MR görüntülemede kullanılan B0 ve B1 gradyanların incelenmesi ve yeni yöntemler

In this thesis, analysis of B0 gradients and B1 fields are performed and novel methods using B1 gradients instead of B0 gradients are proposed. The fi rst contribution of this dissertation is expressing the nature of the interaction between the B0 gradient fi elds and the active implantable medical devices (AIMD). By utilizing the fact that gradient coils produce linear magnetic field in a volume of interest, the simpli ed closed form electric field expressions are de fined inside a homogeneous cylindrical volume. Using these simplifi ed expressions, the induced potential on an implant electrode has been computed approximately for various lead positions on a cylindrical phantom and verifi ed by comparing with the measured potentials for these sample conditions. In addition, the validity of the method has been tested with isolated frog leg stimulation experiments. The results of both phantom and ex vivo experiments show that if the path of the implant lead is known, the induced voltage on the lead can be estimated analytically. The second topic in this dissertation is the Bloch-Siegert (BS) shift based B1 mapping method. The method is analyzed in terms of the e ffects of the o ff-resonance frequency, the RF pulse shape, and the duration of the RF pulse.Based on these analyses, a new theoretical model that relates the Fourier transform of the o ff-resonant BS RF pulse envelope to the phase shift is proposed. Utilizing Bloch simulations and phantom experiments the proposed frequency domain expression is veri fied. The results indicates that the proposed expression works well even for short pulse durations (< 2ms) and low o ffset frequencies (fRF < 500Hz) when the ratio of the RF fi eld and the frequency o ffset of the RF pulse is smaller than 0.5. The last topic of this dissertation is on flow and shear wave imaging with B1 gradients instead of B0 gradients. In flow imaging, a novel sequence using a Bloch-Siegert pulse generated by a spatially dependent B1 fi eld is proposed. The proposed method is experimentally verifi ed by comparing the resultant velocity measurements with those obtained by using bipolar flow encoding B0 gradients. This comparison demonstrates the feasibility of using BS shift with B1 gradients in detecting the flow. The usage of B1 gradients is also proposed to detect shear waves at frequencies in kilohertz range and this method is experimentaly verifi ed for 2kHz, 3kHz and 4kHz shear frequencies. The studies in this thesis indicate that extensive analysis of B0 gradients in Magnetic Resonance Imaging (MRI) is important for safety issues, and for scenarios where B0 gradients prove insu fficient in encoding due to hardware limitations, utilizing B1 gradients can be considered as an alternative.