Ping Wang1,2, Henry Zhu1,2, Hakmook Kang3, Jake Block2, 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
T1ρ imaging
is sensitive to slow macromolecular interactions which may be generally
characterized by a correlation time (τc) but also varies with the
strength of the locking field used (ω1). At higher fields (3T and beyond) T1ρ is also strongly influenced by
chemical exchange processes and the dispersion of the relaxation rate R1ρ (=1/T1ρ)
with locking field may be used to quantify exchange processes. We imaged normal muscles from individuals of
different ages and found that R1ρ
value is negatively correlated with age in normal muscle, and there is a small dispersion
of R1ρ that appears to increase with
age.
Purpose
T1ρ imaging
is sensitive to slow macromolecular interactions typically within the range of 0
- few KHz, but varies with the strength of the locking field used (ω1).
At higher fields (3T and beyond) T1ρ is also strongly influenced by
chemical exchange processes1,2 and the dispersion of the relaxation
rate R1ρ (=1/T1ρ) with locking field may be used to
quantify exchange processes.3 T1ρ is sensitive to tissue composition
and has been widely used to evaluate the status of brain, liver, and cartilage.4-6 However, T1ρ imaging in muscle is
really rare, and there has not been a previous study performing T1ρ dispersion at high
field in muscle for normal aging. The
purpose of this study was to investigate the effect of aging on T1ρ values in normal muscle, and to quantify the R1ρ (= 1/ T1ρ) dispersion at varying locking fields in muscle.Methods
Seven healthy volunteers (ages 24 to 87, median
age 47) participated in this study with written informed consent obtained prior
to MR imaging. Experiments were
performed on a Philips 3T Achieva scanner (Philips Healthcare, Cleveland OH,
USA). Each subject’s lower leg was placed
in a Rapid sodium quadrature knee coil (Rapid Biomedical GmbH, Rimpar, Germany)
for a parallel study (aimed at measuring muscle sodium), and T1ρ
data were acquired using the scanner body coil.
A B0/B1 inhomogeneity self-compensated T1ρ
pre-pulse sequence7 was implemented to create T1ρ
contrast followed by a Turbo Spin Echo (TSE) data acquisition. A single axial slice covering the calf region was
chosen for imaging, with 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 4min4sec. The T1ρ experiment was repeated at
different spin-locking fields (FSL) [0Hz, 100Hz, 300Hz, 500Hz] to evaluate the R1ρ
dispersion in muscle. After acquisition,
an R1ρ map at each spin-locking frequency was calculated by fitting
the signal intensity vs TSL to a
three-parameter mono-exponential model on a pixel-wise basis at the muscle ROI
(see Figure 1). Finally, median values of
the R1ρ in muscle ROIs were used for comparisons.Results
Figure 2 shows an example of R1ρ map
of a 39y old male subject at FSL=500Hz. Figure
3 is the scatter plot of R1ρ vs
age (at each FSL) with linear regression displayed. It is clear that there is an overall decrease
of R1ρ (or increase of T1ρ) with age at each FSL, which
is confirmed by the R2 (> 0.78) in Table 1. Further, as illustrated in Figure 4 there is little
R1ρ dispersion observed in muscle, though the degree of dispersion
appears to increase to some extent with age.Discussion
Previous research has shown T1ρ increases
with age in health articular cartilage,4 but there has not been such a study performed in muscle. Here we found T1ρ positively
correlates with the increase of age in human calf muscle. Also, our study indicates only little R1ρ
dispersion in muscle at 3T (though it is measureable and increases with age), which may indicate there are greater populations
of exchanging protons (such as hydroxyls) in muscle with age. In cartilage, the increase of T1ρ reflects
changes in the extracellular matrix (ECM) - especially the loss of glycosaminoglycan
(GAG). Similarly, GAG is also one of the
components in muscle ECM and is increasingly implicated in the regulation of
biologic processes.8 Our
study infers that T1ρ imaging may be able to reflect the changes of GAG
and/or other exchangeable species in muscle.Conclusion
Aging related R1ρ decrease (or T1ρ
increase) in human calf muscle was found, and there is little R1ρ
dispersion that appears to increase with age. Acknowledgements
No acknowledgement found.References
1.
Cobb J, Xie J, Li K, et al. Exchange-mediated contrast agents for spin-lock
imaging. MRM. 2012; 67(5):1427–1433.
2.
Cobb J, Li K, Xie J, et al. Exchange-mediated contrast in CEST and spin-lock
imaging. MRI. 2014; 32(1):28–40.
3.
Wang P, Block J, Gore J. Chemical Exchange in Knee Cartilage Assessed by R1ρ
(1/T1ρ) Dispersion at 3T. MRI. 2015; 33(1):38-42.
4.
Goto H, Iwama Y, Fujii M, et al. A preliminary study of the T1rho values of
normal knee cartilage using 3T-MRI. Eur J Radiol. 2012; 81(7):e796-803.
5.
Borthakur A, Wheaton A, Gougoutas A, et al. In vivo measurement of T1rho
dispersion in the human brain at 1.5 tesla. JMRI. 2004; 19(4):403-409.
6.
Yuan J, Zhao F, Griffith JF, et al. Optimized efficient liver T1ρ mapping using
limited spin lock times. Phys Med Biol. 2012; 57(6):1631-1640.
7.
Witschey W, Borthakur A, Elliott M, et al.
Artifacts in T1ρ-Weighted Imaging: Compensation for B1 and B0 Field
Imperfections. JMR. 2007; 186(1):75–85.
8.
Negroni E, Henault E, Chevalier F, et al. Glycosaminoglycan Modifications in
Duchenne Muscular Dystrophy: Specific Remodeling of Chondroitin
Sulfate/Dermatan Sulfate. J Neuropathol
Exp Neurol. 2014; 73(8):789-797.