Methods for mapping the B1 field distribution in Magnetic Resonance Imaging

In the diagnostic study of many diseases, Magnetic Resonance Imaging (MRI) is used to evaluate the anatomy or function of an organ under consideration. This technique is based on the physical phenomenon of the Nuclear Magnetic Resonance (NMR), in which magnetic nuclei in magnetic field absorb and re-emit electromagnetic radiation. This happens at a specific resonance frequency which depends on the strength of the magnetic field (B0) and on the magnetic properties of the nuclei; in particular their precession frequency is ν = γ/2π *B0 , where γ is the gyromagnetic ratio and, for proton, assumes the value γ/2π = 42.56 MHz/T . It follows that, for MRI system (where the B0 field is of the order of Tesla), this frequency is within the radio-frequency range. The enormous success of the MRI technique is due to the low energies involved, avoiding the exposure to ionizing radiation, and also to its unique versatility in medicine applications. The MRI techniques are able to extract different types of information from the sample depending on the imaging algorithm (sequence) used. In the past few years, static magnetic fields of increasing strength have been employed to improve image contrast and to increase spatial resolution. Indeed with higher magnetic fields, the signal to noise ratio (SNR) increases. Unfortunately, with increasing fields strength, there are some new effects that require new methods to transmit and receive the signal in the sample. One challenge is the transmission of the RF field within an object. This field, named also the B1+ field, is used to move the magnetization away from equilibrium; when this field is turned off, the magnetization slowly returns to its starting position and, in doing that, it generates a signal that can be detected. The angle the magnetization moves with respect to its equilibrium position, due to the application of the RF field, is called Flip Angle. The frequency of the RF field must be the same of the precession frequency of the nucleus, in order to flip the magnetization. At high magnetic field (B0 > 3 T ), the wavelength of the RF field is comparable with the object dimension, and this leads to effective B1 fields that depend on the position. Furthermore, the non-uniformity of the RF field arises also from the transmit coil and the conductivity, dielectric, and loading differences of the object and errors in flip angle calibration during the prescan routine (which is designed to be correct for only the central portion of the prescribed volume). This thesis treats three different methods for mapping the RF field in MRI systems. The sequences used to obtain these maps have been implemented first on a 1.5 T GE MR Scanner (installed at the Laboratory of Magnetic Resonance IRCCS Stella Maris, Pisa), in order to optimize the various methods, and then tested also on the new research 7 T GE system (installed at the IMAGO 7 Foundation, Pisa). The methods that will be presented are: the Double Angle Method (DAM), the Method of Slopes (MoS) and the Bloch-Siegert Shift Method (BSSM). These methods will be compared with each other, for a detailed analysis. The first one, the Double Angle Method, is the state-of-art of the B1 mapping methods since several years; it gives an accurate estimation of the field within a sample but it requires a long scan time (∼ 25 minutes). The Method of Slopes gives a good estimation with a short scan time (few minutes for 2D acquisitions, ∼ 10 minutes for 3D acquisitions); its B1 estimation is not as accurate as that of the DAM due to the approximations inherent in the method. The last one, the Bloch-Siegert method is a new method that uses the phase of the acquired images to obtain the B1 maps; it has a short scan time (∼ 1 minute for slice) and a good accuracy (comparable to that of the DAM). Finally, an estimation on the variation of the local Specific Absorption Rate (SAR) will be given, to asses whether it does not exceed the standard limits for human experiments