基于图像的梯度非线性特征化，以确定高阶球谐系数，从而提高磁共振成像中的空间位置精度。
Image-based gradient non-linearity characterization to determine higher-order spherical harmonic coefficients for improved spatial position accuracy in magnetic resonance imaging.
作者信息
Weavers Paul T, Tao Shengzhen, Trzasko Joshua D, Shu Yunhong, Tryggestad Erik J, Gunter Jeffrey L, McGee Kiaran P, Litwiller Daniel V, Hwang Ken-Pin, Bernstein Matt A
机构信息
Radiology, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905, United States.
Radiology, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905, United States; Mayo Graduate School, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905, United States.
出版信息
Magn Reson Imaging. 2017 May;38:54-62. doi: 10.1016/j.mri.2016.12.020. Epub 2016 Dec 27.
PURPOSE
Spatial position accuracy in magnetic resonance imaging (MRI) is an important concern for a variety of applications, including radiation therapy planning, surgical planning, and longitudinal studies of morphologic changes to study neurodegenerative diseases. Spatial accuracy is strongly influenced by gradient linearity. This work presents a method for characterizing the gradient non-linearity fields on a per-system basis, and using this information to provide improved and higher-order (9th vs. 5th) spherical harmonic coefficients for better spatial accuracy in MRI.
METHODS
A large fiducial phantom containing 5229 water-filled spheres in a grid pattern is scanned with the MR system, and the positions all the fiducials are measured and compared to the corresponding ground truth fiducial positions as reported from a computed tomography (CT) scan of the object. Systematic errors from off-resonance (i.e., B0) effects are minimized with the use of increased receiver bandwidth (±125kHz) and two acquisitions with reversed readout gradient polarity. The spherical harmonic coefficients are estimated using an iterative process, and can be subsequently used to correct for gradient non-linearity. Test-retest stability was assessed with five repeated measurements on a single scanner, and cross-scanner variation on four different, identically-configured 3T wide-bore systems.
RESULTS
A decrease in the root-mean-square error (RMSE) over a 50cm diameter spherical volume from 1.80mm to 0.77mm is reported here in the case of replacing the vendor's standard 5th order spherical harmonic coefficients with custom fitted 9th order coefficients, and from 1.5mm to 1mm by extending custom fitted 5th order correction to the 9th order. Minimum RMSE varied between scanners, but was stable with repeated measurements in the same scanner.
CONCLUSIONS
The results suggest that the proposed methods may be used on a per-system basis to more accurately calibrate MR gradient non-linearity coefficients when compared to vendor standard corrections.
目的
磁共振成像（MRI）中的空间位置精度是包括放射治疗计划、手术规划以及用于研究神经退行性疾病的形态学变化的纵向研究等各种应用中的一个重要关注点。空间精度受梯度线性的强烈影响。这项工作提出了一种在每个系统基础上表征梯度非线性场的方法，并利用此信息提供改进的和更高阶（第九阶与第五阶）的球谐系数，以在MRI中实现更好的空间精度。
方法
使用MR系统扫描一个包含5229个呈网格状排列的充水球体的大型基准体模，并测量所有基准点的位置，并将其与通过该物体的计算机断层扫描（CT）获得的相应基准点真实位置进行比较。通过使用增加的接收带宽（±125kHz）以及两次具有相反读出梯度极性的采集，将失谐（即B0）效应产生的系统误差降至最低。使用迭代过程估计球谐系数，随后可用于校正梯度非线性。通过在单个扫描仪上进行五次重复测量以及在四个不同的、配置相同的3T大孔径系统上进行跨扫描仪变化评估了重测稳定性。
结果
在将供应商的标准第五阶球谐系数替换为定制拟合的第九阶系数的情况下，此处报告了在直径50cm的球形体积上均方根误差（RMSE）从1.80mm降至0.77mm，并且通过将定制拟合的第五阶校正扩展到第九阶，RMSE从1.5mm降至1mm。最小RMSE在不同扫描仪之间有所不同，但在同一扫描仪中重复测量时是稳定的。
结论
结果表明，与供应商标准校正相比，所提出的方法可在每个系统基础上用于更准确地校准MR梯度非线性系数。
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