418
J.L. Aroyan et al.
used in this study is pictured in Figure 10.2c. The soft tissue range of this
mapping generates good agreement between Cranford's (1992) plots of HU
values through a Tursiops truncatus melon and Norris and Harvey's (1974)
T. truncatus melon velocity measurements. While intact tissue velocities for
D. de/phis have not been measured, it is reasonable to expect that a mapping
based on the measured acoustic properties of mammalian tissues and that
appears to work for T. truncatus will work for other delphinids as well, provided the data ranges for each specimen have been properly determined.
Similar triglyceride and isovalerate wax ester lipid mixtures (roughly 60%
and 40% composition by weight, respectively) are found in the melon and
lower jaw fats across the family Delphinidae (Litchfield et al. 1975). These
ratios differ substantially from blubber compositions. Slight variations of
the delphinid lipid density and velocity mapping presented here should
be sufficient for modeling the odontocete families Phocoenidae and
Monodontidae. The families Platanistidae, Ziphiidae, and Physeteridae,
however, may require different treatments.
Several aspects of Figure 1O.2c merit mention. First, the attenuation range
below -138HU (below all soft tissue structures except air sacs) is again
assigned the velocity of seawater (1,500m/s). Next, soft tissue velocities
above tendon (at 150HU) and up to the bone threshold (at 300HU) are
modeled as a constant 1,730m/s. This was done because the data range 150
to 300HU corresponded to the range of partial-volume blurring of the soft
tissue-bone interface, and because measured soft tissue velocities do not
appear to exceed this value. The "partial-volume" effect is one of several
potential sources of error in X-ray CT data (Cann 1988). It occurs when a
scanned volume element straddles a sharp tissue interface, effectively blurring the interface. A large discontinuity in velocity at the bone threshold
was introduced to correct for the artifact of partial-volume blurring of the
soft tissue-bone interface. We also mention that partial-volume blurring of
the skin-seawater interface was corrected using a simple technique
(Aroyan 1996, p 91), but that no correction was made in the current model
for tissue velocity dependence on temperature near the skin of the animal.
Finally, note that the velocity above the bone threshold was modeled as a
constant 3,450m/s. The threshold for bone at 300HU was determined by
examining the thickness and continuity of skull surfaces crucial to acoustic
behavior. As in previous two-dimensional simulations (Aroyan 1990;
Aroyan et al.I992), the 3-D results are not strongly dependent on the exact
acoustical parameters assigned to bone.
Modeling of air spaces within soft tissue introduces unique difficulties.
While the geometry of skull-soft tissue and external soft tissue-air interfaces is generally well represented in high-resolution X-ray CT data, air sacs
inside soft tissues can suffer distortion due to postmortem changes. Modeling of a dolphin's nasal passages is further complicated by the common
observation that the uppermost nasal sacs in live animals are, to some
extent, mobile during phonation (Dormer 1979). Therefore, the author
J.L. Aroyan et al.
used in this study is pictured in Figure 10.2c. The soft tissue range of this
mapping generates good agreement between Cranford's (1992) plots of HU
values through a Tursiops truncatus melon and Norris and Harvey's (1974)
T. truncatus melon velocity measurements. While intact tissue velocities for
D. de/phis have not been measured, it is reasonable to expect that a mapping
based on the measured acoustic properties of mammalian tissues and that
appears to work for T. truncatus will work for other delphinids as well, provided the data ranges for each specimen have been properly determined.
Similar triglyceride and isovalerate wax ester lipid mixtures (roughly 60%
and 40% composition by weight, respectively) are found in the melon and
lower jaw fats across the family Delphinidae (Litchfield et al. 1975). These
ratios differ substantially from blubber compositions. Slight variations of
the delphinid lipid density and velocity mapping presented here should
be sufficient for modeling the odontocete families Phocoenidae and
Monodontidae. The families Platanistidae, Ziphiidae, and Physeteridae,
however, may require different treatments.
Several aspects of Figure 1O.2c merit mention. First, the attenuation range
below -138HU (below all soft tissue structures except air sacs) is again
assigned the velocity of seawater (1,500m/s). Next, soft tissue velocities
above tendon (at 150HU) and up to the bone threshold (at 300HU) are
modeled as a constant 1,730m/s. This was done because the data range 150
to 300HU corresponded to the range of partial-volume blurring of the soft
tissue-bone interface, and because measured soft tissue velocities do not
appear to exceed this value. The "partial-volume" effect is one of several
potential sources of error in X-ray CT data (Cann 1988). It occurs when a
scanned volume element straddles a sharp tissue interface, effectively blurring the interface. A large discontinuity in velocity at the bone threshold
was introduced to correct for the artifact of partial-volume blurring of the
soft tissue-bone interface. We also mention that partial-volume blurring of
the skin-seawater interface was corrected using a simple technique
(Aroyan 1996, p 91), but that no correction was made in the current model
for tissue velocity dependence on temperature near the skin of the animal.
Finally, note that the velocity above the bone threshold was modeled as a
constant 3,450m/s. The threshold for bone at 300HU was determined by
examining the thickness and continuity of skull surfaces crucial to acoustic
behavior. As in previous two-dimensional simulations (Aroyan 1990;
Aroyan et al.I992), the 3-D results are not strongly dependent on the exact
acoustical parameters assigned to bone.
Modeling of air spaces within soft tissue introduces unique difficulties.
While the geometry of skull-soft tissue and external soft tissue-air interfaces is generally well represented in high-resolution X-ray CT data, air sacs
inside soft tissues can suffer distortion due to postmortem changes. Modeling of a dolphin's nasal passages is further complicated by the common
observation that the uppermost nasal sacs in live animals are, to some
extent, mobile during phonation (Dormer 1979). Therefore, the author