Editorial: Derivation of respiratory gating signals from ECG signals
Venue: Journal of Nuclear Cardiology
Year: 2015

With improvements in the spatial resolution of PET, motion blur is becoming an increasingly dominant limiting factor of image spatial resolution, and is therefore receiving increasing attention. The use of cardiac gating is common practice in cardiac imaging as it enables quantification of ventricular blood volumes and ejection fractions, and is useful for visualization of small cardiac structures by capturing the heart mid stroke. In PET applications involving the lungs, diaphragm, and superior splenic organs, respiratory gating has become common practice to reduce motion blur associated with respiratory motion. Therefore, the technical ability to acquire dual gating signals in conjunction with the PET data has become almost ubiquitous, even if seldom used. Rejection of respiratory motion artifacts in cardiac PET can improve image spatial resolution, enabling visualization of smaller structures (e.g. the thin walls of the atria, focal plaque lesions) especially when applied in conjunction with cardiac gating. Further benefit may be derived in detection of incidental finding such as lung and liver lesions. Perhaps most importantly for the nuclear cardiology audience, however, motion related heterogeneity is reduced in tracer uptake (e.g. perfusion and viability) imaging. Images of myocardial walls with uniform tracer concentrations that run orthogonal to the direction of motion can underestimate tracer uptake intensity, while walls that run parallel to the motion do not suffer of this partial volume effect to the same extent; thus creating a typical pattern of symmetric uptake deficit. This effect is demonstrated in Figure 1 for a simplified LV mimicking ring phantom (left) with blur and without motion (center) and with added horizontal sinusoidal motion with peak to peak amplitude equal to the wall thickness (right) While cardiac gating signals are almost exclusively derived from electrocardiograms, various devices have been utilized to derive respiratory gating signals including optical tracking of surface markers, expansion bands, induction coils, impedance measurements, and pressure sensors. Potential down sides to the introduction of additional devices into the imaging suite is increased operational workload and costs associated with acquiring and maintaining the equipment. Thus, systems that are able to perform multiple functions may be advantageous. List mode derived gating has also been proposed, but requires vendor specific implementation and has not been widely utilized to date. In this issue of JNC, Todica et al evaluate the use of ECG derived respiratory triggers on small animal cardiac PET scans. The triggers were retrospectively embedded into the list mode data, enabling dual gated image reconstruction using the vendor provided reconstruction software. For validation purposes, reference respiratory triggers were also derived using a pressure transducer based sensor positioned beneath rats or pressure sensing belt in healthy human volunteers. In their work, respiratory gating was utilized to reject two thirds of the event data acquired at respiratory phases associated with respiratory motion. Therefore, the reconstructed images consisted of only the end exhalation phase, having less respiratory motion artifacts, but also lower count statistics. The remaining, accepted data were processed to reconstruct ECG gated image sequences that were analyzed to quantify left ventricular volumes and ejection fraction. These parameters were also generated using conventional pressure transducer respiratory gating which did not defer from the ECGderived respiratory gating data. Therefore, the authors concluded that ECG derived respiratory gating was a feasible alternative to dedicated hardware based gating.  Ran Klein, PhD, Department of Nuclear Medicine, The OttawaHospital, Ottawa, ON, K1Y 4E9, Canada; rklein@toh.on.