Cardiff University Brain Research Imaging Centre (CUBRIC), School of Psychology, Cardiff University, Cardiff, UK.
NMR Biomed. 2023 May;36(5):e4876. doi: 10.1002/nbm.4876. Epub 2022 Dec 12.
Specific absorption rate (SAR) relates power absorption to tissue heating, and therefore is used as a safety constraint in magnetic resonance imaging (MRI). This study investigates the implications of initial head positioning on local and whole-head SAR. A virtual body model was simulated at 161 positions inside an eight-channel parallel-transmit (pTx) array. On-axis displacements and rotations of up to 20 mm/degrees and off-axis axial/coronal translations were investigated. Single-channel, radiofrequency (RF) shimming (i.e., single-spoke pTx) and multispoke pTx pulses were designed for seven axial, five coronal and five sagittal slices at each position (the slices were consistent across all positions). Whole-head and local SAR were calculated using safety models consisting of a single (centred) body position, multiple representative positions and all simulated body positions. Positional mismatches between safety models and actual positions cause SAR underestimation. For axial imaging, the actual peak local SAR was up to 4.2-fold higher for both single-channel and 5-spoke pTx, 3.5-fold higher for 3-/4-spoke pTx, and 2-fold higher for RF shimming and 2-spoke pTx, compared with that calculated using the centred body position. For sagittal and coronal imaging, the underestimation of peak local SAR was up to 5.2-fold and 3.8-fold, respectively. Using all body positions to estimate SAR prevented SAR underestimation but yielded up to 11-fold SAR overestimation for RF shimming. Local SAR of single-channel and pTx multispoke pulses showed considerable dependence on the initial patient position. RF shimming yielded much lower sensitivity to positional mismatches for axial imaging but not for sagittal and coronal imaging. This was deemed attributable to the higher degrees-of-freedom of control offered by the investigated coil array for axial imaging. Whole-head SAR is less sensitive to positional mismatches compared with local SAR. Nevertheless, whole-head SAR increased by up to 80% for sagittal imaging. Local and whole-head SAR were observed to be more sensitive to positional mismatches in the axial plane, because of larger variations in coil-tissue proximity. Using all possible body positions in the safety model may become substantially over-conservative and limit imaging performance, especially for the RF shimming mode for axial imaging.
特定吸收率（SAR）与组织加热有关，因此被用作磁共振成像（MRI）中的安全限制。本研究探讨了初始头部定位对局部和全头部 SAR 的影响。在八通道并行传输（pTx）阵列内的 161 个位置模拟了虚拟体模型。研究了高达 20mm/度的轴上位移和旋转，以及轴外轴向/冠状平移。在每个位置设计了用于七个轴向、五个冠状和五个矢状切片的单通道、射频（RF）调谐（即单轮 pTx）和多轮 pTx 脉冲（所有位置的切片都是一致的）。使用由单个（中心）体位置、多个代表性位置和所有模拟体位置组成的安全模型计算全头部和局部 SAR。安全模型与实际位置之间的位置不匹配会导致 SAR 低估。对于轴向成像，与使用中心体位置计算的结果相比，单通道和 5 轮 pTx 的实际峰值局部 SAR 高 4.2 倍，3/4 轮 pTx 的峰值 SAR 高 3.5 倍，RF 调谐和 2 轮 pTx 的峰值 SAR 高 2 倍。对于矢状和冠状成像，峰值局部 SAR 的低估高达 5.2 倍和 3.8 倍。使用所有体位置估计 SAR 可以防止 SAR 低估，但对于 RF 调谐，SAR 高估高达 11 倍。单通道和 pTx 多轮脉冲的局部 SAR 对初始患者位置有很大的依赖性。对于轴向成像，RF 调谐对位置不匹配的敏感性要低得多，但对于矢状和冠状成像则不然。这被认为归因于所研究的线圈阵列为轴向成像提供的更高自由度控制。与局部 SAR 相比，全头部 SAR 对位置不匹配的敏感性较低。然而，对于矢状成像，全头部 SAR 增加了 80%。局部和全头部 SAR 对轴向平面中的位置不匹配更为敏感，因为线圈与组织的接近度变化较大。在安全模型中使用所有可能的体位置可能会变得过于保守，并限制成像性能，尤其是对于轴向成像的 RF 调谐模式。
