A simple correction for B1 field errors in magnetization transfer ratio measurements
A simple correction for B1 field errors in magnetization transfer ratio measurements
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DOI：
10.1016/j.mri.2005.10.025
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发表时间：
2006-04-01
影响因子：
2.5
通讯作者：
Tofts, PS
中科院分区：
文献类型：
--
作者：
Samson, RS;Wheeler-Kingshott, CAM;Tofts, PS
B-1误差是磁化传递比（MTR）测量中的问题，因为MTR值取决于磁化传递（MT）脉冲的幅度。B-1误差可能源于射频（RF）不均匀性（由RF线圈或受试者头部中的趋肤效应和介电共振引起），也可能源于补偿RF线圈负载变化量时发射器输出设置不准确。B-1误差以及因此的MTR误差可能高达5 - 10%，这是定量MR测量中的大误差源。辐射不均匀性会导致MTR直方图变宽，MTR对B-1的依赖关系采用二元自旋浴理论，在连续波（CW）近似下模拟。对于B-1减少高达20%，不同脑组织类型的标准化曲线可以近似为一条直线，表明系统校正可以应用于MTR测量与已知的B-1错误，无论组织类型。MT脉冲幅度从其标称值逐步降低，最高可达20%。对所有对照品和组织类型的数据进行平均，得到直线拟合mtr（标准化）= 0.82 B（1标准化）+0.193，其中mtr（标准化）是MTR的标准化值（相对于其标称值B-1），B（1标准化）是B-1的标准化值（相对于其标称值）。对于MT脉冲幅度降低20%（即B（1归一化）=0.80），18种组织类型的平均MTR值比正确值低7.0%单位（pu）。使用上述单一公式对所有组织类型进行校正后，所有MTR值均在其校正值的1.5 pu范围内[均方根（rms）误差=0.7 pu]。磁化传递比值倾向于略微过度校正，因为简单的线性校正方案仅近似于真实的MTR对B-1的依赖性。基于双角度方法（DAM），采用快速自旋回波（FSE）读出，TR = 15 s，实施了B-1场映射技术;这总共花费了6 min的成像时间。这被用于量化B-1错误和校正MTR图和直方图。然而，脑脊液（CSF）T-1非常长（约4.2 s），因此，为了实现完全的纵向弛豫（DAM B-1标测方法的要求），需要增加TR，因此需要增加采集时间。梗概.然而，我们对计算CSF中的B-1不感兴趣，尽管在CSF周围的部分体积体素中确定B-1是重要的。使用我们的鸟笼头部线圈，全脑B-1直方图被发现具有半高全宽（FWHM），范围仅为标称B-1值的6.8%至11.5%。FSE DAM B-1场标测技术被证明是稳健的，尽管可能需要更长的TR时间以确保完全消除CSF部分体积误差。该程序可以应用于任何可获得Euro-MT序列的扫描仪，或者，其中B-1或MT脉冲的幅度可以被手动减小，以便执行这种类型的“校准”。对于所使用的特定MTR序列进行实验。已知MTR高度依赖于所使用的序列的参数，特别是MT脉冲形状、翻转角、持续时间和偏移频率，以及连续MT脉冲之间的重复时间TR'。因此，对于不同的MTR序列，校正方案将不同，并且将需要新的数据集来计算这些不同的校正方案。(C)2006年爱思唯尔公司All rights reserved.
B-1 errors are a problem in magnetization transfer ratio (MTR) measurements because the MTR value is dependent on the amplitude of the magnetization transfer (MT) pulse. B-1 errors can arise from radiofrequency (RF) nonuniformity (caused by the RF coil, or skin effect and dielectric resonance in the subject's head) and also from inaccurate setting of the transmitter output when compensating for varying amounts of loading of the RF coil. B-1 errors, and hence MTR errors, may be up to 5 - 10%, a large source of error in quantitative MR measurements. Radiofrequency nonuniformity may cause MTR histograms to be broadened.The dependence of MTR on B-1 was modeled using binary spin bath theory, with a continuous wave (CW) approximation. For B-1 reductions of up to 20%, normalized plots for different brain tissue types could be approximated by a single line, indicating that a systematic correction could be applied to MTR measurements with a known B-1 error, regardless of tissue type.On a 1.5-T scanner with a birdcage coil, MTR was measured in IS tissue types in five controls. The MT pulse amplitude was reduced in steps from its nominal value by up to 20%. Averaging data over all controls and tissue types resulted in a line fitting mtr(normalized) = 0.82b(1normalized)+0.193 where mtr(normalized) is the normalized value of MTR (relative to its value at the nominal B-1) and b(1normalized) is the normalized value of B-1 (relative to its nominal value). For a 20% reduction in MT pulse amplitude (i.e. b(1normalized)=0.80), the mean MTR value for the 18 tissue types was 7.0 percent units (pu) below the correct value. After correction using the single equation above for all tissue types, all MTR values were within 1.5 pu of their correct value [root mean square (rms) error=0.7 pu]. Magnetization transfer ratio values tended to be slightly overcorrected because the simple linear correction scheme is only an approximation to the true MTR dependence on B-1.A B-1 field mapping technique was implemented, based on the double angle method (DAM), with fast spin-echo (FSE) readout, and TR = 15 s; this took a total of 6 min of imaging time. This was used to quantify B-1 errors and correct MTR maps and histograms. However, the cerebrospinal fluid (CSF) T-1 is very long (approximately 4.2 s)-, thus, to achieve complete longitudinal relaxation (a requirement of the DAM B-1 mapping method), an increase in TR and, hence, acquisition time would be required. In general. however, we are not interested in calculating the B-1 in the CSF, although it is important that the B-1 is determined in partial volume voxels around the CSF. Using our birdcage head coil, whole-brain B-1 histograms were found to have full-width half maximums (FWHMs) ranging from just 6.8% to 11.5% of the nominal B-1 value. The FSE DAM B-1 field mapping technique was shown to be robust, although a longer TR time may be desirable to ensure complete elimination of CSF partial volume errors.The procedure can be applied on any scanner where the Euro-MT sequence is available, or alternatively, where the amplitude of B-1 or of the MT pulse can be manually reduced in order to perform this type of "calibration" experiment for the particular MTR sequence used. The MTR is known to be highly dependent on the parameters of the sequence used, in particular, the MT pulse shape, flip angle, duration, and offset frequency, and the repetition time TR' between successive MT pulses. Therefore, correction schemes will differ for different MTR sequences, and new data sets would be required to calculate these different correction schemes. (C) 2006 Elsevier Inc. All rights reserved.