J coupling and susceptibility contrast mechanisms in spin echo, fast spin echo, and asymmetric spin echo magnetic resonance imaging.

摘要

In this thesis, two contrast mechanisms are investigated: signal loss due to J coupling and signal loss due to susceptibility inhomogeneities. In particular, the ability to manipulate the magnitude of these effects by varying pulse sequence parameters is examined.;The contrast obtained when using asymmetric spin echo (ASE), spin echo (SE), and gradient echo (GE) pulse sequences to image media containing magnetic inhomogeneities is investigated. The dependence of the SE, GE, and ASE signal on the size of magnetic field perturbers, the magnitude of susceptibility differences, and the echo timings is examined using mean field theory, Monte Carlo random walk simulations and phantom experiments. A theoretical prediction of the ASE signal is obtained using the Anderson-Weiss theory, the results of which are qualitatively supported by computer simulations and experimental studies. It is shown that the ASE sequence can be used to tune the range of perturber sizes that provide the largest contributions to susceptibility contrast effects.;An algorithm is presented for a computer simulation that solves Allerhand's equation for the effects of J coupling on the echotrain of Carr-Purcell-Meiboom-Gill (CPMG) sequences. The program tracks the evolution of the density matrix and can handle spin systems of any size and complexity. Results from the simulation are presented which validate the hypothesis that a decrease in the effects of J coupling is responsible for the bright fat signal seen in fast spin echo (FSE) imaging at high pulse rates. The effects of J coupling on CPMG echotrains are examined for A3B2 and A3B2C2 spin systems over a wide range of J coupling and chemical shift values and pulse spacings. The effects of J coupling on the point spread function of FSE images are also discussed.;The Dual Inerval Echo Train (DIET) sequence, a modification of the FSE sequence that selectively reduces signal from fat in MR images, is also investigated. The sequence is evaluated using theoretical calculations, density matrix simulations, experiments on test objects, and in-vivo imaging. The efficacy of the sequence is compared for different lipid chemical structures, field strengths, and pulse sequence parameters.