Technetium Tc-99m pyrophosphate

Technetium Tc-99m pyrophosphate (Technescan PYP, and generic/branded equivalents such as Phosphotec) is an old bone- and cardiac-imaging kit, FDA-approved for skeletal imaging of altered osteogenesis and, since the 1970s, as an adjunct in diagnosing acute myocardial infarction by direct uptake into necrotic tissue. Its label indications remain that dual scope (plus gated blood pool and gastrointestinal bleeding imaging via in vivo red blood cell labelling); it does not carry an FDA-labelled cardiac amyloidosis indication, and its use for that purpose is guideline-driven, off-label with respect to the original label wording, and now the dominant reason it is ordered in nuclear cardiology.

The evidence base for amyloid imaging is Gillmore et al. (2016, JAMA Cardiology, n=1,217: 857 with confirmed cardiac amyloidosis, 360 controls), which found 99% sensitivity and 86% specificity for ATTR cardiac amyloidosis using grade 2-3 myocardial uptake on planar/SPECT imaging, rising to 100% specificity and positive predictive value when combined with the absence of a monoclonal light-chain clone on serum/urine testing. ASNC/AHA/ASE/EANM/HFSA/ISA/SCMR/SNMMI multisociety expert consensus recommendations (with a 2021 addendum) formalised the protocol: SPECT or SPECT/CT confirmation of myocardial uptake is required (planar imaging alone is not diagnostic), typically read at both 1 hour and 3 hours after injection, with a visual grade of 2 or 3 classed as strongly suggestive of ATTR-CM, grade 1 as equivocal, and grade 0 as not suggestive.

FDA labelled dosing for cardiac imaging is 370-555 MBq (10-15 mCi) intravenously, with imaging historically recommended 60-90 minutes post-injection for acute infarct detection; amyloid imaging protocols from the nuclear cardiology societies use similar activities with imaging windows out to 3 hours to reduce blood-pool interference.