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Quantitative susceptibility mapping and sodium imaging based analysis of susceptibility and sodium concentrations in the basal ganglia1Department of Neuroradiology, University Hospital Heidelberg, Heidelberg, Germany, 2Division of Medical Physics, German Cancer Research Center (DKFZ), Heidelberg, Germany
Synopsis
23Na concentrations and iron deposition in cerebral gray matter have both shown to be increased in degenerative and inflammatory cerebral diseases. This study employs sodium imaging and quantitative susceptibility mapping to assess differences in sodium concentrations and susceptibility within the basal ganglia in healthy volunteers at 7T. Results indicate a fundamentally different distribution of 23Na concentrations compared to the distribution of susceptibility within the nuclei of the basal ganglia and suggest that not only susceptibility but also 23Na concentrations may be physiologically distributed in a characteristic manner.
INTRODUCTION
Quantitative susceptibility mapping (QSM) not only provides a spatially specific image contrast for optimized discrimination of basal ganglia 1,2, but also enables detection of increased iron deposition in the basal ganglia which has become a biomarker for a range of degenerative and inflammatory diseases such as multiple sclerosis, Parkinson’s and Huntington’s disease as well as alcohol use disorder 3-7. Similarly, increased 23Na concentrations in cerebral gray matter have been linked to Alzheimer’s disease and an increasing severity of multiple sclerosis 8,9 as 23Na tissue concentrations are thought to be dependent on volume of extracellular space and cellular membrane integrity. In this study, QSM-based mapping of the basal ganglia and sodium imaging are used to assess differences in sodium concentrations and susceptibility within the basal ganglia.METHODS
The study was conducted in accordance with the
Declaration of Helsinki. Institutional review board approval was obtained and
all subjects provided written informed consent. Five healthy volunteers (mean
age 28.4 ± 6.5 years; three female) were scanned on a 7 T whole-body MR system (MAGNETOM
7 T, Siemens Healthcare GmbH, Germany). A monopolar 3D gradient echo (GRE)
sequence was acquired for susceptibility mapping with a 8Tx/32Rx-channel head
coil (Nova Medical Inc., Wakefield, MA, USA) operated in CP+ mode using an
in-house-built Butler matrix with TR = 21 ms, TE1,2,3 = 6/12/18 ms, flip
angle = 10°, 0.5 mm isotropic resolution, and acquisition time 9:17 min:sec. After
a coil change, 23Na data were acquired using a double-resonant
1H/23Na Tx/Rx quadrature volume head coil integrating a 30-channel 23Na
Rx phased array (Rapid Biomedical GmbH, Rimpar, Germany) and a density adapted
3D radial pulse sequence ((Δx)3=(2.0mm)3, 7000 projections,
TR = 100 ms, TE1 = 0.35 ms, flip angle=90°, TA=11:40 min)12. The data were
reconstructed with adaptive combination (ADC). Sodium data were referenced such
that mean cerebrospinal fluid sodium values equaled the physiological
concentration of 140 mmol/l. A T1-weighted GRE-sequence was acquired
to facilitate image registration.
Susceptibility maps were generated from phase data that
were combined on the scanner using ASPIRE 13.
Brain masks were calculated using FSL-BET 14 from the first
echo of the GRE magnitude data. Laplacian-based phase unwrapping, V-SHARP 15-17
with kernel size up to 12 mm for background field removal and
STAR-QSM 18
were used in Matlab (R2017b, MathWorks, Natick, USA) to calculate
susceptibility maps. Volumes of interest (VOIs) (see Figure 1) for basal
ganglia were manually drawn on susceptibility maps with the Medical Imaging
Interaction Toolkit (MITK) 19,20.
Based
on the recorded
T1-weighted images, VOIs were
registered to sodium data in MITK.RESULTS
Figure 1 shows the segmented basal ganglia on
QSM with the red nucleus in orange, the caudate nucleus in red, the thalamus in
green, the pallidum in blue and the putamen in yellow. Figure 2 shows the
registered masks on axial 23Na images. Mean susceptibility and sodium values are
given in Table 1. The results show that the globus pallidus, the red nucleus
and the thalamus contain sodium concentrations in a similar range while caudate
and the putamen each feature distinctly higher sodium concentrations. In
comparison, susceptibility values of the basal ganglia possess a very different
distribution with values of the thalamus well below all other nuclei and comparatively
high values in the globus pallidus and red nucleus.DISCUSSION and CONCLUSION
Although 23Na concentrations and susceptibility of cerebral gray matter have both been shown to be elevated in degenerative and inflammatory diseases, the distribution of 23Na concentrations appears to be fundamentally different compared to the distribution of susceptibility within the nuclei of the basal ganglia. The interpretation of this study is limited by a low number of studied subjects; the results however suggest that not only susceptibility but also 23Na concentrations may be physiologically distributed in a characteristic manner. A recent study also investigating 23Na concentrations within the basal ganglia at a lower nominal isotropic resolution reported similar concentrations in the thalamus while the distribution of 23Na concentrations in the remaining basal ganglia differed in comparison to this study 21. A possible reason for differing results may be partial volume effects.
The results of this study suggest that basal ganglia may not only distinguish themselves through characteristic susceptibilities but also characteristic 23Na concentrations.
Acknowledgements
The provision of the ASPIRE gradient echo sequence and corresponding ICE program for coil combination of the 7T GRE data by Korbinian Eckstein and Simon D. Robinson is kindly acknowledged.References
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