Interactions between magnetic resonance imaging and dental material

Abstract

Magnetic resonance imaging (MRI) has become a common and important life-saving diagnostic tool in recent times, for diseases of the head and neck region. Dentists should be aware of the interactions of various restorative dental materials and different technical factors put to use by an MRI scanning machine. Specific knowledge about these impacts, at the dentist level and at the level of the personnel at the MRI centers can save valuable time for the patient and prevent errors in MRI images. Artifacts from metal restorations are a major hindrance at such times, as they result in disappearance or distortion of the image and loss of important information.

In addition to palpation and inspection, the imaging modalities used to evaluate the oral cavity include panoramic radiography, ultrasound, computed tomography (CT), Magnetic resonance imaging (MRI), and positron emission tomography (PET). CT and MRI remain the primary methods for evaluation of oral cavity cancers. However, CT and MRI often do not accurately.

Evaluate the primary tumors in patients with dental metallic implants or dentures.[1] MRI is thought to be more useful than CT in diagnosis of soft tissue and blood flow within the bone in implants insertion and osteomyelitis.[2] MR imaging is considered contraindicated in patients with ferromagnetic metallic implants or other ferromagnetic materials, primarily because of the potential risks associated with movement or dislodgment of these objects.[3–6] Artifacts caused by metallic objects, such as dental crowns, dental implants and metallic orthodontic appliances, are a common problem in head and neck MRI.[7–21]

MRI

It’s a diagnostic tool that uses powerful magnetic field, radio waves and a computer, to create images of tissues and organs throughout the body.[7] The powerful magnetic field aligns atomic particles called protons that are present in most of body tissues especially the soft tissues. The applied radio waves then cause these particles to produce signals that are picked up by the receiver within MR scanner.

Magnetic force

It is measured in tesla (T). MRI uses 1.0-1.5 T, more powerful MR scanner uses 3.0 T. Compared to earth′s magnetic force (50 μT); it is 10,000 times more powerful.

Types of MRI

Based on the magnetic field strength:

Low-field MRI scanners (0.23 T-0.3 T): They are typically identified as open MRI scanners. Low-field MRI scanners have decreased image quality and require a longer scan time compared to high-field MRI scanners.

High-field MRI scanners (1.5 T to 3.0 T): These are typically identified as closed MRI scanners. A 1.5 T MRI scanner provides great image quality, fast scan times, and the ability to evaluate how certain structures in the body function. The 3.0 T MRI scanner is great for visualizing very fine detail, such the vessels of the brain or heart.

Ultra-high field MRI scanners (7.0 T to 10 T): It is not widely available and is typically used for research.

Materials Used in Dentistry

Dental materials may be classified in many ways. Here it is classified based on magnetic susceptibility as:[7,8,11]

Ferromagnetic

These are those types of materials which are strongly attracted to a magnet. Their permeability is very high in the range of hundreds and thousands. Examples include chromium oxide, cobalt, ferrite (iron), cadolinium, nickel, rare earth magnet, magnetite, yttrium, etc.

Paramagnetic

These are those materials which are not very strongly attracted to the magnet. They are slightly magnetized when placed in a strong magnetic field and act in the direction of the magnetic field. Their relative permeability is slightly more than one. Examples of such materials are magnesium, tin, platinum, lithium, tantalum, aluminum, molybdenum, etc.

Diamagnetic

These are those materials which are repelled by a magnet. They are slightly magnetized when placed in a strong magnetic field and act in the direction opposite to that of the magnetic field. Their permeability is slightly less than one. For example wood, zinc, copper, bismuth, silver, gold, etc., are diamagnetic materials.

Impact of Magnetic Force on Materials used in Dentistry

Projectile accidents

The powerful magnetic field of MR system will attract iron containing (or ferromagnetic) objects and may cause them to move suddenly and with a great force like a “missile”. This can cause possible risk to patient or anyone in an objects “flight path”. It can pull any ferromagnetic object in the body too.

Thermal heating

Tissue injury can be caused due to heating of prosthesis.[17,18,22] Radio frequency (RF) heating was confirmed to take place at both ends of the implants in spite of their different shapes.[22] The maximum temperature rise was observed at the tip where there is large curvature.[22]

Failure of prosthesis

It also causes failure of prosthesis like crown or FPD dislodgement. Failure of magnets used in overdentures, magnetic keepers, etc., and deflection of orthodontic wires are possible.

Dental restorations/appliances which may cause safety problem

1. Complete denture with metal denture base,[5]

2. Orthodontic brackets, wires,[6]

3. Crowns,[6]

4. Fixed partial dentures,

5. Cast partial denture,

6. Implants,

7. Magnets in overdenture, magnetic keepers, etc.[5]

Impact of Dental Materials in MRI

Artifact

An artifact may be defined as a distortion of signal intensity or void that does not have any anatomic basis in the plane being imaged.[11] It is so defined as the pixels that do not faithfully represent the tissue components being studied.[8] Even a dental bur fragments can cause artifact [Table 1].

Table 1.

Dental materials and artifacts

Factors Affecting the Size of Artifact

Size, shape and position of the material

Larger the size of the material, greater will be the artifact. Maximum area of signal loss will be, when the material is within the radius of 10 cm inside the region of interest. Artifacts are generated even with paramagnetic metals, and that the causative factor is related to the shape of the material.[2] Artifact in clinical case is due to ring-shaped attachments inside the oral cavity, even if they are of the non-magnetic metal, and they may produce artifacts depending on the orientation during MR imaging.

Region that requires MRI

Significant artifact will be produced in orofacial imaging. Importance for neck and brain imaging depends on plane of section.

Factors that influence the risk of using MRI in a patient with implant or ferromagnetic material:[4] (1) The strength of the gradient and static magnetic fields, (2) the degree of ferromagnetism of the implant or material, (3) the geometry of the implant or material, (4) the location and orientation of the implant or material in situ, and (5) the amount of time the implant or material has been in place.

Steps/guidelines to prevent these interactions

1. Knowledge about the interaction of dental materials and MRI is essential for dentists and radiologists. MRI centers can be notified whether the restoration/appliance is MR friendly or not.

2. Also we can prefer nonmetals like all ceramic restorations over metal ceramic.

3. Even in metal ceramic it is better to choose a noble metal alloy.

4. Removable appliances/prosthesis is not a problem, since patient can remove it.

5. Treat all material as MR unsafe, if the dentist is not sure about the type of prosthesis/appliance. It is advisable to remove the prosthesis/appliances prior to MRI.

6. Materials for prosthetic restoration should be selected based not only on their biological compatibility and functional and esthetic qualities, but also on whether they generate minimum artifacts in MRI. Thus ALL ceramic restorations will be a valuable alternative in dentistry.

Conclusions

Further study about the interactions is needed. Undergraduate curriculum should include these interactions and safety measures, especially the magnetic property of various dental materials.

The US Food and Drug Administration reported in 2011 that MRI accidents in the US have risen over 500% from 2000 to 2009. The overwhelming majority of reported injuries fell into one of three categories: Burns, projectiles and hearing damage. However, these injuries are not accidents: They are directly related to the failure to assure proper safety standards.

Most of these accidents were not reported. No records or data is maintained about these accidents. Proper survey in future will reveal exactly the amount of hazards that have happened in the past and will help us to take preventive steps.