组织多普勒超声应用

组织多普勒超声应用

Current Applications of Tissue Doppler Imaging

李道輿醫師

高雄榮民總醫院

Tissue Doppler imaging (TDI) is a relatively new echocardiographic technique that uses Doppler principles to measure the velocity of myocardial motion. We describe the principles behind and the clinical utility of TDI. Principles of TDI

Doppler echocardiography relies on detection of the shift in frequency of ultrasound signals reflected from moving objects. With this principle, conventional Doppler techniques assess the velocity of blood flow by measuring high-frequency, low-amplitude signals from small, fast-moving blood cells. In TDI, the same Doppler principles are used to quantify the higher-amplitude, lower-velocity signals of myocardial tissue motion.

There are important limitations to TD interrogation. As with all Doppler techniques, TDI measures only the vector of motion that is parallel to the direction of the ultrasound beam. In addition, TDI measures absolute tissue velocity and is unable to discriminate passive motion (related to translation or tethering) from active motion (fiber shortening or lengthening). The emerging technology of Doppler strain imaging provides a means to differentiate true contractility from passive myocardial motion by looking at relative changes in tissue velocity.

TDI can be performed in pulsed-wave and color modes. Pulsed-wave TDI is used to measure peak myocardial velocities and is particularly well suited to the measurement of long-axis ventricular motion because the longitudinally oriented endocardial fibers are most parallel to the ultrasound beam in the apical views. Because the apex remains relatively stationary throughout the cardiac cycle,

组织多普勒超声应用

mitral annular motion is a good surrogate measure of overall longitudinal left ventricular (LV) contraction and relaxation.

To measure longitudinal myocardial velocities, the sample volume is placed in the ventricular myocardium immediately adjacent to the mitral annulus. The myocardial velocity above the baseline as the annulus descends toward the apex;

(2) Ea, early diastolic myocardial relaxation velocity below the baseline as the annulus ascends away from the apex; and (3) Aa, myocardial velocity associated with atrial contraction. The lower-case “a” for annulus or “m” for myocardial (Ea or Em) and the superscripted prime symbol (E') are used to differentiate tissue Doppler velocities from conventional mitral inflow. Pulsed-wave TDI has high temporal resolution but does not permit simultaneous analysis of multiple myocardial segments.

Figure 1

With color TDI, a color-coded representation of myocardial velocities is superimposed on gray-scale 2-dimensional or M-mode images to indicate the direction and velocity of myocardial motion. Color TDI mode has the advantage of increased spatial resolution and the ability to evaluate multiple structures and

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segments in a single view.

Clinical Applications of TDI

Assessment of LV Systolic Function

Systolic myocardial velocity (Sa) at the lateral mitral annulus is a measure of longitudinal systolic function and is correlated with measurements of LV ejection fraction and peak dP/dt. A reduction in Sa velocity can be detected within 15 seconds of the onset of ischemia, and regional reductions in Sa are correlated with regional wall-motion abnormalities. Incorporation of TDI measures of systolic function in exercise testing to assess for ischemia, viability, and contractile reserve has been suggested because peak Sa velocity normally increases with dobutamine infusion and exercise and decreases with ischemia. The technical difficulties of timely acquisition of both 2-dimensional and TDI data during exercise represent the major limitations to routine integration in stress testing.

Assessment of Diastolic Function

Traditional echocardiographic assessment of LV diastolic function relied on Doppler patterns of mitral inflow. Reflecting the pressure gradient between the left atrium and LV, transmitral velocities are directly related to left atrial pressure (preload) and independently and inversely related to ventricular relaxation. Because mitral inflow patterns are highly sensitive to preload and can change dramatically as diastolic dysfunction progresses, the use of mitral valve inflow patterns to assess diastolic function remains limited.

TDI assessment of diastolic function is less load dependent than that provided by standard Doppler techniques. Ea reflects the velocity of early myocardial relaxation as the mitral annulus ascends during early rapid LV filling. Peak Ea velocity can be measured from any aspect of the mitral annulus from the apical views, with the lateral annulus most commonly used. Because of intrinsic differences in myocardial fiber orientation, septal Ea velocities are slightly lower

组织多普勒超声应用

than lateral Ea velocities.

Validated against invasive hemodynamic measures, TDI can be correlated with [tau], the time constant of isovolumic relaxation. Lateral Ea velocities can be 20 cm/s or higher in children and healthy young adults, but these values decline with age. In adults >30 years old, a lateral Ea velocity >12 cm/s is associated with normal LV diastolic function. Reductions in lateral Ea velocity to <=8 cm/s in middle-aged to older adults indicate impaired LV relaxation and can assist in differentiating a normal from a pseudonormal mitral inflow pattern. Unlike conventional mitral inflow patterns, Ea is resistant to changes in filling pressure, although preload dependence is more pronounced in structurally normal hearts.

Novel Applications of TDI

A number of emerging applications for TDI are under active investigation. Estimation of LV Filling Pressures

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