Browsing by Subject "Magnetic resonance imaging--Methods."

Now showing 1 - 9 of 9

Open Access
Convection-reaction equation based magnetic resonance electrical properties tomography (cr-MREPT)
(2013) Hafalır, Fatih Süleyman
Tomographic imaging of electrical conductivity and permittivity of tissues may be used for diagnostic purposes as well as for estimating local specific absorption rate (SAR) distributions. Magnetic Resonance Electrical Properties Tomography (MREPT) aims at noninvasively obtaining conductivity and permittivity images at RF frequencies of MRI systems. MREPT algorithms are based on measuring the B1 field which is perturbed by the electrical properties of the imaged object. In this study, the relation between the electrical properties and the measured B + 1 field is formulated, for the first time as, the well-known convection-reaction equation. The suggested novel algorithm, called “cr-MREPT”, is based on the solution of this equation, and in contrast to previously proposed algorithms, it is applicable in practice not only for regions where electrical properties are relatively constant but also for regions where they vary. The convection-reaction equation is solved using a triangular mesh based finite difference method and also finite element method (FEM). The convective field of the convection-reaction equation depends on the spatial derivatives of the B + 1 field. In the regions where the magnitude of convective field is low, a spot-like artifact is observed in the reconstructed conductivity and dielectric permittivity images. For eliminating this artifact, two different methods are developed, namely “constrained cr-MREPT” and “double-excitation cr-MREPT”. In the constrained cr-MREPT method, in the region where the magnitude of convective field is low, the electrical properties are reconstructed by neglecting the convective term in the equation. The obtained solution is used as a constraint for solving electrical properties in the whole domain. In the double-excitation cr-MREPT method, two B1 excitations, which create two convective field distributions having low magnitude of convective field in different locations, are applied separately. The electrical properties are then reconstructed simultaneously using data from these two applied B + 1 field. These methods are tested with both simulation and experimental data from phantoms. As seen from results, successful electrical property reconstructions are obtained in all regions including electrical property transition region. The performance of cr-MREPT method against noise is also investigated.
Open Access
Electromagnetic imaging of three-dimensional conducting objects using the Newton minimization approach
(2013) Etminan, Aslan
The main goal of shape reconstruction is to retrieve the location and shape of an unknown target. This approach is used in a wide range of areas, from detecting cancer tumors to finding buried objects. Various methods can be applied to detect objects in different applications. One of the important challenges in many of these methods is to solve the non-linearity and non-uniqueness of the solutions. Inverse scattering is one of the most efficient ways to retrieve shapes and locations of targets. By illuminating the objects with electromagnetic waves and collecting the scattering fields using appropriate methods, we try to obtain the shape of unknown object. To achieve this goal, we start with an initial guess of the unknown object, then by comparing the scattered far-field patterns of the guess and the real object, we evolve that object and update it iteratively such that we decrease the difference between the patterns and finally achieve the shape of the unknown object. In this thesis, we model the object by one of its parameters, such as the location of the nodes on the surface of the object, or by the conductivity, permittivity, and permeability of the discretized space in which the object is placed. Then, the model parameters are updated iteratively by minimizing the mismatch between the measured data of the target and the collected data from the modeled object. Using surface nodes to model a three-dimensional object is a good choice because we decrease the number of unknowns.
Open Access
Finite element method based simulation, desing, and resonant mode analysis of radio frequency birdcage coils used in magnetic resonance imaging
(2012) Gürler, Necip
Radio Frequency (RF) birdcage coils are widely used in Magnetic Resonance Imaging (MRI) since they can generate very homogeneous RF magnetic field inside the coil and have high signal-to-noise ratio (SNR). In practice, designing a birdcage coil is a time-consuming and difficult task. Calculating the capacitance value, which is necessary for the coil to resonate at the desired frequency, is the starting point of the design process. Additionally, it is also important to know the complete resonance frequency spectrum (or resonant modes) of the birdcage coil that helps the coil designers to be sure that working mode is far away from the other modes and so that tuning and matching procedures of the coil can be done without interfering with the other modes. For this purpose, several studies have been presented in the literature to calculate the capacitance value and the resonant modes of the birdcage coil. Among these studies, lumped circuit element model is the most used technique in capacitance and resonant modes calculations. However, this method heavily depends on the inductance calculations which are made under quasi-static assumptions. As a consequence of this assumption, error in the calculations increases as the frequency increases to a point at which the wavelengths are comparable with the coil dimensions. Additionally, modeling the birdcage coil in a 3D simulation environment and making electromagnetic analysis in the volume of interest is also important in terms of observing the electromagnetic field distributions inside the coil. In this thesis, we have proposed three different Finite Element Method (FEM) based simulation methods which are performed using the developed low-pass and high-pass birdcage coil models in COMSOL Multiphysics. One of these methods is the FEM based optimization method in which magnitude of the port impedance or variance of H+ is used as the objective function and the capacitance value is used as the control variable. This is a new method proposed for calculating the capacitance value of the birdcage coils. The other method is the eigenfrequency analysis which is used to determine not only the resonant modes of the birdcage coil but also the electromagnetic fields distributions inside the coil at these resonant modes. To the best of our knowledge, FEM based eigenfrequency analysis of a birdcage coil is also a new study in the field of MRI. The last method is the frequency domain analysis which is used solve for the electromagnetic fields of a birdcage coil for the specified frequency (or frequencies). One can also use this method to estimate Specific Absorption Rate (SAR) at any object inside the coil. To make these three simulation methods easily and according to the user-specified parameters, we have developed two software tools using MATLAB which have also graphical user interface (GUI). In order to compare the results of the proposed methods and the results of the methods that use lumped circuit element model with the experimental results, we have constructed two handmade birdcage coils and made measurements for different capacitance values. Then, we have compared the measured resonant modes with the calculated resonant modes; used capacitance values with the calculated capacitance values. For the worst case (in which the frequency is the highest), proposed FEM based eigenfrequency analysis method calculates the resonant modes with a maximum of 10% error; proposed FEM based optimization method calculates the necessary capacitance values with 20-25% error. Methods which use lumped circuit element model, on the other hand, calculate the resonant modes and capacitance values with 50-55% error for the worst case.
Open Access
Magnetic resonance electrical impedance tomography based on the solution of the convection equation and 3D Fourier transform-magnetic resonance current density imaging
(2011) Oran, Ömer Faruk
Open Access
A method of decoupling of radio frequency coils in magnetic resonance imaging : application to MRI with ultra short echo time concurrent excitation and acquisition
(2013) Özen, Ali Çağlar
In this thesis, it was both experimentally and theoretically shown that decoupling of transmit and receive coils can be achieved by using a transmit array system such that individual currents induced from transmit coils will cancel each other resulting in a significantly reduced coupling. A novel method for decoupling of radio frequency (RF) coils was developed and implemented in a transmit array system with multiple transmit coil elements driven by RF current sources of different amplitude and phase. It was shown that this method for decoupling provides isolation over 70dB between transmit and receive coils. Decoupling procedure was described and its performance was analyzed in terms of obtained isolation. It was shown that MR signal can be detected during RF excitation with the achieved amount of decoupling. NMR spectroscopy and MRI with concurrent excitation and acquisition (CEA) was implemented. As an alternative to existing CEA methods, this method reduces dynamic range requirements so that CEA sequences can be applied in standard MRI scanners with minimal hardware modification. It was also demonstrated that this method can be used to implement ultra-short echo time (UTE) imaging with shorter acquisition delay. For CEA approach, acquired raw data was formulated as convolution of the free induction decay (FID) signal and the input B1 field. First proof of concept images were reconstructed from nonuniformly sampled k space data using both UTE and CEA sequences. UTE and CEA were shown to be feasible to implement using the same custom made decoupling setup in a clinical 3T MRI scanner. Significance of imaging of samples with ultra short T2* values was discussed.
Open Access
Motion artifact reduction techniques in magnetic resonance imaging
(1991) Atalar, Ergin
It is shown that the expansion/shrinkage and rotational motions of the body cause phase and amplitude distortions and non-rectangular sampling over the A:-domain. If these distortions are not compensated then the reconstructed image will suffer from ''the motion artifact'. The mathematical relation between motion and motion artifact is given. If the motion of the body is known, it is possible to obtain motion artifact free images. The motion is estimated either by using the information in the acquired data or by direct measurement. These estimates and the relation between motion and artifact are used to compensate the phase and amplitude distortions. Using the non-rectangular samples over the ¿-domain the rectangular samples are obtain by the aid of the singular value decomposition method. And finally, the inverse Fourier transform of these calculated samples gives the motion artifact free image. The proposed method is tested by simulations. For the estimation of the motion, two methods are proposed and tested. The first method is an iterative image reconstruction method. The second one uses the navigator echoes to obtain the amount of motion.
Open Access
Novel SAR reduction methods for magnetic resonance imaging
(2011) Eryaman, Yiğitcan
In this thesis, novel methods are presented, which can be used to reduce the heating of the human body due to radiofrequency fields in magnetic resonance imaging (MRI). The proposed methods depend on the modification of the electric field distribution for reducing the specific absorption rate (SAR). These methods can be used to reduce the local SAR in the vicinity of metallic devices and the whole-volume average SAR, as shown by electromagnetic field simulations and phantom, animal and patient experiments. These results can improve the safety of MRI scans performed on patients with metallic implants and MRI-guided interventional procedures. Additionally, by reducing the whole body average SAR, safer and faster MRI scans can be performed.
Open Access
Novel techniques regarding specific absorption rate and field of view reduction in magnetic resonance imaging
(2012) Kopanoğlu, Emre
In this dissertation, novel strategies regarding the reduction of the specific absorption rate (SAR) and the reduction of the total scan time, and analytic calculation methods for the lower limit on the specific absorption rate and the upper limit on the signal-to-noise ratio (SNR) are proposed. The first contribution of this dissertation is on the ultimate intrinsic signal-to-noise ratio (UISNR) and the ultimate intrinsic specific absorption rate (UISAR). Analytic expressions that are valid for arbitrarily shaped subjects are derived for these two parameters at the quasi-static limit. By comparing the UISNR expression to a previously published semi-analytic method for a cylindrical subject, it is shown that the maximum error is below 10%. In the primary contribution of this dissertation, gradient fields with nonlinear variation in space are used for radio-frequency (RF) excitation pulse design. When such fields are used for a pulse design, the relation between the excitation profile and the RF pulse is altered, which leads to a different RF envelope and hence, a different SAR value. Using simulations and experiments, SAR reductions between 15% - 54% are demonstrated, in three case studies. Another topic of this dissertation is on the reduction of the total scan time using nonlinear gradient fields. It is demonstrated that, when nonlinear gradient fields are used for excitation, the excitation region can be focused along more than a single direction. Furthermore, with a careful selection of readout encoding direction, reduced field-of-view imaging can be made without changing the SAR or the echo time. In a volunteer experiment, 60% reduction in the total scan time is obtained. The last topic of this dissertation is on curved slice imaging. It is shown using simulations that RF encoding can be used for imaging a curved field-of-view with non-rectangular and nonuniform voxels that may conform to the region-of-interest better. It is proposed that when the method is used with multi-dimensional excitation pulses, curved regions may be imaged in a shorter time
Open Access
A novel verse optimal RF pulse design method for parallel transmission in magnetic resonance imaging
(2009) Bayındır, Haldun Özgür
A novel radio frequency (RF) pulse design method for magnetic resonance imaging (MRI) and an improvement to an existing method that reduces specific absorption rate (SAR) in MRI are presented. The new RF pulse design method, variable rate selective excitation optimal RF pulse design method for parallel transmission (VERSEp), is developed for parallel transmission and aim of the method is to design RF pulses with lowest SAR after SAR reduction with variable rate selective excitation (VERSE) method. This is achieved by modifying the SAR optimal RF pulse deisgn method for parallel transmission. Performance of the VERSEp method is tested by comparing VERSE-SAR reduced SAR of the RF pulses designed with SAR optimal RF pulse design method and VERSE-SAR reduced SAR of the RF pulses designed using VERSEp. In the simulations, SAR reductions up to 47% are obtained. Different aspects of VERSEp are also shown with simulations. The second contribution of this work is an improvement made to an existing constrained VERSE-SAR reduction method. The existing VERSE-SAR reduction method uses a peak RF constaint for SAR reduction. In this work, peak square root power constraint is used instead of peak RF constraint in the VERSE-SAR reduction method. In the simulation results, the SAR of the RF pulses designed using the improved method were compared with SAR of the RF pulses designed using the method before improvement. SAR reductions up to 50% are obtained by using peak square root power constrained SAR reduction instead of peak RF constrained SAR reduction.