Browsing by Subject "B1 mapping"

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    ItemOpen Access
    Approximate fourier domain expression for bloch-siegert shift
    (John Wiley and Sons Inc., 2015) Turk, E. A.; Ider, Y. Z.; Ergun, A. S.; Atalar, Ergin
    Purpose: In this study, a newsimple Fourier domain-based analytical expression for the Bloch-Siegert (BS) shift-based B1 mapping method is proposed to obtain |B1+| more accurately while using short BS pulse durations and small off-resonance frequencies.Theory and Methods: A new simple analytical expression for the BS shift is derived by simplifying the Bloch equations. In this expression, the phase is calculated in terms of the Fourier transform of the radiofrequency pulse envelope, and thus making the off- and on-resonance effects more easily understandable. To verify the accuracy of the proposed expression, Bloch simulations and MR experiments are performed for the hard, Fermi, and Shinner-Le Roux pulse shapes.Results: Analyses of the BS phase shift-based B1 mapping method in terms of radiofrequency pulse shape, pulse duration, and off-resonance frequency show that |B1+| can be obtained more accurately with the aid of this new expression.Conclusions: In this study, a new simple frequency domain analytical expression is proposed for the BS shift. Using this expression, |B1+| values can be predicted from the phase data using the frequency spectrum of the radiofrequency pulse. This method works well even for short pulse durations and small offset frequencies.
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    ItemOpen 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.
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    ItemOpen Access
    Convection-reaction equation based magnetic resonance electrical properties tomography (cr-MREPT)
    (Institute of Electrical and Electronics Engineers Inc., 2014) Hafalir, F. S.; Oran, O. F.; Gurler, N.; Ider, Y. Z.
    Images of electrical conductivity and permittivity of tissues may be used for diagnostic purposes as well as for estimating local specific absorption rate distributions. Magnetic resonance electrical properties tomography (MREPT) aims at noninvasively obtaining conductivity and permittivity images at radio-frequency frequencies of magnetic resonance imaging 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 B1 field is formulated for the first time as a well-known convection-reaction equation. The suggested novel algorithm, called 'cr-MREPT,' is based on the solution of this equation on a triangular mesh, 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 convective field of the convection-reaction equation depends on the spatial derivatives of the B1 field, and in the regions where its magnitude is low, a spot-like artifact is observed in the reconstructed electrical properties images. For eliminating this artifact, two different methods are developed, namely 'constrained cr-MREPT' and 'double-excitation cr-MREPT.' Successful reconstructions are obtained using noisy and noise-free simulated data, and experimental data from phantoms.