Adiabatic T2-Prepared 3D Fast Gradient Echo Imaging for Brain Tumor Studies at 7T
Peng Cao1, Angela Jakary1, Yan Li1, Sarah J. Nelson1, Doug Kelley2, and Peder E. Z. Larson1

1Department of Radiology, University of California at San Francisco, San Francisco, CA, United States, 2GE Healthcare, Waukesha, WI, United States

Synopsis

Mechanisms to create a homogenous T2 contrast at 7T should be robust to both B0 and B1 inhomogeneities. This issue was addressed by applying a zero-degree BIR-4 adiabatic pulse for T2 preparation. Simulation and in vivo experiments in mouse brain verified the robustness and contrast of the preparation scheme at 7T. Application of this method was further demonstrated in a study of a patient with a brain tumor. The adiabatic T2 weighting showed high intensity in the region of the tumor, suggesting that the proposed method is likely to be useful in screening and characterizing tumors.

Purpose

Mechanisms to create homogenous T2 contrast at 7T should be robust to both B0 and B1 inhomogeneities. Previous studies showed the T2 contrast can be generated by a long and segmented 0°- B1-independent rotation (0°-BIR4) adiabatic pulse [1-4]. In this study, we applied this strategy to provide adiabatic T2 preparation in combination with the widely used fast 3D gradient echo imaging (GRE). The resulting adiabatic T2-prepared 3D GRE was further applied to a patient with a brain tumor in order to evaluate its potential for improving diagnostic performance.

Methods

Adiabatic T2 preparation design: A BIR4 RF pulse was designed according to the parameters: bandwidth = 1 kHz, duration = 56 ms, maximum B1 = 9 μT, sechn modulations, β = 184 rad/s, μ = 8, order = 8 and flip angle = 0°. Bloch simulations for using this as a T2 preparation pulse were performed with varied B0 and B1.

3D fast gradient echo (GRE) sequence: The sequence was modified from a standard segmented fast GRE sequence (GE Healthcare) with the inversion pulse being replaced by the T2 preparation pulse, acquiring multiple ky and kz lines per each T2 preparation.

In vivo mouse brain experiment: This was performed on a 7T whole-body MRI scanner (GE Healthcare, Waukesha, WI) with an RF surface coil, which induced a varied B1 field. The 3D GRE images with and without T2-preparation were acquired with TE/TR = 3.7/20.3 ms, bandwidth = 62.5 Hz, matrix size = 256×128×28, FOV = 60×60×10 mm3, NEX = 1 and FA = 1°.

Patient study: Brain tumor patient scans were performed on the same 7T MRI. The study was performed with informed consent under a protocol approved by the UCSF Committee on Human Research. A 32-channel volume RF coil (NOVA Medical) was used. 3D GRE images with and without T2 preparation were acquired with parameters: TE/TR = 1.3/3.7 ms, bandwidth = 62.5 Hz, matrix size = 256×256×32, FOV = 240×240×30 mm3, NEX = 1 and FA = 4°. High resolution T2-prepared 3D GRE images were also acquired, with TE/TR = 2.3/7.1 ms, matrix size = 512×512×32, FOV = 240×240×60 mm3 and NEX = 2, and other parameters were identical with the previous scans. A 2D fast spin echo (FSE) scan was also performed with effective TE/TR = 85.5/3000 ms, matrix size = 512×512, number of slices = 12, slice thickness = 3 mm, slice gap = 4 mm, FOV = 240×240 mm3, NEX = 2, echo train = 16 and FA = 90°.

Results

In Figure 1, Bloch simulations show that homogenous T2 contrast can be achieved across a large region with varied B0 (i.e., -500 Hz < Δ f < 500 Hz) and B1 (70% < RF amplitude < 110%). Figure 2 shows results of the in vivo mouse brain experiment at 7T with inhomogeneous B1 field that was caused by the surface coil used. The adiabatic T2 preparation pulse provided homogeneous T2 contrast, demonstrating robustness to B1 variations. Brain tumor patient scans shown in Figures 3 and 4 demonstrate a similar high intensity in the tumor region with either T2-weighted spin echo or adiabatic T2 weighting. It should be noted that homogenous T2 contrast was achieved with the adiabatic preparation even in the presence of the B0 inhomogeneity in the frontal lobe (≈72 Hz) and B1 inhomogeneity of 14% (std/mean) across the brain (known as “central brightening”). In Figure 4, the T2 contrast generated by the adiabatic T2 preparation was observed to be comparable with that of T2-weighted FSE.

Discussion

B0 and B1 inhomogeneities are central concerns for clinical MRI studies at high field. Although adiabatic pulses are known to be effective tools for addressing this problem, intensively applying adiabatic pulses can drastically increase the SAR, and limitations in the RF hardware at high field (e.g., B1 threshold = 9 μT in this study) impose a further challenge in applying adiabatic pulses. To overcome these issues, we integrated a long (and strongly time modulated) adiabatic T2 preparation with a fast 3D GRE sequence, which is an extension of the conventional hard-pulse-based T2 preparation scheme. This approach can be applied for 7T patient studies and is immune to both B0 and B1 inhomogeneities.

Conclusion

Animal and human brain experiments verified the robustness of adiabatic T2 preparation at 7T. The value of the adiabatic T2 preparation was demonstrated on a clinical brain tumor study at 7T. Adiabatic T2 weighting showed high intensity in the tumor region, suggesting that the proposed method may be useful for tumor screening and characterization.

Acknowledgements

No acknowledgement found.

References

[1] Jenista E. R., Magnetic resonance in medicine 2013;70:1360-8.

[2] Guo J. and E. C. Wong Magnetic resonance in medicine 2012;68:1458-71.

[3] Nezafat R., Magnetic resonance in medicine 2009;61:1326-35.

[4] Nguyen T. D., Magnetic resonance in medicine 2015.

Figures

Figure 1 Adiabatic T2 preparation design and simulated T2 contrast, compared with hard pulses (90º-180º-180º-90º). Note that the dash box on the top right indicates homogenous T2 contract achieved in the region with varied B0 (i.e., -500 Hz < Δ f < 500 Hz) and B1 (70% < RF amplitude < 110%).

Figure 2 Results of in vivo mouse brain experiment at 7T with a surface RF transmission coil (illustrated by the white bar in the top-left figure). Proton density-weighted 3D fast GRE with adiabatic T2 preparation (NEX=4, right panel) provided homogeneous T2 contrast, in contrast to that without such preparation (NEX=1, left panel).

Figure 3 Proton density-weighted 3D fast GRE images acquired from a brain tumor patient without (NEX=1, left panel) and with adiabatic T2 preparation (NEX=1, right panel). The latter showed homogenous T2 contrast and high intensity at the tumor region (arrows). Top, middle and bottom rows show coronal, axial and sagittal views, respectively.

Figure 4 Results from the same patient. The first two columns illustrate large variations in the frontal lobe and center brightening on B0 and B1 maps. Adiabatic T2-prepared 3D fast GRE provided homogenous T2 contrast and high intensity at the tumor region (4th column) comparable to standard 2D T2-weighted FSE (3rd column).



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
4344