Hsieh C, Stafford R, Reeve D
UT MD Anderson Cancer Center, Houston, TX.
Med Phys. 2012 Jun;39(6Part28):3977. doi: 10.1118/1.4736228.
The ACR MRI accreditation program requires measurement of the magnetic field homogeneity (MFH) in the annual QA. Full implementation of vendor methods can be very time consuming and incorporates techniques not available to clinical physicists. Conversely, many of the less involved techniques proposed tend to be less inaccurate and/or precise. Here, we propose a robust approach to MFH analysis using a simple phase mapping acquisition which is a time efficient compromise.
The root mean square (r.m.s.) for MFH measurement is commonly used over multiple slices (∼24). In regions with high SNR, the MR signal can be reasonably assumed Gaussian distributed. Therefore, the standard deviation of phase values in ROI is equivalent to the r.m.s. of those. The standard deviations of phases in x, y and z are assumed uncorrelated. Thus, only axial, sagittal and coronal planes need to be acquired to approximate the MFH as opposed to a full 3D acquisition. To investigate this concept, phase images were acquired on four 1.5T clinical scanners and one 3.0 clinical scanner (MAGNETOM Espree, Siemens Medical Systems, Signa Excite HD 1.5T, and 3.0T GE Healthcare Technologies). The manufacturer phantoms were scanned using two echo times (delta_TE=TE2-TE1<5ms) using a simple 2D gradient echo acquisition to produce phase images. After acquiring phase difference images in three orthogonal planes, the standard deviation was calculated in three circular ROIs (Diameter=10, 20 and 30cm) in each plane, respectively, to estimate the MFH for the effective DSV.
The MFH values in five scanners using this method were within vendor specifications for the DSV. Additionally, the measured MFH values compared favorably with vendor planned maintenance records with <0.1 ppm discrepancy.
This proposed method may be a reliable and practical for regular MFH measurement in QA programs and providing an independent check of the vendor measurement.
美国放射学会（ACR）的MRI认证计划要求在年度质量保证（QA）中测量磁场均匀性（MFH）。完全采用供应商的方法可能非常耗时，并且包含临床物理学家无法使用的技术。相反，许多提出的不太复杂的技术往往不太准确和/或精确。在此，我们提出一种使用简单相位映射采集进行MFH分析的稳健方法，这是一种时间高效的折衷方案。
MFH测量的均方根（r.m.s.）通常在多个切片（约24个）上使用。在高信噪比区域，可以合理地假设MR信号呈高斯分布。因此，感兴趣区域（ROI）中相位值的标准差等同于这些值的r.m.s.。假设x、y和z方向上相位的标准差不相关。因此，与完整的三维采集相反，只需要采集轴向、矢状面和冠状面来近似MFH。为了研究这个概念，在四台1.5T临床扫描仪和一台3.0T临床扫描仪（MAGNETOM Espree，西门子医疗系统公司，Signa Excite HD 1.5T和3.0T通用电气医疗技术公司）上采集了相位图像。使用简单的二维梯度回波采集，采用两个回波时间（delta_TE = TE2 - TE1 < 5ms）对制造商的体模进行扫描以生成相位图像。在三个正交平面上采集相位差图像后，分别在每个平面的三个圆形ROI（直径 = 10、20和30cm）中计算标准差，以估计有效视野（DSV）的MFH。
使用此方法在五台扫描仪中测得的MFH值在供应商规定的DSV范围内。此外，测得的MFH值与供应商计划维护记录相比，差异<0.1 ppm，表现良好。
该方法对于QA程序中的常规MFH测量可能是可靠且实用的，并且可对供应商测量进行独立检查。
