【作者】 曹筝；

【导师】 陈莉丽；

【作者基本信息】 浙江大学 ， 口腔临床医学， 2007， 硕士

【摘要】 磁共振成像(magnetic resonance imaging,MRI)技术是近二十年来随着计算机技术的飞速发展,在X线CT的基础上发展起来的一种新型的医学成像技术,由于其具有任意方向断层、多参数成像、无创伤及对软组织分辨率高等特点,已显示出巨大的优越性,特别是在神经外科、头颈外科、口腔颌面外科的术前诊断、定位、指导手术方案的设计、术后复查等应用方面发挥着不可替代的作用。在口腔领域,金属体常见于颌面部,以嵌体、贴面、冠、固定桥及固定夹板等形式存在,通常采用高贵金属、贵金属(黄金、白金、钯、银等)及非贵金属合金(钴、铬、镍)材料。其中某些金属材料可造成局部磁场不均匀,对磁共振成像产生干扰,使得图像上出现伪影,干扰正常解剖结构和病变组织的显示,因此部分患者在需要作磁共振检查时受到一定程度的限制而影响正确诊断治疗。减少和避免金属伪影的影响是目前正在研究和亟待解决的问题。近年来MRI在磁场强度方面的发展,趋向于向低场强(0.3T)及高场强(3.0T)发展,高磁场MRI机对人体的一些生理运动及金属异物等,都将产生更明显的伪影。3.0T MRI设备临床上应用以来,由于其卓越的优势使其临床应用日趋广泛,了解这些金属材料产生伪影严重程度以及同MRI检查序列的关系对将来的临床检查有重要指导意义。目的:探讨1.5T、3.0T场强下口腔内常用三种金属材料在磁共振检查时是否产生伪影和伪影的严重程度,并比较不同扫描序列对伪影的影响。方法:用3种口腔内常用金属材料(钴铬合金、镍铬合金、金合金)组成的烤瓷冠制备成琼脂模型,分别使用磁场强度为1.5T、3.0T的磁共振扫描仪进行扫描,扫描序列为快速自旋回波序列(FSE)T1加权、T2加权成像,液体衰减反转恢复序列(FLAIR),梯度回波序列(GRE 3D TOF)和平面回波扩散加权序列(EPI DWI)五种临床常用的头部扫描序列。对MRI伪影进行定量分析,利用SPSS12.0统计学软件对结果进行分析,统计采用金属材料、磁场强度、扫描序列三因素析因实验设计的方差分析。结果:不同金属材料、不同磁场强度、不同扫描序列对金属伪影的影响各不相同。钴铬合金和镍铬合金在各种场强及扫描序列中都产生明显伪影,且伪影随磁场强度的增加而增大;金合金产生伪影极小,磁场强度的增加对伪影无影响;相同合金中1.5T场强下FSE序列产生的伪影最小,FLAIR序列次之,3D TOF序列伪影较大,DWI序列伪影最大;3.0T场强下3D TOF序列例外,伪影大小与1.5T场强下3D TOF序列相似。析因试验方差分析结果显示主效应中,金属材料、磁场强度、扫描序列对伪影的影响均有统计学意义(P＜0.05);各因素之间交互作用无统计学意义(P＞0.05)。结论:1、伪影的大小取决于金属合金的类型。钴铬合金、镍铬合金伪影较大,金合金伪影很小。合理选择金属材料可以减轻伪影。2、不同扫描序列对金属伪影的大小影响也各不相同。FSE序列产生伪影最小,DWI序列产生伪影最大。口腔内有金属修复体的患者应尽可能采用速度快且伪影小的FSE序列,尽量避免使用GRE序列,EPI序列则不宜使用。3、通常情况下,场强越高,产生的金属伪影越大,为了降低伪影的影响,可选择低场机器;但另一方面伪影不随磁场强度的增加而成倍增加,量化指标显示,3.0T场强下伪影增幅只在10%左右,因此,为了临床的需要选择高场机器也是可以的。更多还原

【Abstract】 During the last decades magnetic resonance images （MRI） has become a new medical imaging technique based on X-rays and CT with the development of the computer system. The advantages of MRI include its ability to provide sectional images of anatomic regions in any arbitrary plane, non-invasive, non-ionizing and its excellent soft-tissue contrast resolution. MRI is also used for diagnostics, treatment planning and therapy process control including neurodegenerative diseases, orthopedic surgery, craniomaxillofacial area. Metal objects are frequently located in the orofacial area. They are mostly inlays, crowns, fixed bridges and fixed dental splints. They are usually made of high-noble and noble alloys （including: gold, platinum, palladium, silver） as well as base metal alloys （cobalt, chromium, nickel）. In MRI, degradation of image quality and its impact on image interpretation in the vicinity of metallic material are due to a combination of signal voids, image distortion, and signal inhomogeneity. Artifacts in MR images do not faithfully represent the tissue components being studied. Reducing and avoiding the effect of the metal artifacts is the problem to be solved. Recently, the development of MRI system is to the direction of lower field strength （0.3T） and the higher field strength （3.0T）, and it can be anticipated that metal artifacts will be more extensive than at the more commonly used field strength of 1.5 T. Clinical use of magnetic field strengths of 3.0 Tesla has been approved in many countries. Understanding the extent of the metal artifacts and the relation with the MRI pulse sequences have great instruction on the practise of the 3.0T MRI system. Objective: To evaluate artifact sizes at 3.0 T compared to at 1.5 T caused by three kinds of metallic dental materials, and to evaluate the influence of different magnetic resonance imaging （MRI） sequences on artifacts.Methods: metal-ceramic crown samples of various dental materials（ Ni-Cr alloy, Co-Cr alloy and Gold alloy ） were placed in a cylindrical plastic vessel filled with agarose gel. Images were performed by means of 1.5 T and 3.0 T MRI apparatus with five different sequences.The pulse sequences are fast spin echo T₁WI and T₂WI （FSE T₁WI, FSE T₂WI）sequences, fluid attenuated inversion recovery （FLAIR） sequence, 3D TOF of gradient echo （GRE） sequence and echo planar imaging DWI （EPI DWI） sequence, of which are all the common head sequences in clinical practise. A linear region of interest were defined across the centre of the sample in the transverse direction, and the susceptibility artifact diametre was calculated and compared quantitatively. The data were analyzed with factorial experiment’s analysis of variance using SPSS 12.0. factorial design was used to test three factors （metal materials, magnetic field strengths and sequences）.Results: Susceptibility artifact developed around all metal materials. The artifact diametres caused by Co-Cr alloy and Ni-Cr alloy were big and increased substantially at 3.0 T, compared to at 1.5 T. Gold alloy’s artifact was very small and not influenced by magnetic field strengths. Different pulse sequences had different susceptibility artifacts. At 1.5T, FSE sequence produced the least artifacts, FLAIR sequence produced the second, 3D TOF sequence produced the third and DWI sequence produced the greatest artifact. At 3.0T, 3D TOF sequence produced the similar size of the artifacts with that of 1.5T. Analysis of variance showed that metal materials, magnetic field strengths and sequences were independently significant factors （P<0.05）. Interactions between the three factors were not statistically significant （P >0.05）.Conclusions:1. The metal artifacts on MRI depends on the composition of the metal alloy, with Gold alloy creating significantly less artifact than Co-Cr alloy and Ni-Cr alloy. Selection of specific metal materials could minimize the metal artifacts in MRI. 2. Sequences were important factors that affected the size of metal artifacts. FSE sequence produced the least artifacts and EPI sequence produced the greatest. The patient with the metal restoration should select FSE sequence, avoid GRE sequence and EPI sequence is strictly prohibited.3. Usually higher field strength results in greater imaging artifacts, so the lower field strength MR scanner is considered, but the increment of artifacts are only 10% at 3.0T compared to at 1.5T, while 3.0T MRI system has great advantages, it is eligible on the patient with metal restoration.更多还原