Willeke A. Traa’s research while affiliated with Eindhoven University of Technology and other places

p>Background: deep tissue injury is a type of pressure ulcer which originates subcutaneously due to sustained mechanical loading. The relationship between mechanical compression and damage development has been extensively studied in 2D. However, recent studies have suggested that damage develops beyond the site of indentation. The objective of this study was to compare mechanical loading conditions to the associated damage in 3D. Methods: an indentation test was performed on the tibialis anterior muscle of rats (n = 39). Changes in the form of oedema and structural damage were monitored with MRI in an extensive region. The internal deformations were evaluated using MRI based 3D finite element models. Findings: damage propagates away from the loaded region. The 3D analysis indicates that there is a subject specific tolerance to compression induced deep tissue injury. Interpretation: Individual tolerance is an important factor when considering the mechanical loading conditions which induce damage.</p

Pressure-induced deep tissue injury is a form of pressure ulcer which is difficult to detect and diagnose at an early stage, before the wound has severely progressed and becomes visible at the skin surface. At the present time, no such detection technique is available. To test the hypothesis that muscle damage biomarkers can be indicative of the development of deep tissue injury after sustained mechanical loading, an indentation test was performed for 2 h on the tibialis anterior muscle of rats. Myoglobin and troponin were analysed in blood plasma and urine over a period of 5 days. The damage as detected by the biomarkers was compared to damage as observed with T2 MRI to validate the response. We found that myoglobin and troponin levels in blood increased due to the damage. Myoglobin was also increased in urine. The amount of damage observed with MRI immediately after loading had a strong correlation with the maximal biomarker levels: troponin in blood rs = 0.94; myoglobin in blood rs = 0.75; and myoglobin in urine rs = 0.57. This study suggests that muscle damage markers measured in blood and urine could serve as early diagnosis for pressure induced deep tissue injury.

Pressure ulcers occur due to sustained mechanical loading. Deep tissue injury is a severe type of pressure ulcer, which is believed to originate in subcutaneous tissues adjacent to bony prominences. In previous experimental-numerical studies the relationship between internal tissue state and damage development was investigated using a 2D analysis. However, recent studies suggest that a local analysis is not sufficient. In the present study we developed a method to create animal-specific 3D finite element models of an indentation test on the tibialis anterior muscle of rats based on MRI data. A detailed description on how the animal specific models are created is given. Furthermore, two indenter geometries are compared and the influence of errors in determining the indenter orientation on the resulting internal strain distribution in a defined volume of tissue was investigated. We conclude that with a spherically-shaped indenter errors in estimating the indenter orientation do not unduly influence the results of the simulation.

Early diagnosis of deep tissue injury remains problematic due to the complicated and multi-factorial nature of damage induction, and the many processes involved in damage development and recovery. In this paper we present a comprehensive assessment of deep tissue injury development and remodeling in a rat model by multi-parametric magnetic resonance imaging (MRI) and histopathology. The tibialis anterior muscle of rats was subjected to mechanical deformation for 2 h. Multi-parametric in vivo MRI, consisting of T2, T2∗, mean diffusivity (MD), and angiography measurements, was applied before, during, and directly after indentation, as well as at several time points during a 14 days follow-up. MRI readouts were linked to histological analyses of the damaged tissue. The results showed dynamic change in various MRI parameters, reflecting the histopathological status of the tissue during damage induction and repair. Increased T2corresponded with edema, muscle cell damage, and inflammation. T2∗ was related to tissue perfusion, hemorrhage, and inflammation. MD increase and decrease reported on the tissue's microstructural integrity and reflected muscle degeneration, edema, as well as fibrosis. Angiography provided information on blockage of blood flow during deformation. Our results indicate that the effects of a single damage causing event of only 2 h deformation were present up to 14 days. The initial tissue response to deformation, as observed by MRI, starts at the edge of the indentation. The quantitative MRI readouts provided distinct and complementary information on the extent, temporal evolution, and microstructural basis of deep tissue injury related muscle damage.

Deformation of skeletal muscle in the proximity of bony structures may lead to deep tissue injury category of pressure ulcers. Changes in mechanical properties have been proposed as a risk factor in the development of deep tissue injury and may be useful as a diagnostic tool for early detection. MRE allows for the estimation of mechanical properties of soft tissue through analysis of shear wave data. The shear waves originate from vibrations induced by an external actuator placed on the tissue surface. In this study a combined Magnetic Resonance (MR) compatible indentation and MR Elastography (MRE) setup is presented to study mechanical properties associated with deep tissue injury in rats. The proposed setup allows for MRE investigations combined with damage-inducing large strain indentation of the Tibialis Anterior muscle in the rat hind leg inside a small animal MR scanner. An alginate cast allowed proper fixation of the animal leg with anatomical perfect fit, provided boundary condition information for FEA and provided good susceptibility matching. MR Elastography data could be recorded for the Tibialis Anterior muscle prior to, during, and after indentation. A decaying shear wave with an average amplitude of approximately 2 μm propagated in the whole muscle. MRE elastograms representing local tissue shear storage modulus Gd showed significant increased mean values due to damage-inducing indentation (from 4.2 ± 0.1 kPa before to 5.1 ± 0.6 kPa after, p<0.05). The proposed setup enables controlled deformation under MRI-guidance, monitoring of the wound development by MRI, and quantification of tissue mechanical properties by MRE. We expect that improved knowledge of changes in soft tissue mechanical properties due to deep tissue injury, will provide new insights in the etiology of deep tissue injuries, skeletal muscle damage and other related muscle pathologies.

Detail of indentation and MRE actuator part of setup. Following parts are labeled: u-shaped profile (a), cutout for the rat’s groin (b), indentor (c), movable indentor holder (d), rotatable half arch (e), dovetail profile (f), spacer plates (g), MRE piston (h), drive rod (i), cantilever (j). (TIF)

MRE pre and post indentation movie. Movie of a 16 offsets 900 Hz SE-EPI-MRE (A) before and (B) after indentation. (AVI)

Photograph of a rat positioned in the setup. Rat, Indentation and MRE component are indicated with arrows. In A, the indentor, put through the surface RF coil, positioned on top of TA muscle in rats hindleg, and the MRE piston attached at distal side of TA muscle are shown. Indentation component is removed in B, revealing the MRE component and surface coil. Pre-amplifier block of the surface coil, the respiratory sensor and the rectal temperature probe are also visible. Anesthesia mask and rat’s head are underneath the pre-amplifier block. (TIF)

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 quantiဠed 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 speciဠcally wounds that develop in skeletal muscle (deep tissue injury). MRE measurements were performed before and after damage-inducing indentation of the tibialis anterior (TA) muscle of Sprague Dawley (SD) rats. 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 ဠeld of the MR scanner, and cantilever. The indentor can apply sustained mechanical loading to the TA muscle of rats. 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 proဠle ဠlled with alginate molding substance for ဠxation, 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 T-mapping MRI (Spin-Echo, 18 slices, FOV = 3 x 6 cm , 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 cm , 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 G , and quantitative T-maps were calculated. 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 1,2 3 1 4 3 5 6 1 2 1 2 3 4 5 6 [1][2][3] [4] [5][6] 2 2 2 d 2 [7] elastograms (A) before, as well as (B) 45 min after 2 h of indentation are shown together with the corresponding T-maps (C-D) of two diတerent animals. Increased shear stiတness was observed in the TA in the elastograms that were measured following 2 hours of indentation. T-maps after indentation revealed elevated T values indicative for muscle damage in areas that co-localized with increased shear stiတness. The region of increased stiတness was less diတuse and smaller compared to the region of elevated T. We believe that the region of high shear stiတness highlights a spot with severe muscle damage, whereas elevated T results from edema surrounding the injury. We are currently performing histology to correlate MRI ဠndings 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 quantiဠed by MRE. Interesting observations such as the hot spot of increased shear stiတness 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.