Combination of Motion Adapted Gating with Non-Rigid Motion Correction for Free-Breathing MRI

Introduction The navigator gating technique reduced respiratory motion artifacts and is suited to abdominal MRI with patients who have difficulty in breath-holding. The motion adapted gating (MAG) approach [1] employs varying gating threshold depending on phase encoding steps, which accelerates gating to reduce scan time. However, even the MAG approach results in considerably longer scan time compared to a single breath-hold scan (e.g. 1.7-fold) to get images with sufficiently suppressed motion artifacts. In this work, we combined the non-rigid motion correction technique with the MAG approach to reduce scan time to the level comparable to breath-hold, while preventing image degradation. Methods Navigator Gating Threshold Setting Our navigator gating method used three acceptance thresholds depending on phase encoding steps (ky values) to be scanned. To accelerate gating, each threshold was calculated larger than conventional MAG approach with the use of the navigator pre-scan histogram data. For example, we decided the lowest threshold to include 10 percent of the pre-scan navigator data set counted from the position of end-expiration, and the data with the ky values of the central 5 percent were acquired when the diaphragm position fell under the threshold during the imaging period. Non-Rigid Motion Correction We applied non-rigid motion correction to sagittal abdominal imaging with linear expansion model in the SI direction. After inverse FFT of the k-space signals in the frequency encoding (SI) direction, each ky line data f0(x,ky) was corrected to f(x,ky) according to the displacement d from the end-expiration position as ( ) ( ) ( ) ⎩ ⎧ ≤ − = otherwise k x f x x if k x dx x f k x f