Adil Al-Mayah

Contact surface and material nonlinearity modeling of human lungs

A finite element model has been developed to investigate the effect of contact surfaces and hyperelastic material properties on the mechanical behavior of human lungs of one lung cancer patient. The three-dimensional model consists of four parts, namely the left lung, right lung, tumor in the left lung and chest wall. The interaction between the lungs and chest wall was modeled using frictionless surface-based contact. Hyperelastic material properties of the lungs are used in the model. The effect of the two parameters is investigated by tracking the tumor movement, and by comparing the analytical results to the patient bifurcation points: 45 points in each lung and 18 points around the tumor. The accuracy of the model is improved by including the contact surface and hyperelastic material properties. The average error and the standard deviation (SD) in modeling the displacement in the SI direction are reduced from 0.68 (SD = 0.34) cm in the elastic model to 0.09 (0.21) cm in the contact-hyperelastic model. Similarly, the average error (SD) of tumor location decreases from 0.71 (0.21) cm in the elastic material without contact to -0.03 (0.24) cm in the hyperelastic material with contact model.

Validation of a method for measuring the volumetric breast density from digital mammograms

The purpose of this study was to evaluate the performance of an algorithm used to measure the volumetric breast density (VBD) from digital mammograms. The algorithm is based on the calibration of the detector signal versus the thickness and composition of breast-equivalent phantoms. The baseline error in the density from the algorithm was found to be 1.25 +/- 2.3% VBD units (PVBD) when tested against a set of calibration phantoms, of thicknesses 3-8 cm, with compositions equivalent to fibroglandular content (breast density) between 0% and 100% and under x-ray beams between 26 kVp and 32 kVp with a Rh/Rh anode/filter. The algorithm was also tested against images from a dedicated breast computed tomography (CT) scanner acquired on 26 volunteers. The CT images were segmented into regions representing adipose, fibroglandular and skin tissues, and then deformed using a finite-element algorithm to simulate the effects of compression in mammography. The mean volume, VBD and thickness of the compressed breast for these deformed images were respectively 558 cm(3), 23.6% and 62 mm. The displaced CT images were then used to generate simulated digital mammograms, considering the effects of the polychromatic x-ray spectrum, the primary and scattered energy transmitted through the breast, the anti-scatter grid and the detector efficiency. The simulated mammograms were analyzed with the VBD algorithm and compared with the deformed CT volumes. With the Rh/Rh anode filter, the root mean square difference between the VBD from CT and from the algorithm was 2.6 PVBD, and a linear regression between the two gave a slope of 0.992 with an intercept of -1.4 PVBD and a correlation with R(2) = 0.963. The results with the Mo/Mo and Mo/Rh anode/filter were similar.

Sliding characteristic and material compressibility of human lung: Parametric study and verification

PURPOSE To find and verify the optimum sliding characteristics and material compressibility that provide the minimum error in deformable image registration of the lungs. METHODS A deformable image registration study has been conducted on a total of 16 lung cancer patients. Patient specific three dimensional finite element models have been developed to model left and right lungs, chest (body), and tumor based on 4D CT images. Contact surfaces have been applied to lung-chest cavity interfaces. Experimental test data are used to model nonlinear material properties of lungs. A parametric study is carried out on seven patients, 20 conditions for each, to investigate the sliding behavior and the tissue compressibility of lungs. Three values of coefficient of friction of 0, 0.1, and 0.2 are investigated to model lubrication and sliding restriction on the lung-chest cavity interface. The effect of material compressibility of lungs is studied using Poissons ratios of 0.35, 0.4, 0.45, and 0.499. The model accuracy is examined by calculating the difference between the image-based displacement of bronchial bifurcation points identified in the lung images and the calculated corresponding model-based displacement. Furthermore, additional bifurcation points around the tumor and its center of mass are used to examine the effect of the mentioned parameters on the tumor localization. RESULTS The frictionless contact model with 0.4 Poissons ratio provides the smallest residual errors of 1.1 +/- 0.9, 1.5 +/- 1.3, and 2.1 +/- 1.6 mm in the LR, AP, and SI directions, respectively. Similarly, this optimum model provides the most accurate location of the tumor with residual errors of 1.0 +/- 0.6, 0.9 +/- 0.7, and 1.4 +/- 1.0 mm in all three directions. The accuracy of this model is verified on an additional nine patients with average errors of 0.8 +/- 0.7, 1.3 +/- 1.1, and 1.7 +/- 1.6 mm in the LR, AP, and SI directions, respectively. CONCLUSIONS The optimum biomechanical model with the smallest registration error is when frictionless contact model and 0.4 Poissons ratio are applied. The overall accuracies of all bifurcation points in all 16 patients including tumor points are 1.0 +/- 0.7, 1.2 +/- 1.0, and 1.7 +/- 1.4 mm in the LR, AP, and SI directions, respectively.

Toward efficient biomechanical-based deformable image registration of lungs for image-guided radiotherapy

Both accuracy and efficiency are critical for the implementation of biomechanical model-based deformable registration in clinical practice. The focus of this investigation is to evaluate the potential of improving the efficiency of the deformable image registration of the human lungs without loss of accuracy. Three-dimensional finite element models have been developed using image data of 14 lung cancer patients. Each model consists of two lungs, tumor and external body. Sliding of the lungs inside the chest cavity is modeled using a frictionless surface-based contact model. The effect of the type of element, finite deformation and elasticity on the accuracy and computing time is investigated. Linear and quadrilateral tetrahedral elements are used with linear and nonlinear geometric analysis. Two types of material properties are applied namely: elastic and hyperelastic. The accuracy of each of the four models is examined using a number of anatomical landmarks representing the vessels bifurcation points distributed across the lungs. The registration error is not significantly affected by the element type or linearity of analysis, with an average vector error of around 2.8 mm. The displacement differences between linear and nonlinear analysis methods are calculated for all lungs nodes and a maximum value of 3.6 mm is found in one of the nodes near the entrance of the bronchial tree into the lungs. The 95 percentile of displacement difference ranges between 0.4 and 0.8 mm. However, the time required for the analysis is reduced from 95 min in the quadratic elements nonlinear geometry model to 3.4 min in the linear element linear geometry model. Therefore using linear tetrahedral elements with linear elastic materials and linear geometry is preferable for modeling the breathing motion of lungs for image-guided radiotherapy applications.

Biomechanical-based image registration for head and neck radiation treatment*

Deformable image registration of four head and neck cancer patients has been conducted using a biomechanical-based model. Patient-specific 3D finite element models have been developed using CT and cone-beam CT image data of the planning and a radiation treatment session. The model consists of seven vertebrae (C1 to C7), mandible, larynx, left and right parotid glands, tumor and body. Different combinations of boundary conditions are applied in the model in order to find the configuration with a minimum registration error. Each vertebra in the planning session is individually aligned with its correspondence in the treatment session. Rigid alignment is used for each individual vertebra and the mandible since no deformation is expected in the bones. In addition, the effect of morphological differences in the external body between the two image sessions is investigated. The accuracy of the registration is evaluated using the tumor and both parotid glands by comparing the calculated Dice similarity index of these structures following deformation in relation to their true surface defined in the image of the second session. The registration is improved when the vertebrae and mandible are aligned in the two sessions with the highest average Dice index of 0.86 ± 0.08, 0.84 ± 0.11 and 0.89 ± 0.04 for the tumor, left and right parotid glands, respectively. The accuracy of the center of mass location of tumor and parotid glands is also improved by deformable image registration where the errors in the tumor and parotid glands decrease from 4.0 ± 1.1, 3.4 ± 1.5 and 3.8 ± 0.9 mm using rigid registration to 2.3 ± 1.0, 2.5 ± 0.8 and 2.0 ± 0.9 mm in the deformable image registration when alignment of vertebrae and mandible is conducted in addition to the surface projection of the body.

Deformable image registration of heterogeneous human lung incorporating the bronchial tree

PURPOSE To investigate the effect of the bronchial tree on the accuracy of biomechanical-based deformable image registration of human lungs. METHODS Three dimensional finite element models have been developed using four dimensional computed tomography image data of ten lung cancer patients. Each model is built of a body, left and right lungs, tumor, and bronchial trees. Triangular shell elements are used for the bronchial trees while tetrahedral elements are used for other components. Hyperelastic material properties based on experimental investigation on human lungs are used for the lung parenchyma. Different material properties are assigned for the bronchial tree using five values for the modulus of elasticity of 0.01, 0.12, 0.5, 10, and 18 MPa. Lungs are modeled to slide inside chest cavities by applying frictionless contact surfaces between each lung and corresponding chest cavity. The accuracy of the models is examined using an average of 40 bronchial bifurcation points identified on inhale and exhale images. Relative accuracy is evaluated by comparing the displacement of all nodes within the lungs as well as the dosimetric difference at the exhale position predicted by the model. RESULTS There is no significant effect of bronchial tree on the model accuracy based on the bifurcation points analysis. However, on the local level, using an average of 38 000 nodes, there is a maximum difference of 8.5 mm in the deformation of the bronchial trees, as the modulus of elasticity of the bronchial trees increases from 0.01 to 18 MPa; however, more than 96% of nodes are within a 2.5 mm difference in each direction. The average dose difference at the predicted exhale position is less than 35 cGy between the models. CONCLUSIONS The bronchial tree has little effect on the global deformation and the accuracy of deformable image registration of lungs. Hence, the homogenous model is a reasonable assumption. Since there are some local deformation differences between nodes as the material properties of the bronchial tree change that may affect the accuracy of dosimetric results, heterogeneity may be required for a smaller scale modeling of lungs.

Effect of Friction and Material Compressibility on Deformable Modeling of Human Lung

A three dimensional finite element model has been developed to investigate the sliding mechanics and compressibility of human lungs of seven lung cancer patients. The model consists of both lungs, tumor, and chest wall. The interaction between lungs and chest cavities is modeled using surface-based contact with coefficient of friction of 0, 0.1 and 0.2. Experimentally measured hyperelastic material properties of the lungs are applied in the model with different degrees of compressibility using Poissons ratio (i¾?) of 0.35, 0.4, 0.45 and 0.499. The analytical results are compared to actual measurements of the bifurcation of the vessels and bronchi in the lungs and tissues. The least absolute average error of 0.21(±0.04) cm is reached when frictionless contact surfaces with hyperelastic material and Poissons ratio of 0.35 and 0.4 are applied. The error slightly changes in contact models as the coefficient of friction and Poissons ratio increases. However, Poissons ratio has more effect in models without contact surfaces where the average error changes from 0.33(±0.11) cm to 0.26(±0.07) cm as the Poissons ratio increased from 0.35 to 0.499.

Quasi-static magnetic resonance elastography at 7 T to measure the effect of pathology before and after fixation on tissue biomechanical properties

Evaluation of imaging for cancer detection and localization can be achieved by correlation of gold‐standard histopathology with imaging data. Usage of a 3D biomechanical‐based deformable registration for correlation of the histopathology of whole‐tissue specimens with ex vivo imaging necessitates measurement of the distribution of biomechanical properties in the ex vivo tissue specimen and changes that occur during pathology fixation. To measure high‐resolution 3D distributions of Youngs modulus (E) prefixation and postfixation, a quasi‐static magnetic resonance elastography method was developed at 7 T. Use of echo‐planar imaging allowed for shorter imaging times, in line with limited time frames allowable for pathology specimens. The finite element modeling algorithm produced voxel‐wise E measures, and mechanical indentation was used for comparison. An initial preclinical evaluation with canine prostate specimens (n = 5) demonstrated a consistent increase in E with fixation (P < 0.002) by a factor of 4 (±1). Increases were a function of distance from the tissue edge and correlated with fixation time (ρ = 1, P < 0.02). The technique will be used to generate population‐averaged data of E from clinical ex vivo specimens prefixation and postfixation to inform registration of whole‐mount histopathology with in vivo imaging. Magn Reson Med, 2012.

Simplified Anchor System for CFRP Rods

AbstractThe increased use of carbon fiber–reinforced polymer (CFRP) rods in prestressed concrete applications has been challenged by identifying a suitable anchor system. To overcome such a challenge, the design of a simplified anchor system composed of three wedges and a barrel, without a soft sleeve, is presented to duplicate the simplicity of the widely utilized anchor systems in steel strands. A numerical study and experimental verification of the simplified anchor system for a CFRP rod are presented. Three-dimensional (3D) finite-element modeling has been conducted on the anchor system consisting of isotropic steel wedges and barrel, in addition to an orthotropic CFRP rod. The wedges and barrel are modeled as elastoplastic materials of different hardness. The rod/wedge and wedge/barrel interfaces are simulated using surface-based contact models having different coefficients of friction. Two hardness levels [171 and 319 Vickers hardness numbers (VPN)] are considered for the wedges. When the softer wed...

Development of a peripheral thickness estimation method for volumetric breast density measurements in mammography using a 3d finite element breast model

A method was developed to determine the area in a mammogram where the breast is not in contact with the compression paddle (the periphery), and to predict the breast thickness in that peripheral region The periphery is determined by evaluating the variation of the signal intensity along radial lines, and the peripheral thickness is modeled assuming the breast has a semi-circular shape The method was tested on 26 simulated mammograms for which the volumetric information was available The mammograms were obtained using CT data that were deformed to simulate mammographic compression and then projected using a physical model The method predicted the thickness in the periphery to within 3.3 mm of the actual value and the volumetric breast density within 4.3 percentage points The method was also tested on 209 digital mammograms, and on average it was estimated that thickness errors occurred on 9% of the breast image, and the average absolute thickness error on those points was estimated to be approximately 2.0 mm in the periphery and central region of the breast but as large as 10.5 mm in the extreme periphery where the thickness is small.