Contrast-manipulation techniques for steady-state magnetic resonance imaging

Magnetic resonance imaging (MRI) has become a dependable tool for the clinical assessment of a range of medical disorders. As commercially available MRI hardware improves, increasingly rapid (sub-second 2D and sub-minute 3D) scans become possible. Steady-state free precession (SSFP) image acquisition protocols, which maintain coherent magnetization from RF excitation to excitation, are especially useful in these fast scans because they can retain high signal even as imaging time decreases. Unfortunately, the contrast between different soft tissues is difficult to control in SSFP scans because conventional contrast-manipulation strategies are not directly compatible with SSFP acquisitions. This dissertation introduces a set of new techniques for manipulating contrast in SSFP scans, providing the clinician with flexible control over soft-tissue contrast in addition to the speed and image-quality benefits that are characteristic of SSFP acquisitions.; Two of the techniques described here exploit the motion of blood to generate images with velocity-dependent contrast. In one of these techniques, an SSFP sequence is modified such that flowing material reaches a steady state that oscillates between two equilibrium values, while stationary material attains a single, non-oscillatory steady state. Subtraction of adjacent echoes results in large, uniform signal from all flowing spins and zero signal from stationary spins. In the other technique, velocity is quantified in the complex phase of an MR image by exploiting the linear-phase characteristic of the SSFP response. Both techniques are shown to possess more than double the SNR of the corresponding conventional technique in the same scan time.; A technique is also described that can be used to generate SSFP pulse sequences with arbitrary magnetization profiles as a function of free precession. This is accomplished by appropriate periodic modulation of the RF magnitude and phase from echo to echo. The technique is applied to the design of SSFP sequences with flat profiles, providing the opportunity for banding-artifact-free imaging with steady-state contrast. The algorithm is also applied to the design of frequency-selective filters, which can be used to provide fat-suppressed image contrast. Experimental results confirm the technique's suitability for lipid-suppressed SSFP imaging with no increase in scan time over the corresponding standard-SSFP experiment.