Patrick Alexander Liebig^{1,2}, Robin Martin Heidemann^{2}, and David Andrew Porter^{3}

^{1}Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany, ^{2}Siemens Healthcare GmbH, Erlangen, Germany, ^{3}Fraunhofer MEVIS, Bremen, Germany

### Synopsis

**A new approach to Echo-Planar Imaging (EPI) is
introduced under the name Zigzag-Aligned-Projections (ZAP) that replaces the
blipped phase-encoding (PE) gradient with the modulus of the readout (RO). This
comes with two significant advantages: the reduction of acoustic noise due to
the modified PE gradient and the higher efficiency due to continuous data
sampling. ZAP EPI is the only EPI derivate that combines Cartesian GRAPPA using
a fixed Kernel size with continuous data sampling. The reduction in acoustic noise
was verified experimentally and volunteer images were acquired and processed with
two reconstruction techniques.**### Purpose

In
standard echo-planar Imaging (EPI)

^{1}, an oscillating trapezoidal
readout (RO) gradient is combined with a blipped phase-encoding (PE) gradient,
resulting in a rectilinear k-space trajectory, which can be easily combined
with parallel imaging

^{2}. No data are acquired during the blipped PE
gradient because the trajectory would deviate from a Cartesian path at the
edges of k-space in the RO direction. A significant reduction in sound pressure
can be achieved by using a sinusoidal waveform

^{3}, which preserves the
rectilinear k-space trajectory (Fig. 1A). Acoustic noise is reduced further by
using a constant PE gradient

^{3}. However, this presents challenges for
parallel imaging because the k-space trajectory is no longer Cartesian, but an
imperfect zigzag (Fig. 1B). Data acquisition however can now take place
continuously, allowing a more efficient use of the gradient system. In this
paper we introduce a new EPI sampling scheme – the Zigzag-Aligned-Projections
(ZAP) EPI - which adapts the constant PE scheme to give a true zigzag
trajectory in k-space. Therefore the two advantages (higher efficiency and
lower acoustic noise) of the constant PE EPI are maintained and can now be combined
with standard parallel imaging techniques, such as GRAPPA

^{2}.

### Methods

The new sampling scheme replaces the constant PE
gradient with a low-amplitude variable gradient that is the modulus of the RO waveform
to give a true zigzag trajectory (Fig. 1C). The data are processed with a 1D phase
correction, a 1D interpolation for the variable density sampling along the
zigzag and are finally time reversed . Due to time reversal, the
data are now aligned on a Cartesian grid. However, phase errors of higher order appear
between odd and even echoes . This will result in
folding artifacts in PE direction. The zigzag sampled data can be reconstructed
using an interlaced Fourier Transformation (iFT)

^{4}. In addition, for
parallel imaging, the data can be separated into odd and even echoes to give
two data sets with consistent phase, which can be processed using a fixed
GRAPPA kernel as in the Cartesian-sampled case

^{2}. Afterwards the data
are combined to yield the final image.

### Results

An image reconstructed using iFT is shown in
Fig. 2A and an image reconstructed using GRAPPA with an acceleration factor of
2 is shown in Fig. 2B. Images were acquired from healthy volunteers using a
Siemens MAGNETOM Prisma 3T system and acoustic noise measurements were
performed on a Siemens MAGNETOM Skyra 3T system.

### Discussion

The image reconstructed with iFT is prone to
artifacts . The GRAPPA reconstructed
image shows no artifacts and this is therefore the preferred reconstruction
technique. The acoustic noise measurements showed that the effect of the PE
gradient of the ZAP sampling scheme on the overall acoustic noise is negligible
and at the same level as the acoustic noise generated by the constant PE EPI
scheme.

### Conclusion

We have shown that a true zigzag k-space trajectory
can be achieved with EPI by replacing the constant-amplitude PE gradient with a
waveform which is the modulus of a sinusoid. The phase errors relating to the
alternating RO gradient can be corrected by either an iFT or a GRAPPA-based
reconstruction.
Acoustic noise is on the same level as the constant PE EPI – regarded as the
quietest EPI. Further measurements are
planned to verify the effect of higher efficiency of the ZAP EPI readout compared
to the standard blipped-PE approach to EPI.

### Acknowledgements

We would like to thank Dr. Annette Stein for conducting the noise measurements with us and Dr. Mario Zeller for his valuable advice.

### References

[1] Mansfield P., J Phys C: Solid State Physics 1977; 10:L55-L58.

[2] Griswold et al, Magn Reson Med 2002, 47:1201-1210.

[3] Zapp et al, J Magn Reson Imaging 2012, 36:581-588.

[4] Sekihara and Kohno, Magn Reson Med 1987, 5:485-491.