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極化時(shí)間一到，立即將RF脈沖串用于地層。第一個(gè)RF脈沖稱為9O°脈沖，這是因?yàn)樗馨炎畛跖cB。平行的磁化矢量旋轉(zhuǎn)到垂直于B0 的橫向平面上。一旦磁化在橫向平面內(nèi)進(jìn)行，它就會(huì)繞著B。旋轉(zhuǎn)，就在原來產(chǎn)生脈沖的同一天線上產(chǎn)生一個(gè)隨時(shí)問變化的信號(hào)。緊跟著9O。脈沖，首先產(chǎn)生一個(gè)NMR 自由感應(yīng)衰減(FID)信號(hào)，但由于其衰減太快而無法探測(cè)到。900脈沖之后是一系列間隔均勻的180。脈沖，用來使氫核的磁矩重新聚焦，形成連貫的自旋回波信號(hào)。在每對(duì)180。脈沖信號(hào)之間記錄自旋回波信號(hào)。之所以把信號(hào)稱之為回波，是因?yàn)樗鼈冊(cè)诿恳粚?duì)180。脈沖的中間點(diǎn)能夠達(dá)到最大幅度，然后在下一個(gè)脈沖到來之前快速衰減為零，下一脈沖重聚磁矩以產(chǎn)生下一個(gè)回波。
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