Correction of B0 inhomogeneity Induced by Orthodontic Appliances in MRI

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 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.