Influence of orthodontic appliance-derived artifacts on 3-T MRI movies
Ozawa et al. Progress in Orthodontics
Influence of orthodontic appliance-derived artifacts on 3-T MRI movies
Erika Ozawa 0
Ei-ichi Honda 2
Kulthida Nunthayanon Parakonthun 1
Hiroko Ohmori 0
Kazuo Shimazaki 0
Tohru Kurabayashi 3
Takashi Ono 0
0 Department of Orthodontic Science, Tokyo Medical and Dental University (TMDU) Graduate School , 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549 , Japan
1 Department of Pedodontics & Preventive Dentistry, Srinakharinwirot University , Bangkok , Thailand
2 Department of Oral and Maxillofacial Radiology, The University of Tokushima , Tokushima , Japan
3 Department of Oral and Maxillofacial Radiology, Tokyo Medical and Dental University (TMDU) Graduate School , Tokyo , Japan
Background: Magnetic resonance imaging (MRI) has been used to study configurations of speech organs in the resting state. However, MRI is sensitive to metals, and numerous types of metallic appliances, most of which have a large magnetic susceptibility, are used in orthodontic treatment and may cause severe artifacts on MRI. We have developed techniques for obtaining MRI movies of the oral region, to evaluate articulatory changes, especially movement of the tongue, palate, and teeth, pre- and post-orthodontic/orthognathic treatment. We evaluated the influence of artifacts caused by orthodontic appliances, including fixed retainers, metal brackets, and wires, on measurements in 3-T MRI movies. Methods: Sixteen healthy young adults (nine males, seven females; average age, 27 years) with normal occlusion were recruited. Four types of customized maxillary and mandibular plates were prepared by incorporating one of the following into the plate: (a) nothing, (b) a fixed canine-to-canine retainer, (c) metal brackets for the anterior and molar teeth, or (d) clear brackets for the anterior teeth and metal brackets for molars. A 3-T MRI movie, in segmented cine mode, was generated for each plate condition while participants pronounced a vowel-consonant-vowel syllable (/asa/). The size of the artifact due to the metallic brackets was measured. The face size and orthodontically important anatomical structures, such as the velum, the hard palate, and the laryngeal ventricle, were also measured. Results: A large artifact was observed over the entire oral region around orthodontic appliances, altering regional visibility. The velopharyngeal height was measured as significantly longer in the presence of metal brackets. The maximum artifact size due to a metallic bracket was > 8 cm. Our results show that even if it is possible to obtain the measurements of palate length, nasion to sella, and nasion to basion in individuals wearing metal brackets for molars, the measurements might be affected due to the presence of artifacts. Conclusions: Orthodontic appliances, including metallic materials, sometimes produce significant measurement error in speech evaluation using MRI movies, which often become invisible or distorted by metallic orthodontic appliances. When the distorted image is measured, caution should be exercised, as the measurement may be affected. Based on the study, it is concluded that orthodontists should not necessarily remove all metallic appliances before MRI examination because the influence varies among the appliances and should also know that a significant measurement error in speech evaluation using MRI movie may occur by image distortion caused by metallic artifacts.
Magnetic resonance imaging (MRI) produces images of
soft tissues with higher contrast resolution than computed
tomography (CT), without radiation exposure, and various
MRI software techniques and hardware have been
developed. This technique has been applied to assess the
dynamic movement of the heart. Moreover, by combining
several technologies, observation of the movement of the
speech organ also became possible [
At present, many heart studies [
] and some speech
organ studies [
] have been performed using MRI
movies, but delineation of hard tissues, including the
bone and teeth, remains difficult, although conventional
methods for teeth delineation in MRI have been
]. Our group researches MRI movie
techniques for oral lesions, to evaluate articulation changes,
especially movement of the tongue, palate, and teeth,
before and after orthodontic and oral surgical treatment
] and have reported many relevant findings [
Articulation disorders are caused by various factors,
including congenital deformation, malocclusion,
developmental maxillofacial disturbances, and tongue
habits. Orthodontics is used to treat these conditions.
Treatment starts in childhood, in many cases, and
radiographic imaging is frequently used. For
protection from radiation, patient exposure should be
reduced as much as possible, especially in children.
MRI is therefore suitable because it does not require
the use of radiation, and MRI movies are the best
approach to observe articulation over an extended
period. Nevertheless, MRI is more sensitive to metals
than radiography, and metallic artifacts caused by
metallic fillings, such as crowns or inlays often appear in
the oral maxillofacial region, complicating diagnosis.
Moreover, numerous types of metallic appliances are
used in orthodontic treatment and most have a large
magnetic susceptibility, leading to severe artifacts on
MRI. Although it is ideal to remove all metallic
appliances before MRI examination, debonding and
rebonding may harm the tooth enamel and extend
the treatment period [
]. Knowledge of the effect of
various orthodontic appliances on MRI can
circumvent unnecessary removal of orthodontic appliances
and is important for orthodontists and radiologists.
The aim of this study was to examine the influence of
orthodontic appliance-derived artifacts on MRI movie
images and to evaluate their influence on diagnosis. The
null hypothesis was that the orthodontic metallic
appliances do not lead to artifacts in MRI examinations.
Nine healthy males and seven healthy females with
normal occlusion were recruited. The mean age of the male
group was 26.8 ± 0.6 (mean ± standard deviation [SD])
years, and that of the female group was 26.3 ± 0.5 years.
Males and females did not significantly differ with
respect to overjet, overbite, upper arch length, or lower
arch length (Table 1). Written informed consent was
obtained from all 16 subjects prior to their participation
in the study. All procedures in this study were approved
by the Ethics Committee (No. 1282) of Tokyo Medical
and Dental University and complied with the Code of
Ethics of the World Medical Association (Declaration of
Description of procedures
Four types of customized maxillary (Mx) and mandibular
(Md) plates made of Biostar® (DURASOFT PD, Rocky
Mountain Morita, Tokyo, Japan) were prepared (Fig. 1). The
plate types were divided as follows: nothing incorporated in
the plate (type 1), a fixed canine-to-canine retainer (HI-T™ II
TWIST-WIRE, 0.015" (0.381-mm) in diameter, 3M, Tokyo)
incorporated in the plate (type 2), 0.018" (0.4572-mm)-slot
metal brackets (CLEAR BRACKET SL+, Dentsply, Tokyo)
for anterior teeth and metal brackets for molars (OPA-K,
TOMY INTERNATIONAL INC., Tokyo) incorporated (type
3), and 0.018”-slot clear brackets (CrystaBrace3, Dentsply,
Tokyo) for anterior teeth and metal brackets for molars
(OPA-K, TOMY INTERNATIONAL INC.) incorporated
(type 4; Fig. 1). No archwire was applied.
Magnetic resonance imaging of subjects
A 3-Tesla (3-T) MRI scanner (Magnetom Spectra, Siemens,
Munich, Germany) was used, and movie images were taken
following the method reported by Nunthayanon and
]. The mid-sagittal plane was imaged
using the following parameters: TR = 22.5 ms, echo
time (TE) = 2.07 ms, field of view = 256 × 256 mm,
pixel size = 1 × 2 mm, slice thickness = 4 mm, and the
total acquisition time was 24 s.
Speech task and speech condition
The subjects were required to repeat the
vowel-consonantvowel (VCV) syllable (/asa/) 16 times at 1500-ms intervals.
The consonant was chosen because a previous study
showed that during production of /s/, the velopharyngeal
structures are not affected by upright or supine body
]. Subjects were scanned under six conditions with
the plate: (1) neither Mx nor Md (control), (2) type 1 for
both Mx and Md (C1), (3) type 1 for Mx and type 2 for Md
(C2), (4) type 2 for both Mx and Md (C3), (5) type 3 for
both Mx and Md (C4), and (6) type 4 for both Mx and Md
(C5; Fig. 1). The subjects were not aware of the plate type
used in the experiment. Recorded sound data were
manipulated by Kulthida’s method [
]. Sound stage was
determined by the manipulated sound data. Images obtained at
the rest phase (100 ms before pronunciation; stage A) and
the consonant phase (the middle of consonant /s/; stage B)
were chosen for analysis (Fig. 2).
Definition of measurements
Twelve images of stages A and B were obtained in the
six conditions. Eight linear parameters were determined
based on the study by Perry [
] (Table 2, Fig. 3). The
standard line (Sella-Nasion [SN] reference) was based on
the T1-weighted image by turbo spin echo (TSE)
sequence. Five facial measurements were also performed
on the TSE image by Perry (Table 3 and Fig. 3). There
were significant sex differences in two parameters
(Nasion to Sella and face height; Table 5; p < 0.01). The
measurements were performed using Osirix MD
(Pixmeo Sarl, Geneva, Switzerland).
Magnetic resonance imaging with orthodontic appliances
Each bracket was placed on a pole in an acrylic cylinder
container (diameter: 4.5 mm) filled with water. The
sagittal, coronal, and transverse direction images were
obtained by TSE and gradient-echo (FLASH). The range
of artifacts surrounding each bracket was evaluated
based on the method by Imai et al. [
Four kinds of subtracted images were generated. First,
the control image was subtracted from the C1 image
to confirm similarity. Because there was no clear
difference, the C1 image was defined as a standard, and
three kinds of subtraction imaging (C1–C3, C1–C4, and
C1–C5) were performed. Paired images were visually
aligned with the reference to an outline of the skull and
cropped to the same size by customized software (Image
Rugle 2009 Medic Engineering Inc., Kyoto, Japan). After
processing, the images were subtracted by Adobe Photoshop
CS6 software (Adobe Systems Inc., San Jose, CA).
Each organ on MRI movie images was measured
three times over 5 days. A single examiner (EO)
performed measurements to avoid interobserver error.
Intraobserver reliability was assessed by intraclass
correlation coefficients (ICC). The measurement errors
determined by ICC were very small (range: p < 0.05),
indicating the measurements were reproducible.
Analysis of variance (ANOVA) was used to evaluate
the difference of the length between stages A and B
and among each condition. Dunnett’s test, for
multiple comparisons between the control and each
condition, was used when a significant difference was
observed. Significance level was defined as p < 0.05.
With regard to velar size, the logistic analysis of the
relation between the number of measurable cases and
the face size in C4 and C5 was used. The analysis
was performed in two dimensions of the velar depth
(VD) and velar height (VH), given that the velum
plays an important role in pronunciation.
Typical MRI movie images taken in the six
conditions are shown in Fig. 4, and representative sound
data combined with the image of subject are shown
in Fig. 2. At stage A, the articulators were in the
rest position, in which the velum was placed on the
posterior aspect of the tongue. At stage B, the
tongue actively moved anteriorly toward the
premaxillary area. No artifact was observed in C1 at either
stage. In C2 and C3, clear artifacts were observed
around the lips and incisors, and the surrounding
soft tissues disappeared. In C4 and C5, more severe
artifacts were observed all over the oral cavity, and
the tongue and palate disappeared completely.
The number of measurable cases is shown in Tables 4.
The number in both stages was similar. All distances
could be measured in C1 at both stages. A significant
sex difference was observed in RS in C4 and C5 at stage
A and in RS in C4 at stage B (p < 0.05).
Regional visibility was different between the fixed
retainer and the orthodontic brackets (Table 5). In the
fixed retainer condition, only the anterior part of the
tongue was invisible in all cases. Although the
velopharynx, the posterior part of the tongue, and the vocal fold
were clearly visible, the hard palate (HPL) was
occasionally visible. In contrast, with the orthodontic
brackets, most organs were invisible; only the vocal fold
was always clearly visible.
The eight length measurements where significant
differences were observed at stages A and B in the six
conditions are shown in Figs. 5 and 6. Only the VH in C4 at
stage A and in C4 and C5 at stage B was significantly
longer than that of the control, because the upper
margin was located superiorly (p < 0.05). It was impossible
to measure the HPL in C4 and C5 and the lingual apex
−pharyngeal (LAPL) and lingual apex−hard palate
(LAHL) in C2–C5 because anatomical structures
disappeared due to the severe artifacts.
Typical MRI images of the brackets for the anterior
and molar teeth are shown in Fig. 7, and the
measurements are shown in Fig. 8. The most severe artifact was
observed in a gradient echo image involving a molar
bracket. The artifact size was greater than 8 cm in
diameter. Parameters of the brackets for the incisor could be
measured, but most parameters of the molar bracket
could not be measured. There was a significant
difference between turbo echo and gradient echo images in
the incisor bracket.
There was no significant difference in any of the items
(Tables 6). However, there was a tendency for an
Table 5 Visibility of each organ in subjects wearing orthodontic
Anterior part of tongue
Posterior part of tongue
Symbols: ○, clearly visible; △, occasionally visible; ×, completely invisible
increase in the length of the Nasion (N) to Basion (Ba)
and the palate length (ANS to PNS), which
corresponded with the increase in the number of measurable
Figure 9 shows the subtracted images (orthodontic
appliance vs. no orthodontic appliance). The lips and the
anterior part of the tongue were affected by the fixed
retainer. Artifacts from the brackets affected the lips, the
whole tongue, and the chin, while the larynx was not
The measurable range in subjects wearing orthodontic metallic appliances
The severity of artifacts mainly depends on the size,
shape, and magnetic susceptibility of the metal of the
orthodontic appliance [
]. The influence of metallic
materials used in orthodontic treatment on MRI has
been reported in a number of studies [
Nonetheless, no studies have reported quantitative
data of the artifact range generated by an orthodontic
bracket, including the range of organs influenced by
In our study, the null hypothesis was rejected: the
orthodontic metallic bracket for the molars used in
this study caused a very severe artifact. The results
indicated that tissues located within a 5-cm radius
from a bracket could be influenced by an artifact,
and observation and measurement of organs and
tissues in this range would be distorted. The tongue,
HPL, velopharynx, cervical vertebrae, epiglottis, and
hyoid bone are included in the area, and the range
could be clearly seen in the subtraction images.
Thus, caution should be exercised when measuring
the length of organs in patients wearing orthodontic
appliances during MRI. In terms of the type of
metallic bracket, there was a significant difference in
artifact size between brackets used on the anterior
and on the molar teeth, probably due to the weight
and composition of the brackets. The metallic
weights of the anterior and molar brackets are
approximately 0.01 and 0.1 g, respectively. While both
types of brackets are made of stainless steel, the
crystal structure of stainless steel in the anterior
bracket is austenitic and that of the molar bracket is
martensitic. Many studies have reported that
martensitic stainless steel has markedly higher magnetic
susceptibility than austenitic stainless steel [
Thus, the molar brackets caused an immense artifact
because of its heavier weight and higher magnetic
Measurable range due to individual difference
Since metallic artifacts caused image distortion, it was
difficult to measure the target region influenced by the
artifact accurately. Despite a clear metallic artifact, the
length of the velum was measurable in some cases.
There was no significant sex difference in the length of
the palate (Table 3). However, the upper pharyngeal
space in males was wider than that in females. No
reports discuss this difference, and future research on this
topic is necessary.
During general pronunciation, articulatory organs,
such as the tongue and the soft palate, change form. By
observing these organs in MRI movie images during
pronunciation of /asa/, we found that the velar apex was
elevated with mouth opening and contacted the
posterior wall of the pharynx. In some subjects, the velar apex
did not contact the posterior pharyngeal wall. In
contrast, the velar apex in all subjects contacted the
posterior pharyngeal wall during production of /s/, and
velopharyngeal closure was completed. The tongue apex
also approached the lingual surface of the maxillary
When returned to the position for production of /a/,
the tongue blade sagged, and the velar slowly separated
from the posterior wall of the pharynx. In phonetics, the
velopharyngeal closure is completed by the
posterosuperior elevation of the velum, the contraction of the
pharyngeal constrictor muscles, and the formation of
Passavant’s ridge toward the pharyngeal cavity. The
velopharyngeal closure is sometimes not completed during
production of /a/.
The velar apex is elevated by the construction of the
levator veli palatini muscle and changes form.
Production of /s/ is carried out by the formation of a
narrow space between the tongue apex and the lingual
side of the maxillary anterior teeth [
]. Since the
movement of the tongue as well as the velum is related
to pronunciation, orthodontic concerns, such as overjet
and overbite, which determine tongue position, influence
pronunciation. The tongue approaches the lingual side
of the maxillary anterior teeth on production of /s/.
Excessive overjet increased the length of LAPL and RS
in this study, whereas the values of LAPL and RS
showed no change, because there was no significant
difference in overjet.
A tongue-thrusting habit influences pronunciation,
and the tongue position and the RS sometimes change
]. Future studies will be able to diagnose
tonguethrusting habit by using MRI movies.
The velopharynx was rostrocaudally prolonged by metallic
artifacts. The subtraction image showed that the artifact
Table 6 The relationship between number of measurable cases
and individual linear measurements in C4 and C5 at rest and
Nasion to Sella rest
Nasion to Basion rest
Face height rest
Palate length rest
PNS to Basion rest
p > 0.1
NS not significant
p < 0.1
p < 0.1
p < 0.1
p < 0.1
p < 0.1
p < 0.1
p < 0.1
p < 0.1
p < 0.1
p < 0.1
spread in the direction from the head to the mandible.
Although there was a significant difference in the VD
between the rest phase (i.e., stage A) and the consonant
phase (i.e., stage B) in control, C1, C2, and C3, this
difference was not observed in C4 and C5. This may be due to
image distortion by metallic artifacts from the molar
brackets. The VH in C4 and C5 was significantly longer
than that in control. This difference may be attributable to
the weight of the metal. As a metal was included in the
anterior and premolar brackets as well as in the molar
brackets in C4, the size of the artifact in C4 would increase.
In terms of the VH, during production of /s/, the velum
partially contacted the HPL, and the tip was pushed
to the posterior wall of the pharynx. The velar shape
became vertically longer and was easily influenced
by a metallic artifact caused by premolar and molar
brackets in C4 and C5. Consequently, the
measurement was altered, differing from that in the control.
In some cases, the velar length could not be
measured because of the metallic artifact. Whether the
velar length is measurable depends on face size,
because the range of orthodontic appliance-derived
artifacts is almost constant. In this study, there was
no clear relation between face size and measurable
cases, but a trend was noted for the length of N to
Ba and ANS to PNS. This indicates that as the
antero-posterior velar length increases, the
probability of measurable cases also increases.
Phonetic analysis of articulatory movements by MRI
movie is difficult in children who wear orthodontic
appliances because the face is small, and the
articulators are easily influenced by metallic artifacts. In
contrast, orthodontic appliance is not always a
problem in patients with larger faces.
Orthodontic appliances, including metallic materials,
may produce a significant error in measurement of
the articulatory organs when using MRI movie
images. Metallic orthodontic appliances such as the
fixed retainer and metal brackets often make
anatomical structures disappear and become distorted by
signal void caused by metallic artifact. When the velum
is evaluated in a distorted image, caution should be
exercised as the measurement may be altered. In
conclusion, the influence on MRI examination varies
among orthodontic metallic appliances and
orthodontists should not necessarily remove all metallic
appliances before MRI examination. Meanwhile,
orthodontists should know the fact that a significant
measurement error in speech evaluation using MRI
movie may occur by image distortion caused by
metallic artifacts and share the information with
EO made substantial contributions to conception and design. HO made the
ethical document, explained to subjects and gave informed consent. EH
made acquisition of data. KS made the phantom. KN designed the phantom,
selected and supplied materials. TK performed statistical analyisis. TO gave
final approval of the version to be submitted and any revised version. All
authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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