Purpose

Several groups have investigated the role of magnetic resonance elastography (MRE), a non-invasive quantitative stiffness imaging technique capable of measuring the viscoelastic properties of tissues in vivo, for the diagnosis of neurological diseases. MRE has shown significant changes in brain stiffness in brain tumors (1,2), normal pressure hydrocephalus (3,4), multiple sclerosis (5,6), and different types of dementia (7,8). Furthermore, groups have established baseline measurements of brain stiffness in healthy volunteer populations by investigating cross-sectional changes in brain viscoelasticity with respect to age and sex (9-11). A majority of this work has been performed using a 2D echo-planar or spiral pulse sequences in order to measure a 3-dimensional displacement field. Two-dimensional acquisitions are limited by slice-to-slice phase variation, which can lead to artifacts and potential underestimation of tissue stiffness. Recently a 3D GRE (Gradient Recalled Echo) elastography pulse sequence has been developed to overcome these limitations. However, translating this into patient studies has not been possible due to low displacement sensitivity arising from gradient performance limitations on whole body MRI scanners(12). A novel C3T MRI scanner with gradients simultaneously capable of 80mT/m amplitude and 700 T/m/s slew rate on each axis has recently been developed (13-15). The objective of this study was to determine if 3D GRE could be performed on the C3T, localize small inclusions of elevated stiffness in a brain phantom, and on a patient with a vestibular Schwannoma.

Methods

A volunteer and a patient (recruited under an IRB-approved protocol) were imaged with 3D GRE MRE after providing written informed consent. A 3D sagittal MRE was performed on a compact 3T scanner with the following parameters, 7.2 mT/m motion encoding gradients (MEG), mechanical vibration=60Hz, TE=20.4ms, TR=24.1ms, a 1 cycle 1-2-1 MEG pulse, motion encoding sensitivity (MENC) = 7.1 μm/rad, RBW = ±25kHz, FOV = 240x240x192mm3, acquisition matrix= 120x120x96, resolution = 2x2x2mm3, flip angle = 12, 2D ARC reduction=2(slice)x2(phase), 8 channel receiver array (Invivo, Gainesville Fl), scan time = 12:25. Realtime gradient pre-emphasis (16) and frequency shifting compensated additional concomitant fields from the asymmetric gradient design. The healthy volunteer was also scanned twice on a standard clinical 3T scanner (Signa HDxt, GE Healthcare) to compare displacement sensitivity between the two scanners. The first scan used the same imaging parameters as the C3T but with a maximum MEG amplitude of 40 mT/m, which resulted in a MENC = 14.3 μm/rad, with a scan time of 12:40 minutes. In order to try and match MENC values a second scan was done using a 2 cycle MEG pulse (MENC=7.1, scan time = 21:23 minutes). To evaluate the ability to resolve localized regions of stiffness, 3D GRE MRE was performed on the C3T using a polyvinyl chloride brain phantom with background stiffness of ~3.4 kPa and embedded spherical inclusions (~7.4 kPa stiffness) with diameters of 1.75cm, 2cm, 2.25cm, 2.5cm, 2.75cm, and 3cm (Figure 1). All acquisition parameters were the same as for the human subjects, but the vibration frequency was set to 80Hz in order to reduce standing wave effects and phase wrapping artifacts which were observed even when using the minimum possible driving amplitudes at 60Hz.

Results

The shear wave displacement sensitivity difference between the C3T and the conventional 3T scanner in the healthy volunteer is shown in Figure 2. The C3T scanner produces the SNR and motion sensitivity necessary to capture the shear wave amplitudes, but the conventional, whole-body, 3T scanner does not. 3D GRE MRE elastograms overlaid on T1-wieghted images in a patient with a vestibular Schwannoma is shown in Figure 3. This demonstrates the feasibility of using the C3T head-only scanner for applications in localizing stiffness within and around a tumor. All inclusions in the brain phantom could be visualized and discerned from the background gel (Figure 4). This demonstrates the potential for high resolution 3D GRE exams to localize small regions of interest.

Discussion and Conclusions

This study demonstrated that a C3T scanner has sufficient gradient performance to successfully perform high resolution 3D GRE MRE, which is not possible on conventional 3T scanners due to limitations like peripheral nerve stimulation(15). 3D GRE MRE was able to delineate the heterogeneous nature of a tumor including stiff and cystic regions in a tumor, and localize inclusions as small as 1.75cm in diameter in a phantom.

Acknowledgements

This research was supported by National Institutes of Health R01 grants EB010065 and EB001981 (R.L.E.).

References

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