Browsing by Subject "transmit array"

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    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.