MR安全性和禁忌症

nullMRI的安全性和禁忌症MRI的安全性和禁忌症北京同仁医院 牛延涛设 备设 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 关于同志近三年现实表现材料材料类招标技术评分表图表与交易pdf视力表打印pdf用图表说话 pdf 面线圈对SNR的作用。nullClinical0.5T Interventionalnull 磁体种类磁体种类超高场（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）。 磁体种类磁体种类永磁型磁体 阻抗型磁体 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“开放 设计 领导形象设计圆作业设计ao工艺污水处理厂设计附属工程施工组织设计清扫机器人结构设计 ”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）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 null扫描间计算机室操作间（控制间）磁体匀场线圈梯度线圈RF 线圈发射 & 接收X, Y ,Z 梯度浸泡在制冷剂中的超导线圈RF & 梯度放大器 RF 和梯度脉冲编程器 RF 探测器/数字转换器电源 (PDU) 患者床计算机 照相机采集和显示控制显示器储存设备键盘工作站六面 RF 屏蔽MR Site Layout 带电线圈，产生在某一个方向上变化的磁场。梯 度 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) 。null层面选择梯度 相位编码梯度 频率编码或读出梯度梯 度 梯度线圈 Z轴梯度梯度线圈 Z轴梯度X 和Y 梯度线圈X 和Y 梯度线圈yx – 梯度y – 梯度梯 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。B0RF 系统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）null表面线圈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RF 线圈头线圈肢体线圈相控阵线圈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线性肩关节线圈相控阵线圈相控阵线圈心脏线圈相控阵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相控阵线圈乳腺相控阵线圈相控阵线圈安 全 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 属金 属铁磁性:和主磁场轻微反向 金 铜 锌 水银顺磁性:反磁性:轻微被主磁场吸引 铱 锰 钛 钆 铂被主磁场吸引： 铁 镍 钴 一些合金SafetySafety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SafetySafety听力Safety金属SafetySafety面部和眼部SafetySafety起搏器SafetySafety内部损伤SafetySafetyRF 加热SafetySafety电缆和线圈Safety安 全安 全不要将金属带进扫描间! 安 全安 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!null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。null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。null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。null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。null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。null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。null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。null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。null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射频能量的特殊吸收率美国国家 标准 excel标准偏差excel标准偏差函数exl标准差函数国标检验抽样标准表免费下载红头文件格式标准下载 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 对教师党员的评价套管和固井爆破片与爆破装置仓库管理基本要求三甲医院都需要复审吗 。 确认无禁忌证后，发给预约单，其 内容 财务内部控制制度的内容财务内部控制制度的内容人员招聘与配置的内容项目成本控制的内容消防安全演练内容 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。null谢 谢nullnull高斯（gauss, G） Gauss (1777-1855)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德国著名数学家，于1832年首次测量了地球的磁场。5安培1厘米1高斯null地球的磁场强度分布图null特斯拉（Tesla,T） Nikola Tesla (1857-1943), 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 FieldsGeneral 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 performed 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 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 EffectsTemperature 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 EffectsElectrical 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 cardiac-gating 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 EffectsNeurological 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 whole-body 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 ConsiderationsCryogen 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 QuenchElectrical 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 FIELDSBIOEFFECTS 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