Sharif Digital Repository

We study mix-nets with randomized partial checking (RPC) as proposed by Jakobsson, Juels, and Rivest (2002). RPC is a technique to verify the correctness of an execution both for Chaumian and homomorphic mix-nets. The idea is to relax the correctness and privacy requirements to achieve a more efficient mix-net. We identify serious issues in the original description of mix-nets with RPC and show how to exploit these to break both correctness and privacy, both for Chaumian and homomorphic mix-nets. Our attacks are practical and applicable to real world mix-net implementations, e.g., the Civitas and the Scantegrity voting systems  

Teeth segmentation in computed tomography (CT) images is a major and challenging task for various computer assisted procedures. In this paper, we introduced a hybrid method for quantification of teeth in CT volumetric dataset inspired by our previous experiences and anatomical knowledge of teeth and jaws. In this regard, we propose a novel segmentation technique using an adaptive thresholding, morphological operations, panoramic re-sampling and variational level set algorithm. The proposed method consists of several steps as follows: first, we determine the operation region in CT slices. Second, the bony tissues are separated from other tissues by utilizing an adaptive thresholding technique... 

The configuration of landmarks is an important issue in minimizing the target registration error (TRE). In this paper the effect of different landmark configurations on the accuracy of pedicle screw placement during image guided spine surgery (IGSS) is investigated. Since the spine is deformed in intra-operative conditions compared to the preoperative situation, an accurate alignment of each vertebra is crucial to compensate for the deformation. CT compatible markers are placed over anatomical landmarks which are feasible and routinely used in surgical procedures. The TRE is obtained directly for the markers which are placed on the right and left pedicle of the vertebra. The estimated TRE... 

Traumatic brain injury (TBI) has long been known as one of the most anonymous reasons for death around the world. This phenomenon has been under study for many years and yet it remains a question due to physiological, geometrical and computational complexity. Although the modeling facilities for soft tissue have improved, the precise CT-imaging of human head has revealed novel details of the brain, skull and meninges. In this study a 3D human head including the brain, skull, and meninges is modeled using CT-scan and MRI data of a 30-year old human. This model is named "Sharif University of Technology Head Trauma Model (SUTHTM)". By validating SUTHTM, the model is then used to study the... 

Spinal cord is a long, thin, tubular bundle of nervous tissue located in the vertebral column. Since the spinal cord is one of the most crucial pathways for information connecting of the central nervous system, it has to stay safe under any blunt impact loading, i.e., accident, punch, gunshot, burst, etc. Therefore, it is important to investigate the possible injuries that may occur in the vertebral column, especially the spinal cord, as a result of blunt loading. However, due to various experimental limitations, numerical modelling, specifically finite element (FE) models, has been beneficial in predicting the injury to the spinal cord. This study, thus, aimed at predicting the stresses and... 

Introduction: Utilization of computed tomography (CT) scans is increasing annually. This study aimed to reduce the absorbed dose of sensitive organs in the head (eye lenses and thyroid) and to assess changes in resultant images quality in head scans when the radiation dose is decreased. Material and Methods: An anthropomorphic phantom was examined with head protocols in both helical and sectional modes using two 16-slice CT scanners. The entrance surface dose of eye lenses and thyroid was measured with standard protocols and after reducing the mAS and kilo-voltage using thermo-luminescence dosimeters (TLDs). Results: In sectional mode with standard protocol, the highest surface dose was 2.3... 

Segmentation of the human airway tree from volumetric computed tomography images is an important stage for many clinical applications such as virtual bronchoscopy. The main challenges of previously developed methods are to deal with two problems namely, leaking into the surrounding lung parenchyma during segmentation and the need to manually adjust the parameters. To overcome these problems, a multi-seeded fuzzy based region growing approach in conjuction with the spatial information of voxels is proposed. Comparison with a commonly used region growing segmentation algorithm shows that the proposed method retrieves more accurate results by achieving the specificity and sensitivity of 98.81%... 

In this paper, we divide the imaging geometry of a photoacoustic computed tomography (PACT) structure that have a circular geometry and have focused transducers of positive or negative focus, into different zones of varying resolution. With using the presented analysis, it is possible to find the maximum range in which the system resolution is at its best value, according to the device specifications. The following equations are used to find the blind spot angles of the system for each source point inside the chamber. These are important for two reasons: the first is that the user can calculate the range in which the imaging system has the best quality by entering the specifications of his... 

This study considers blood flow in total cavopulmonary connection (TCPC) morphology created in Fontan surgical procedure in patients with a single ventricle heart disease. Ordinary process of TCPC operation reduces pulmonary blood flow pulsatility since the right ventricle being bypassed. This reduction may limit the long term outcome of Fontan circulation. There is an idea of increasing pulmonary flow pulsations by keeping Main Pulmonary Artery (MPA) partially open while it was closed in ordinary TCPC operation. The purpose of the present study is to verify the effects of Antegrade Flow (AF) coming through stenosed MPA on pulmonary flow pulsations. The 3D geometry is reconstructed from CT... 

Porosity and specific surface are two prominent factors in describing the hydraulic properties of porous media. Determination of these two important parameters leads to identify the capability of porous media to conduct the fluids. In the present study, a new relationship between porosity and specific surface of sandstones has been developed. Micro-CT data from 10 types of sandstones has been utilized in order to present a porosity-specific surface correlation. This correlation also contains the average grain radius of each rock obtained by image processing algorithms. Finally, the correlation is tested on the provided data to evaluate its precision. The simplicity and applicability of the... 

The Fontan surgery is performed on patients with a single ventricle heart defect to prevent the combination of highlyoxygenated and poorly-oxygenated blood. Blood flow in total cavopulmonary connection (TCPC) which culminates an ordinary Fontan operation is practically steady-state but this flow is not appropriate for respiratory systems. This article investigates an approach in Fontan surgery that has been recently proposed in order to make the pulmonary blood flow pulsating. Moreover, for investigating the compliance of vessels and its effects on blood flow in TCPC, we have used the FSI (Fluid Structure Interaction) method as well as rigid wall assumption for comparison purposes. Our TCPC... 

Traumatic brain injury (TBI) has long been known as one of the most anonymous reasons for death around the world. A presentation of a model of what happens in the process has been under study for many years; and yet it remains a question due to physiological, geometrical and computational complications. Although the facilities for soft tissue modeling have improved and the precise CT-imaging of human head has revealed novel details of brain, skull and the interface (the meninges), a comprehensive FEM model of TBI is still being studied. This study aims to present an optimized model of human head including the brain, skull, and the meninges after a comprehensive study of the previous models.... 

This study considers blood fl ow in Total Cavopulmonary Connection (TCPC) morphology, created in Fontan surgical procedure in patients with single ventricle heart disease. Ordinary process of TCPC operation reduces pulmonary blood fl ow pulsatility; because of right ventricle being bypassed. This reduction may limit the long term outcome of Fontan circulation. There is an idea stating that keeping Main Pulmonary Artery (MPA) partially open, would increase pulmonary fl ow pulsations. MPA gets closed in ordinary TCPC operation. The purpose of current study is to verify effects of Antegrade Flow (AF) coming through stenosed MPA on pulmonary fl ow pulsations, by means of Computational Fluid... 

In this paper, a Kalman filter technique which is operated in time is introduced for noise reduction on CT set of projections to reconstruct medical images. The experiments were done on medical image of kidneys and the simulated projections are captured by CT scanner. Evaluation results indicated that as the number of projections increase in the collected ray sums corrupted by noise the quality of reconstructed image becomes better in terms of contrast and transparency. However, for the comparison issue, the same conditions are applied for reconstruction of medical image in frequency domain using filter back projection technique. It observes that filter back projection technique does not... 

Computed Tomography (CT) medical images show a specific part of human body and present it in a digital form. Lossless image compression is one of the medical imaging applications. To implement such application on hardware we need a simple and fast algorithm. Differential pulse code modulation (DPCM) is a simple and efficient method for transforming image. In this paper best predictor for DPCM introduce in a manner in which has the best result in compression and also be efficient for hardware implementation. After transforming image by DPCM, Huffman encoding used to compress image. We introduce this method with application to brain CT images  

We propose a new nonblind multiresolution watermarking method for still images based on the contourlet transform (CT). In our approach, the watermark is a grayscale image which is embedded into the highest frequency subband of the host image in its contourlet domain. We demonstrate that in comparison to other methods, this method enables us to embed more amounts of data into the directional subbands of the host image without degrading its perceptibility. The experimental results show robustness against several common watermarking attacks such as compression, adding noise, filtering, and geometrical transformations. Since the proposed approach can embed considerable payload, while providing... 

A novel CT imaging structure based on Compressive Sensing (CS) is proposed. The main goal is to mitigate the CT imaging time and, thus, X-ray radiation dosage without compromising the image quality. The utilized compressive sensing approach is based on radial Fourier sampling. Thanks to the intrinsic relation between captured radon samples in a CT imaging process and the radial Fourier samples, partial Fourier sampling could be implemented systematically. This systematic compressive sampling helps in better control of required conditions such as incoherence and sparsity to guarantee adequate image quality in comparison to previous CS-based CT imaging structures. Simulation results prove the...