Renato Natal Jorge

A review of algorithms for medical image segmentation and their applications to the female pelvic cavity

This paper aims to make a review on the current segmentation algorithms used for medical images. Algorithms are classified according to their principal methodologies, namely the ones based on thresholds, the ones based on clustering techniques and the ones based on deformable models. The last type is focused on due to the intensive investigations into the deformable models that have been done in the last few decades. Typical algorithms of each type are discussed and the main ideas, application fields, advantages and disadvantages of each type are summarised. Experiments that apply these algorithms to segment the organs and tissues of the female pelvic cavity are presented to further illustrate their distinct characteristics. In the end, the main guidelines that should be considered for designing the segmentation algorithms of the pelvic cavity are proposed.

Analysis of Functionally Graded Plates by a Robust Meshless Method

The analysis of static deformations of functionally graded plates is performed by using the collocation method, the radial basis functions and a higher-order shear deformation theory. The collocation method is truly meshless, allowing a fast and simple domain and boundary discretization. We select the shape parameter in the radial basis functions by an optimization procedure based on the cross-validation technique, and use the Mori-Tanaka homogenization technique to deduce effective properties of functionally graded materials. Numerical tests show that the method is reliable, robust and produces accurate results.

Static analysis of functionally graded sandwich plates according to a hyperbolic theory considering Zig-Zag and warping effects

In this paper, a variation of Murakamis Zig-Zag theory is proposed for the analysis of functionally graded plates. The new theory includes a hyperbolic sine term for the in-plane displacements expansion and accounts for through-the-thickness deformation, by considering a quadratic evolution of the transverse displacement with the thickness coordinate. The governing equations and the boundary conditions are obtained by a generalization of Carreras Unified Formulation, and further interpolated by collocation with radial basis functions. Numerical examples on the static analysis of functionally graded sandwich plates demonstrate the accuracy of the present approach. The thickness stretching effect on such problems is studied.

Novel Approach to Segment the Inner and Outer Boundaries of the Bladder Wall in T2-Weighted Magnetic Resonance Images

Diagnosis of bladder-related conditions needs critical measurements which require the segmentation of the inner and outer boundaries of the bladder wall. In T2-weighted MR images, the low-signal intensity bladder wall can be identified due to the large contrast with the high-signal intensity urine and perivesical fat. In this article, two deformable models are proposed to segment the bladder wall. Based on the imaging features of the bladder, a modified geodesic active contour is proposed to segment the inner boundary. This method uses the statistical information of the bladder lumen and can handle the intensity variation in MR images. Having obtained the inner boundary, a shape influence field is formed and integrated with the Chan–Vese (C–V) model to segment the outer boundary. The shape-guided C–V model can prevent the overlapping between the two boundaries when the appearance of the bladder wall is blurred. Segmentation examples are presented and analyzed to demonstrate the effectiveness of this novel approach.

Experimental study and constitutive modeling of the viscoelastic mechanical properties of the human prolapsed vaginal tissue

In this paper, the viscoelastic mechanical properties of vaginal tissue are investigated. Using previous results of the authors on the mechanical properties of biological soft tissues and newly experimental data from uniaxial tension tests, a new model for the viscoelastic mechanical properties of the human vaginal tissue is proposed. The structural model seems to be sufficiently accurate to guarantee its application to prediction of reliable stress distributions, and is suitable for finite element computations. The obtained results may be helpful in the design of surgical procedures with autologous tissue or prostheses.

Free Vibration Analysis of Composite and Sandwich Plates by a Trigonometric Layerwise Deformation Theory and Radial Basis Functions

In this study trigonometric layerwise deformation theory is used for the analysis of free vibration of symmetric composite plates. A meshless discretization method based on global multiquadric radial basis functions is used. The equations of motion and the boundary conditions are derived and interpolated by radial basis functions. This method is applied to the free vibration analysis of composite and sandwich plates. The results are then compared with analytical and numerical solutions. The results show that the use of trigonometric layerwise deformation theory discretized with multiquadrics provides very good solutions for the free vibration of composite and sandwich plates.

Computational modeling approach to study the effects of fetal head flexion during vaginal delivery

OBJECTIVE The purpose of this study was to investigate the influence of fetal head flexion during vaginal delivery with a 3-dimensional computational finite element model. STUDY DESIGN A finite element model of the pelvic skeletal structure, pelvic floor, and fetus was developed. The movements of the fetus during birth were simulated in engagement, descent, flexion, internal rotation, and extension of the fetal head. The opposite forces against the fetal descendent and the stress of the pelvic floor muscles were obtained on simulations with different degrees of head flexion. RESULTS The simulated increase in fetal head flexion is associated with lower values of opposite forces against the fetal descent. The descending fetus with abnormal head flexion also meets resistance in later stations. Lower stress on the pelvic floor was demonstrated with simulated increase in fetal head flexion during vaginal delivery. CONCLUSION This analytic evidence suggests that the fetal head flexion during vaginal delivery may facilitate birth and protect the pelvic floor.

The influence of pelvic muscle activation during vaginal delivery.

OBJECTIVE: To estimate the influence of pelvic floor muscle activation during vaginal delivery using a three-dimensional computational finite element model. METHODS: A computational finite element model of the pelvic skeletal structure, pelvic floor, and fetus was developed. The movements of the fetus during birth, in vertex position, were simulated; namely, the engagement, descent, flexion, internal rotation, and extension of the fetal head. The opposite forces against the fetal descent and the stress on the pelvic floor muscles were obtained in passive, 5%, 10%, and 15% pelvic floor muscle simulated activations. RESULTS: The increase in pelvic floor muscle activation was associated with higher values of forces against the fetal descent. The descending fetus encountered increasing resistance in higher stations with the increase in pelvic floor muscle activation. The maximum values of stress of the pelvic floor muscles were obtained in +4 station. The increase in pelvic floor muscle activation was also followed by higher values of pelvic floor stress. CONCLUSION: This study demonstrates the feasibility of using a computational modeling approach to study parturition. This experimental evidence suggests that the pelvic floor muscle activation during vaginal delivery may represent an obstacle to fetal descent and increase the risk for pelvic floor injuries. LEVEL OF EVIDENCE: III

Biomechanical study on the bladder neck and urethral positions: Simulation of impairment of the pelvic ligaments

Excessive mobility of the bladder neck and urethra are common features in stress urinary incontinence. We aimed at assessing, through computational modelling, the bladder neck position taking into account progressive impairment of the pelvic ligaments. Magnetic resonance images of a young healthy female were used to build a computational model of the pelvic cavity. Appropriate material properties and constitutive models were defined. The impairment of the ligaments was simulated by mimicking a reduction in their stiffness. For healthy ligaments, valsalva maneuver led to an increase in the α angle (between the bladder neck-symphysis pubis and the main of the symphysis) from 91.8° (at rest) to 105.7°, and 5.7 mm of bladder neck dislocation, which was similar to dynamic imaging of the same woman (α angle from 80° to 103.3°, and 5mm of bladder neck movement). For 95% impairment, they enlarged to 124.28° and 12 mm. Impairment to the pubourethral ligaments had higher effect than that of vaginal support (115° vs. 108°, and 9.1 vs. 7.3mm). Numerical simulation could predict urethral motion during valsalva maneuver, for both healthy and impaired ligaments. Results were similar to those of continent women and women with stress urinary incontinence published in the literature. Biomechanical analysis of the pubourethral ligaments complements the biomechanical study of the pelvic cavity in urinary incontinence. It may be useful in young women presenting stress urinary incontinence without imaging evidence of urethral and muscle lesions or organ descend during valsalva, and for whom fascial damage are not expected.

A meshless microscale bone tissue trabecular remodelling analysis considering a new anisotropic bone tissue material law.

In this work, a novel anisotropic material law for the mechanical behaviour of the bone tissue is proposed. This new law, based on experimental data, permits to correlate the bone apparent density with the obtained level of stress. Combined with the proposed material law, a biomechanical model for predicting bone density distribution was developed, based on the assumption that the bone structure is a gradually self-optimising anisotropic biological material that maximises its own structural stiffness. The strain and the stress field required in the iterative remodelling process are obtained by means of an accurate meshless method, the Natural Neighbour Radial Point Interpolation Method (NNRPIM). Comparing with other numerical approaches, the inclusion of the NNRPIM presents numerous advantages such as the high accuracy and the smoother stress and strain field distribution. The natural neighbour concept permits to impose organically the nodal connectivity and facilitates the analysis of convex boundaries and extremely irregular meshes. The viability and efficiency of the model were tested on several trabecular benchmark patch examples. The results show that the pattern of the local bone apparent density distribution and the anisotropic bone behaviour predicted by the model for the microscale analysis are in good agreement with the expected structural architecture and bone apparent density distribution.