化學(xué)位移偽影

1、化學(xué)位移偽影 節(jié)選自：醫(yī)學(xué)影像物理學(xué)教程第三篇 磁共振成像 學(xué)習(xí)內(nèi)容2. MR圖像是怎樣重建出的？1. 什么是化學(xué)位移？（重點(diǎn)） （難點(diǎn)） 學(xué)習(xí)目標(biāo)2. 從K空間的角度解釋MR圖像重建準(zhǔn)確描述K空間 化學(xué) 位移分子中的H核被電子云包圍Results in a shift of resonant frequency which affects the imagee.g. 脂肪 (CH2) vs 水(H2O)?Since the secondary fields are as a result of the application of B0, they are proportional to B

2、0. Theres nothing you can do about them ! Spatial position is frequency encoded in one direction in the imaging process, and so signals from fat and water in the same spatial location will be assigned to different spatial locations on the image. This is called misregistration, and has nothing to do

3、with relaxation times.The external applied magnetic field is modified by the local structure of the molecule a proton resides in. Local variations in field mean different resonant frequencies (c.f. Larmor equation). This is sometimes stated as shielding by the electron cloud of the molecule. Shieldi

4、ng refers to the induction of secondary magnetic fields that oppose B0 by the movement of electrons due to B0.Lets look at how this happens.化學(xué)位移：在MRI中，脂肪和水的H質(zhì)子的進(jìn)動(dòng)頻率之間的差異。H 質(zhì)子共振頻率與主磁場(chǎng)B0有關(guān)水脂肪頻率編碼梯度MR信號(hào)是通過(guò)施加頻率編碼梯度造成不同相位質(zhì)子的頻率差異來(lái)空間定位的脂肪中的H 質(zhì)子的共振頻率與主磁場(chǎng)B0有關(guān)圖像中脂肪的位置會(huì)發(fā)生輕度偏移0化學(xué)位移：由化學(xué)環(huán)境不同引起的共振頻率的偏移現(xiàn)象。 化學(xué)位移偽影：由

5、于頻率差異會(huì)隨外部磁場(chǎng)強(qiáng)度 (B0)的變化而變化，化學(xué)位移常用ppm(parts-per-million) 來(lái)表示， 水和脂肪的化學(xué)位移是3.5 ppm，當(dāng)B0.一定時(shí)，可以計(jì)算出頻率 。（頻率編碼方向）脂肪水水化學(xué)位移與頻率信號(hào)疊加信號(hào)丟失樣品圖像MR信號(hào)是通過(guò)施加頻率編碼梯度造成不同相位質(zhì)子的頻率差異實(shí)現(xiàn)空間定位;脂肪的頻率低于水（中心頻率），因此圖像上脂肪信號(hào)向低頻方向移動(dòng)脂肪水植物油（塑料試管）（頻率編碼方向）低接收帶寬高接收帶寬化學(xué)位移與接收機(jī)帶寬這幅圖像的化學(xué)位移有多大?多少個(gè)像素?（接收）帶寬頻率編碼矩陣大小化學(xué)位移e.g. 16 kHz= 32 kHze.g. 25632 x 103(H z)256 (pixels)125 Hz / pixel3.5 ppm224 Hz ( 1.5