Prevalence and characteristics of intra-frame motion in 13N-ammonia PET/CT dynamic acquisitions

Heart motion negatively impacts PET/CT dynamic acquisitions, leading to estimation errors up to 5 times in myocardial blood flow (MBF) and myocardial flow reserve (MFR) values. The shifting of the heart from one frame to another (Inter-frame Motion) has already been described in literature with several conventional motion correction tools . A prototype data driven motion correction (DDMC) algorithm tracks heart-contour motion for each second of acquisition time, including the motion that takes place within each frame [Intra-frame motion (IntraMo) as well. This project aims to describe the prevalence and characteristics of cardiac IntraMo in 13NH3 PET/CT dynamic acquisitions with the use of DDMC.

25 patients without myocardial ischemia (normal) and 27 patients with ischemia according to cardiac PET/CT examination were retrospectively included. Additional data considered for the inclusion of patients was CAD-history, follow-up and PCI results (if performed)]. DDMC was used to retrieve motion information in all frames Some frames directly after radiotracer injection were discarded from analysis as heart-wall motion tracking was suboptimal. Data was retrieved in left-right transaxial (x), anterior-posterior transaxial (y) and axial (z) directions. Values of Motion Speed (speed) and Absolute Average Motion Amplitude (amplitude) were calculated as measures of IntraMo. Amplitude was characterized according to its intensity (>3mm, >6mm and >9mm). Differences in prevalence and characteristics of IntraMo between axes/phases were tested using independent t-test.

The z direction was the axis with statistically significant (p<0.05) higher motion speed, irrespective of the phase. Also, this direction presented significant higher motion amplitude, reflected by a higher prevalence of amplitudes >3mm (z=52.4%, y=7.04%, x=2.6%), >6mm (z=18.16%, y=0.57%, x=0.15%) and >9mm (z=6.61%, y=0.04%, x=0.09%), irrespective of the phase. Prevalence of amplitudes >6mm and >9mm was neglectable (<1%) in rest and stress for the x and y directions. Stress phase showed significantly higher motion speed, regardless the axis. Also, there was significant higher prevalence of amplitudes >3mm in the stress phase for the x (rest=0.77%, stress=2.6%) and y axis (rest=2.82%, stress=7.22%). The z axis shows an equal presence of amplitudes >3mm, >6mm and >9mm in the rest and stress phase. All findings were similar in the normal and ischemic groups.

In dynamic 13NH3 PET/CT, heart-wall motion is unrelated to the presence of ischemia. Movement is more prevalent in the axial axis (breathing motion) and has the highest speed and amplitude. Breathing amplitude is similar in rest and stress, suggesting that this motion is not affected by stressor drugs. However, pharmacological stress could play a role in other types of motion (e.g. discomfort movement) as amplitude and speed are increased in the other two axis during the stress phase.