Probing the microscopic environment of <sup>23</sup>Na ions in brain tissue by MRI: On the accuracy of different sampling schemes for the determination of rapid, biexponential  decay at low signal‐to‐noise ratio

Purpose Parameter mapping in sodium MRI data is challenging due to inherently low SNR and spatial resolution, prompting the need to employ robust models and estimation techniques. This work aims to develop a continuum model of sodium T2*‐decay to overcome the limitations of the commonly employed bi‐exponential models. Estimates of mean T2*‐decay and fast component fraction in tissue are emergent from the inferred continuum model. MethodsA closed‐form continuum model was derived assuming a gamma distribution of T2* components. Sodium MRI was performed on four healthy human subjects and a phantom consisting of closely packed vials filled with an aqueous solution of varying sodium and agarose concentrations. The continuum model was applied to the phantom and in vivo human multi‐echo 7T data. Parameter maps by voxelwise model‐fitting were obtained. ResultsThe continuum model demonstrated comparable estimation performance to the bi‐exponential model. The parameter maps provided improved contrast between tissue structures. The fast component fraction, an indicator of the heterogeneity of localised sodium motion regimes in tissue, was zero in CSF and high in WM structures. ConclusionsThe continuum distribution model provides high quality, high contrast parameter maps, and informative voxelwise estimates of the relative weighting between fast and slow decay components.

“…However as the bi‐exponential model can be envisaged as sampling

T_{2}^{*}

distribution at two discrete points, the fast and slow

T_{2}^{*}

estimates should lie reasonably close to mean

T_{2}^{*}

in voxels with relatively narrow

T_{2}^{*}

distributions. For example in CSF voxels without partial voluming, a relatively smaller number of compartments are observed and the mean

T_{2}^{*}

values are reasonably closer to the previous studies . The wide spread of mean

T_{2}^{*}

Section: Discussionsupporting

confidence: 84%

“…The wide spread of mean

T_{2}^{*}

T_{2}^{*}

combines contributions from both fast and slow transverse relaxation components in various compartmentalized motion regimes, the average mean

T_{2}^{*}

values of 5 ms (WM) and 7 ms (GM) lie expectedly in the range (

T_{2_{italicshort}}^{*}

T_{2_{italiclong}}^{*}

) derived from previously reported values …”

Section: Discussionmentioning

confidence: 60%

A continuum of components: Flexible fast fraction mapping in sodium MRI

Syeda

Blunck

Kolbe

et al. 2019

“…It is worth mentioning that recently a number of studies have shown the feasibility of voxel-wise T * 2 mapping in the brain using UTE-3D acquisition techniques. [16][17][18][19][20] However, these techniques were conducted with fewer time points (8-38 echoes), and require long acquisition times (26 min-1 h). Moreover, many of these methods were either done with lower spatial resolution, at a higher magnetic field, or done with both.…”

mentioning

confidence: 99%

Fast in vivo ²³Na imaging and mapping using accelerated 2D‐FID UTE magnetic resonance spectroscopic imaging at 3 T: Proof of concept and reliability study

Alhulail

Xia

Shen

et al. 2020

Self Cite

To implement an accelerated MR-acquisition method allowing to map T * 2 relaxation and absolute concentration of sodium within skeletal muscles at 3T. Methods: A fast-UTE-2D density-weighted concentric-ring-trajectory 23 Na-MRSI technique was used to acquire 64 time points of FID with a spectral bandwidth of 312.5 Hz with an in-plane resolution of 2.5 × 2.5 mm 2 in ~15 min. The fast-relaxing 23 Na signal was localized with a single-shot, inversion-recovery-based, non-echo (SIRENE) outer volume suppression (OVS) method. The sequence was verified using simulation and phantom studies before implementing it in human calf muscles. To evaluate the 2D-SIRENE-MRSI (UTE = 0.55 ms) imaging performance, it was compared to a 3D-MRI (UTE = 0.3 ms) sequence. Both data sets were acquired within 2 same-day sessions to assess repeatability. The T * 2 values were fitted voxelby-voxel using a biexponential model for the 2D-MRSI data. Finally, intra-subject coefficients of variation (CV) were estimated. Results: The MRSI-FID data allowed us to map the fast and slow components of T * 2 in the calf muscles. The spatial distributions of 23 Na concentration for both MRSI and 3D-MRI acquisitions were significantly correlated (P < .001). The test-retest analysis rendered high repeatability for MRSI with a CV of 5%. The mean T * 2Fast in muscles was 0.7 ± 0.1 ms (contribution fraction = 37%), whereas T * 2Slow was 13.2 ± 0.2 ms (63%). The mean absolute muscle 23 Na concentration calculated from the T * 2-corrected data was 28.6 ± 3.3 mM. 1784 | ALHULAIL et AL. Conclusion: The proposed MRSI technique is a reliable technique to map sodium's absolute concentration and T * 2 within a clinically acceptable scan time at 3T.

“…In multi-echo data acquisitions the time gain allows for faster refocusing and consequently closer echo spacing that is beneficial for the estimation of signal decay parameter, in particular for the short T 2 decay component. 27,37,38 The zG RF -RHE acquisition approach shows image quality improvements across investigated T RO , whereas for shorter T RO an improvement in edge delineation was observed, SNR improvements were found particularly for longer T RO .…”

Section: Discussionmentioning

confidence: 91%

Zero‐gradient‐excitation ramped hybrid encoding (zG_RF‐RHE) sodium MRI

Blunck

Moffat

Kolbe

et al. 2018

Purpose Fast bi‐exponential transverse signal decay compounds sodium image quality. This work aims at enhancing image characteristics using a special case of ramped hybrid encoding (RHE). Zero‐gradient‐excitation (zGRF)‐RHE provides (1) gradient‐free excitation for high flip angle, artifact‐free excitation profiles and (2) gradient ramping during dead‐time for the optimization of encoding time (tenc) to reduce T2* signal decay influence during acquisition. Methods Radial zGRF‐RHE and standard radial UTE were investigated over a range of receiver bandwidths in simulations, phantom and in vivo brain experiments. Central k‐space in zGRF‐RHE was acquired through single point measurements at the minimum achievable TE. T2* blurring artifacts and image SNR and CNR were assessed. Results zGRF ‐RHE enabled 90° flip angle artifact‐free excitation, whereas gradient pre‐ramping provided greater tenc efficiency for any readout bandwidths. Experiments confirmed simulation results, revealing sharper edge characteristics particularly at short readout durations (TRO). Significant SNR improvements of up to 4.8% were observed for longer TRO. Conclusion zGRF ‐RHE allows for artifact‐free high flip angle excitation with time‐efficient encoding improving on image characteristics. This hybrid encoding concept with gradient pre‐ramping is trajectory independent and can be introduced in any center‐out UTE trajectory design.