Heart failure affects more than 5 million people in the United States, and the number increases by more than 0.5 million every year.Acute decompensated heart failure accounts for over one million hospital admissions per year, and the estimated direct and indirect cost for heart failure in 2006 is $29.6 billion.Cardiac resynchronization therapy (CRT) has been approved by FDA as a treatment of heart failure patients refractory to conventional therapy.However, 20-40% of the patients who selected for CRT based on the conventional criteria do not respond to CRT.We developed a quantitative technique, phase analysis, which automatically measure left ventricular dyssynchrony from ECG-gated single-photon emission computed tomography myocardial perfusion imaging study.We have validated this technique with tissue Doppler imaging and shown that this technique can reproducibly predict the response to CRT in patients with heart failure induced conduction disturbances and ventricular dyssynchrony.This speech will introduce the technical background of phase analysis, Demonstrate the tool with patient examples, and discuss its clinical indications and future directions.