Evaluation of Left Ventricular Structure and Function using 3D Echocardiography

Summary

In this article, we provide a step-by-step acquisition and analysis protocol for the volumetric assessment and speckle-tracking analysis of the left ventricle by 3D echocardiography, particularly focusing on practical aspects that maximize the feasibility of this technique.

Abstract

Three-dimensional (3D) quantification of the left ventricle (LV) provides significant added value in terms of diagnostic accuracy and precise risk stratification in various cardiac disorders. Recently, 3D echocardiography became available in routine cardiology practice; however, high-quality image acquisition and subsequent analysis have a steep learning curve. The present article aims to guide the reader through a detailed 3D protocol by presenting tips and tricks and also by highlighting the potential pitfalls to facilitate the widespread but technically sound use of this important technique concerning the LV. First and foremost, we show the acquisition of a high-quality 3D dataset with optimal spatial and temporal resolution. Then, we present the analytical steps toward a detailed quantification of the LV by using one of the most widely applied built-in software. We will quantify LV volumes, sphericity, mass and also systolic function by measuring ejection fraction and myocardial deformation (longitudinal and circumferential strain). We will discuss and provide clinical examples about the essential scenarios where the transition from a conventional echocardiographic approach to a 3D-based quantification is highly recommended.

Introduction

The assessment of left ventricular (LV) morphology and function is the predominant purpose of general and even more specific investigations in cardiology¹. The widely available and noninvasive transthoracic echocardiography (TTE), which can provide dense amounts of information, is the method of choice for a convenient, fast, and cost-effective evaluation.

Measurement of LV mass, volumes, and subsequent ejection fraction holds significant diagnostic and also prognostic value². The more accurate a given measure is, the higher its value will be. A better correlation with gold standard cardiac magnetic resonance (CMR) imaging derived values is an ongoing chase for echocardiographic techniques. Generally, clinical practice guidelines recommend the biplane Simpson's method for LV volume and ejection fraction measurement³. However, the LV is a three-dimensional (3D) structure with an often irregular shape, and therefore, several tomographic planes will undoubtedly fail in some clinical scenarios to accurately delineate LV morphology and function. Recent advancements in ultrasonic hardware and software technology permitted the development of real-time 3D imaging, which revolutionalizes echocardiographic protocols.

Moreover, the need for a quantitative approach concerning wall motion abnormalities resulted in the rise of deformation imaging⁴. Strain and strain rate parameters can be calculated by speckle tracking using standard grey-scale images. 3D echocardiography may also overcome several shortcomings of a two-dimensional strain assessment⁵. From an expensive scientific tool, 3D echocardiography started to become a powerful technique used in everyday clinical practice, and the quantification of the LV is certainly in the first line in this breakthrough.

The present article aims to guide the reader through a detailed 3D protocol by presenting tips and tricks and also by highlighting the potential pitfalls to facilitate the widespread but technically sound use of this important technique concerning the LV.

Protocol

This protocol follows the guidelines of the Semmelweis University Regional and Institutional Committee of Science and Research Ethics. The present protocol applies to a specific vendor. Although some steps remain valid regardless of the ultrasound machine and postprocessing software, important differences may exist if using other vendors' solutions.

1. Technical requirements

1. Utilize an echocardiography machine capable of 3D imaging.
2. Connect a 3D transthoracic echocardiography capable phased array transducer.
3. Apply the built-in 3-lead ECG of the ultrasound system to allow the system to synchronize the recordings and analyses to the cardiac cycle.

2. Acquisition of the 3D echocardiographic images

1. Position the patient in the left lateral decubitus position (patient lying on the left side with the left arm stretched above the head).
2. Ensure that the ECG tracing on the screen is of good quality.
   NOTE: This is a prerequisite for postprocessing as the software will detect the different points of the cardiac cycle based on the ECG signal.
3. Unfreeze the image, and start to examine the patient with the transducer. Visualize a conventional apical four-chamber view.
4. Optimize the image quality by adjusting sector width to LV, lowering the depth to truncate the left atrium, and by using a slight overgain.
   NOTE: Ensure the entire LV endo- and also the epicardial surface is visible.
5. Press the 4D button to switch to 3D mode.
   NOTE: By pressing the Multi-Slice... button on the touch screen, four options will be available (5, 7, 8, 12 slices) to overview the 3D dataset using standard short- and long axis cuts. If needed, transducer positioning can be corrected to ensure the inclusion of the entire LV wall thickness from apical to mitral valve level into the pyramidal 3D dataset. The use of 12 slices (with nine adjustable short-axis views) is recommended.
6. Acquire 3D images using Multi Beat or Single Beat mode.
   1. Use the Multi Beat mode to achieve higher spatial and temporal resolution, where the dataset will be reconstructed from 2, 3, 4 or 6 cardiac cycles (this can be set-up on the screen) - end-expiratory breath-hold of the patient and stable transducer positioning needed to minimize stitching artifacts.
      NOTE: Single Beat acquisition is of lower spatial and temporal resolution; however, most modern transducers have better quality and, therefore, can be used to acquire proper 3D datasets without reconstruction to undergo further analysis. As a general recommendation, volume rates over 15 volumes per second are recommended for further analysis.
   2. When the full-volume is reconstructed from the subvolumes, and the entire LV is visible, freeze the image. Using the Cycle Select and Number of Cycles knobs, select the optimal acquired cardiac cycle(s) and press Image Store.
      NOTE: Stitching artifacts are spatially or temporarily misaligned subvolumes next to each other. Datasets with a significant dropout of LV walls or with stitching artifacts are generally not suitable for further analysis. The quality of the already acquired 3D dataset can be double-checked using the Multi-Slice mode.

3. Postprocessing to quantify LV morphology and function

1. Select a 3D dataset appropriate for further analysis.
   NOTE: This part of the protocol requires the previously acquired and saved good-quality 3D images and can be performed on the ultrasound machine and a separate workstation, either.
2. Click on Measure | Volume, and then select 4D Auto LVQ.
3. On the quad-screen (three apical views: four-, two-, and three-chamber views, and one short-axis view, the latter can be adjusted by a horizontal plane on long-axis views), the software asks Modify alignment of apical slices to standard views. If required, correct the apical views manually by tilting and rotation to show the corresponding standard view, thereby eliminating foreshortening. Set tilting to align the caliper with the long axis of the LV by dragging and moving the calipers on long-axis views. Set rotation by the corresponding or the Rotate All knobs on the machine or by adjusting the calipers on the short-axis image.
   NOTE: Software recommendation can be reset by pushing the Auto Align button.
4. After finishing view alignment, click to the next step EDV. The end-diastolic (ED) frame is automatically detected using the ECG signal, but can be manually corrected if necessary.
5. Semi-automatic detection of LV endo- and epicardial surface
   1. Select two landmark points manually on any apical views. Firstly, identify the LV apex and then the middle of the LV base (mitral annulus level) in any apical view. The algorithm will automatically contour the endocardial border of the entire LV.
      NOTE: There are two more options: Manual, which means that two basal and one apical landmark should be set in every apical view, and Auto Init, which will automatically contour the LV without any user interaction.
   2. Check contour credibility in three apical views, three short-axis views of different levels, and a fourth user-controlled short axis, to allow visual verification of the detected surface. Contour correction is possible by manually adding points that will then be incorporated in the contour line.
      NOTE: With Undo, the previously added point can be deleted. Reset button resets the contouring to start the entire section from the beginning. Contour visibility can be adjusted to allow the appreciation of the endocardial surface on the grey-scale image. Endocardial and epicardial contouring should be performed in an accurate and consistent manner. For a detailed recommendation, please check the following reference⁶.
   3. Choose the next step, which is the ESV.
   4. Repeat the same procedure (3.5.1-3.5.2) as mentioned in the previous points to identify and correct the endocardial contour on the end-systolic frame.
      NOTE: The end-systolic (ES) frame is automatically detected using the ECG signal, but can be manually corrected if necessary. Values of end-diastolic volume (EDV), end-systolic volume (ESV), ejection fraction (EF), heart rate (HR), stroke volume (SV), cardiac output (CO) and sphericity index (SpI) are already displayed on the screen.
   5. Press Volume waveform for the next step. The software displays a dynamic 3D model of the LV and also time-volume curve as it traces the endocardial surface throughout the cardiac cycle frame-by-frame (Figure 1).
      NOTE: Here, there is a possibility to edit the endocardial border at any frame.
   6. For the next step, press LV Mass. The software automatically contours LV epicardial contour on the end-diastolic frame and calculates LV mass (EDMass).
      NOTE: If necessary, edit the contour of the epicardial surface by adding points to include (same method as previously described) in any short- or long-axis plane. It can be selected which contour to adjust: Endo, Epi, or Endo+Epi.
   7. Press 4D Strain ROI for the next step. The software automatically contours LV epicardial contour on the end-systolic frame and calculates LV end-systolic mass (ESMass).
      NOTE: If necessary, edit the end-systolic contour of the epicardial surface by adding points to include (same method as previously described) in any short- or long-axis plane. ESMass should be of similar value than EDMass. This step is essential to calculate 3D strain values by speckle tracking.
   8. Press 4D Strain Results for the next step. The software visualizes the 3D myocardial tracking on multiple short- and long-axis planes and corresponding strain values of the 17 standard LV segments throughout the cardiac cycle, frame-by-frame. Time-strain curves and bull's eye plot are also displayed. The following parameters are calculated and can be demonstrated: longitudinal strain, circumferential strain, radial strain, area strain, rotation, and torsion.
      NOTE: There is a possibility to exclude a particular LV segment from analysis if it is considered as having a low tracking quality by visual observation of images or based on the time-strain curve. However, the software recommends by default on segment approval or rejection. Color-coded strain values can be visualized on a dynamic 3D model of the LV by changing the "Layout".
6. To terminate the analysis, press Approve & Exit.

Results

3D analysis of the LV is feasible in the majority of patients. Case 1 is a healthy volunteer with normal ventricular volumes and function (Figure 1). Case 2 (Figure 2) is a 64-year old male patient with dilated cardiomyopathy and a wide QRS complex (160 ms) of left bundle branch block morphology. Gold standard CMR measurements were the following: end-diastolic volume: 243 mL, end-systolic volume: 160 mL, ejection fraction: 34%, L...

Discussion

LV morphological and functional measurements represent cornerstones of diagnosis, management, and follow-up of cardiac diseases; moreover, they are powerful predictors of outcome. Generally, 2D echocardiography-based evaluation of the LV is recommended by current practice guidelines; however, 3D echocardiography has been proven to be more accurate as it is free from geometrical assumptions concerning LV shape^7,8. Deformation imaging by speckle tracking is a robus...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
3V-D/4V-D/4Vc-D	General Electric	n.a.	ultrasound probe
4D Auto LVQ	General Electric	n.a.	software for analysis
E9/E95	General Electric	n.a.	ultrasound machine
EchoPac v203	General Electric	n.a.	software for analysis