BodyBuilder Legs Up & Sitting Postures
The MIRD phantoms are defined in a standing posture with the legs below
and parallel to the torso. BodyBuilder includes the two additional
postures shown above. In the Legs Up posture, the straight legs are in
front of and perpendicular to the torso. In the Sitting posture, the
legs are bent at a 90° angle at the midpoint of the legs; the upper
half of the legs are the same as in the Legs Up posture. We refer to
the lower half of the legs in the Sitting posture as the shins; legs
will refer to the upper portion. Checking the Legs Up or Sitting item
in the Organs menu selects an alternate posture.
The legs are positioned by applying a transformation (TR10) to the legs
in the original standing model. The transformation positions the plane
bounding the top of the legs to be just outside of the torso surface.
The height (along the torso vertical axis) is chosen so that the bottom
of the thickest portion of the legs is at the same level as the bottom
of the original torso. The male genitalia and testicles are transformed
along with the legs (above figure).
The leg transformation leaves a gap between the large end of the legs
and the torso. The legs are extended to the midplane of the torso to
fill the gap. The figure above shows the extensions. (The extensions
are part of the leg cell; they are shown separately in the Figure for
illustrative purposes only.) No extension is applied to the leg bones.
The repositioning of the legs leaves the bottom of the torso bare. The
alternate posture models add a bottom layer to the posterior of the
torso. The thickness is 5 times the skin thickness. The outer layer
(both sides and bottom) is skin if the model includes skin.
The leg extensions and bottom layer add additional soft tissue volume
not present in the original MIRD specification. The legs were modified
so that the combined volumes of the legs with extensions and the bottom
are the same as the legs volume in the original model. The legs are
truncated cones. The volume was adjusted by changing the opening angle
of the cones. The volume fraction capability of the Moritz program was
used to calculate both the original and modified volumes. The
resolution of the volume calculation (set primarily by the number of
paths used in the ray tracing technique) was set high enough to give an
estimated accuracy of at least 0.1% for the leg volumes. The original
(standing) and modified (legs up) volumes agree to within 1%. The
volume matching procedure was performed for each of the BodyBuilder
ages.
In the sitting model, the upper legs and shins are separated by a
45° plane (surface 853) that intersects the middle of the legs at
1/2 of their length. The shins are translated (TR11) from the original
position. The translation parameters were chosen so that the shin
length is 1/2 of the original leg length and the leg and shin bones are
contiguous. The figure above shows the bones and outlines of the legs,
shins, and torso for a sitting adult male. The leg surfaces do not
match exactly at the leg/shin interface, but the model should be
sufficient for most dose estimation studies. The leg volumes are within
10% of the original leg volumes.
The user can modify the angle of the legs and shins by adjusting their
respective transformations (10 and 11). If either or both is modified,
the models should be plotted and the translation parameters of the
transformations adjusted if necessary. If the angle between the legs
and shins is changed from 90°, the user may also want to consider
changing the angle of the separating plane (853).
The legs up and sitting positions cannot be used with the 6 and 9 month
pregnant models—the legs interfere with the extended belly. We could
move the legs to accommodate the pregnant models if there is user
demand. A more refined model would have the top of the leg bones closer
to the pelvis (lowermost bone in torso in Figure 15). The alternate
postures should work with extra torso tissue layers but have not been
fully tested with thick extra layers.
Last modified: September, 2004
Kenneth A. Van Riper / email
