Single shot whole brain imaging using spherical stack of spirals trajectories Jakob Assländer a, ⁎, Benjamin Zahneisen a , Thimo Hugger a, b , Marco Reisert a , Hsu-Lei Lee a , Pierre LeVan a , Jürgen Hennig a a Department of Radiology, Medical Physics, University Medical Center Freiburg, Breisacher Str. 60a, 79106 Freiburg, Germany b Bruker BioSpin MRI GmbH, Rudolf-Plank-Straße 23, 76275 Ettlingen, Germany abstract article info Article history: Accepted 24 January 2013 Available online 4 February 2013 Keywords: Magnetic resonance encephalography MREG Fast fMRI B 0 inhomogeneities Susceptibility gradients Off-resonance MR-encephalography allows the observation of functional signal in the brain at a frequency of 10 Hz, permitting filtering of physiological “noise” and the detection of single event activations. High temporal resolution is achieved by the use of undersampled non-Cartesian trajectories, parallel imaging and regularized image reconstruction. MR-encephalography is based on 3D-encoding, allowing undersampling in two dimensions and providing advan- tages in terms of signal to noise ratio. Long readout times, which are necessary for single shot whole brain imaging (up to 75 ms), cause off-resonance artifacts. To meet this issue, a spherical stack of spirals trajectory is proposed in this work. By examining the trajectories in local k-space, it is shown that in areas of strong susceptibility gradients spatial information is fundamentally lost, making a meaningful image reconstruction impossible in the affected areas. It is shown that the loss of spatial information is reduced when using a stack of spirals trajectory compared to concentric shells. © 2013 Elsevier Inc. All rights reserved. Introduction Echo planar imaging (EPI) (Mansfield, 1977) is the established technique for functional magnetic resonance imaging (fMRI). For whole brain imaging EPI has a temporal resolution of typically 2–3 s. This is sufficiently fast compared to the blood oxygenation level depen- dent response (BOLD) (Ogawa et al., 1990), but there are several issues making it desirable to achieve high temporal resolution for fMRI. A single event hemodynamic response function (HRF) has significant sig- nal fluctuations lasting approximately 20 s. Therefore about 10 data points are measured in the meantime when acquiring with a standard EPI protocol. With MR-encephalography (MREG) 200 data points are supplied for the same period of time, allowing a better analysis of the onset and shape of the HRF (Zahneisen et al., 2011). Furthermore the increased amount of data points improves the statistical power from which single event fMRI can benefit(LeVan et al., 2012), as well as the study of functional connectivity of networks (Lee et al., 2013; Lin et al., 2008). Fast acquisition also allows direct filtering of respiration and cardiac artifacts, since they are not aliased in the frequency domain anymore (Hennig et al., 2007; Posse et al., 2012). While in the original implementation of MREG (Hennig et al., 2007) only coil sensitivities were used for spatial encoding, there have been several approaches for combining a small amount of gradient encoding with the spatial information multi coil arrays provide. An early attempt was to perform a fully sampled sagittal 2D-EPI measurement, while encoding the third dimension purely by coil sensitivities, which is referred to as inverse imaging (Lin et al., 2006). Further approaches were to use a small number of projections for 2D functional imaging (Grotz et al., 2009) or a rosette k-space trajectory for single shot 3D imaging (Zahneisen et al., 2011). For all single shot acquisition techniques (and this includes standard EPI) the image quality is strongly affected by susceptibility induced local field inhomogeneities. For EPI this is reasonably well understood and leads to the well-known susceptibility artifacts — primarily signal attenuation and geometric distortions. Distortions predominantly occur along the phase encoding direction and can be corrected with suitable methods (Jezzard and Balaban, 1995). For non-Cartesian trajectories the off-resonance behavior is more complex. As outlined by (Zahneisen et al., 2012), self-intersecting tra- jectories like rosettes (Zahneisen et al., 2011) and single shot radial trajectories (Hugger et al., 2011) suffer from a high sensitivity to off-resonance, T 2 ⁎ -decay and gradient imperfections. Concentric shells (Zahneisen et al., 2012) are designed to have no intersections. They have a more benign off-resonance behavior, allowing longer readout times and therefore higher spatial resolution, while keeping the tem- poral resolution below 100 ms. Like rosettes, concentric shells sample k-space symmetrically. As a consequence of the symmetry their point spread functions (PSF) do not show any off-resonance dependent shifts and therefore no image distortions. On the downside off-resonance leads to blurring and signal dropout. In practice geometric distortions are easier to NeuroImage 73 (2013) 59–70 ⁎ Corresponding author. Fax: +49 761 270 38310. E-mail addresses: jakob.asslaender@uniklinik-freiburg.de (J. Assländer), zahneisen@gmx.de (B. Zahneisen), thimo.hugger@bruker-biospin.de (T. Hugger), marco.reisert@uniklinik-freiburg.de (M. Reisert), hsu-lei.lee@uniklinik-freiburg.de (H.-L. Lee), pierre.levan@uniklinik-freiburg.de (P. LeVan), juergen.hennig@uniklinik-freiburg.de (J. Hennig). 1053-8119/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.neuroimage.2013.01.065 Contents lists available at SciVerse ScienceDirect NeuroImage journal homepage: www.elsevier.com/locate/ynimg