Introduction

In magnetic resonance elastography (MRE), mechanical tissue properties are assessed by gathering information about mechanical wave propagation induced by externally applied vibrations [1-5]. In conventional MRE, tissue displacement is assessed by using bipolar motion encoding gradients at distinct offsets to the mechanical vibration in order to get a time-resolved series of phase offset images showing the propagation of the waves. Alternatively, tissue displacement imaging can be performed via displacement encoding with stimulated echoes (DENSE) [6] which has been proposed for human heart MRE [3]. We recently have presented an accelerated DENSE-MRE approach with multi phase offset readout acquisition [4]. This approach is very beneficial for low frequency excitations which are receiving increased interest in human brain MRE [1,5]. Such vibrations penetrate deeper into the brain due to less damping and thereby regions of low wave amplitude are minimized. Consequently they allow the evaluation of larger cross-sectional areas in the calculated elastograms. While conventional MRE needs long echo times (TEs) for the motion encoding of low frequency mechanical excitations, the encoding gradient duration in DENSE-MRE is not coupled to the wave period [3,4]. Additionally, using the multi phase offset readout strategy, the complete time resolved wave image series can be acquired in a single sequence run. Thus besides TE also the acquisition time (TA) and thus possible unwanted patient movement related artifacts can be reduced. We here investigate the application of the multi phase offset readout DENSE approach for multi-slice MRE in the human brain.

Methods

MRE acquisitions and viscoelastic properties were investigated in the brain of two healthy volunteers using the multi phase offset readout DENSE approach. A 3T MRI scanner (MAGNETOM Prisma fit, Siemens Healthcare, Erlangen, Germany) and a 20ch head coil were used for all experiments. Sinusoidal mechanical vibrations at 20Hz were excited via a piezoelectric actuator and released to a head cradle. The excitation was triggered by the scanner each 50ms to achieve a continuous vibration over the whole sequence run. All of the 4 time resolved phase offset images for one encoding direction were acquired in a single run with proper mixing time (TM) settings (TM=8.01ms+[0/12.5/25/37.5]ms). A variable flip angle scheme was used to take an average T1 signal decay at the different TMs into account (α=30°/35°/45°/90°). Further imaging parameters were: FOV=300x300mm², matrix=128x128, slice thickness=4mm, TE=7.71ms, TR=2500ms, 5 slices, GRAPPA parallel imaging factor 2, Partial Fourier 6/8, TA=2:30min per encoding direction. The sequence was repeated three times with altered displacement encoding axis x, y and z. All raw phase images were spatially and temporally mean filtered to eliminate background phase information and ensure clear visibility of the imaged waves. First harmonics were extracted after temporal Fourier Transform of the time resolved images for each encoding direction. Estimates of the complex shear modulus were obtained using an MDEV [5] reconstruction.

Results

Low frequency excited displacement encoded phase images could be acquired in all encoding directions using the multi phase offset readout DENSE-MRE approach. The phase images are superimposed with background phase information in the first place as shown for the 4 time resolved phase offsets of one encoding direction of a volunteer (Fig. 1a). Elimination of the background phase by mean filtering revealed the wave propagation information (Fig. 1b). First harmonic selection revealed the 20Hz-corresponding complex wave images (Fig. 2). Estimates of the magnitude of the complex shear modulus were obtained for all 5 slices (Fig. 3). Brain tissue’s mean values of the magnitude of the complex shear modulus over all slices were 0.37 kPa and 0.32 kPa for the two healthy volunteers.

Discussion and conclusion

Our results show that brain MRE acquisitions at low frequencies can be reliably performed using the proposed multi phase offset readout DENSE approach. The acquisition scheme is able to capture low vibration wave fields in the brain in a fast way while keeping TE short and with no susceptibility artifacts. Multi phase offset as well as interleaved multi-slice acquisition is possible in a single sequence run.

Acknowledgements