Background: Heart failure (HF), the leading cause of death in the westernworld, develops when a cardiac injury or insult impairs the ability of the heart to pump blood and maintain tissue perfusion. Cardiacresynchronization therapy (CRT) is a well-established treatment in selectedpatients with drug-refractory heart failure. In order to improve the responserate of patients treated with CRT, imaging can provide information onmechanical dyssynchrony, viability, and cardiac venous anatomy. Variousechocrdiographic methods have been proposed to quantify intra-ventriculardyssynchrony, including M-mode echocardiography and tissue Dopplerimaging (TDI). Nuclear imaging techniques can also be used for theassessment of mechanical dyssynchrony. Chen and coworkers developeda novel approach for assessing LV dyssynchrony from gated myocardialperfusion SPECT.Aim of the work: The aim of this study is to compare the degree of LVdyssynchrony as assessed with phase analysis from gated myocardialperfusion SPECT (GMPS) and the degree of LV dyssynchrony as assessedwith TDI in patients with HF, depressed LVEF, and wide QRS complex, tovalidate the use of phase image in gated SPECT technique in selection ofpatients eligible for CRT. Subjects and Methods: 30 patients were included, all scheduled for CRT,ATL-HDI 5000 colored echocardiograph was utilized forEchocardiographic imaging, TDI was measured as standard deviation (SD) of time for beginning of QRS to peak systolic velocity in 12 segment 6basal and 6 mid segment all done by expert Echocardiographer.Myocardial perfusion imaging (MPI) and phase analysis of Gated SPECTacquisition was performed within 1 hr of the injection of 20 mci TC99msestaMIBI using dual head Siemens Gamma Camera (Symbia E) utilizing cedar Sinai Gated SPECT 8 frames/ cycle 32 view each 20second over 180degree arc, software phase analysis parameters is histogram bandwidthwhich include 95% of the element of the phase distribution. Studypopulation was divided into two groups: responders and non-respondersaccording to increase of at least 15% of LVEF after 3 month by 2D Echo.Results: After 3 month follow up patients were divided into 23 (76%) wereresponders and 7 (24%) were non-responders according to 2 D echo withLVEF 32.4±5.9 for responders versus 25.3±4.8 for non responders, p value0.007. Histogram bandwidth pre-implantation in all 30 patients mean138.2±92.1° to be 116.0±75.7° post- implantation with p value 0.049. InCRT responders pre-implantation 150.4±96.5° to be post-implantation122±80.9° p value 0.012 , in non responders group 98±66.4° to be postimplantation96.4±56.3° p value 0.962. TDI PSV in 30 patients66.4±30.3msec to be 45.7±21.8 msec post-impamtation p value 0.001, inCRT responders 70.6±32.3 msec to be 45,6±22.3 p value 0.001, in nonresponders 52.9±17.6 to be post implantation 46±22.1 p value 0.06. ROCanalysis was done to reveal that Phase analysis parameter acted in betterway to predict CRT response with histogram bandwidth 55.5°Area undercurve 68.9% sensitivity 87% specificity 42.9% positive predictive value(PPV)83.3% negative predictive value (NPV) 50% compared to TDIsensitivity 52.25% , specificity 71.4% PPV85.7% NPV 31.3% Whenapplying histogram bandwidth cuttoff 55.5 degree dyssynchrony wasillustrated in 20 (87%) patients in comparison to 14 (60%) patients withEcho TDI, there was significant difference in sensitivity of histogrambandwidth compared to TDI with p value 0.043.Conclusion: The LV dyssynchrony assessed from GMPS may be morepredictive to response to CRT. Histogram bandwidth of GMPS Tc99mSestMIBI may be more predictive of significant response to CRT as compared to TDI. This parameter seems more optimal for assessment ofLV dyssynchrony with gated SPECT.