Chengchuan Wu1, Yasmin Blunck1, and Leigh Johnston1
1Biomedical Engineering, The University of Melbourne, Parkville, Australia
Synopsis
This work
presents SELA, a 23Na MRI sequence that can simultaneously acquire spin-density-weighted
(SDW) and SNR-enhanced fluid-attenuated (FLAIR) images in one scan. The sequence
was examined by numerical simulation and phantom experiment in a 7T preclinical
scanner. Preliminary results support the design purpose to improve 23Na MRI efficiency and 23Na-FLAIR image quality.
INTRODUCTION
Spin-density-weighted
(SDW) and fluid-attenuated (FLAIR) 23Na MRI provide information on total sodium concentration and potential
insight into intracellular sodium distribution, respectively1. However,
low SNR and SAR limitations in 23Na MRI necessitate long acquisition
times, making the sequential acquisition of these two forms of image contrast very time-consuming. To address
this issue, SELA is proposed for the simultaneous acquisition of Spin-density-weighted
and SNR-Enhance fLuid-attenuated 23NA images, providing more information
in one scan under SAR values equivalent to a conventional hard-pulse
inversion recovery (HIR) acquisition. In addition to the efficiency advantage, SELA
also produces stronger signal in the FLAIR image, indicating its potential of
improving intracellular 23Na imaging which is generally hampered by poor SNR.METHODS
The SELA sequence (block diagram is shown in Figure 1) consists of three $$$90^\circ$$$ hard pulses along the same axis and two UTE readouts. The first
pulse tips the magnetisation into the transverse plane, followed by a UTE readout
acquiring SWD data and a subsequent rewinder gradient. During the delay time $$$\tau$$$ between the first two
pulses, the magnetisations in different types of environment are differentiated
by $$$T^*_2$$$ relaxation. The second
pulse completes the inversion. Subsequently, the third pulse is applied at the inversion
time that selectively nullifies the fluid signal, producing FLAIR signals from
non-nulled environments. Both SDW and FLAIR acquisitions are based on 3D radial
UTE readouts.
Simulations
23Na spin dynamical simulation was done
in MATLAB based on the tensor operator formalism2.
Three sets of parameters were obtained from the literature3: i)
spectral density $$$J_0/J_1/J_2 = 190/21/14\,$$$Hz, residual quadrupolar interaction frequency $$$\omega_0 = 0\,$$$Hz for simulation of 50
mM 23Na in 3% agar; ii) $$$J_0/J_1/J_2 =
300/31/18\,$$$Hz, $$$\omega_0 = 0\,$$$Hz for 50 mM 23Na
in 6% agar; iii) $$$J_0/J_1/J_2 = 9.4/9.4/9.4\,$$$Hz, $$$\omega_0 = 0\,$$$Hz for 50 mM 23Na
in saline. SELA and HIR were simulated with sequence parameters listed in Table
1. The inversion pulse duration and the excitation pulse duration for HIR were respectively
chosen to be 1 ms and 500
µs, resulting in SAR equivalent to SELA which consists of
three 500-µs pulses. The delay time $$$\tau$$$ was optimised for 23Na
in 6% agar through simulation.
7T Experimental Acquisition
Phantom experiment
was performed on a research 7T MRI scanner (Siemens Healthineers,
Erlangen,Germany) with a dual-tuned 1H-23Na
transmit/receive head coil (QED, USA). The phantom consisted of 5 vials with
varying concentrations of sodium and agar submerged in a cylindrical container
with saline (Figure 3a). All images were reconstructed offline via re-gridding
in MATLAB onto an isotropic 3.1 mm resolution grid. Prior to image acquisition,
shimming was performed using a vendor-provided 1H-based shimming
procedure. Images were corrected for Rician noise and analysed for SNR4.RESULTS
The
simulated 23Na transverse magnetisations are plotted in Figure 2. Peaks
occurring around 40 ms represent FLAIR signals. In 3% agar environment, the peak
signal produced by SELA is 31% higher than the one by HIR. In 6% agar, the peak
produced by SELA is 18% higher.
Phantom
schematic, the SDW image and the FLAIR images acquired by SELA and HIR are presented
in Figure 3a-d. SNR of the phantom images is shown in the bar graph in Figure
3e. SNR is at least 12 and as high as 40 in the SDW image. In terms of FLAIR images, significant signal
attenuation is seen in saline and 1% agar while signals in 3% and 6% agar vials
are mostly retained. SNR in FLAIR image is improved by 6 – 30% using SELA sequence.
Both SELA and HIR take 8.3 minutes to acquire 2000 projections.DISCUSSION
Although the delay time $$$\tau$$$, optimised in the simulation,
limits the achievable readout duration for the SDW UTE readout, phantom image shows
good contrast and SNR, demonstrating that the time window is sufficient for SWD
readout.
Analysing FLAIR image quality, both simulation and phantom experiment
indicate that the signals from slow-motion environments are stronger using SELA
sequence. This is the result of the $$$T^*_2$$$ relaxation during the delay $$$\tau$$$ which expands the difference
between magnetisations in fast-motion and slow-motion environments. SELA improves SNR in FLAIR images,
and further increase is expected by extending the readout duration (currently
a short 6-ms readout duration was employed). Increasing the number of
projections or averaging is another way to enhance SNR, however in the cost of increasing acquisition time. These aspects will be optimised in future phantom
experiments and validated in in-vivo experiments.CONCLUSION
This work
demonstrates that the proposed sequence SELA yields SDW and FLAIR images in one
scan at no additional SAR expense. The sequence improves 23Na MRI scan efficiency and SNR, offering multiple markers of sodium content with potential clinical
relevance for both diagnosis and treatment monitoring5.Acknowledgements
We acknowledge
the facilities, and the scientific and technical assistance of the Australian
National Imaging Facility, a National Collaborative Research Infrastructure
Strategy (NCRIS) capability, at the Melbourne Brain Centre Imaging Unit of the
University of Melbourne. The work was also supported by a research
collaboration agreement with Siemens Healthcare Australia.References
[1] Madelin G, Regatte
RR. Biomedical applications of sodium MRI in vivo. Journal of Magnetic
Resonance Imaging. 2013 Sep;38(3):511-29.
[2] Van der Maarel JR.
Thermal relaxation and coherence dynamics of spin 3/2. I. Static and
fluctuating quadrupolar interactions in the multipole basis. Concepts in
Magnetic Resonance Part A: An Educational Journal. 2003;19(2):97-116.
[3] Stobbe RW, Beaulieu
C. Exploring and enhancing relaxation-based sodium MRI contrast. Magnetic
Resonance Materials in Physics, Biology and Medicine. 2014 Feb 1;27(1):21-33.
[4] Gudbjartsson H, Patz S. The Rician distribution of noisy MRI data. Magnetic resonance in medicine. 1995 Dec;34(6):910-4.
[5] Thulborn KR. Quantitative sodium MR imaging: a
review of its evolving role in medicine. Neuroimage. 2018 Mar 1;168:250-68.