Synopsis

Keywords: Pulse Sequence Design, CEST & MT, GRE FLASH TrueFISP

For clinical implementation, chemical exchange saturation transfer (CEST) imaging must be fast with high signal‐to‐noise ratio (SNR), 3D coverage, and produce robust contrast. In general, CEST imaging with fast low angle shot GRE sequence (FLASH) can achieve fast acquisition, but may yield low SNR efficiency. This study evaluated APT and NOE maps for single saturation 3D CEST imaging with True FISP and FLASH sequences, respectively. We found that True FISP could achieve better SNR of Z-spectrum and obtain more reliable CEST effects contrast. 3D True FISP sequence is expected to become a more promising 3D CEST-GRE sequence for clinical application.

Introduction

Chemical exchange saturation transfer (CEST) allows for indirect detection of diluted molecules by their saturation transfer to the abundant water pool such as peptides and proteins, glutamate, and glucose derivatives^1,2. Spatial correlations between isolated amide CEST and gadolinium ring enhancement have been reported³. Changes in nuclear Overhauser enhancement (NOE) were also shown to correlate with histology and be a measure for brain tumor therapy response⁴. Many previously published applications of CEST evaluated Z‐spectrum asymmetry (MTR_asym) to identify CEST contrast while removing direct water saturation effects⁵. MTR_asym approaches can be performed with a limited number of scans and are therefore relatively fast, which can be performed clinically in 3D whole brain. But MTR_asym can not separate APT and NOE effects. In order to obtain reliable CEST comparison, it is often necessary to collect dozens of frequency offset points to form the Z-spectrum.

To this end, various fast readout sequences have been deployed, such as TSE⁶, EPI⁷ and GRE⁸. The TSE CEST sequence yields high image quality with minimal artifacts, but it is relatively slow and suffers from a relatively high SAR. Fast imaging with EPI readout yields superior acquisition efficiency^9,10, however, it suffers from susceptibility-related image distortion and ghosting artifacts from lipid signals, which may interfere with CEST imaging. It has been proposed to use single saturation 3D fast low angle shot GRE (FLASH) for rapid sampling^11,12.FLASH can achieve faster acquisition than TSE CEST with relatively low SAR, but may yield lower SNR efficiency. True FISP sequence can achieve higher SNR than FLASH at the same spatial and temporal resolution, so the purpose of this study is to verify that True FISP can collect Z-spectrum with better SNR and achieve more reliable CEST comparison.

Methods

Imaging was performed on a 3T whole‐body MRI system (MAGNETOM Prisma; Siemens Healthcare, Erlangen, Germany) on 5 healthy volunteers. The vendor’s Head/Neck 64-channel coil was used for signal reception.

The CEST sequence consisted of a presaturation module followed by a gradient echo readout using centric spiral reordering (Figure 1). The presaturation module consisted of a train of 28 Gaussian‐shaped RF pulses with pulse time t_pulse= 100 ms, and B_1,mean= 0.6 µT. After each train of CEST saturation pulses, a crusher gradient was applied to destroy residual transversal magnetization. At the end of CEST saturation module, fat saturation was applied, resulting in total saturation time t_Sat = 3 seconds. Z‐spectrum data were obtained after saturation at 55 irradiation frequency offsets: two –300 ppm for unsaturated reference images, ±50 ppm, ±35 ppm, ±20 to ±11 ppm in steps of 3 ppm, -10 to 10 ppm in steps of 0.5 ppm. The imaging parameters were field of view = 220 × 180 × 90 mm³, matrix size = 128 × 104 × 18, voxel size = 1.7 × 1.7 × 5 mm³, TR = 3.6 ms, TE = 1.8 ms, flip angle = 6 ° (FLASH sequence) or 26 ° (True FISP sequence), bandwidth = 700 Hz/pixel. These settings resulted in a readout time of t_RO = 2.2 seconds. Recovery time after each acquisition was t_Re = 0.6 seconds. The acquisition time of each offset was TA = t_Sat+t_RO+ t_Re = 5.8 seconds. A generalized auto-calibrating partially parallel acquisition (GRAPPA) with an acceleration factor of 2 in the right-left direction and elliptical sampling were used to reduce acquisition time. For 55 irradiation frequency offsets, the total scanning time of Z-spectrum was about 5.5 minutes.

Neural network fitting was used to fit Z-spectrum to obtain images of APT and NOE effects¹³.

The standard deviation of the difference between the fitted Z-spectrum and the collected Z-spectrum far from the CEST effects was used to measure the noise of the Z-spectrum. The smaller the standard deviation is, the lower the noise of Z-spectrum is.

Results

Figure 2 shows the APT, NOE effects, Z-spectrum noise maps of a healthy volunteer collected by True FISP and FLASH sequences. Compared with FLASH sequence, the APT and NOE effects maps collected by True FISP sequence show a more obvious contrast of gray white matter in the brain. It can be seen that the noise of Z-spectrum collected by True FISP was obviously smaller than FLASH.

Figure 3 shows the mean noise of Z-spectrum in 9^th slice of each volunteer, which demonstrates that the noise of Z-spectrum collected by True FISP was significantly reduced compared with FLASH.

Discussion

In this study, we used 3D True FISP and 3D FLASH sequences to collect Z-spectrum. The two acquisition methods used the same saturation mode, acquisition time and spatial resolution. The results show that 3D True FISP sequence can collect Z-spectrum with better SNR compared with 3D FLASH sequence. This study is only a preliminary result, parameter optimization and clinical application evaluation are required in the future study.

Conclusion

In conclusion, True FISP sequence produces more reliable CEST comparison, thus is more suitable for 3D CEST-GRE acquisition than FLASH.

Acknowledgements

No acknowledgement found.

References

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