Reducing Brain MRI Artifacts Caused by Ferromagnetic Orthodontic Appliances Using Permanent Magnets
Zhiyue J Wang1,2, Yong Jong Park1,2, Youngseob Seo1,2, Michael C Morriss1,2, and Nancy K Rollins1,2

1UT Southwestern Medical Center, Dallas, TX, United States, 2Children's Medical Center, Dallas, TX, United States


Stainless steel orthodontic appliances are commonly found in adolescents undergoing clinical brain MRI examinations. They cause severe magnetic susceptibility artifacts and failure to obtain diagnostic information from many MR techniques. The B0 shimming capability present on clinical MR scanners cannot remove these artifacts. We have constructed devices for the correction of these artifacts at 1.5 T using small pieces of permanent magnets mounted on intra-oral mouth guards or an extra-oral mouth-band. The magnetic field from the permanent magnets cancels the B0 inhomogeneity induced by ferromagnetic orthodontic appliances, resulting in drastic improvement of MR image quality.


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.

Materials and Methods

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.


This work receives support from NIDCR R21 DE023916-01A1.


1. New PF, et al: Radiology 147, 139-148 (1983).

2. Coey JMD (editor): Rare-Earth Iron Permanent Magnets, Oxford Press (1996)

3. Jauch EC, et al: Stroke 44, 870-947 (2013).


Figure 1. Intraoral field correction device. (A) An upper teeth model with orthodontic brackets; (B) Upper device embedding permanent magnets; (C) The device of (B) fits the model in (A). (D,E,F) corresponding figures for the lower teeth.

Figure 2. External field correction device. This device is intended to decrease the effort on patient's part. Correction magnets with different strengths can be mounted. Correction for unilateral implants is possible.

Figure 3. Measured magnetic moment of permanent magnets of different volumes is shown (blue diamonds). The measurement was done at 1.5T with the magnetization opposing to the external magnetic field. The red line is the calculated value using the residual magnetic flux (Br) from the data sheet for zero external field.

Figure 4. Results of the stability test for two magnets at 1.5T over a one hour period with the magnetization opposing to the external field. The magnetic moment showed no trend of drifting. The initial transient could be related to the instability of the setup in the beginning of measurement.

Figure 5. ADC maps from one healthy subject. (A) Without wearing the device, the anterior areas were stretched and posterior regions compressed. (B) Use of the device decreased these geometric distortions.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)