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Abstract:
Purpose/Introduction: Short acquisition times and short effective echo
times are of prime importance for functional MR imaging studies of the
brain to reduce distortion artifacts and increase SNR. The spiral sequence,
specifically in the segmented version, offers these features and presents itself
in specific applications as an alternative to conventional EPI. Spiral imaging
is also less prone to flow and motion artifacts. But this method is more
demanding on the exact gradient performance and correction mechanisms
need to be implemented to achieve necessary readout gradient corrections.
Subjects and Methods: To test and illustrate the performance of the
implemented corrections, a geometry phantom was measured with a spin
echo sequence as a reference and compared with a segmented spiral-out
sequence. The measurements were performed on a 7T/ 60 cm Bruker
Biospec vertical wide bore monkey MR system. A saddle coil was used
in transmit and receive mode. Measurements of the k-space trajectories
and images were performed[1]. With the k-space trajectory of the readout
gradient the deviation from the theoretical course were calculated. From
these the k-space offset, the time delay and the slope and baseline of the
gradient courses were derived. The latter is done by comparing trajectories
with positive and negative amplitudes. These corrections were then applied
for spiral image acquisition and reconstruction.
Results: The image where no corrections are applied (Fig.1, middle) shows
that the edges of the rectangle inside the phantom are reproduced sharp and
at the first look not distorted. It reveals although that the geometry is not at
all correct compared to the reference image gained by the MSME sequence
(Fig.1, left). Regarding the edges of the phantom in the lower part of the
image a strong blurring occurs. Performing now the corrections before
acquiring and before reconstructing the image the geometry is reproduced
satisfactorily (Fig.1, right). The blurring artifact at the lower edge of the
phantom is nearly fully eliminated.
Discussion/Conclusion: The measurements of the trajectories and images
on a geometry phantom show the efficiency of the corrections. Blurring
artifacts are reduced. A misleading geometry like for an image measured
with an uncorrected spiral-out EPI sequence can be avoided by applying
the corrections. The corrections enable the spiral imaging sequence to be an
alternative to conventional EPI in specific applications utilizing short echo
times. [1] Takahashi A. et al. MagnResonMed 1995;34:446-456.
