Browsing by Author "Kidane, Tesfaye (PhD)"

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    "Electromagnetc Fields In- Duced In a Solid Cylindrical Volume Conductor Due to Switching of Mri Z Gradient Field: Application to Peripheral Nerve Stimulation"
    (Addis Ababa University, 2018-06-18) Debela, Getachew; Kidane, Tesfaye (PhD)
    The rapid switching frequency of MRI gradient _elds has a direct relationship to the resolution and hence to the quality of the image. Hence, there is a tendency in the research area to increase the frequency of switching even more. However, the rapid switching frequency can create unwanted peripheral nerve stimulation on the patient's body due to the eddy current induction. In this report a model of the induction of the magnetic and electric _elds as well as the eddy current in a solid cylindrical volume conductor (human body model) due to the switching of the Z gradient coils have been explored analytically. The coils are assumed to carry time harmonic variable current at low frequency. For the analysis, Maxwell's Equations are approximated for this quasi-static situation. Starting from the vector potential expression and taking the _rst order asymptotic approximations of the modi_ed Bessel functions, the solutions for the _eld vectors and the eddy current expression in the cylinder have been found. The application of the problem to peripheral nerve stimulation and the mechanism of PNS in the aspect of its useful application has been discussed. The plot of the eddy current density against the radial distance in the cylinder has shown that the peripheral nerve stimulation is con_ned to the peripheral surface of the patient's body. The solutions have important implication in the design of MRI gradient coils to avoid PNS during scanning and ensure patient safety and comfort.
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    Numerical Simulation of Bloch Equation for Protons in Water and Fat Molecules
    (Addis Ababa University, 2018-07-03) Asega, Birtukan; Kidane, Tesfaye (PhD)
    The phenomenological time dependent Bloch equations have been a source of insight about the magnetic resonance phenomena and the purpose of this study was to solve the Bloch equations in the laboratory frame. First, the time- dependent Bloch equations were reduced to a homogeneous linear differential equation, and then a simple equation was derived to solve it using a matrix operation (diagonalization method). Then, as an example, this method is applied to the time dependent Bloch equations for protons in water and fat molecules in uniform static field. The radiofrequency (RF) pulse is first applied to disturb the protons so that they fall out of alignment with the static magnetic field. This disturbance occurs through the transfer of energy from the RF pulse to the protons, that can only occur when the RF pulse has the same frequency as the Larmor frequency of the protons. The representative trajectories of transverse component describe the complete time evolution of magnetization in the transverse component. The transverse magnetization behaviour can be described by transient sinusoidal oscillation. This has been observed due to the interaction with neighbouring spin, the amplitude magnetization damped and oscillates sinusoidal with time that can considerably vary for different matters, depending on the size of the time constant T2 . For example, for water, T2 are quite high, dephasing of the transverse magnetization are slow. For fat, however, relaxation process are considerably faster. Hence, this work clearly describes the effect of tissue properties (T2 relaxation time) of the proton of both water and fat if they are exposed in a homogeneous field in the sample. The Fourier transformation of the time domain leads Lorentzian peak in the frequency spectrum in given sample.The matlab simulation results give the more physical insight about the behaviour of magnetization.
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    Paleomagnetic Investigation of Saha and Surrounding Area Volcanic Rocks, Central Afar, Northeast Ethiopia
    (Addis Ababa Universty, 2017-05) Kendie, Temesgen; Kidane, Tesfaye (PhD)
    The paleomagnetic investigation was located in the area of Saha and surrounding area, central Afar. The paleomagnetic samples were collected from nine lava flow unit outcrop of basalt (Gulf basalt) which exposed Saha and surrounding area (Central Afar). Five to nine samples were collected from each lava flow. The specimens conducted to partial demagnetize by using Alternating field (AF) and Thermal (TH) demagnetization techniques, as well as magnetize by Impulse magnetizer techniques (IM). The paleomagnetic results were measured by using JR- 6ASpinner Magnetometer from School of Earth Science paleomanetic laboratory in Addis Ababa University. The Natural Remenant magnetization (NRM) direction explains the specimens which display components of magnetizations were low stability component (secondary NRM) was removed by AF about 10mT- 30mT, by heating about 120°C- 300°C were as high stability components (primary NRM or characteristic remenant magnetization) above these steps and the vector straight line is directed towards the origin from orthogonal vector diagrams. The results of magnetization curve plots for magnetic rock analysis shows that magnetic mineralogy dominated by Ti- poor titanomagnetite, magnetite and few presence of maghemite. Principal component analysis and site mean direction calculated from both normal and reverse polarities. An overall mean direction was calculated from 9 individual site mean, the result obtained: D= 352.5°, I= 12.5°, K= 13.2, α95=14.7°, N= 9 and comparison with expected Geomagnetic Axial Dipole based on Apparent Polar Wander Path curve for 1Ma Mile- Gewane Ali Sabieh, (Kidane et al.,2003), as stable Africa plate Besse and Courtillot (2002). The mean paleomagnetic pole was calculated from the combined individual VGP, λ= 80.9°., ф =277.8°. , K = 22.8, α95 =11°. The declination difference which explains rotation of blocks about vertical axis was: =-7.2 ±6.6; difference in inclination was: = 3.9±3.2 obtained. This declination difference explains counter clock wise rotation about vertical axis due to the young propagation rift.
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    Paleomagnetic Study and Structural Mapping of Dofan Magmatic Segment of Northern Main Ethiopian Rift in Afar, Ne Ethiopia
    (Addis Ababa Universty, 2015-12) Nugsse, Kahsay; Kidane, Tesfaye (PhD)
    The paleomagnetic investigation was carried out on outcropping Dofan volcanic rocks, southern Afar, Main Ethiopian Rift. The aim of this paleomagnetic study was to quantify vertical axis crustal block rotation and identifying the potential magnetic minerals carrier the remanence of rock sampled. Structurally the area is characterized by intense Quaternary faulting and fracturing. Two sets of faults are mapped in the study area namely; NNE–SSW and N-S trending youngest rift faults and few NNW–SSE striking rift margin normal faults. A total of 26 paleomagnetic sites were sampled within the well exposed Dofan volcanic rocks (outcrop of basalt, ignimbrite and trachyte) for the first ever paleomagnetic investigations. After routine paleomagnetic sample collection and preparation procedures 170 specimens were subjected to stepwise progressive alternative field and thermal demagnetizations techniques. The careful directional analysis of individual specimens revealed one or two components of natural remanent magnetization (NRM) at the end of the demagnetization process as: 1) A pervasive overprint carried by low blocking temperature (<300°C) and low coercivity (<20mT) grains which complicate and contribute uncertainty to the interpretation of Characteristic remanent magnetization (ChRM) directions which is effectively, eliminated by alternative demagnetization 2) ChRMs are recognized above those steps. After removal of overprinted components most magnetization directions displaying straight lines directed toward the origin of the orthogonal plot. Paleomagnetic directional analysis derived from the 24 reliable sites in Dofan volcano are distributed in cluster about a normal polarity mean direction.The blocking temperature and coercivity ranges and isothermal remanent magnetization examination representative specimens indicates that the critical magnetic minerals that do have a significant response for paleomagnetism are pure magnetite as dominant remanence carrier mineralogy. The ChRM obtained at 500-580°C, showing an evidence of magnetite and titanomagnetite. However, the high temperature component was isolated between 600-640°C; indicating hematites are also present as subordinate constituents. The overall observed mean directions from 24 accepted sites calculated using high quality ChRM directions calculated by excluding the anomalous direction (DF7 and DF3 sites) (Ds = 351.8°, Is = 11.5°, N = 24, K = 21.4, α95 = 6.5°) compared with the expected mean geomagnetic dipole reference field directions of Africa for an average age of 1.5Ma. This result suggests that statistically significant rotation about the vertical axis has occurred at the Dofan locality which indicate∼9.2° counterclockwise crustal block rotations. This result is in prefect agreement with counterclockwise block rotation linked to transtensional deformation of the Fentale-magmatic segment reported by (Tesfaye Kidane et al., 2009).
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    Paleomagnetism and Tectonics of Manda Hararo Rift of North Afar, Ethiopia
    (Addis Ababa Universty, 2010-01) Atnafu, Ameha; Kidane, Tesfaye (PhD)
    This study explains tectonics and Paleomagnetism of Manda Hararo rift in north afar. The field trip which was organized by scientists from New Zealand, Ethiopia and UK was made to study the Dabbahu magmatic segment in northern part of Manda Hararo rift. A total of 271 oriented core samples were collected for paleomagnetic study from 34 sites within Manda Hararo rift. Available age data of Basalts from the sampled localities show that the age ranges between 0.09Ma to 1.11Ma. 175 specimens were subjected to progressive Alternating Field (AF) demagnetization after routine sample preparation in the laboratory. Almost all specimens show fairly simple demagnetization behavior. AF between 0 to 30mT often isolates the first component. Median Destructive Field for most specimens ranges between 20 and 35mT, suggesting that Pseudo single domain grains as remnant magnetization carriers. Among 34 sites, 30 sites show normal polarities and four sites show reversed polarities. The normal and reversed polarities are 1800 apart, showing that the ChRM are free of secondary NRM components. Magnetic polarities were found to be coincident with Geomagnetic Polarity Time Scale (GPTS) of Cande and Kent (1995). Geomagnetic Polarity Time Scale of Langeries et al., (1997), Lund et al., (1998) and Nowaczyk & Frederichs (1999) were used to explain and determine age of the reversed polarity site PMAF04 from Afdera. The overall mean direction for 33 sites is D=357.10, I=10.60 (k=32.6 and α95=4.40). Comparison of observed mean direction with predicted direction of Kidane et al., 2003 for stable Africa for ~2Ma (D=359.70, I=16.40, α95=5.70), indicates no statistically significant vertical axis rotation has occurred in Manda Hararo rift. Comparison of mean direction from Kori and Saha with the predicted direction of Kidane et al., 2003 indicates that significant amount of counter clock wise rotation (~5.90±8.60) has occurred. Further comparison of mean values from the two sites with the Apparent Polar Wander Path curve of Besse and Courtillot, 1991, shows a counter clock wise rotation of 7.10±6.70. This local rotation is due to the overlap of Red Sea propagators, Erta’ale-Dader in the east and Erta’ale-Manda Hararo in the west. A reference pole is determined based on 33 sites. It is located at λ=83.10N, ф=238.80E(α95=4.00).
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    Paleomagnetism and Tectonics of Northern Munessa, Ethiopia
    (Addis Ababa Universty, 2012-06) Alemayehu, Diriba; Kidane, Tesfaye (PhD)
    The paleomagnetic investigation is carried out in the eastern part of lake langano-ziway, Main Ethiopian Rift. The aim of this paleomagnetic study is to quantify vertical axis rotation.A total of 11 paleomagnetic samples were sampled from outcrop of basalt, ignimbrites and rhyolite flows ranging in age from 2ma to 0.1ma. The paleomagnetic specimens are subjected to progressive alternating field demagnetization, thermal demagnetization and impulse magnetization. In general, the directional analysis showed either one or two components of Natural Remanent Magnetization (NRM) to characterize the wholemeasurements. The low stability components are removed by alternating fields up to 15mt or by heating up to 300oC. The high stability components are isolated above those steps and vector is directed towards origin, and it represented by Characteristic Remanent Magnetization (ChRM). The principal component analysis and site mean directions calculation yielded normal and reversed polarities. An overall mean direction calculated from individual site mean directions are Ds=354.1o, Is=8.3o, N=11, K =22, α95=10o, with an angular difference of -7.8 ± 8.24 and the negative value indicate the rotation is counter clock wise.The comparison of this result with the mean expected reference dipole geomagnetic field direction (Ds=1.9, Is=13.5, N=32, K=105.6, andα95 =2.5), from the apparent polar wander path reference curve for Africa at 1.5 ma (Besse&Courtillot 2003), found to be statistically different. This would imply that the area is at the contact of central and rift margins with small rotation. These results indicate that the late Pliocene-Pleistocene rocks of the MER in the study area show small vertical axis rotation, argue transtensional and seafloorspreading- transform kinematic models. It is therefore, suggested that large offset faults, produce the right stepping, en echelon magmatic segments of the MER, which is at the transition from continental to oceanic extensionrather thanmagma intrusion.
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    Visualizing Electromagnetic Vacuum by MRI
    (Addis Ababa University, 2017-11-04) Kelbiso, Berhanu; Kidane, Tesfaye (PhD)
    Based upon Maxwell‟s equations, it has long been established that oscillating EM fields due to current carrying wire incident upon a metal surface, decay exponentially inside the conductor and this leads to the absence of EM fields at sufficient depths. Magnetic resonance imaging (MRI) utilizes radiofrequency (r.f.) EM fields to produce images. Here the first visualization of a virtual EM vacuum inside a bulk metal strip by MRI has been discussed and reviewed using the basic principle of RF EM field in MRI physics. This work is based on radio frequency field calculation inside cylindrical shell of finite thickness and infinite length due to excitation by concentric coils placed inside the shell. The B1 field calculation is made analytically using mathematical methods of separation of variable and the result is shown in terms of modified Bessel function and the skin depth effect and the result is simulated. Finally B1 field is the key component in MRI system and it also determines the EM vacuum in the case of low signal.