Incremental value of dedicated [201Tl] cadmium-zinc-telluride thallium SPECT over rest/stress [99mTc] technetium sestamibi SPECT MPI: a comparative analysis of fixed perfusion defects.

BACKGROUND: Rest/stress [99mTc]technetium sestamibi single-photon emission computed tomography myocardial perfusion imaging (SPECT MPI) is a mainstay for evaluating coronary artery disease. However, fixed defects on standard MPI present a well-documented clinical challenge, as they can represent either non-viable scars or viable hibernating myocardium. We aimed to examine the incremental value and rate of diagnostic reclassification provided by performing subsequent dedicated cadmium-zinc-telluride (CZT) [201Tl]thallium myocardial SPECT in patients presenting with fixed defects on standard sestamibi MPI.

METHODS: In this retrospective study, we analyzed 174 patients-totaling 2,958 myocardial segments-who underwent both protocols. Intermodality correlation was assessed, and the rates of segmental and patient-level reclassification (segments converting from <50% maximum uptake on sestamibi to ≥50% maximum uptake on thallium) were quantified.

RESULTS: While a moderately high correlation was observed between the baseline uptake scores of the two modalities (Spearman's r = 0.817 for rest sestamibi vs. thallium), thallium imaging provided substantial additional information, with 48.0% of segments classified as fixed defects on sestamibi SPECT demonstrating improved uptake on thallium SPECT. At the patient level, 78.9% of individuals who had fixed defects on the initial sestamibi SPECT had at least one segment reclassified. Receiver operating characteristic analysis showed that a rest sestamibi uptake score of <20% predicted a lack of reclassification with 99% specificity, although with low sensitivity (12%).

CONCLUSION: In a selected cohort of patients presenting with fixed defects on standard rest/stress sestamibi SPECT MPI, supplementary imaging with thallium myocardial CZT SPECT provided incremental data, altering the clinical interpretation in nearly 80% of patients. However, in the absence of an independent viability reference standard, this finding represents intertracer diagnostic reclassification rather than proven functional recovery. A rest sestamibi uptake of <20% demonstrated high specificity for non-reclassification; however, this cutoff should be interpreted cautiously as a potential gatekeeping finding when considering downstream testing rather than a rigid rule for omitting further myocardial viability evaluation.