磁共振成像的安全性

对磁共振成像(MRI)的安全性进行了综述,主要涉及五个方面:静磁场、梯度场、射频场、噪声和造影剂.在没有铁磁性外源性物质的条件下,静磁场对人体没有明显的损害,有较高的安全系数.随时间变化的梯度场(dB/dt)可在受试者体内诱导出电场而兴奋神经或肌肉.当梯度上升时间只有数毫秒时,外周神经兴奋是梯度场安全的上限指标.在MRI测定过程中,射频场发射的功率在患者组织内转化成热能,使组织温度升高.MRI运行过程中可产生各种噪声,可能使某些患者的听力受到损伤,使用耳塞仍是削弱噪声最简单和最经济的方法.目前使用的造影剂主要为含钆的化合物,副作用发生率在2%~4%.

SAFETY OF MAGNETIC RESONANCE IMAGING

Safety of MRI for both patients and employees is reviewed. The safety concerns and considerations of exposure to static magnetic fields, radio frequency fields, switching magnetic field gradients , acoustic noise and image enhanced contrast agents are described. In the absence of ferromagnetic foreign bodies, there is no replicated scientific study showing a health hazard associated with static magnetic field exposure and no evidence for hazards associated with cumulative exposure to the field. In magnetic resonance, time varying gradient magnetic fields (d B /d t ) may stimulate nerves or muscles by inducing electric fields in patients. For gradient ramp duration of less than a few milliseconds, mean peripheral nerve stimulation is a safe indicator of high d B /d t . During an MR procedure, most of the transmitted RF power is transformed into heat within the patient's tissue as a result of resistive losses, and the primary bioeffects associated with the RF radiation are directly related to the thermogenic qualities of this electromagnetic field. During the operation of MR systems, various types of acoustic noise produced, and they may produce substantial hearing problems in patients. The simplest and least expensive means of preventing problems associated with acoustic noise is to encourage the routine use of earplugs. The total incidence of adverse reactions of Gadolinium based MR imaging contrast agents ranges from approximately 2% to 4%, cautions should be taken during MRI measurement.