Zigzag-Aligned-Projections in Echo-Planar Imaging
Patrick Alexander Liebig1,2, Robin Martin Heidemann2, and David Andrew Porter3

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


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.


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 imaging2. 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 waveform3, which preserves the rectilinear k-space trajectory (Fig. 1A). Acoustic noise is reduced further by using a constant PE gradient3. 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 GRAPPA2.


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 case2. Afterwards the data are combined to yield the final image.


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.


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.


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.


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


[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.


Fig.1: (A) Blipped EPI. (B) EPI with constant PE. (C) EPI with modulus sinusoidal PE. Note, non-equidistant sampling along RO in all three cases.

Fig.2: Images reconstructed using iFT (A) and GRAPPA (B).

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)