A Rapid Method for Constructing an Approximate Knee Model for Local SAR Estimation Based on a Convolutional Neural Network
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Purpose: The local specific absorption rate (SAR) of tissues is a key consideration in high-field MRI of knee joint. To automatically and rapidly construct individual's knee model for SAR simulation and estimation, a segmentation method for MR images using U-Net network and tissue simplification is proposed in this paper. Materials and methods: Based on a data set of low-field MR Tl-weighted images in the axial plane, the knee tissues were simplified and classified as muscle, fat, and bone for labeling and segmentation. Single U-net was adopted to segmented images, the number of convolutional layers was 13, the convolution kernel size was 7x7, and the mini batch was 16. The data set consisted of 30 volunteers' knee joint images (in the axial plane): 14, 4, and 12 for the training, validation, and test set respectively. To overcome the limitation of imaging slice number and make the SAR simulation result more accurate, the constructed knee model was extended in the axial direction using slice extrapolating. Based on the test set, electromagnetic simulation in a 3T coil was carried out and local SAR was evaluated using the models constructed with the proposed method and manual delineation. Result and discussions: The two constructed models had similar SARlOg distribution, the locations of hot spots were basically unchanged, and the mean and standard deviation of the errors between their maximum SARlOg values were relatively small. Conclusions: The method by using single U-Net networks, tissue simplification and model extending shows promising potential for constructing knee model on which approximate local SAR could be estimated.Keywords:
Convolution (computer science)
Kernel (algebra)
Specific absorption rate
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This paper compares the specific absorption rate (SAR) values induced in human tissues once obtained when using the equivalent liquid dielectric properties of the equivalent liquid filling the phantom used in SAR measurements and from a model that proposes realistic continuation of tissues in the human body. The calculations are performed for the case of a dipole close to a multilayer structure representing the different body tissues using integral equations techniques solved by the method of moments.
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In our earlier study we reported that 900 MHz continuous wave (CW) radiofrequency radiation (RFR) exposure (2 W/kg specific absorption rate [SAR]) had no significant effect on the hematopoietic system of rats. In this paper we extend the scope of the previous study by testing for possible effects at: (i) different SAR levels; (ii) both 900 and 1800 MHz, and; (iii) both CW and pulse modulated (PM) RFR.Excised long bones from rats were placed in medium and RFR exposed in (i) a Transverse Electromagnetic (TEM) cell or (ii) a waveguide. Finite-difference time-domain (FDTD) numerical analyses were used to estimate forward power needed to produce nominal SAR levels of 2/10 and 2.5/12.4 W/kg in the bone marrow. After exposure, the lymphoblasts were extracted and assayed for proliferation rate, and genotoxicity.Our data did not indicate any significant change in these end points for any combination of CW/PM exposure at 900/1800 MHz at SAR levels of nominally 2/10 W/kg or 2.5/12.4 W/kg.No significant changes were observed in the hematopoietic system of rats after the exposure of CW/PM wave 900 MHz/1800 MHz RF radiations at different SAR values.
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The influence of the hand on the SAR (Specific Absorption Rate) assessments of mobile phones is investigated. A possible increase of the SAR value in the presence of the hand has been reported by several authors but the electromagnetic phenomenon is still unexplained. Numerical simulations are herein performed using four different mobile phones using the SAM (Specific Anthropomorphic Mannequin) phantom and a hand model at three different frequencies in order to understand the key parameters - mobile phone design, antenna type, frequency, etc. - which may influence the SAR value in the presence of the hand. An increase of the SAR is observed for one mobile phone model at 1950 MHz and for a configuration such that the distance between the antenna and the palm of the hand is about 20 mm. Additional numerical simulations and measurements performed using a dipole antenna placed in-between a flat head phantom and a flat hand phantom confirm that an increase of the SAR is obtained for some specific values of the distance between the antenna and the hand at a given frequency.
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In this study, the effects of medical physiotherapy devices, cellular phones and wireless communication devices on human muscle tissue was examined in different studying frequency. Different muscle models were formed using HFSS program by taking into consideration electrical properties of the muscle tissue for different operating frequencies. Specific absorption rate values were computed on these models. Also the temperature rise amount and Spesific absorbation rate in the tissue from caused by exposure to electromagnetic fields were determined mathematically.
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Experimental tests of specific absorption rate (SAR) reduction on a mobile phone have been performed. To protect a human head from exposure to electromagnetic fields and comply with exposure guidelines, the electromagnetic bandgap (EBG) structures are inserted in a commercial personal communication services (PCS) mobile phone. The measured results demonstrate the movement of a hot spot and the reduction of SAR in the human head.
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This paper gives a study and comparison of specific absorption rate (SAR) induced in a child-sized head and an adult head using a dual band mobile phone. Both the average SAR over a mass of 10g in the head and the power budget are determined in the two cases using the finite difference in time domain (FDTD) method. The differences between the results for adult and child-sized heads are discussed at 900 MHz and 1800 MHz.
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Multi-band device
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In this paper, a specific absorption rate (SAR) evaluation on human exposure from wireless communication devices inside a vehicle is investigated. A finite-difference time-domain method (FDTD) is used for simulating the electromagnetic field distribution inside the vehicle. Results have shown that the maximum SAR induced for a mobile phone user in the vehicle is only increased by 5% when compared to free space. Results have also indicated that other passengers in the vehicle are also inducing various levels of the SAR values in their bodies. The induced SAR values can be as high as about 40% of the maximum SAR value induced in the mobile phone user.
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Abstract The influence of the hand on the specific absorption rate (SAR) assessments of mobile phones is investigated.An increase of the SAR value in the presence of the hand has been observed but the electromagnetic phenomenon is still unexplained. The distance between the palm of the hand and the antenna is herein shown to be an important parameter. Numerical simulations and measurements performed using a dipole antenna placed in‐between a flat head phantom and a flat hand phantom confirm that an increase of the SAR is expected for some specific values of the distance between the antenna and the hand at a given frequency. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:654–656, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26644
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