A SPIRAL IN & OUT PULSE SEQUENCE DESIGN FOR RETROSPECTIVE CORRECTION SENSE

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Introduction Correction of motion artifacts still remains to be one of the most essential topics in MR. Especially in the case of uncooperative patients such as children and patients suffering from a medical condition that prevents them from staying stationary, accurate determination and correction of motion becomes a must for good image quality. In this study, we propose a spin-echo spiral in & out sequence for retrospective motion correction that is aimed to remove motion related artifacts in the case of in-planar rigid body motion, which includes only translational and rotational motion. The spiral in & out sequence designed for this study can be used to get low resolution navigator data for each interleave with no extra penalty in scan time. The modified SENSE reconstruction procedure uses that navigator data to find the motion parameters and eliminates the effects of undersampling in k-space. Materials and Methods A spin echo Archimedian spiral in & out pulse sequence is designed for this study according to the algorithm described in [1]. Due to the gradient system limitations, a spiral trajectory mostly starts off in slew rate limited region and switches to amplitude limited region after a certain time which is determined by the scan parameters. In the case of the spiral in & out trajectory used for this study, a spiral in trajectory is used to get a fully sampled low resolution image for each interleave, and the spiral out part constitutes one of the interleaves of the final high resolution image. One advantage of this pulse sequence is that the spiral in portion makes use of the dead time after the 180o degree pulse up to the echo time TE and this introduces no penalty for scan time in case of T2 weighting. The matrix size of the low resolution navigator data can be adjusted interactively before scan by the operator. This sequence has been tested on two normal volunteers using a 1.5T scanner (GE Signa LX, 11.0) with a high performance gradient system (Gmax = 50mT/m, SR = 150 mT/m/s) and an 8 channel head array (MRI Devices). The volunteers were asked to move their head inside the head coil by approximately 10-20 degrees for every 10 seconds during the scan to simulate in-planar rigid body motion. All human studies were approved by the review board of our institution. Other parameters used for the pulse sequence are as follows: TR/TE = 4000/56 ms, slice thickness/gap = 5/0 mm, 12 slices, FOV = 24 cm, matrix size = 256, interleaves = 32, NEX = 1, navigator matrix size = 32 and BW = 125 kHz. The data obtained from the scans were fed into a motion correction algorithm that uses the navigator images to accomplish co-registration and to obtain the amount of rotation and translation. After the determination of motion parameters, k-space trajectory, k-space data and the coil sensitivities are corrected accordingly by counter-rotating the k-space trajectories and applying a linear phase to k-space data. This motion correction introduces some gaps in k-space and causes aliasing in image domain. A modified version of the generalized SENSE algorithm that has a channel for each coil and for each interleave is used to remove aliasing and reconstruct the image. Results The results of motion correction are shown. The artifacts resulting from rigid body motion are significantly removed by the application of motion correction algorithm. The modified SENSE algorithm provides improvement in the final image quality by filling in the gaps in k-space resulting from the counter-rotation of k- space trajectories. This is apparent from the difference between the initial image and final image in the SENSE iteration. The spiral trajectory used in this study allows for better utilization of SENSE reconstruction due to the constant undersampling factor throughout k-space after rotation correction. In case of other trajectories like EPI, rotation of individual interleaf leaves large and arbitrarily spaced gaps in k-space which cannot be corrected by SENSE. An effective reduction factor, Reff, is used as a measure of the k-space undersampling and is defined as the ratio of the maximum distance between two spiral arms to the original k-space sampling density. For the motion corrupted data sets, Reff is 1.65, which is a reasonable value that can be corrected with SENSE. The navigator images obtained for all 32 interleaves and the motion corrected images are shown. Because of the motion, the subject is exposed to different combined coil sensitivity for each interleave. This results in navigator images having a slightly different intensity variation which might affect the registration. Acknowledgements This work was supported in part by the NIH (1R01EB002771), the Center of Advanced MR Technology at Stanford (P41RR09784), Lucas Foundation and Oak Foundation. References [1] Glover GH, MRM, 42:412-415 (1999). [2] Atkinson D, et al., MRM, 42:963-969 (1999) [3] Pruessmann et al, MRM, 46:638-651 (2001