Synopsis

In vivo human brain imaging under highly inhomogeneous B₀ field of 250 kHz off-resonance variation over 20 cm is demonstrated. The B₀ field inhomogeneity was generated by mounting a head gradient coil at 36 cm off the isocenter of a 90-cm 4T magnet. Brain imaging was performed with 3D missing-pulse steady-state free precession using the inhomogeneous field gradient for spatial (readout) encoding. Frequency-modulated pulses were employed to excite the widely distributed spin frequencies in the inhomogeneous field with easily achievable RF peak power (~1 kW). By providing combined T₁, T₂, and high diffusion weighting, images with clear delineation of brain anatomy were produced.

Purpose

Methods

A permanently inhomogeneous static field was introduced by mounting a head gradient coil at 36 cm away from the magnet center of a 4T human MRI system (Agilent Technologies, Inc.), which resulted in approximately 250 kHz off-resonance variation over 20 cm. To preserve spin signals under the highly inhomogeneous field, a 3-dimensional MP-SSFP sequence was employed (Fig.1), where echoes consisting of overlapping spin and stimulated echoes refocus at the missing pulse timings regardless of the field inhomogeneity (Fig.2). The permanent B₀ field gradients mostly along the z direction were used as spatial encoding (readout) gradients. For RF excitation, a frequency-modulated (FM) pulse that is a stretched hyperbolic secant pulse (HS2)² was employed to cover the wide frequency range generated by the inhomogeneous field (>100 kHz). The dependence of the steady-state magnetization on the missing pulse interval was investigated using simulations, and the transverse magnetization reaching the steady-state was calculated from a phase diagram³ using the following parameters: pulse interval (τ)=3 ms, flip angle (FA)=4°, and T₁/T₂=1200/80 ms. To study the dependence of the steady-state magnetization on T₁ and T₂ relaxation, simulations were performed using one missing pulse in every 6 pulses (TR=6τ) for three T₁ and T₂ combinations (T₁/T₂=1200/80, 1600/100 and 900/80 ms).

Under the inhomogeneous B₀ field, in vivo human brain imaging was performed with the following sequence parameters: τ=2.6 ms, pulse missing is every 6 pulses (TR=15.6 ms), HS2 pulse excitation with FA=4°, BW=140 kHz and T_p=700 μs (corresponding to a time-bandwidth product (TBP)=100), readout bandwidth = 180 kHz, cylindrical k-space sampling, and scan time = 5.5 min. Pulsed slab-selective and readout gradients, G_ss and G_r, were applied to mitigate a portion of the B₀ inhomogeneity during excitation and acquisition. SAR was within the FDA guideline. A quadrature TEM coil was used for transmit and receive. Since the field gradient associated with the inhomogeneous B₀ field was not linear, a reconstructed image showed image distortion along the z direction. The image distortion was corrected based on a measured B₀ field map. Image reconstruction was performed with a reconstruction routine written with C++ and image distortion correction was done with Insight Toolkit (ITK)⁴.

Results

The simulation shows a rapidly increasing amplitude of the refocused transverse magnetization at the beginning of the MP-SSFP sequence, that reaches a maximum and then slowly decreases to a steady-state level (Fig.3a). In addition, the steady-state transverse magnetization increases as the missing pulse interval becomes longer (i.e. longer TR). SAR per unit time also increases gradually as the missing pulse interval increases, but the steady-state magnetization increases at a faster rate (Fig.3b). One drawback of using longer missing pulse interval is an increase of total scan time due to longer TR. The steady-state magnetization attains different steady-state magnetization levels, depending on T₁ and T₂ values (Fig.3c), which generate contrasts in images.

A brain image reconstructed with inverse Fourier transformation showed significant image distortion resulting from the nonlinear inhomogeneous B₀ field (Fig.4a,b). Distortion correction using a measured B₀ field map nicely eliminated the image distortion (Fig.4c-f).

Discussion

The proposed 3D MP-SSFP technique uses the permanent inhomogeneous B₀ field gradient as a frequency encoding (readout) gradient. The nonlinear inhomogeneous gradient field generates not only image distortion, but also spatially varying diffusion weighting in reconstructed images. There was a gradual signal drop toward the top of the brain because of the steeper B₀ field gradient (Fig.4a-c). In this study, relatively short τ (2.6 ms) was used to avoid excessively strong diffusion weighting in those regions.

Signal excitation with broad bandwidth (>100 kHz) is another challenging aspect of MRI under highly inhomogeneous fields, which makes conventional spin-echo sequences using 180 degree refocusing pulses impractical. The achievable excitation bandwidth is a severe limitation for conventional amplitude modulated (AM) pulses. Specifically, the required pulse width of common AM pulses (e.g., sinc and Gaussian) is <50 μs for 100 kHz bandwidth, which may require extremely high RF peak power even for low flip angles. FM pulses used in this study can significantly alleviate the peak power demand by using high TBP pulses.

Conclusions

Acknowledgements

References

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