Stainless steel orthodontic appliances (commonly known as braces) are frequently found in adolescents undergoing brain MRI examinations. They cause severe magnetic susceptibility artifacts resulting in failure to obtain diagnostic information from many MR techniques [1]. Currently, the B₀ shimming capability of clinical MR scanners cannot remove these artifacts. We are developing and testing devices comprising small pieces of rare-earth iron permanent magnets [2] for correcting these artifacts. The magnetic field from the permanent magnets cancels the B₀ inhomogeneity induced by ferromagnetic orthodontic appliances.

Permanent Magnets: Custom made NdFeB rectangular prism magnets (N38EH grade with an intrinsic coercivity of 2387 kA/m which corresponds to 3T in a vacuum) of various sizes with nickel coating were purchased from Dexter Magnetic Technologies. The magnetization is along the long axis of the magnets.

Magnetic Moments of the Magnets: The magnetic moment of the magnets were measured in 1.5T MRI scanner. A magnet was glued on the tip of a plastic rod and inserted into a 2 liter spherical flask filled with water. B₀ field distribution was mapped with 3D gradient echo imaging and the magnetic moment was obtained through fitting the data using a dipole model.

Intraoral Field Correction Device: The device was made in the shape of a mouth guard with small permanent magnets tightly sealed between 2 layers of dental plastic sheets. Multiple pieces were made for the upper and lower teeth (Figure 1), each with a different total magnetic moment. A combination of an upper- and a lower-piece will provide correction with a wide range of magnetic moment, accommodating a variety of braces. As a precaution, the device was wrapped inside a thin plastic sheet to protect the patient in case the device breaks. Devices were sterilized using Biocide G30 solution for repeated use.

External Field Correction Device: The device resembles a facemask. Permanent magnets were embedded inside plastic strips which can be mounted on the device using nylon screws (Figure 2). The device is secured to the head using Velcro straps.

Human Subject: Brain MRI scans have been acquired from 3 healthy subjects and 3 patients undergoing clinically indicated brain MRI using intra-oral devices. For healthy subjects, the scans included DWI, MRA and B₀ mapping without and with the field correction device. Patient study involves B₀ mapping for calibration and the clinical protocol.

Measurements on permanent magnets: Our tests have shown no loss of magnetization after repeatedly moving these magnets in and out of a 1.5T MRI scanner, with the magnetization of the permanent magnet opposing to the main field of the MRI magnet. The magnetic moment of the magnets of different sizes are shown in Figure 3. The magnetic moment of 2 magnets measured repeatedly over a one hour period demonstrated good stability (Figure 4).

Human Studies: For healthy subjects undergoing orthodontic treatment, the intraooral device was well tolerated during the MRI session. Patients undergoing clinical brain MRI had variable degree of tolerence to the device. A main complaint was difficulty in breathing. Improvement in MRI quality was obtained in each subject. Figure 5 shows decreased geometric distortions in DWI in a healthy subject.

Removal of dental braces is often required for patients undergoing a brain MRI examination. However, this may not be practical in emergencies such as acute stroke where MRI is very important and there is only a narrow therapeutic time window [3]. Furthermore, patients undergoing frequent follow-up MRI’s often terminate orthodontic treatment prematurely. A field correction device will have an important impact on patient care.

We have developed two types of field correction devices and have until now acquired data with the intraoral device. The intraoral device allows closer distance between the permanent magnets and orthodontic brackets and wires, and this is advantageous for optimal correction. On the other hand, the external device is expected to require less patient collaboration and should not interfere with breathing.

When the device is aligned correctly with the B₀ field, the device is in an unstable equilibrium and experiences small magnetic force and torque. Otherwise, the subject may feel a manageable force and torque on the device.

The work has been done at 1.5T mainly because these scanners are most common, and our permanent magnets can better resist irreversible demagnetization at this field strength. In addition, the induced magnetic moment in orthodontic appliances is weaker at 1.5T. This situation is more favorable from a safety point of view.

Susceptibility artifacts caused by ferromagnetic orthodontic appliances at 1.5T can be drastically decreased using a field correction device employing permanent magnets.