Fig. 6.1
Different modalities of 3D Strain graphic representations. Left: Plastic Bag view;Middle: Wires view; Right: Doughnuts view. With the current settings, yellow colour indicates high values of strain for all visible segments. All these views can be rotated and tilted for assessing the contraction of segments of interest
Technical Aspects
3DST technology is capable of tracking 3D volumes in any desired direction. Usually 3D images are acquired through a 3D matrix-array transducer with a single acquisition from apical window [25], and processed in a specifically designed ultrasound machine, providing a relevant reduction of acquisition and processing time: a complete processed study is achieved in one third of the time when compared with 2DST [26, 27]; in other cases a generic 3D echocardiography machine is used and off-line processing with an external workstation and specific software will be necessary [28].
Acquisition Protocol
In the following section we describe step by step the way to acquire and process a 3DST study of the LV. However, it is also possible to study other structures like the right ventricle or the atria, but commercial software is mainly optimized for LV at the moment.
The fist step for acquiring a 3DST study is to assess that all required conditions are OK:
Acceptable acoustic window. Septal, lateral, inferior and anterior walls must be visible on echo image. Use the multiplane application for best assessing.
ECG must be shown properly on screen and free from artifacts.
No contrast can be used during acquisition.
The second step is to improve the settings for acquisition.
Adjust the depth in order to include the whole LV. The less depth needed, the more volume rate and quality of data. It is useful to select a 2 plane view (pre-full 4D mode on Toshiba Artida system) and check the good visualization of all endocardial and epicardial borders.
If needed, adjust the scan range, by widening or shortening the pyramidal sector. Again, shortest ranges will provide better quality, but dilated LV’s may need more range for avoiding loss of apical segments.
The number of subvolumes used for composing the image usually range from 2 to 6. In the presence of irregular rhythm, lower values may help to avoid stitching, but some of the image quality will be lost.
The third step is the acquisition.
Ask the patient to hold his breath.
Start 4D protocol (“Full 4D” for Toshiba Artida)
Wait as many cardiac cycles as the number of sub-volumes selected for composing the image, then store the resulting loop. (“Clips Store” for Artida)
It is desirable to have several acquired loops and choose the best quality one for post-processing.
Processing
Several vendors have developed 3DST software for processing 3D echocardiography data. This software may be included in the ultrasound system allowing an almost “on-line” analysis of the data (e.g. Toshiba Artida), or it may be loaded into a separate workstation for off-line processing (e.g. General Electric Echo-Pac). We will explain the processing protocol for both cases.
Toshiba Artida
A correct alignment of the LV axis must be checked and corrected is necessary.
Before semi-automated detection of the endocardial and epicardial border, the system will ask for landmarks setting, usually at both sides of the mitral annulus and at the apex. Manual delineation is also possible.
When semi-automated detection is performed, manual corrections are also possible in cases with poor automatic results.
Perform automatic wall motion tracking process.
Play the loop and check the tracking. Some cases may require frame by frame corrections at this stage.
Display the tables with the individual values for each segment and the global values. The system allows selection of a number of parameters, with a different table for each one.
The definitive processed dataset after manual corrections should be saved in order to use always the same settings for any needed data from the patient.
See Fig. 6.2 and legends for additional details.
Fig. 6.2
Protocol for 3DST on ARTIDA. 1- The “Pre-4D” screen shows a biplanar 2D view. It allows depth and azimuth adjustment and number of subvolumes (from 2 to 6) selection; 2- The “Full 4D” mode allows acquisition of a 3D echo loop. The clip should be stored. 3- The saved clips can be selected and processed by pressing again “Full 4D”. Axis adjustment is performed at this stage. 4- “3DT” (3D- tracking) mode will ask for reference points at the mitral annulus and the apex. Manual corrections and even manual delineation instead of points setting are also possible. Pressing “Start” will begin the automatic tracking process
Also see Figs. 6.3 and 6.4 for finding explanation of examples for right ventricle and left atria 3DST processing.
Fig. 6.3
“Plastic Bag” and “Wires” image modalities for the right ventricle (RV) modeling.When processing chambers like the RV or the atria, the method is similar to the protocol we have described for theLV, but the desired chamber must be properly centered in the middle of the screen during acquisition, and “Other” must be specified instead of“LV”
Fig. 6.4
3DST of the left atrium (LA). (a) Full 4D volume acquisition of both LV and LA with corrected alignment of the LA axis; (b) Multiplane images of LA 3D strain as a result of automatic tracking after drawing endocardial borders of LA; (c) “Wires” representation of the LA model; (d) “Plastic Bag” view
GE EchoPac
Only datasets with enough volume rate (generally over 30 vps) will be available for processing. The 4D Auto Left Ventricular Quantification tool menu includes the following consecutive steps for getting at the end the 3D strain analysis.
Aligning views (similar to Artida)
EDV: Setting landmarks for the endocardial borders at diastole
ESV: Same process at systole.
Volume wave: Internal endocardial borders and LV volume data are displayed.
LV mass: external epicardial borders and LV mass data are displayed.
4D Strain: Regional and global values for longitudinal, radial, circumferential and area strain may be selected for display. If any segment fails to be tracked properly, it may be excluded for analysis. More than 3 excluded segments will make not possible the global value estimations.
See Fig. 6.5 and Video 6.2 for additional details.
Fig. 6.5
Radial strain 3DST analysis with GE. 3 long-axis and 3 short-axis views from 3D data are shown for assessing endocardial and epicardial tracking.Strain/time curves show the segmental strain from the 17segments (yellow curved lines)and global strain (white line)through the cardiac cycle. The polar map displays the maximum radial strain values. One of the segments (basal lateral) has been excluded for analysis due to suboptimal quality. The global value of RS (42%) is shown as “G42” besides the polar map
Interpretation
On Echo-PAC software, the global values and the graphic representations for the main strain parameters are displayed in a simple way when reaching the final step of the process.
On Toshiba Artida the results are displayed on a complex table (Figs. 6.6 and 6.7) including among other data the maximum and minimum value of strain (in its different modalities) for every segment (with 16 or 17 segments models) and the time to achieve this peak. Also global values are provided. It’s important to know that when assessing thickening parameters such as radial strain and 3D strain, the desired peak value will be placed as maximum value (positive), whereas in the case of shortening parameters like longitudinal, circumferential or area strain, the relevant value is the minimum one (negative) (Fig. 6.6).
Fig. 6.6
Data table for the 16 myocardial segments and global longitudinal strain. As a shortening parameter, LS is a negative value at systole and its peak value for each segment will appear on the “minimum” column (Min1), highlighted in red box. The global value for the whole left ventricle appears below them (in white). The next row (Min1T) shows the time to reach the peak. This information is useful for dyssynchrony analysis
Fig. 6.7
Data table for radial strain in a patient with an anterolateral infarction. Medium lateral and medium anterior segments show low strain values (red circles) andglobal radial strain is also decreased. As a thickening parameter, RS reaches positive maximum values during systole, which are displayed on the first column of the table (Max1). Time to reach peak (Max1T) and second peak value (Max 2) are also available
Data about peak twist or twist degree at a certain segment are also available, however if untwist values are needed, they must be calculated [29]:
Untwist (%) = (peak LV twist − Twist1*)/(Peak LV twist) × 100* Twist1= twist at a specific point along diastole, usually mitral valve opening.
Besides the numerical data, 3DST software usually offers several different modalities of graphic representation of LV 3D model (Fig. 6.1), multiplane 2D views of the 3D echocardiography with volumes and ejection fraction information (Fig. 6.8) a 16 or 17 segments polar map with regional values of strain or other parameters (Fig. 6.9 and Video 6.3) and many other features.
Fig. 6.8
Multiplane representation of radial strain at early systole (“a” image) and late systole (“b” image) frames. The “Hold” function is disabled, allowing to represent both positive and negative strain. Yellow color indicates high positive strain which appears first on septal apical wall and later on lateral and anterior wall. Blue color indicates negative strain on the opposite wall in each frame. With the “Hold” function enabled, only positive strain would be coded (from black to yellow). On this screen, also LV volumes, mass and ejection fraction are shown
Fig. 6.9
Polar map and Plastic Bag view in a patient with a myocardial infarction.Low values of 3D strain at peak systole are shown specially at mid- lateral (2%) and mid-anterior (9%) segments. An anterolateral 3D strain defect is evident on the 3D model. 3D Strain with “Hold” setting on (yellow/black coding) is particularly useful for visual assessment of contractility defects
Old and New Parameters
3DST technology offers a number of different parameters for assessing myocardial mechanics. The basic information is “displacement” and from that data, strain and rotation parameters can be obtained. The usual 2DST parameters (RS, LS, CS, torsion, rotation, twist…) are also available from 3D to ST [4] and additionally new parameters like area tracking/area strain and 3D strain have been described. The practical definitions of these parameters are displayed on Table 6.1.
Table 6.1
Frequently used 3DST- derived parameters with their simplified definitions
Variable | Units | Practical definition |
|---|---|---|
Radial strain | % | Thickening (direction: normal to endocardial contour) |
Longitudinal strain | -% | Shortening (tangential to endocardium) |
Circumferential strain | -% | Shortening (circumferential to the endocardial contour) |
Rotation | º | Rotation angle (counterclockwise) of endocardium around the center |
Twist | º | Angle difference between apex (or a segment) and base |
Torsion | º/cm | Twist change per distance |
Area strain | -% | Endocardial area change |
3D Strain (Toshiba) | % | Thickening (in the wall thickening direction) |
3D Strain (Philips) | -% | Tangential shortening. Vector sum of longitudinal and circumferential components. Similar to Area Strain. |
Area change rate | %/s | Velocity of area change |
During systole, myocardial fibers are shortened in both longitudinal and circumferential directions [30]. Longitudinal strain is defined as SL=100*(L–L0)/L0 where L is the instantaneous longitudinal length of the segment and L0 is the initial length at end systole. Circumferential strain is the same concept applied to circumferential length of the segment. As myocardial tissue is incompressible, the result of longitudinal and circumferential strain is thickening in the radial direction for conservation of the mass. Radial strain is a estimation of that component of myocardial deformation.
As for rotational mechanics, counter-clockwise rotation of the LV apex is normally observed and it can be expressed as positive degrees, and clockwise rotation of the LV base will result in negative degrees (Fig. 6.10, Video 6.4). We also may want to measure the degrees of difference in rotation between apex (or any segment) and the LV base, which would be called “Twist”. If we measure the change in twist per distance we will call it “Torsion”.
Fig. 6.10
Physiological LV rotation. Polar map and doughnuts view, with “averaged levels” setting in a healthy subject: average values of rotation for basal, midventricular and apical segments are shown. Positive numbers on mid(4 degrees) and specially on apical segments (6 degrees) indicatecounter-clockwise rotation. Negative numbers on basal segments (-6 degrees) confirm the clockwise rotation of the LV base
The new 3D–WMT based systems also provide some new parameters, which were not available with the 2D approach.
Area Strain
This index estimates the degree of change of the sub-endocardial area (Fig. 6.11 ). It can be obtained from the combined information of longitudinal and circumferential shortening when both measures are obtained simultaneously from a 3D dataset. The result is a negative value which is called Area Change (Toshiba) [31, 32] or Area Strain (GE) [28], and also a similar concept is used for a parameter called “3D Strain” on Philips platforms [33]. This “3D Strain” should not be confused with 3DS from Toshiba, which is a different concept, as it will be explained below. These area change parameters are expected to be useful for detection of ischaemia, given that sub-endocardial surface is particularly sensitive in that context.
Fig. 6.11
Area Strain or Area Tracking is obtained from 3D longitudinal and circumferential strain information. The objective is to estimate the percentage of change between end-diastolic (ED) and end-systolic (ES) endocardial areas in a segment (on this example, medium inferior wall) or in the whole left ventricle.IF ES area is 30% shorter than ED, area strain is -30% in that segment
3D Strain
As stated previously, on Philips platforms the denomination”3D Strain” is applied to a parameter which is equivalent to AS from other vendors. However on Toshiba systems 3D Strain (3DS) is a totally different index, with positive values like RS, and indicates strain in any wall thickening direction [34]. Its range of values is usually similar to RS, but it is expected to overcome its limitations, as RS takes as reference the endocardial contour and 3DS takes into account the direction of thickening in 3D space. Though, few data support the possible clinical advantage of 3DS over RS at the moment.
Reliability, Normal Values and Differences Between Vendors
Kleijn et al. described reliability data for 3DST [35]. LV volumes, EF and global CS measurements demonstrated good intra-observer, inter-observer and test-retest reliability with intraclass correlation coefficients (ICC) 0.85–0.99; however, global LS and RS, while maintaining good intra-observer ICC (0.92 and 0.88) showed lower inter-observer (0.74 and 0.58) and test-retest (0.66, 0.52) results.
It is important to mention that, as usual on 3DST studies, these results exclude patients with atrial fibrillation or insufficient image quality (23 of the 140 patients, 16%). In the real clinical practice, image quality may lower the reproducibility in some cases. Also, in our experience, depending on the need for manual adjustments of the automatic tracking this reliability may also change. The absence of manual corrections when automatic tracking is not optimal may result in better reproducibility within the same dataset, but it may not reflect the real motion and strain.
There have been a few attempts to set the normal values for some of these indexes [33, 36, 37]. As noticed on Table 6.2, discrepancies between different vendors are significant. Furthermore, Muraru et al. also compared data from the same platform processed by vendor-specific software and the same data processed with vendor-independent software, and they found also significant differences in RS and CS results [36]. Overall there seems to be a need for agreement between vendors for setting standard methods for obtaining and processing 3DST data before clinical use of these data and comparisons between studies from different platforms in the daily practice can be a reality.
Table 6.2
Normal values of usual 3DST parameters according to different authors
Variable | Kleijn et al. | Kaku et al. | Muraru et al. |
|---|---|---|---|
Radial strain | 35.6±10.3 | 47.1 ± 20.3 | Median 52 (Q1 47, Q3 59) LLN 38 |
Longitudinal strain | -15.9±2.4 | −11.3 ± 4.4 | Median 19 (Q1 21, Q3 17) LLN15
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