Transthyretin cardiac amyloidosis (ATTR) cardiac amyloidosis

Discussion:

Tc-99m pyrophosphate (PYP) scan is used to diagnose transthyretin cardiac amyloidosis (ATTR) cardiac amyloidosis. The bone agent relies on calcium in amyloid deposits to bind to phosphate. 

The Perugini score is a qualitative measure comparing myocardial to rib uptake. A grade greater than 2 is considered a positive finding for ATTR cardiac amyloidosis.

The heart to contralateral lung (HCL) ratio is a quantitative measure comparing the counts of the myocardium with the contralateral lung. It is vital to not include the sternum, liver or fractures in the region of interest of the contralateral lung as it would confound the ratio. A ratio less than 1 is considered negative. A ratio greater than 1.5 is strongly suggestive. A ratio between 1 and 1.5 is equivocal.

The main differential diagnosis for this appearance includes myocardial infarction although it is usually more focal. 

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