Four-dimensional MRI using three-dimensional radial sampling with respiratory self-gating to characterize temporal phase-resolved respiratory motion in the abdomen

Abstract

Purpose: To develop a four-dimensional MRI (4D-MRI) technique to characterize the average respiratory tumor motion for abdominal radiotherapy planning.

Methods: A continuous spoiled gradient echo sequence was implemented with 3D radial trajectory and 1D self-gating for respiratory motion detection. Data were retrospectively sorted into different respiratory phases based on their temporal locations within a respiratory cycle, and each phase was reconstructed by means of a self-calibrating CG-SENSE program. Motion phantom, healthy volunteer and patient studies were performed to validate the respiratory motion detected by the proposed method against that from a 2D real-time protocol.

Results: The proposed method successfully visualized the respiratory motion in phantom and human subjects. The 4D-MRI and real-time 2D-MRI yielded comparable superior-inferior (SI) motion amplitudes (intraclass correlation = 0.935) with up-to one pixel mean absolute differences in SI displacements over 10 phases and high cross-correlation between phase-resolved displacements (phantom: 0.985; human: 0.937-0.985). Comparable anterior-posterior and left-right displacements of the tumor or gold fiducial between 4D and real-time 2D-MRI were also observed in the two patients, and the hysteresis effect was shown in their 3D trajectories.

Conclusion: We demonstrated the feasibility of the proposed 4D-MRI technique to characterize abdominal respiratory motion, which may provide valuable information for radiotherapy planning.

Keywords: 3D radial sampling; 4D-MRI; hysteresis; radiotherapy planning; respiratory motion; self-gating.

Figure 1

The sample Self-Gating (SG) projection profile time series for the first (a) and second (b) SG line. The effect of eddy current is apparent in the first SG series (a) (shown as the superimposed high frequency signal variation), which may reduce the robustness of motion estimation. This artifact is greatly suppressed in the second SG series (b).

Figure 2

The proposed spoiled gradient recalled echo (GRE) sequence with 3DPR trajectory, 2D golden means ordering and one-dimensional (1D) self-gating (SG). (a) 4D-MRI imaging sequence showing each SG k-space group (dashed arrow) inserted in the superior-inferior (SI) direction at every segment of 15 radial projections, giving a temporal interval of ~98 ms between each SG group. A total of 73,005 projections were collected with 4867 SG lines after an approximately 8 minute scan. (b) 3D k-space trajectory showing data collection via radial 2D golden means ordering corresponding to (a). Note that the radial lines collected in previous segments are grayed and the SG lines are represented via dashed arrows.

Figure 3

Retrospective respiratory phase sorting in k-space demonstrated in a healthy volunteer (a) and a patient (b). The superior-inferior (SI) respiratory motion displacements represented in the respiratory curve were extracted via a principle component analysis (PCA) based method and served as a surrogate for respiratory phase throughout the acquisition (position index vs. time). Each peak (circle), representing end-expiration, was identified. Projection group outliers such as those involved in the respiratory cycles with abnormal time period (rectangle) and inconsistent expiratory amplitude (vertical oval) or those with large respiratory phase drift (horizontal oval) were discarded while only valid projection groups were assigned to respiratory phases between 1 and 10, shown as the black circles on the bottom graph. As shown here, the healthy volunteer showed a relatively stable breathing pattern, while the patient showed occasional irregularities.

Figure 4

Phantom study. a) A commercial Dynamic Breathing Phantom system placed outside the MR scanner room (I) was used to produce simulation signals mimicking human respiratory motion. Through an air pump (II) and tube, the signals were used to drive a box filled by gadolinium-doped water, which served as an imaging target (III). During the scan, the box executed reciprocating motion along the z-axis of the magnet at a frequency of 18 cycles/min, giving a set of phase resolved images (phase 1, 3, …, 9) reformatted from 4D-MRI, where the dashed line is drawn for a better visualization of the target motion at each respiratory phase (b). (c) A single frame of the real-time 2D-MRI image series. d) Measured SI displacement series, comparing between 4D-MRI and real-time 2D-MRI at each respiratory phase.

Figure 5

Healthy volunteer. a) Phase-resolved sagittal and coronal images (phase 1, 3, …, 9) reformatted from the 4D MRI image series throughout the entire respiratory cycle. Dashed lines are drawn for better visualization of organ motion at each respiratory phase. b) A single frame of the corresponding real-time 2D-MRI image series. c) Measured SI displacement series, comparing between 4D-MRI and real-time 2D-MRI at each respiratory phase.

Figure 6

Patient A. a) Phase-resolved sagittal and coronal images (phase 1, 3, …, 9) reformatted from the 4D MRI image series showing well delineated gold fiducial (arrows) throughout the entire respiratory cycle. Dashed lines are drawn for better visualization of organ motion at each respiratory phase. b) A single frame of the corresponding real-time 2D-MRI image series.

Figure 7

Patient A. a–c) Measured displacement series, comparing between 4D-MRI and real-time 2D-MRI at each respiratory phase for SI (a), AP (b), and LR (c) directions. d) 3D visualization of the fiducial trajectories over ten respiratory phases, showing the hysteresis effect.

Figure 8

Patient B. a) Phase-resolved sagittal and coronal images (phase 1, 3, …, 9) reformatted from the 4D MRI image series, showing well delineated tumor (arrows) throughout the entire respiratory cycle. Dashed lines are drawn for better visualization of organ motion at each respiratory phase. b) A single frame of the corresponding real-time 2D-MRI image series.

Figure 9

Patient B. a–c) Measured displacement series, comparing between 4D-MRI and real-time 2D-MRI at each respiratory phase for SI (a), AP (b), and LR (c) directions. d) 3D visualization of the tumor trajectories over ten respiratory phases, showing the hysteresis effect.