Department of Radiation Oncology, Academic Medical Center University of Amsterdam, 1105 AZ Amsterdam, The Netherlands.
Med Phys. 2011 Jan;38(1):142-50. doi: 10.1118/1.3523624.
A biomechanical model was constructed to give insight into pelvic organ motion as a result of bladder filling changes.
The authors used finite element (FE) modeling to simulate bladder wall deformation caused by urine inflow. For ten volunteers, a series of MRI scans of the pelvic area was recorded at regular intervals of 10 min over 1 h. For the series of scans, the bladder volume gradually increased while the rectal volume was constant. The MR image with the bladder volume closest to 250 ml was selected as the reference in each volunteer. All pelvic structures were defined from the reference image including bladder wall, small bowel, prostate (male), uterus (female), rectum, pelvic bone, and the rest of the body. These structures were translated to FE meshes. Using appropriate material properties for all organs, deformations of these organs as a response to changing bladder pressure were computed.
The computation results showed realistic anisotropic deformation of the bladder wall: The bladder became more elongated in the cranial and anterior directions with increasing bladder volume. After fitting the volume of the computed bladder to the actual bladder volume on the test images, the computed bladder shape agreed well with the real bladder shape (overlap from 0.79 to 0.93). The average mean bladder wall prediction errors of all the volunteers were 0.31 cm average and 0.29 cm SD.
In conclusion, a FE based mechanical bladder model shows promise for the prediction of the short-term bladder shape change using only one pelvic scan and volume change of the bladder as input. The accuracy levels achieved with this method are likely mostly limited by inaccuracies in material properties and sliding tissue between organs, which has not been modeled. This model can potentially be used to improve image-guided radiotherapy for bladder cancer patients, i.e., by prediction short-term bladder deformation.
构建生物力学模型，深入了解膀胱充盈变化引起的盆腔器官运动。
作者使用有限元（FE）建模来模拟尿液流入引起的膀胱壁变形。对十名志愿者进行了一系列盆腔区域的 MRI 扫描，每隔 10 分钟记录一次，共 1 小时。在这一系列扫描中，膀胱容量逐渐增加，而直肠容量保持不变。在每个志愿者中，选择最接近 250ml 膀胱容量的 MRI 图像作为参考。从参考图像中定义了所有的盆腔结构，包括膀胱壁、小肠、前列腺（男性）、子宫（女性）、直肠、骨盆和身体的其他部分。这些结构被转化为 FE 网格。使用所有器官的适当材料特性，计算了这些器官对膀胱压力变化的响应变形。
计算结果显示出膀胱壁的逼真各向异性变形：随着膀胱容量的增加，膀胱在颅侧和前侧方向变得更加拉长。在将计算出的膀胱体积拟合到测试图像上的实际膀胱体积后，计算出的膀胱形状与真实膀胱形状吻合较好（重叠度为 0.79 至 0.93）。所有志愿者的平均膀胱壁预测误差均为 0.31cm 平均和 0.29cm SD。
总之，基于 FE 的机械膀胱模型有望仅使用一次盆腔扫描和膀胱体积变化作为输入，预测短期膀胱形状变化。该方法达到的准确性水平可能主要受到材料特性和器官间滑动组织的不准确性限制，这些组织尚未建模。该模型可用于改善膀胱癌患者的图像引导放疗，例如通过预测短期膀胱变形。
