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Introduction
Pressure ulcers are tissue degenerations caused by sustained
mechanical loading. Sustained mechanical loading induces ischemia by
pinching off blood vessels, blocks the lymphatic drainage, and causes
tissue deformation (Fig. 1). Frequent loading and unloading can also
cause ischemia-reperfusion injury. Pressure ulcers are often seen in
wheelchair or bedridden patients, for example when the tissue is
compressed between a protruding bone and the bed or seat. In hospitals
1 out of 10 patients develop early signs of pressure ulcers while
hospitalized [1].
A special type of pressure ulcers are ulcers which start deep under the
skin, at the bone tissue interface. This type of ulcers are called deep
tissue injury. Deep tissue injury type of pressure ulcers are of particular
research interest because they most often lead to the severe grade 3 and
4 ulcers, of which examples are shown in Figure 2. An additional
problem of this type of pressure ulcers is that they are initially invisible.
To get more insight in the aetiology of deep tissue injury, studies in a
Brown-Norway rat model were performed combined with finite element
modelling. The role of deformation, ischemia and reperfusion in the
development of deep tissue injury were investigated in detail and lead to
a hypothesis for the development of deep tissue injury, see Loerakker et
al [2].
Additional research is needed to better understand the mechanical
properties related to the development of deep tissue injuries and if these
changes can be used to develop an early diagnosis method [3].
Therefore, the aim of this study is to develop a Magnetic Resonance
Elastography (MRE) based early diagnosis method for deep tissue injury.
Towards early diagnosis of pressure ulcers with
Magnetic Resonance Elastography
Jules Nelissen1, Tom Schreurs1, Cees Oomens2, Klaas Nicolay1, Gustav Strijkers1
1Biomedical NMR, Biomedical Engineering, TU/e, The Netherlands
2Soft Biomechanics and Tissue Engineering, Biomedical Engineering, TU/e, The Netherlands
[1] Schoonhoven et al. Int J Nurs Stud, 44, 927-935 (2007). [2] Loerakker et al. PhD thesis TU/e, ISBN 978-90-386-2550-8 (2011)
[3] Loerakker et al. Comp Meth BioMech BioMed Eng , 2012. [4] Mariappan et al. Clinical Anatomy, 23, 497-511 (2010)
Methods
MRE is a motion sensitive MRI method which is used for the
quantification of soft tissue mechanical properties such as stiffness.
When tissue becomes diseased this is often accomplished with a change
in mechanical properties, which can be detected with MRE. As illustrated
in Fig. 3 MRE needs 3 basic steps: 1) Shear waves introduced with
external driver. 2) Shear wave measurement by using motion sensitive
MRE technique to detect small displacements when the waves propagate
through the tissue. 3) From the wave images the velocity, wavelength
and attenuation of the shear waves can be determined and an
elastogram representing the mechanical properties can be reconstructed
[4].
As external driver system an electromagnetic shaker was build based on
a system used by Sinkus et al. (Fig. 4b) and tested on a gel phantom
with stiff inclusions in a Bruker 6.3T small animal MRI scanner. An
adapted MSME (Spin Echo) sequence was used to make the wave
images.
Preliminary Results
Fig. 5 shows promising MRE results of the gel phantom. The elastogram
shows that all three inclusion were detected. Peaks in the elastogram are
not exactly in the same positions as the inclusions, because the used
reconstruction method fails due to interference effects.
Outlook
Adapt MRE setup build for 6.3T scanner to setup for new 7T scanner
Phantom tests and mechanical properties validation
Design of new MRE sequences
MRE on Sprague-Dawley rats before and after deep tissue injury
Fig. 3 Typical MRE example; shear waves are generated with an external driver
in a gel with stiff inclusions. Shear waves are detected with motion sensitive MRI
technique. Elastogram is reconstructed afterwards and shows the inclusions with
different stiffness. Adapted from Mariappan et al.
Gel sample Wave image Elastogram
Fig. 4 Computer aided design drawing of external driver setup.
Wave image Elastogram
Gel sample
Fig. 5 MRE results of gel sample with stiff inclusions.
Fig. 1 Pressure ulcers can develop at every interface where there is contact
between a hard surface and the body. The pressure of the bone against a hard
surface leads to damage of the skin and soft tissue.
Fig. 2 Grading scheme and progression of pressure ulcer. Adapted from EPUAP
grading scheme