Christian Guenthner^{1}, Sweta Sethi^{2}, Ayse Sila Dokumaci^{3}, Ralph Sinkus^{3}, and Sebastian Kozerke^{1}

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.

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.

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 encoding^{7}, and the high SNR of 3D acquisitions, 4D MRE of
the breast and axial lymph nodes becomes feasible in under 2 minutes.

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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 encoding^{7}. 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.