相位循环bSSFP MRI参数映射的约束椭圆拟合
Constrained Ellipse Fitting for Efficient Parameter Mapping with Phase-cycled bSSFP MRI
摘要：
平衡稳态自由旋进（bSSFP）成像能够在MRI中实现高扫描效率，但与传统序列不同的是，对主场不均匀性和非标准T2/T1加权组织对比度的灵敏度提高。为了解决这些局限性，通常使用一组不同的RF相位循环增量采集相同解剖结构的多个bSSFP图像。相位循环采集的联合处理用于减轻对场不均匀性的敏感性。最近，相位循环bSSFP采集也被用于基于显式信号模型估计弛豫参数。虽然有效，但这些基于模型的方法通常涉及大量采集（N~10-16），降低了扫描效率。在这里，我们提出了一种新的约束椭圆拟合方法（CELF）的参数估计，提高了效率和准确性的相位循环bSSFP MRI。CELF基于复bSSFP信号的椭圆信号模型框架;并且它引入对椭圆属性的几何约束以提高估计效率，并且引入基于字典的识别以提高估计精度。CELF通过采用单独的B1图生成T1、T2、非谐振和谐振bSSFP信号的图，以减轻对翻转角变化的敏感性。我们的研究结果表明，CELF可以产生准确的非共振和带自由bSSFP地图与少到N=4收购，而松弛参数的估计精度是显着的bSSFP成像的微观结构的灵敏度的偏差是有限的。
英文摘要：
Balanced steady-state free precession (bSSFP) imaging enables high scan efficiency in MRI, but differs from conventional sequences in terms of elevated sensitivity to main field inhomogeneity and nonstandard T2/T1-weighted tissue contrast. To address these limitations, multiple bSSFP images of the same anatomy are commonly acquired with a set of different RF phase-cycling increments. Joint processing of phase-cycled acquisitions serves to mitigate sensitivity to field inhomogeneity. Recently phase-cycled bSSFP acquisitions were also leveraged to estimate relaxation parameters based on explicit signal models. While effective, these model-based methods often involve a large number of acquisitions (N~10-16), degrading scan efficiency. Here, we propose a new constrained ellipse fitting method (CELF) for parameter estimation with improved efficiency and accuracy in phase-cycled bSSFP MRI. CELF is based on the elliptical signal model framework for complex bSSFP signals; and it introduces geometrical constraints on ellipse properties to improve estimation efficiency, and dictionary-based identification to improve estimation accuracy. CELF generates maps of T1, T2, off-resonance and on-resonant bSSFP signal by employing a separate B1 map to mitigate sensitivity to flip angle variations. Our results indicate that CELF can produce accurate off-resonance and banding-free bSSFP maps with as few as N=4 acquisitions, while estimation accuracy for relaxation parameters is notably limited by biases from microstructural sensitivity of bSSFP imaging.
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