FRACTIONAL REPETITION TIME (TR) ENCODING IN MAGNETIC RESONANCE (MR) ELASTOGRAPHY
Publication Date: 2017-Feb-01
The IP.com Prior Art Database
A data acquisition scheme is proposed. The scheme uses scan repetition time (TR) to contain fractions of the driver motion cycle. The change of phase offsets in different phases is achieved by the constantly shifting the relative positions of the motion encoding gradient (MEG) and the driver motion between consecutive TRs. Change in the TR and the driver triggering scheme eliminates abrupt phase changes and discarded data acquisitions.
The present disclosure relates generally to magnetic resonance elastography and more particularly to technique for fractional repetition time (TR) encoding in MR elastography.
Generally, multi-phase acquisitions are obtained in magnetic resonance (MR) elastography. Each phase features a distinctive phase or time offset between the motion encoding gradient (MEG) and the driver motion cycle. In order to appropriately synchronize the MEG and the driver motion for achieving the desired phase offset, the scan repetition time (TR) is set as multiples of the driver motion cycle. Accordingly, a triggering pulse for the motion driver is played out in each TR.
However, such fixation of the TR unnecessarily prolongs the overall scan time. In addition, abrupt phase changes are observed in transition from one phase to the other. Further, to accommodate for the phase change, discarded data acquisitions (DDA), in which no data acquisition is made are carried out to achieve a steady state.
Figure 1 depicts a conventional encoding scheme. For ease of illustration, only the z-axis gradient where is MEG is played is shown along with simultaneous driver motion. The trigger pulse on the sloped slab profile (SSP) board shows when the driver motion is reset. The TR is set as a multiple of the driver cycles, 2 cycles in the illustrated example. As a result, the phase offset between the MEG and driver motion stays constant in the consecutive TRs. If a change of the phase offset of π/2 (0.25 cycle) is desired, the position of the triggering pulse needs to be shifted, as seen in TR = 2. This leads to discontinuity in the driver motion. Hence, motion encoding in TR = 2 is incorrect and considered as discarded data acquisition DDA. The actual data acquisition for the new phase offset is then made in the next TR.
DDA also add to the scan time. For instance, for an echo planar imaging (EPI) based MR touch protocol, where TR = 1 second, the total scan time is 16 seconds out of which 6 seconds is attributed to the DDAs.
It would be desirable to have an improved technique for repetition time (TR) encoding in MR elastography.
BRIEF DESCRIPTION OF DRAWINGS...