Motion compensated spiral FISP MRI

#This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to the use of fast imaging with steady precession (FISP) and with motion compensated spiral k-space trajectories.

#In cardiac imaging using MRI techniques, rapid acquisition of MRI signals is desired. MR ventriculography is often performed in a gated mode using conventional gradient echo 2DFT techniques. These techniques often suffer from inadequate signal and inadequate contrast between blood and myocardium. Recently, MR scanners have been including a modification of these techniques using steady-state free precession (SSFP) or “FISP” techniques, which refocus all gradient lobes between excitations to preserve transverse magnetization for a longer time. These techniques lead to increased signal and increased blood-myocardium contrast, but cardiac-gating and breath-holding are still required to get good spatial and temporal resolution, because of the inherent inefficiencies of 2DFT scanning.

#Fluoroscopic ventriculography is advantageous because cardiac gating and breath-holding are not needed and because it can be used in a real-time interactive mode, similar to echocardiography. Fluoroscopic ventriculography requires an efficient scanning technique, such as spiral scanning. However, conventional spiral ventriculography sometimes suffers from inadequate contrast between blood and myocardium and inadequate signal.

#FIG. 1 illustrates an excitation pulse and magnetic field gradients for real-time spiral SSFP (FISP) cardiac imaging in accordance with an embodiment of the invention.

#FIGS. 2A and 2B illustrate a conventional magnetic field gradient spiral rewinder and a rewinder with a separate bipolar for motion compensation.

#FIG. 3 illustrates a magnetic field gradient with built-in motion compensation in accordance with an embodiment of the invention.

#Referring now to the drawings, FIG. 1 illustrates a pulse sequence in accordance with the invention for combining the contrast of SSFP (FISP) with the efficiency of spiral k-space scanning. An RF pulse is applied in combination with Gz gradient pulses for tilting nuclear spins in an object of interest with slide selection determined by the Gz gradient. Signals from the precession of the tilted nuclear spins are read out in the presence of Gx and Gy gradients. Each of the Gx and Gy gradients includes at the end thereof a rewinder with built-in motion compensation which optimizes for a minimum repetition time, TR, for decreased scan time and decreased sensitivity to inhomogeneity. Further, higher order moments are reduced to thereby reduce sensitivity to acceleration.

#Conventionally, spiral rewinders are provided with Gx and Gy gradients to bring the k-space scan back to the k-space origin as shown in FIG. 2A. Further, a spiral rewinder plus separate bipolar portions have been provided for motion compensation as shown in FIG. 2B. However, in accordance with the invention the rewinder for the Gx and Gy gradients as shown in FIG. 3 has built-in motion compensation which is optimized for a minimum repetition time and decreased scan time and provides decreased sensitivity to inhomogeneity. Further, the zero and first order moments are nulled to zero while higher order moments are reduced for a reduced sensitivity to acceleration.

#For the motion compensated rewinder design, each of the x and y rewinders are separately optimized for a constant slew rate, S. The time origin for the moment calculations is at the end of readout which is the start of the rewinder.