Biomedical Engineering Reference
In-Depth Information
Fig. 5.29 Transversal MR-images of the indentation region showing gluteal adipose (white) and
skeletal muscle (dark) tissue deformation at constant indenter displacement of a 10 mm, and
b 40 mm; position [1]: tablet marker position at skin level, [2]: pelvic bone
(TR = 54 ms, TE = 1.55 ms, FOV = 420 9 302.6 mm, matrix = 272 9 196,
5 mm slice thickness, flip angle = 80 deg, iPAT factor = 2) gradient echo turbo flash
2D-sequence was used. The slice in the indenter axis plane was scanned 400 times
without interruption in transversal orientation resulting in a total creep deformation
time of 270 s. Longer examination periods were not chosen since significant changes
in creep deformation were not apparent thereafter (cf. Fig.
5.28
b and c).
Separating Procedure (Fat and Muscle): Tissue deformation allowed a clear
reconstruction and evaluation from the MR-images. The actual thicknesses l
i
;
F
ð
t
Þ
at
time t of the skin/fat layer and the muscle layer l
i
;
M
ð
t
Þ
was measured along the
indentation axis from the particular image, Fig.
5.28
a. With respect to the unde-
formed initial lengths of both layers (skin/fat: l
0
;
F
¼
27 mm
;
muscle: l
0
;
M
¼
56 mm
;
the actual change in thickness of the skin/fat layer and the muscle layer along the
indenter axis could be derived.
Displacement-Time-Data: Assigning the measured points in time provided by
cine-imaging to the corresponding tissue displacements, individual displacement-
time data, as depicted in Fig.
5.28
b for gluteal skin/fat and muscle tissue, were
obtained. A detailed view of the initial creep data is shown in Fig.
5.28
c. Fat and
muscle tissue exhibit a distinct initial elastic response (spontaneous elasticity),
characterized by a steep displacement increase, (until approx. 0.7 s in Fig.
5.28
c)
followed by the actual creep process with gradual increasing displacements.
The separated creep data further show that skin/fat tissue marginally contributes
to the overall creep displacement under compressive loading. In this investigation
and linearly related to the initial tissue thickness of fat and muscle, transversally
loaded passive gluteal muscle tissue exhibited a 50 % higher creep deformation
along the indenter axis, compared to skin/fat tissue. A total displacement of
5.4 mm was found for muscle and 1.3 mm for skin/fat (along the indenter axis)
after 270 s of testing. In this case, the particular creep process can be considered as
nearly complete, approaching steady state after approximately 4 min.
