Synopsis

Diffusion-weighted imaging (DWI) has been widely used in brain structure and connectivity investigations and other neuroscience applications. To implement an optimized protocol for high spatial resolution DW imaging at ultra-high field, we have conducted macaque brain imaging by using readout segmented echo planar imaging at 7T. Our results indicate that, the rsEPI has potential to achieve DWI with improved signal-to-noise ratio and with the intent of minimizing geometric distortion.

Introduction

Methods

All MRI scans were performed on a 7T research system (Siemens Healthcare, Erlangen, Germany) equipped with a 1Tx/28Rx knee coil (QED, Mayfield Village, OH, USA). Three healthy female macaques (2-3 years old with weights of 3.6, 3.2 and 3.7 kg) were included in the study, and were maintained and anesthetized by 1.5~2% isoflurane – all procedures were in accordance with NIH standards and approved by local Institutional Animal Care Committee. All monkeys were fixed in the sphinx position with its head centered within the knee coil.

A prototype sequence^4,5 was used to acquire 1 mm isotropic rsEPI DW images with different diffusion encoding schemes, acceleration modes, and number of readout segments. Evaluation of the image SNR (illustrated in Table 1) with optimized parameters such as TR, TE, EPI-factor, receiver bandwidth and echo spacing was performed. The diffusion encoding scheme include the conventional bipolar gradient scheme (twice refocused bipolar gradient scheme), the traditional Stejskal-Tanner sequence (single refocused, monopolar gradient) and a modified Stejskal-Tanner sequence (optimized monopolar scheme, monopolar+). The monopolar+ is an optimized Stejskal-Tanner scheme with a single bipolar gradient⁶. The SNR values were calculated by using the difference method and a two-factor repeated measures analysis of variance test⁷.

In addition, 3D MPRAGE T1-weighted images were obtained as structure reference, with isotropic spatial resolution of 0.5 mm, TR/TE 3,500/3.74 ms, TI 1,500 ms, flip angle 7 deg, bandwidth 170 Hz/pixel, FOV 96 mm and matrix size 192x192.

Results and Discussion

Figure 1 shows the SNR comparison results of DW images at b-value = 0 and 1,000 s/mm² with 7 readout segments using different diffusion encoding schemes and accelerate modes. With GRAPPA acceleration, the mean SNRs of grey matter and white matter were consistently higher than those without GRAPPA. Similarly, the mean SNRs were higher under GRAPPA acceleration mode with monopolar and monopolar+ when compared with bipolar scheme, but the difference between monopolar and monopolar+ scheme did not reach statistical significance. The SNR benefit of using GRAPPA comes from the reduction of TE, despite of g-factor noise penalty. The GRAPPA technique reduces also the effective echo train length and thus improves the image quality³. Corresponding images are shown in Fig. 2.

Figure 3 and 4 show the comparison of 1 mm isotropic DW images with monopolar and monopolar+ scheme between 7, 13, 19 readout segments with an GRAPPA acceleration factor of two. These DW images showed slight geometric distortion at air-tissue interfaces such as paranasal sinus, the nasal cavity and the ear cavity. Increasing readout segments of rsEPI leads to longer scan time but shorter readout echo spacing (see Table 1), the latter of which is supposed to alleviate image distortions due to the susceptibility effect⁸. However, the geometric distortion observed in Figure 4 were not apparently reduced with increased readout segments. In addition, there is still no significant SNR difference between monopolar and monopolar+ scheme.

The standard bipolar diffusion encoding is commonly used to reduce eddy currents, but limits the minimum achievable TE, because of an extra refocusing pulse and additional corresponding crusher gradients. The low eddy current environment provided by modern gradient coil designs allow the use of monopolar and monopolar+ diffusion encoding. Particularly, the monopolar+ diffusion encoding is designed to improve image SNR, shorten TE and minimize eddy current distortion, which may effectively merit diffusion-related investigations with higher sensitivity to eddy current effects (e.g., tractography)⁶.

Conclusion

Acknowledgements

References

1. Mansfield P, Pykett IL. Biological and medical imaging by NMR. Journal of Magnetic Resonance, 1969 1978:29:355–373.

2. Griswold M A, Jakob P M, Heidemann R M, et al. Generalized autocalibrating partially parallel acquisitions (GRAPPA). Magn Reson Med, 2002, 47(6):1202-1210.

3. Porter DA, Heidemann RM. High resolution diffusion-weighted imaging using readout-segmented echo planar imaging, parallel imaging and a two-dimensional navigator-based reacquisition. Magn Reson Med. 2009:62:468–475.

4. Pipe JG, Farthing VG, Forbes KP. Multishot diffusion-weighted FSE using PROPELLER MRI. Magn Reson Med. 2002; 47:42–52.

5. Robert Frost, et al. High-Resolution Diffusion-Weighted Neuroimaging at 3T and 7T with Simultaneous Multi-Slice RESOLVE. MAGNETOM Flash | (63) 3/2015

6. Morelli J N, Runge V M, Feiweier T, et al. Evaluation of a modified Stejskal-Tanner diffusion encoding scheme, permitting a marked reduction in TE, in diffusion-weighted imaging of stroke patients at 3 T. Investigative Radiology, 2010, 45(1):29-35.

7. Fritz J, Fritz B, Zhang J, et al. Simultaneous Multislice Accelerated Turbo Spin Echo Magnetic Resonance Imaging: Comparison and Combination With In-Plane Parallel Imaging Acceleration for High-Resolution Magnetic Resonance Imaging of the Knee. Investigative Radiology, 2017, 52(9):529.

8. Wang Y, Ma X, Zhang Z, et al. A comparison of readout segmented EPI and interleaved EPI in high-resolution diffusion weighted imaging. [J]. Mag Reson Imag, 2017, 47:39.