Collins Christopher M, Liu Wanzhan, Wang Jinghua, Gruetter Rolf, Vaughan J Thomas, Ugurbil Kamil, Smith Michael B
Center for NMR Research, Department of Radiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA.
J Magn Reson Imaging. 2004 May;19(5):650-6. doi: 10.1002/jmri.20041.
To examine relationships between specific energy absorption rate (SAR) and temperature distributions in the human head during radio frequency energy deposition in MRI.
A multi-tissue numerical model of the head was developed that considered thermal conductivity, heat capacity, perfusion, heat of metabolism, electrical properties, and density. Calculations of SAR and the resulting temperature increase were performed for different coils at different frequencies.
Because of tissue-dependent perfusion rates and thermal conduction, there is not a good overall spatial correlation between SAR and temperature increase. When a volume coil is driven to induce a head average SAR level of either 3.0 or 3.2 W/kg, it is unlikely that a significant temperature increase in the brain will occur due to its high rate of perfusion, although limits on SAR in any 1 g of tissue in the head may be exceeded.
Attempts to ensure RF safety in MRI often rely on assumptions about local temperature from local SAR levels. The relationship between local SAR and local temperature is not, however, straightforward. In cases where high SAR levels are required due to pulse sequence demands, calculations of temperature may be preferable to calculations of SAR because of the more direct relationship between temperature and safety.
研究磁共振成像（MRI）中射频能量沉积过程中人体头部比吸收率（SAR）与温度分布之间的关系。
建立了一个考虑热导率、比热容、灌注、代谢热、电学特性和密度的头部多组织数值模型。针对不同频率下的不同线圈，进行了SAR计算以及由此产生的温度升高计算。
由于组织依赖的灌注率和热传导，SAR与温度升高之间不存在良好的整体空间相关性。当使用容积线圈驱动以诱导头部平均SAR水平达到3.0或3.2 W/kg时，尽管头部任何1 g组织中的SAR可能会超过限值，但由于大脑的高灌注率，大脑不太可能出现显著的温度升高。
在MRI中确保射频安全的尝试通常依赖于根据局部SAR水平对局部温度的假设。然而，局部SAR与局部温度之间的关系并不直接。在因脉冲序列要求而需要高SAR水平的情况下，由于温度与安全性之间的关系更为直接，温度计算可能比SAR计算更可取。
