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Ristretto MRE: A Generalized Multi-Shot GRE-MRE Sequence

Christian Guenthner¹, Sweta Sethi², Ayse Sila Dokumaci³, Ralph Sinkus³, and Sebastian Kozerke¹

¹Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland, ²Division of Research Oncology, Guy's and St Thomas' NHS Foundation Trust, London, United Kingdom, ³Division of Imaging Sciences and Biomedical Engineering, King's College London, London, United Kingdom

Synopsis

The purpose of this work is to increase GRE-MRE sequence flexibility by generalizing the multi-shot eXpresso approach. We show that Ristretto MRE allows for the fine-tuning of imaging shot durations in both multi-slice and 3D-MRE acquisitions permitting significant scan time reductions without loss of image quality.

Introduction

In multi-slice MRE¹,$$$N_S$$$ slice excitations are interleaved to maximize the per-slice repetition time, while synchronization to the wave period$$T=f^{-1}$$needs to be ensured. In order to reduce total scan duration, the “eXpresso” sequence was proposed acquiring an integer number of $$$x$$$ slices per wave period² as illustrated in Figure 1a. Different wave offsets are acquired for each k-line by introducing a delay $$$T/N_P$$$, which is depicted in red. Since each slice is acquired with a different phase shift relative to the onset of wave generation, phase correction is necessary before wave inversion. While the eXpresso scheme has been used in numerous MRE applications^2–5, it lacks flexibility, which in turn may result in extended scan durations. The objective of the present work is to show that the eXpresso scheme is overly restrictive and to propose a generalized multi-shot approach that allows for the fine-tuning of image shot durations in multi-slice and 3D GRE-MRE acquisitions without compromising image quality.

Theory

The “Ristretto” concept generalizes the eXpresso approach by acquiring an arbitrary number of slices $$$N_S$$$ within an arbitrary number of wave periods $$$N_W$$$ (Figure 1b). Hence, the number of slices per period is allowed to be a rational number, which is given by$$x=N_S/N_W,$$while each imaging shot is of duration$$T_R=T/x.$$In addition, the delay can be omitted by distributing it evenly over all imaging shots (Figure 1c). By interleaving the wave phases through extending the delay by an integer factor $$$N_D$$$ (Figure 1d), the flexibility of the imaging shot duration can be further increased to $$T_R=T\left(\frac{N_W}{N_S}+\frac{N_D}{N_PN_S}\right).$$N.B.: $$$N_D$$$ must be chosen such that all phase offsets are acquired. The allowed $$$N_D$$$ are given by integers not divisible by any prime factor of $$$N_P$$$. If $$$N_P$$$ is a power of 2, the delay $$$N_D$$$ can be any uneven number and the repetition time can be chosen in steps of$$\Delta{}T_R=\frac{2T}{N_SN_P}.$$In a typical 10 slice, 8 phase acquisition, this amounts to $$\Delta{}T_R^{N_S=10,N_P=8}=\frac{T}{40}.$$ The extended-no-delay-Ristretto scheme can be applied to 3D encoding (Figure 2), where a slab is excited and phase encoding is employed along the slice direction with interleaved acquisition of the motion phases for the same $$$k_y$$$/$$$k_z$$$-lines. The shot duration is given by$$T_R=T\frac{N_D}{N_P},$$ with $$$N_W=N_D$$$, while the same rule applies for $$$N_D$$$ as with multi-slice Ristretto.

Methods

The proposed Ristretto sequences were implemented on different Philips platforms and compared to eXpresso MRE in an ultrasound gel phantom, in the breast and the thigh of one volunteer. Sequence parameters in each of the three experiments were held constant, except for Ristretto parameters and slice TRs, respectively. Scan parameters and actuation modalities are included in the result figures for each scan. Shear wave velocities were obtained using a FEM-based inversion algorithm⁶.

Results & Discussion

In Figure 3, the real-part of the complex displacement field of an ultrasound gel phantom is shown. eXpresso, Ristretto and no-delay-Ristretto show excellent agreement. With the chosen echo time of 9.2ms and 30 Hz wave frequency, eXpresso MRE only permits two shots per period. The use of Ristretto allows for 2.5 slices per period, reducing scan duration to 80%, while interleaved acquisition of the eight phases further reduces scan duration to 65%.

In Figure 4, fractional GRE-MRE is compared to multi-slice Ristretto and 3D Ristretto in the thigh. Displacement fields are in good agreement between all three techniques. eXpresso MRE cannot be applied as the high wave frequency does not allow for the acquisition of multiple slices per period. Multi-slice Ristretto can acquire 12 slices within 10.625 wave periods by interleaving the phase offsets. 3D Ristretto with $$$N_W=7$$$ acquires the eight phases in reverse ordering.

In Figure 5, the standard eXpresso breast protocol using 3 shots per period is compared to multi-slice Ristretto, 3D Ristretto and a rapid 3D Ristretto scheme. Displacement fields and reconstructed shear wave velocities are in good agreement within the breast, however, the increased SNR in the 3D scans allows for the reconstruction of the stiffer areas in the region of the axial lymph nodes. By reducing the number of phases to four, making use of the increased encoding efficiency of Hadamard encoding⁷, and the high SNR of 3D acquisitions, 4D MRE of the breast and axial lymph nodes becomes feasible in under 2 minutes.

Conclusions

We have demonstrated, that the multi-shot concept of eXpresso MRE can be extended to allow for flexible tuning of imaging shot durations and permits reduction of total scan time without limiting constraints on the number of shots, slices, or the specific choice of the echo time. The concept can be further extended to 3D GRE-MRE and in combination with Hadamard encoding and the acquisition of four wave phases, allows for 4D MRE of the breast in under 2 minutes.

Acknowledgements

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 668039.

References

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2. Garteiser P, Sahebjavaher RS, Ter Beek LC, et al. Rapid acquisition of multifrequency, multislice and multidirectional MR elastography data with a fractionally encoded gradient echo sequence. NMR Biomed. 2013;26(10):1326-1335. doi:10.1002/nbm.2958.

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Figures

Figure 1: Timing diagram of the proposed multi-shot sequences, Ristretto, no-delay Ristretto, and extended no-delay Ristretto compared to eXpresso MRE for conventional fractional, multi-slice GRE-MRE. Red squares above the diagrams denote the shift in wave-phase. Red markers within the sequence diagram denote delay objects, which are introduced into the sequence to shift the multi-slice acquisition with respect to the trigger of the wave. Extended no-delay Ristretto allows for flexible tuning of imaging shot durations by $$$2T/(N_PN_S)$$$ for $$$N_P=2^n, n\in\mathbb{N}$$$ phase offsets and $$$T$$$ being the wave period.

Figure 2: Timing diagram of the Ristretto scheme for phase-interleaved scans, which allows for fast acquisition of 3D-encoded MRE datasets. Red squares denote the amount of phase offset per imaging shot. The phase offset is designed such that after $$$N_P$$$ imaging shots, the net phase offset is 0. Here, for $$$N_P=8$$$ phases, the delay can be any uneven number.

Figure 3: Real-part of the complex displacement field of an ultrasound phantom, comparing the multi-slice Ristretto sequences to the original multi-shot eXpresso MRE. All three measurements show excellent agreement of wave displacement fields. Scan durations could be reduced to 80% by acquisition of all 10 slices within 4 instead of 5 wave periods (middle) and further to 65% by interleaved acquisition of the phases using extended no-delay Ristretto (right).

Figure 4: Comparison of eXpresso, multi-slice Ristretto and 3D Ristretto MRE in the thigh at 100 Hz actuation. The high wave frequency does not allow for the acquisition of multiple shots in the standard eXpresso sequence. Ristretto can reduce acquisition time by 12% by interleaved acquisition of the wave phases, i.e. incorporating the acquisition of all 12 slices into 10.625 wave periods. 3D Ristretto is able to capture the same wave field as both multi-slice MRE schemes with similar non-linearity and reconstructed shear wave veloctities. The 3D acquisition used 1.3x slice oversampling and no-elliptical shutter.

Figure 5: Comparison of conventional eXpresso, multi-slice Ristretto, 3D Ristretto, and a fast 3D Ristretto scheme employing 4 wave offsets together with Hadamard encoding⁷. Displacement maps are in good agreement between all scans, though non-linearity is slightly elevated within the parenchymal areas in the 3D Ristretto scan originating from partial saturation, which can be decreased using smaller flip angles. The high SNR of the 3D scans in the area of the axial lymph nodes allows for the reconstruction of the elevated stiffness in these regions. Application of Hadamard encoding and acquisition of 4 wave offsets only, allows for 4D-Breast MRE in under 2 minutes.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)

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