The majority of MRI experiments use amplitude modulated RF pulses with a constant carrier frequency to manipulate the spins in the object. Frequency swept, or frequency modulated, pulses have, for the most part, been limited to cases where a B₁-independent flip angle is desired. This work describes two projects that use the phase profile created by frequency swept pulses to improve MR imaging.

The hallmark pathology of Alzheimer's disease is the accumulation of amyloid plaques on the neurons. Amyloid plaques have been visualized using MRI in both ex vivo human brain specimens and transgenic mouse brain specimens. Recently, in vivo imaging of amyloid plaques in transgenic mice has been demonstrated. Unfortunately, the high resolution needed to visualize plaques results in a very long scan time, so previously reported studies have been limited to a single imaging method. This work describes a quantitative comparison of a variety of imaging techniques in their ability to create plaque contrast. A few of the sequences are novel imaging sequences using frequency swept pulses to improve the images.

Time encoding is a novel encoding method that uses the phase profile of the magnetization directly. It is described in general terms, and it is described as it is implemented in Rapid Acquisition by Sequential Excitation and Refocusing (RASER). RASER is a novel imaging sequence capable of generating single-shot images in regions of the brain that are problematic for EPI.