MRI provides different types of exquisite soft tissue and functional contrast. However, depending on the desired contrast and spatial resolution properties, the acquisition time can be quite long. The long-term objective of this work is to improve methods to quickly and accurately diagnose human disease using MRI and provide resources that can be used for image-guided therapy and real-time imaging.
Balanced steady state free precession (bSSFP) or True-FISP provides the highest signal-to-noise ratio efficiency of any pulse sequence. However, in bSSFP, contrast is a mixture of T1 and T2 weightings, which is often not desired clinically, and the signal from flowing blood is hyperintense, which can obscure the vessel wall and create artifacts. Also, there are obstructive saturation band artifacts at intersections of rapidly-acquired multiplanar images. The objective of this work was to modify the magnetization preparation and readout properties of the bSSFP sequence to: 1) improve methods that eliminate T1 and isolate T2 contrast, 2) suppress the signal from flowing blood, and 3) characterize and reduce the saturation banding artifacts.
A new T-One insensitive Steady State Imaging (TOSSI)-bSSFP combined acquisition technique, Resolution Enhanced TOSSI (RE-TOSSI), has been developed by using a partial Fourier acquisition and eliminating the inversion pulses from TOSSI after the data around the center of k-space is acquired. Results show that TOSSI contrast is maintained, while spatial resolution degradation is reduced. Additional benefits include reduced RF power deposition and faster imaging time. Application to high-resolution, non-subtraction thermal ablation monitoring is demonstrated.
An improved dark blood bSSFP pulse sequence (HEFEWEIZEN) has been developed by introducing spatial saturation in True-FISP. This method does not increase the repetition time (TR) or substantially alter stationary tissue contrast and allows for directional suppression of blood flow (e.g. arterial vs. venous). Comparison to diffusion-prepared SSFP in the common carotid artery demonstrated significantly improved vessel wall-lumen contrast-to-noise ratio efficiency (p = 0.02).
Intersecting-plane saturation band artifacts were characterized in three common steady-state pulse sequences (FLASH, FISP, and bSSFP). Substantial temporal and pulse sequence dependencies were found. Reverse centric phase encoding is demonstrated to be a simple and effective way of minimizing this artifact.