^99mTc-Pyrophosphate Scintigraphy for the Diagnosis of ATTR Cardiac Amyloidosis: Early Phase Planar and SPECT Images versus Late Phase SPECT Images

Objectives: ^99mTc-PYP bone scintigraphy can accurately diagnose transthyretin cardiac amyloidosis (ATTR-CA) and distinguish it from light-chain cardiac amyloidosis (AL-CA), with diffuse myocardial uptake of ^99mTc-PYP being very specific for ATTR-CA. Two methods have been validated to analyze ^99mTc-PYP scintigraphy: a quantitative heart-to-contralateral (H/CL) ratio calculated on planar images obtained 1h post injection, and a visual grading system based on planar or SPECT images obtained at 3h. The purpose of this multicentric study was to compare the 1h H/CL ratio to the visual grading system at 3h and to compare the visual grading system at 1h and 3h.

Methods: A total of 122 consecutive subjects, from 2 centers, who underwent both 1h and 3h planar and SPECT ^99mTc-PYP imaging were retrospectively included. Circular regions of interest drawn over the heart and mirrored on the contralateral thorax were used to calculate the H/CL on 1h planar anterior images. Myocardial uptake intensity was compared to bone activity on both 1h and 3h SPECT images using the visual grading system. H/CL between 1.0 and 1.5 were considered equivocal for ATTR-CA, as was Grade 1 uptake on 3h imaging, as per current ASNC Practice Points. Agreement was evaluated using Cohen’s Kappa.

Results: 77 (63%), 10 (8%), 12 (10%), and 23 (19%) subjects had grade 0, 1, 2, and 3 myocardial uptake respectively at 3h. 10 (8%), 81 (66%) and 31 (25%) subjects had 1h H/CL ratio <1.0, between 1.0 and 1.5, and >1.5 respectively. Amongst those with H/CL between 1.0 and 1.5, 9 (11%) had grade 1 uptake at 3h while 67 (83%) had grade 0 and 5 (6%) and grade 2 or 3. The proportion of equivocal studies was significantly less when using the 3h grade compared to the H/CL method (p<0.0001). For two subjects, 3h grading and H/CL yielded discordant results; one had biopsy proven ATTR-CA with a H/CL of 0.91 and a grade 2 uptake while the other had biopsy proven AL-CA with a H/CL of 1.96 and a grade 0 uptake. Uptake grade was the same at 1h and at 3h on SPECT imaging in most patients, with 5 subjects having different grading at 1h and 3h. One subject had grade 1 uptake at 1h and grade 0 at 3h, 3 had grade 3 uptake at 1h and grade 2 at 3h, and 1 had grade 2 uptake at 1h and grade 3 at 3h. The agreement between uptake grade at 1h and at 3h was 99% when combining grade 2 and 3 (both considered positive for ATTR-CA as per current ASNC Practice Points).

Conclusions: A significant proportion of PYP scans yielded equivocal H/CL on 1h imaging, with significantly fewer equivocal results using the visual grading system on SPECT images at 3h, which in turn seems very comparable to the visual grading system on SPECT images at 1h. These results indicate that the use of 1h imaging H/CL alone, with current ASNC Practice Points interpretation thresholds, may lead to high proportion of equivocal studies, and 3h imaging remains necessary in a large proportion of patients.

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