A glimpse into the future of orthodontic imaging
Drs. Ryan J. Cox and Chung H. Kau discuss the potential role of MRI in orthodontics
Educational aims and objectives
The aim of this article is to describe the current status of magnetic resonance imaging use in orthodontics, its advantages and disadvantages, and its potential future applications.
Expected outcomes
Correctly answering the quiz questions following this article, worth 2 hours of CE, will demonstrate the reader can:
• discuss the history, advantages, and disadvantages of MRI use in dentistry.
• describe how MRI technology basically works.
• compare the MRI to other current imaging modalities.
• list the contraindications to using MRI in certain patients.
Introduction
Imaging in orthodontics is emerging at an astonishing speed due to the development of three-dimensional (3-D) imaging modalities. After their development in the 1990s, custom-built craniofacial machines began to appear on the market in the early 2000s. Other sophisticated imaging, with a variety of applications to the maxillofacial dental and skeletal regions, has followed. Orthodontists currently have the option to image their patients with a multitude of imaging modalities, which include computed tomography (CT), cone beam computed tomography (CBCT), cephalometric head films, panoramic films, intraoral and extraoral photography, soft-tissue laser scanning, stereo-photogrammetry, and magnetic resonance imaging (MRI).
Although there have been many developments and much significant research focusing on imaging modalities for skeletal structures and soft-tissue morphology, the use of MRI is diminutive in comparison.1 The use of non-ionizing MRI has not been considered as an imaging modality for routine orthodontic diagnosis due in part to its poor performance in imaging dental and skeletal hard tissue.2-4 Traditional MR imaging is better adapted for imaging the soft-tissue components, including temporomandibular joint (TMJ) pathology,5,6 tumors,7,8 muscles, and attachments.7 However, recent literature has been published exemplifying the use of MRI in dental hard-tissue applications, showing quality images without the exposure to ionizing radiation.1-4,9-16
Magnetic resonance imaging
Magnetic resonance imaging began with the ideas of an Irish physicist, Sir Joseph Larmor,17 and the first in vivo medical MRI was published in 1977 by Damadian.18 The magnet is the biggest and most important part in an MRI system. Most MRI machines are graded on the strength of the magnet, which is measured in Tesla units. To put these numbers into perspective, 1 Tesla unit is the equivalent of 20,000 times the magnetic field strength of Earth.7 Currently, MRI units are in the range of 1.5 to 3 Tesla units for in vivo applications, but the magnetic strength may be much higher for research.
Magnetic resonance imaging essentially utilizes hydrogen atoms to capture an image. By aligning individual atoms with magnetization, the machine applies a radiofrequency pulse to depolarize the atoms, capturing the signal emitted on excitation with a receiver coil.19 A basic depiction of MRI can be seen in Figure 1. Hydrogen is found in abundance in soft tissue, but is lacking in most hard tissues. Because the head and neck region are primarily dental and skeletal hard tissues, MRI has been perceived to have limited applications. In recent years, upon a chance finding during research of four-dimensional cardiac MRI, an MRI sequence capable of hard tissue differentiation was discovered.4 Multiple MRI sequences exist and are selected for each diagnostic procedure to help enhance the contrast of the tissue that is being studied. Each sequence consists of different radiofrequency pulses and associated mathematical gradients. Along with these parameters, timing of signal capture also plays a critical role when imaging hard tissue. With further testing and optimization of the sequence, it was realized that the new technique of ultrashort echo time magnetic resonance (UTE-MRI) could maximize hard tissue capture, and, hence, differentiate enamel from dentin and pulpal tissue.2-4 Standard MRI sequences are not capable of such differentiation and characterize enamel, dentin, and skeletal tissues equally.
The application of MRI in dentistry first appeared in 1981 in a paper entitled “NMR: dental imaging without x-rays?”20 Since that time, 
the advancement of MRI in dental applications has been relatively insignificant. Current applications include assessment of the TMJ5,6 and evaluation of soft tissue tumors of the head and neck.7,8 Recent advancements have shown that MRI technology in orthodontics may be used to take indirect impressions of the oral cavity,10,15 assess impactions of the dentition,12 detect root resorption,12 detect demineralization and caries,1,2,9,13 and diagnose and treatment plan the three-dimensional patient. Unfortunately, imaging patients with metallic orthodontic appliances in active treatment has been shown to cause strong local image artifacts.21-23 Nevertheless, the use of ceramic appliances and the development of composite wires may void this as a concern in imaging in the future.
Risk versus benefit
To obtain 3-D images for diagnosis and treatment planning in orthodontics, we currently utilize a limited- or full-field CBCT. Although the total radiation exposure to the patient is typically 20% of a traditional medical CT, sources have quoted that it can be as high as 44 times the typical panoramic dose, especially in unmodified devices.24 The resulting effective radiation dose is dependent on the quality of the image desired, which can be easily controlled with the kilovolt peak (kVp) and milliamp (mA) settings. This relative increase in radiation dose limits CBCT as a routine screening and supervision supplement when we recognize that the benefit should outweigh the cost to the patient. It is well known that children are at a higher risk of radiation effects because their tissues are more radiosensitive.25,26 Hence, the extensive use of radiation for imaging in young children should be minimized. The judicial use of CBCT has best been summarized in the statement, “Responsibility is the key factor for the utilization of cone beam technology.”26 It is the opinion of the authors that CBCT is a remarkable technology when properly utilized, and there is without a doubt an increase in the standard of delivery of patient care when a third dimension is used.26 However, with the advancement of MRI systems, this technology will definitely have a role in filling in some of the gaps of imaging systems that use radiation sources.
Image manipulation
Once an image has been captured, it may be exported into a DICOM format. This format may be used by most available 3-D software and manipulated with its tools. Multi-planar views and soft-tissue contrasting are possible in these DICOM formats and are very similar to CBCT derived images. 
Limitations
At present, there is no dedicated MRI machine in orthodontics. This is in part the result of an undiscovered usage of the technology. Tymofiyeva et al compared the economic feasibility of MRI vs CBCT at the University of Wuerzburg, Germany.1 This comparison as of 2009 showed that a CBCT dento-maxillofacial examination cost 175 euro, while a similar exam with an MRI would cost 206 euro.12 Unfortunately, the cost of an MRI is based on time, and can be lengthened or shortened. The article also showed that MRI is inferior in spatial resolution, but MRI can provide the information necessary for diagnosis.1 It was also noted that metal artifacts are noted in both imaging modalities.1
Bracher et al states that acquisition times equivalent to x-ray or CBCT may never be feasible, but times of “less than 15 minutes for a complete assessment of the status of the teeth and the head and neck appears feasible.”2
The greatest hindrances to this rising technology include relatively long image acquisition times, high development and operating costs, and poorer spatial resolution compared to CBCT.1,2 Additional disadvantages of MRI include claustrophobia, noise, and image artifacts from ferromagnetic items.1,7 In orthodontics, patient positioning in a natural position also becomes a dilemma. If traditional MRI machines are used, patients will have a soft-tissue pull from gravity that will be 90 degrees to the gravitational pull at natural head position, which may allow excessive relaxation of the mandible and attached tissues. Another disadvantage to any 3-D technique is the accuracy of the 3-D image over the full field of view.25 Lastly, there is a lack of in vivo applications of dental MRI due to the “need for high magnetic fields, long measurement times, and a lack of dedicated hardware.”1
Contraindications
Contraindications include cardiac pacemakers, implanted cardiac defibrillators, aneurysm clips, metallic foreign bodies, neurostimulators, and other implantable devices.1-7 Some companies are currently producing devices that are MRI safe, but the device in question should be referenced prior to taking a scan.
Current work
Current research is being carried out at the Department of Orthodontics at the University of Alabama at Birmingham and the University of Ulm on the use of MRI in orthodontics. Figure 2 shows the ability to contrast the different dental hard tissues including enamel, dentin, and pulp. The first image is that of a UTE-MRI showing a clear separation between enamel and dentin. The later of the two images is an inversion of the first, which appears in a more familiar format more closely resembling traditional radiography. Figure 3 shows the imaging of two different ceramic brackets, as well as a non-bracketed tooth. The UTE-MRI images were captured in plastic test tubes in which the teeth were mounted in agarose gel. One tooth has a ceramic bracket with a metal slot, while the other is a traditional monocrystalline bracket. It can be seen that the incorporation of a metal slot in the bracket will cause a localized “blow out” or distortion of the image, rendering the image useless without complex algorithms to minimize the distortion. The UTE-MRI of the non-metallic bracket shows good resolution of the bracket. The image of the tooth is included to show the resolution of the dental tissues with UTE-MRI. For comparison, photographic images as well as CBCT images of the same teeth are included.
Conclusions
As our diagnostic skills continue to grow with these multi-planar techniques, we must keep in mind the cost, benefit, and risk ratio to our patients when selecting imaging modalities. The use of dental MRI appears to be a safe tool in orthodontics for 3-D imaging without ionizing radiation, and may be considered a radiation-free imaging technique in the growing trend of organic dentistry. The techniques that are established and those being perfected at research institutes have shown us that we can indeed use this exciting technology in many aspects of our profession without the concerns of radiation, allowing the practitioner more freedom with progress and/or supervision records. Future research is still warranted, as the current findings are preliminary. With continued research and the development of dedicated dental MRIs, the dental community may see an in-office dental MRI unit in the future.
Acknowledgements
The authors would like to thank Volker Rasche and the Department of Internal Medicine II at the University of Ulm for their collaboration in this work.
Ryan J. Cox, DMD, is an orthodontic graduate student at the University of Alabama at Birmingham and will enter private practice in early 2012. This article has been derived from his master’s thesis on the imaging of teeth and ceramic brackets with the use of ultrashort echo time magnetic resonance imaging.
Dr. Chung H. Kau is Chairman and Professor in the Department of Orthodontics, School of Dentistry, at the University of Alabama at Birmingham. He has a keen interest in imaging and has written two books on 3-D imaging.
References
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