Fragilidad relativa de fémures osteoporóticos evaluados con DXA y simulación de caídas con elementos finitos guiados por radiografías de urgencias / Relative fragility of osteoporotic femurs assessed withDXA and simulation of finite element falls guided byemergency X-rays

OBJETIVO: El diagnóstico de osteoporosis se ha fundamentado en la medición de la densidad mineral ósea, si bien esta variable tiene una capacidad limitada en la discriminación de pacientes con o sin fracturas. La aplicación del análisis de elementos finitos (FE) sobre imágenes volumétricas de tomografía computarizada ha mejorado la clasificación de sujetos hasta 90%, aunque la dosis de radiación, complejidad y coste no aconsejan su práctica regular. Nuestro objetivo es aplicar el análisis FE a modelos tridimensionales con absorciometría radiológica dual (3D-DXA), para clasificar pacientes con fractura osteoporótica de fémur proximal y sin fractura. MATERIAL Y MÉTODOS: Se seleccionó una cohorte de 111 pacientes con osteoporosis densitométrica: 62 con fractura y 49 sin ella. Se utilizaron modelos FE sujeto-específicos para el impacto, como la simulación estática de la caída lateral. Las simulaciones de impacto permiten identificar la región crítica en el 95% de los casos, y la respuesta mecánica a una fuerza lateral máxima. Se realizó un análisis mediante un clasificador discriminativo (Support Vector Machine) por tipo de fractura, tejido y género, utilizando las mediciones DXA y parámetros biomecánicos. RESULTADOS: Los resultados mostraron una sensibilidad de clasificación del 100%, y una tasa de falsos negativos de 0% para los casos de fractura de cuello para el hueso trabecular en las mujeres. Se identifica la variable tensión principal mayor (MPS) como el mejor parámetro para la clasificación. CONCLUSIÓN: Los resultados sugieren que el uso de modelos 3D-DXA podría ayudar a discriminar mejor a los pacientes con elevado riesgo de fracturarse

OBJETIVE: The diagnosis of osteoporosis has been based on the measurement of bone mineral density, although this variable has a limited capacity in discriminating patients with or without fractures. The application of finite element analysis (FE) on computed tomography volumetric images has improved the classification of subjects by up to 90%, although the radiation dose, complexity, and cost do not favor their regular practice. Our objective is to apply FE analysis to three-dimensional models with dual-energy x-ray absorptiometry (3D-DXA), to classify patients who present osteoporotic fracture of the proximal femur and those without fracture. MATERIAL AND METHODS: A cohort of 111 patients with densitometric osteoporosis was selected: 62 with fracture and 49 without it. Subject-specific FE models for impact were used, such as static simulation of lateral fall. Impact simulations allow identifying the critical region in 95% of cases, and the mechanical response to maximum lateral force. An analysis was performed using a discriminative classifier (Support Vector Machine) by fracture type, tissue and gender, using DXA measurements and biomechanical parameters. RESULTS: The results showed a classification sensitivity of 100%, and a false negative rate of 0% for cases of neck fracture for trabecular bone in women. The variable major main stress (MPS) is identified as the best parameter for the classification. CONCLUSION: The results suggest that using 3D-DXA models help in order to better discriminate patients with raised fracture risk

Influence of segmentation on micro-CT images of trabecular bone.

Segmentation of biomedical images is of great importance in various studies aiming to both the identification of regions of interests within the image and the performance of quantified measurements. Nevertheless, the segmentation of the biomedical images represents a wide range of medical cases and there is not a unique technique applicable to all kinds of medical images. In this study, three popular techniques for segmenting micro-CT images of bone microstructures are evaluated. Fixed threshold, Otsu's algorithm and a modified version of the Chan-Vese segmentation technique have been applied on micro-CT images and have been compared to higher resolution golden standard, that is histological images. The modification of the Chan-Vese technique is based on the novel implementation of a new initialization process called the Branch Point Initialization. Stereological measurements were performed on all the segmented images and statistically compared to the golden standard. Fixed threshold and the modified Chan-Vese technique have shown comparable results, with a maximum significant error of about 10%. However, Chan-Vese showed an easier, faster and more reliable segmentation procedure for optimal settings identification. The Otsu's method showed a maximum error larger than 20%. Given the limits and advantages of the known segmentation techniques, the proposed modified Chan-Vese active contour technique shows high potential for use in the segmentation of micro-CT images as well as in other high-resolution X-ray images. This potential is augmented by the recent introduction of high-resolution clinical technologies for which standard techniques have already shown to be insufficient.

Quality control protocol for in vitro micro-computed tomography.

The aim of this work was to present and discuss a quality control protocol for in vitro micro-computed tomography (microCT), based on the adaptation of the quality control protocols for medical computed tomography. The importance of establishing a quality control protocol is related to the opportunity to identify problems on time comparing the microCT images acquired in different time points, and in this way to verify the performance of the device. The proposed quality control protocol was applied for a long-time monitoring period to verify the stability of the micro-tomographic system over time. The protocol proposed in this study was applied to the histomorphometric characterization of bone tissue, but it can be used on a wide range of in vitro microCT applications. Noise and uniformity tests, taken and adapted to micro-tomographic system by medical standard guidelines of quality control, were performed by the use of a water phantom. An accuracy test was designed and performed by the use of a morphometric calibrated phantom. All these tests were performed during a long-time monitoring period to control the stability of the system. Specific control charts and monitoring parameters for each test were used to represent the monthly measures collected during 20 months and an out of control condition was defined. The reference values (baseline), calculated to control the stability of micro-tomographic system over time, were calculated during acceptance/status test. During the period, no out of control conditions in noise, uniformity and accuracy tests were recorded. However, a changing condition was found in noise test, as showed by using statistical C (P < 0.01) and Kruskal-Wallis (P < 0.05) tests. In particular, a Wilcoxon rank sum test with Bonferroni correction (P < 0.0125) was applied in noise test to investigate which of the comparisons among first five acquisitions of year 2004 (group B.L.) and each group was significant (P < 0.0125). The noise showed a slight but significant increase over the years compared to baseline value; however, no out of control conditions were recorded. Nonetheless, a maintenance service to control the performance of mechanical components of microCT was required and performed.