BodyBuilder Legs Up & Sitting Postures

Different 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.

Lap Detail

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).

Leg Extensions

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.

Sitting Detail

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.

White Rock Science Logo Last modified: September, 2004
Kenneth A. Van Riper / email