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Introduction of Tissue Doppler Imaging Echocardiography—Based on Pulsed-wave Mode Fen-Chiung Lin*, I-Chang Hsieh, Cheng-Hung Lee, Ming-Sien Wen Noninvasive evaluation of global left ventricular (LV) myocardial contractile function using echocardiography has depended on M-mode and two-dimensional measurement of the LV dimension and area changes between diastolic and systolic phases, which provide an estimation of volume changes between diastole and systole, to calculate the ejection fraction as an index of LV contractile function. The LV ejection fraction represents a contractile function in summary of several LV myocardial segments. Noninvasive evaluation by echocardiography for regional myocardial contractile function depends mainly on visual assessment of the segmental myocardial motion velocity, especially the endocardial inward motion velocity. Human visual assessment is subjective and, hence, has a certain degree of intraobserver and interobserver variability. Tissue Doppler imaging (TDI) echocardiography, a technique which can have a frame rate of up to 200 frames per second (fps), is far beyond the capability of human vision (30 fps). TDI can detect the high-intensity, lowvelocity motion of regional myocardium, mainly on the LV longitudinal axis using three apical views. The myocardium of the LV basal segments moves towards the apex during systole; hence, the systolic motion velocity is highest in the basal segments, middle in the LV mid-segments, and lowest in apical segments. The strain (S) image calculates regional myocardial deformation and is defined as the change in length divided by the original length, (L1–L0)/L0. The S is expressed as the percentage change, that is, the degree of regional deformation. A negative S represents a regional myocardial shortening, and a positive S represents a regional myocardial lengthening. The rate at which the S change occurs is called the strain rate (SR) image. The SR is expressed as 1/second and a negative value represents a regional rate of change in deformation during systolic shortening. A total displacement (D) at end-systole of a certain segment can also represent a regional myocardial contractile function. A segmental mean D can be acquired quickly and coded in colors to represent a semi-quantitative regional myocardial contractile function index display using a color bar. Recently, a tissue synchronization image mode has been developed for a more convenient quantification of time-to-peak velocity on several myocardial segments. In this context, examples on each mode acquired from normal and diseased

Received: August 19, 2008 Accepted: August 25, 2008 Second Section of Cardiology, Chang Gung Memorial Hospital, Lin-Kou Medical Center, Chung Gung University, Taoyuan, Taiwan. *Address correspondence to: Dr. Fen-Chiung Lin, Second Section of Cardiology, Department of Internal Medicine, Chang Gung Medical Center, Lin-Kou Medical Center, Chang Gung University, Medical College, 5 Fu-Xing Street, Guwei-San Siang, Taoyuan Sien, Taiwan. E-mail: [email protected]

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Introduction to Tissue Doppler Echocardiography

regional myocardium will be shown. Thus, TDI will hopefully be more easily and widely accepted by echocardiographers and in clinical applications.

KEY WORDS — strain and strain rate, tissue Doppler image, tissue synchronization image ■ J Med Ultrasound 2008;16(3):202–209 ■

Introduction Conventional evaluation of global and regional myocardial contractile function using noninvasive echocardiography mainly depends on visual assessment of the degree of myocardial thickening and endocardial inward motion, as well as M-mode and two-dimensional echocardiographic measurements of left ventricular (LV) ejection fraction (LVEF) [1]. Visual assessment is known to be very subjective and, hence, has a high degree of intraobserver and interobserver variability. A certain degree of disagreement has also been noted for LVEF measurements made by echocardiography [2–4]. Tissue Doppler imaging (TDI) echocardiography has been used for several decades; however, its clinical application has been controversial. In contrast to conventional Doppler color ultrasound, TDI can detect low-velocity, high-intensity myocardial tissue motion to depict low-intensity, high-velocity blood flow, mainly red blood cell flow motion velocity. TDI has a frame rate of 90–200 frames per second (fps), thus has the advantages of high temporal resolution up to 5 milliseconds and high spatial resolution up to 1 mm. This high spatial and temporal resolution is far beyond the capabilities of human vision but has the disadvantage of invisibility, and thus limits its clinical applicability [5–9].

Tissue Doppler Imaging (TDI) The digitized data sets acquired by various TDI modes, defined by various names, and by different echo instrument companies, have mostly emphasized

the convenience of data retrieval and its off-line analysis capability using various analytical software [8,9]. However, the data acquired by those TDI modes are the mean myocardial motion velocities [10,11]. A tiny variation in the sample volume placement during off-line analysis can result in a big difference in the velocity-time integral (VTI) expression, which then causes further variation in the VTI-derived parametric data. The parametric data includes strain (S), and strain rate (SR) which are derived from the TDI examination [12–20]. Hence, the introduction of various modes of TDI and its derived parametric images in the present study will be based on the pulsed-wave TDI (PWTDI) mode [10]. TDI examination can detect LV myocardial motion velocity in its longitudinal axis. Therefore, a TDI examination is usually performed via an apical approach using three apical views. The LV myocardium is divided into 16 segments according to the coronary artery supply, as recommended by the American Society of Echocardiography [1]. A segmental longest time-to-peak velocity (TpV) of the basal septal and mid-septal segments can be detected on the medial side myocardium by an apical four-chamber view, along with basal and mid-lateral segments on the left lateral side myocardium by an apical four-chamber view. The basal, middle segmental TpV of the inferior and anterior segmental TpV can be detected by an apical two-chamber view. Likewise, the TpV of the basal, middle posterior segments and anteroseptal segments of the LV myocardium can be detected on an apical long-axis view (Figs. 1 and 2). The transducer should be aligned to the myocardial region of interest as parallel as possible to obtain an optimal J Med Ultrasound 2008 • Vol 16 • No 3

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Mid septum

Mid lateral Mid inferior

Basal septum

Basal lateral

Basal inferior

Apical four-chamber view

Mid anterior

Basal anterior

Apical two-chamber view

Mid posterior

Mid anteroseptal

Basal posterior

Basal anteroseptal

Apical long-axis view

Fig. 1. Three apical views (the apical four-chamber, the apical two-chamber view and the apical long-axis view). The left ventricle is divided into 16 segments as recommended by the American Society of Echocardiography. The nomenclature of the basal and mid segments shown on the figure relate to the 12 non-apical segments.

Diastole

Systole

The total displacement of the basal anterior segment is between 4 and 6 mm at end-systole, shown as green, which correlates with the semiquantitative scale color bar shown on the left upper two-dimensional sector frame (Fig. 5).

Strain and Strain Rate Image Longitudinal axis

Fig. 2. Figure illustrates the principle direction of left ventricular myocardial deformation during systole (black arrows) and diastole (red arrows) on the longitudinal axis, view from the apex. maximal VTI, so as to define an optimal peak velocity for a precise TpV measurement [12–19]. In normal functioning regional myocardial segments, the myocardium of the base moves towards the apex during systole, on a longitudinal axis. The myocardial motion velocity is maximal at the base, lower in the mid-segment, and least at the apex, which can be obtained by TDI mode [20–26]. The PWTDI mode acquires the maximal velocity (Fig. 3A), the color flow M-mode depicts the mean velocity of the entire sector image [27–32] (Fig. 4).

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The strain (S) is defined as the percent change, shortening or lengthening, relative to its original length. A normal S on the longitudinal axis is a negative % value, representing normal myocardial shortening during systole (Fig. 6). The myocardial tissue motion velocity gradient is used to calculate the strain rate (SR). SR is the velocity difference between two points along the ultrasound beam parallel to the myocardial wall, divided by the distance between the two points (Figs. 7 and 8). The two points move closer to each other in systole and away from each other in diastole. Tissue synchronization image mode is a semiquantitative color-coded mode which depicts the segmental peak mean velocity, using color bar to code the TpV as green to red according to the time sequence.


A

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Fig. 3. (A) Pulsed-wave tissue Doppler imaging (PWTDI) mode acquired segmental time-to-peak velocity (TpV) of the basal anterior segment on apical two-chamber view. TpV = 85 milliseconds by PWTDI. The peak velocity (Vp) = 6.66 cm/s, as shown on the left upper column. This figure was designed to compare these values with those in (B). The TpV of the basal anterior segment was shorter than the mid-inferior segment, and there was no postsystolic shortening (PSS) in (A). The Vp was higher in the basal anterior segment shown in (A), than the Vp in the mid-inferior segment shown in (B). (B) PWTDI mode examination, with the sample volume placed at the mid-inferior segment to acquire a TpV of 439.9 milliseconds with a PSS, and Vp was 3.59 cm/s as shown on the left upper column.

The following figures show examples in a patient case, which were used to illustrate the possible clinical application of each image mode. All the parametric images were calculated and derived from PWTDI. The figures shown in pairs were designed to compare a normal functioning myocardial segment (basal anterior segment) and an abnormal VTI acquired by PWTDI on the LV mid-inferior segment of this patient.

Fig. 4. (A) Color flow M (CFM) mode depicted the mean velocity of the entire sector of the myocardium. The velocitytime integral shown on the left, with the time-to-peak mean velocity (t) of 280 milliseconds, the peak mean velocity was 3.07 cm/s. Mean velocity is lower than the peak velocity acquired by pulsed-wave tissue Doppler imaging (PWTDI) mode. The yellow circle indicated the myocardial region of interest in the basal anterior segment on apical two-chamber view. The region of interest by CFM mode correlated with the sample volume placement location using PWTDI mode in Fig. 3A. (A) was designed to contrast with (B), to demonstrate a higher-peak mean velocity of the basal anterior segment compared with the lower-peak mean velocity of the mid-inferior segment, and to demonstrate (B) the tissue Doppler imaging examination by CFM mode. The CFM mode demonstrated a delay in time-to-peak velocity (TpV)(t) of the mid-inferior segment measured from the onset of electrocardiogram R-wave to the peak mean velocity on time-velocity integral was 440 milliseconds, shown on the left frame. The yellow circle in both two-dimensional frames indicates the region of interest in the mid-inferior myocardial segment, which correlated with the sample volume placement on PWTDI mode shown in Fig. 3B. The TpV showed that a delay occurred after aortic valve closure. The peak velocity (v) was 3.69 cm/s, as shown in the right upper area of the figure.


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Fig. 5. (A) Total displacement at end-systole of the basal-anterior segment was between 4 and 6 mm, expressed in green, and correlated with the color bar shown on the right of the left frame. (A) was designed to contrast with (B), which showed a smaller total displacement (1.9 mm) of the mid-inferior segment, compared with the larger total displacement (4–6 mm) of the basal-anterior segment at end-systole. The time point at the end of the electrocardiogram T-wave is shown at the bottom of the right frame. (B) The total segmental displacement (D) of the mid-inferior segment was 1.9 mm at end-systole. The time frame was frozen and is shown on the electrocardiogram at the bottom of each frame. In the right upper area of the figure, the time to maximal displacement is shown. The color bar to the right on the left upper area represents the total displacement by color display.

A

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Fig. 6. (A) Strain image (SI) of the basal anterior segment shows that the peak strain (S) was −19% before aortic valve closure (AVC), and was −20% after AVC. When (A) and (B) were compared, the peak S was −31% for the mid-inferior segment, which was higher than the peak S for the basal anterior segment. (B) The SI of the mid-inferior segment showed an S of −31% on longitudinal axis, before AVC. A delayed second peak S was −28% at the time (t) of postsystolic shortening after AVC. The time to delayed second peak S was 305 milliseconds, shown in the right upper area of the figure. The SI measures the percentage change in dimension or length, normalized to the initial length, and was negative in percentage during systole on longitudinal axis on apical two-chamber view, which represented myocardial shortening during systole. The S value would be positive in percentage if the myocardial showed an abnormal lengthening during systole, which would usually represent myocardial aneurysm formation.

The legend for each figure illustrates the different results obtained using each parametric mode derived from TDI. A sample volume was placed at the same region of interest on the same segment, as similar as possible, in the different modes, which made the comparison more meaningful.

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Explaining Various TDI Modes, Including Parametric Derivatives, Using the Following Patient as an Example The 50-year-old male patient suffered an acute anterior wall myocardial infarction 4 years previously.


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Fig. 7. (A) Strain rate (SR) image showed a normal peak SR of −1.0 for the basal anterior segment. In a comparison of (A) and (B), the second peak SR shown on (A) occurred at the time of aortic valve closure (AVC). The mid-inferior segment had a higher positive SR of +1.5 on (B) than the smaller +0.3 for the basal anterior segment on (A) at the turning point before the second negative SR. This was suggestive of a higher myocardial deformation in the mid-inferior segment, more than that in the basal anterior segment. (B) SR imaging of the mid-inferior segment on longitudinal axis showed a peak SR of −2.2 before AVC, and there was another peak SR of −1.3 after AVC, at the time of postsystolic shortening. SRI depicts the rate of velocity change (1/second) along the distance between the two velocities.

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Fig. 8. (A) Tissue synchronization image (TSI) mode examination of the basal anterior segment depicted a shorter time-to-peak velocity (TpV) of around 85 milliseconds, coded as green, comparable to the color bar scale shown at the upper part of the left frame in the two-dimensional color sector. Comparing the color coded as red by TSI mode in the mid-inferior segment shown on both (A) and (B), the time velocity integral on the left in (B) showed a delayed TpV of 440 milliseconds at the time of postsystolic shortening. (B) TSI mode depicted the time scale of segmental TpV expressed by color bar green to red, as shown in the right upper area on the left upper frame. The electrocardiogram phasic scale is shown at the bottom of the right frame. The red mark on the electrocardiogram represented the total time scale on the present TSI mode. The TpV of the mid-inferior segment was coded as red, which correlated with a TpV of around 440 milliseconds, more than 330 milliseconds.

The patient received percutaneous coronary intervention for left anterior descending coronary artery stenosis. LVEF was 46% by ventriculography, and the patient showed hypokinesia of the anterior and anteroseptal segments. LVEF improved to 65%

on ventriculography at 1-year follow-up, and coronary angiography showed no restenosis at the previous stent site. Two-dimensional echocardiography 4 years later showed an adequate global LV contractility, without any segmental wall motion J Med Ultrasound 2008 • Vol 16 • No 3

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Conclusion Using PWTDI mode as the basic TDI examination to derive other parametric images would allow this novel technique more clinical applicability. Possible tedious and time-consuming work off-line after processing for data analysis may also become easier. More importantly, the fixed sample volume location inside the same myocardial segment performed by PWTDI may increase the intraobserver and interobserver agreement, thus making TDI a more comprehensive tool in interpreting the parametric data derived from PWTDI examinations. The data analysis in each mode would be more meaningful and could be helpful in patient decision making, and would be more widely accepted in clinical applications.

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