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Rapid Anatomical Imaging of the Neonatal Brain Using T2-Prepared 3D Steady-State Free Precession
Jinho Park1, Catherine Limperopoulos1,2,3,4, and Zungho Zun1,2,3,4

1Division of Diagnostic Imaging and Radiology, Children's National Medical Center, Washington, DC, United States, 2Division of Fetal and Transitional Medicine, Children's National Medical Center, Washington, DC, United States, 3Department of Pediatrics, George Washington University, Washington, DC, United States, 4Department of Radiology, George Washington University, Washington, DC, United States

Synopsis

Anatomical imaging of the neonatal brain is typically acquired using T2-weighted imaging based on either 3D fast spin echo (FSE) or 2D single shot FSE (SSFSE). While 3D imaging provides higher signal-to-noise ratio (SNR) and spatial resolution than 2D, its scan time is relatively long.In this study, we developed rapid 3D anatomical imaging for the neonatal brain using 3D steady-state free precession (SSFP) with T2 preparation. Our proposed method demonstrated similar T2 contrast to that of 3D FSE and 2D SSFSE while achieving shorter scan time than 3D FSE and higher through-plane resolution and higher SNR than 2D SSFSE.

Introduction

Anatomical imaging of the neonatal brain is routinely acquired in the clinic using T2-weighted (T2-W) imaging based on either 3D fast spin echo (FSE)1-3 or 2D single shot FSE (SSFSE) imaging.4 While 3D imaging provides higher signal-to-noise ratio (SNR) and higher spatial resolution than 2D, its scan time is relatively long, which increases the sensitivity to motion.5 To overcome this, faster 3D imaging may be achieved using gradient echo (GRE) or steady-state free precession (SSFP) imaging that use very short TR; however, these rapid imaging methods lack T2 weighing, which offers the most optimal contrast in the neonatal brain.In this study, we developed rapid T2-W 3D anatomical imaging using T2 preparation and SSFP readout.

Method

Figure 1 shows the schematic of our proposed T2-prepared 3D SSFP sequence. SSFP sequence was chosen for its high SNR efficiency, and was performed immediately after T2 preparation before the prepared T2 weighting decays. T2 preparation was composed of a 90° excitation pulse, followed by MLEV-8 type of refocusing pulses and a -90° of tip-up pulse.6 Figure 2-a shows simulated transient signals of CSF, white matter and gray matter using proposed T2-prepared 3D SSFP. We designed a k-space trajectory based on a Cartesian ring shape to achieve smooth signal variation around the k-space center (Fig 2-b, c). For in-vivo experiment, we performed 2D SSFSE, 3D FSE (CUBE on GE systems), 3D SSFP and our proposed method in a healthy neonate (postmenstrual age = 43 weeks) with the same effective TE of 80 ms. While effective TE was determined by the time interval between 90° and -90° pulses in the proposed method, it was determinedbased on the time of k-space center sampling in 2D SSFSE and 3D FSE. There was no equivalent TE in 3D SSFP because of no T2 weighting. While slice thickness was 1 mmin 3D sequences, it was limited to 2 mm in 2D SSFSE due to imperfect slice profile and lower SNR. Scan parameters of the sequences are summarized in Table 1. Method

Result

Figure 3 shows that our proposed scheme of T2-prepared 3D SSFP obtained T2 contrast similar to those of 3D FSE and 2D SSFSE. Our approach also achieved shorter scan time than 3D FSE and higher through-plane resolution and higher SNR than 2D SSFSE. Product 3D SSFP sequence, although showing comparable scan time and spatial resolution to the proposed method, did not provide T2 contrast, which is critical to distinguishing gray and white matters of the neonatal brain.

Discussion

T2 preparation has been used with 2D SSFP in previous studies. 7,8 In this study, we demonstrated the integration of T2 preparation with 3D readout and showed that T2 contrast similar to that of FSE was achievable using our approach, without generating significant blurring. This was attributed in part to longer relaxation times of the neonatal brain (vs adult). In the current setup, we used 8 refocusing pulses in T2 preparation with effective TE of 80 ms but we found that more number of refocusing pulses was desirable with longer effective TE. One of the limitations of this method is the slightly increased specific absorption rate (SAR) compared to the FSE due to SSFP readout. The SAR level of our method was still lower than the FDA limit, but future work on SAR reduction or close monitoring of SAR during scan may be required.

Conclusion

We developed T2-prepared 3D SSFP for T2-W anatomical imaging of the neonatal brain at 3T. This approach offered significantly reduced scan time, making it more optimal for neonatal MRI.

Acknowledgements

R01 HL116585-01

References

  1. Busse RF, Hariharan H, Vu A, et al. Fast spin echo sequences with very long echo trains: Design of variable refocusing flip angle schedules and generation of clinical T2 contrast. Magn Reson Med. 2006;55:1030–1037.
  2. Kitajima M, Hirai T, Shigematsu Y et al. Comparison of 3D FLAIR, 2D FLAIR, and 2D T2-weighted MR imaging of brain stem anatomy. AJNR Am J Neuroradiol. 2012; 33:922-927.
  3. Mugler JP III. Optimized three-dimensional fast-spin-echo MRI. J Magn Reson Imaging. 2014; 39:745-767.
  4. Patel MR. Klufas RA, Alberico RA et al. Half-Fourier acquisition single-shot turbo spin-echo (HASTE) MR: Comparison with fast spin-echo MR in diseases of the brain. AJNR Am J Neuroradiol. 1997; 18:1635-1640.
  5. Malamateniou C, Malik SJ, Counsell SJ, et al. Motion-compensation techniques in neonatal and fetal MR imaging. AJNR Am J Neuroradiol. 2013; 34(6):1124–36.
  6. Levitt MH, Ernst RR. Composite pulses constructed by a recursive expansion procedure. J Magn Reson. 1983;55:247–254.
  7. Brittain JH, Hu BS, Wright GA, et al. Coronary angiography with magnetization-prepared T2 contrast. Magn Reson Med. 1995 May;33(5):689-96.
  8. Rodríguez-Soto AE, Langham MC, Abdulmalik O, et al. MRI quantification of human fetal O2delivery rate in the second and third trimesters of pregnancy. Magn Reson Med. 2018;80:1148–1157.

Figures

Figure 1. Sequence diagram of proposed T2-prepared 3D SSFP. T2 preparation based on Michael-Levitt-8 (MLEV) was used immediately prior to SSFP readout with SSFP preparation to achieve T2 weighing

Figure 2. (a) Transverse component of magnetization of CSF, white matter and gray matter across RF excitations simulated using relaxation times of the neonatal brain. (b) Proposed cartesian ring trajectory of first 145 point. (c) Depostied weighting in the ky-kz plane for CSF.

Figure 3. Anatomical images acquired in a healthy neonate (postmenstrual age = 43 weeks) using (a) 2D SSFSE, (b) 3D FSE, (c) T2-prepared 3D SSFP (proposed) and (d) 3D SSFP. Effective TE was 80 ms for a-c. Our proposed method showed similar T2 contrast to 3D T2 FSE and 2D SSFSE.

Table 1. Imaging parameters of our protocol

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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