Fig 1.
Muscle-lipid system (MLS) of the thigh.
Cross section of the femur (left) and schematic composition (right) with muscle (dark grey) and adipose tissue (light grey). The fascia is shown in yellow and the periosteal bone surface in green. AT within the fascia is termed intermuscular AT (IMAT), outside subcutaneous adipose tissue (SAT). IMAT within the anatomical muscle M (magenta contours define the border of the Semitendinosus) is termed extramyocellular lipids (EML) and outside perimuscular AT (PAT). In CT images intramyocellular lipids (IML) can only be measured indirectly by a lower average CT value compared to that of pure muscle tissue.
Fig 2.
Definition of the analysis VOI.
VOIUS defined by 1: The center of mass of the femur in plane A, which is perpendicular to the shaft axis and intersects the center of the trochanter minor and 2.: The voxel of the femoral head with maximum distance from A. The projected distance between the two points onto the scanner z-axis is used as an anatomic size- and pose-specific distance d. VOIUS contains n = 0.5 d / s (s: slice thickness) slices; the most proximal slice contained the distal end of the acetabulum (not visible here).
Fig 3.
Multi step 3D hierarchical segmentation of the volume of interest inside the fascia (VOIIF) exemplified in one 2D axial slice.
Step 1: Connected components of right leg (VOIRC) for G = 1 (a) and G = 3 (b) (G: Grade of muscle appearance, see text). Step 2: Probable muscle voxels within processed left leg (VOILC) (c) and compactification resulting in VOIIFA (d and e) (IFA: Approximation of IF). Step 3: refinement of VOIIFA. Candidate voxels close to FA that may belong to VOIIF (f and g), seed points to select clusters that belong to VOIIF (h), clusters that were not selected stay porous (i) and are removed by morphological smoothing yielding the final result (j). User-dependence is indicated by blue color.
Fig 4.
Approximation of the intrafascia VOI (yellow), candidate voxels (purple) and seed point (blue) for the ray reflection model: The space between IFA and candidate voxels that were connected with the seed were either x: shielded, -: not-shielded or h: filled by contour-filling.
Fig 5.
Image-specific definition of thresholds for anatomical muscle and muscle tissue.
Left graph: definition of the adipose tissue threshold TAT from SAT*; SD: standard deviation. Right graph: CT value spectrum of the combined subcutaneous adipose tissue and muscle tissue VOIs (VOIMLS) (black). Gaussian mixture model used to fit adipose (yellow) and muscle tissue (magenta) distributions; a: intersection between the two curves, b: peak of the muscle tissue curve.
Fig 6.
Segmentation results for three subjects with washed-out and moth-eaten muscles and strong edema.
First row: native images; second row: segmented fascia (yellow).
Table 1.
Reanalysis precision errors as CVrms in per cent and biological variance as CV/100 in percent for density, relative volume and features for different VOIs.
In each row intra/interoperator CVrms results are on top and CV100 results below. D: density; DIfGMM, DMLSGMM: density derived from Gaussian mixture model applied to of VOIIF or VOIMLS, respectively; rVMLS, rVIF: Volume relative to VOIIF or VOIMLS, respectively; Gavg: average grain size; FD: fractal dimension; Ψ: sphericity. VOIs: SAT: subcutaneous adipose tissue; IF: intrafascia; M: muscle; MT: muscle tissue; HDM: high density muscle; IMAT: intermuscular adipose tissue. Note: not all features are determined for all VOIs.
Table 2.
Percentage changes for density, relative volume and features for different VOIs when an exposure level of 135 mAs was simulated.
D: density; DIFGMM, DMLSGMM: density derived from Gaussian mixture model applied to of VOIIF or VOIMLS, respectively; rVMLS, rVIF: Volume relative to VOIIF or VOIMLS, respectively; Gavg: average grain size; FD: fractal dimension; Ψ: sphericity. VOIs: SAT: subcutaneous adipose tissue; IF: intrafascia; M: muscle; MT: muscle tissue; HDM: high density muscle; IMAT: intermuscular adipose tissue.
Table 3.
A simulated addition / subtraction of 5 HU from CTH2O.caused the percent change shown in the table. D: density; DIFGMM, DMLSGMM: density derived from Gaussian mixture model applied to of VOIIF or VOIMLS, respectively; SAT: subcutaneous adipose tissue; IF: intrafascia; M: muscle; MT: muscle tissue; HDM: high density muscle muscle.