MRI Investigation of the Linkage Between Respiratory Motion of the Heart and Markers on Patient's Abdomen and Chest: Implications for Respiratory Amplitude Binning List-Mode PET and SPECT Studies

Abstract

Respiratory motion of the heart impacts the diagnostic accuracy of myocardial-perfusion emission-imaging studies. Amplitude binning has come to be the method of choice for binning list-mode based acquisitions for correction of respiratory motion in PET and SPECT. In some subjects respiratory motion exhibits hysteretic behavior similar to damped non-linear cyclic systems. The detection and correction of hysteresis between the signals from surface movement of the patient's body used in binning and the motion of the heart within the chest remains an open area for investigation. This study reports our investigation in nine volunteers of the combined MRI tracking of the internal respiratory motion of the heart using Navigators with stereo-tracking of markers on the volunteer's chest and abdomen by a visual-tracking system (VTS). The respiratory motion signals from the internal organs and the external markers were evaluated for hysteretic behavior analyzing the temporal correspondence of the signals. In general, a strong, positive correlation between the external marker motion (AP direction) and the internal heart motion (SI direction) during respiration was observed. The average ± standard deviation in the Spearman's ranked correlation coefficient (ρ) over the nine volunteer studied was 0.92 ± 0.1 between the external abdomen marker and the internal heart, and 0.87 ± 0.2 between the external chest marker and the internal heart. However despite the good correlation on average for the nine volunteers, in three studies a poor correlation was observed due to hysteretic behavior between inspiration and expiration for either the chest marker and the internal motion of the heart, or the abdominal marker and the motion of the heart. In all cases we observed a good correlation of at least either the abdomen or the chest with the heart. Based on this result, we propose the use of marker motion from both the chest and abdomen regions when estimating the internal heart motion to detect and address hysteresis when binning list-mode emission data.

Keywords: Cardiac respiratory motion; MRI; emission tomography; hysteresis; signal processing.

Fig. 1

(a) Layout of 7 external marker positions on the chest and abdomen of subjects. (b) Illustration of a subject undergoing imaging positioned in supine and feet-first orientation in the MR scanner with 3 cameras of the VTS employed for tracking external marker motion. The MR scanner and the VTS are temporally synchronized by having a signal from the MRI trigger the start of motion tracking. ECG and respiratory information from the pressure sensor is acquired simultaneously during MRI acquisition.

Fig. 2

(a) Illustrated is the positioning of navigator beams (shown as boxes) on the dome of the right hemi-diaphragm, the superior wall of the left ventricle, and the chest wall. The vertical lines in the transaxial and the coronal slices indicate the position of sagittal slices acquired along with the navigator signals. (b) Shown is example output from the 3 navigator beams as plots of the 1D gray-scale information perpendicular to each body interface versus time. These portray respiratory motion of the chest wall, heart, and diaphragm. (c) Illustrated are the 300 dynamic sagittal slices acquired when the heart was at mid-diastole. (d) Shown at the bottom along with portions of the EKG signal is the signal from the pressure sensor about the abdomen of the volunteers.

Fig. 3

(a) Shown is the plot of the displacements of the external chest and abdomen markers as a function of time for about 5 minutes as measured by the VTS for volunteer 3. (b) Plotted are the displacements of the internal heart, diaphragm and chest wall respiratory motions obtained by the respective navigator as a function of time. (c) Plot of the respiratory pressure sensor data during MR acquisition. The plots demonstrate the well defined spatial and temporal correlation between the internal and the external motion for this subject.

Fig. 4

Shown are the Spearman ranked correlation coefficients between the MRI Navigator heart motion and the VTS (a) chest and (b) abdomen markers versus marker number for 5 volunteers.

Fig. 5

The time series plots for Volunteers 2 (a), 4 (b) and 5 (c) show the VTS determined AP displacements of selected external chest and abdomen markers as a function of time during MRI acquisition. Also shown is the internal SI respiratory motion of the heart superior LV wall as determined by the Navigator. The Spearman ranked correlation coefficients between the MRI Navigator heart motion and the VTS chest (ρC) and abdomen (ρA) markers are also shown.

Fig. 6

Shown in (a) through (e) are 2D scatter plots of pair-wise comparisons of the navigator data for the heart and the diaphragm, and the VTS data for the external chest and abdomen markers for five volunteers. The straight lines are linear fits to the data with Pearson’s correlation coefficient r in upper left corner of each plot. The respiratory signals acquired from the navigator and VTS are separated into inspiration and expiration for better visualization of the inspiratory and expiratory trajectories of the heart and external markers. Note scales vary for each plot and that the higher sampling rate of the external markers is evident in the increased density of the points plotted.

Fig. 7

2D scatter plots of pair-wise comparisons between the navigator data of the heart and the down-sampled VTS data of the external chest and abdomen markers for two cases: (a) linear pattern (Volunteer 2), and (b) hysteretic pattern (Volunteer 4). The plots illustrate the correlation between the external markers (chest and abdomen) and the internal respiratory motion of the heart.

Fig. 8

2D scatter plots of pair-wise comparisons of the navigator data for the heart and the diaphragm, and the VTS data for the external chest and abdomen markers for one volunteer 4 are shown for (a) deep breathing and (b) shallow breathing acquired on different days. Note how these plots clearly show the hysteretic (a) and linear (b) behavior internally and externally.