shape assumptions (more reliable especially for less experienced users).

LV volume 3-D quantification is more reproducible than 2-D, and correlates well with MRI-obtained volumes. For regional LV function/wall motion, the 3-D LV cast is automatically divided into 16 wedges plus an apical cap and is based on a change in LV chamber volume over time. High sensitivity and specificity. 17 segments! Where have we seen this before? A full-volume 3-D dataset can be cut in multiple 2-D planes and displayed simultaneously as a series of parallel SAX planes similar to a MRI view.

Conclusion

RT3-D is one of the most significant developments of the last decade in cardiac US imaging. The challenges are that it requires new basic skills to manipulate 3-D datasets and properly orient 3-D images. Intraoperative real-time 3-D transesophageal echocardiography elevates experience to a new fascinating and challenging dimension. Over time there will be quicker acquisition of 3-D full-volume and 3-D color flow Doppler. This combined with on-line automatic 3-D LV and RV reconstruction will offer the most precise tool to assess and monitor LV and RV volumes. The next generation of echocardiographers will, “grow up” on 3-DE, and in the same way that 2-D echocardiographers today can’t imagine how people managed with M mode, the next generation is going to say “how did we manage with 2-D?”.

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