Pulse sequence timing diagram for spiral cine-DENSE MR imaging. The displacement-encoding module is played out immediately following the ECG trigger. The spiral -space imaging sequence modified to include the DENSE encoding and unencoding gradients is used to rapidly sample the displacement-encoded longitudinal magnetization at multiple cardiac phases. The angle notation such as means the rotation about the axis by an angle of 90° in the left-hand convention. The RF pulse of is accomplished by setting the phase of the RF pulse of to be 180°. means the rotation about the axis by a small flip angle of . is the spatial frequency imparted by the displacement encoding gradient, which is proportional to the area of the gradient and is given by .
Example of phase-reconstructed DENSE images in a healthy volunteer (subject 1). Left: MR magnitude image. Brain tissue appears white, and CSF appears gray. Right: Phase-reconstructed DENSE images acquired at multiple time points after the R wave. The time interval between two neighboring cardiac phases is 102 ms. Displacement was encoded in the horizontal direction in the top row and in the vertical direction in the bottom row.
Magnitude image and colored displacement maps of the brain in the middle sagittal plane (top row, from subject 2) and in the coronal plane (bottom row, from subject 3) at 340 ms after the R wave. The directions of the motion are indicated by the arrows. (Square box in the top left image is a nine-pixel region to be shown in Fig. 4.)
The pixelwise motion trajectories in a nine-pixel region within the pons (corresponds to the square box in the sagittal proton image in Fig. 3). The black dots indicate the initial positions at the first cardiac phase, with the resulting trajectories in lighter black curve. The scale of the motion amplitude with respect to the pixel size is modified for better visualization.
Dynamic displacement maps of the brain of a healthy volunteer (subject 2) at multiple cardiac phases from the R wave of ECG. The time interval between two neighboring cardiac phases is 102 ms. Displacement was encoded in the horizontal direction in the top row and in the vertical direction in the bottom row.
Example of displacements as a function of time after the R wave for selected ROIs in the midsagittal plane (from subject 2). A total of 18 cardiac phases after the R wave of ECG were acquired in this example.
Measurements of peak 2D displacement and time to peak for selected regions of interest. The results were shown as deviation (STD).
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