Browsing by Subject "CEA"

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Open Access
Full-duplex MRI for zero TE imaging
(2016-05) Salim, Maryam
In this thesis a new method for decoupling of RF transmit and receive coils in MRI is presented. A modified version of isolation concept used in the full-duplex radios in communication systems is applied to acquire MRI signal using concurrent excitation and acquisition (CEA) method. Since in MRI transmit power is many orders of magnitude larger than receive signal, a weak coupling might dominate the MR signal during CEA in MRI. In our new method, a small copy of RF transmit signal is attenuated and delayed to generate the same coupling signal which is available in the receiver coil then it is subtracted from the receive signal in order to detect the MRI signal. The proposed decoupling method is developed and implemented in two designs. First a semi-automatic controllable decoupling design which uses a programmable attenuator and coaxial cables for the purpose of time delay. After estimating the length of coaxial cables an optimization algorithm finds the amount of attenuation factor. Using this method we could achieve more than 75 dB decoupling. Second design is a fully-automatic controllable decoupling design which contains four delay and attenuator lines. In this design four fixed phase shifters are used in order to generate the same phase delay between transmit and receive coils. A genetic optimization algorithm is used to find the amount of attenuation factors of each line. It is shown that this method provides more than 100 dB decoupling between transmit and receive coils which is good enough for detecting MRI signals during excitation from tissues with very short relaxation time. This study shows feasibility of applying full duplex electronics which is used in telecommunications, to decouple transmit and receive coils for MRI with CEA, using a clinical MRI system. This device can automatically tune the cancellation circuit and it is a potential tool for recovering signal from tissues with extremely short T2 in clinical MR systems.
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.