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J.L. Aroyan et al.
recur over a range of ensonification directions for all frequencies tested
and are quite robust with respect to biologically reasonable variation of
the HU-to-density and HU-to-velocity model mappings discussed in
Section 2.2.
We now proceed to examine in detail the focal behavior of the forehead
tissues suggested by Figure lOAb. Results of inverse and forward simulations are presented below for three different models of the forehead of the
common dolphin: for a skull-only model, for a skull and soft tissues (including the melon) model, and for a complete skull, air sacs, and soft tissue
model. We will also present the results of several moved source simulations
using the complete model.
3.1 Skull-Only Simulations
The region of the CT data set selected for more detailed forehead simulations is illustrated by an isosurface at the CT data bone threshold (300 HU)
in Figure 10.5a and by bone and skin isosurface in Figure lO.5b. This tissue
region is a rectangular volume having x,y,z side-lengths of 27.0, 12.6, and
12.6cm, represented on a 1.5-mm cubic grid. In the skull-only model, the
HU-to-density and HU-to-velocity mappings of Section 2.2 were used to
assign velocity and density values only to tissue coordinates with Hounsfield
numbers equal to or greater than the bone threshold. All other model
coordinates were mapped to background seawater density (1.03 g/cc) and
velocity (1,500m/s). The last few points of the density and velocity grids
along the back, bottom, and side edges of the tissue region were graded
back to seawater values in order to reduce spurious reflections at the model
boundary.
The maxima of the inverse forehead simulations all occurred within a
small volume of the supranarial region in all models. We will therefore illustrate the detailed inverse simulation data only within a subvolume of the
forehead tissue model here referred to as the "potential source region"
(PSR), which includes the supranarial region and extends forward to
include about half of the melon body. The rectangular PSR volume (x,y,z
side-lengths of 12.0, 9.0 and 9.0cm) of the forehead model is outlined
(boxed) in Figures lO.5a and b. To help establish positional reference with
respect to the skull and soft tissue structures, Figure 1O.5c illustrates a
parasagittal slice through the center of the right MLDB complex within the
PSR. Cranford et al. (1996) provide a review of relevant delphinid forehead
tissue structures. The center of the left MLDB complex lies within the PSR,
approximately 3.3 cm to the left and about 0.7 cm downward from the center
of the right MLDB.
Figures 10.6a and b illustrate the acoustic potential energy density
Wp
J.L. Aroyan et al.
recur over a range of ensonification directions for all frequencies tested
and are quite robust with respect to biologically reasonable variation of
the HU-to-density and HU-to-velocity model mappings discussed in
Section 2.2.
We now proceed to examine in detail the focal behavior of the forehead
tissues suggested by Figure lOAb. Results of inverse and forward simulations are presented below for three different models of the forehead of the
common dolphin: for a skull-only model, for a skull and soft tissues (including the melon) model, and for a complete skull, air sacs, and soft tissue
model. We will also present the results of several moved source simulations
using the complete model.
3.1 Skull-Only Simulations
The region of the CT data set selected for more detailed forehead simulations is illustrated by an isosurface at the CT data bone threshold (300 HU)
in Figure 10.5a and by bone and skin isosurface in Figure lO.5b. This tissue
region is a rectangular volume having x,y,z side-lengths of 27.0, 12.6, and
12.6cm, represented on a 1.5-mm cubic grid. In the skull-only model, the
HU-to-density and HU-to-velocity mappings of Section 2.2 were used to
assign velocity and density values only to tissue coordinates with Hounsfield
numbers equal to or greater than the bone threshold. All other model
coordinates were mapped to background seawater density (1.03 g/cc) and
velocity (1,500m/s). The last few points of the density and velocity grids
along the back, bottom, and side edges of the tissue region were graded
back to seawater values in order to reduce spurious reflections at the model
boundary.
The maxima of the inverse forehead simulations all occurred within a
small volume of the supranarial region in all models. We will therefore illustrate the detailed inverse simulation data only within a subvolume of the
forehead tissue model here referred to as the "potential source region"
(PSR), which includes the supranarial region and extends forward to
include about half of the melon body. The rectangular PSR volume (x,y,z
side-lengths of 12.0, 9.0 and 9.0cm) of the forehead model is outlined
(boxed) in Figures lO.5a and b. To help establish positional reference with
respect to the skull and soft tissue structures, Figure 1O.5c illustrates a
parasagittal slice through the center of the right MLDB complex within the
PSR. Cranford et al. (1996) provide a review of relevant delphinid forehead
tissue structures. The center of the left MLDB complex lies within the PSR,
approximately 3.3 cm to the left and about 0.7 cm downward from the center
of the right MLDB.
Figures 10.6a and b illustrate the acoustic potential energy density
Wp