MR安全性和禁忌癥課件

1、MRI的安全性和禁忌癥北京同仁醫(yī)院 牛延濤MRI的安全性和禁忌癥北京同仁醫(yī)院 牛延濤設(shè) 備鑒別3種磁體。認(rèn)識包圍超導(dǎo)磁體的多層結(jié)構(gòu)。認(rèn)識MR室的所有設(shè)備。認(rèn)識表面線圈對SNR的作用。設(shè) 備鑒別3種磁體。Clinical3.0T0.35T Ovation0.7T OpenSpeed1.5T TwinSpeed3.0T Whole Body0.5T InterventionalClinical3.0T0.35T Ovation0.7T Bo磁體內(nèi)孔磁體勻場線圈梯度線圈RF 線圈 RF線圈梯度線圈勻場線圈磁體脈沖序列靜磁場 Bo磁體內(nèi)孔磁體勻場線圈梯度線圈RF 線圈 RF線圈磁體種類超高場（4.07

2、.0T）；主要用于研究；高場（1.53.0T）;中場（0.51.4T）;低場（0.20.4T）;超低場（小于0.2T）。 磁體種類超高場（4.07.0T）；主要用于研究；磁體種類永磁型磁體阻抗型磁體超導(dǎo)型磁體磁體種類永磁型磁體永磁型磁體使用永磁型材料制作的磁體“開放設(shè)計”的垂直磁場。內(nèi)含塊狀或條狀天然鐵制材料。磁場強(qiáng)度 0.06T 0.35T。邊緣磁場較低。掃描間溫度會影響磁場強(qiáng)度，從而導(dǎo)致共振頻率改變。永磁型磁體使用永磁型材料制作的磁體“開放設(shè)計”的垂直磁場。阻抗型磁體（常導(dǎo)型） 磁場由普通電導(dǎo)體內(nèi)電流產(chǎn)生的磁體。電線內(nèi)電流感應(yīng)產(chǎn)生的磁場。 需使用直流電，增加電流會增加磁場強(qiáng)度并使電線產(chǎn)熱。

3、需要冷卻。不用時可以關(guān)閉。對溫度敏感。阻抗型磁體（常導(dǎo)型） 磁場由普通電導(dǎo)體內(nèi)電流產(chǎn)生的磁體。超導(dǎo)型磁體 在超導(dǎo)材料內(nèi)流動的電流感應(yīng)產(chǎn)生磁場的磁體。 這種磁體必須被包圍在制冷設(shè)備中。水平磁場需要直流電超導(dǎo)線由鈮鈦合金制成，浸泡在液氦（絕對零度4.2K或270）中去除電阻?？僧a(chǎn)生高磁場強(qiáng)度 (FDA 4.0T) 高SNR, 掃描時間短，空間分辨率高 超導(dǎo)型磁體 在超導(dǎo)材料內(nèi)流動的電流感應(yīng)產(chǎn)生磁場的磁體。水掃描間計算機(jī)室操作間（控制間）磁體勻場線圈梯度線圈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 探測器/數(shù)字轉(zhuǎn)換器電源 (

4、PDU) 患者床計算機(jī) 照相機(jī)采集和顯示控制顯示器儲存設(shè)備鍵盤工作站六面RF 屏蔽MR Site Layout掃描間計算機(jī)室操作間（控制間）磁體勻場線圈梯度線圈RF 線圈 梯 度 帶電線圈，產(chǎn)生在某一個方向上變化的磁場。對數(shù)據(jù)進(jìn)行空間編碼。在3個方向上產(chǎn)生圖像。梯度幅度:每距離單位的磁場變化 (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間 (T/m/s) 。 梯 度 帶電線圈，產(chǎn)生在某一個方向上變化的磁場。對數(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x

5、zyx 梯度y 梯度梯 度通過輕微改變磁場強(qiáng)度來加快或減慢質(zhì)子的進(jìn)動頻率。用于選層或?qū)邮盏降男盘栠M(jìn)行空間定位。梯度34012-1-2B0梯 度通過輕微改變磁場強(qiáng)度來加快或減慢質(zhì)子的進(jìn)動頻率。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（廣播電臺的發(fā)射天線） - 采集MR信號（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。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,同時減少來自FOV外的噪音，使

6、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。使用表面線圈時體線圈用來發(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圍血管線圈相控陣線圈周圍血管線圈神經(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MR對患者的

7、損傷。認(rèn)識MR技師可采用哪些方法減輕對患者的損傷。Safe examination安 全 認(rèn)識MR對患者的損傷金 屬鐵磁性:和主磁場輕微反向金銅 鋅水銀順磁性:反磁性:輕微被主磁場吸引銥錳鈦釓鉑被主磁場吸引：鐵鎳鈷一些合金金 屬鐵磁性:和主磁場輕微反向順磁性:反磁性:輕微被Safety對患者的損傷聽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Screening FormSafety對患者的損傷Screening Form聽力Safety聽力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)

8、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

9、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)閾值的金屬探測器。鐵磁性投射物投射效應(yīng)是在強(qiáng)磁場作用下鐵磁性物體從磁體以外的地常見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)、助聽器等。MRI室應(yīng)建立一整套安全防范措施。常見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)植入物梯度場噪聲孕婦的MRI檢查不良心

10、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在磁力和磁扭矩的體內(nèi)植入物通過各種渠道置入體內(nèi)并長期駐留體內(nèi)的異物。彈片、鐵砂、假牙、動脈夾、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)等是最常見的體內(nèi)植入物。非鐵磁性植入物患者可接受MRI檢查，但會產(chǎn)生嚴(yán)重的金屬偽影；鐵磁性植入物患者一般來說不宜接受MRI檢查。研究表明，大約1/3的體內(nèi)植入物將在靜磁場中發(fā)生偏倚或移位，但不見得把所有鐵磁性植入物都看作MR

11、I禁忌癥。體內(nèi)植入物通過各種渠道置入體內(nèi)并長期駐留體內(nèi)的異物。彈片、鐵體內(nèi)植入物的安全性MRI對鐵磁性體內(nèi)植入物的影響主要表現(xiàn)在以下幾個方面：位置變化；功能紊亂；局部升溫。強(qiáng)磁場可使腦動脈瘤治療中放置的動脈夾移動甚至脫落；靜磁場和RF場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性決定于它們在體內(nèi)的位置。眼內(nèi)的金屬異物被拉出時容易造成傷害，已經(jīng)有眼內(nèi)金屬異物致盲的報告。透視或拍片是對金屬異物進(jìn)行預(yù)檢查的一種既敏感又廉價的方法，在X線片上可發(fā)現(xiàn)小到0.1mm的金屬異物。金

12、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噪聲MRI裝置的音頻噪聲可分為靜態(tài)及動態(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。動態(tài)噪聲即梯度場噪聲，指掃描過程中由梯度場的不斷開啟或關(guān)閉而形成的。由于的主磁場的存在，梯度線圈工作時將產(chǎn)生很強(qiáng)的洛侖茲力，使線圈載體在梯度場轉(zhuǎn)換期間發(fā)生劇烈振蕩，從而產(chǎn)生掃描時的特殊噪聲。梯度場噪聲MRI裝置的音頻噪聲可分為靜態(tài)及動態(tài)兩種。梯度場噪聲系統(tǒng)的靜磁場越高、梯度上升速度越快或梯度脈沖的頻率越高，它

13、發(fā)出的噪聲就會越大。1.02.0T時，梯度場達(dá)到25mT/m時，噪聲可高達(dá)110dB。心理傷害是可誘發(fā)癲癇和幽閉恐懼癥。生理傷害是暫時性聽力下降或永久性聽力損害。梯度場噪聲系統(tǒng)的靜磁場越高、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有爭議的話題。建議“在妊娠的頭3個月謹(jǐn)慎應(yīng)用”MRI檢查。孕期的工作人員對MRI電磁場的接觸也應(yīng)受到限制。一般來說，活動范圍要盡量在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M

14、RI是否有致畸作用一直是一個有爭議的話題。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；在

15、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)靜磁場的生物效應(yīng)梯度磁場的生物效應(yīng)射頻場的生物效應(yīng)磁共振成像系統(tǒng)的生物效應(yīng)靜磁場的生物效應(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、梯度磁場和射頻磁場的輻射。理論上講，任一種磁場都將產(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、梯度磁共振成像系統(tǒng)的生物效應(yīng)近20年來，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)

16、和計算機(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，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。三種磁場的復(fù)合作用結(jié)果很難評價，動物模型與人體的差異較大。生物效應(yīng)的影響因素多。三種磁場的影響因素都很多。MRI系統(tǒng)千差萬別。每一型號都需要相當(dāng)長的時間來積累研究資料或臨床數(shù)據(jù)。硬件發(fā)展過快，許多新技術(shù)的生物效應(yīng)尚未開始評價就已在臨床應(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以下的場強(qiáng)）中可以得到這樣的結(jié)論：常規(guī)MR

17、I成像不會給人類健康造成任何有臨床意義的威脅，它對人體健康的影響遠(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)行評價。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)靜磁場的生物效應(yīng)梯度磁場的生物效應(yīng)射頻場的生物效應(yīng)磁共振成像系統(tǒng)的生物效應(yīng)靜磁場的生物效應(yīng)靜磁場的生物效應(yīng)隨著超導(dǎo)磁體技術(shù)的日益成熟，場強(qiáng)有不斷提高的趨勢。靜磁場對生物體的影響至今沒有完全闡明，表明超高場（2T以上）對人體影響的資料就更少。FDA明確規(guī)定，因場強(qiáng)超過規(guī)定限值而造成的一切后果由MRI制造商承擔(dān)。靜磁場的生物效應(yīng)隨著超導(dǎo)磁體技術(shù)的日益成熟，場強(qiáng)有不斷提高

18、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，出現(xiàn)過磁體使體溫升高、磁場不影響體溫甚至磁場使身體某部位的體溫下降等多種觀點?，F(xiàn)已證明，靜磁場對人的體溫不產(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。磁流體動力學(xué)效應(yīng)磁場中的血流以及其他流動液體產(chǎn)生的生物效應(yīng)。靜磁場能使紅細(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。場強(qiáng)對ECG的影響不是非常明顯。磁流體動力學(xué)效應(yīng)磁場中的血流以及其他流動液體產(chǎn)生的生物效應(yīng)。中樞神經(jīng)系統(tǒng)效應(yīng)磁場有可能引起神經(jīng)活動的誤傳導(dǎo)。目前公認(rèn)，短期的暴露在2.0T以下的靜磁場對人的中樞神經(jīng)系統(tǒng)沒有明顯不良影響。但在4.0T以上的MRI系統(tǒng)

19、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，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)磁場有可能引起神經(jīng)活動的誤傳導(dǎo)。磁共振成像系統(tǒng)的生物效應(yīng)靜磁場的生物效應(yīng)梯度磁場的生物效應(yīng)射頻場的生物效應(yīng)磁共振成像系統(tǒng)的生物效應(yīng)靜磁場的生物效應(yīng)梯度場及其感應(yīng)電流梯度磁場是一種時變場，變化的磁場在導(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的切換率、最大磁通強(qiáng)度（梯度場強(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。靜磁場中運動的導(dǎo)電物體也會產(chǎn)生電流，病人被送入磁體的過程中體內(nèi)有感生電流出現(xiàn)。梯度場及其感應(yīng)電流梯度

20、磁場是一種時變場，變化的磁場在導(dǎo)體中將梯度場的心血管效應(yīng)強(qiáng)電流對心血管系統(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）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。梯度場的心血管效應(yīng)強(qiáng)電流對心血管系統(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)胞后形成的視覺紊亂，是梯度場最敏感的生理反應(yīng)之一。光幻視與梯度場變化率和靜磁場強(qiáng)度均有關(guān)系，且在梯度場停止后自動消失，1.5T和20T/s以下不出現(xiàn)這種幻覺，但在4T中2040Hz時很容易使正常人產(chǎn)生磁幻

21、視。磁致光幻視又叫光幻視或磁幻視，是在梯度場作用下眼前出現(xiàn)閃光感磁共振成像系統(tǒng)的生物效應(yīng)靜磁場的生物效應(yīng)梯度磁場的生物效應(yīng)射頻場的生物效應(yīng)磁共振成像系統(tǒng)的生物效應(yīng)靜磁場的生物效應(yīng)射頻能量的特殊吸收率人體受到電磁波照射時將其能量轉(zhuǎn)換為熱。MRI掃描時RF激勵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（specific absorption rate）指單位重量生物組織中RF功率的吸收量，是對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將其能量轉(zhuǎn)換為熱。M射頻

22、能量的特殊吸收率在MRI中，SAR的大小與共振頻率（靜磁場強(qiáng)度）、RF脈沖的類型（90或180 ）、重復(fù)時間和脈寬、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場最主要的生物效應(yīng)是溫度效應(yīng)，但RF照射引起的實際組織溫升還決定于照射時間、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國家標(biāo)準(zhǔn)協(xié)會和FDA規(guī)定：接受連續(xù)電磁波輻射時，全身平均SAR不能超過0.4W/kg，或每克組織的SAR空間峰值不超過8.0W/kg。射頻能量的特殊吸收率美國國家標(biāo)準(zhǔn)協(xié)會和FDA規(guī)定：接受連續(xù)電射頻場對體溫的影響靜

23、磁場與體溫?zé)o關(guān)，MRI檢查時病人體溫的變化完全是射頻場作用的結(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。射頻場對體溫的影響靜磁場與體溫?zé)o關(guān)，MRI檢查時病人體溫的變射頻場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。過量電磁輻射可能導(dǎo)致患者暫時甚至永久不育和白內(nèi)障，但有人認(rèn)為臨床MRI成像一般不會造成眼組織的熱損傷。高SAR的MRI檢查或長時間的MRI檢查所致熱效應(yīng)是一個需要進(jìn)一步研究的課題。射頻場最易損傷的器官人體中散熱功能不好的器官，如睪丸、眼等對禁忌證有心臟起搏器的患者。手術(shù)后動脈夾存留

24、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個月。以上各項有疑問有患者要進(jìn)行調(diào)研，弄清情況，再決定是否做MRI檢查。否則應(yīng)謝絕做此項檢查。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個方面：接診時核對資料、病史、明確檢查目的和要求。確認(rèn)無禁忌證后，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個方面：磁共振檢查前的準(zhǔn)備

25、對預(yù)約檢查登記患者，要核對資料、登記建檔，并詢問是否做過MRI及CT檢查。有“老號”者，認(rèn)真查找老片，以利于對比。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，爭取患者密切配合與合作。磁共振檢查前的準(zhǔn)備對預(yù)約檢查登記患者，要核對資料、登記建檔，磁共振檢查前的準(zhǔn)備對嬰兒及躁動患者，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)師說明發(fā)生意外不能在機(jī)器房

26、內(nèi)搶救。磁共振檢查前的準(zhǔn)備對嬰兒及躁動患者，應(yīng)在臨床醫(yī)師指導(dǎo)下適當(dāng)給謝 謝謝 謝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厘米處檢測到的磁場強(qiáng)度德國著名數(shù)學(xué)家，于1832年首次測量了地球的磁場。5安培1厘米1高斯高斯（gauss, G） Gauss (1777-1855)地球的磁場強(qiáng)度分布圖地球的磁場強(qiáng)度分布圖特斯拉（Tesla,T）Nikola Tesla (1857-1943), 奧地利電器工程師，物理學(xué)家，旋轉(zhuǎn)磁場原理及其應(yīng)用的先驅(qū)者之一。1 T = 10000G 特斯拉（Tesla,T）1 T = 10000G G

27、eneral 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

28、 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 potentia

29、lly 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

30、 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.General Bioeffects of Static MTemperature EffectsThere are conflictin

31、g 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 i

32、n 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 w

33、ork been substantiated. In addition, studies that reported static magnetic fieldinduced skin and/or body temperature changes used eitherlaboratory animals known to have labile temperatures or instrumentation that may have been affected by the static magnetic fields.72,203A recent investigation indic

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

35、emperatures of human subjects are believed to be unaffected by exposure to static magnetic fields of up to 1.5T.Temperature EffectsElectrical Induction and Cardiac EffectsInduced biopotentials may be observed during exposure to static magnetic fields and are caused by blooda conductive fluidflowing

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

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

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

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

40、al risks are believed to be associated with themagnetohydrodynamic effect that occurs in conjunction with static magnetic field strengths of up to 2 T.Electrical Induction and CardiNeurological EffectsTheoretically, electrical impulse conduction in nerve tissue may be affected by exposure to static

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

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

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

44、several 3 and 4 T whole-body MR systems in operation at various researchsites around the world. One study indicated that workers and volunteer subjects exposed to a 4 T MR system experienced vertigo, nausea,headaches, a metallic taste in their mouths, and magnetophosphenes (visual flashes).157 As a

45、result, considerable research is under way worldwideto study the mechanisms responsible for these bioeffects and to determine possible means, if any, to counterbalance them.Neurological EffectsCryogen ConsiderationsAll superconductive MR systems in clinical use today use liquid helium. Liquid helium

46、, which maintains the magnet coils in their superconductivestate, will achieve the gaseous state (“boil off”) at approximately 268.93 C (4.22 K).96 If the temperature within the cryostat precipitously rises, thehelium will enter the gaseous state. In such a situation the marked increase in volume of

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

48、(and loud) egress of gaseous helium as it escapesfrom the cryostat. In normal situations this gas should be vented out of the imaging room and into the external atmosphere. It is possible, however,that during such venting some helium gas might accidentally be released into the ambient atmosphere of

49、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, itsventilation capacity, and the total amount of gas released will determine whether the helium gas will reach the patient or the health pr

50、actitioner, whois 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

51、ofhelium 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. Insuch a circumstance the first response should be to evacuate the area until the offending helium vapor is adequately removed from the imaging roomenvi

52、ronment and safely redirected to an outside environment away from patients, pedestrians, or any temperature-sensitive material.96Better cryostat design and insulation materials have allowed the use of liquid helium alone in many of the newer superconducting magnets.Nevertheless, a great number of ma

53、gnets in clinical use still use liquid nitrogen as well. Liquid nitrogen within the cryostat acts as a buffer betweenthe liquid helium and the outside atmosphere, boiling off at 77.3 K. In the event of an accidental release of liquid nitrogen into the ambientatmosphere of the imaging room, there is

54、a potential for frostbite, similar to that encountered with gaseous helium release. Gaseous nitrogen isroughly the same density as air and is certainly much less buoyant than gaseous helium.In the event of an inadvertent venting of nitrogen gas into the imaging room, the gas could easily settle near

55、 floor level; the amount of nitrogen gaswithin the room would continue to increase until venting ceased. The total concentration of nitrogen gas contained within the room would bedetermined on the basis of the total amount of the gas released into the room, the dimensions of the room, and its ventil

56、ation capacity (i.e., theexistence and size of other routes of egressdoors, windows, ventilation ducts, and fans). A pure nitrogen environment is exceptionally hazardous,and unconsciousness generally results as soon as 5 to 10 sec after exposure.96 It is imperative that all patients and health pract

57、itioners evacuate thearea as soon as it is recognized that nitrogen gas is being released into the imaging room. They should not return until appropriate measures havebeen taken to clear the gas from the room.96Dewar (cryogen storage containers) storage should also be within a well-ventilated area,

58、lest normal boil-off rates increase the concentration of inertgas within the storage room to a dangerous level.71 At least one reported death has occurred in an industrial setting during the shipment ofcryogens,70 although to our knowledge no such fatality has occurred in the medical community. Ther

59、e is one report of a sudden loss ofconsciousness of unexplained cause by an otherwise healthy technologist (with no prior or subsequent similar episodes) passing through a cryogenstorage area where multiple dewars were located.4 Although there is no verification of ambient atmospheric oxygen concent

60、ration to confirm anyrelationship 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.96Proper handling and storage of cryogens,