Hongyan Liu^{1}, Tom Bruijnen^{1}, Maaike van Haandel^{1}, Oscar van den Heide^{1}, Miha Fuderer^{1}, Cornelis A.T. van den Berg^{1}, and Alessandro A.T. Sbrizzi^{1}

^{1}Computational Imaging Group for MR diagnostics & therapy, Center for Image Sciences, UMC Utrecht, Utrecht, Netherlands

We propose a new method to increase the T2 encoding ability of MR-STAT sequences, according to a recently developed strategy for T2 mapping. Recent work has shown that RF phase modulated GRE sequences with small quadratic phase increments can effectively encode T2 information into the phase of the signal. In this abstract, we show that by incorporating a simple and similar RF modulation strategy in 2D MR-STAT sequences, T2 sensitivity of transient-state gradient-spoiled sequences is improved, and therefore T2 maps as well as the proton density maps (PD) can be reconstructed with higher accuracy.

In this work, we propose a new method to increase the T2 encoding ability of the transient-state, gradient-spoiled GRE sequences according to a recently developed strategy for T2 mapping. Wang et al

- Sequence design

$$$\phi(n)=\begin{cases}0\text{ degree},&n=0,448,\\\phi(n-1)+2n\text{ degree},&0<n<448,\\\phi(n-1)-2n\text{ degree},&448<n<1120.\end{cases}$$$

The Cartesian phase-encoding pattern is also shown in Fig.1(a). Five k-spaces are sampled. RF phases are quadratically increasing in the first two k-spaces and then quadratically decreasing in the last three k-spaces.

Fig.1(b) shows the amplitude and phase of the transient-state signals using both MR-STAT sequences for two different sets of parameter values: T1=1000ms, T2=60/90ms. When using the RF phase modulated sequence, a small time-varying phase difference for two signals with different T2 values can be observed, while using the no RF phase sequence, the phases are constantly zero.

- Numerical experiment

- Gel phantom and in-vivo experiment

Fig.3(a) shows the T1 and T2 maps reconstructed from gel-phantom data, and Fig.3(b) summarizes the T1 and T2 mean values as well as standard deviations. It can be observed for the standard deviations are approximately 50% lower for tubes with relatively low T2 values (tube 1 to 3) when using RF modulated sequence.

Fig.4 shows the quantitative maps reconstructed from in-vivo data. T2 and PD maps from RF phase modulated sequence show improved contrast-to-noise. Lower CSF values and no hyper-intense blood vessel (in-flow effects) are also observed in T2 maps using the RF modulated sequence.

We believe that the T2 improvement using RF phase encoding generalizes to other transient-state sequences, such as MR Fingerprinting and therefore warrant further investigation and optimization by the quantitative MRI community. Currently Cramer-Rao-based optimizations are used for sequence design in quantitative imaging such as MR-STAT and MRF

[1] van der Heide, Oscar, et al. “Extension of MR-STAT to non-Cartesian and gradient-spoiled sequences.” Proc. Intl. Soc. Mag. Reson. Med. 28 (2020), 0886.

[2] Jiang, Yun, et al. "MR fingerprinting using fast imaging with steady state precession (FISP) with spiral readout." Magnetic resonance in medicine 74.6 (2015): 1621-1631.

[3] Hamilton, Jesse I., et al. "MR fingerprinting for rapid quantification of myocardial T1, T2, and proton spin density." Magnetic resonance in medicine 77.4 (2017): 1446-1458.

[4] Wang, Xiaoke, Diego Hernando, and Scott B. Reeder. "Phase‐based T2 mapping with gradient echo imaging." Magnetic resonance in medicine 84.2 (2020): 609-619.

[5] Aubert-Broche, Berengere, Alan C. Evans, and Louis Collins. "A new improved version of the realistic digital brain phantom." NeuroImage 32.1 (2006): 138-145.

[6] Wang, Charlie Yi, et al. "Magnetic resonance fingerprinting with quadratic RF phase for measurement of T2* simultaneously with δf, T1, and T2." Magnetic resonance in medicine 81.3 (2019): 1849-1862.

[7] Fuderer, Miha, et al. “ BLAKJac - A computationally efficient noise-propagation performance metric for the analysis and optimization of MR-STAT sequences.” Proc. Intl. Soc. Mag. Reson. Med. 28 (2020), 3059.

[8] Zhao, Bo, et al. "Optimal experiment design for magnetic resonance fingerprinting: Cramer-Rao bound meets spin dynamics." IEEE transactions on medical imaging 38.3 (2018): 844-861.

Fig1.(a) The flip-angle train, RF phase modulation and
phase-encoding pattern used for simulations and experiments. (b) The amplitude and phase of the transient-state
signals using either RF phase modulated MR-STAT sequence or no RF phase
sequence for two different T2 values. T1=1000ms
for all simulations. Other sequence parameters are: TE=4.0ms, TR=7.8ms,
readout frequency=191.7Hz, image size = 224x224, resolution=1x1x3mm^3. An
adiabatic inversion pulse is used at the beginning of the sequence for better
T1 encoding. The scan time is 8.8 seconds.

Fig.2. T1, T2 and PD maps reconstructed from simulation datasets (SNR = 20) using no RF phase modulation sequence and RF phase modulated sequence with MR-STAT reconstruction. Absolute relative error maps are shown and MAPEs (mean absolute percentage error) are computed. We observe that data using RF phase encoding result in ~50% lower errors in T2 and PD maps.

Fig.3. Gel phantom
results using the no RF phase sequence and RF phase modulated sequence. (a) T1
and T2 maps using both sequences. (b) Bar plots with mean T1 and T2 values as
well as standard deviations for each gel phantom. Note that results from RF
phase modulated sequence show lower standard deviations for T2.

Fig. 4. In-vivo results
using the no RF phase sequence and RF phase modulated sequence. It can be
observed that T2 and proton density (PD) maps from RF phase modulated sequence
show better contrast (black arrow for example) and lower noise. Lower values of CSF and the blood vessel (red arrow) are also observed in T2 maps using
the RF modulated sequence.

DOI: https://doi.org/10.58530/2022/0625