Ping Wang1,2, Henry Zhu1,2, Hakmook Kang3, and John C. Gore1,2
1Institute of Imaging Science, Vanderbilt University Medical Center, Nashville, TN, United States, 2Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Nashville, TN, United States, 3Department of Biostatistics, Vanderbilt University Medical Center, Nashville, TN, United States
Synopsis
Simultaneous acquisitions of sodium
concentrations and T1ρ in muscles from different aged individuals
show that sodium values increase with age and are accompanied by increases in
the dispersion of spin-lock relaxation rates (i.e. the difference in R1ρ
= 1/T1ρ at low and high locking frequencies). A previous study has suggested that such differences
in R1ρ at different fields reflects the contribution of chemical
exchange to relaxation, which is known to dominate transverse relaxation at
high fields, and potentially reflects GAG concentration in cartilage. In this study, we found ΔR1ρ in muscle
was smaller than in cartilage at 3T but was measureable and showed a strong
correlation with sodium content in muscle.
The increase in sodium with age possibly corresponds to the loss of
muscle mass and increase in extracellular volume within a voxel, but this
appears to be accompanied by an increase in exchangeable protons as well.Introduction
Both sodium and T1ρ imaging have demonstrated
their potentials to explore tissue biochemical composition prior to morphologic
changes,1, 2 and their
correlation has been verified in cartilage osteoarthritis,2 in which cartilage degeneration is
characterized by the loss of glycosaminoglycan (GAG) associated with
proteoglycan within cartilage extracellular matrix.3 In cartilage, T1ρ value is known
to increase with GAG loss, and also, because GAG is negatively charged, it
endows cartilage with a negative fixed charge density which attracts
free-floating positive counter-ions, such as Na+.4 The purpose of this study is to investigate
the correlation between sodium content and T1ρ values in human
muscle, which might provide more specific and complementary information for
muscular diseases.Methods
Six healthy subjects (ages 24 to 87, median age
49) were recruited for this study with written informed consent obtained. MR imaging was performed on a Philips 3T
Achieva system (Philips Healthcare, Cleveland OH, USA) with a Rapid sodium
quadrature knee coil (Rapid Biomedical GmbH, Rimpar, Germany) for sodium
imaging. Four calibration phantoms (NaCl
aqueous solution with [Na]: 10mM, 20mM, 30mM, and 40mM) were scanned together
with calf for computing tissue relative sodium concentration through linear
trend analysis. Sodium imaging
parameters: 3D GRE, FOV: 192×192mm2, 7 slices at a thickness of
30mm, bandwidth=434Hz/pixel, TR/TE/FA=50ms/0.99ms/85°, NEX=64, resulting in a
total scan time of 15min02sec. T1ρ
data were acquired by the scanner body coil (T1ρ pre-pulse5
followed by a Turbo Spin Echo (TSE) sequence), a single axial slice (with the
same isocenter as sodium imaging) was scanned with parameters: FOV: 192×192mm2,
pixel size: 1×1mm2, slice thickness: 4mm, TR/TE=4000ms/10ms, TSE
factor=15, NEX=1. Five spin-locking
times (TSL) [2ms, 22ms, 42ms, 62ms, 82ms] were combined into a single scan for
T1ρ calculations, resulting in a scan time of 4min04sec. The T1ρ measurement was repeated
at different spin-locking fields (FSL) [0Hz, 100Hz, 300Hz, 500Hz] to evaluate
the T1ρ dispersion with locking frequency in muscle. After acquisition, a T1ρ map at
each spin-locking frequency was calculated by fitting the signal intensity vs TSL to a three-parameter
mono-exponential model. Both sodium
content and T1ρ value were calculated on a pixel-wise basis at the
muscle ROI (see Figure 1), and the median values were used for the correlation
analysis. The variation of the
relaxation rate R1ρ (= 1/T1ρ) with locking field was
plotted and the dispersion between low and high locking frequencies was used as
a measure of chemical exchange effects within muscle by analogy to previous
studies in cartilage.6 We
specifically evaluated the correlation between ΔR1ρ = R1ρ(0Hz)
- R1ρ(500Hz) and sodium content in muscle, which likely reflects the
correlation between exchangeable protons and sodium in muscle.Results
Figure 2 shows an example of sodium image and R1ρ
map (FSL=300Hz) of a 24y old male subject.
Figure 3 plots the R1ρ dispersion in muscle for all the
six subjects, note that little dispersion is observed, though the degree of
dispersion appears to increase with age.
Further, Figure 4 illustrates the correlation between ΔR1ρ =
R1ρ(0Hz) - R1ρ(500Hz) vs
sodium content, a strong linearship is observed (R2=0.961).Discussion
GAG is an important component in the extracellular
matrix of both cartilage and muscle. A previous
study has suggested that ΔR1ρ reflects the contribution of chemical exchange
to relaxation, which is known to dominate transverse relaxation at high fields7,8,
and potentially reflects GAG concentration in cartilage. In this study, we found ΔR1ρ was
smaller than in cartilage at 3T but was measureable and showed a strong
correlation with sodium content in muscle.
The increase in sodium with age possibly corresponds to the loss of
muscle mass and increase in extracellular volume within a voxel, but this appears
to be accompanied by an increase in exchangeable protons as well. Whether this increase corresponds to GAG in
muscle remains to be investigated.Acknowledgements
No acknowledgement found.References
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