Volumetric structural magnetic resonance imaging (MRI) is commonly used to determine the extent of neuronal loss in aging, indicated by cerebral atrophy. The brain, however, exhibits other ...biophysical characteristics such as mechanical properties, which can be quantified with magnetic resonance elastography (MRE). MRE is an emerging noninvasive imaging technique for measuring viscoelastic tissue properties, proven to be sensitive metrics of neural tissue integrity, as described by shear stiffness, μ and damping ratio, ξ parameters. The study objective was to evaluate global and regional MRE parameter differences between young (19–30 years, n = 12) and healthy older adults (66–73 years, n = 12) and to assess whether MRE measures provide additive value over volumetric magnetic resonance imaging measurements. We investigated the viscoelasticity of the global cerebrum and 6 regions of interest (ROIs) including the amygdala, hippocampus, caudate, pallidum, putamen, and thalamus. In older adults, we found a decrease in μ in all ROIs, except for the hippocampus, indicating widespread brain softening; an effect that remained significant after controlling for ROI volume. In contrast, the relative viscous-to-elastic behavior of the brain ξ did not differ between age groups, suggesting a preservation of the organization of the tissue microstructure. These data support the use of MRE as a novel imaging biomarker for characterizing age-related differences to neural tissue not captured by volumetric imaging alone.
Manual palpation is a clinical methodology to determine tissue mechanical properties, such as viscoelasticity (i.e. stiffness and viscosity), which is a primary indicator of the development of tissue ...pathology. Advancing medical imaging technology means it is now possible to reliably non-invasively measure tissue stiffness in-vivo through the use of Magnetic Resonance Elastography (MRE). Muscle pathology is traditionally assessed in the clinic through measurement of muscle morphology and function (e.g. Maximum Voluntary Contraction MVC). However, MRE has been shown to be an effective method to study muscle pathology and may offer novel biomechanical insight into, for example, muscle engagement, injury and recovery, which cannot be obtained through conventional testing. The aim of this thesis is to perform a series of exploratory investigations to determine the precision, sensitivity and reliability of the muscle MRE technique for studying the relationships between muscle mechanical properties and morphology. This is especially relevant to the clinical application of the technique which is investigated in two pilot studies. Specific interests are to investigate whether muscle MRE offers reliable insight regarding muscle ageing, injury and loading and has potential clinical application such as in monitoring recovery after time in Critical Care and the effects of Total Knee Replacement (TKR) in patients with osteoarthritis of the knee. This thesis begins with a review of musculoskeletal biomechanics, Magnetic Resonance Imaging (MRI) and MRE research to date. A limited number of clinical musculoskeletal elastography research studies were identified and which motivated several investigations conducted in this thesis. A musculoskeletal MRE analysis pipeline was developed to accurately acquire and analyse MRE data and consists of image co-registration, quantification of muscle mechanical (i.e. stiffness) and morphological properties (i.e. muscle cross-sectional area and a shape measure referred to as circularity), which may be related to clinical measures and relevant functional indices such as MVC. The pipeline includes quality control procedures to detect image artefacts and provides results which can be potentially reliably compared with those of other research groups. The first two investigations to be reported concern the study of changes in the mechanical properties of muscles that have occurred passively. In particular, the effects of ageing are studied together with the effect of time spent in Critical Care and subsequent rehabilitation. The effect of ageing was primarily evident in the quadriceps muscle group which decreased significantly in cross-sectional area and significantly increased in stiffness. The effects produced by immobilisation were also predominantly in the quadriceps but here a significant decrease in muscle cross-sectional area was associated with a decrease in muscle stiffness. The next three exploratory studies all involve an intervention or manipulation in terms of an eccentric exercise protocol which produces muscle injury as well as muscle loading. The former was based on a re-analysis of previously published work with the aim of determining whether there was a significant difference in muscle stiffness in subjects in whom injury was shown to be associated with muscle oedema on T2-weighted MR images. Here the new pixel-wise analysis of the data showed that although the two groups of subjects performed a similar workload, subjects who developed oedema may have used a different combination of muscles to perform the task, and especially may have additionally recruited medial muscles rather than efficiently co-contracting the quadriceps and hamstrings. A loading study revealed a significant relationship between the stiffness and shape (i.e. circularity) of especially rectus femoris and first steps were taken to investigate whether this relationship may show insight into the recovery of patients following TKR surgery. Taken together these exploratory investigations demonstrate the precision, sensitivity and viability of the muscle MRE technique and its promise for potential clinical application.
Purpose
To investigate the effect of warmup by application of the thermal agent Deep Heat (DH) on muscle mechanical properties using magnetic resonance elastography (MRE) at 3T before and after ...exercise‐induced muscle damage (EIMD).
Materials and Methods
Twenty male participants performed an individualized protocol designed to induce EIMD in the quadriceps. DH was applied to the thigh in 50% of the participants before exercise. MRE, T2‐weighted MRI, maximal voluntary contraction (MVC), creatine kinase (CK) concentration, and muscle soreness were measured before and after the protocol to assess EIMD effects. Five participants were excluded: four having not experienced EIMD and one due to incidental findings.
Results
Total workload performed during the EIMD protocol was greater in the DH group than the control group (P < 0.03), despite no significant differences in baseline MVC (P = 0.23). Shear stiffness |G*| increased in the rectus femoris (RF) muscle in both groups (P < 0.03); however, DH was not a significant between‐group factor (P = 0.15). MVC values returned to baseline faster in the DH group (5 days) than the control group (7 days). Participants who displayed hyperintensity on T2‐weighted images had a greater stiffness increase following damage than those without: RF; 0.61 kPa vs. 0.15 kPa, P < 0.006, vastus intermedius; 0.34 kPa vs. 0.03 kPa, P = 0.06.
Conclusion
EIMD produces increased muscle stiffness as measured by MRE, with the change in |G*| significantly increased when T2 hyperintensity was present. DH did not affect CK concentration or soreness; however, DH participants produced greater workload during the EIMD protocol and exhibited accelerated MVC recovery.
Level of Evidence: 1
Technical Efficacy: Stage 2
J. Magn. Reson. Imaging 2017;46:1115–1127.
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