Odd-number hybrid EPI
Michael H. Buonocore, David C. Zhu
Department of Radiology, UC Davis Medical Center, Sacramento, CA 95817
Introduction: This abstract introduces and illustrates the advantages
of the "odd-number hybrid" echo planar imaging sequence, defined
by using an odd number of interleaved EPI k-space acquisitions to
generate images with high-spatial resolution. In "hybrid" (also referred to as "interleaved", or "segmented") EPI, two or more separate interleaved acquisitions are used to acquire different sets of ky
lines of data that are combined into one full data set. Usually, an
even number of acquisitions is used to build the data set (1,2).
Hybrid EPI is particularly sensitive to the generation of ghost artifacts, which arise from the differences in eddy currents and gradient
ramping that occur during even (left-to- right trajectory along kx) ky
lines versus the odd (right-to-left trajectory along kx) ky lines. In single shot EPI, these effects cause phase errors (denoted "e" and "o"
below) that result, upon 2D Fourier Transformation, in N/2 ghost artifacts. These can often be eliminated using phase error information
obtained from non-ky encoded reference scans. In even-number hybrid EPI sequences, more ghost artifacts occur because, after combining the data, more complicated modulation of eddy current and
gradient ramping phase errors exists as a function of ky. For example,
a two-hybrid sequence will produce e-e- o-o-e-e-o-o, etc. pattern of
phase errors as a function of ky, and separate image ghosts at N/4 and
3N/4 of the FOV will be generated.
Methods: The ghost correction algorithm based on image phase correction (3) does not require reference scans, and instead uses images
reconstructed separately from even and odd echos to provide estimates of the eddy current and gradient ramping phase errors. However, it cannot be used for hybrid EPI sequences unless the phase errors follow an alternating pattern (i.e. e-o-e-o-e-o etc.) as a function
of ky (3). Even-number hybrid sequences cannot produce this alternating pattern, but odd-number hybrid sequences do. For example, a
three-hybrid sequence will produce this pattern by running the first
hybrid for the Oth, 3d, 6th, etc. lines, the second hybrid for the 1st, 4 th,
7th, etc. lines, and the third hybrid for the 2nd, 5 , 8th, etc. lines. The
first and third hybrids are started in the even kx direction, the second
hybrid in the odd kx direction, and the first acquired lines of the first
and second hybrid are not used in image reconstruction (see Fig 1).
Results: Fig. 2 shows a 128 x 128 two-hybrid EPI sequence (composed of two 64 x 128 acquisitions) with two ghosts (N/4 and 3N/4
spatial shifts) arising from the e-e- o-o- pattern of phase errors. The
ghosts can, in principle, be removed with phase information from
four reference scans, but good cancellation is not reliable. Ghost correction by image phase correction is not possible, because the parent
image is everywhere overlapping with ghost. Fig. 3 shows the 128 x
128 odd-number hybrid sequence (composed of three 44 x 128 acquisitions) causes a single ghost (N/2 spatial shift) to be generated.
Fig. 4 shows that the image phase correction algorithm removes the
ghost well.
Conclusion: The odd-number hybrid EPI sequences have a unique
advantage, relative to the even-number hybrid sequences, in that they
generate only a single ghost (shifted by N/2) that can be reliably removed with image phase correction. Odd-number hybrid EPI might
be successful in systems using high-performance body gradient coils
in which eliminating phase and eddy current ghosts via reference
scan has been unreliable.
References:
1. Butts K., Riederer SJ, Ehman RL, et. al. Magn. Reson. Imaging.
31, 67-72 (1994). 2. McKinnon GC. Mag. Reson. Med. 30, 609-616
(1993). 3. Buonocore MH, Gao L., Mag. Reson. Med. 38 (1): 89100.
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Fig. 1: The 3-hybrid EPI
sequence generates alternating even-odd traversals
of k-space. 3 x44 = 132 ky
lines (0-131) are acquired,
of which 2-129 are used for
image reconstruction.
Fig. 2: Two-hybrid EPI: 128
x 128 image acquired with
two 128 x 64 interleaved EPI
acquisitions. Two ghosts are
generated from an e-e-o-oetc. pattern of phase errors in
the ky direction.
Fig. 3: Three-hybrid EPI:
128 x 128 image acquired
with three 128 x 44 interleaved EPI acquisitions. One
ghost is generated from an eo-e-o- etc. pattern of phase
errors in the k, direction.
Fig. 4: Image phase correction alg6rithm,applied to
Fig. 3 image, removes ghost.
The algorithm depends on at
least some of the "parent"
image (i.e. central strip) not
overlapping ghost.