Jules Laurent Nelissen1,2, Willeke Traa3, Larry de Graaf1, Kevin Moerman4, Cees Oomens3, Aart Nederveen5, Ralph Sinkus6, Klaas Nicolay1, and Gustav Strijkers2
1Biomedical NMR, Eindhoven University of Technology, Eindhoven, Netherlands, 2Preclinical and Translational MRI, Academic Medical Center, Amsterdam, Netherlands, 3Biomechanics of Soft Tissues, Eindhoven University of Technology, Eindhoven, Netherlands, 4MIT media lab, Massachusetts Institute of Technology, Cambridge, MA, United States, 5Radiology, Academic Medical Center, Amsterdam, Netherlands, 6Imaging Sciences & Biomedical Engineering, King's College London, London, United Kingdom
Synopsis
We have investigated the feasibility of using Magnetic Resonance
Elastography (MRE) to quantify muscle-tissue mechanical properties and changes
therein related to the development of deep tissue injury type of pressure
ulcers. MRE measurements were performed before and after
damage-inducing indentation of the tibialis-anterior muscle of Sprague Dawley
rats. Current study demonstrates that changes in muscle-tissue mechanical
properties associated with deep tissue injury can be quantified by MRE. We
expect that better knowledge of changes in soft tissue mechanical properties
due to damage, measured with MRE, will provide new insights in the aetiology of
deep tissue injury and other muscle pathologies.Target audience
Scientists interested in pressure ulcer research,
skeletal muscle damage, musculoskeletal MRI, and MR Elastography.
Purpose
We have investigated the feasibility of using Magnetic Resonance Elastography (MRE) to quantify muscle-tissue mechanical properties and changes therein related to the development of pressure ulcers, more specifically wounds that develop in skeletal muscle (deep tissue injury).
[1][2][3] MRE measurements were performed before and after damage-inducing indentation of the tibialis anterior (TA) muscle of Sprague Dawley (SD) rats.
[4]Materials & Methods
MR Compatible indentation and MRE setup:
A detail of the setup for both indentation and MRE is shown in Fig. 1. The MRE piston is brought into motion via the drive rod attached to an electromagnetic shaker, placed outside the magnetic field of the MR scanner, and cantilever. The indentor can apply sustained mechanical loading to the TA muscle of rats. [5][6]
Rat model:
11 to 13 week-old SD rats (female, n=5) were measured. The right leg of the rat was shaved and placed in a plastic profile filled with alginate molding substance for fixation, while keeping the TA muscle accessible for the indentor and the MRE piston. The MRE piston was gently placed against the tendon at the distal side of the TA muscle. Indentation of the TA muscle, for a period of 2 hours, took place in the MR scanner.
In vivo MRI:
A Bruker 7.0 T small animal scanner was used with a 2 cm diameter receive surface coil, placed on top of the TA muscle inside the MR compatible indentation setup, in combination with a 86 mm diameter excitation coil. Skeletal muscle injury was assessed with T2-mapping MRI (Spin-Echo, 18 slices, FOV = 3 x 6 cm2, MTX = 256 x 512, TE = 10.2 – 203.6 ms, 20 echoes, TR = 4s, fat suppression) and changes in mechanical properties with an in-house implemented fast MRE sequence (SE-MRE-EPI, 18 slices of 1 mm, FOV = 3 x 6 cm2, MTX = 96 x 192, 16 MRE frames, s|p|f|ref-directions, acquisition time: 16 min). Measurements were performed before and up to 90 min after indentation.
Data Analysis:
MRE elastograms representing the real part of the complex shear modulus Gd, and quantitative T2-maps were calculated. [7]
Results and Discussion
Fig. 2 and
Fig. 3 show animated gifs of the 16 frames MRE phase movies acquired pre and 45 min post 2h damage inducing indentation, respectively. A distinct change in wave pattern was observed, which can be explained by a change in tissue mechanical properties at the indentation site. In
Fig. 4 and
Fig. 5 the MRE elastograms (A) before, as well as (B) 45 min after 2 h of indentation are shown together with the corresponding T
2-maps (C-D) of two different animals. Increased shear stiffness was observed in the TA in the elastograms that were measured following 2 hours of indentation. T
2-maps after indentation revealed elevated T
2 values indicative for muscle damage in areas that co-localized with increased shear stiffness. The region of increased stiffness was less diffuse and smaller compared to the region of elevated T
2. We believe that the region of high shear stiffness highlights a spot with severe muscle damage, whereas elevated T
2 results from edema surrounding the injury. We are currently performing histology to correlate MRI findings with the histopathological status of the tissue.
Conclusion
This study demonstrates that changes in muscle-tissue mechanical properties associated with deep tissue injury can be quantified by MRE. Interesting observations such as the hot spot of increased shear stiffness after indentation should be studied in more detail and validated with histology. We expect that improved knowledge of changes in soft tissue mechanical properties due to damage, measured with MRE, will provide new insights in the etiology of pressure ulcers and other related muscle pathologies.
Acknowledgements
This research was supported by the Dutch Technology Foundation STW (NWO)References
[1] Linder-Ganz et al., Stiffening of Muscle Tissue Under Bone Compression is a Key Factor in Formation of Pressure Sores, Conf Proc IEEE Eng Med Biol Soc, 2003
[2] Gefen et al., In Vivo Muscle Stiffening Under Bone Compression Promotes Deep Pressure Sores, J. Biomech. Eng., 2005
[3] Deprez et al., On the potential of ultrasound elastography for pressure ulcer early detection, Med. Phys. ,2011
[4] Loerakker et al., The effects of deformation, ischemia, and reperfusion on the development of muscle damage during prolonged loading, J. Appl. Physiol, 2011
[5] Nelissen et al., A Novel MR compatible indentation setup to study the etiology of pressure ulcers and related deep tissue injury, Proc. Int. Soc. Mag. Res. Med. 2014
[6] Nelissen et al., A small animal MR Elastography setup to study skeletal muscle damage and the etiology of pressure ulcers and related deep tissue injury, Proc. Int. Soc. Mag. Res. Med. 2015
[7] Sinkus et al., MR elastography of breast lesions: understanding the solid/liquid duality can improve the specificity of contrast-enhanced MR mammography, Magn Reson Med. 2007 Dec; 58(6):1135-44.