Vastly undersampled Isotropic Projection Reconstruction imaging with Multi-half-Echo (VIPR ME). A. Lu1, O. Wieben2, T. M. Grist1, W. F. Block1. 1University of ...
Vastly undersampled Isotropic Projection Reconstruction imaging with Multi-half-Echo (VIPR ME) A. Lu1, O. Wieben2, T. M. Grist1, W. F. Block1 1
University of Wisconsin-Madison, Madison, WI, United States, 2University of Freiburg, Freiburg, Germany
A multi-half-echo technique is presented that dramatically improves the data sampling efficiency of 3D PR sequences. K-space trajectory deviations are measured quickly and are corrected on a per sample basis. These corrections allow for sampling throughout the gradient waveform, including ramps and the half echoes formed during the gradient dephaser and rephaser. This capability was implemented in VIPR sequences to significantly increase the data acquisition efficiency with only slight increases in TR. INTRODUCTION 2D projection sampling techniques which acquire multiple radial lines per TR have achieved greater success as more attention has been placed on determining the deviations in the k-space trajectory that are caused by eddy currents [1-4]. Current 2D multi-echo PR sequences calculate the phase difference in the image domain between projections of near opposite polarity. The phase difference indicates a k-space shift that can be applied across the projection to align the k-space data with other projections. Since the single correction factor does not indicate how the k-space error accrued, only k-space diameters or partial diameters can be acquired. By extending the trajectory characterization of Duyn from 2 to 3 dimensions [5-6], we can measure the k-space trajectories for the entire gradient waveform for each physical axis in less than one second. The accuracy of the measured trajectories allows us to view the gradient dephaser, readouts, and rephaser as a series of half echoes that each collects a different radial line. When implemented with an undersampled technique such as VIPR [7], the multiple echoes increase SNR by increasing acquisition time, but also improve SNR and CNR dramatically by minimizing the undersampling artifact. MATERIALS AND METHODS The VIPR Multi-half-Echo sequence (VIPR ME) was implemented in both RF-spoiled gradient and SSFP mode. The pulse sequence diagram and k-space trajectory for a four-half-echo train are shown in Fig. 1a and 1b respectively. Data acquisition occurs continuously after beginning at the k-space origin, proceeding to the edge of k-space, rotating slightly to a new angle, and then returning to the origin. This process can be repeated multiple times. Ramp sampling considerably shortens the TR, and thus the fourhalf-echo sequence shown has about the same TR as the normal full echo sequence. The gradient waveforms of a four-half-echo train are similar to the full echo VIPR sequence except that two small blips are used to vary the projection angles. RESULTS AND DISCUSSION Studies were performed on a 1.5 T scanner (Signa CVi and Echospeed Plus; GE Medical Systems, Milwaukee, WI). The effect of eddy currents is shown in Fig. 2 where a four-half-echo train is investigated. The deviations from the ideal k-space trajectories along the echo train in all 3 axes are shown in Fig 2a. Significant improvement in image quality can be seen in the eddy current corrected image (Fig. 2c). In Fig. 3, a 24 cm FOV was imaged using the four-half-echo sequence with a readout matrix equivalent to 256 × 256 × 256. These images demonstrate high SNR and high contrast between different tissues that allows for quick analysis of brain anatomy.
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Fig. 1 (a) Gradient waveforms and b) k-space trajectories for 4-half-echo train (2 echo trains from 2 TRs are shown)
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Fig. 2 a) Measured k-space deviations for each axis b) Uncorrected image and c) Corrected image shows dramatic improvement (arrows).
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Fig. 3 T2 volume of entire head and neck in only 40 s with 0.9 mm isotropic res. a) Coronal and b) sagittal reformats TR:4ms
CONCLUSIONS The VIPR ME sequence made possible by effective eddy current correction can significantly increase the data acquisition efficiency while maintaining a reasonable TR. The benefits of increased SNR were demonstrated in the volunteer studies. This technique makes it possible to acquire volumetric fast SSFP images with excellent spatial resolution, coverage, and SNR. REFERENCES 1. V. Rasche et al., MRM 42 324 (1999) 2. D. Peters et al., Proc. 9th ISMRM, 1882 (2001) 3. T. Schaeffter et al. 10th ISMRM, 21 (2002) 4. A. Larson et al., MRM 46 1059, (2001) 5. J. Duyn et al., JMR, 132 150 (1998) 6. B. Dale, 10th ISMRM, 2334 (2002) 7. A. Barger et al. MRM 48 297 (2002)
ACKNOWLEDGEMENT
Research was supported by NIH R01-HL 62425, GE Medical Systems and the Whitaker Foundation.
Proc. Intl. Soc. Mag. Reson. Med. 11 (2003)
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