If you use SPECT-CT, you can image at 1 hour and do just fine.
Better actually, because it is easier on patients. Case below shows it may be less likely to have a false negative.
@NotNoninvasive has done nice work on this
Case report of HMDP washout at 3hrs in cardiac amyloidosis.
Key points:
1. 99mTc has more stable tagging to HMDP than PYP, leading to less free 99mTc in blood pool
2. HMDP may wash out of the myocardium more quickly than PYP
3. Consider imaging with HMDP prior to 3hrs