Heart cath showed 52% left ven. Ejection fracture, while echo two weeks later showed 12%.  what could cause difference?  muga scan is scheduled soon. 

Technical error? Echocardiograms are not too accurate at times. Orher possibility is that you had a temporal condition of the heart but most likely is a technical error. Cardiac cath is more accurate and muga scan will confirm it.
Different techniques. These are different techniques that have some variability (usually 5%) between them. 40% off is a lot - unless some intervening event it's hard to understand. Muga is the gold standard to determine ejection fraction.
Operator dependent. Estimating ejection fraction is operator dependent with many techniques. It often depends upon experience. Muga (multi gated acquisition) scan is the most accurate since it depends on end diastolic and end systolic counts rather than the geometry of the ventricle.