Correction of B0 inhomogeneity Induced by Orthodontic Appliances in MRI

 描述(申请人提供)：在接受脑部核磁共振检查的正畸金属矫治器的患者中，硬件会导致口腔附近区域的信号丢失，以及大脑上的严重图像失真。眼眶、脑垂体腺、下丘脑和额叶等关键区域常受影响。在常规的临床脑部检查中，弥散加权成像和磁共振血管成像伪影最为严重。在大多数儿童医院，神经成像检查通常占磁共振成像容量的80%以上，其中10%或更多的患者配备了正畸硬件。在95%以上的病例中，正畸托槽由铁磁性不锈钢制成。在某些情况下，受损的核磁共振扫描会导致原本可以避免的死亡和发病。神经成像研究通常使用扩散张量成像、功能磁共振成像和脑体积测量技术，这些技术在正畸矫治器的受试者中都会受到严重的图像失真的影响。我们的目标是开发牙科视野矫正设备，将正畸金属伪影大幅减少到大多数诊断性MRI扫描所能容忍的低水平。这是可行的，因为最近商用的NdFeB永磁体具有很强的本征矫直力，可以抵抗1.5T下的不可逆退磁。当磁化时，这类材料具有与MRI扫描仪内不锈钢牙托中感应的磁化相当的磁化。我们假设，通过在铁磁支架附近沿与MRI B0场相反的方向分布小的永久磁体，可以消除源于支架的脑组织中感应的不均匀磁场。这将导致B0不均匀性的降低和MR图像质量的提高。据我们所知，这项建议是纠正牙套引起的B0不均一性的第一次尝试。这项研究分几个步骤进行。首先，需要对托槽的磁特性进行表征，包括托槽感应磁偶极子的方向和幅度对磁特性的影响。我们已经在这一领域做了大量工作。其次，将构建两种类型的场校正器样机。第一种类型适合戴牙套的患者的口腔。这些设备必须能够适应儿童患者颌骨大小的变化。该装置的设计类似于一个普通的护口器，可以很容易地放置在嘴里，上面粘着许多小的永久磁铁。第二种类型的装置在口腔外部佩戴，并基于同一MRI会话中的B0标测扫描将校正磁铁安装在装置上。第三，30名戴着不锈钢牙套的受试者将分别在没有和有场校正设备的情况下接受一次脑MRI检查，利用各种扫描技术，包括扩散加权成像、磁共振血管成像、磁化率加权成像、磁共振波谱和全脑B0场映射。这些扫描将证明该设备在临床核磁共振检查中的有效性。

 DESCRIPTION (provided by applicant): In patients with metallic orthodontic appliances undergoing MRI examination of the brain, the hardware causes signal loss in regions within the immediate vicinity of the oral cavity as well as significant image distortion over the brain. Critial regions such as the orbits, pituitary gland, hypothalamus and frontal lobe are commonly affected. In routine clinical brain examinations, the artifacts are most severe for diffusion weighted images and MR angiography. Neuroimaging examinations are typically over 80% of the volume of MR imaging in most children's hospitals and 10% or more of these patients have orthodontic hardware. In over 95% of cases, orthodontic brackets are made of ferromagnetic stainless steel. In some cases, compromised MRI scans lead to fatality and morbidity that are otherwise avoidable. Neuroimaging research studies commonly utilize diffusion tensor imaging, functional MRI and brain volume measurement techniques that are all subject to significant image distortion in subjects with orthodontic braces. We aim to develop dental field correction devices that will drastically reduce orthodontic metal artifacts to low levels that are tolerable fr most diagnostic MRI scans. This is feasible due to recently commercially available NdFeB permanent magnets with strong intrinsic coercivities that can resist irreversible demagnetization at 1.5T. When magnetized, this class of materials has a magnetization comparable to that induced in the stainless steel dental bracket inside MRI scanners. We hypothesize that by distributing small permanent magnets near the ferromagnetic brace brackets in an orientation opposing the MRI B0 field, the induced inhomogeneous magnetic field in the brain tissue originating from the braces can be cancelled. This will lead to the decreasing of B0 inhomogeneity and improvement of MR image quality. This proposal, to our knowledge, is the first attempt to correct B0 inhomogeneity caused by dental braces. The study proceeds through several steps. First, magnetic properties of the dental brace brackets need be characterized, including the orientation dependence of direction and amplitude of the induced magnetic dipole of brackets. We have already done substantial amount of work in this area. Secondly, 2 types of field correction device prototypes will be constructed. The first type fits into the mouth of a patient who wears dental braces. The devices must be able to accommodate the variability of jaw sizes in pediatric patients. The device design resembles a common mouth guard which can be easily positioned in the mouth, with many small permanent magnets bonded to it. The second type of device is worn externally outside the mouth, and correction magnets are mounted on the device based on a B0 mapping scan in the same MRI session. Third, 30 subjects wearing stainless steel dental braces will each undergo one session of brain MRI without and with a field correction device, utilizing a variety of scanning techniques including diffusion weighted imaging, MR angiography, susceptibility weighted imaging, MR spectroscopy, and whole brain B0 field mapping. These scans will demonstrate the efficacy of the devices in clinical MRI examinations.