Ultra-high field systems provide phenomenal intrinsic signal sensitivity. Moreover, a number of contrasts (BOLD, SWI, ASL perfusion) disproportionally benefit from the increased field. Often, however, much of this sensitivity advantage is lost in uneconomical acquisition schemes due to low acquisition duty-cycle, inefficient k-space coverage, or suboptimal TE. In many instances spiral acquisitions optimally address all these issues. Unfortunately, being critically affected by all sorts of encoding imperfections spiral imaging has not yet made it into the MR scientist’s toolbox. However recent advances in enhanced signal modelling and reconstruction promise to break the barrier and render non-cartesian sampling schemes attainable for broader use. At 3T the mentioned obstacles have already been overcome [1-3]. In this work we explore single-shot spiral images at 7T with 1mm resolution, appealing SNR and image quality, acquired in 35ms. Images are algebraically reconstructed using a conjugate-gradient (CG) based algorithm that is based on an encoding model that incorporates static off-resonance maps and concurrently monitored encoding field dynamics.Data were acquired from a healthy volunteer on a Philips 7T Achieva system (Philips Healthcare) using a 32 channel head receive array (Nova Medical, Wilmington). 12 NMR field probes data were mounted on the head array and the signals acquired using a dedicated MR acquisition system [4]. K-space was covered by a five-fold undersampled Archimedean spiral, FOV 22cm, 0.9x0.9mm resolution, 1.5mm slice thickness, starting in the k-space center at a TE of 3ms and 41ms acquisition duration. Moreover an Archimedean outside-in spiral directly followed by an inside-out spiral was acquired (TE = 28ms, FOV 22cm, 1.25x1.25mm, 1.5mm slice thickness) [5]. In each scan ten slices were acquired with a 3s volume repetition time. The readouts were concurrently monitored up to full 2nd order in space. From a dual-echo (1ms TE spacing, water and fat in-phase) spin-warp pre-scan with identical geometry, off-resonance and coil sensitivity maps were computed. Algebraic reconstruction was performed with a conjugate-gradient algorithm incorporating concurrently measured field dynamics up to 1st order in space as well as the off-resonance and sensitivity maps [6, 7]. As the spin-warp and the spiral images are both reconstructed based on monitored field dynamics, they intrinsically share an identical geometry which is key for this image reconstruction based on additional parameter maps.

The resulting single shot spiral out images are displayed in Figure 1. All images independent of the actual readout trajectory show high quality regarding resolution and geometric consistency. The spiral image matches accurately the overlayed mask of the spin warp image used for off-resonance correction, here exemplarily shown for the first slice. In Fig. 1 the comparison of a fully sampled k-space (upper panel) and the undersampled single-shot case (lower panel) proves the advanced reconstruction modalities to be capable of handling challenging single-shot spirals without quality loss. This fact is again confirmed regarding the even more elaborate spiral in-out trajectories (Figure 2). The scheme was already published in earlier times, but benefits tremendously from the advanced reconstruction procedure.

Single-shot spiral imaging at high field strength is feasible. By measuring and monitoring respectively all relevant image encoding mechanisms (static off-resonance, gradient field dynamics including all delays and coupling terms, as well as coil sensitivities) and fusing them into the encoding model, high quality single-shot spiral images can be reconstructed. Only in this concerted manner, they are able to overcome the common hindrances of spiral imaging. Blurring and distortions are kept in check by off-resonance correction and field monitoring, whereas ghosting is tackled by parallel imaging. The sensitivity gain from robust single-shot spiral imaging at ultra-high field will benefit a number of applications, amongst them being BOLD, fQSM, and ASL perfusion.