Myocardial perfusion imaging (MPI) is a main noninvasive method for detecting myocardial ischemia with high diagnostic accuracy for identifying significant coronary artery disease (CAD). A normal MPI, especially in those with an intermediate-to-high CAD likelihood, indicates a very low risk of cardiac death or nonfatal myocardial infarction, bolstering MPI's role in patient selection for revascularization. Nevertheless, traditional MPI, which focuses on relative myocardial perfusion defects, has its limitations. This method can miss cases, particularly in patients with microvascular dysfunction or multivessel disease, due to its reliance on the best-perfused region as a reference, potentially leading to underestimation of risks.1

Recent advances have embraced positron-emission tomography (PET) for quantitative assessments of myocardial blood flow (MBF) and myocardial flow reserve (MFR).2 PET, with its high temporal resolution, offers a more precise representation of regional tracer kinetics, making it invaluable for determining the functional significance of coronary lesions.3,4 The increasing availability of hybrid PET and CT systems and the usage of the 82-Rubidium (82-Rb) tracer, which does not necessitate an onsite cyclotron, has fueled interest in transitioning PET from research to routine clinical use.5

Nonetheless, there are technical challenges to address, including misalignment between PET data and attenuation correction maps, which could lead to diagnostic inaccuracies.6 Cardiorespiratory motion during scans presents a significant challenge in maintaining consistent imaging results. As a patient breathes and the heart contracts and relaxes, the precise positioning and orientation of cardiac structures can shift. This can lead to blurred images or potential misinterpretations of the data, especially when high-resolution imagery is crucial for accurate diagnoses.7

Another challenge that complicates the imaging process is the phenomenon known as “myocardial creep” which was first described by Friedman at al.8 This occurs as the stress agents begin to wear off leading to changes in the respiratory depth and frequency of the patient. This change in respiratory pattern causes a slight repositioning or shifting of the heart within the thoracic cavity. As a result, there can be discrepancies in the time-activity curves that are essential for evaluating MBF. Such discrepancies might lead to potential misinterpretations or the need for repeated scans to ensure accuracy.

Despite a known phenomenon, myocardial creep has seen limited proposed correction techniques. The few techniques that exist primarily cater to dynamic reconstructions used for myocardial blood flow assessments, focusing only on inter-frame motion correction. Despite integrating an averaged image of later frames in a motion corrected, dynamic image series might present a solution, such images could suffer from increased noise and the residual blur due to absent intra-frame motion correction.

Amidst the prevailing challenges in quantitative MBF assessment using cardiac PET, the latest edition of the Journal of Nuclear Cardiology features a study by Lassen et al.9 The study sets out with two main objectives in focus: first, to determine the feasibility of retrospectively extracting data on myocardial creep using solely 82Rb-PET raw (listmode) data, and second, to delve into the ramifications of motion correction on these myocardial creep events. In addressing the challenge of myocardial creep, the paper utilized triple-gated reconstructions. This technique encompassed a dual-gated cardiorespiratory protocol for every pinpointed myocardial creep occurrence. After this, image co-registration was carried out, leading to a triple-motion corrected image series (3xMC). Results derived from the 82Rb-PET MPI were then benchmarked against the fractional flow reserve (FFR), acknowledged as the gold standard in coronary intervention.

The current study underscores the potential of PET raw data, especially listmode files, to retrospectively extract pivotal insights on both respiratory motion and myocardial creep. The study revealed an average of two myocardial creep events during stress MPI sessions, emphasizing the need for refined detection and correction strategies. Implementing these corrections, particularly for cardiorespiratory and myocardial creep events, resulted in significant improvements in the Stress Total Perfusion Deficit (sTPD) and Ischemic Total Perfusion Deficit (iTPD) assessments, both pivotal metrics in myocardial perfusion evaluation.

However, certain limitations were evident. One key constraint was the small cohort size of the study, which primarily consisted of healthy individuals. This could introduce a bias, potentially tilting the results towards motion-induced perfusion deficits due to the stressing agent. Another limitation pertains to the duration of the Triple-Motion Correction (3xMC) protocol. While this approach, which corrects for cardiac, respiratory, and myocardial creep motions, has proven effective, its time-intensive nature, necessitating more than three hours for reconstruction, might pose challenges in routine clinical applications.

In conclusion, the investigation by Lassen et al offers valuable insights into the evolving landscape of cardiac imaging techniques. While considerable advancements have been made, there remains a consistent drive for further improvement and refinement. This study serves as a testament to the ongoing efforts in the field, setting the stage for future research aimed at enhancing the precision, efficiency, and applicability of cardiac PET techniques in varied clinical contexts.