Davide Romani

An SVM classifier to separate false signals from microcalcifications in digital mammograms

In this paper we investigate the feasibility of using an SVM (support vector machine) classifier in our automatic system for the detection of clustered microcalcifications in digital mammograms. SVM is a technique for pattern recognition which relies on the statistical learning theory. It minimizes a function of two terms: the number of misclassified vectors of the training set and a term regarding the generalization classifier capability. We compare the SVM classifier with an MLP (multi-layer perceptron) in the false-positive reduction phase of our detection scheme: a detected signal is considered either microcalcification or false signal, according to the value of a set of its features. The SVM classifier gets slightly better results than the MLP one (Az value of 0.963 against 0.958) in the presence of a high number of training data; the improvement becomes much more evident (Az value of 0.952 against 0.918) in training sets of reduced size. Finally, the setting of the SVM classifier is much easier than the MLP one.

MicroCT Scan in paleobiology: application to the study of dental tissues

State of the art in paleoanthropological and paleoprimatological research foresees the use of advanced non-destructive investigative approaches. Microcomputed tomography (microCT) is a fundamental tool, since it offers the opportunity to get high quality morphological information with high spatial resolution. We carried out the set-up of an experimental microCT system able to examine paleobiological samples. The equipment can operate on small objects (size up to 3 cm) with a nominal spatial resolution of 30 μm, allowing their 3D volume reconstruction and morphometric analysis. This approach represents a forefront technique in paleobiological studies, successfully employed only in a limited number of advanced research centers. A specific program of microCT analysis has been planned on a sample of human and non-human fossil primate dentitions, in order to assess the specific nature of a number of tooth lesions (e.g. caries versus abrasion). This currently in progress experimental activity represents the first step for the set-up of a research center specifically devoted to the realization of advanced studies in the field of archaeo-paleobiology.

Experimental micro-CT system for x-ray NDT

This work describes the setup of an experimental system for microtomography developed in the framework of a collaboration between the Physics Department of the University of Bologna (Italy) and the Geosphaera Research Center of Moscow (Russia). The main goal of this inspection system is to carry out high-resolution analysis in vitro of biomedical samples as well as nondestructive testing (NDT) of industrial components. The detection system consists of a 30x15 mm2 rectangular fiberoptic taper (ratio 2:1) optically coupled to a cooled 12-bit CCD camera (1024x512 pixels). On the entrance window of the taper is deposited a thin layer of Gd2O2S:Tb phosphor which provides the X-light conversion. The image readout is carried out by means of a commercial frame grabber installed on a personal computer and specific software is used for data acquisition and control of the tomographic process. The object under investigation is arranged on a 3-degree micro-positioning system (x-y translation and rotation) and irradiated by an X-ray microfocus beam (up to 200 kVp). The sample can be positioned easily along the source-detector axis in order to obtain a large magnification of details of interest. The X-ray detector has been intensively tested in order to determine its performance in terms of MTF, NPS, and DQE. Moreover, preliminary tests have been carried out on several samples in order to evaluate the performance of the micro-CT system.

An Innovative CCD-Based High-Resolution CT System for Analysis of Trabecular Bone Tissue

Synchrotron-based digital radiography and microtomography devices are powerful, nondestructive, high-resolution research tools. In this paper, we present a linear system with a pixel size of 22.5 mum and a field-of-view (FOV) 13 cm long and about 1 mm high. The system is composed of a linear converter GOS screen coupled to an intensified electron-bombarded CCD (EBCCD) camera, by means of a rectangular-to-linear fiber optic adapter. This optical guide is composed of seven bundles, each one transporting light in a coherent way to preserve spatial information. In this way, a high spatial resolution over an extended FOV is obtained. The detector works as an X-ray scanner by means of a high-precision translation mechanical device with 18 cm travel range. The total FOV obtained this way is 13 cm long and 18 cm high. The aim of this paper is to demonstrate the feasibility of this system to investigate a large area of a bone and to calculate the appropriate histomorphometric parameters. Here we present an investigation gained at ELETTRA synchrotron facility at Trieste, Italy. A monochromatic 34-keV beam has been used for imaging a human proximal femur, about 9 cm in width, with our system. The reconstructed images (13 cmtimes13 cm) were cross sections containing femoral head, femoral neck, and greater trochanter. The local variations in trabecular and cortical structure of the examined bone were clearly visible at a level not obtainable with medical CT scanners. The used spatial resolution allowed the visualization of thin trabeculae, which typically lie in a range of 100 mum or lower. The quality of the reconstructed cross-section images confirmed that the system presented is a novel tool for high resolution three-dimensional (3-D) imaging of bone structure, with a pixel size over a volume of interest not achievable with conventional microCT scanners

SYSTEM FOR AUTOMATIC DETECTION OF CLUSTERED MICROCALCIFICATIONS IN DIGITAL MAMMOGRAMS

In this paper, we investigate the performance of a Computer Aided Diagnosis (CAD) system for the detection of clustered microcalcifications in mammograms. Our detection algorithm consists of the combination of two different methods. The first, based on difference-image techniques and gaussianity statistical tests, finds out the most obvious signals. The second, is able to discover more subtle microcalcifications by exploiting a multiresolution analysis by means of the wavelet transform. We can separately tune the two methods, so that each one of them is able to detect signals with similar features. By combining signals coming out from the two parts through a logical OR operation, we can discover microcalcifications with different characteristics. Our algorithm yields a sensitivity of 91.4% with 0.4 false positive cluster per image on the 40 images of the Nijmegen database.

X-ray computed tomography of an ancient large globe

Computed Tomography (CT) is one of the principal non-invasive techniques for the investigation of the inner structure of works of art. The main advantage of using CT is that it provides high resolution 3D information of the analyzed object. CT of large objects can be hampered by the long time needed and by the difficulties regarding the experimental arrangements required. In this paper we present a CT study of an ancient large globe (diameter of about 2.2 m). We set-up an ad hoc system for the analysis of the globe in situ. The system consists of an X-ray tube, a detector made of a GOS scintillator and an EBCCD camera, the movement axes, a vertical moving axis for the tube, a horizontal-vertical axis for the detector, and a rotating platform for the globe. The investigation of the entire globe has required the acquisition of about 32000 planar images, for providing the 3D tomographic reconstruction. The analysis of the reconstructed volume has allowed to estimate the composition of the inner structure of the globe.

3D micro-CT analysis of cancellous bone architecture

Osteoporosis is the most common type of metabolic bone disease generating morphological alterations of the bone structure. Therefore, an intensive and accurate study of cancellous bone architecture is required to individuate the origin of the disease and to estimate its evolution. For this purpose, a specific project is in progress at the Physics Department of Bologna to carry out microtomography analysis in vitro of animal bone samples. The inspection system mainly consists of an X-ray microfocus source to irradiate the sample and of a CCD-based detector for the detection of the radiographic image. A sample with a diameter up to 30 mm and a height of 15 mm can be investigated over its whole volume. Three-dimensional images of the bone structure can be obtained with a spatial resolution variable from 30 up to few microns. A pig bone specimen has been investigated to test the equipment and to carry out intensive analysis and modeling of bone architecture. Moreover, a specific volume data processing procedure has been developed and tested to extract morphometric measurements of the bone structure.

Study of an appropriate reconstruction algorithm for an innovative electron beam imaging system for dosimetry in IORT (intra operative radiation therapy)

Intra operative radiation therapy (IORT) is a technique based on delivery of a high dose of ionising radiation to the cancer tissue, after tumour ablation, during surgery. The Novac7 is a new linear accelerator expressly conceived for IORT that supplies electron beams at several energy, with high dose rate. These peculiar characteristics give rise to some complications with classical dosimetric techniques. In the framework of a research contract between ENEA and the Physics Department of Bologna, Italy, an original digital system has been developed to study and visualise the Novac7 electron beam in real time. The system is conceived as a grid consisting of two bundles of scintillating optical fibres (SOP) over-crossing each other, optically coupled with two arrays of photodiodes as read-out system. The problem of image reconstruction can be expressed as follows: there are only two profiles, or data arrays, which correspond respectively to the light emitted along the fibres perpendicular to the X-axis for the X-profile, and along the fibres perpendicular to the Y-axis for the Y-profile. This problem is not dissimilar to the reconstruction problem in tomography where several projections should be composed to trace them back to the original image. Unfortunately, here we have only two profiles: we have two one-dimensional profiles and are seeking one bidimensional image that could produce them. We selected a known beam image acquired by another digital instrument, then we extracted from it the two profiles simulating the acquisition arrays. Subsequently, we tested several reconstruction algorithms on these profiles, comparing the reconstructed image with the original one. We started from the simple sum algorithm until to iterative algorithms, searching the best compromise between the computational complexity and an high precision. We found that the iterative method is the best solution: it respects the geometrical characteristics and the absolute intensity values of the original image. Moreover it can reconstruct the image in a time of less than one second, a very good result.

Characterization of an FFDM unit based on a-Se direct conversion detector

The aim of this paper is to investigate the properties of a clinical FFDM unit (Giotto - Image MD, IMS Italy). The digital detector consists of a flat panel using the amorphous selenium technology (ANRAD Corporation, Canada). The active area of the imager is 17.4 cm × 23.9 cm (2048×2816 pixels) with a pixel pitch of 85 µm. The direct conversion of X-rays into charge provides excellent imaging performance. In this work we present an objective and complete characterization of such system: detector response, MTF, NPS and DQE calculation will be presented. MTF and DQE at Nyquist frequency (5.88 lp/mm) are equal to 38% and 15%, respectively. The detector linearity is very good under the typical mammographie tested exposure conditions.

A CCD-based high resolution CT system for analysis of trabecular bone tissue

This paper illustrates the effectiveness of a new multi-slice CT system to study the trabecular bone tissue. The system is capable of acquiring 3D images of 5600/spl times/5600/spl times/52 voxels on specimens up to 130 mm with a spatial resolution of 22.5 micrometers. This new detector is coupled to a CCD intensified camera (EBCCD) and was patented by the University of Bologna. The CT acquisitions were performed with an experimental setup at Elettra facilities at beamline SYRMEP. The reconstructed images were sections containing the femoral head, femoral neck and trochantere. The used spatial resolution allows to visualize also thin trabeculae, which typically lie in a range below 100 microns. The morphometric trabecular characterization parameters as BV/TV, Tb.Th, Tb.Sp, Tb.N were calculated over three regions of interest. The local variations in trabecular and cortical structure of the examined bone are clearly visible at a level not obtainable with medical CT scanners. The quality of the reconstructed cross sections images confirm that this investigation technique is an advanced tool for high resolution three-dimensional imaging of bone structure.