The Use of 3D Echocardiography in Surgical Planning of the Mitral Valve in Pediatric Cardiology

Summary

3D echocardiography of the mitral valve in pediatric cardiology produces full anatomic reconstructions that contribute to improved surgical management. Here, we outline a protocol for 3D acquisition and post-processing of the mitral valve in pediatric cardiology.

Abstract

Mitral valve disease in pediatric cardiology is complex and can involve a combination of annular, leaflet, chordae tendineae and papillary muscle abnormalities. Transthoracic two-dimensional echocardiography (2DE) remains the primary diagnostic imaging technique utilized in pediatric surgical planning. However, given that the mitral valve is a three-dimensional (3D) structure, the addition of 3D echocardiography (3DE) to better define the mechanisms of stenosis and/or regurgitation is advantageous. Transthoracic 3DE technology has improved with advances in probe technology and ultrasound scanners, producing images with good spatial resolution and adequate temporal resolution. Specifically, the addition of pediatric 3D transducers with higher frequencies and a smaller footprint provides better 3DE imaging in children. Improved efficiency of 3DE acquisition and analysis allow 3D assessment of the mitral valve to be more easily integrated by the sonographer, the cardiologist and the surgeon in mitral valve assessment. This improvement was also made possible by the postprocessing software optimization.

In this method paper, we aim to describe the transthoracic 3DE assessment of the mitral valve in children and its use in the surgical planning of pediatric mitral valve disease. Firstly, the 3DE assessment begins by selecting the correct probe and by obtaining a view of the mitral valve. Then, the appropriate data acquisition method should be selected based on the individual patient. Next, optimization of the data set is critical in order to properly balance spatial and temporal resolution. During live scanning or following acquisition, the data set can be cropped using innovative tools that allow the user to quickly obtain an infinite number of cut planes or volumetric reconstructions. The cardiologist and surgeon can view the mitral valve en face; thus, accurately reconstructing its morphology in order to support medical or surgical planning. Finally, a review of some clinical applications is proposed, showing examples in pediatric mitral valve managements.

Introduction

The mitral valve apparatus is a complex structure consisting of the mitral valve annulus, leaflets, chordae tendineae and left ventricular papillary muscles^1,2. Pediatric mitral valve disease consists of an extensive range of morphologic abnormalities associated with congenital and acquired heart anomalies³. The description of the morphology of mitral valve disease and its underlying mechanisms are key parameters for the surgical planning⁴. This requires the use of accurate diagnostic imaging modalities. Echocardiography is established as one of the primary diagnostic techniques used in pediatric mitral valve disease⁵. Specifically, two-dimensional (2D) echocardiography in pediatric mitral valve disease remains the most widely used diagnostic method. However, due to the nature of 2D imaging, the sonographer, the cardiologist and the surgeon must mentally reconstruct this complex 3D structure to determine the pathological mechanisms.

With the ability to produce anatomically correct views and an infinite number of cut planes, three-dimensional (3D) echocardiography has the ability to enhance mitral valve imaging. The value of 3D echocardiography is shown in its ability to provide specific information about annular shape and dynamics, leaflet scallop prolapse and the zone of leaflet coaptation^6,7. While 3D transesophageal echocardiography (TEE) has been shown to be the most accurate ultrasound modality in identifying adult mitral valve pathology⁸, 3D transthoracic echocardiography (TTE) is more feasible in children due to a better acoustic window. 3D TTE has been proven to be highly accurate in discerning simple vs. complex mitral valve lesions and the need for surgical intervention⁹. Additionally, acquiring a 3D volumetric dataset allows surgeons and cardiologists to collaborate in post-processing, further enhancing surgical planning.

3D TTE technology has continued to improve with advancement in probe technology, ultrasound processing power, and post-processing efficiencies. The current 3D matrix probes can now acquire a full volume single-beat data set at a volume rate of approximately 25 volumes per second¹⁰. It is possible to further increase the volume rate of a single-beat data set above 25 volumes per second depending on the ultrasound vendor, probe technology and volume optimization. However, if the ECG gated (sub volumes) full volume method is used, this number can more than double, providing volumes rates that are needed in children. The higher heart rates in children compared to adults require higher temporal 3D resolution for diagnostic accuracy. Additionally, the development of specific pediatric 3D probe technology allowed for a higher scanning frequency, providing better spatial resolution that is crucial regarding the small size of the mitral valve and its apparatus¹¹. Despite all these technological improvements, the vendors have managed to produce probes with footprints adapted to the anatomy of small children to maintain an optimal acoustic window. Lastly, new post-processing features, such as a quick cropping tools, allow for efficient post-processing.

In this paper, we describe the technique for 3D TTE assessment of the mitral valve in children, which can be applied to any ultrasound system with 3D TTE application. Additionally, post-processing of the 3D data will be reviewed and its benefit in the surgical planning. Finally, we will discuss some clinical applications of 3D imaging in children and include some examples.

Protocol

This protocol follows the guidelines of our institution's human research ethics committee.

NOTE: For the implementation of this protocol, a General Electric (GE) Vivid E95 or Philips Epiq 7C ultrasound system is used. On the GE Vivid E95 system, the user has a choice between the 4Vc-D (adult probe) or 6Vc-D (pediatric probe). On the Philips Epiq 7C, the user has a choice between the X5-1 (adult probe) or X7-2 (pediatric probe). See Figure 1.

1. Patient setup and probe selection

1. Position the patient in a left lateral decubitus position when possible. See Figure 1, step A.
2. Select the appropriate 3D matrix probe, pediatric or adult, based on the patient size and imaging window quality. In the majority of pediatric patients under the age of ten, a high frequency (pediatric) probe can be used when imaging from a parasternal imaging window due to the close proximity of the mitral valve. Over the age of ten, use of a pediatric probe can be attempted, however with excellent image quality on older children, the adult probe is more ideal. See Figure 1, step B.
   ​NOTE: If the user only has access to an adult 3D matrix probe, for smaller pediatric patients, increase the scanning frequency for optimal spatial resolution.

2. Probe positioning and 2D image optimization

1. Apply a generous amount of gel to the selected 3D matrix probe.
   NOTE: The optimal imaging window for 3D mitral assessment is a modified low parasternal long axis view. From this view, the mitral valve apparatus is in close proximity to the probe and the mitral valve leaflets to be relatively perpendicular to the ultrasound beam. In addition, a low parasternal long axis view provides full visualization of the entire mitral valve apparatus. See Figure 1, step C.
2. To obtain a modified low parasternal long axis view, position the probe on the chest in a standard parasternal long axis echocardiography view.
   1. Slide the probe laterally on the chest until the mitral valve leaflets are more perpendicular to the ultrasound beam and the 2D imaging window is optimal (this position will be between the standard parasternal window and standard apical window).
      ​NOTE: If the patient does not have an optimal modified low parasternal view, a standard parasternal window and apical window in combination will allow full visualization of the mitral valve anatomy.
   2. Center the mitral valve in the ultrasound sector by rocking the probe. Rocking the probe involves motion in the long axis of the probe along a fixed point while changing the angle of insonation away from 90 degrees. In 3D imaging, center the area of interest in the ultrasound sector to allow for a narrower volume and therefore better temporal resolution.

3. 3D Volume acquisition method

1. Begin by activating the 3D button on the ultrasound console (may also be labelled 4D by some vendors) to enter a full volume display. The full volume display should begin as a real-time full volume.
   NOTE: 3D Zoom can also be used to obtain a 3D data set of the mitral valve, however with its limited region of interest, would not be recommended because including surrounding structures can be important for surgical management.
2. If the patient is cooperative and able to hold their breath, use ECG gated full volume acquisition (see Figure 1, step E). Choose the number of sub volumes (heart beats) to use for the acquisition; on most ultrasound systems the number of sub volumes can be set between 2-6 (see Figure 1, step H). The higher number of sub volumes used during acquisition will result in a higher volume rate (increased temporal resolution) but can result in stitch artifacts related to breathing or motion as the sub volumes are put together.
3. If the patient is uncooperative or unable to hold their breath, real-time 3D full volume acquisition will eliminate the potential for "stitch" artifacts (see Figure 1, step F). However, the reduced temporal resolution is not ideal in children and will require the user to either sacrifice volume size (region of interest) or spatial resolution to compensate (both discussed in the next step).

4. 3D volume optimization (see Figure 1, step G)

1. Optimize the full volume size to include all the mitral valve annulus, chordae, papillary muscles and aortic valve where possible.
   NOTE: With ECG gated acquisition, a larger volume of data can be acquired because of an increase in volume rate achieved via sub volumes.
   1. A smaller volume of data will be required for real-time acquisition, in order to maintain a reasonable frame rate. Do this by narrowing the elevation plane and imaging in a parasternal short axis to allow for full visualization of the mitral valve leaflets and annulus (see Figure 2).
2. Optimize 3D signal-to-noise ratio (quality of the images) by increasing the ultrasound line density when possible. An increase in ultrasound line density will result in a decrease in volume rate. Different vendors have variable terminology for this function. On the GE Vivid E95 ultrasound system, optimize the line density using the Frame Rate knob. On the Philips Epiq 7C ultrasound system, optimize the line density using the Image Quality touch screen button.
   1. With ECG gated acquisition, increase the 3D volume line density because the use of sub volumes will maintain a good volume rate.
   2. With real time acquisition, balance the 3D volume line density with an acceptable volume rate for the patient's heart rate.
3. Set the 3D gain settings higher than 2D gain settings to minimize drop-out in the mitral valve leaflets. Gain can be decreased during post-processing to further optimize the cropped image if needed.

5. Storing the 3D full volume acquisition (see Figure 1, step I)

1. If using ECG gated acquisition, ask the patient to hold their breath and remain still. Then activate the number of sub volumes (heart beats) selected. Wait at least the number of beats selected before pressing Store (the more sub volumes that are selected will result in a longer storing process)
   1. Ensure there are no "stitch" artifacts present and the entire mitral valve is visible in the 3D volume before storing the final volume.
2. If using real time acquisition, store the final volume once all optimization is complete.

6. 3D color Doppler acquisition

1. Separately obtain a color Doppler 3D volume acquisition by adding color Doppler and following steps 3-5 of the protocol. Optimize the color Doppler box size as narrow as possible while including the entire mitral valve annulus. Set the color Doppler velocity scale between 60-80 cm/s.
2. Use ECG gated acquisition to maintain an adequate volume rate. Follow step 5.1 to store the 3D color Doppler volume.
   ​NOTE: The addition of color Doppler to a 3D volume significantly reduces temporal resolution, making its feasibility in children difficult.

7. Post processing and cropping of the mitral valve

NOTE: Post processing and cropping of the mitral valve can be performed directly on the ultrasound system for immediate results. However, there is also dedicated GE software (EchoPAC) and Philips software (QLAB) that provide the same functions from a reviewing station. In addition, TomTec provides a universal software for post processing and cropping 3D datasets from both vendors.

1. Load the stored 3D volume of the mitral valve in a 3-panel multi-planar display (2D lateral plane, 2D elevation plane, and 3D reconstruction) and activate the quick cropping tool. The quick cropping tool requires two clicks and allows the user to crop in any plane.
   NOTE: Different vendors will have variable terminology for the quick cropping tool. On the GE Vivid E95 ultrasound system, this cropping tool is labelled "2 Click Crop". On the Philips Epiq 7C ultrasound system, this cropping tool is labelled "Quick Vue".
2. To obtain an en face view of the mitral valve viewing from the left atrium (Surgeon's view) follow the below steps (see Figure 3, step E).
   1. Working from the 2D lateral plane (low parasternal long axis in this protocol), position the first curser within the left atrium, just above the mitral annulus. After the first position is set, drag the curser across the mitral valve towards the ventricular side and align the crop line parallel to the mitral valve annulus. Position the second curser within the left ventricle, ensuring the mitral valve leaflets are captured within your crop lines, and set this point (see Figure 3 step B).
   2. The recommended display orientation for the mitral valve en face is anterior up¹². Using the trackball, rotate the 3D mitral valve to position the aortic valve at the top of the screen.
3. To obtain an en face view of the mitral valve viewing from the left ventricle, simply flip the previous step cropped image 180 degrees (on some vendor systems there is a flip crop function that accomplishes this quickly) (see Figure 3, step F).
   1. Crop the color Doppler 3D volume of the mitral valve in the same orientation as step 7.3.
4. Obtain a view of the mitral valve sub-valvar apparatus including the chordae tendineae and papillary muscles.
   1. Working from the 2D lateral plane (low parasternal long axis in this protocol), position the first curser in the middle of the left ventricle. After the first position is set, drag the curser towards the posterior wall of the left ventricle and align the crop lines parallel with the long axis of the left ventricle. Position the second curser below the posterior wall and set this point (see Figure 4).
5. Optimize the 3D gain and compression settings.
   1. Optimize the 3D gain settings to its lowest setting while maintaining minimal to no mitral valve leaflet drop-out.
   2. Optimize 3D compression settings to include a wider or narrower range of color shades. 3D compression can improve 3D depth perception. On the Philips Epiq 7 system, adjusting the 3D compression is performed by rotating the Compression knob. On the GE Vivid E95 system, adjusting the 3D compression is performed by rotating the Active Mode gain knob.
6. Store the optimized, cropped 3D views of the mitral valve as separate cine loop clips.

Results

A good quality 3D data set of the mitral valve in pediatric echocardiography will have an optimal volume rate that is appropriate for assessing leaflet motion and excellent spatial resolution that utilizes superior axial resolution. To assess the success of the protocols 3D ECG gated acquisition, first determine whether any significant "stitch" artifact is present. In the presence of no artifact and if the acquisition was made using an excellent quality 2D low parasternal long-axis view, this 3D data set will pro...

Discussion

For the operator/sonographer, 3D echocardiography is often met with several challenges. First, by nature there is significant variation in patient size, heart rate and cooperation during a pediatric echocardiography exam. These parameters make it difficult to have 3D specific protocols and therefore make the 3D acquisition operator dependent. Often training for sonographers is focused primarily on 2D imaging, leaving a gap in knowledge with regards to 3D image acquisition and interpretation. In addition, 3D temporal reso...

Materials

Name	Company	Catalog Number	Comments
4Vc-D probe	General Electric		Ultraspound probe (GE)
6Vc-D probe	General Electric		Ultraspound probe (GE)
Epiq 7C	Philips		Ultrasound system
Vivid E95	General Electric		Ultrasound system
X5-1	Philips		Ultraspound probe (Philips)
X7-2	Philips		Ultraspound probe (Philips)