最新MR安全性和禁忌癥課件

MR安全性和禁忌癥MR安全性和禁忌癥設(shè)備鑒別3種磁體。認(rèn)識(shí)包圍超導(dǎo)磁體的多層結(jié)構(gòu)。認(rèn)識(shí)MR室的所有設(shè)備。認(rèn)識(shí)表面線圈對(duì)SNR的作用。設(shè)備鑒別3種磁體。最新MR安全性和禁忌癥課件最新MR安全性和禁忌癥課件最新MR安全性和禁忌癥課件最新MR安全性和禁忌癥課件最新MR安全性和禁忌癥課件最新MR安全性和禁忌癥課件超導(dǎo)型磁體在超導(dǎo)材料內(nèi)流動(dòng)的電流感應(yīng)產(chǎn)生磁場(chǎng)的磁體。這種磁體必須被包圍在制冷設(shè)備中。水平磁場(chǎng)需要直流電超導(dǎo)線由鈮鈦合金制成，浸泡在液氦（絕對(duì)零度4.2K或270）中去除電阻?？僧a(chǎn)生高磁場(chǎng)強(qiáng)度(FDA–4.0T)高SNR,掃描時(shí)間短，空間分辨率高超導(dǎo)型磁體在超導(dǎo)材料內(nèi)流動(dòng)的電流感應(yīng)產(chǎn)生磁場(chǎng)的磁體。水掃描間計(jì)算機(jī)室操作間（控制間）磁體勻場(chǎng)線圈梯度線圈RF線圈發(fā)射&接收X,Y,Z梯度浸泡在制冷劑中的超導(dǎo)線圈RF&梯度放大器RF和梯度脈沖編程器RF探測(cè)器/數(shù)字轉(zhuǎn)換器電源(PDU)患者床計(jì)算機(jī)照相機(jī)采集和顯示控制顯示器儲(chǔ)存設(shè)備鍵盤(pán)工作站六面RF屏蔽MRSiteLayout掃描間計(jì)算機(jī)室操作間（控制間）磁體勻場(chǎng)線圈梯度線圈RF線圈

梯度帶電線圈，產(chǎn)生在某一個(gè)方向上變化的磁場(chǎng)。對(duì)數(shù)據(jù)進(jìn)行空間編碼。在3個(gè)方向上產(chǎn)生圖像。梯度幅度:每距離單位的磁場(chǎng)變化(mT/m)。梯度切換率：梯度性能的表示方法。梯度幅度除以梯度爬升時(shí)間(T/m/s)。梯度帶電線圈，產(chǎn)生在某一個(gè)方向上變化的磁場(chǎng)。對(duì)數(shù)層面選擇梯度相位編碼梯度頻率編碼或讀出梯度梯度層面選擇梯度梯度梯度線圈

Z軸梯度梯度線圈

Z軸梯度X和Y梯度線圈xzyx–梯度y–梯度X和Y梯度線圈xzyx–梯度y–梯度梯度通過(guò)輕微改變磁場(chǎng)強(qiáng)度來(lái)加快或減慢質(zhì)子的進(jìn)動(dòng)頻率。用于選層或?qū)邮盏降男盘?hào)進(jìn)行空間定位。梯度34012-1-2B0梯度通過(guò)輕微改變磁場(chǎng)強(qiáng)度來(lái)加快或減慢質(zhì)子的進(jìn)動(dòng)頻率。RF系統(tǒng)RF系統(tǒng)產(chǎn)生能量使質(zhì)子共振，并接收質(zhì)子釋放的能量。RF系統(tǒng)包括下列組件：組成射頻放大器射頻通道脈沖線圈發(fā)射線圈接收線圈作用（如同天線）-激發(fā)人體產(chǎn)生共振（廣播電臺(tái)的發(fā)射天線）-采集MR信號(hào)（收音機(jī)的天線）RF系統(tǒng)RF系統(tǒng)產(chǎn)生能量使質(zhì)子共振，并接收質(zhì)子釋放的能量。最新MR安全性和禁忌癥課件表面線圈表面線圈可放置在感興趣解剖部位表面，增加小范圍薄層掃描的SNR,同時(shí)減少來(lái)自FOV外的噪音，使MR圖像的SNR得到很大的改善。SNR和線圈半徑成反比。表面線圈只能接收信號(hào)。使用表面線圈時(shí)體線圈用來(lái)發(fā)射RF脈沖。發(fā)射/接收線圈(i.e.肢體線圈)表面線圈表面線圈可放置在感興趣解剖部位表面，增加小范圍薄層掃RF線圈頭線圈肢體線圈RF線圈頭線圈肢體線圈相控陣線圈腕關(guān)節(jié)相控陣線圈相控陣線圈腕關(guān)節(jié)相控陣線圈表面線圈肩關(guān)節(jié)相控陣線圈線性肩關(guān)節(jié)線圈表面線圈肩關(guān)節(jié)相控陣線圈線性肩關(guān)節(jié)線圈相控陣線圈心臟線圈相控陣線圈心臟線圈相控陣線圈周?chē)芫€圈相控陣線圈周?chē)芫€圈神經(jīng)血管相控陣線圈相控陣線圈神經(jīng)血管相控陣線圈相控陣線圈乳腺相控陣線圈相控陣線圈乳腺相控陣線圈相控陣線圈安全認(rèn)識(shí)MR對(duì)患者的損傷。認(rèn)識(shí)MR技師可采用哪些方法減輕對(duì)患者的損傷。Safeexamination安全認(rèn)識(shí)MR對(duì)患者的損傷金屬鐵磁性:和主磁場(chǎng)輕微反向

金 銅鋅 水銀順磁性:反磁性:輕微被主磁場(chǎng)吸引

銥 錳 鈦 釓 鉑被主磁場(chǎng)吸引： 鐵 鎳 鈷 一些合金金屬鐵磁性:和主磁場(chǎng)輕微反向順磁性:反磁性:輕微被Safety對(duì)患者的損傷聽(tīng)力損傷金屬面部和眼部起搏器內(nèi)部損傷RF加熱電纜和線圈ScreeningFormSafety對(duì)患者的損傷ScreeningForm聽(tīng)力Safety聽(tīng)力Safety金屬Safety金屬Safety面部和眼部Safety面部和眼部Safety起搏器Safety起搏器Safety內(nèi)部損傷Safety內(nèi)部損傷SafetyRF加熱SafetyRF加熱Safety電纜和線圈Safety電纜和線圈Safety不要將金屬帶進(jìn)掃描間!安全不要將金屬帶進(jìn)掃描間!安全MR技師在允許任何人（不僅僅是患者）進(jìn)入掃描間前都要篩查嚴(yán)防任何禁忌發(fā)生的可能性！安全YOU!MR技師在允許任何人安全YOU!磁共振成像的安全性鐵磁性投射物體內(nèi)植入物梯度場(chǎng)噪聲孕婦的MRI檢查不良心理反應(yīng)及其預(yù)防磁共振成像的安全性鐵磁性投射物鐵磁性投射物投射效應(yīng)是在強(qiáng)磁場(chǎng)作用下鐵磁性物體從磁體以外的地方以一定的速度投向磁體的現(xiàn)象，是磁體強(qiáng)大吸引力的外在表現(xiàn)。鐵磁性投射物既可以是縫衣針、別針、螺絲刀、扳手等小物體，也可能是氧氣瓶、吸塵器、工具箱等大物體。投射效應(yīng)是MRI系統(tǒng)最大的安全問(wèn)題之一。有必要在磁體室入口處安裝可調(diào)閾值的金屬探測(cè)器。鐵磁性投射物投射效應(yīng)是在強(qiáng)磁場(chǎng)作用下鐵磁性物體從磁體以外的地常見(jiàn)鐵磁性投射物典型的鐵磁性投射物含有鐵的成分，但鎳和鈷等元素也具有較強(qiáng)的鐵磁性。非鐵磁性物品雖然不產(chǎn)生投射效應(yīng)，卻能形成金屬偽影而干擾圖像。外科手術(shù)器械、氧氣瓶、醫(yī)療儀器、擔(dān)架、輪椅等；小刀、金屬拉鏈、鈕扣、指甲刀、鋼筆、鑰匙、硬幣、手表、打火機(jī)、手機(jī)、助聽(tīng)器等。MRI室應(yīng)建立一整套安全防范措施。常見(jiàn)鐵磁性投射物典型的鐵磁性投射物含有鐵的成分，但鎳和鈷等元磁共振成像的安全性鐵磁性投射物體內(nèi)植入物梯度場(chǎng)噪聲孕婦的MRI檢查不良心理反應(yīng)及其預(yù)防磁共振成像的安全性鐵磁性投射物體內(nèi)植入物MRI受檢者體內(nèi)的各種鐵磁性物體會(huì)在磁力和磁扭矩的作用下發(fā)生移位或傾斜。MRI的射頻電磁波有可能使植入體內(nèi)的某些電子設(shè)備失靈。體內(nèi)植入物MRI受檢者體內(nèi)的各種鐵磁性物體會(huì)在磁力和磁扭矩的體內(nèi)植入物通過(guò)各種渠道置入體內(nèi)并長(zhǎng)期駐留體內(nèi)的異物。彈片、鐵砂、假牙、動(dòng)脈夾、人工股骨頭、起搏器、人工心臟瓣膜、電子耳蝸、藥物泵、避孕環(huán)等是最常見(jiàn)的體內(nèi)植入物。非鐵磁性植入物患者可接受MRI檢查，但會(huì)產(chǎn)生嚴(yán)重的金屬偽影；鐵磁性植入物患者一般來(lái)說(shuō)不宜接受MRI檢查。研究表明，大約1/3的體內(nèi)植入物將在靜磁場(chǎng)中發(fā)生偏倚或移位，但不見(jiàn)得把所有鐵磁性植入物都看作MRI禁忌癥。體內(nèi)植入物通過(guò)各種渠道置入體內(nèi)并長(zhǎng)期駐留體內(nèi)的異物。彈片、鐵體內(nèi)植入物的安全性MRI對(duì)鐵磁性體內(nèi)植入物的影響主要表現(xiàn)在以下幾個(gè)方面：位置變化；功能紊亂；局部升溫。強(qiáng)磁場(chǎng)可使腦動(dòng)脈瘤治療中放置的動(dòng)脈夾移動(dòng)甚至脫落；靜磁場(chǎng)和RF場(chǎng)都可能干擾人工心臟起搏器使其失效或停搏。體內(nèi)植入物的安全性MRI對(duì)鐵磁性體內(nèi)植入物的影響主要表現(xiàn)在以金屬異物的預(yù)檢查體內(nèi)可能存留諸如彈片、金屬屑、鐵砂等金屬碎片患者的危險(xiǎn)性決定于它們?cè)隗w內(nèi)的位置。眼內(nèi)的金屬異物被拉出時(shí)容易造成傷害，已經(jīng)有眼內(nèi)金屬異物致盲的報(bào)告。透視或拍片是對(duì)金屬異物進(jìn)行預(yù)檢查的一種既敏感又廉價(jià)的方法，在X線片上可發(fā)現(xiàn)小到0.1mm的金屬異物。金屬異物的預(yù)檢查體內(nèi)可能存留諸如彈片、金屬屑、鐵砂等金屬碎片磁共振成像的安全性鐵磁性投射物體內(nèi)植入物梯度場(chǎng)噪聲孕婦的MRI檢查不良心理反應(yīng)及其預(yù)防磁共振成像的安全性鐵磁性投射物梯度場(chǎng)噪聲MRI裝置的音頻噪聲可分為靜態(tài)及動(dòng)態(tài)兩種。靜態(tài)噪聲是由于磁體冷卻系統(tǒng)即冷頭的工作而引起的噪聲，一般比較小。動(dòng)態(tài)噪聲即梯度場(chǎng)噪聲，指掃描過(guò)程中由梯度場(chǎng)的不斷開(kāi)啟或關(guān)閉而形成的。由于的主磁場(chǎng)的存在，梯度線圈工作時(shí)將產(chǎn)生很強(qiáng)的洛侖茲力，使線圈載體在梯度場(chǎng)轉(zhuǎn)換期間發(fā)生劇烈振蕩，從而產(chǎn)生掃描時(shí)的特殊噪聲。梯度場(chǎng)噪聲MRI裝置的音頻噪聲可分為靜態(tài)及動(dòng)態(tài)兩種。梯度場(chǎng)噪聲系統(tǒng)的靜磁場(chǎng)越高、梯度上升速度越快或梯度脈沖的頻率越高，它發(fā)出的噪聲就會(huì)越大。1.0~2.0T時(shí)，梯度場(chǎng)達(dá)到25mT/m時(shí)，噪聲可高達(dá)110dB。心理傷害是可誘發(fā)癲癇和幽閉恐懼癥。生理傷害是暫時(shí)性聽(tīng)力下降或永久性聽(tīng)力損害。梯度場(chǎng)噪聲系統(tǒng)的靜磁場(chǎng)越高、梯度上升速度越快或梯度脈沖的頻率磁共振成像的安全性鐵磁性投射物體內(nèi)植入物梯度場(chǎng)噪聲孕婦的MRI檢查不良心理反應(yīng)及其預(yù)防磁共振成像的安全性鐵磁性投射物孕婦的MRI檢查MRI是否有致畸作用一直是一個(gè)有爭(zhēng)議的話題。建議“在妊娠的頭3個(gè)月謹(jǐn)慎應(yīng)用”MRI檢查。孕期的工作人員對(duì)MRI電磁場(chǎng)的接觸也應(yīng)受到限制。一般來(lái)說(shuō)，活動(dòng)范圍要盡量在1mT線（10高斯線）以外，以避免接受MRI產(chǎn)生的小劑量慢性輻射。孕婦的MRI檢查MRI是否有致畸作用一直是一個(gè)有爭(zhēng)議的話題。磁共振成像的安全性鐵磁性投射物體內(nèi)植入物梯度場(chǎng)噪聲孕婦的MRI檢查不良心理反應(yīng)及其預(yù)防磁共振成像的安全性鐵磁性投射物不良心理反應(yīng)及其預(yù)防MRI檢查中，由于磁體孔洞比較狹小，加之梯度場(chǎng)噪聲的干擾，患者可能出現(xiàn)焦慮、恐慌或情緒低落等心理反應(yīng)，甚至誘發(fā)幽閉恐懼癥。需要采取以下措施來(lái)降低其發(fā)生率：事先向患者講解MRI檢查的特殊性，如磁體孔洞的大小及梯度場(chǎng)的噪聲水平等；允許被檢者的親屬或朋友進(jìn)入磁體室陪同；不良心理反應(yīng)及其預(yù)防MRI檢查中，由于磁體孔洞比較狹小，加之不良心理反應(yīng)及其預(yù)防改變體位：仰臥位改為俯臥位、頭先進(jìn)改為腳先進(jìn)；提供MRI兼容耳機(jī)并播放音樂(lè)；在磁體孔洞內(nèi)設(shè)置鏡片或反光鏡，分散病人注意力；掃描中同病人保持對(duì)講等某種類型的通訊聯(lián)系。不良心理反應(yīng)及其預(yù)防改變體位：仰臥位改為俯臥位、頭先進(jìn)改為腳磁共振成像系統(tǒng)的生物效應(yīng)靜磁場(chǎng)的生物效應(yīng)梯度磁場(chǎng)的生物效應(yīng)射頻場(chǎng)的生物效應(yīng)磁共振成像系統(tǒng)的生物效應(yīng)靜磁場(chǎng)的生物效應(yīng)磁共振成像系統(tǒng)的生物效應(yīng)MRI檢查中，受檢者受到靜磁場(chǎng)、梯度磁場(chǎng)和射頻磁場(chǎng)的輻射。理論上講，任一種磁場(chǎng)都將產(chǎn)生相關(guān)的生物效應(yīng)。目前，諸多研究還不能得出MRI對(duì)機(jī)體存在潛在危害的結(jié)論。磁共振成像系統(tǒng)的生物效應(yīng)MRI檢查中，受檢者受到靜磁場(chǎng)、梯度磁共振成像系統(tǒng)的生物效應(yīng)近20年來(lái)，MRI技術(shù)得到飛速發(fā)展，超導(dǎo)技術(shù)、磁體技術(shù)、低溫技術(shù)、電子技術(shù)和計(jì)算機(jī)等相關(guān)技術(shù)的最新成果均在MRI中得到應(yīng)用。但是，MRI的生物效應(yīng)研究卻大大滯后，原因如下。磁共振成像系統(tǒng)的生物效應(yīng)近20年來(lái)，MRI技術(shù)得到飛速發(fā)展，磁共振成像系統(tǒng)的生物效應(yīng)生物效應(yīng)研究的難度大。三種磁場(chǎng)的復(fù)合作用結(jié)果很難評(píng)價(jià)，動(dòng)物模型與人體的差異較大。生物效應(yīng)的影響因素多。三種磁場(chǎng)的影響因素都很多。MRI系統(tǒng)千差萬(wàn)別。每一型號(hào)都需要相當(dāng)長(zhǎng)的時(shí)間來(lái)積累研究資料或臨床數(shù)據(jù)。硬件發(fā)展過(guò)快，許多新技術(shù)的生物效應(yīng)尚未開(kāi)始評(píng)價(jià)就已在臨床應(yīng)用。磁共振成像系統(tǒng)的生物效應(yīng)生物效應(yīng)研究的難度大。三種磁場(chǎng)的復(fù)合磁共振成像系統(tǒng)的生物效應(yīng)目前的觀察資料（僅限于1.5T以下的場(chǎng)強(qiáng)）中可以得到這樣的結(jié)論：常規(guī)MRI成像不會(huì)給人類健康造成任何有臨床意義的威脅，它對(duì)人體健康的影響遠(yuǎn)遠(yuǎn)小于X射線CT。MRI是安全的。生物效應(yīng)的存在又是肯定的，有必要深入地進(jìn)行評(píng)價(jià)。磁共振成像系統(tǒng)的生物效應(yīng)目前的觀察資料（僅限于1.5T以下的磁共振成像系統(tǒng)的生物效應(yīng)靜磁場(chǎng)的生物效應(yīng)梯度磁場(chǎng)的生物效應(yīng)射頻場(chǎng)的生物效應(yīng)磁共振成像系統(tǒng)的生物效應(yīng)靜磁場(chǎng)的生物效應(yīng)靜磁場(chǎng)的生物效應(yīng)隨著超導(dǎo)磁體技術(shù)的日益成熟，場(chǎng)強(qiáng)有不斷提高的趨勢(shì)。靜磁場(chǎng)對(duì)生物體的影響至今沒(méi)有完全闡明，表明超高場(chǎng)（2T以上）對(duì)人體影響的資料就更少。FDA明確規(guī)定，因場(chǎng)強(qiáng)超過(guò)規(guī)定限值而造成的一切后果由MRI制造商承擔(dān)。靜磁場(chǎng)的生物效應(yīng)隨著超導(dǎo)磁體技術(shù)的日益成熟，場(chǎng)強(qiáng)有不斷提高的溫度效應(yīng)MRI出現(xiàn)后最早受到關(guān)注的生物效應(yīng)之一。多年來(lái)，出現(xiàn)過(guò)磁體使體溫升高、磁場(chǎng)不影響體溫甚至磁場(chǎng)使身體某部位的體溫下降等多種觀點(diǎn)?，F(xiàn)已證明，靜磁場(chǎng)對(duì)人的體溫不產(chǎn)生影響。溫度效應(yīng)MRI出現(xiàn)后最早受到關(guān)注的生物效應(yīng)之一。磁流體動(dòng)力學(xué)效應(yīng)磁場(chǎng)中的血流以及其他流動(dòng)液體產(chǎn)生的生物效應(yīng)。靜磁場(chǎng)能使紅細(xì)胞的沉積速度加快、心電圖發(fā)生改變，并有可能感應(yīng)出生物電位。場(chǎng)強(qiáng)對(duì)ECG的影響不是非常明顯。磁流體動(dòng)力學(xué)效應(yīng)磁場(chǎng)中的血流以及其他流動(dòng)液體產(chǎn)生的生物效應(yīng)。中樞神經(jīng)系統(tǒng)效應(yīng)磁場(chǎng)有可能引起神經(jīng)活動(dòng)的誤傳導(dǎo)。目前公認(rèn)，短期的暴露在2.0T以下的靜磁場(chǎng)對(duì)人的中樞神經(jīng)系統(tǒng)沒(méi)有明顯不良影響。但在4.0T以上的MRI系統(tǒng)中，大多數(shù)志愿者出現(xiàn)眩暈、惡心、頭痛、口中有異味等主觀感覺(jué)，顯然超高磁體可導(dǎo)致人體某種顯著的生理變化。中樞神經(jīng)系統(tǒng)效應(yīng)磁場(chǎng)有可能引起神經(jīng)活動(dòng)的誤傳導(dǎo)。磁共振成像系統(tǒng)的生物效應(yīng)靜磁場(chǎng)的生物效應(yīng)梯度磁場(chǎng)的生物效應(yīng)射頻場(chǎng)的生物效應(yīng)磁共振成像系統(tǒng)的生物效應(yīng)靜磁場(chǎng)的生物效應(yīng)梯度場(chǎng)及其感應(yīng)電流梯度磁場(chǎng)是一種時(shí)變場(chǎng)，變化的磁場(chǎng)在導(dǎo)體中將感應(yīng)出電流。感應(yīng)電流在人體內(nèi)部構(gòu)成回路。感應(yīng)電流的大小與梯度場(chǎng)的切換率、最大磁通強(qiáng)度（梯度場(chǎng)強(qiáng)度）、平均磁通強(qiáng)度、諧波頻率、波形參數(shù)、脈沖極性、體內(nèi)電流分布等諸多因素均有關(guān)系。靜磁場(chǎng)中運(yùn)動(dòng)的導(dǎo)電物體也會(huì)產(chǎn)生電流，病人被送入磁體的過(guò)程中體內(nèi)有感生電流出現(xiàn)。梯度場(chǎng)及其感應(yīng)電流梯度磁場(chǎng)是一種時(shí)變場(chǎng)，變化的磁場(chǎng)在導(dǎo)體中將梯度場(chǎng)的心血管效應(yīng)強(qiáng)電流對(duì)心血管系統(tǒng)的作用為直接刺激血管和心肌纖維等電敏感細(xì)胞。引起心律不起、心室或心房纖顫等。一般將皮膚（感覺(jué)）神經(jīng)或外周骨骼肌神經(jīng)受到刺激（抽搐或收縮）看作心律不齊或心室纖顫出現(xiàn)的先兆。梯度場(chǎng)的心血管效應(yīng)強(qiáng)電流對(duì)心血管系統(tǒng)的作用為直接刺激血管和心磁致光幻視又叫光幻視或磁幻視，是在梯度場(chǎng)作用下眼前出現(xiàn)閃光感或色環(huán)的現(xiàn)象。電刺激視網(wǎng)膜感光細(xì)胞后形成的視覺(jué)紊亂，是梯度場(chǎng)最敏感的生理反應(yīng)之一。光幻視與梯度場(chǎng)變化率和靜磁場(chǎng)強(qiáng)度均有關(guān)系，且在梯度場(chǎng)停止后自動(dòng)消失，1.5T和20T/s以下不出現(xiàn)這種幻覺(jué)，但在4T中20~40Hz時(shí)很容易使正常人產(chǎn)生磁幻視。磁致光幻視又叫光幻視或磁幻視，是在梯度場(chǎng)作用下眼前出現(xiàn)閃光感磁共振成像系統(tǒng)的生物效應(yīng)靜磁場(chǎng)的生物效應(yīng)梯度磁場(chǎng)的生物效應(yīng)射頻場(chǎng)的生物效應(yīng)磁共振成像系統(tǒng)的生物效應(yīng)靜磁場(chǎng)的生物效應(yīng)射頻能量的特殊吸收率人體受到電磁波照射時(shí)將其能量轉(zhuǎn)換為熱。MRI掃描時(shí)RF激勵(lì)波的功率將全部或大部被人體所吸收，其生物效應(yīng)主要是體溫的變化。SAR（specificabsorptionrate）指單位重量生物組織中RF功率的吸收量，是對(duì)組織中電磁能量吸收值或RF功率沉積值的度量。由局部和全身SAR之分，分別對(duì)應(yīng)于局部組織和全身組織平均的射頻功率吸收量。射頻能量的特殊吸收率人體受到電磁波照射時(shí)將其能量轉(zhuǎn)換為熱。M射頻能量的特殊吸收率在MRI中，SAR的大小與共振頻率（靜磁場(chǎng)強(qiáng)度）、RF脈沖的類型（90或180

）、重復(fù)時(shí)間和脈寬、線圈效率、成像組織容積、組織類型（電特性）、解剖結(jié)構(gòu)等許多因素有關(guān)。RF場(chǎng)最主要的生物效應(yīng)是溫度效應(yīng)，但RF照射引起的實(shí)際組織溫升還決定于照射時(shí)間、環(huán)境溫度以及被檢者自身的溫度調(diào)節(jié)功能。射頻能量的特殊吸收率在MRI中，SAR的大小與共振頻率（靜磁射頻能量的特殊吸收率美國(guó)國(guó)家標(biāo)準(zhǔn)協(xié)會(huì)和FDA規(guī)定：接受連續(xù)電磁波輻射時(shí)，全身平均SAR不能超過(guò)0.4W/kg，或每克組織的SAR空間峰值不超過(guò)8.0W/kg。射頻能量的特殊吸收率美國(guó)國(guó)家標(biāo)準(zhǔn)協(xié)會(huì)和FDA規(guī)定：接受連續(xù)電射頻場(chǎng)對(duì)體溫的影響靜磁場(chǎng)與體溫?zé)o關(guān)，MRI檢查時(shí)病人體溫的變化完全是射頻場(chǎng)作用的結(jié)果。MRI掃描可導(dǎo)致溫度的顯著升高，但有人認(rèn)為此升高不構(gòu)成臨床有害影響。射頻場(chǎng)對(duì)體溫的影響靜磁場(chǎng)與體溫?zé)o關(guān)，MRI檢查時(shí)病人體溫的變射頻場(chǎng)最易損傷的器官人體中散熱功能不好的器官，如睪丸、眼等對(duì)溫度的升高非常敏感，這些部位是最容易受MRI射頻輻射損傷的部位。過(guò)量電磁輻射可能導(dǎo)致患者暫時(shí)甚至永久不育和白內(nèi)障，但有人認(rèn)為臨床MRI成像一般不會(huì)造成眼組織的熱損傷。高SAR的MRI檢查或長(zhǎng)時(shí)間的MRI檢查所致熱效應(yīng)是一個(gè)需要進(jìn)一步研究的課題。射頻場(chǎng)最易損傷的器官人體中散熱功能不好的器官，如睪丸、眼等對(duì)禁忌證有心臟起搏器的患者。手術(shù)后動(dòng)脈夾存留患者。鐵磁性異物患者，如體內(nèi)存留有彈片、眼內(nèi)存留有金屬異物等。換有人工金屬心臟瓣膜患者。金屬假肢、金屬關(guān)節(jié)患者。體內(nèi)置有胰島素泵或神經(jīng)刺激器者。妊娠不足3個(gè)月。

以上各項(xiàng)有疑問(wèn)有患者要進(jìn)行調(diào)研，弄清情況，再?zèng)Q定是否做MRI檢查。否則應(yīng)謝絕做此項(xiàng)檢查。

禁忌證有心臟起搏器的患者。磁共振檢查前的準(zhǔn)備磁共振檢查前的準(zhǔn)備應(yīng)包括以下8個(gè)方面：接診時(shí)核對(duì)資料、病史、明確檢查目的和要求。確認(rèn)無(wú)禁忌證后，發(fā)給預(yù)約單，其內(nèi)容為MR宣傳資料，囑患者認(rèn)真閱讀。對(duì)腹部盆腔部位檢查者，檢查當(dāng)日早晨控制小量進(jìn)食水。置有金屬避孕環(huán)患者，囑取環(huán)后再行檢查。磁共振檢查前的準(zhǔn)備磁共振檢查前的準(zhǔn)備應(yīng)包括以下8個(gè)方面：磁共振檢查前的準(zhǔn)備對(duì)預(yù)約檢查登記患者，要核對(duì)資料、登記建檔，并詢問(wèn)是否做過(guò)MRI及CT檢查。有“老號(hào)”者，認(rèn)真查找老片，以利于對(duì)比。進(jìn)入MR室前應(yīng)囑患者除去攜帶的一切金屬物品、磁性物品及電子元件，以免引起偽影，傷害患者。對(duì)于體內(nèi)有金屬異物及安裝心臟起搏器者禁止檢查，以防發(fā)生意外。消除患者恐懼心理，爭(zhēng)取患者密切配合與合作。磁共振檢查前的準(zhǔn)備對(duì)預(yù)約檢查登記患者，要核對(duì)資料、登記建檔，磁共振檢查前的準(zhǔn)備對(duì)嬰兒及躁動(dòng)患者，應(yīng)在臨床醫(yī)師指導(dǎo)下適當(dāng)給予鎮(zhèn)靜處理。對(duì)于危重患者，除早期腦梗塞患者外，原則上不做MR檢查，如果特別需要，一必須檢查，應(yīng)由有經(jīng)驗(yàn)的臨床醫(yī)師陪同。備齊搶救器械和藥品，并向臨床醫(yī)師說(shuō)明發(fā)生意外不能在機(jī)器房?jī)?nèi)搶救。磁共振檢查前的準(zhǔn)備對(duì)嬰兒及躁動(dòng)患者，應(yīng)在臨床醫(yī)師指導(dǎo)下適當(dāng)給謝謝謝謝最新MR安全性和禁忌癥課件高斯（gauss,G）

Gauss(1777-1855)1高斯為距離5安培電流的直導(dǎo)線1厘米處檢測(cè)到的磁場(chǎng)強(qiáng)度德國(guó)著名數(shù)學(xué)家，于1832年首次測(cè)量了地球的磁場(chǎng)。5安培1厘米1高斯高斯（gauss,G）Gauss(1777-1855)地球的磁場(chǎng)強(qiáng)度分布圖地球的磁場(chǎng)強(qiáng)度分布圖特斯拉（Tesla,T）NikolaTesla(1857-1943),奧地利電器工程師，物理學(xué)家，旋轉(zhuǎn)磁場(chǎng)原理及其應(yīng)用的先驅(qū)者之一。1T=10000G特斯拉（Tesla,T）1T=10000GGeneralBioeffectsofStaticMagneticFieldsThereisapaucityofdataconcerningtheeffectsofhigh-intensitystaticmagneticfieldsonhumans.SomeoftheoriginalinvestigationsonhumansubjectsexposedtostaticagneticfieldswereperformedbyVyalov,227,228whostudiedworkersinvolvedintheermanent-magnetindustry.Thesesubjectswereexposedtostaticmagneticfieldsrangingfrom0.0015to0.35Tesla(T)andreportedfeelingsofheadache,chestpain,fatigue,vertigo,lossofappetite,insomnia,itching,andother,morenonspecificailments.227,228However,exposuretootherpotentiallyhazardousenvironmentalworkingconditions(elevatedroomtemperature,airbornemetallicdust,chemicals)mayhavebeenpartiallyesponsibleforthereportedsymptomsinthesestudysubjects.Andbecausethisinvestigationlackedanappropriatecontrolgroup,itisdifficulttoascertainwhethertherewasadefinitecorrelationbetweentheexposuretothestaticmagneticfieldandthereportedabnormalities.Subsequentstudiesperformedwithmorescientificrigorhavenotsubstantiatedmanyoftheaforementionedfindings.GeneralBioeffectsofStaticMTemperatureEffectsThereareconflictingstatementsintheliteratureregardingtheeffectofstaticmagneticfieldsonthebodyandtheskintemperaturesofmammals.Reportshavevariouslyindicatedthatstaticmagneticfieldseitherincreaseorbothincreaseanddecreasetissuetemperature,dependingontheorientationoftheorganisminthestaticmagneticfield.72,203Otherarticlesstatethatstaticmagneticfieldshavenoeffectontheskinandthebodytemperaturesofmammals.Noneoftheinvestigatorswhoidentifiedastaticmagneticfieldeffectontemperaturesproposedaplausiblemechanismforthisresponse,norhasthisworkbeensubstantiated.Inaddition,studiesthatreportedstaticmagneticfield–inducedskinand/orbodytemperaturechangesusedeitherlaboratoryanimalsknowntohavelabiletemperaturesorinstrumentationthatmayhavebeenaffectedbythestaticmagneticfields.72,203Arecentinvestigationindicatedthatexposuretoa1.5Tstaticmagneticfielddoesnotaltertheskinandthebodytemperaturesinhumanbeings.213Thisstudywasperformedbyusingaspecialfluoropticthermometrysystemdemonstratedtobeunperturbedbyhigh-intensitystaticmagneticfields;thereforetheskinandthebodytemperaturesofhumansubjectsarebelievedtobeunaffectedbyexposuretostaticmagneticfieldsofupto1.5T.TemperatureEffectsElectricalInductionandCardiacEffectsInducedbiopotentialsmaybeobservedduringexposuretostaticmagneticfieldsandarecausedbyblood—aconductivefluid—flowingthroughamagneticfield.InducedbiopotentialsareexhibitedbyanaugmentationofT-waveamplitudeandbyother,nonspecificwaveformchangesontheelectrocardiogram(ECG).Theyhavebeenobservedatstaticmagneticfieldstrengthsaslowas0.1T.11,15,214TheincreaseinT-waveamplitudeisdirectlyrelatedtotheintensityofthestaticmagneticfield,suchthatatlowstaticmagneticfieldstrengthstheeffectsarenotaspredominantasthoseathigherfieldstrengths.ThemostmarkedeffectontheTwaveisthoughttobecausedwhenthebloodflowsthroughthethoracicaorticarch.ThisT-waveamplitudechangecanbesignificantenoughtofalselytriggertheRFexcitationduringacardiac-gatedMRexamination.OtherportionsoftheECGalsomaybealteredbythestaticmagneticfield,andthisvarieswiththeplacementoftherecordingelectrodes.Alternateleadpositionscanbeusedtoattenuatethestaticmagneticfield–inducedECGchangestofacilitatecardiac-gatingstudies.43Oncethepatientisnolongerexposedtothestaticmagneticfield,theseECGvoltageabnormalitiesreverttonormal.BecausenocirculatoryalterationsappeartocoincidewiththeseECGchanges,nobiologicalrisksarebelievedtobeassociatedwiththemagnetohydrodynamiceffectthatoccursinconjunctionwithstaticmagneticfieldstrengthsofupto2T.ElectricalInductionandCardiNeurologicalEffectsTheoretically,electricalimpulseconductioninnervetissuemaybeaffectedbyexposuretostaticmagneticfields;however,thisisanareainthebioeffectsliteraturethatcontainscontradictoryinformation.Somestudieshavereportedremarkableeffectsonboththefunctionandthestructureofthoseportionsofthecentralnervoussystemassociatedwithexposuretostaticmagneticfields,whereasothershavefailedtoshowanysignificantchanges.*FurtherinvestigationsofpotentialunwantedbioeffectsareneededbecauseoftherelativelackofclinicalstudiesinthisfieldthataredirectlyapplicabletoMRI.Atpresent,exposuretostaticmagneticfieldsofupto2Tdoesnotappeartosignificantlyinfluencebioelectricalpropertiesofneuronsinhumans.96,177,184Insummary,thereisnoconclusiveevidenceofirreversibleorhazardousbiologicaleffectsrelatedtoacute,short-termexposureofhumanstostaticmagneticfieldsofstrengthsupto2T.However,asof1998,therewereseveral3and4Twhole-bodyMRsystemsinoperationatvariousresearchsitesaroundtheworld.Onestudyindicatedthatworkersandvolunteersubjectsexposedtoa4TMRsystemexperiencedvertigo,nausea,headaches,ametallictasteintheirmouths,andmagnetophosphenes(visualflashes).157Asaresult,considerableresearchisunderwayworldwidetostudythemechanismsresponsibleforthesebioeffectsandtodeterminepossiblemeans,ifany,tocounterbalancethem.NeurologicalEffectsCryogenConsiderationsAllsuperconductiveMRsystemsinclinicalusetodayuseliquidhelium.Liquidhelium,whichmaintainsthemagnetcoilsintheirsuperconductivestate,willachievethegaseousstate(“boiloff”)atapproximately–268.93°C(4.22°K).96Ifthetemperaturewithinthecryostatprecipitouslyrises,theheliumwillenterthegaseousstate.Insuchasituationthemarkedincreaseinvolumeofthegaseousversustheliquidcryogen(withgas-liquidvolumeratiosof760:1forheliumand695:1fornitrogen)willdramaticallyincreasethepressurewithinthecryostat.96Apressure-sensitivecarbon“pop-off”valvewillgiveway,sometimeswitharatherloudpoppingnoise,followedbytherapid(andloud)egressofgaseousheliumasitescapesfromthecryostat.Innormalsituationsthisgasshouldbeventedoutoftheimagingroomandintotheexternalatmosphere.Itispossible,however,thatduringsuchventingsomeheliumgasmightaccidentallybereleasedintotheambientatmosphereoftheimagingroom.Gaseousheliumisconsiderablylighterthanair.Ifanyheliumgasisinadvertentlyreleasedintotheimagingroom,thedimensionsoftheroom,itsventilationcapacity,andthetotalamountofgasreleasedwilldeterminewhethertheheliumgaswillreachthepatientorthehealthpractitioner,whoisinthelowerpartoftheroomnearthefloor.96Heliumvaporlookslikesteamandisodorlessandtasteless,butitmaybeextremelycold.Asphyxiationandfrostbitearepossibleifapersonisexposedtoheliumvaporforaprolongedtime.Inasystemquenchaconsiderablequantityofheliumgasmaybereleasedintotheimagingroom.Thismightmakeitdifficulttoopenthedooroftheroombecauseofthepressuredifferential.Insuchacircumstancethefirstresponseshouldbetoevacuatetheareauntiltheoffendingheliumvaporisadequatelyremovedfromtheimagingroomenvironmentandsafelyredirectedtoanoutsideenvironmentawayfrompatients,pedestrians,oranytemperature-sensitivematerial.96Bettercryostatdesignandinsulationmaterialshaveallowedtheuseofliquidheliumaloneinmanyofthenewersuperconductingmagnets.Nevertheless,agreatnumberofmagnetsinclinicalusestilluseliquidnitrogenaswell.Liquidnitrogenwithinthecryostatactsasabufferbetweentheliquidheliumandtheoutsideatmosphere,boilingoffat77.3°K.Intheeventofanaccidentalreleaseofliquidnitrogenintotheambientatmosphereoftheimagingroom,thereisapotentialforfrostbite,similartothatencounteredwithgaseousheliumrelease.Gaseousnitrogenisroughlythesamedensityasairandiscertainlymuchlessbuoyantthangaseoushelium.Intheeventofaninadvertentventingofnitrogengasintotheimagingroom,thegascouldeasilysettlenearfloorlevel;theamountofnitrogengaswithintheroomwouldcontinuetoincreaseuntilventingceased.Thetotalconcentrationofnitrogengascontainedwithintheroomwouldbedeterminedonthebasisofthetotalamountofthegasreleasedintotheroom,thedimensionsoftheroom,anditsventilationcapacity(i.e.,theexistenceandsizeofotherroutesofegress—doors,windows,ventilationducts,andfans).Apurenitrogenenvironmentisexceptionallyhazardous,andunconsciousnessgenerallyresultsassoonas5to10secafterexposure.96Itisimperativethatallpatientsandhealthpractitionersevacuatetheareaassoonasitisrecognizedthatnitrogengasisbeingreleasedintotheimagingroom.Theyshouldnotreturnuntilappropriatemeasureshavebeentakentoclearthegasfromtheroom.96Dewar(cryogenstoragecontainers)storageshouldalsobewithinawell-ventilatedarea,lestnormalboil-offratesincreasetheconcentrationofinertgaswithinthestorageroomtoadangerouslevel.71Atleastonereporteddeathhasoccurredinanindustrialsettingduringtheshipmentofcryogens,70althoughtoourknowledgenosuchfatalityhasoccurredinthemedicalcommunity.Thereisonereportofasuddenlossofconsciousnessofunexplainedcausebyanotherwisehealthytechnologist(withnopriororsubsequentsimilarepisodes)passingthroughacryogenstorageareawheremultipledewarswerelocated.4Althoughthereisnoverificationofambientatmosphericoxygenconcentrationtoconfirmanyrelationshiptothecryogensperse,thehistoryisstronglysuggestiveofsucharelationship.CryogenspresentapotentialconcerninclinicalMRIdespiteanoverwhelminglysaferecordoverthepast7ormoreyearsofclinicalservice.96Properhandlingandstorageofcryogens,aswellastheappropriatebehaviorinthepresenceofpossibleleaks,shouldbeemphasizedateachsite.Anoxygenmonitorwithanaudiblealarm,situatedatanappropriateheightwithineachimagingroom,shouldbeamandatoryminimumsafetymeasureforallsites;automaticlinkingtoandactivationofanimagingroomventilationfansystemwhentheoxygenmonitorregistersbelow18%or19%shouldbeconsideredateachmagnetinstallation.CryogenConsiderationsElectricalConsiderationsofaQuenchInadditiontothepotentialforcryogenrelease,thereisalsoaconcernaboutthecurrentsthatmaybeinducedinconductors(suchasbiologicaltissues)neartherapidlychangingmagneticfieldassociatedwithaquench.96Inonestudy,physiologicalmonitoringofapigandmonitoringoftheenvironmentwereperformedduringanintentionalquenchfrom1.76T;thereseemedtobenosignificanteffectonthebloodpressure,pulse,temperature,andelectroencephalographicandECGmeasurementsofthepigduringorimmediatelyafterthequench.41Althoughasingleobservationdoesnotprovesafetyforhumansundergoingexposuretoaquench,thedatadosuggestthattheexperiencewouldindeedbesimilar,andthattherewouldbenodeleteriouselectricaleffectsonhumansundergoingasimilarexperienceandexposure.ElectricalConsiderationsofaBIOEFFECTSOFGRADIENTMAGNETICFIELDSMRIexposesthehumanbodytorapidvariationsofmagneticfieldsasaresultofthetransientapplicationofmagneticfieldgradientsduringtheimagingsequence.Gradientmagneticfieldscaninduceelectricalfieldsandcurrentsinconductivemedia(includingbiologicaltissue)accordingtoFaraday'slawofinduction.Thepotentialforinteractionbetweengradientmagneticfieldsandbiologicaltissueisinherentlydependentonthefundamentalfieldfrequency,themaximumfluxdensity,theaveragefluxdensity,thepresenceofharmonicfrequencies,thewaveformcharacteristicsofthesignal,thepolarityofthesignal,thecurrentdistributioninthebody,andtheelectricalpropertiesandsensitivityoftheparticularcellmembrane.96,177,184Foranimalandhumansubjects,theinducedcurrentisproportionaltotheconductivityofthebiologicaltissueandtherateofchangeofthemagneticfluxdensity.18,96,161,177Intheorythelargestcurrentdensitieswillbeproducedinperipheraltissues(i.e.,atthegreatestradius)andwilllinearlydiminishtowardthebody'scenter.18,96,161,177Thecurrentdensitywillbeenhancedathigherfrequenciesandmagneticfluxdensitiesandwillbefurtheraccentuatedbyalargertissueradiuswithagreatertissueconductivity.Currentpathsareaffectedbydifferencesintissuetypes,suchthattissueswithlowconductivity(e.g.,adiposeandbone)willchangethepatternoftheinducedcurrent.Bioeffectsofinducedcurrentscanresultfromeitherthepowerdepositedbytheinducedcurrents(thermaleffects)ordirecteffectsofthecurrent(nonthermaleffects).ThermaleffectscausedbyswitchedgradientsusedinMRIarenegligibleandarenotbelievedtobeclinicallysignificant.30,96,177Possiblenonthermaleffectsofinducedcurrentsarestimulationofnerveormusclecells,inductionofventricularfibrillation,increasedbrainmannitolspace,epileptogenicpotential,stimulationofvisualflashsensations,andbonehealing.*ThethresholdcurrentsrequiredfornervestimulationandventricularfibrillationareknowntobemuchhigherthantheestimatedcurrentdensitiesthatwillbeinducedunderroutineclinicalMRconditions.30,96,161,177,184Theproductionofmagnetophosphenesisconsideredtobeoneofthemostsensitivephysiologicalresponsestogradientmagneticfields.30,96,177,184Magnetophosphenesaresupposedlycausedbyelectricalstimulationoftheretinaandarecompletelyreversiblewithnoassociatedhealtheffects.30,96,177,184Thesehavebeenelicitedbycurrentdensitiesofroughly17mA/cm2.Incontrasttothislevel,thecurrentsrequiredfortheinductionofnerveactionpotentialsisroughly3000mA/cm2,andthoserequiredforventricularfibrillationinductionofhealthycardiactissuearecalculatedtobe100to1000mA/cm2.30Althoughtoourknowledgetherehavebeennoreportedcasesofmagnetophosphenesforfieldsof1.95Torless,magnetophospheneshavebeenreportedinvolunteersworkinginandarounda4Tresearchsystem.157Inaddition,ametallictasteandsymptomsofvertigoalsoseemtobereproducibleandassociatedwithrapidmotionwithinthestaticmagneticfieldofthese4Tsystems.157Time-varying,extremelylow-frequencymagneticfieldshavebeendemonstratedtobeassociatedwithmultipleeffects,includingclusteringandalteredorientationoffibroblasts,aswellasincreasedmitoticactivityoffibroblastgrowth,alteredDNAsynthesis,andreducedfentanyl-inducedanesthesia.96,152,200Possibleeffectsinmultipleotherorganisms,includinghumans,havealsobeenmentioned.96Althoughnostudyhasconclusivelydemonstratedcarcinogeniceffectsfromexposuretotime-varyingmagneticfieldsofvariousintensitiesanddurations,severalreportssuggestthatanassociationbetweenthetwoisplausible.BIOEFFECTSOFGRADIENTMAGNETIGeneralBioeffectsofRadiofrequencyElectromagneticFieldsRFradiationiscapableofgeneratingheatintissuesasaresultofresistivelosses.ThereforethemainbioeffectsassociatedwithexposuretoRFradiationarerelatedtothethermogenicqualitiesofthiselectromagneticfield.?ExposuretoRFradiationalsomaycauseathermic,field-specificalterationsinbiologicalsystemsthatareproducedwithoutasignificantincreaseintemperature.?Thistopicissomewhatcontroversialbecauseofassertionsconcerningtheroleofelectromagneticfieldsinproducingcanceranddevelopmentalabnormalities,alongwiththeconcomitantramificationsofsucheffects.?AreportfromtheU.S.EnvironmentalProtectionAgency(EPA)claimedthattheexistingevidenceonthisissueissufficienttodemonstratearelationshipbetweenlow-levelelectromagneticfieldexposuresandthedevelopmentofcancer.144Todate,therehavebeennospecificstudiesperformedtostudypotentialathermalbioeffectsofMRI.Thoseinterestedinathoroughreviewofthistopic,particularlyasitpertainstoMRI,arereferredtotheextensivearticlewrittenbyBeers.14RegardingRF-powerdepositionconcerns,investigatorshavetypicallyquantifiedexposuretoRFradiationbymeansofdeterminingthespecificabsorptionrate(SAR).§TheSARisthemassnormalizedrateatwhichRFpoweriscoupledtobiologicaltissueandisindicatedinunitsofwattsperkilogram.MeasurementsorestimatesofSARarenottrivial,particularlyinhumansubjects,andthereareseveralmethodsofdeterminingthisparameterforRF-energydosimetry.50,65-67,119,124TheSARproducedduringMRIisacomplexfunctionofnumerousvariables:thefrequency(which,inturn,isdeterminedbythestrengthofthestaticmagneticfield),typeofRFpulse(i.e.,90or180degrees),repetitiontime,pulsewidth,typeofRFcoilused,volumeoftissuewithinthecoil,resistivityofthetissue,configurationoftheanatomicalregionimaged,andotherfactors.30,96,177,184TheactualincreaseintissuetemperaturecausedbyexposuretoRFradiationisdependentonthesubject'sthermoregulatorysystem(e.g.,skinbloodflow,skinsurfacearea,andsweatrate).96,177,184TheefficiencyandabsorptionpatternofRFenergyaredeterminedmainlybythephysicaldimensionsofthetissueinrelationtotheincidentwavelength.50,65-67,124Thereforeifthetissuesizeislargerelativetothewavelength,energyispredominantlyabsorbedonthesurface;ifitissmallrelativetothewavelength,thereislittleabsorptionofRFpower.50,65-67,124BecauseoftherelationshipbetweenRFenergyandphysicaldimensions,studiesdesignedtoinvestigatetheeffectsofexposuretoRFradiationduringMRIthatareintendedtobeapplicabletotheclinicalsettingrequiretissuevolumesandanatomicalshapescomparabletothoseofhumansubjects.Nolaboratoryanimalsufficientlymimicsorsimulatesthethermoregulatorysystemorresponsesofman.Thusresultsobtainedinlaboratoryanimalexperimentscannotsimplybe“scaled”orextrapolatedtohumansubjects.GeneralBioeffectsofRadiofreMagneticResonanceImagingandExposuretoRadiofrequencyRadiationLittlequantitative