Overview and Analysis of the Utility of 1- and 3-Hour 99mTechnetium-Pyrophosphate Planar and SPECT/CT Imaging in the Diagnosis of Transthyretin Cardiac Amyloidosis

Objectives: The use of 99mTechnetium-Pyrophosphate (99mTc-PYP) for the diagnosis of transthyretin cardiac amyloidosis (ATTR) is a relatively new indication for a radiotracer that has been in use for decades. The American Society of Nuclear Cardiology (ASNC) has set forth guidelines for the interpretation of 99mTc-PYP imaging for the evaluation of ATTR, including both 1-hour quantitative and 3-hour semi-quantitative assessments (Tables 1 and 2). However, since introduction of this study at our medical center in October 2018, discrepancies between the 1-hour and 3-hour methods of image assessment have arisen. In this work we provide an overview of the current ASNC guidelines for the diagnosis of ATTR with 99mTc-PYP, review data from our institution and discuss guideline modifications which may optimize diagnostic accuracy and allocation of resources while performing this study.

Methods: 99mTc-PYP imaging was introduced at our medical center in October 2018. Between October 2018 and December 2019, thirty-five patients underwent evaluation for ATTR with 99mTc-PYP imaging. Analysis included 1-hour quantitative comparison of myocardial and contralateral lung radiotracer uptake utilizing planar imaging (H/L ratio), 3-hour semi-quantitative visual comparison of myocardial and rib radiotracer uptake utilizing planar and SPECT imaging, and 3-hour SPECT/CT imaging to assess the presence or absence of radiotracer within the myocardium. Per ASNC guidelines, studies were deemed “positive” or “strongly suggestive” for ATTR with H/L ratio 1.5 or greater (Table 1) or semi-quantitative visual grade of 2 or greater (Table 2). Studies were deemed “equivocal” for ATTR if a semi-quantitative visual score of 1 or H/L ratio 1-1.5 were recorded. Studies were deemed “negative” if a semi-quantitative visual score of 0 or H/CL ratio less than 1 were recorded. Dictated reports of the 35 patients were reviewed and the various methods of analysis were compared. Results: Of the 35 patients evaluated, 10 patients had a 1-hour H/L ratio of 1.5 or greater (28.6%). Of these 10, 1 had a 3-hour planar visual grade of 0, 1 had a visual grade of 1, and 8 had a visual grade of 3, therefore 2/10 had negative semi-quantitative visual grade that contradicts positive H/L value. Further analysis of these 10 patients with H/L greater than 1.5 revealed 7/10 exhibited myocardial radiotracer uptake on SPECT/CT, while 3/10 had no myocardial uptake on SPECT/CT (2 of these 3 represent the patients with a 3-hour visual grade of 1 or 0). Given this data, 30% of the patients with a 1-hour H/L ratio suggestive of ATTR were determined to be false positives given the lack of myocardial uptake seen on SPECT/CT. Of the 25 patients with 1-hour H/L ratios less than 1.5, none were found to have myocardial radiotracer uptake at 3 hours on SPECT/CT. Conclusion: 1-hour 99mTc-PYP planar imaging may be useful for excluding ATTR but cannot be relied upon to accurately diagnose ATTR. Discrepancies between 1-hour and 3-hour imaging are likely caused by cardiac blood pooling in patients with other etiologies of cardiac dysfunction which falsely elevates the 1-hour H/L ratio. We suggest studies with a 1-hour planar imaging H/L ratio less than 1.5 be considered negative for ATTR and the study terminated. Furthermore, we suggest institutions with the capability of SPECT/CT or SPECT imaging rely solely on the 3-hour planar and SPECT/CT or SPECT imaging to diagnose ATTR, with positive studies requiring a semi-quantitative visual grade of 2 or greater and SPECT/CT or SPECT evidence of myocardial radiotracer uptake.