Parameter estimation approach to banding artifact reduction in balanced steady-state free precession.

PURPOSE: The balanced steady-state free precession (bSSFP) pulse sequence has shown to be of great interest due to its high signal-to-noise ratio efficiency. However, bSSFP images often suffer from banding artifacts due to off-resonance effects, which we aim to minimize in this article. METHODS: We present a general and fast two-step algorithm for 1) estimating the unknowns in the bSSFP signal model from multiple phase-cycled acquisitions, and 2) reconstructing band-free images. The first step, linearization for off-resonance estimation (LORE), solves the nonlinear problem approximately by a robust linear approach. The second step applies a Gauss-Newton algorithm, initialized by LORE, to minimize the nonlinear least squares criterion. We name the full algorithm LORE-GN. RESULTS: We derive the Cramér-Rao bound, a theoretical lower bound of the variance for any unbiased estimator, and show that LORE-GN is statistically efficient. Furthermore, we show that simultaneous estimation of T1 and T2 from phase-cycled bSSFP is difficult, since the Cramér-Rao bound is high at common signal-to-noise ratio. Using simulated, phantom, and in vivo data, we illustrate the band-reduction capabilities of LORE-GN compared to other techniques, such as sum-of-squares. CONCLUSION: Using LORE-GN we can successfully minimize banding artifacts in bSSFP.

SNR dependence of optimal parameters for apparent diffusion coefficient measurements.

Optimizing the diffusion-weighted imaging (DWI) parameters (i.e., the b-value and the number of image averages) to the tissue of interest is essential for producing high-quality apparent diffusion coefficient (ADC) maps. Previous investigation of this optimization was performed assuming Gaussian noise statistics for the ADC map, which is only valid for high signal-to-noise ratio (SNR) imaging. In this work, the true statistics of the noise in ADC maps are derived, followed by an optimization of the DWI parameters as a function of the imaging SNR. Specifically, it is demonstrated that the optimum b-value is a monotonically increasing function of the imaging SNR, which converges to the optimum b-value from previously proposed approaches for high-SNR cases, while exhibiting a significant deviation from this asymptote for low-SNR situations. Incorporating the effects of T(2) weighting further increases the SNR dependence of the optimal parameters. The proposed optimization scheme is particularly important for high-resolution DWI, which intrinsically suffers from low SNR and therefore cannot afford the use of the conventional high b-values. Comparison scans were performed for high-resolution DWI of the spinal cord, demonstrating the improvements in the resulting images and the ADC maps achieved by this method.

Noninvasive assessment of coronary vasodilation using magnetic resonance angiography.

OBJECTIVES: The purpose of this study was to investigate the use of coronary magnetic resonance angiography (MRA) for assessing human epicardial coronary artery vasodilation. BACKGROUND: Coronary vasodilation plays a vital role in the human coronary circulation. Previous studies of epicardial coronary vasodilation have used invasive coronary angiography. Coronary MRA may provide an alternative noninvasive method to directly assess changes in coronary size. METHODS: Thirty-two subjects were studied: 12 patients (age 55 +/- 18 years) and 20 healthy subjects (age 34 +/- 4 years). High-resolution multi-slice spiral coronary MRA (in-plane resolution of 0.52 to 0.75 mm) was performed before and after sublingual nitroglycerin (NTG). Quantitative analysis of coronary vasodilation was performed on cross-sectional images of the right coronary artery (RCA). A time-course analysis of coronary vasodilation was performed in a subset of eight subjects for 30 min after NTG. Signal-to-noise ratio was also measured on the in-plane RCA images. RESULTS: Coronary MRA demonstrated a 23% increase in cross-sectional area after NTG (16.9 +/- 7.8 mm2 to 20.8 +/- 8.9 mm2, p <0.0001), with significant vasodilation between 3 and 15 min after NTG on time-course analysis. The MRA measurements had low interobserver variability (< or =5%) and good correlation with X-ray angiography (r=0.98). The magnitude of vasodilation correlated with baseline cross-sectional area (r=0.52, p=0.03) and age (r=0.40, p=0.019). Post-NTG images also demonstrated a 31% improvement in coronary signal-to-noise ratio (p = 0.002). CONCLUSIONS: Nitroglycerin-enhanced coronary MRA can noninvasively measure coronary artery vasodilation and is a promising noninvasive technique to study coronary vasomotor function.