Transcript 阻抗型磁体
MRI的安全性和禁忌症 北京同仁医院 牛延涛 设 备 鉴别3种磁体。 认识包围超导磁体的多层结构。 认识MR室的所有设备。 认识表面线圈对SNR的作用。 静磁场 脉冲序列 磁体 匀场线圈 梯度线圈 RF 线圈 磁体内孔 Bo RF线圈 梯度线圈 匀场线圈 磁体 磁体种类 超高场（4.0~7.0T）；主要用于研究 ； 高场（1.5~3.0T）; 中场（0.5~1.4T）; 低场（0.2~0.4T）; 超低场（小于0.2T）。 磁体种类 永磁型磁体 阻抗型磁体 超导型磁体 永磁型磁体 使用永磁型材料制作的磁体 “开放设计”的垂直磁场。 内含块状或条状天然铁制材料。 磁场强度 0.06T ~ 0.35T。 边缘磁场较低。 扫描间温度会影响磁场强度，从而导致共振 频率改变。 阻抗型磁体（常导型） 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。 需使用直流电，增加电流会增加磁场强度 并使电线产热。 需要冷却。 不用时可以关闭。 对温度敏感。 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。 这种磁体必须被包围在制冷设备中。 水平磁场 需要直流电 超导线由铌钛合金制成，浸泡在液氦（绝对 零度4.2K或270）中去除电阻。 可产生高磁场强度 (FDA – 4.0T) 高SNR, 扫描时间短，空间分辨率高 六面 RF 屏蔽 MR Site Layout 扫描间 磁体 键盘 显示器 采集和显示控制 匀场线圈 梯度线圈 RF 线圈 照相机 储存设备 工作站 计算机室 浸泡在制 冷剂中的 超导线圈 X, Y ,Z 梯度 患者床 操作间（控制间） 发射 & 接收 RF 探测器/数字转换 器 电源 (PDU) RF & 梯度 放大器 RF 和梯度脉 冲编程器 计算机 梯 度 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。 在3个方向上产生图像。 梯度幅度: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 幅度除以梯度爬升时间 (T/m/s) 。 梯 度 层面选择梯度 相位编码梯度 频率编码或读出梯度 梯度线圈 Z轴梯度 X 和Y 梯度线圈 x x – 梯度 y z y – 梯度 梯 度 通过轻微改变磁场强度来加快或减慢质子的进 动频率。 用于选层或对接收到的信号进行空间定位。 梯度 -2 B0 -1 0 1 2 3 4 RF 系统 RF系统产生能量使质子共振，并接收质子释放的 能量。RF系统包括下列组件： • 组成 – 射频放大器 – 射频通道 – 脉冲线圈 发射线圈 接收线圈 • 作用（如同天线） - 激发人体产生共振（广播电 台的发射天线） - 采集MR信号（收音机的天 线） 表面线圈 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SNR,同时减少来自FOV外 的噪音，使MR图像的SNR得到很大的改善。 SNR 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 圈用来发射RF脉冲。 发射/接收线圈 (i.e. 肢体线圈) RF 线圈 肢体线圈 头线圈 相控阵线圈 腕关节相控阵线圈 表面线圈 线性肩关节线圈 肩关节相控阵线圈 相控阵线圈 心脏线圈 相控阵线圈 周围血管线圈 相控阵线圈 神经血管相控阵线圈 相控阵线圈 乳腺相控阵线圈 安 全 认识MR对患者的损伤。 认识MR技师可采用哪些方法减轻对患者的损伤。 金 属 反磁性: 和主磁场轻微反向 金 铜 锌 水银 顺磁性: 轻微被主磁场吸引 铱 钛 铂 铁磁性: 被主磁场吸引： 铁 镍 钴 一些合金 锰 钆 Safet y 对患者的损伤 听力损伤 金属 面部和眼部 起搏器 内部损伤 RF 加热 电缆和线圈 Safety •听力 Safet y •金属 Safet y •面部和眼部 Safet y •起搏器 Safet y •内部损伤 Safet y •RF 加热 Safet y •电缆和线圈 安 全 • 不要将金属带进扫描间! 安 全 MR技师在允许任何人 （不仅仅是患者）进 入扫描间前都要筛 查严防任何禁忌发 生的可能性！ YOU! 磁共振成像的安全性 铁磁性投射物 体内植入物 梯度场噪声 孕妇的MRI检查 不良心理反应及其预防 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 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。 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 大物体。 投射效应是MRI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。 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 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。 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、 硬币、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。 磁共振成像的安全性 铁磁性投射物 体内植入物 梯度场噪声 孕妇的MRI检查 不良心理反应及其预防 体内植入物 MRI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。 MRI的射频电磁波有可能使植入体内的某 些电子设备失灵。 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 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 常见的体内植入物。 非铁磁性植入物患者可接受MRI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 接受MRI检查。 研究表明，大约1/3的体内植入物将在静磁场中发 生偏倚或移位，但不见得把所有铁磁性植入物都看 作MRI禁忌症。 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 几个方面：位置变化；功能紊乱；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 脱落；静磁场和RF场都可能干扰人工心脏起搏器 使其失效或停搏。 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 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 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 又廉价的方法，在X线片上可发现小到0.1mm的 金属异物。 磁共振成像的安全性 铁磁性投射物 体内植入物 梯度场噪声 孕妇的MRI检查 不良心理反应及其预防 梯度场噪声 MRI装置的音频噪声可分为静态及动态两种。 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。 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， 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 时的特殊噪声。 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 的频率越高，它发出的噪声就会越大。 1.0~2.0T时，梯度场达到25mT/m时，噪声可高 达110dB。 心理伤害是可诱发癫痫和幽闭恐惧症。 生理伤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 铁磁性投射物 体内植入物 梯度场噪声 孕妇的MRI检查 不良心理反应及其预防 孕妇的MRI检查 MRI是否有致畸作用一直是一个有争议的话题。 建议“在妊娠的头3个月谨慎应用”MRI检查。 孕期的工作人员对MRI电磁场的接触也应受到限 制。一般来说，活动范围要尽量在1mT线（10高 斯线）以外，以避免接受MRI产生的小剂量慢性 辐射。 磁共振成像的安全性 铁磁性投射物 体内植入物 梯度场噪声 孕妇的MRI检查 不良心理反应及其预防 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 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。 需要采取以下措施来降低其发生率： 事先向患者讲解MRI检查的特殊性，如磁体孔洞的大 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； 不良心理反应及其预防 改变体位：仰卧位改为俯卧位、头先进改为脚先进； 提供MRI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； 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 静磁场的生物效应 梯度磁场的生物效应 射频场的生物效应 磁共振成像系统的生物效应 MRI检查中，受检者受到静磁场、梯度磁 场和射频磁场的辐射。 理论上讲，任一种磁场都将产生相关的生物 效应。 目前，诸多研究还不能得出MRI对机体存 在潜在危害的结论。 磁共振成像系统的生物效应 近20年来，MRI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 算机等相关技术的最新成果均在MRI中得 到应用。 但是，MRI的生物效应研究却大大滞后， 原因如下。 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 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 多。 MRI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。 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。 磁共振成像系统的生物效应 目前的观察资料（仅限于1.5T以下的场强 ）中可以得到这样的结论：常规MRI成像 不会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X射线CT 。 MRI是安全的。 生物效应的存在又是肯定的，有必要深入地 进行评价。 磁共振成像系统的生物效应 静磁场的生物效应 梯度磁场的生物效应 射频场的生物效应 静磁场的生物效应 随着超导磁体技术的日益成熟，场强有不断 提高的趋势。 静磁场对生物体的影响至今没有完全阐明， 表明超高场（2T以上）对人体影响的资料 就更少。 FDA明确规定，因场强超过规定限值而造 成的一切后果由MRI制造商承担。 温度效应 MRI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 响体温甚至磁场使身体某部位的体温下降等 多种观点。 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 磁场中的血流以及其他流动液体产生的生物 效应。 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。 场强对ECG的影响不是非常明显。 中枢神经系统效应 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2.0T以下的静磁场对 人的中枢神经系统没有明显不良影响。 但在4.0T以上的MRI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 ，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 静磁场的生物效应 梯度磁场的生物效应 射频场的生物效应 梯度场及其感应电流 梯度磁场是一种时变场，变化的磁场在导体中将感 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 （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 系。 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。 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 心肌纤维等电敏感细胞。 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 到刺激（抽搐或收缩）看作心律不齐或心室纤 颤出现的先兆。 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 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 场最敏感的生理反应之一。 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，1.5T和20T/s以下不出 现这种幻觉，但在4T中20~40Hz时很容易使正常 人产生磁幻视。 磁共振成像系统的生物效应 静磁场的生物效应 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。MRI 扫描时RF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。 SAR（specific absorption rate）指单位重量 生物组织中RF功率的吸收量，是对组织中电磁能 量吸收值或RF功率沉积值的度量。 由局部和全身SAR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。 射频能量的特殊吸收率 在MRI中，SAR的大小与共振频率（静磁场强度 ）、RF脉冲的类型（90或180 ）、重复时间和 脉宽、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。 RF场最主要的生物效应是温度效应，但RF照射 引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。 射频能量的特殊吸收率 美国国家标准协会和FDA规定：接受连续 电磁波辐射时， 全身平均SAR不能超过0.4W/kg， 或每克组织的SAR空间峰值不超过8.0W/kg 。 射频场对体温的影响 静磁场与体温无关，MRI检查时病人体温 的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 认为此升高不构成临床有害影响。 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 的升高非常敏感，这些部位是最容易受MRI射频 辐射损伤的部位。 过量电磁辐射可能导致患者暂时甚至永久不育和白 内障，但有人认为临床MRI成像一般不会造成眼 组织的热损伤。 高SAR的MRI检查或长时间的MRI检查所致热效 应是一个需要进一步研究的课题。 禁忌证 有心脏起搏器的患者。 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 金属异物等。 换有人工金属心脏瓣膜患者。 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。 妊娠不足3个月。 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MRI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应包括以下8个方面： 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MR宣传 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 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 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MRI及CT检查。有“老号”者，认 真查找老片，以利于对比。 进入MR室前应嘱患者除去携带的一切金属物品、 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 ，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。 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 镇静处理。 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 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 不能在机器房内抢救。 谢 谢 高斯（gauss, G） Gauss (1777-1855) 德国著名数学家，于1832年首次测量了地球的磁场。 1高斯为距离5安培电流的直导线1厘米处检测到的磁场强度 5安培 1厘米 1高斯 地球的磁场强度分布图 特斯拉（Tesla,T） Nikola Tesla (18571943), 奥地利电器工程 师，物理学家，旋转磁 场原理及其应用的先驱 者之一。 1 T = 10000G General Bioeffects of Static Magnetic Fields There 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 performed by Vyalov,227,228 who studied workers involved in the ermanentmagnet 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 nonspecific 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 appropriate 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 Effects There 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 orientation 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 this response, nor has this work been substantiated. In addition, studies that reported static magnetic field–induced skin and/or body temperature changes used either laboratory animals known to have labile temperatures or instrumentation that may have been affected by the static magnetic fields.72,203 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.213 This study was performed by using a special fluoroptic thermometry system demonstrated to be unperturbed by high-intensity static magnetic fields; therefore the skin and the body temperatures of human subjects are believed to be unaffected by exposure to static magnetic fields of up to 1.5 T. Electrical Induction and Cardiac Effects Induced biopotentials may be observed during exposure to static magnetic fields and are caused by blood—a conductive fluid—flowing through a magnetic field. Induced biopotentials are exhibited by an augmentation of T-wave amplitude and by other, nonspecific waveform changes on the electrocardiogram (ECG). They have been observed at static magnetic field strengths as low as 0.1 T.11,15,214 The increase in T-wave amplitude is directly related to the intensity of the static magnetic field, such that at low static magnetic field strengths the effects 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 blood flows through the thoracic aortic arch. This T-wave amplitude change can be significant enough to falsely trigger the RF excitation during a cardiac-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. Alternate lead positions can be used to attenuate the static magnetic field–induced ECG changes to facilitate cardiacgating studies.43 Once the patient is no longer 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 biological risks are believed to be associated with the magnetohydrodynamic effect that occurs in conjunction with static magnetic field strengths of up to 2 T. Neurological Effects Theoretically, electrical impulse conduction in nerve tissue may be affected by exposure to static magnetic fields; however, this is an area in the bioeffects literature that contains contradictory information. Some studies have reported remarkable effects on both the function and the structure of those portions of the central nervous system associated with exposure to static magnetic fields, whereas others have failed to show any significant changes.* Further investigations of potential unwanted bioeffects are needed because of the relative lack of clinical studies in this field that are directly applicable to MRI. At present, exposure to static magnetic fields of up to 2 T does not appear to significantly influence bioelectrical properties of neurons in humans.96,177,184 In summary, there is no conclusive evidence of irreversible or hazardous biological effects related to acute, short-term exposure of humans to static magnetic fields of strengths up to 2 T. However, as of 1998, there were several 3 and 4 T wholebody MR systems in operation at various research sites 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 research is under way worldwide to study the mechanisms responsible for these bioeffects and to determine possible means, if any, to counterbalance them. Cryogen Considerations All superconductive MR systems in clinical use today use liquid helium. Liquid helium, which maintains the magnet coils in their superconductive state, will achieve the gaseous state (“boil off”) at approximately –268.93° C (4.22° K).96 If the temperature within the cryostat precipitously rises, the helium will enter the gaseous state. In such a situation the marked increase in volume of the gaseous versus the liquid cryogen (with gas-liquid volume 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 escapes from 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 considerably lighter than air. If any helium gas is inadvertently released into the imaging room, the dimensions of the room, its ventilation capacity, and the total amount of gas released will determine whether the helium gas will reach the patient or the health practitioner, who is in the lower part of the 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 of helium gas may be released into the imaging room. This might make it difficult to open the door of the room because of the pressure differential. In such a circumstance the first response should be to evacuate the area until the offending helium vapor is adequately removed from the imaging room environment and safely redirected to an outside environment away from patients, pedestrians, or any temperature-sensitive material.96 Better 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 nitrogen as well. Liquid nitrogen within the cryostat acts as a buffer between the liquid helium and the outside atmosphere, boiling off at 77.3° K. In the event of an accidental release of liquid nitrogen into the ambient atmosphere of the imaging room, there is a potential for frostbite, similar to that encountered with gaseous helium release. Gaseous nitrogen is roughly 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 gas within the room would continue to increase until venting ceased. The total concentration of nitrogen gas contained within the room would be determined 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., the existence and size of other routes of egress—doors, 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 the area as soon as it is recognized that nitrogen gas is being released into the imaging room. They should not return until appropriate measures have been taken to clear the gas from the room.96 Dewar (cryogen storage containers) storage should also be within a well-ventilated area, lest normal boil-off rates increase the concentration of inert gas within the storage room to a dangerous level.71 At least one reported death has occurred in an industrial setting during the shipment of cryogens,70 although to our knowledge no such fatality has occurred in the medical community. There is one report of a sudden loss of consciousness of unexplained cause by an otherwise healthy technologist (with no prior or subsequent similar episodes) passing through a cryogen storage area where multiple dewars were located.4 Although there is no verification of ambient atmospheric oxygen concentration to confirm any relationship to the 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.96 Proper handling and storage of cryogens, as well as the appropriate behavior in the 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 safety measure for all sites; automatic linking to and activation of an imaging room ventilation fan system when the oxygen monitor registers below 18% or 19% should be considered at each magnet installation. Electrical Considerations of a Quench In addition to the potential for cryogen release, there is also a concern about the currents that may be induced in conductors (such as biological tissues) near the rapidly changing magnetic field associated with a quench.96 In one study, physiological monitoring of a pig and monitoring of the environment were performed during an intentional quench from 1.76 T; there seemed to be no significant effect on the blood pressure, pulse, temperature, and electroencephalographic and ECG measurements of the pig during or immediately after the quench.41 Although a single observation does not prove safety for humans undergoing exposure to a quench, the data do suggest that the experience would indeed be similar, and that there would be no deleterious electrical effects 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 the imaging sequence. Gradient magnetic fields can induce electrical fields and currents in conductive media (including biological tissue) according to Faraday's law of induction. The potential for interaction between gradient magnetic fields and biological tissue is inherently dependent on the fundamental field frequency, the maximum flux density, the average flux density, the presence of harmonic frequencies, the waveform characteristics of the signal, the polarity of the signal, the current distribution in the body, and the electrical properties and sensitivity of the particular cell membrane.96,177,184 For animal and human subjects, the induced current is proportional to the conductivity of the biological tissue and the rate of change of the magnetic flux 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 linearly diminish toward the body's center.18,96,161,177 The current density will be enhanced at higher frequencies and magnetic flux densities and will be further accentuated by a larger tissue radius with a greater tissue conductivity. Current paths are affected by differences in tissue types, such that tissues with low conductivity (e.g., adipose and bone) will 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 believed to be clinically significant.30,96,177 Possible nonthermal effects of induced currents are stimulation of nerve or muscle cells, induction of ventricular fibrillation, increased brain mannitol space, epileptogenic potential, stimulation of visual flash sensations, and bone healing.* The threshold currents required for nerve stimulation and ventricular fibrillation are known to be much higher than the estimated current densities that will be induced under routine clinical MR conditions.30,96,161,177,184 The production of magnetophosphenes is considered to be one of the most sensitive physiological responses to gradient magnetic fields.30,96,177,184 Magnetophosphenes are supposedly caused by electrical stimulation of the retina and are completely reversible with no associated health effects.30,96,177,184 These have been elicited by current densities of roughly 17 mA/cm2. In contrast to this level, the currents required for the induction of nerve action potentials is roughly 3000 mA/cm2, and those required for ventricular fibrillation induction of healthy cardiac tissue are calculated to be 100 to 1000 mA/cm2.30 Although to our knowledge there have been no reported cases of magnetophosphenes for fields of 1.95 T or less, magnetophosphenes have been reported in volunteers working in and around a 4 T research system.157 In addition, a metallic taste and symptoms of vertigo also seem to be reproducible and associated with rapid motion within the static magnetic field of these 4 T systems.157 Time-varying, extremely low-frequency magnetic fields have been demonstrated to be associated with multiple effects, including clustering and altered orientation of fibroblasts, as well as increased mitotic activity of fibroblast growth, altered DNA synthesis, and reduced fentanyl-induced anesthesia.96,152,200 Possible effects in multiple other organisms, including humans, have also been mentioned.96 Although no study has conclusively demonstrated carcinogenic effects from exposure to time-varying magnetic fields of various intensities and durations, several reports suggest that an association between the two is plausible. General Bioeffects of Radiofrequency Electromagnetic Fields RF radiation is capable of generating heat in tissues as a result of resistive losses. Therefore the main bioeffects associated with exposure to RF radiation are related to the thermogenic qualities of this electromagnetic field.† Exposure to RF radiation also may cause athermic, field-specific alterations in biological systems that are produced without a significant increase in temperature.‡ This topic is somewhat controversial because of assertions concerning the role of electromagnetic fields in producing cancer and developmental abnormalities, along with the concomitant ramifications of such effects.‡ A report from the U.S. Environmental Protection Agency (EPA) claimed that the existing evidence on this issue is sufficient to demonstrate a relationship between low-level electromagnetic field exposures and the development of cancer.144 To date, there have been no specific studies performed to study potential athermal bioeffects of MRI. Those interested in a thorough review of this topic, particularly as it pertains to MRI, are referred to the extensive article written by Beers.14 Regarding RF-power deposition concerns, investigators have typically quantified exposure to RF radiation by means of determining the specific absorption 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 per kilogram. Measurements or estimates of SAR are not trivial, particularly in human subjects, and there are several methods of determining this parameter for RF-energy dosimetry.50,65-67,119,124 The SAR produced during MRI is a complex function of numerous variables: the frequency (which, in turn, is determined by the strength of the 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 caused by exposure to RF radiation is dependent on the subject's thermoregulatory system (e.g., skin blood flow, skin surface area, and sweat rate).96,177,184 The efficiency and absorption pattern of RF energy are determined mainly by the physical dimensions of the tissue in relation to the incident wavelength.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 small relative to the wavelength, there is little absorption of RF power.50,65-67,124 Because of the relationship between RF energy and physical dimensions, studies designed to investigate the effects of exposure to RF radiation during MRI that are intended to be applicable to the clinical setting require tissue volumes and anatomical shapes comparable to those of human subjects. No laboratory animal sufficiently mimics or simulates the thermoregulatory system or responses of man. Thus results obtained in laboratory animal experiments cannot simply be “scaled” or extrapolated to human subjects. Magnetic Resonance Imaging and Exposure to Radiofrequency Radiation Little quantitative data have been previously available on thermoregulatory responses of humans exposed to RF radiation before the studies performed with MRI. The few studies that existed did not directly apply to MRI because these investigations examined either thermal sensations or therapeutic applications of diathermy, usually involving only localized regions of the body.40,50,65,124 Several studies of RF-power absorption during MRI have been performed recently and have yielded useful information about tissue heating in humans.* During MRI, tissue heating results primarily from magnetic induction, with a negligible contribution from the electric fields, so that ohmic heating is greatest at the surface of the body and approaches zero at the center of the body. Predictive calculations and measurements obtained in phantoms and human subjects exposed to MRI support this pattern of temperature distribution.21,22,180,185,187 Although one paper reported significant temperature rises produced by MRI in internal organs,201 this study was conducted on anesthetized dogs and is unlikely to be applicable to conscious adult human subjects because of factors related to the physical dimensions and dissimilar thermoregulatory systems of these two species. However, these data may have important implications for the use of MRI in pediatric patients because this patient population 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 to MRI at SAR levels 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 clinically hazardous.194,238 These results imply that the suggested exposure level of 0.4 W/kg for RF radiation during MRI is too conservative for individuals with normal thermoregulatory function.194 Additional studies are needed, however, to assess physiological responses of patients with conditions that may impair thermoregulatory function (e.g., elderly patients; patients with underlying health conditions, such as fever, diabetes, obesity, or cardiovascular disease; and patients taking medications that affect thermoregulation, such as calcium blockers, beta blockers, diuretics, and vasodilators) before subjecting them to MRI procedures that require high SARs. Temperature-Sensitive Organs Certain human organs that have reduced capabilities for heat dissipation, such as the testis and the eye, are particularly sensitive to elevated temperatures. Therefore these are primary sites of potential harmful effects if RF-radiation exposures during MRI are excessive. Laboratory investigations 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 radiation–induced heating from exposures sufficient to raise scrotal or testicular tissue temperatures to 38° to 42° C.17 Scrotal skin temperatures (i.e., an index of intratesticular temperature) were measured in volunteer subjects undergoing MRI at a whole-body–averaged SAR of 1.1 W/kg.192 The largest change in scrotal skin temperature was 2.1° C, and the highest scrotal skin temperature recorded was 34.2° C.192 These temperature changes were below the threshold known to impair testicular function. However, excessively heating the scrotum during MRI could exacerbate certain preexisting disorders associated with increased scrotal or testicular temperatures (e.g., acute febrile illnesses and varicocele) in patients who are already oligospermic and lead to possible temporary or permanent sterility.192 Therefore additional studies designed to investigate these issues are needed, particularly if patients are scanned at whole-body–averaged SARs higher than those previously evaluated. Dissipation of heat from the eye is a slow and inefficient process because of the eye's relative lack of vascularization. Acute near-field exposures of RF radiation to the eyes or heads of laboratory animals have been demonstrated to be cataractogenic as a result of the thermal disruption of ocular tissues if the exposure is of a sufficient intensity and duration.108,124 An investigation conducted by Sacks et al171 revealed that there were no discernible effects on the eyes of rats by MRI at exposures that far exceeded typical clinical imaging levels. However, it may not be acceptable to extrapolate these data to humans, considering the coupling of RF radiation to the anatomy and tissue volume of the laboratory rat eyes compared with 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 3.1 W/kg.180 The largest corneal temperature change was 1.8° C, and the highest temperature measured was 34.4° C. Because the temperature threshold for RF radiation–induced cataractogenesis in animal models has been demonstrated to be between 41° to 55° C for acute, near-field exposures, it does not appear that clinical MRI using a head coil has the potential to cause thermal damage in ocular tissue.180 The effect of MRI at higher 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 distribution of RF power may arise whenever current concentrations are produced in association with restrictive conductive patterns. It has been suggested that RF radiation hot spots may generate thermal hot spots under certain conditions during MRI. Because RF radiation is mainly absorbed by peripheral tissues, thermography has been used to study the heating pattern associated with MRI at high whole-body SARs.196 This study demonstrated no evidence of surface thermal hot spots related to MRI of human subjects. The thermoregulatory system apparently responds to the heat challenge by distributing the thermal load, producing a “smearing” effect of the surface temperatures. However, there is a possibility that internal thermal hot spots may develop as a result of MRI.1 FOOD AND DRUG ADMINISTRATION GUIDELINES FOR MAGNETIC RESONANCE DEVICES In 1988, MR diagnostic devices were reclassified from class III, in which premarket approval is required, to class II, which is regulated by performance standards, as long as the device (or devices) are within the “umbrella” of certain defined limits, addressed later.52 Subsequent to this reclassification, new devices had to demonstrate only that they were “substantially equivalent” to any class II device that was brought to market using the premarket notification process (510[k]) or, alternatively, to any of the devices described by the 13 MR-system manufacturers that had petitioned the FDA for such a reclassification. Four areas relating to the use of MR systems have been identified for which safety guidelines have been issued by the FDA. These include the static magnetic 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 guidance document for MR devices.52 The new wording for those criteria 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 are considered significant risk investigations, 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 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) sufficient 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 during MRI represents a potential risk to patients. Acoustic noise is associated with the activation and deactivation of electrical current that induces vibrations of the gradient coils. This repetitive sound is enhanced by higher-gradient duty cycles and sharper pulse transitions. Thus acoustic noise is likely to increase with decreases in section thicknesses and decreased fields of view, repetition times, and echo times. Gradient magnetic field–related noise levels measured on several commercial MR scanners were in the range of 65 to 95 dB, considered to be within the recommended safety guidelines set forth by the FDA.52 However, there have been reports that acoustic noise generated during MRI has caused patient annoyance, interference with oral communication, and reversible hearing loss in patients who did not wear ear protection.28,189 A study of patients undergoing MRI without earplugs reported temporary hearing loss in 43% of the subjects.28 Furthermore, the possibility exists that significant gradient coil–induced noise may produce permanent hearing impairment in certain patients who are particularly susceptible to the damaging effects of relatively loud noises.28,189 The safest and least expensive means of preventing problems associated with acoustic noise during clinical MRI is to encourage the routine use of disposable earplugs.28,189 The use of hearing protection has been demonstrated to successfully avoid the potential temporary hearing loss that can be associated with clinical 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 effectively reduces noise and permits better patient communication.63 This technique calls for a real-time Fourier analysis of the noise emitted from the MR system.63 Next, a signal is produced that possesses the same physical characteristics of the sound generated by the MR system but of the opposite phase. The two opposite-phase signals are then combined, resulting in a cancellation of the repetitive noise, while allowing other sounds, such as music and voice, to be transmitted to the patient.63 A recent investigation demonstrated no significant degradation of image quality when MRI is performed with MR systems that use this antinoise method.63 Although this technique has not yet found widespread clinical application, it nevertheless 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 though few investigations have examined its teratogenic potential (see Chapter 31). By comparison, thousands of studies have examined the possible hazards of ultrasound during pregnancy, and controversy still exists concerning the safe use of this nonionizing-radiation imaging technique. Most of the earliest studies conducted to determine possible unwanted biological effects of MRI during pregnancy showed negative results.* More recently, 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-rump length.82 In another study, performed by Tyndall and Sulik,222 exposure to the electromagnetic fields used for a simulated clinical MRI examination caused eye malformations in a genetically prone mouse strain. Therefore it appears that the electromagnetic fields used for MRI have the potential to produce developmental abnormalities. A variety of mechanisms could produce deleterious biological effects with respect to the developing fetus and the use of electromagnetic fields during MRI.† In addition, it is well known that cells undergoing division, as in the case of the developing fetus during the first trimester, are highly susceptible to damage from different types of physical agents. Therefore, because of the limited data available at present, a cautionary approach is recommended for the use of MRI on pregnant patients. In the United States the FDA requires the labeling of MRI devices to indicate 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 Radiological Protection Board in 1983 specify that “it might be prudent to exclude pregnant women during the first three months of pregnancy.”113 According to the Safety Committee of the Society for Magnetic Resonance Imaging (this information also has been adopted recently by the American College of Radiology), MRI is indicated for use in pregnant women if other nonionizing forms of diagnostic imaging are inadequate or if the 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 produced deleterious effects. However, as noted by the FDA, the safety of MRI during pregnancy has not been proved.52 Patients who are pregnant or suspect they are pregnant must be identified to assess the risks versus the benefits of the MR examination. Because there is a high spontaneous abortion rate in the general population during the first trimester of pregnancy (>30%), particular care should be exercised with the use of MRI during the first trimester because of associated potential medicolegal implications. MAGNETIC RESONANCE IMAGING AND CLAUSTROPHOBIA, ANXIETY, AND PANIC DISORDERS Claustrophobia 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, the duration of the 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 who tolerated MRI with great difficulty and had persistent claustrophobia that required long-term psychiatric treatment.55 Because adverse psychological responses 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 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 headphones 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 the 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. 7.Use a large light at either end of the scanner to decrease the patient's 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 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 in reducing MRI-induced anxiety or claustrophobia.103,154,155 One study demonstrated that providing detailed information about the MRI procedure, in addition to “relaxation exercises,” successfully reduced the anxiety level of a group of patients both before and during MRI. A similar anxiety reduction could not be shown in patients provided with only information or “stress reduction” counseling. Relaxation methods have also been shown to significantly decrease anxiety during other medical procedures. Certain MR-system architectures employing a vertical magnetic field offer a more open design that might reduce the frequency of psychological problems associated with MRI procedures.