| Frequency-swept pulses have widely been used in many nuclear magnetic resonance (NMR) applications because of the immunity to radiofrequency (RF) field inhomogeneity and ability of covering broad bandwidths with modest RF peak power. However, when used for excitation or refocusing, frequency-swept pulses have the disadvantage of yielding non-linear phase variation across a slice or slab. Recently, it has been recognized that 3D MRI can benefit from the non-linear phase variation because signal-to-noise ratio (SNR) can be maintained by reducing the peak echo amplitude in the event that the dynamic range in the digital receiver is limiting.; In this thesis, a general theory and methods are described for exciting or refocusing spins using frequency-swept pulses, specifically the hyperbolic secant family (HSn pulses) which deliver much better slice profiles than other pulses. The phase profiles of HSn pulses are analytically described for basic sequences, i.e., gradient-echo and spin-echo sequences, in MRI. When the HSn pulse is used for excitation or (single) refocusing, it produces a "pseudo-echo", where each isochromat has a unique local rephasing time unlike a conventional echo. Imaging pseudo-echoes can be performed by conventional fast Fourier transform (FFT). A new image reconstruction method is also developed to allow an apodization using a sliding window function, which provides better SNR than a conventional apodization using a fixed window in the pseudo-echo. In addition, the exact condition to compensate non-linear phase is proposed in spin-echo imaging when HS n pulses are used for both excitation and refocusing. The methods presented in this thesis make it possible to take full advantage of frequency-swept pulses for multi-slice 2D and slab-selective 3D MRI. |
