Chen Hua-Hsuan, Boykin Rex D, Clarke Geoffrey D, Gao Jia-Hong T, Roby John W
Department of Radiology, The University of Texas Health Science Center at San Antonio, San Antonio, Texas 78284, USA.
Med Phys. 2006 Nov;33(11):4299-306. doi: 10.1118/1.2359229.
Poor main magnetic field (B0) homogeneity (H(B)) leads to artifacts and signal losses in magnetic resonance imaging (MRI). The American College of Radiology's MRI quality control manual mandates annual checks of H(B), suggesting tests using spectral linewidth and phase-difference (delta phi) maps. A new method, the bandwidth-difference (deltaBW) method, which compares the distortion for small and large BW acquisitions to determine the HB, is proposed. The deltaBW method has the advantage that it can be used to measure multiple diameters of spherical volumes (DSV) in a single phantom. A phantom has been developed to exploit this method and results obtained with it are compared to those using three standard methods. Small receiver BW in the presence of poor H(B) leads to geometric distortions because gradients are reduced to the level of the B0 inhomogeneities. Data were acquired using seven MRI systems from different manufacturers, ranging in field strength from 0.2 to 3.0 T. Fast gradient echo pulse sequences were scanned twice using small and large BWs. H(B) was measured from the shift of landmarks between the two BW acquisitions. Results were compared with data from the full width at half maximum (FWHM) method, the delta phi method and one manufacturer's resonant frequency mapping data. The FWHM method was available on two systems and the detla phi method was available on one. The deltaBW method could be performed in all scanners investigated. The H(B) measured ranged 0.11-0.32 ppm to 6.7-12.9 ppm for DSV of 13-22.6 cm. Direct comparisons of the data obtained using the deltaBW method showed good agreement with data obtained using the FWHM method. Data obtained using the deltaBW method compared favorably with the manufacturer's resonant frequency map. The deltaBW method produces measurements of H(B) at various DSV values that can be obtained from a single set of phantom images. The accuracy of deltaBW B0 homogeneity measurements are comparable to the other methods tested.
主磁场（B0）均匀性差（H(B)）会导致磁共振成像（MRI）中出现伪影和信号损失。美国放射学会的MRI质量控制手册要求每年检查H(B)，建议使用谱线宽度和相位差（δφ）图进行测试。本文提出了一种新方法——带宽差（δBW）方法，该方法通过比较小带宽和大带宽采集时的失真来确定H(B)。δBW方法的优点是可以在单个体模中测量球形体积的多个直径（DSV）。已经开发了一种体模来应用此方法，并将其得到的结果与使用三种标准方法得到的结果进行比较。在H(B)较差的情况下，小接收带宽会导致几何失真，因为梯度降低到了B0不均匀性的水平。使用来自不同制造商的7个MRI系统采集数据，场强范围为0.2至3.0 T。使用小带宽和大带宽对快速梯度回波脉冲序列进行两次扫描。根据两次带宽采集之间地标位置的偏移来测量H(B)。将结果与半高宽（FWHM）方法、δφ方法以及一个制造商的共振频率映射数据进行比较。FWHM方法在两个系统上可用，δφ方法在一个系统上可用。δBW方法可以在所有研究的扫描仪上执行。对于13 - 22.6 cm的DSV，测量得到的H(B)范围为0.11 - 0.32 ppm至6.7 - 12.9 ppm。使用δBW方法获得的数据与使用FWHM方法获得的数据直接比较显示出良好的一致性。使用δBW方法获得的数据与制造商的共振频率图相比具有优势。δBW方法可以在从单组体模图像中获得的各种DSV值下测量H(B)。δBW B0均匀性测量的准确性与其他测试方法相当。
