Simultaneous Multi-Slice Acquisition with Multi-Contrast Segmented EPI for Dynamic Contrast-Enhanced Imaging
Klaus Eickel1,2,3, Lutz Lüdemann3, David Porter1, and Matthias Günther1,2

1Fraunhofer MEVIS, Bremen, Germany, 2mediri GmbH, Heidelberg, Germany, 3University Hospital Essen, Essen, Germany

Synopsis

The application of simultaneous multi-slice imaging to a segmented EPI allows the acquisition of multiple contrasts while retaining sufficient temporal resolution and spatial coverage. Contrast-enhanced perfusion measurements were performed on a pig’s hip/leg resulting in mainly muscle-perfusion images with separated S0 and T2* maps. The separation of the different signal contributions potentially allows for a quantitative analysis in contrast-enhanced dynamic imaging.

Introduction

A number of acceleration strategies have been proposed for MRI, especially for dynamic imaging, where there is a trade-off between temporal and spatial resolution. To acquire multiple contrasts within one dataset for simultaneous T1 and T2* measurements1,2,3 additional acceleration is required. Simultaneous multi-slice imaging (SMS) is a complimentary technique4,5 which can be combined with standard k-space sampling strategies, such as parallel imaging and keyhole methods. CAIPIRINHA-like6 signal shifting for more efficient use of coil-sensitivity profiles improves SMS reconstruction7. A multi-shot segmented EPI yields sufficient signal also for longer echo-times and allows a flexible acquisition scheme with respect to the desired echo times. Additionally it enables integrated controlled slice-aliasing to reduce g-factor penalty5. To satisfy these aspects we introduce a novel incorporation of SMS to a multi-contrast, segmented EPI sequence8 (Fig.1) which allows extended spatial coverage in contrast-enhanced dynamic imaging to fully cover larger parts of the body (e.g. leg’s muscles) without exceeding temporal constrains.

Subjects and Methods

One sedated pig was studied with an 1.5T MR-scanner (Aera, Siemens Healthcare/Germany). After bolus-injection of a contrast-agent (CA) (dose of 0.1 mmol/kg at 5ml/s injection-rate of Dotarem, Guebert/France) dynamic images of the hip and right leg were acquired for 11 minutes according to 500 measurements at a temporal resolution of 1.3 s. For a FOV=400x200 mm2 at a spatial resolution of 3.1 x 3.1mm2 with 24 slices of 5 mm thickness (20% gap, interleaved acquisition scheme to reduce inter-slice crosstalk) could be achieved due to the 4 times (R=MB) accelerated imaging speed. Despite the sampling rate and a reasonable FOV three contrasts (TE1=9 ms, TE2=1.5 ms, TE3=34 ms) with a TR=261 ms could be acquired without additional in-plane acceleration. The 4 simultaneously excited slices (MB=4) were shifted by ¼ FOV along phase-encoding direction (PE) with blipped CAIPI where gradient blips were integrated into multi-shot scheme8,9. Sagittal slice orientation was chosen such that the aorta and femoral artery were covered for detection of the arterial input function (AIF). For offline SMS reconstruction a 5x5 MB-kernel was trained with MB and Single-Band (SB) data which was acquired before CA administration and with identical imaging parameters (TRACS=1500 ms and TEACS=TESMS). Slice-GRAPPA reconstruction of the SMS data potentially regularizes the SNR of the SMS data according to the SNR of the auto-calibration-signal (ACS)5,10 which reduces apparent T1 weighting in the SMS images. For comparison two SMS-reconstruction approaches were tested: SB & MB (both TR=1500 ms) and SB & synthetic MB from SB (TR=1500 ms), in both cases 2 prescans were run for steady-state conditions. Time penalty for ACS data acquisition was 20s prior to the dynamic scan. Collecting multiple contrasts in one acquisition allows signal separation into a S0 (TE=0 ms) and T2* signal by fitting the data to the exponential T2* decay after SMS reconstruction.

Results

The application of SMS yields sufficient temporal resolution for AIF detection in a large vessel while anatomical coverage allows perfusion analysis of manually segmented muscles. As addressed by the FLEET-ACS approach10 echo-spacing is crucial for correct contrast recovery during slice-GRAPPA reconstruction, in addition to echo-spacing also the TR has to be kept constant for MB-kernel training data. The quality of the reconstructed images meets the requirements for post-processing (Fig.2) such that signal separation by fitting the different acquired contrasts (Fig.3) can be successfully performed. By correcting the signal for the T2* contribution a correction of the susceptibility related changes can be achieved which becomes especially important for the distribution phase with high CA concentrations1. Furthermore T1 effects of CA leakage can be quantified11. Figure 4 shows the S0(t) signal inside the femoral artery for the first 50 measurements, enabling AIF extraction. In contrast to regions with fast CA-dynamics the S0(t) time-curve in muscle-tissue represents the extravasation process into the tissue and the wash-out after a maximum CA-concentration has be reached (Fig.5).

Discussion and Conclusion

By using a segmented EPI acquisition scheme echo-times can be selected flexibly and reasonable signal in late contrasts can be achieved. TR and TE issues in SMS reconstruction have been addressed and the resulting SMS perfusion images show expected signal dynamics. Contrast-enhanced perfusion measurements were performed on a pig’s hip/leg resulting in mainly muscle-perfusion images with separated S0(t) and T2* maps which allow a more complex pharmacokinetic analysis1,2. For further acceleration and/or shorter minimal TEs SMS segmented EPI can be combined with (in-plane) undersampling strategies5.

It has be demonstrated that the incorporation of SMS into multi-contrast, segmented EPI can overcome existing limitations for dynamic imaging while retaining desired acquisition parameters such as temporal and spatial resolution even for imaging of relatively large FOVs.

Acknowledgements

Experimental setup and facility provided by University Hospital Essen and Deutsche Forschungsgesellschaft. Support for sequence development was also recieved from David Feinberg and Liyong Chen.

References

[1] Heilmann, M. et al. Simultaneous dynamic T1 and T2* measurement for AIF assessment combined with DCE MR in a mouse tumor model. Magnetic Resonance Materials in Physics, Biology and Medicine 2007;20(4):193-203.

[2] Lüdemann, L. et al.: Simultaneous Quantification of Perfusion and Permeability in the Prostate Using Dynamic Contrast-Enhanced Magnetic Resonance Imaging with an Inversion-Prepared Dual-Contrast Sequence. Annals of Biomedical Engineering 2009;37(4):749-762.

[3] Sourbron, S. et al. Bolus-tracking MRI with a Simultaneous T1- and T2*-Measurement. Magnetic Resonance in Medicine 2009;62(3):672-681.

[4] Feinberg, D. et al. Multiplexed Echo Planar Imaging for Sub-Second Whole Brain FMRI and Fast Diffusion Imaging. PloSone 2010; 5(12):e15710.

[5] Setsompop, K. et al. Blipped-Controlled Aliasing in Parallel Imaging for Simultaneous Multislice Echo Planar Imaging With Reduced g-Factor Penalty. Magnetic Resonance in Medicine 2012;67(5):1210-1224.

[6] Breuer, F. et al. Controlled Aliasing in Parallel Imaging Results in Higher Acceleration (CAIPIRINHA) for Multi-Slice Imaging. Magnetic Resonance in Medicine 2005;53(3):684-691.

[7] Larkmann, D. et al. Use of Multicoil Arrays for Separation of Signal from Multiple Slices Simultaneously Excited. Journal of Magnetic Resonance Imaging 201;13(2):312-317

[8] Eickel, K. et al. Multi-Contrast Segmented EPI Sequence using Simultaneous Multi-Slice Acquisition with Controlled Aliasing Readout for Dynamic Imaging: First Results. Abstract ISMRM Workshop on SMS Imaging, Pacific Grove, USA 2015.

[9] Polimeni, J. et al. Rapid multi-shot segmented EPI using the Simultaneous Multi-Slice acquisition method. Abstract ISMRM, Melbourne, Australia 2012.

[10] Polimeni, J. et al. Reducing Sensitivity Losses Due to Respiration and Motion in Accelerated Echo Planar Imaging by Reordering the Autocalibration Data Acquisition. Magnetic Resonance in Medicine 2015; doi: 10.1002/mrm

[11] Eichner, C. Slice Accelerated Gradient-Echo Spin-Echo Dynamic Susceptibility Contrast Imaging with Blipped CAIPI for Increased Slice Coverage. Magnetic Resonance in Medicine 2014;72(3):770-778.

Figures

Fig. 1: For MB segmented EPI phase correction the phase-correction scans were played out with the identical blipped CAIPI scheme indicated by the blue area as the later readouts themselves. After iteration over all interleaved multibands the consecutive segments are acquired. For clarity the ADC events are not displayed on a separate axis but indicated by different colors (red: TE1, yellow: TE2, green: TE3).

Fig. 2: By taking signal regularization by the slice-GRAPPA reconstruction into account the MB-kernel was trained with MB and SB data with the same imaging parameters (TR, TE). The reconstructed MB images (left) are in good agreement with the reference images from SB acquisition (right).

Fig. 3: Selected slice during CA-arrival in femoral artery for the 3 different contrasts: TE1=9 ms, TE2=21.5 ms, TE3=34 ms at TR = 1500 ms.

Fig. 4: From the resulting S0 data set a separated S0 time-curve can be extracted to recover the AIF and removing susceptibility effects (first 50 measurements displayed). The shown MR image is from an anatomical scan for orientation purposes.

Fig. 5: After signal separation the S0 signal in muscle-tissue rises after CA extravasates out of the vascular bed and drops after wash-out started at about 5 minutes after injection.



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