Cardiac Amyloidosis Imaging, Part 3: Interpretation, Diagnosis, and Treatment

Single- Versus Dual-Time-Point Imaging for Transthyretin Cardiac Amyloid Using 99mTc-Pyrophosphate

Nuclear medicine scintigraphy using 99mTc-pyrophosphate has proven valuable in the diagnosis of cardiac transthyretin amyloidosis in recent years. However, there is still confusion over the optimal imaging time points. The American Society of Nuclear Cardiology has recommended different imaging time points over the last decade. We aimed to determine whether single- or dual-time-point imaging is required for reporting purposes and which time point would be the most appropriate if a single time point was to be considered. Methods: Cardiac amyloid scans using 99mTc-pyrophosphate acquired from 2017 to 2023 were retrieved from our Picture Archiving and Communications System. Scans with static views and SPECT/CT images of the chest for both imaging time points, at 1 h (early) and 3 h (delayed) after injection, were included. Each study was independently read by 3 nuclear medicine physicians. Original clinical reports using both imaging time points were used as a reference to calculate the accuracy of a single time point. Results: In total, 70 patients were included in this study. Reports of cardiac amyloid studies using any single-time-point imaging were highly sensitive, accurate, and specific. There was agreement among all readers. Of the 140 datasets reported by each reader, 4 scans were classified as equivocal, requiring more imaging for confident reporting. Conclusion: Single-time-point imaging showed an accuracy comparable to the dual-time-point imaging in diagnosing cardiac transthyretin amyloidosis. This was further validated by agreement among the 3 readers. Early time-point imaging is preferred, and additional delayed imaging can be acquired when the early result is equivocal.

Cardiac Amyloidosis: Role of the Endomyocardial Biopsy

Amyloidosis is characterized by protein misfolding and extracellular deposition of insoluble beta-sheet fibrils. It represents a heterogeneous disease with different aetiologies and clinical manifestations. Cardiac involvement (ie, cardiac amyloidosis) has an adverse prognosis, early diagnosis and amyloid typing being crucial for treatment. Recently, the non-invasive diagnosis of CA has increased using imaging techniques, particularly in TTR forms with no monoclonal protein. In the other settings and with inconclusive peripheral biopsy, the EMB allows the demonstration of amyloid by Congo red staining under polarized light, whereas amyloid typing is performed by using immunohistochemistry, immune-electron microscopy and/or proteomics.

Cardiac Amyloidosis Imaging, Part 2: Quantification and Technical Considerations

^99mTc-pyrophosphate imaging has been around for a long time. In the 1970s, it was used to image recent myocardial infarction. However, it has recently been recognized for its value in detecting cardiac amyloidosis, leading to widespread use across the United States. Increased use led to considerable procedure variability. As the evidence base to support formal guidelines was being developed, experts from several professional medical societies issued imaging and interpretation recommendations titled "ASNC/AHA/ASE/EANM/HFSA/ISA/SCMR/SNMMI Expert Consensus Recommendations for Multimodality Imaging in Cardiac Amyloidosis: part 1 of 2-Evidence Base and Standardized Methods of Imaging." To reach a consensus on a protocol that would benefit the bulk of laboratories, the experts considered several parameters and radiotracer kinetics. The most critical parameters concerned injection-to-imaging delay and planar imaging versus SPECT. Accordingly, the standardized protocol recommends the injection of 370-740 MBq (10-20 mCi) of ^99mTc-pyrophosphate with imaging 3 h later. Planar images of the chest are acquired in the anterior and lateral views accompanied by SPECT images. Both the planar and the SPECT images are used to semiquantitatively grade the degree of myocardial uptake compared with the amount of uptake in the ribs using a 0-3 scale. A grade of 2 or 3 on the SPECT images is considered positive for cardiac amyloidosis. The planar images are used to calculate a heart-to-contralateral-lung ratio. A ratio greater than 1.3 at 3 h helps to confirm the diagnosis of cardiac amyloid if the SPECT images have positive findings. This article is part of a 3-part series in this issue of the Journal of Nuclear Medicine Technology Part 1 details the etiology of cardiac amyloidosis and ^99mTc-pyrophosphate imaging acquisition parameters. Part 2, this article, describes the procedure evolution over 50 y, image processing, and quantification. It further discusses radiotracer kinetics and 2 important technical considerations: injection-to-imaging delay and planar imaging versus SPECT. Part 3 covers study interpretation along with cardiac amyloidosis diagnosis and treatment.

Cardiac Amyloidosis Imaging, Part 1: Amyloidosis Etiology and Image Acquisition

Cardiac amyloidosis is a systemic form of amyloidosis in which protein-based infiltrates are deposited in myocardial extracellular space. The accumulation of amyloid fibrils causes the myocardium to thicken and stiffen, leading to diastolic dysfunction and, eventually, heart failure. Until recently, cardiac amyloidosis was considered rare. However, the recent adoption of noninvasive diagnostic testing, including ^99mTc-pyrophosphate imaging, has revealed a previously undiagnosed sizable disease prevalence. Light-chain amyloidosis (AL) and transthyretin amyloidosis (ATTR), the 2 primary types, account for 95% of cardiac amyloidosis diagnoses. AL results from plasma cell dyscrasia and has a very poor prognosis. The usual treatment for cardiac AL is chemotherapy and immunotherapy. Cardiac ATTR is more chronic, usually resulting from age-related instability and misfolding of the transthyretin protein. ATTR is treated by managing heart failure and using new pharmacotherapeutic drugs. ^99mTc-pyrophosphate imaging can efficiently and effectively distinguish between ATTR and cardiac AL. Although the exact mechanism of myocardial ^99mTc-pyrophosphate uptake is unknown, it is believed to bind to amyloid plaque microcalcifications. ^99mTc-pyrophosphate imaging has a 97% sensitivity and nearly 100% sensitivity for identifying cardiac ATTR when the AL form of the disease is ruled out through serum free light-chain and serum and urine protein electrophoresis with immunofixation testing. Although there are no published ^99mTc-pyrophosphate cardiac amyloidosis imaging guidelines, the American Society of Nuclear Cardiology, Society of Nuclear Medicine and Molecular Imaging, and others have published consensus recommendations to standardize test performance and interpretation. This article, part 1 of a 3-part series in this issue of the Journal of Nuclear Medicine Technology, describes amyloidosis etiology and cardiac amyloidosis characteristics, including the types, prevalence, signs and symptoms, and disease course. It further explains the scan acquisition protocol. Part 2 of the series focuses on image/data quantification and technical considerations. Finally, part 3 describes scan interpretation, along with the diagnosis and treatment of cardiac amyloidosis.