Fast and contrast-enhanced phase-sensitive magnetic resonance imaging
Phase-sensitive magnetic resonance (MR) imaging has a number of important clinical applications, such as phase-sensitive inversion recovery (PSIR) and Dixon water/fat imaging. PSIR and Dixon techniques are widely used in neurological and body imaging to improve tissue-contrast, the former by extending the dynamic range of image intensity and the later by suppressing unnecessary fat signals. Several important limitations, however, occur in these techniques: (1) Dixon techniques cannot decompose two signals if the resonance frequencies are close. For example, in MR mammography, it is difficult to separate silicone breast implants signals (4.0 ppm) from fat signals (3.5 ppm); (2) the signal dynamic range of images acquired using Dixon techniques is limited by the equilibrium magnetization; and (3) long image acquisition time. These limitations have hindered the applications of phase-sensitive Dixon imaging techniques on breast implant imaging or as a screening tool where fast acquisition is required. In this work, novel phase-sensitive MRI techniques were developed to enhance the capability, image-contrast, and scan-efficiency of Dixon imaging techniques. Specifically, we developed (1) a generalized chemical-shift imaging technique to separate spectrally overlapped signals both T1-contrast and chemical-shift; (2) a contrast-enhanced Dixon technique to extend the signal dynamic range of Dixon images; and (3) a single-echo acquisition (SEA) imaging technique integrated with phase-sensitive MR imaging to provide ultra-fast image acquisitions. Phantom studies, performed on 1.5 T and 4.7 T MR scanners, demonstrated the developed generalized chemical-shift imaging technique could clearly separate breast silicone implant signals (4.0 ppm) from fat (3.5 ppm). The contrast-enhanced Dixon technique, by extending the dynamic range of signal intensity from positive levels to positive/negative levels, could improve image-contrast by 1.6 times, compared with a conventional single-point Dixon technique. Phantom studies, using a 64-channel SEA imaging system, showed the integrated Dixon technique with SEA could acquire decomposed 2-D water-only and fat-only images with ultra-fast frame-rates up to 1/TR, while providing improved image-contrast (by 2.4 times in this experiment) compared with a conventional SEA imaging technique.