Nuclear Cardiology: A 53 year old woman being evaluated for dyspnea

Nuclear Cardiology case image

The above images represent the exercise dual isotope study (stress sestamibi/rest thallium) of a 53-year old woman being evaluated for dyspnea. In 1991 transthoracic echocardiogram had suggested an ASD, which was not confirmed by transesophageal echo. What is the abnormality?
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