xSPECT-based myocardial perfusion scintigraphy: feasibility of myocardial uptake quantitation in patients with suspected coronary artery disease – a pilot study

Introduction: Myocardial perfusion SPECT (MPS) is generally analyzed for the presence of relative regional perfusion defects, which does not account for possible interindividual differences in myocardial blood flow (MBF). Several attempts have been made to absolutely quantify MBF with SPECT/CT, with inconsistent results mainly due to the suboptimal kinetic of 99mTc-labelled perfusion tracers. An independent, standardized, and reliable parameter able to assess the presence of perfusion defects in clinical practice is highly warranted, and a quantitation of myocardial uptake at the “steady state” may fit this need. We performed a pilot study in phantoms and humans.

Methods: MPS imaging was acquired with a quantitatively calibrated Symbia Intevo 16 (Siemens Healthineers) using a prototype of xSPECT Cardiac (xSc). xSc employs retrospective gating, automated motion correction, and data driven respiratory motion correction and yields voxelized uptake in Bq/ml. We used an anthropomorphic thorax phantom with simulated ECG-gating to assess consistency of quantified myocardial uptake. We performed initial human studies on 6 patients (3 males, aged 50-83yrs). All underwent on the same day a stress/rest 82Rb-PET/CT (1110 MBq each), a Ca-score CT and a stress 99mTc-Sestamibi quantitative MPS (250 MBq). A quantitative MPS in rest condition was performed on the following day. Quantified uptake (Bq/ml, SUVmax and SUVpeak) was calculated for the myocardium (40% threshold segmentation). MPS-based coronary reserve (MPS-CR) was defined as stress-SUVmax/rest-SUVmax. Quantified 99mTc-Sestamibi uptake was compared to PET-derived global MBF.

Results: The quantitation of ^99mTc-activity was accurate using the torso phantom. In clinical studies, 5 Patients had unremarkable MBF on PET, while one patient had 3-vessel disease with globally reduced MBF. Patients with unremarkable PET had both higher stress MBF (3.51±0.60 ml/g/min) and stress-SUVmax (8.27±1.92) compared to the patient with 3-vessel disease (MBF 2.15 ml/g/min, stress-SUVmax 6.3). Similarly, patients with normal PET had higher myocardial flow reserve (2.95±0.96) and MPS-CR (1.19±0.06) compared to the patient with 3-vessel disease (2.12 and 0.88, respectively)

Conclusions: Absolute quantitation of myocardial uptake is feasible using the defined protocol and xSPECT Cardiac prototype. This is expected to yield improved diagnostic accuracy over standard MPS and will be evaluated in our subsequent study (ClinicalTrials.gov Identifier: NCT04583787)..