Numerical modelling of bladder filling and prostate positioning in modern radiotherapy

The human bladder is an important organ within the pelvic system of the human body. It routinely collects urine excreted by the kidneys prior to disposal via urination causing significant variation in the shape and volume of the bladder. The mechanics of the filling process directly affects the function of the bladder and its interaction with the surrounding organs, in particular the position of the prostate that can influence the diagnosis and planning of radiotherapy treatments of prostate cancer. The success of prostate radiotherapy depends on the delivery of high doses of radiation to a defined tissue volume with a high degree of positional accuracy. A detailed study is required to establish the volume and shape change of the bladder during the course of the radiotherapy treatment, over a long period of time, and to develop an effective modelling tool to simulate this complex mechanical process to provide guidance to diagnosis and treatment planning. In this work, systematic data of the bladder dimension are presented based on measurement of Computed Tomography (CT) images of a group of 10 patients with prostate cancer, over the course of radiotherapy treatment. A method of measuring the bladder wall distance in the anterior-posterior (AP), superiorinferior (SI) and left-right (LR) directions is implemented. Systematic data obtained from scanned images taken on the transaxial, sagittal and coronal planes are analysed and compared to assess the potential influence of the scanning sequence on the shape and size of the bladder. The variation of the bladder dimensions, volume and shape were analysed between different arms for the same subject and the upper limit of bladder volume variation were determined. The relationship between some key dimensional parameters (such as wall distance and aspect ratio) with the volume change is established. Key systematic results of prostatic displacement are determined, and inter subject difference of prostate positional change is analysed. The process of bladder filling and potential inter and intra subject difference and their influence on the diagnosis and planning process are discussed. A systematic numerical modelling method based on MR images has been developed to simulate the mechanics of bladder filling and its effects on the position of the prostate. A new approach for constructing detailed three dimensional (3D) Finite Element (FE) models that were specific to each patient were developed using multiple magnetic resonance (MR) images taken in three different planes (transaxial, sagittal, and coronal). Detailed sensitivity studies have been performed in comparing the 2-D and 3-D model, mesh size, materials models and parameters. The overall model of bladder deformation was compared with repeated images of the filled bladder that were obtained using computed tomography (CT) to validate the FE models. FE bladder deformation was found to be comparable to repeated CT images of the same patient, thus validating the FE models and the approach used in this work. The relationship between bladder deformation and its volume change has been established. The numerical results showed that the bladder dimensions increased linearly with its volume and the predicted coefficients is comparable to some published clinical results. The movement of the prostate in the anterior-posterior (AP), superior-inferior (SI), left-right (LR) directions, as well as its rotational movement, that is associated with changes in bladder volume was predicted. The numerical results showed that the movement of the prostate in the AP and SI directions (3-4mm and 1..S-4mm) was much greater than that in the LR direction (O.5-1mm). The scale of the movement of the prostate with the increase in bladder volume varied considerably among subjects. The numerical predictions were compared with published clinical data and the significance of the results in medical application is discussed. The work showed that subject specific Finite Element modelling could potentially provide crucial guidance for the medical practice on the treatment planning process of the prostate cancer

An Experimental and Numerical Program to Study the Properties of Thin Biological Membranes and Water Filling Process

Many organs such as the bladder consists of a water filled structure. An accurate measurement of the properties of the wall tissues and the liquid filling process is very important for the study of their mechanics and interaction with other organs. In this work, a new method has been developed to test thin membranes and inversely predict their mechanical properties based on indentation bending tests. A testing frame has been developed to test thin sheet of different length scales with finite element (FE) model mimicking each testing condition developed. The material properties of the membrane were predicted based on a parametric study approach. Tests have been performed using thin rubber sheet as a model material and the elastic property has been successfully predicted by matching the numerical and experimental data. The predicted material properties were then used in modelling water filling process of a balloon mimicking the bladder filling process

Numerical study of the mechanics of indentation bending tests of thin membranes and inverse materials parameters prediction

Indentation bending tests are an important testing method for thin rubber-like materials. The choice of material laws in finite element (FE) simulations of the test directly influences the accuracy of the numerical model and material properties predicted through inverse FE modelling. In this work, the effect of using a linear elastic or hyperelastic model on the material parameters predicted from indentation bending tests of a thin rubber sheet over a low strain range were studied. An inverse program has been developed based on the Kalman filter method to predict the material properties from experimental tests and to assess the uniqueness of the converged results for different material models. The predicted results were compared to standard tests carried out on the same material. Results showed that the Young's modulus of the material with the linear elastic model can be accurately predicted while the converged parameters (C 10 and C 01) for the Mooney-Rivlin model were not unique; data analysis showed that parameters C 10 and C 01 of the converged data were associated with the shear modulus of the material. © 2011 Elsevier B.V. All rights reserved

Numerical Study of Effects of Bladder Filling on Prostate Positioning In Radiotherapy

The success of radical prostate radiotherapy depends on delivery of high dose radiotherapy to a defined tissue volume with a high degree of positional accuracy. Modern techniques such as conformal and intensity-modulated radiotherapy (IMRT) can deliver increased radiation dose to the tumour without increased toxicity, but at the expense of requiring smaller delineated treatment margins, which is often difficult to control due to complex organ interactions. In prostate cancer radiotherapy, the position of the prostate is influenced by many factors in particular bladder and rectum filling. It is essential to study the mechanics of these processes and its interaction with adjacent structures to improve the understanding of their influences on the position of the intended irradiated area. In this work, a detailed 3D finite element model has been developed using patient specific MRI images to study the interaction between the bladder, prostate and rectum. Bladder filling was simulated by modelling the physical process to predict deformation fields with volume changes. The movement of the prostate associated with Bladder volume change in the anterior-posterior (AP), superior-inferior (SI) and right-left (RL) directions as well its rotational movement has been predicted. The numerical results were compared to repeated images and showed good agreement with some published clinical data. © 2009 Springer Berlin Heidelberg

Modeled and observed N-2 Lyman-Birge-Hopfield band emissions: A comparison

A thorough understanding of how the N-2 Lyman-Birge-Hopfield (LBH) band emissions vary with altitude is essential to the use of this emission by space-based remote sensors. In this paper, model-to-model comparisons are first performed to elucidate the influence of the solar irradiance spectrum, intrasystem cascade excitation, and O-2 photoabsorption on the limb profile. Next, the observed LBH emissions measured by the High resolution Ionospheric and Thermospheric Spectrograph aboard the Advanced Research and Global Observation Satellite are compared with modeled LBH limb profiles to determine which combination of parameters provides the best agreement. The analysis concentrates on the altitude dependence of the LBH (1,1) band, the brightest LBH emission in the observations. In the analysis, satellite drag data are used to constrain the neutral densities used for the data-to-model comparisons. For the average limb profiles on two of the three days analyzed (28, 29, and 30 July 2001), calculations using direct excitation alone give slightly better agreement with the observations than did calculations with cascading between the singlet electronic N-2 states (a(1)Pi(g), a\u27Sigma(-)(u), and w(1)Delta(u)); however, the differences between the observed profiles and either model are possibly greater than the differences between the models. Nevertheless, both models give excellent agreement with the observations, indicating that current models provide an adequate description of the altitude variation of the N-2 LBH (1,1) band emissions. Consequently, when using the LBH bands to remotely sense thermospheric temperatures, which can be used to provide an unprecedented view of the thermosphere, the temperatures derived have a negligible dependence on the model used

Remote sensing of neutral temperatures in the Earth\u27s thermosphere using the Lyman-Birge-Hopfield bands of N-2: Comparisons with satellite drag data

This paper presents remotely sensed neutral temperatures obtained from ultraviolet observations and compares them with temperatures from the NRLMSISE-00 version of the Mass Spectrometer and Incoherent Scatter (MSIS) model (unconstrained and constrained to match the total densities from satellite drag). Latitudinal profiles of the temperatures in the Earth\u27s thermosphere are obtained by inversion of high-resolution (similar to 1.3 angstrom) observations of the (1,1) and (5,4) Lyman-Birge-Hopfield (LBH) bands of N-2. The spectra are from the High resolution Ionospheric and Thermospheric Spectrograph (HITS) instrument aboard the Advanced Research and Global Observation Satellite (ARGOS). The results indicate that on each day examined there was consistency between the remotely sensed thermospheric temperatures, the densities from coincident satellite drag measurements at adjacent altitudes, and the NRLMSISE-00 model

MRI image-based FE modelling of the pelvis system and bladder filling

In this study, high-resolution magnetic resonance imaging was performed in the transaxial, coronal and sagittal planes to provide comprehensive structural details of the bladder and surrounding systems. Detailed finite-element (FE) models that were specific to each participant were developed by rendering the images, and the process of bladder filling was simulated. The overall model of bladder deformation was compared with repeated images of the filled bladder that were obtained using computed tomography to validate the FE models. The relationship between the changes in the key dimensions of the bladder and the increase in bladder volume during the filling process was also investigated. The numerical results showed that the bladder dimensions increased linearly with its volume during the filling process and the predicted coefficients are comparable to some of the published clinical results