Matthias Utzschneider1,2, Sebastian Lachner1, Nicolas G.R. Behl3, Lena V. Gast1, Andreas Maier2,4, Michael Uder1, and Armin Nagel1
1Institute of Radiology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany, 2Pattern Recognition Lab, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany, 3Division of Medical Physics in Radiology, German Cancer Research Centre (DKFZ), Heidelberg, Germany, 4Erlangen Graduate School in Advanced Optical Technologies, Erlangen, Germany
Synopsis
Quantitative sodium MRI could be a sensitive tool for therapy monitoring in muscular diseases. However, sodium MRI suffers a low signal-to-noise ratio (SNR). 3D dictionary-learning compressed-sensing (3D-DLCS) enables SNR improvement and acceleration of sodium MRI, but it is dependent on parameterization. In this work a simulation based optimization method for 3D-DLCS is presented, which finds the most suitable parameters for 3D-DLCS in the context of sodium quantification. The method is applied in an in vivo study to quantify sodium in the skeletal muscle. The optimized 3D-DLCS yields a lower quantification error than the reference reconstruction method (Nonuniform FFT).
Introduction
Tissue sodium concentration
(TSC) is potentially a useful measure for muscle tissue constitution and could
be an impactful tool for therapy monitoring in muscular diseases1,2. Sodium MRI (23Na-MRI) is a non-invasive
method to quantify TSC2. However, 23Na-MRI suffers from low signal-to-noise
ratio (SNR) due to the low gyromagnetic ratio, low in vivo concentration and fast
relaxation times of sodium. Compressed sensing3 (CS) based approaches4,5 have been shown to be very effective to improve SNR for 23Na-MRI. However, these iterative methods are dependent on parameterization. In particular
sparsity weighting is still a frequently discussed topic and there is no gold
standard for assessment. In this work, a simulation based assessment method of
CS reconstructions for the application of TSC quantification is proposed. The
method is applied in an in vivo study to optimize parameters for reconstruction
with 3D dictionary-learning compressed-sensing6 (3D-DLCS). Quantitative in vivo TSC maps are reconstructed,
which are undersampled to decrease measurement time and facilitate clinical
applicability.Methods
The assessment approach is based on simulation
of an analytical phantom of the human calf (see Fig. 1). Different tissue types
are simulated with assigned concentrations and T2* relaxation times
corresponding to literature6,7 (fat tissue: 10 mMol/L, blood vessels: 80 mMol/L, muscle
tissue: 12-25 mMol/L, see Figure 1). Four reference tubes (10, 20, 30, 40 mMol/L)
are simulated for normalization and complex white Gaussian noise is added to
match the SNR of the in vivo measurements. The assessment method refers to the
phantom as ground truth (GT) and uses a region-of-interest (ROI) based
determination of the TSC. The normalized maximum (mxEnorm) and mean error (mEnorm) w.r.t. the GT and the normalized mean
standard deviation (mSDnorm) are evaluated inside each ROI. An error
metric (em) is applied to assess reconstructions:
em=√(mxEnorm)2+(mEnorm)2+(mSDnorm)2=√max(¯Xi−¯Xi,ref¯Xi,ref)2+mean(¯Xi−¯Xi,ref¯Xi,ref)2+(σi¯Xi,ref)2,i ϵ [1,#ROI],
where Xi,σi, are the mean intensity and SD of
a chosen ROI in the reconstructed TSC map and Xi,ref the mean intensity in the same ROI of the GT. em weights the SD against the quantification
errors to find the result with lowest uncertainty (low mSDnorm) without over smoothing (low mxEnorm,mEnorm).
The assessment method uses em to find an optimized sparsity weighting factor λem.
To emulate multiple acquisitions, N acquisitions
with different white Gaussian noise distributions are simulated and
reconstructed for every λ.
The reconstruction with the lowest em score determines λem for the dataset.
Simulations: The analytical calf phantom (see Fig. 1) was
simulated with different undersampling factors (USF: 1, 3.2, 4.4, 6.7) and
reconstructed with 3D-DLCS and nonuniform FFT with a Hamming filter (hNUFFT) for reference.
Values for λem were determined for each USF (see Fig. 2) by
the proposed method for optimized 3D-DLCS (optDLCS). Parameters:
block-size: 3x3x2, dictionary size: 300.
In
vivo study: 23Na-MRI was conducted on a 3-T whole body
system (MAGNETOM Skyra, Siemens Healthcare GmbH, Erlangen, Germany). TSC maps
were acquired from the right calf muscle of four healthy volunteers (2 female, 2
male, 28 +/- 4.7 years old) with four reference tubes containing NaCl (10, 20, 30,
40 mMol/L) for normalization. A density-adapted 3D radial acquisition sequence
with an anisotropic field of view8 was used to acquire images with a nominal
spatial resolution of 3x3x15mm3. Acquisition Parameters: TE/ TR = 0.30/150 ms; α = 90°; readout duration TRO = 10 ms. TSC maps
with the same USFs as used in the simulations were acquired and the same
reconstruction parameters were applied (Acquistion times (TA): USF=1: 22:42
min, USF=3: 6:53 min, USF=5: 4:40 min, USF=7: 3:05 min). The most
suitable sparsity weighting factor λem determined in the simulations for each USF was
chosen for the optDLCS reconstructions (see Fig. 2).
Results
For simulations,
the mEnorm stays below 5% for TSC maps reconstructed with
optDLCS. The SD and mEnorm is
lower than using hNUFFT (see Fig. 3). In the in vivo study the mean
quantification error (mEref, USF = 1 as reference) stays
within 3% using optDLCS for USF = 3, 4.4 (USF 6.7: 6%, see Fig. 4). hNUFFT reconstructions with
USF > 1 yield a mEref of more
than 5% and a higher SD than optDLCS. The increases of mEref and
SD with increasing USF are more
pronounced for hNUFFT compared to optDLCS (see
Fig. 4).Conclusion
In this work, we demonstrated that it is possible to accurately quantify TSC from
undersampled 23Na-MRI data using 3D-DLCS with priorly optimized
parameters. Application of the method for undersampled in vivo TSC maps show
promising results, which might enhance clinical applicability of sodium
quantification using 23Na-MRI.Acknowledgements
No acknowledgement found.References
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