Body motion detection and correction in cardiac PET: Phantom and human studies

Abstract

Purpose: Patient body motion during a cardiac positron emission tomography (PET) scan can severely degrade image quality. We propose and evaluate a novel method to detect, estimate, and correct body motion in cardiac PET.

Methods: Our method consists of three key components: motion detection, motion estimation, and motion-compensated image reconstruction. For motion detection, we first divide PET list-mode data into 1-s bins and compute the center of mass (COM) of the coincidences' distribution in each bin. We then compute the covariance matrix within a 25-s sliding window over the COM signals inside the window. The sum of the eigenvalues of the covariance matrix is used to separate the list-mode data into "static" (i.e., body motion free) and "moving" (i.e. contaminated by body motion) frames. Each moving frame is further divided into a number of evenly spaced sub-frames (referred to as "sub-moving" frames), in which motion is assumed to be negligible. For motion estimation, we first reconstruct the data in each static and sub-moving frame using a rapid back-projection technique. We then select the longest static frame as the reference frame and estimate elastic motion transformations to the reference frame from all other static and sub-moving frames using nonrigid registration. For motion-compensated image reconstruction, we reconstruct all the list-mode data into a single image volume in the reference frame by incorporating the estimated motion transformations in the PET system matrix. We evaluated the performance of our approach in both phantom and human studies.

Results: Visually, the motion-corrected (MC) PET images obtained using the proposed method have better quality and fewer motion artifacts than the images reconstructed without motion correction (NMC). Quantitative analysis indicates that MC yields higher myocardium to blood pool concentration ratios. MC also yields sharper myocardium than NMC.

Conclusions: The proposed body motion correction method improves image quality of cardiac PET.

Keywords: body motion; bulk motion; cardiac PET; image reconstruction; motion correction; motion detection; motion estimation.

Figure 1:

(a) Illustration of the positioning of ROIs and line profiles for the calculation of myocardium TBR and wall thickness. Circular ROIs (4 mm in radius) were placed approximately at the center of the anterior, inferior, septal, and lateral regions. Another same-size ROI was drawn in the middle of the LV blood pool, (b) Five short-axis planes (from base to apex), in which a total number of 20 ROIs/profiles were made.

Figure 2:

(a–c) COM in x, y, and z directions, respectively, versus time for the phantom study with manually induced slow motion. Left and right-most vertical dashed lines indicate when motion started and ended, respectively, (d) The motion index of the COMs in a 25-second sliding window versus time. The parameter ρ is the threshold that separates MF from SF frames.

Figure 3:

Short axis-views of NMC, MC, and REF images for the phantom studies.

Figure 4:

COM (after removing outliers) in x, y, and z directions versus time for human subjects 1 (a), 2 (b), and 3 (c).

Figure 5:

(a) Motion index versus time for the subject 1. (b) Histogram of motion index for the same subject. The adaptive threshold ρ = 0.015 separates SF from MF.

Figure 6:

NMC, MC and REF images for subject 1 (FDG) in short-axis and horizontal long-axis views. The REF images were obtained by reconstructing the data in the selected reference frame. Arrows indicate the papillary muscle, which is visible in both MC and REF, but not in NMC images. The profiles on the right were made along the dashed line shown on the short-axis image on the left.

Figure 7:

NMC, MC and REF images for subjects 2 and 3 (TPP) in short-axis view. Arrows indicate the delineation of the structures.

Figure 8:

Myocardium TBR and wall thickness comparison between NMC, MC and REF images for human subjects. Overall, MC yields higher TBR and lower wall thickness than NMC.