Ultra-high field MRI holds considerable promise for advanced diagnostic imaging due to its unique contrast properties and high baseline sensitivity. This superior sensitivity enables shorter scans and can be traded for higher spatial or temporal resolution. In practice, however, many high-field protocols make grossly inefficient use of the available sensitivity. Cause are high-field specific conditions (SAR limitations, long T1/short T2) that enforce lengthy repetition times, in combination with short readouts. In the ensuing small acquisition duty cycle the superior high-field sensitivity is squandered. To leverage high-field imaging, full use must be made of the available acquisition windows, as can be achieved with efficient and long k-space readouts. In this work, 7T anatomical whole-brain imaging in under 1 minute is shown, presenting a 5-10 fold scan time reduction as compared to conventional readouts. To this end, spiral acquisitions are used, combining efficient k-space readouts with flexible acquisition times. Impeccable image quality is achieved by controlling for physiology- and system-induced encoding field imperfection, combining iterative, off-resonance-map-based image reconstruction with concurrent magnetic field monitoring (Fig. 1, [1]).

SETUP: The brains of 3 healthy volunteers were scanned on a 7T system (Philips Achieva, Best), using a 1-channel transmit, 32-channel receive coil (Nova Medical, Wilmington). 16 NMR field probes were placed in optimal positions (cf. below) around the receive (and inside the transmit) coil using a clip mount system and connected to a dedicated MR acquisition system [2].

SEQUENCE AND TRAJECTORIES: For the anatomical scans, a set of multi-slice sequences was designed utilizing spiral readouts, which achieved time optimality by fully exploiting the system’s slew-rate and gradient strength limits [3]. The selected sequences covered different contrasts (PD/T2*; spiral in/out) and timing constraints (36-90s total duration, see Fig. 2 for details), but shared slice geometry (thickness 1.5-3mm; -10 deg tilt oblique transverse). A multi-slice 2D setup allowed for whole-brain coverage by slice-interleaved acquisition of up to 50 slices within one TR (3s). For determining the sensitivity and off-resonance maps, a spin-warp multi-echo sequence (delta TE 1ms) was acquired with identical slice geometry, but 1mm in-plane resolution. In all scans the field probes were excited before the start of the readout trajectory and sampled with 1MHz bandwidth during the readout. MR signals from the head-coil were acquired via the scanner spectrometer.

SIGNAL PROCESSING AND IMAGE RECONSTRUCTION (Fig. 1): For every time point during the imaging readout, 16 k-space coefficients (k0-k15) – corresponding to a real-valued spherical harmonic basis function set up to 3rd order – were computed from the probe signals. To minimize error in the measurements of the encoding dynamic fields, the probe positions were chosen such as to reduce the error in the fitted k-space coefficients [4,5] , respecting the coil geometry. On the coil raw data, an iterative, CG-based image reconstruction was performed (SENSE with multi-frequency interpolation, [6,7]). For the encoding model, the measured field dynamics up to first order (k0-k3) were used, in combination with the sensitivity- and static off-resonance maps [8,9], which were smoothed via a CG algorithm.

Field evolutions during spiral encoding of several tens of ms could be successfully monitored (Fig. 3). Combined with static B0 off-resonance maps, virtually artifact-free T2* spiral-out images (Fig.4) could be reconstructed from multiple slices and volunteers, exhibiting both high contrast-to-noise ratio (CNR) and resolution (0.5mm). Furthermore, the geometric congruence of the images with the undistorted multi-echo spin-warp reference scan was confirmed for all proposed spirals (Fig. 5). Total acquisition durations of 90s were sufficient for convincing 0.5mm resolution images. Slight reduction in resolution (0.7mm) enabled sub-minute scan time. Alternatively, a SENSE reconstruction (R=2) with half of the interleaves of a 0.5 mm spiral-out trajectories succeeded, enabling a flexible trade-off between CNR and scan time.

Oftentimes poor use is made of the sensitivity available in ultra-high field imaging, resulting in lengthy scan protocols, particularly for high-resolution scans. Spiral readouts with optimal acquisition windows provide a powerful remedy, if the image reconstruction is performed on the actual encoding fields (monitored k-trajectory, static off-resonance and sensitivity maps). Anatomical whole brain coverage is shown in a sub-minute scan with image quality identical to conventional spin-warp acquisitions. Proton-density (TE 3ms) as well as a T2*-weighted contrast is shown, the latter coming in two flavors: outside-in and inside-out spirals.

Spiral imaging is suitable to other ultra-high field applications. As single-shot readout for fMRI, ASL, fQSM, and in multi-shot acquisitions for angiography, susceptibility imaging and different anatomical contrasts. Optimized spiral trajectories such as variable density spirals have the potential of further increasing imaging speed and image SNR, leveraging the superb sensitivity of ultra-high field MRI.