One of the major limitations of CRT is the objective assessment of whether cardiac output and ventricular function are improved. As well, the precise location of where to pace the heart in order to optimize hemodynamic function needs to be determined. The objective assessment of successful CRT is a difficult clinical issue and should ideally be performed non-invasively. Traditional two-dimensional echocardiographic and Doppler indices have been used to assess the efficacy of CRT and include measuring cardiac output, looking at ventricular ejection times, visually assessing wall motion, and measuring the length of diastole using the mitral "E" and "A" waves. As most of the existing techniques are limited to assessing global function, more detailed methods of assessment are necessary in order to fully assess and optimize CRT.10, 11 Tissue Doppler Imaging (TDI) offers a more detailed analysis of regional cardiac function and allows quantitative measures to be obtained. TDI operates at high frame rates and can non-invasively map cardiac activation and add information related to the degree and location of cardiac dyssychrony. TDI and its derivatives allow: (1) measurement of myocardial velocities, which is based on the detection of the Doppler shift caused by the motion of myocardial tissue during the cardiac cycle; (2) visualization of tissue tracking, which color-codes tissue segments with similar displacements according to a color map; (3) measurement of regional strain rates, which describes the rate of deformation, or how quickly a segment of tissue shortens or lengthens; and (4) measurement of regional strain, which describes the deformation of an object (in this case, tissue) relative to its original state. 12, 13