MR安全性和禁忌症

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1、MRI的的安全性安全性和和禁忌症禁忌症北京同仁医院北京同仁医院 牛延涛牛延涛设设 备备鉴别鉴别3种磁体。种磁体。认识包围超导磁体的多层结构。认识包围超导磁体的多层结构。认识认识MR室的所有设备。室的所有设备。认识表面线圈对认识表面线圈对SNR的作用。的作用。 Bo磁体内孔磁体磁体匀场线圈匀场线圈梯度线圈梯度线圈RF 线圈线圈 RF线圈线圈梯度线圈梯度线圈匀场线圈匀场线圈磁体磁体脉冲序列脉冲序列静磁场静磁场磁体种类磁体种类超高场（超高场（4.07.0T）；主要用于研究；）；主要用于研究；高场（高场（1.53.0T）;中场（中场（0.51.4T）;低场（低场（0.20.4T）;超低场（小于超低场（

2、小于0.2T）。）。 磁体种类磁体种类永磁型磁体永磁型磁体阻抗型磁体阻抗型磁体超导型磁体超导型磁体永磁型磁体永磁型磁体使用永磁型材料制作的磁体使用永磁型材料制作的磁体“开放设计开放设计”的垂直磁场。的垂直磁场。内含块状或条状天然铁制材料内含块状或条状天然铁制材料。磁场强度磁场强度 0.06T 0.35T。边缘磁场较低。边缘磁场较低。扫描间温度会影响磁场强度，从而导致共振频扫描间温度会影响磁场强度，从而导致共振频率改变。率改变。阻抗型磁体（常导型）阻抗型磁体（常导型） 磁场由普通电导体内电流产生的磁体。磁场由普通电导体内电流产生的磁体。电线内电流感应产生的磁场。电线内电流感应产生的磁场。 需使用

3、直流电，增加电流会增加磁场强度并需使用直流电，增加电流会增加磁场强度并使电线产热。使电线产热。需要冷却。需要冷却。不用时可以关闭。不用时可以关闭。对温度敏感。对温度敏感。超导型磁体超导型磁体 在超导材料内流动的电流感应产生磁场的磁体。在超导材料内流动的电流感应产生磁场的磁体。 这种磁体必须被包围在制冷设备中。这种磁体必须被包围在制冷设备中。水平磁场水平磁场需要直流电需要直流电超导线由铌钛合金制成，浸泡在液氦（绝对超导线由铌钛合金制成，浸泡在液氦（绝对零度零度4.2K或或 270 ）中去除电阻。）中去除电阻。可产生高磁场强度可产生高磁场强度 (FDA 4.0T) 高高SNR, 扫描时间短，空间分

4、辨率高扫描时间短，空间分辨率高 扫描间扫描间计算机室计算机室操作间（控制间）操作间（控制间）磁体磁体匀场线圈匀场线圈匀场线圈匀场线圈梯度线圈梯度线圈梯度线圈梯度线圈RF RF 线圈线圈线圈线圈发射发射 &接收接收X, Y ,Z 梯度梯度浸泡在制浸泡在制冷剂中的冷剂中的超导线圈超导线圈RF & 梯度梯度放大器放大器 RF 和梯度脉和梯度脉冲编程器冲编程器 RF 探测器探测器/数字转换数字转换器器电源电源 (PDU) 患者床患者床计算机计算机 照相机照相机采集和显示控制采集和显示控制显示器显示器储存设备储存设备键盘键盘工作站工作站六面六面RF 屏蔽屏蔽MR Site Layout 梯梯 度度 带电

5、线圈，产生在某一个方向上变化的磁场。带电线圈，产生在某一个方向上变化的磁场。对数据进行空间编码。对数据进行空间编码。在在3个方向上产生图像。个方向上产生图像。梯度幅度梯度幅度:每距离单位的磁场变化每距离单位的磁场变化 (mT/m)。梯度切换率：梯度性能的表示方法。梯度幅梯度切换率：梯度性能的表示方法。梯度幅度除以梯度爬升时间度除以梯度爬升时间 (T/m/s) 。层面选择层面选择梯度梯度相位编码相位编码梯度梯度频率编码频率编码或或读出读出梯度梯度梯梯 度度 梯度线圈梯度线圈Z轴梯度轴梯度X 和和Y 梯度线圈梯度线圈xzyx 梯度梯度y 梯度梯度梯梯 度度通过轻微改变磁场强度来加快或减慢质子的进通

6、过轻微改变磁场强度来加快或减慢质子的进动频率。动频率。用于选层或对接收到的信号进行空间定位。用于选层或对接收到的信号进行空间定位。梯度梯度34012-1-2B0RF 系统系统RF系统产生能量使质子共振，并接收质子释放的能系统产生能量使质子共振，并接收质子释放的能量。量。RF系统包括下列组件：系统包括下列组件：组成组成射频放大器射频放大器射频通道射频通道脉冲线圈脉冲线圈发射线圈发射线圈接收线圈接收线圈 作用（如同天线）作用（如同天线） - 激发人体产生共振（广播电激发人体产生共振（广播电台的发射天线）台的发射天线） - 采集采集MR信号（收音机的天信号（收音机的天线）线）表面线圈表面线圈表面线圈

7、可放置在感兴趣解剖部位表面，增加表面线圈可放置在感兴趣解剖部位表面，增加小范围薄层扫描的小范围薄层扫描的SNR,同时减少来自同时减少来自FOV外的外的噪音，使噪音，使MR图像的图像的SNR得到很大的改善。得到很大的改善。SNR 和线圈半径成反比。和线圈半径成反比。表面线圈只能接收信号。使用表面线圈时体线表面线圈只能接收信号。使用表面线圈时体线圈用来发射圈用来发射RF脉冲。脉冲。发射发射/接收线圈接收线圈 (i.e. 肢体线圈肢体线圈)RF 线圈线圈头线圈头线圈肢体线圈肢体线圈相控阵线圈相控阵线圈腕关节相控阵线圈腕关节相控阵线圈表面线圈表面线圈肩关节相控阵线圈肩关节相控阵线圈线性肩关节线圈线性肩

8、关节线圈相控阵线圈相控阵线圈心脏线圈心脏线圈相控阵线圈相控阵线圈周围血管线圈神经血管相控阵线圈神经血管相控阵线圈相控阵线圈相控阵线圈乳腺相控阵线圈乳腺相控阵线圈相控阵线圈相控阵线圈安安 全全 认识认识MR对对患者的损伤。患者的损伤。认识认识MR技师可采用哪些方法减轻对患者的损伤。技师可采用哪些方法减轻对患者的损伤。Safe Safe examinationexamination金金 属属铁磁性铁磁性铁磁性铁磁性: :和主磁场轻微反向和主磁场轻微反向金金铜铜 锌锌水银水银顺磁性顺磁性顺磁性顺磁性: :反磁性反磁性反磁性反磁性: :轻微被主磁场吸引轻微被主磁场吸引铱铱锰锰钛钛钆钆铂铂被主磁场吸引：

9、被主磁场吸引：铁铁镍镍钴钴一些合金一些合金Safety对患者的损伤对患者的损伤听力损伤听力损伤金属金属面部和眼部面部和眼部起搏器起搏器内部损伤内部损伤RF 加热加热电缆和线圈电缆和线圈Screening Form听力听力Safety金属金属Safety面部和眼部面部和眼部Safety起搏器起搏器Safety内部损伤内部损伤SafetyRF 加热加热Safety电缆和线圈电缆和线圈Safety不要将金属带进扫描间不要将金属带进扫描间!安安 全全MR技师在允许任何人技师在允许任何人（不仅仅是患者）进（不仅仅是患者）进入扫描间前都要筛查入扫描间前都要筛查严防任何禁忌发生的严防任何禁忌发生的可能性！可

10、能性！安安 全全YOU!磁共振成像的安全性磁共振成像的安全性铁磁性投射物铁磁性投射物体内植入物体内植入物梯度场噪声梯度场噪声孕妇的孕妇的MRI检查检查不良心理反应及其预防不良心理反应及其预防铁磁性投射物铁磁性投射物投射效应是在强磁场作用下铁磁性物体从磁体以外投射效应是在强磁场作用下铁磁性物体从磁体以外的地方以一定的速度投向磁体的现象，是磁体强大的地方以一定的速度投向磁体的现象，是磁体强大吸引力的外在表现。吸引力的外在表现。铁磁性投射物既可以是缝衣针、别针、螺丝刀、扳铁磁性投射物既可以是缝衣针、别针、螺丝刀、扳手等小物体，也可能是氧气瓶、吸尘器、工具箱等手等小物体，也可能是氧气瓶、吸尘器、工具箱

11、等大物体。大物体。投射效应是投射效应是MRI系统最大的安全问题之一。有必要系统最大的安全问题之一。有必要在磁体室入口处安装可调阈值的金属探测器。在磁体室入口处安装可调阈值的金属探测器。常见铁磁性投射物常见铁磁性投射物典型的铁磁性投射物含有铁的成分，但镍和钴等元典型的铁磁性投射物含有铁的成分，但镍和钴等元素也具有较强的铁磁性。非铁磁性物品虽然不产生素也具有较强的铁磁性。非铁磁性物品虽然不产生投射效应，却能形成金属伪影而干扰图像。投射效应，却能形成金属伪影而干扰图像。外科手术器械、氧气瓶、医疗仪器、担架、轮椅等；外科手术器械、氧气瓶、医疗仪器、担架、轮椅等；小刀、金属拉链、钮扣、指甲刀、钢笔、钥匙

12、、硬小刀、金属拉链、钮扣、指甲刀、钢笔、钥匙、硬币、手表、打火机、手机、助听器等。币、手表、打火机、手机、助听器等。MRI室应建立一整套安全防范措施。室应建立一整套安全防范措施。磁共振成像的安全性磁共振成像的安全性铁磁性投射物铁磁性投射物体内植入物体内植入物梯度场噪声梯度场噪声孕妇的孕妇的MRI检查检查不良心理反应及其预防不良心理反应及其预防体内植入物体内植入物MRI受检者体内的各种铁磁性物体会在磁力受检者体内的各种铁磁性物体会在磁力和磁扭矩的作用下发生移位或倾斜。和磁扭矩的作用下发生移位或倾斜。MRI的射频电磁波有可能使植入体内的某些的射频电磁波有可能使植入体内的某些电子设备失灵。电子设备失

13、灵。体内植入物体内植入物通过各种渠道置入体内并长期驻留体内的异物。弹通过各种渠道置入体内并长期驻留体内的异物。弹片、铁砂、假牙、动脉夹、人工股骨头、起搏器、片、铁砂、假牙、动脉夹、人工股骨头、起搏器、人工心脏瓣膜、电子耳蜗、药物泵、避孕环等是最人工心脏瓣膜、电子耳蜗、药物泵、避孕环等是最常见的体内植入物。常见的体内植入物。非铁磁性植入物患者可接受非铁磁性植入物患者可接受MRI检查，但会产生严检查，但会产生严重的金属伪影；铁磁性植入物患者一般来说不宜接重的金属伪影；铁磁性植入物患者一般来说不宜接受受MRI检查。检查。研究表明，大约研究表明，大约1/3的体内植入物将在静磁场中发的体内植入物将在静磁

14、场中发生偏倚或移位，但不见得把所有铁磁性植入物都看生偏倚或移位，但不见得把所有铁磁性植入物都看作作MRI禁忌症。禁忌症。体内植入物的安全性体内植入物的安全性MRI对铁磁性体内植入物的影响主要表现在以下几对铁磁性体内植入物的影响主要表现在以下几个方面：个方面：位置变化位置变化；功能紊乱功能紊乱；局部升温局部升温。强磁场可使脑动脉瘤治疗中放置的动脉夹移动甚至强磁场可使脑动脉瘤治疗中放置的动脉夹移动甚至脱落；静磁场和脱落；静磁场和RF场都可能干扰人工心脏起搏器场都可能干扰人工心脏起搏器使其失效或停搏。使其失效或停搏。金属异物的预检查金属异物的预检查体内可能存留诸如弹片、金属屑、铁砂等金属碎片体内可能

15、存留诸如弹片、金属屑、铁砂等金属碎片患者的危险性决定于它们在体内的位置。患者的危险性决定于它们在体内的位置。眼内的金属异物被拉出时容易造成伤害，已经有眼眼内的金属异物被拉出时容易造成伤害，已经有眼内金属异物致盲的报告。内金属异物致盲的报告。透视或拍片是对金属异物进行预检查的一种既敏感透视或拍片是对金属异物进行预检查的一种既敏感又廉价的方法，在又廉价的方法，在X线片上可发现小到线片上可发现小到0.1mm的金的金属异物。属异物。磁共振成像的安全性磁共振成像的安全性铁磁性投射物铁磁性投射物体内植入物体内植入物梯度场噪声梯度场噪声孕妇的孕妇的MRI检查检查不良心理反应及其预防不良心理反应及其预防梯度场

16、噪声梯度场噪声MRI装置的音频噪声可分为装置的音频噪声可分为静态静态及及动态动态两种。两种。静态噪声是由于磁体冷却系统即冷头的工作而引起静态噪声是由于磁体冷却系统即冷头的工作而引起的噪声，一般比较小。的噪声，一般比较小。动态噪声即梯度场噪声，指扫描过程中由梯度场的动态噪声即梯度场噪声，指扫描过程中由梯度场的不断开启或关闭而形成的。由于的主磁场的存在，不断开启或关闭而形成的。由于的主磁场的存在，梯度线圈工作时将产生很强的洛仑兹力，使线圈载梯度线圈工作时将产生很强的洛仑兹力，使线圈载体在梯度场转换期间发生剧烈振荡，从而产生扫描体在梯度场转换期间发生剧烈振荡，从而产生扫描时的特殊噪声。时的特殊噪声。

17、梯度场噪声梯度场噪声系统的静磁场越高、梯度上升速度越快或梯度脉冲系统的静磁场越高、梯度上升速度越快或梯度脉冲的频率越高，它发出的噪声就会越大。的频率越高，它发出的噪声就会越大。1.02.0T时，梯度场达到时，梯度场达到25mT/m时，噪声可高达时，噪声可高达110dB。心理伤害心理伤害是可诱发癫痫和幽闭恐惧症。是可诱发癫痫和幽闭恐惧症。生理伤害生理伤害是暂时性听力下降或永久性听力损害。是暂时性听力下降或永久性听力损害。磁共振成像的安全性磁共振成像的安全性铁磁性投射物铁磁性投射物体内植入物体内植入物梯度场噪声梯度场噪声孕妇的孕妇的MRI检查检查不良心理反应及其预防不良心理反应及其预防孕妇的孕妇的

18、MRI检查检查MRI是否有致畸作用一直是一个有争议的话题。是否有致畸作用一直是一个有争议的话题。建议建议“在妊娠的头在妊娠的头3个月谨慎应用个月谨慎应用”MRI检查。检查。孕期的工作人员对孕期的工作人员对MRI电磁场的接触也应受到限制。电磁场的接触也应受到限制。一般来说，活动范围要尽量在一般来说，活动范围要尽量在1mT线（线（10高斯线）高斯线）以外，以避免接受以外，以避免接受MRI产生的小剂量慢性辐射。产生的小剂量慢性辐射。磁共振成像的安全性磁共振成像的安全性铁磁性投射物铁磁性投射物体内植入物体内植入物梯度场噪声梯度场噪声孕妇的孕妇的MRI检查检查不良心理反应及其预防不良心理反应及其预防不良

19、心理反应及其预防不良心理反应及其预防MRI检查中，由于磁体孔洞比较狭小，加之梯度场噪检查中，由于磁体孔洞比较狭小，加之梯度场噪声的干扰，患者可能出现焦虑、恐慌或情绪低落等心声的干扰，患者可能出现焦虑、恐慌或情绪低落等心理反应，甚至诱发幽闭恐惧症。理反应，甚至诱发幽闭恐惧症。需要采取以下措施来降低其发生率：需要采取以下措施来降低其发生率：事先向患者讲解事先向患者讲解MRI检查的特殊性，如磁体孔洞的大检查的特殊性，如磁体孔洞的大小及梯度场的噪声水平等；小及梯度场的噪声水平等；允许被检者的亲属或朋友进入磁体室陪同；允许被检者的亲属或朋友进入磁体室陪同；不良心理反应及其预防不良心理反应及其预防改变体位

20、：仰卧位改为俯卧位、头先进改为脚先进；改变体位：仰卧位改为俯卧位、头先进改为脚先进；提供提供MRI兼容耳机并播放音乐；兼容耳机并播放音乐；在磁体孔洞内设置镜片或反光镜，分散病人注意力；在磁体孔洞内设置镜片或反光镜，分散病人注意力；扫描中同病人保持对讲等某种类型的通讯联系。扫描中同病人保持对讲等某种类型的通讯联系。磁共振成像系统的生物效应磁共振成像系统的生物效应静磁场的生物效应静磁场的生物效应梯度磁场的生物效应梯度磁场的生物效应射频场的生物效应射频场的生物效应磁共振成像系统的生物效应磁共振成像系统的生物效应MRI检查中，受检者受到检查中，受检者受到静磁场静磁场、梯度磁场梯度磁场和和射频磁场射频磁

21、场的辐射。的辐射。理论上讲，任一种磁场都将产生相关的生物理论上讲，任一种磁场都将产生相关的生物效应。效应。目前，诸多研究还不能得出目前，诸多研究还不能得出MRI对机体存在对机体存在潜在危害的结论。潜在危害的结论。磁共振成像系统的生物效应磁共振成像系统的生物效应近近20年来，年来，MRI技术得到飞速发展，超导技技术得到飞速发展，超导技术、磁体技术、低温技术、电子技术和计算术、磁体技术、低温技术、电子技术和计算机等相关技术的最新成果均在机等相关技术的最新成果均在MRI中得到应中得到应用。用。但是，但是，MRI的生物效应研究却大大滞后，原的生物效应研究却大大滞后，原因如下。因如下。磁共振成像系统的生

22、物效应磁共振成像系统的生物效应生物效应研究的难度大。三种磁场的复合作用结果生物效应研究的难度大。三种磁场的复合作用结果很难评价，动物模型与人体的差异较大。很难评价，动物模型与人体的差异较大。生物效应的影响因素多。三种磁场的影响因素都很生物效应的影响因素多。三种磁场的影响因素都很多。多。MRI系统千差万别。每一型号都需要相当长的时间系统千差万别。每一型号都需要相当长的时间来积累研究资料或临床数据。来积累研究资料或临床数据。硬件发展过快，许多新技术的生物效应尚未开始评硬件发展过快，许多新技术的生物效应尚未开始评价就已在临床应用。价就已在临床应用。磁共振成像系统的生物效应磁共振成像系统的生物效应目前

23、的观察资料（仅限于目前的观察资料（仅限于1.5T以下的场强）以下的场强）中可以得到这样的结论：中可以得到这样的结论：常规常规MRI成像不会成像不会给人类健康造成任何有临床意义的威胁，它给人类健康造成任何有临床意义的威胁，它对人体健康的影响远远小于对人体健康的影响远远小于X射线射线CT。MRI是安全的。是安全的。生物效应的存在又是肯定的，有必要深入地生物效应的存在又是肯定的，有必要深入地进行评价。进行评价。磁共振成像系统的生物效应磁共振成像系统的生物效应静磁场的生物效应静磁场的生物效应梯度磁场的生物效应梯度磁场的生物效应射频场的生物效应射频场的生物效应静磁场的生物效应静磁场的生物效应随着超导磁体

24、技术的日益成熟，场强有不断随着超导磁体技术的日益成熟，场强有不断提高的趋势。提高的趋势。静磁场对生物体的影响至今没有完全阐明，静磁场对生物体的影响至今没有完全阐明，表明超高场（表明超高场（2T以上）对人体影响的资料就以上）对人体影响的资料就更少。更少。FDA明确规定，因场强超过规定限值而造成明确规定，因场强超过规定限值而造成的一切后果由的一切后果由MRI制造商承担。制造商承担。温度效应温度效应MRI出现后最早受到关注的生物效应之一。出现后最早受到关注的生物效应之一。多年来，出现过磁体使体温升高、磁场不影多年来，出现过磁体使体温升高、磁场不影响体温甚至磁场使身体某部位的体温下降等响体温甚至磁场使

25、身体某部位的体温下降等多种观点。多种观点。现已证明，静磁场对人的体温不产生影响。现已证明，静磁场对人的体温不产生影响。磁流体动力学效应磁流体动力学效应磁场中的血流以及其他流动液体产生的生物磁场中的血流以及其他流动液体产生的生物效应。效应。静磁场能使红细胞的沉积速度加快、心电图静磁场能使红细胞的沉积速度加快、心电图发生改变，并有可能感应出生物电位。发生改变，并有可能感应出生物电位。场强对场强对ECG的影响不是非常明显。的影响不是非常明显。中枢神经系统效应中枢神经系统效应磁场有可能引起神经活动的误传导。磁场有可能引起神经活动的误传导。目前公认，短期的暴露在目前公认，短期的暴露在2.0T以下的静磁场

26、对以下的静磁场对人的中枢神经系统没有明显不良影响。人的中枢神经系统没有明显不良影响。但在但在4.0T以上的以上的MRI系统中，大多数志愿者出系统中，大多数志愿者出现眩晕、恶心、头痛、口中有异味等主观感觉，现眩晕、恶心、头痛、口中有异味等主观感觉，显然超高磁体可导致人体某种显著的生理变化。显然超高磁体可导致人体某种显著的生理变化。磁共振成像系统的生物效应磁共振成像系统的生物效应静磁场的生物效应静磁场的生物效应梯度磁场的生物效应梯度磁场的生物效应射频场的生物效应射频场的生物效应梯度场及其感应电流梯度场及其感应电流梯度磁场是一种时变场，变化的磁场在导体中将感梯度磁场是一种时变场，变化的磁场在导体中将

27、感应出电流。感应电流在人体内部构成回路。应出电流。感应电流在人体内部构成回路。感应电流的大小与梯度场的切换率、最大磁通强度感应电流的大小与梯度场的切换率、最大磁通强度（梯度场强度）、平均磁通强度、谐波频率、波形（梯度场强度）、平均磁通强度、谐波频率、波形参数、脉冲极性、体内电流分布等诸多因素均有关参数、脉冲极性、体内电流分布等诸多因素均有关系。系。静磁场中运动的导电物体也会产生电流，病人被送静磁场中运动的导电物体也会产生电流，病人被送入磁体的过程中体内有感生电流出现。入磁体的过程中体内有感生电流出现。梯度场的心血管效应梯度场的心血管效应强电流对心血管系统的作用为直接刺激血管和强电流对心血管系统

28、的作用为直接刺激血管和心肌纤维等电敏感细胞。心肌纤维等电敏感细胞。引起心律不起、心室或心房纤颤等。引起心律不起、心室或心房纤颤等。一般将皮肤（感觉）神经或外周骨骼肌神经受一般将皮肤（感觉）神经或外周骨骼肌神经受到刺激（抽搐或收缩）看作心律不齐或心室纤到刺激（抽搐或收缩）看作心律不齐或心室纤颤出现的先兆。颤出现的先兆。磁致光幻视磁致光幻视又叫光幻视或磁幻视，是在梯度场作用下眼前出现又叫光幻视或磁幻视，是在梯度场作用下眼前出现闪光感或色环的现象。闪光感或色环的现象。电刺激视网膜感光细胞后形成的视觉紊乱，是梯度电刺激视网膜感光细胞后形成的视觉紊乱，是梯度场最敏感的生理反应之一。场最敏感的生理反应之一

29、。光幻视与梯度场变化率和静磁场强度均有关系，且光幻视与梯度场变化率和静磁场强度均有关系，且在梯度场停止后自动消失，在梯度场停止后自动消失，1.5T和和20T/s以下不出以下不出现这种幻觉，但在现这种幻觉，但在4T中中2040Hz时很容易使正常人时很容易使正常人产生磁幻视。产生磁幻视。磁共振成像系统的生物效应磁共振成像系统的生物效应静磁场的生物效应静磁场的生物效应梯度磁场的生物效应梯度磁场的生物效应射频场的生物效应射频场的生物效应射频能量的特殊吸收率射频能量的特殊吸收率人体受到电磁波照射时将其能量转换为热。人体受到电磁波照射时将其能量转换为热。MRI扫扫描时描时RF激励波的功率将全部或大部被人体

30、所吸收，激励波的功率将全部或大部被人体所吸收，其生物效应主要是体温的变化。其生物效应主要是体温的变化。SAR（specific absorption rate）指单位重量生物组）指单位重量生物组织中织中RF功率的吸收量，是对组织中电磁能量吸收功率的吸收量，是对组织中电磁能量吸收值或值或RF功率沉积值的度量。功率沉积值的度量。由局部和全身由局部和全身SAR之分，分别对应于局部组织和全之分，分别对应于局部组织和全身组织平均的射频功率吸收量。身组织平均的射频功率吸收量。射频能量的特殊吸收率射频能量的特殊吸收率在在MRI中，中，SAR的大小与共振频率（静磁场强度）、的大小与共振频率（静磁场强度）、RF

31、脉冲的类型（脉冲的类型（90 或或180 ）、重复时间和脉宽、）、重复时间和脉宽、线圈效率、成像组织容积、组织类型（电特性）、线圈效率、成像组织容积、组织类型（电特性）、解剖结构等许多因素有关。解剖结构等许多因素有关。RF场最主要的生物效应是温度效应，但场最主要的生物效应是温度效应，但RF照射引照射引起的实际组织温升还决定于照射时间、环境温度以起的实际组织温升还决定于照射时间、环境温度以及被检者自身的温度调节功能。及被检者自身的温度调节功能。射频能量的特殊吸收率射频能量的特殊吸收率美国国家标准协会和美国国家标准协会和FDA规定：接受连续电规定：接受连续电磁波辐射时，磁波辐射时，全身平均全身平均

32、SAR不能超过不能超过0.4W/kg，或每克组织的或每克组织的SAR空间峰值不超过空间峰值不超过8.0W/kg。射频场对体温的影响射频场对体温的影响静磁场与体温无关，静磁场与体温无关，MRI检查时病人体温的检查时病人体温的变化完全是射频场作用的结果。变化完全是射频场作用的结果。MRI扫描可导致温度的显著升高，但有人认扫描可导致温度的显著升高，但有人认为此升高不构成临床有害影响。为此升高不构成临床有害影响。射频场最易损伤的器官射频场最易损伤的器官人体中散热功能不好的器官，如睾丸、眼等对温度人体中散热功能不好的器官，如睾丸、眼等对温度的升高非常敏感，这些部位是最容易受的升高非常敏感，这些部位是最容

33、易受MRI射频辐射频辐射损伤的部位。射损伤的部位。过量电磁辐射可能导致患者暂时甚至永久不育和白过量电磁辐射可能导致患者暂时甚至永久不育和白内障，但有人认为临床内障，但有人认为临床MRI成像一般不会造成眼组成像一般不会造成眼组织的热损伤。织的热损伤。高高SAR的的MRI检查或长时间的检查或长时间的MRI检查所致热效应检查所致热效应是一个需要进一步研究的课题。是一个需要进一步研究的课题。禁忌证禁忌证有心脏起搏器的患者。有心脏起搏器的患者。手术后动脉夹存留患者。手术后动脉夹存留患者。铁磁性异物患者，如体内存留有弹片、眼内存留有铁磁性异物患者，如体内存留有弹片、眼内存留有金属异物等。金属异物等。换有人

34、工金属心脏瓣膜患者。换有人工金属心脏瓣膜患者。金属假肢、金属关节患者。金属假肢、金属关节患者。体内置有胰岛素泵或神经刺激器者。体内置有胰岛素泵或神经刺激器者。妊娠不足妊娠不足3个月。个月。以上各项有疑问有患者要进行调研，弄清情况，再以上各项有疑问有患者要进行调研，弄清情况，再决定是否做决定是否做MRI检查。否则应谢绝做此项检查。检查。否则应谢绝做此项检查。磁共振检查前的准备磁共振检查前的准备磁共振检查前的准备应包括以下磁共振检查前的准备应包括以下8个方面：个方面：接诊时核对资料、病史、明确检查目的和要求。接诊时核对资料、病史、明确检查目的和要求。确认无禁忌证后，发给预约单，其内容为确认无禁忌证

35、后，发给预约单，其内容为MR宣传宣传资料，嘱患者认真阅读。资料，嘱患者认真阅读。对腹部盆腔部位检查者，检查当日早晨控制小量进对腹部盆腔部位检查者，检查当日早晨控制小量进食水。置有金属避孕环患者，嘱取环后再行检查。食水。置有金属避孕环患者，嘱取环后再行检查。磁共振检查前的准备磁共振检查前的准备对预约检查登记患者，要核对资料、登记建档，并对预约检查登记患者，要核对资料、登记建档，并询问是否做过询问是否做过MRI及及CT检查。有检查。有“老号老号”者，认者，认真查找老片，以利于对比。真查找老片，以利于对比。进入进入MR室前应嘱患者除去携带的一切金属物品、室前应嘱患者除去携带的一切金属物品、磁性物品及

36、电子元件，以免引起伪影，伤害患者。磁性物品及电子元件，以免引起伪影，伤害患者。对于体内有金属异物及安装心脏起搏器者禁止检查对于体内有金属异物及安装心脏起搏器者禁止检查 ，以防发生意外。，以防发生意外。消除患者恐惧心理，争取患者密切配合与合作。消除患者恐惧心理，争取患者密切配合与合作。磁共振检查前的准备磁共振检查前的准备对婴儿及躁动患者，应在临床医师指导下适当给予对婴儿及躁动患者，应在临床医师指导下适当给予镇静处理。镇静处理。对于危重患者，除早期脑梗塞患者外，原则上不做对于危重患者，除早期脑梗塞患者外，原则上不做MR检查，如果特别需要，一必须检查，应由有经检查，如果特别需要，一必须检查，应由有经

37、验的临床医师陪同。验的临床医师陪同。备齐抢救器械和药品，并向临床医师说明发生意外备齐抢救器械和药品，并向临床医师说明发生意外不能在机器房内抢救。不能在机器房内抢救。谢谢 谢谢高斯（高斯（gauss, G） Gauss (1777-1855)1高斯为距离高斯为距离5安培电流的直导线安培电流的直导线1厘米处检测到的磁场强度厘米处检测到的磁场强度德国著名数学家，于德国著名数学家，于1832年首次测量了地球的磁场。年首次测量了地球的磁场。5安培安培1厘米1高斯高斯地球的磁场强度分布图特斯拉（特斯拉（Tesla,T）Nikola Tesla (1857-1943), 奥地利电器工程奥地利电器工程师，物理

38、学家，旋转磁师，物理学家，旋转磁场原理及其应用的先驱场原理及其应用的先驱者之一。者之一。1 T = 10000G General Bioeffects of Static Magnetic FieldsThere is a paucity of data concerning the effects of high-intensity static magnetic fields on humans. Some of the original investigations on human subjects exposed to static agnetic fields were perfo

39、rmed by Vyalov,227,228 who studied workers involved in the ermanent-magnet industry. These subjects were exposed to static magnetic fields ranging from 0.0015 to 0.35 Tesla (T) and reported feelings of headache, chest pain, fatigue, vertigo,loss of appetite, insomnia, itching, and other, more nonspe

40、cific ailments.227,228 However, exposure to other potentially hazardous environmental working conditions (elevated room temperature, airborne metallic dust, chemicals) may have been partially esponsible for the reported symptoms in these study subjects. And because this investigation lacked an appro

41、priate control group, it is difficult to ascertain whether there was a definite correlation between the exposure to the static magnetic field and the reported abnormalities. Subsequent studies performed with more scientific rigor have not substantiated many of the aforementioned findings.Temperature

42、 EffectsThere are conflicting statements in the literature regarding the effect of static magnetic fields on the body and the skin temperatures of mammals.Reports have variously indicated that static magnetic fields either increase or both increase and decrease tissue temperature, depending on the o

43、rientation of the organism in the static magnetic field.72,203 Other articles state that static magnetic fields have no effect on the skin and the body temperatures of mammals.None of the investigators who identified a static magnetic field effect on temperatures proposed a plausible mechanism for t

44、his response, nor has this work been substantiated. In addition, studies that reported static magnetic fieldinduced skin and/or body temperature changes used eitherlaboratory animals known to have labile temperatures or instrumentation that may have been affected by the static magnetic fields.72,203

45、A recent investigation indicated that exposure to a 1.5 T static magnetic field does not alter the skin and the body temperatures in human beings.213This study was performed by using a special fluoroptic thermometry system demonstrated to be unperturbed by high-intensity static magnetic fields;there

46、fore the skin and the body temperatures of human subjects are believed to be unaffected by exposure to static magnetic fields of up to 1.5T.Electrical Induction and Cardiac EffectsInduced biopotentials may be observed during exposure to static magnetic fields and are caused by blooda conductive flui

47、dflowing through amagnetic field. Induced biopotentials are exhibited by an augmentation of T-wave amplitude and by other, nonspecific waveform changes on theelectrocardiogram (ECG). They have been observed at static magnetic field strengths as low as 0.1 T.11,15,214The increase in T-wave amplitude

48、is directly related to the intensity of the static magnetic field, such that at low static magnetic field strengths theeffects are not as predominant as those at higher field strengths. The most marked effect on the T wave is thought to be caused when the bloodflows through the thoracic aortic arch.

49、 This T-wave amplitude change can be significant enough to falsely trigger the RF excitation during acardiac-gated MR examination.Other portions of the ECG also may be altered by the static magnetic field, and this varies with the placement of the recording electrodes. Alternatelead positions can be

50、 used to attenuate the static magnetic fieldinduced ECG changes to facilitate cardiac-gating studies.43 Once the patient is nolonger exposed to the static magnetic field, these ECG voltage abnormalities revert to normal.Because no circulatory alterations appear to coincide with these ECG changes, no

51、 biological risks are believed to be associated with themagnetohydrodynamic effect that occurs in conjunction with static magnetic field strengths of up to 2 T.Neurological EffectsTheoretically, electrical impulse conduction in nerve tissue may be affected by exposure to static magnetic fields; howe

52、ver, this is an area in thebioeffects literature that contains contradictory information. Some studies have reported remarkable effects on both the function and the structure ofthose portions of the central nervous system associated with exposure to static magnetic fields, whereas others have failed

53、 to show any significantchanges.* Further investigations of potential unwanted bioeffects are needed because of the relative lack of clinical studies in this field that aredirectly applicable to MRI. At present, exposure to static magnetic fields of up to 2 T does not appear to significantly influen

54、ce bioelectricalproperties of neurons in humans.96,177,184In summary, there is no conclusive evidence of irreversible or hazardous biological effects related to acute, short-term exposure of humans to staticmagnetic fields of strengths up to 2 T. However, as of 1998, there were several 3 and 4 T who

55、le-body MR systems in operation at various researchsites around the world. One study indicated that workers and volunteer subjects exposed to a 4 T MR system experienced vertigo, nausea,headaches, a metallic taste in their mouths, and magnetophosphenes (visual flashes).157 As a result, considerable

56、research is under way worldwideto study the mechanisms responsible for these bioeffects and to determine possible means, if any, to counterbalance them.Cryogen ConsiderationsAll superconductive MR systems in clinical use today use liquid helium. Liquid helium, which maintains the magnet coils in the

57、ir superconductivestate, will achieve the gaseous state (“boil off”) at approximately 268.93 C (4.22 K).96 If the temperature within the cryostat precipitously rises, thehelium will enter the gaseous state. In such a situation the marked increase in volume of the gaseous versus the liquid cryogen (w

58、ith gas-liquidvolume ratios of 760:1 for helium and 695:1 for nitrogen) will dramatically increase the pressure within the cryostat.96 A pressure-sensitive carbon“pop-off” valve will give way, sometimes with a rather loud popping noise, followed by the rapid (and loud) egress of gaseous helium as it

59、 escapesfrom the cryostat. In normal situations this gas should be vented out of the imaging room and into the external atmosphere. It is possible, however,that during such venting some helium gas might accidentally be released into the ambient atmosphere of the imaging room.Gaseous helium is consid

60、erably lighter than air. If any helium gas is inadvertently released into the imaging room, the dimensions of the room, itsventilation capacity, and the total amount of gas released will determine whether the helium gas will reach the patient or the health practitioner, whois in the lower part of th

61、e room near the floor.96 Helium vapor looks like steam and is odorless and tasteless, but it may be extremely cold.Asphyxiation and frostbite are possible if a person is exposed to helium vapor for a prolonged time. In a system quench a considerable quantity ofhelium gas may be released into the ima

62、ging room. This might make it difficult to open the door of the room because of the pressure differential. Insuch a circumstance the first response should be to evacuate the area until the offending helium vapor is adequately removed from the imaging roomenvironment and safely redirected to an outsi

63、de environment away from patients, pedestrians, or any temperature-sensitive material.96Better cryostat design and insulation materials have allowed the use of liquid helium alone in many of the newer superconducting magnets.Nevertheless, a great number of magnets in clinical use still use liquid ni

64、trogen as well. Liquid nitrogen within the cryostat acts as a buffer betweenthe liquid helium and the outside atmosphere, boiling off at 77.3 K. In the event of an accidental release of liquid nitrogen into the ambientatmosphere of the imaging room, there is a potential for frostbite, similar to tha

65、t encountered with gaseous helium release. Gaseous nitrogen isroughly the same density as air and is certainly much less buoyant than gaseous helium.In the event of an inadvertent venting of nitrogen gas into the imaging room, the gas could easily settle near floor level; the amount of nitrogen gasw

66、ithin the room would continue to increase until venting ceased. The total concentration of nitrogen gas contained within the room would bedetermined on the basis of the total amount of the gas released into the room, the dimensions of the room, and its ventilation capacity (i.e., theexistence and si

67、ze of other routes of egressdoors, windows, ventilation ducts, and fans). A pure nitrogen environment is exceptionally hazardous,and unconsciousness generally results as soon as 5 to 10 sec after exposure.96 It is imperative that all patients and health practitioners evacuate thearea as soon as it i

68、s recognized that nitrogen gas is being released into the imaging room. They should not return until appropriate measures havebeen taken to clear the gas from the room.96Dewar (cryogen storage containers) storage should also be within a well-ventilated area, lest normal boil-off rates increase the c

69、oncentration of inertgas within the storage room to a dangerous level.71 At least one reported death has occurred in an industrial setting during the shipment ofcryogens,70 although to our knowledge no such fatality has occurred in the medical community. There is one report of a sudden loss ofconsci

70、ousness of unexplained cause by an otherwise healthy technologist (with no prior or subsequent similar episodes) passing through a cryogenstorage area where multiple dewars were located.4 Although there is no verification of ambient atmospheric oxygen concentration to confirm anyrelationship to the

71、cryogens per se, the history is strongly suggestive of such a relationship.Cryogens present a potential concern in clinical MRI despite an overwhelmingly safe record over the past 7 or more years of clinical service.96Proper handling and storage of cryogens, as well as the appropriate behavior in th

72、e presence of possible leaks, should be emphasized at each site.An oxygen monitor with an audible alarm, situated at an appropriate height within each imaging room, should be a mandatory minimum safetymeasure for all sites; automatic linking to and activation of an imaging room ventilation fan syste

73、m when the oxygen monitor registers below 18% or19% should be considered at each magnet installation.Electrical Considerations of a QuenchIn addition to the potential for cryogen release, there is also a concern about the currents that may be induced in conductors (such as biologicaltissues) near th

74、e rapidly changing magnetic field associated with a quench.96 In one study, physiological monitoring of a pig and monitoring of theenvironment were performed during an intentional quench from 1.76 T; there seemed to be no significant effect on the blood pressure, pulse,temperature, and electroenceph

75、alographic and ECG measurements of the pig during or immediately after the quench.41 Although a singleobservation does not prove safety for humans undergoing exposure to a quench, the data do suggest that the experience would indeed be similar, andthat there would be no deleterious electrical effect

76、s on humans undergoing a similar experience and exposure.BIOEFFECTS OF GRADIENT MAGNETIC FIELDS MRI exposes the human body to rapid variations of magnetic fields as a result of the transient application of magnetic field gradients during theimaging sequence. Gradient magnetic fields can induce elect

77、rical fields and currents in conductive media (including biological tissue) according toFaradays law of induction. The potential for interaction between gradient magnetic fields and biological tissue is inherently dependent on thefundamental field frequency, the maximum flux density, the average flu

78、x density, the presence of harmonic frequencies, the waveformcharacteristics of the signal, the polarity of the signal, the current distribution in the body, and the electrical properties and sensitivity of the particularcell membrane.96,177,184For animal and human subjects, the induced current is p

79、roportional to the conductivity of the biological tissue and the rate of change of the magneticflux density.18,96,161,177 In theory the largest current densities will be produced in peripheral tissues (i.e., at the greatest radius) and will linearlydiminish toward the bodys center.18,96,161,177 The

80、current density will be enhanced at higher frequencies and magnetic flux densities and will befurther accentuated by a larger tissue radius with a greater tissue conductivity. Current paths are affected by differences in tissue types, such thattissues with low conductivity (e.g., adipose and bone) w

81、ill change the pattern of the induced current.Bioeffects of induced currents can result from either the power deposited by the induced currents (thermal effects) or direct effects of the current(nonthermal effects). Thermal effects caused by switched gradients used in MRI are negligible and are not

82、believed to be clinicallysignificant.30,96,177Possible nonthermal effects of induced currents are stimulation of nerve or muscle cells, induction of ventricular fibrillation, increased brain mannitolspace, epileptogenic potential, stimulation of visual flash sensations, and bone healing.* The thresh

83、old currents required for nerve stimulation andventricular fibrillation are known to be much higher than the estimated current densities that will be induced under routine clinical MRconditions.30,96,161,177,184The production of magnetophosphenes is considered to be one of the most sensitive physiol

84、ogical responses to gradient magnetic fields.30,96,177,184Magnetophosphenes are supposedly caused by electrical stimulation of the retina and are completely reversible with no associated healtheffects.30,96,177,184 These have been elicited by current densities of roughly 17 mA/cm2. In contrast to th

85、is level, the currents required for theinduction of nerve action potentials is roughly 3000 mA/cm2, and those required for ventricular fibrillation induction of healthy cardiac tissue arecalculated to be 100 to 1000 mA/cm2.30 Although to our knowledge there have been no reported cases of magnetophos

86、phenes for fields of 1.95 T orless, magnetophosphenes have been reported in volunteers working in and around a 4 T research system.157 In addition, a metallic taste andsymptoms of vertigo also seem to be reproducible and associated with rapid motion within the static magnetic field of these 4 T syst

87、ems.157Time-varying, extremely low-frequency magnetic fields have been demonstrated to be associated with multiple effects, including clustering andaltered orientation of fibroblasts, as well as increased mitotic activity of fibroblast growth, altered DNA synthesis, and reduced fentanyl-inducedanest

88、hesia.96,152,200 Possible effects in multiple other organisms, including humans, have also been mentioned.96 Although no study has conclusivelydemonstrated carcinogenic effects from exposure to time-varying magnetic fields of various intensities and durations, several reports suggest that anassociat

89、ion between the two is plausible.General Bioeffects of Radiofrequency Electromagnetic FieldsRF radiation is capable of generating heat in tissues as a result of resistive losses. Therefore the main bioeffects associated with exposure to RFradiation are related to the thermogenic qualities of this el

90、ectromagnetic field. Exposure to RF radiation also may cause athermic, field-specificalterations in biological systems that are produced without a significant increase in temperature. This topic is somewhat controversial because ofassertions concerning the role of electromagnetic fields in producing

91、 cancer and developmental abnormalities, along with the concomitantramifications of such effects. A report from the U.S. Environmental Protection Agency (EPA) claimed that the existing evidence on this issue issufficient to demonstrate a relationship between low-level electromagnetic field exposures

92、 and the development of cancer.144 To date, there havebeen no specific studies performed to study potential athermal bioeffects of MRI. Those interested in a thorough review of this topic, particularly asit pertains to MRI, are referred to the extensive article written by Beers.14Regarding RF-power

93、deposition concerns, investigators have typically quantified exposure to RF radiation by means of determining the specificabsorption rate (SAR). The SAR is the mass normalized rate at which RF power is coupled to biological tissue and is indicated in units of watts perkilogram. Measurements or estim

94、ates of SAR are not trivial, particularly in human subjects, and there are several methods of determining thisparameter for RF-energy dosimetry.50,65-67,119,124The SAR produced during MRI is a complex function of numerous variables: the frequency (which, in turn, is determined by the strength of the

95、static magnetic field), type of RF pulse (i.e., 90 or 180 degrees), repetition time, pulse width, type of RF coil used, volume of tissue within the coil,resistivity of the tissue, configuration of the anatomical region imaged, and other factors.30,96,177,184 The actual increase in tissue temperature

96、caused by exposure to RF radiation is dependent on the subjects thermoregulatory system (e.g., skin blood flow, skin surface area, and sweatrate).96,177,184The efficiency and absorption pattern of RF energy are determined mainly by the physical dimensions of the tissue in relation to the incidentwav

97、elength.50,65-67,124 Therefore if the tissue size is large relative to the wavelength, energy is predominantly absorbed on the surface; if it is smallrelative to the wavelength, there is little absorption of RF power.50,65-67,124 Because of the relationship between RF energy and physical dimensions,

98、studies designed to investigate the effects of exposure to RF radiation during MRI that are intended to be applicable to the clinical setting requiretissue volumes and anatomical shapes comparable to those of human subjects. No laboratory animal sufficiently mimics or simulates thethermoregulatory s

99、ystem or responses of man. Thus results obtained in laboratory animal experiments cannot simply be “scaled” or extrapolated tohuman subjects.Magnetic Resonance Imaging and Exposure to Radiofrequency RadiationLittle quantitative data have been previously available on thermoregulatory responses of hum

100、ans exposed to RF radiation before the studiesperformed with MRI. The few studies that existed did not directly apply to MRI because these investigations examined either thermal sensations ortherapeutic applications of diathermy, usually involving only localized regions of the body.40,50,65,124Sever

101、al studies of RF-power absorption during MRI have been performed recently and have yielded useful information about tissue heating inhumans.* During MRI, tissue heating results primarily from magnetic induction, with a negligible contribution from the electric fields, so that ohmicheating is greates

102、t at the surface of the body and approaches zero at the center of the body. Predictive calculations and measurements obtained inphantoms and human subjects exposed to MRI support this pattern of temperature distribution.21,22,180,185,187 Although one paper reported significant temperature rises prod

103、uced by MRI in internal organs,201 this study was conducted on anesthetized dogs andis unlikely to be applicable to conscious adult human subjects because of factors related to the physical dimensions and dissimilar thermoregulatorysystems of these two species. However, these data may have important

104、 implications for the use of MRI in pediatric patients because this patientpopulation is typically sedated or anesthetized for MR examinations.An investigation using fluoroptic thermometry probes that are unperturbed by electromagnetic fields demonstrated that human subjects exposed toMRI at SAR lev

105、els up to 4 W/kg (i.e., 10 times higher than the level currently recommended by the U.S. Food and Drug Administration FDA)have no statistically significant increases in body temperatures and elevations in skin temperatures and are not believed to be clinicallyhazardous.194,238 These results imply th

106、at the suggested exposure level of 0.4 W/kg for RF radiation during MRI is too conservative for individualswith normal thermoregulatory function.194 Additional studies are needed, however, to assess physiological responses of patients with conditions thatmay impair thermoregulatory function (e.g., e

107、lderly patients; patients with underlying health conditions, such as fever, diabetes, obesity, orcardiovascular disease; and patients taking medications that affect thermoregulation, such as calcium blockers, beta blockers, diuretics, andvasodilators) before subjecting them to MRI procedures that re

108、quire high SARs.Temperature-Sensitive OrgansCertain human organs that have reduced capabilities for heat dissipation, such as the testis and the eye, are particularly sensitive to elevatedtemperatures. Therefore these are primary sites of potential harmful effects if RF-radiation exposures during MR

109、I are excessive. Laboratoryinvestigations have demonstrated detrimental effects on testicular function (i.e., a reduction or cessation of spermatogenesis, impaired sperm motility,degeneration of seminiferous tubules) caused by RF radiationinduced heating from exposures sufficient to raise scrotal or

110、 testicular tissuetemperatures to 38 to 42 C.17Scrotal skin temperatures (i.e., an index of intratesticular temperature) were measured in volunteer subjects undergoing MRI at awhole-bodyaveraged SAR of 1.1 W/kg.192 The largest change in scrotal skin temperature was 2.1 C, and the highest scrotal ski

111、n temperaturerecorded was 34.2 C.192 These temperature changes were below the threshold known to impair testicular function. However, excessively heatingthe scrotum during MRI could exacerbate certain preexisting disorders associated with increased scrotal or testicular temperatures (e.g., acutefebr

112、ile illnesses and varicocele) in patients who are already oligospermic and lead to possible temporary or permanent sterility.192 Thereforeadditional studies designed to investigate these issues are needed, particularly if patients are scanned at whole-bodyaveraged SARs higher thanthose previously ev

113、aluated.Dissipation of heat from the eye is a slow and inefficient process because of the eyes relative lack of vascularization. Acute near-field exposures ofRF radiation to the eyes or heads of laboratory animals have been demonstrated to be cataractogenic as a result of the thermal disruption of o

114、culartissues if the exposure is of a sufficient intensity and duration.108,124 An investigation conducted by Sacks et al171 revealed that there were nodiscernible effects on the eyes of rats by MRI at exposures that far exceeded typical clinical imaging levels. However, it may not be acceptable toex

115、trapolate these data to humans, considering the coupling of RF radiation to the anatomy and tissue volume of the laboratory rat eyes comparedwith those of man.Corneal temperatures have been measured in patients undergoing MRI of the brain by using a send-receive head coil at local SARs up to 3.1W/kg

116、.180 The largest corneal temperature change was 1.8 C, and the highest temperature measured was 34.4 C. Because the temperaturethreshold for RF radiationinduced cataractogenesis in animal models has been demonstrated to be between 41 to 55 C for acute, near-fieldexposures, it does not appear that cl

117、inical MRI using a head coil has the potential to cause thermal damage in ocular tissue.180 The effect of MRI athigher SARs and the long-term effects of MRI on ocular tissues remain to be determined.Radiofrequency Radiation and “Hot Spots”Theoretically, RF radiation “hot spots” caused by an uneven d

118、istribution of RF power may arise whenever current concentrations are produced inassociation with restrictive conductive patterns. It has been suggested that RF radiation hot spots may generate thermal hot spots under certainconditions during MRI. Because RF radiation is mainly absorbed by periphera

119、l tissues, thermography has been used to study the heating patternassociated with MRI at high whole-body SARs.196 This study demonstrated no evidence of surface thermal hot spots related to MRI of humansubjects. The thermoregulatory system apparently responds to the heat challenge by distributing th

120、e thermal load, producing a “smearing” effect ofthe surface temperatures. However, there is a possibility that internal thermal hot spots may develop as a result of MRI.1FOOD AND DRUG ADMINISTRATION GUIDELINES FOR MAGNETIC RESONANCE DEVICES In 1988, MR diagnostic devices were reclassified from class

121、 III, in which premarket approval is required, to class II, which is regulated byperformance standards, as long as the device (or devices) are within the “umbrella” of certain defined limits, addressed later.52 Subsequent to thisreclassification, new devices had to demonstrate only that they were “s

122、ubstantially equivalent” to any class II device that was brought to marketusing the premarket notification process (510k) or, alternatively, to any of the devices described by the 13 MR-system manufacturers that hadpetitioned the FDA for such a reclassification.Four areas relating to the use of MR s

123、ystems have been identified for which safety guidelines have been issued by the FDA. These include the staticmagnetic field, the gradient magnetic fields, the RF power of the examination, and the acoustical considerations.On September 29, 1997, the office of Device Evaluation of the FDA issued a new

124、 guidance document for MR devices.52 The new wording for thosecriteria that are considered signifcant risk investigations follow.Patient studies utilizing magnetic resonance diagnostic devices which are conducted under any one of the following operating conditions areconsidered significant risk inve

125、stigations, and require approval of an investigational device exemption (IDE) by the Food and Drug Administration(FDA) Center for Devices and Radiological Health (CDRH): 1. Main static magnetic field greater than 4 tesla; 2. Specific absorption rate (SAR) greater than 8. a. 4 W/kg averaged over the

126、whole body for any period of 15 minutes; 8. b. 3 W/kg averaged over the head for any period of 10 minutes; or 8. c. 8 W/kg in any gram of tissue in the head or torso, or 12 W/kg in any gram of tissue in the extremities, for any period of 5 minutes; 3. Time rate of change of gradient fields (dB/dt) s

127、ufficient to produce severe discomfort or painful nerve stimulation; or 4. Peak unweighted sound pressure level greater than 140 dB or A-weighted r.m.s. sound pressure level greater than 99 dBA with hearing protection in place.MAGNETIC RESONANCE IMAGING AND ACOUSTIC NOISE The acoustic noise produced

128、 during MRI represents a potential risk to patients. Acoustic noise is associated with the activation and deactivation ofelectrical current that induces vibrations of the gradient coils. This repetitive sound is enhanced by higher-gradient duty cycles and sharper pulsetransitions. Thus acoustic nois

129、e is likely to increase with decreases in section thicknesses and decreased fields of view, repetition times, and echotimes.Gradient magnetic fieldrelated noise levels measured on several commercial MR scanners were in the range of 65 to 95 dB, considered to bewithin the recommended safety guideline

130、s set forth by the FDA.52 However, there have been reports that acoustic noise generated during MRI hascaused patient annoyance, interference with oral communication, and reversible hearing loss in patients who did not wear ear protection.28,189 Astudy of patients undergoing MRI without earplugs rep

131、orted temporary hearing loss in 43% of the subjects.28 Furthermore, the possibility exists thatsignificant gradient coilinduced noise may produce permanent hearing impairment in certain patients who are particularly susceptible to thedamaging effects of relatively loud noises.28,189The safest and le

132、ast expensive means of preventing problems associated with acoustic noise during clinical MRI is to encourage the routine use ofdisposable earplugs.28,189 The use of hearing protection has been demonstrated to successfully avoid the potential temporary hearing loss that can beassociated with clinica

133、l MR examinations.28,189 MR-compatible headphones that significantly muffle acoustic noise are also commercially available.An acceptable alternative strategy for reducing sound levels during MRI is to use an antinoise or destructive interference technique that effectivelyreduces noise and permits be

134、tter patient communication.63 This technique calls for a real-time Fourier analysis of the noise emitted from the MRsystem.63 Next, a signal is produced that possesses the same physical characteristics of the sound generated by the MR system but of the oppositephase. The two opposite-phase signals a

135、re then combined, resulting in a cancellation of the repetitive noise, while allowing other sounds, such asmusic and voice, to be transmitted to the patient.63 A recent investigation demonstrated no significant degradation of image quality when MRI isperformed with MR systems that use this antinoise

136、 method.63 Although this technique has not yet found widespread clinical application, itnevertheless has considerable potential for minimizing acoustic noise and its associated problems.PERFORMING MAGNETIC RESONANCE IMAGING ON PREGNANT PATIENTS MRI is not believed to be hazardous to the fetus, even

137、though few investigations have examined its teratogenic potential (see Chapter 31). Bycomparison, thousands of studies have examined the possible hazards of ultrasound during pregnancy, and controversy still exists concerning the safeuse of this nonionizing-radiation imaging technique.Most of the ea

138、rliest studies conducted to determine possible unwanted biological effects of MRI during pregnancy showed negative results.* Morerecently, one study examined the effects of MRI on mice exposed during midgestation. No gross embryotoxic effects were observed; however,there was a reduction in crown-rum

139、p length.82 In another study, performed by Tyndall and Sulik,222 exposure to the electromagnetic fields used for asimulated clinical MRI examination caused eye malformations in a genetically prone mouse strain. Therefore it appears that the electromagneticfields used for MRI have the potential to pr

140、oduce developmental abnormalities. A variety of mechanisms could produce deleterious biological effects with respect to the developing fetus and the use of electromagnetic fieldsduring MRI. In addition, it is well known that cells undergoing division, as in the case of the developing fetus during th

141、e first trimester, are highlysusceptible to damage from different types of physical agents. Therefore, because of the limited data available at present, a cautionary approach isrecommended for the use of MRI on pregnant patients.In the United States the FDA requires the labeling of MRI devices to in

142、dicate that the safety of MRI when used to image the fetus and the infant“has not been established.”113 In Great Britain the acceptable limits of exposure for clinical MRI recommended by the National RadiologicalProtection Board in 1983 specify that “it might be prudent to exclude pregnant women dur

143、ing the first three months of pregnancy.”113According to the Safety Committee of the Society for Magnetic Resonance Imaging (this information also has been adopted recently by theAmerican College of Radiology), MRI is indicated for use in pregnant women if other nonionizing forms of diagnostic imagi

144、ng are inadequate or ifthe examination provides important information that would otherwise require exposure to ionizing radiation (i.e., x-ray, CT).189 For pregnant patients,it is recommended to inform them that, to date, there has been no indication that the use of clinical MRI during pregnancy has

145、 produced deleteriouseffects. However, as noted by the FDA, the safety of MRI during pregnancy has not been proved.52Patients who are pregnant or suspect they are pregnant must be identified to assess the risks versus the benefits of the MR examination. Becausethere is a high spontaneous abortion ra

146、te in the general population during the first trimester of pregnancy (30%), particular care should be exercisedwith the use of MRI during the first trimester because of associated potential medicolegal implications.MAGNETIC RESONANCE IMAGING AND CLAUSTROPHOBIA, ANXIETY, AND PANIC DISORDERS Claustrop

147、hobia and a variety of other psychological reactions, including anxiety and panic disorders, may be encountered by as many as 5% to 10%of patients undergoing MRI. These sensations originate from several factors, including the restrictive dimensions of the interior of the scanner, theduration of the

148、examination, the gradient-induced noises, and the ambient conditions within the bore of the scanner.Adverse psychological responses to MRI are usually transient. However, there was a report of two patients with no history of claustrophobia whotolerated MRI with great difficulty and had persistent cl

149、austrophobia that required long-term psychiatric treatment.55 Because adverse psychologicalresponses to MRI typically delay or even cause cancellation of the examination, several techniques have been developed to avert these problems.These techniques include the following: 1.Brief the patient on the

150、 specific aspects of the MR examination, including the level of gradient-induced noise to expect, the internal dimensions of the scanner, and the length of the examination. 2.Allow an appropriately screened relative or friend to remain with the patient during the procedure. 3.Give the patient headph

151、ones with calming music to decrease the repetitive noise created by the gradient coils. 4.Maintain physical or verbal contact with the patient throughout the examination. 5.Place the patient in a prone position with the chin supported by a pillow. In this position the patient is able to visualize th

152、e opening of the bore and thus alleviate the “closed-in” feeling. An alternative method to reduce claustrophobia is to place the subject feet-first instead of head-first into the scanner. 6.Use scanner-mounted mirrors or prism glasses within the scanner to allow the patient to see out of the scanner

153、. 7.Use a large light at either end of the scanner to decrease the patients anxiety about being in a long, dark enclosure. 8.Use a blindfold on the patient so that he or she is unaware of the close surroundings. 9.Use relaxation techniques, such as controlled breathing and mental imagery.113 Several

154、 case reports have shown hypnotherapy to be successful in reducing MRI-related claustrophobia and anxiety. 10.Use psychological “desensitization” techniques before the MR examination.Recently, several investigators have attempted to compare the effectiveness of some of the aforementioned techniques

155、in reducing MRI-inducedanxiety or claustrophobia.103,154,155 One study demonstrated that providing detailed information about the MRI procedure, in addition to “relaxationexercises,” successfully reduced the anxiety level of a group of patients both before and during MRI. A similar anxiety reduction

156、 could not be shownin patients provided with only information or “stress reduction” counseling. Relaxation methods have also been shown to significantly decreaseanxiety during other medical procedures. Certain MR-system architectures employing a vertical magnetic field offer a more open design that mightreduce the frequency of psychological problems associated with MRI procedures.