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High Resolution T2W imaging using Multi-shot Reduced Field-of-View PROPELLER
Xinzeng Wang1, Daniel V Litwiller2, Lloyd Estkowski3, Kang Wang4, Arnaud Guidon5, Jason R Stafford6, and Ersin Bayram1

1Global MR Applications & Workflow, GE Healthcare, Houston, TX, United States, 2Global MR Applications & Workflow, GE Healthcare, New York, NY, United States, 3Global MR Applications & Workflow, GE Healthcare, Menlo Park, CA, United States, 4Global MR Applications & Workflow, GE Healthcare, Madison, WI, United States, 5Global MR Applications & Workflow, GE Healthcare, Boston, MA, United States, 6M. D. Anderson Cancer Center, Houston, TX, United States

Synopsis

Multi-shot fast-spin-echo based PROPELLER (FSE-PROPELLER) combines the robustness to off-resonance and bulk motion and has been increasingly used in body imaging. However, FSE-PROPELLER methods require longer acquisition times compared with conventional FSE methods, especially in high resolution imaging with small FOV, since the oversampling is not confined to a single axis. Outer volume suppression alone can provide a modest reduction in scan time. In this work, we optimized the rotating outer volume suppression method and combined it with variable refocusing flip angles to further reduce scan time for high resolution T2W imaging.

Introduction

High-resolution T2W imaging can depict fine anatomical details and small lesions. However, high-resolution T2W imaging methods often suffer from long scan times and are sensitive to motion, especially in body imaging. While multi-shot FSE-PROPELLER is useful for correcting bulk motion, it requires longer acquisition times compared with conventional FSE methods, since the phase-encoding direction in PROPELLER is changed with each blade, and the oversampling is not confined to a single axis. Although inner-volume imaging (IVI)1 and outer-volume suppression (OVS)2,3 have been incorporated into PROPELLER to reduce the FOV, they cannot dramatically reduce the scan time, because interleaved slice acquisition is not compatible with IVI, and OVS increases the TR, which partially offsets the reduced oversampling factor. Thus, the purpose of this work was to optimize the rotating outer volume suppression method3 and to use variable refocusing flip angles4 to further reduce the scan time for high resolution T2W imaging by reducing SAR and increasing echo train length (ETL).

Materials and Methods

PROPELLER with rotating outer volume suppression (PROPELLER FOCUS)3 and variable refocusing flip (VRF) angles acquisition4 was implemented on a 3.0T MRI scanner (Discovery MR750, GE Healthcare, Waukesha, WI) and is shown in Figure 1a, 1b. The proposed method was optimized and evaluated in phantoms and prostate of 2 healthy volunteers with IRB approval and written informed consent. The number of OVS saturation pulses was varied from 1 to 3 to evaluate saturation efficiency, SAR and scan time. The typical in-vivo imaging parameters included: axial orientation; FOV = 280 × 280 mm2 (Large FOV) / 140 × 140 mm2 (Small FOV); Resolution = 1 × 1 mm2 (Low Resolution) / 0.5 × 0.5 mm2 (High Resolution); slice thickness = 4 mm; number of slices = 22; Oversampling Factor (OSF) = 1.5 (Large FOV) /3 (Small FOV); FA = 120 degrees; ETL = 28; TE = 103 ms and auto TR. Same parameters were used in PROPELLER with VRF, except ETL is 46 for VRF = 130 (first), 60 (minimum), 120 (middle),160 (last) degrees to match the TE. For conventional PROPELLER, the scan times for T2W low-resolution imaging and high-resolution image were 2:31 min and 5:40 min respectively. For PROPELLER FOCUS, the scan times were 3:53 min (3 OVS pulses), 3:30 min (2 OVS pulses) and 2:36 min (2 OVS pulses with VRF).

Results and Discussion

The spatial profile of saturation band using the rotating outer volume suppression is shown in Figure 1d. Using conventional PROPELLER, the bright spots in the ACR phantom were well separated in the high-resolution images with reduced FOV (Fig. 2b vs 2a), but the scan time was doubled due to the increased oversampling factor. With VRF angles, the ETL was increased to 46 to match the TE, and images with similar quality were generated with reduced scan times (Fig. 2c). The aliasing artifacts due to reduced FOV (Fig. 2d) were suppressed using the rotating OVS. However, minor aliasing artifacts were observed in images acquired using 1 OVS pulse (Fig. 2e) and not observed using 2 or 3 OVS pulses (Fig. 2f, 2g). Figure 3 also shows efficient outer-volume suppression using 2 or 3 OVS pulses in prostate imaging. The conventional PROPELLER images with reduced FOV (Fig. 4b) demonstrate a shaper profile of the prostate compared to the low-resolution large-FOV images (Fig. 4a), but significantly increased the scan time (2:31 min vs 5:40 min). PROPELLER FOCUS with 2 OVS pulses (Fig. 4c) also achieved high resolution, but with reduced scan time (3:30 min). The scan time of high-resolution imaging was further reduced by using VRF with PROPELLER FOCUS (Fig. 4d, 2:36 min). While images acquired using PROPELLER FOCUS showed reduced SNR, the SNR could be probably improved by combining VRF, low receiver bandwidth and post-processing methods in the future. Figure 5 shows the scan time of the in-vivo protocol with different acquisition parameters. Smaller FOV often requires a larger oversampling factor, where the PROPELLER FOCUS can save more scan time (Fig. 5b). Compared to standard PROPELLER with oversampling factor of 3, PROPELLER FOCUS with VRF can save at least 60% scan time (Fig. 5a, 5c, 5d). However, PROPELLER FOCUS can save more scan time if <10 slices are acquired, because the scan time is more limited by the repetition time than the SAR.

Conclusion

PROPELLER with rotating outer volume suppression and variable refocusing flip angles can be used to generate high-resolution T2W images in shorter scan times by effectively suppressing outer volume signals, and mitigating SAR.

Acknowledgements

No acknowledgement found.

References

[1] Deng J and Larson A. Multishot targeted PROPELLER magnetic resonance imaging: description of the technique and initial applications. Invest Radiol. 2009; 44(8): 454-462.

[2] Kojima S, Morita S, Ueno E, et al. Aliasing artifacts with the BLADE technique: Causes and effective suppression. J Magn Reson Imaging. 2011; 33(2):432-440.

[3] Litwiller DV, Taviani V, Banerjee S, et al. Rotating Outer Volume Suppression for Reduced Field of View PROPELLER Imaging. Joint Annual Meeting ISMRM-ESMRMB. 2685 (2018).

[4] Busse RF, Hari H, Anthony V, 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(5):1030-7

Figures

Figure 1. Diagram of PROPELLER with rotating outer volume suppression (a) and variable refocusing flip angles (b). Double-headed arrows in (a) present the phase encoding directions of each blade. The saturation bands are applied symmetrically in the phase-encoded direction of each blade. Phantom images were acquired without/with OVS RF pulses (c/d). By reducing the spacing between OVS RF pulses, the expected saturation band was shown in the middle of the phantom (d).

Figure 2, Bright spots cannot be separated in the low-resolution image (a), but well separated in the high-resolution image (b). Conventional PROPELLER used oversampling factor of 2 to reduce the aliasing artifacts for reduced FOV imaging (b). Long ETL was used in VRF, which generated image (c) with similar quality compared to constant refocusing flip angles (b). Without applying oversampling and OVS pulse, strong aliasing artifacts were observed (d, red arrow). While the aliasing artifacts can be suppressed using OVS pulses (e - g), 1 OVS pulse is not sufficient to suppress all the aliasing artifacts (e, red arrow).

Figure 3. High-resolution reduced Field-of-View prostate image (14 × 14 mm2, 0.5 × 0.5 mm2) was acquired using conventional PROPELLER with an oversampling factor of 3 (a). Strong aliasing artifacts were expected in image acquired with an oversampling factor of 1 using conventional PROPELLER (b). PROPELLER FOCUS with 2 or 3 OVS pulses (c, d) can effectively suppress the aliasing artifacts without phase oversampling.

Figure 4. Large FOV and small FOV prostate images acquired using conventional PROPELLER (a, 1 × 1 mm2, b, 0.5 × 0.5 mm2), PROPELLER FOCUS (c, 0.5 × 0.5 mm2) and PROPELLER FOCUS with VRF (d, 0.5 × 0.5 mm2). The edges of prostate anatomical structures are blur in low-resolution image, and shaper in high-resolution images (b - d). PROPELLER FOCUS with/without VRF can be acquired 60% and 40% faster than the corresponding high-resolution conventional PROPELLER, with an expected reduction in SNR.

Figure 5. a) Scan time of large-FOV low-resolution and small-FOV high-resolution images acquired using conventional PROPELLER with different oversampling factors (OSF) and PROPELLER FOCUS with/without VRF (FOCUS, FOCUS VRF). b) Depending on the oversampling factor, FOCUS can save more scan time if a larger oversampling factor is applied. c, d) Scan time for the high-resolution imaging sequences with respect to the number of slices. FOCUS with/without VRF reduces about 60% and 40% scan times of high-resolution conventional PROPELLER. FOCUS can save more scan time if number of slices <10, since the scan time is initially limited by the minimum TR.

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