Respiratory phase-matched attenuation correction improves uniformity of hypercapnea-stress PET myocardial perfusion images

Objectives The safety profile of adenosine (&analogs) has been questioned recently as a pharmacologic stress agent for myocardial perfusion imaging (MPI). Increased end-tidal partial pressure of CO₂ (PetCO₂), termed hypercapnea was investigated as an alternative hyperemic stressor.

Methods Ten healthy volunteers (9 male) underwent rest + hypercapnea (P_ETCO2 = 60 mmHg) + adenosine MPI using Rb-82 PET. A low-dose CT scan was acquired at normal end-expiration for attenuation correction of the PET images. ECG- and respiratory-gated images were reconstructed from 1.5 to 8 min after tracer injection. End-expiration gated and ungated (summed) stress images were scored from 0 to 4, using the AHA standard 17-segment model. A sum-stress-score (SSS) ≥ 4 (out of 68 maximum) was considered abnormal. End-expiration gated and ungated CO₂ SSS values were compared to ungated adenosine as the clinical standard.

Results Ungated perfusion images acquired at peak CO₂-stress demonstrated high apparent tracer uptake in the inferior wall, due to over-correction of attenuation effects from PET-CT misalignment due to the increased respiratory tidal volume (12 to 22 L/min; p<0.001). Ungated SSS values were higher using CO₂ vs adenosine stress (2.9 vs 1.4; p=0.004) consistent with an apparent increase in heterogeneity of tracer uptake in the myocardium. Respiratory gating allowed reconstruction of end-expiration CO₂-stress images with phase-matched attenuation correction. Myocardial uptake uniformity was improved in these end-expiration CO₂-stress images; the SSS was not significantly different vs adenosine (1.5 vs 1.4; p=0.4), suggesting that this approach may be used for stress MPI with rubidium PET-CT.

Conclusions Controlled CO₂ inhalation may be a viable alternative to adenosine for stress MPI, but requires respiratory-gating for accurate end-expiration imaging at peak stress. Post-stress imaging may be more practical for routine clinical application, following the return to normal breathing.

Research Support NSERC-ENGAGE grant