Positive EKG Negative Thallium Test

My husband is 49, non-smoker, gets fairly regular exercise, has cholesterol of 200 and no symptoms of chest pain etc. He does have strong family history, mother died young from blocked arteries, 7 siblings with bypass surgery, but many of them smoked.

Recently he had an EKG and Thallium stress test for preventative reasons.  The EKG while on the treadmill showed blockages.  The pictures from the thallium scan showed nothing.  He is scheduled for an angiogram and we wonder if this is truly necessary.  The only other anomaly he has is that his heart rate is consistently low, getting into the low 50's at rest.  He takes no medicine of any kind and does not drink.  What could cause the difference between low heart rate, abnormal EKG and the thallium pictures other than a blockage, that is somehow not seen on the scan?   thanks for any advice.