Download Image artifact in dental cone

Image artifact in dental cone-beam CT
Akitoshi Katsumata, DDS, PhD,a Akiko Hirukawa, RT,b Marcel Noujeim, DDS, MS,c
Shinji Okumura, RT,d Munetaka Naitoh, DDS, PhD,e Masami Fujishita, DDS, PhD,f
Eiichiro Ariji, DDS, PhD,g and Robert P. Langlais, DDS, MS,h Gifu and Nagoya,
Japan, and San Antonio, Texas
ASAHI UNIVERSITY SCHOOL OF DENTISTRY, AICHI-GAKUIN UNIVERSITY DENTAL HOSPITAL,
AND THE UNIVERSITY OF TEXAS HEALTH SCIENCE CENTER AT SAN ANTONIO
Purpose. The purpose of this study was to investigate the appearance and possible cause of an artifact seen
in limited-volume cone-beam CT imaging.
Methods. A water-filled plastic cylinder was used as a phantom of the head. A test object was constructed
as a bone-equivalent phantom to be imaged. The test object was variously positioned at the center of the
phantom and near its margins. CT images of the test object were acquired using a 3DX Accuitomo system.
Results. In slice images with the test object positioned near the margin of the phantom, arch-shaped defects or deformities
were observed on the side of the object. There was a negative correlation between the artifact and the CT value
of the object. The artifact was larger in images scanned with a higher voltage.
Conclusion. The probability that this artifact is caused by halation from the image intensifier (II) system is suggested.
(Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2006;101:652-7)
Limited-volume cone-beam computed tomography
(CBCT) systems for dental use include the 3DX
Accuitomo (Morita, Kyoto, Japan) and the PSR9000N
(Asahi Roentgen, Kyoto, Japan). The usefulness in dentistry of limited-volume CBCT has been reported.1-6
With progress in the clinical and experimental applications of this imaging modality, a unique-shaped defect
in the CBCT images of the object was observed. We
noted this image defect appeared often in images of solid
nonanatomical objects placed on the dental arch, such as
diagnostic stents (guide splint) for the precise placement
of dental implants and rectangular-shaped radiopaque
reference markers for the quantitative assessment of
a
Associate Professor, Department of Oral Radiology, Asahi University School of Dentistry, Gifu, Japan.
b
Radiologic Technologist, Aichi-Gakuin University Dental Hospital,
Nagoya, Japan.
c
Chief Resident, Department of Dental Diagnostic Science, Dental
School, The University of Texas Health Science Center at San Antonio, San Antonio, Texas.
d
Chief Radiologic Technologist, Aichi-Gakuin University Dental
Hospital, Nagoya, Japan.
e
Associate Professor, Department of Oral and Maxillofacial Radiology, Aichi-Gakuin University School of Dentistry, Nagoya, Japan.
f
Professor, Department of Oral Radiology, Asahi University School
of Dentistry, Gifu, Japan.
g
Professor, Department of Oral and Maxillofacial Radiology, AichiGakuin University School of Dentistry, Nagoya, Japan.
h
Professor, Department of Dental Diagnostic Science, Dental School,
The University of Texas Health Science Center at San Antonio, San
Antonio, Texas.
Received for publication Apr 21, 2005; returned for revision Jul 14,
2005; accepted for publication Jul 19, 2005.
1079-2104/$ - see front matter
Ó 2006 Mosby, Inc. All rights reserved.
doi:10.1016/j.tripleo.2005.07.027
652
periodontal disease. This phenomenon was seen predominantly in the images of radiopaque materials such
as hydroxyapatite (HAp) containing resin and aluminum. In addition, this phenomenon was most often
seen when these intraoral objects were positioned near
the facial surface. Fig. 1 demonstrates a CBCT image
of a rectangular-shaped radiopaque test object placed
on the upper molar teeth. Arch-shaped or curved defects
in the images were observed on the mesial and distal
margins of a rectangular test object (Fig. 1).
Van Daatselaar et al.7 reported that data discontinuity
in limited area CBCT imaging causes specific artifacts.
As this phenomenon was thought to be a CBCT artifact
related to halation from the image intensifier (II) we subsequently designed an in vitro investigation to study this
artifact. The purpose of this preliminary report is to discuss the properties of this presumed II halation artifact.
MATERIALS AND METHODS
Test objects and fan-beam CT imaging
Several 10 3 10 3 20 mm-sized test objects were
prepared from the following materials: inner trabecular
and cortical bone-equivalent hydroxyapatite (HAp) content resinous phantom (Tough Bone Phantom, Kyoto
Kagaku, Kyoto, Japan), aluminum, acrylic radiationfiltering sheets (KYOWAGLAS-XA, Kuraray, Tokyo,
Japan), and copper.
A 150-mm diameter 3 200-mm tall water-filled plastic cylinder was used as the head phantom. As shown
in Fig. 2, a test object was positioned at 3 points in the
water-filled phantom: the center, near the lateral margin,
and near the frontal margin. The CT values (Hounsfield
Unit: HU) of the test objects were measured using a
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Katsumata et al. 653
Fig. 1. A rectangular shaped radiopaque test object was placed on the occlusal plane of the upper molar (A). In limited cone-beam
CT images (B), arch-shaped defects (arrows) were observed on the medial and distal side of the test object.
Fig. 2. Schematic drawing demonstrating the water-filled phantom and the variation of test objects positioned on the phantom.
whole-body fan-beam CT scanner (High Speed NX/
iPro, GE Yokogawa Medical Systems, Tokyo, Japan).
A 2-mm slice thickness was selected for the images of
each object positioned at the center of the water-filled
phantom. The scan was set for 120 kV, 150 mA, and
0.7 s/rotation. The CT value was measured using a circular region of interest (ROI) with a diameter of 8
mm. The circular ROI was set as the center of the image.
CBCT imaging
A limited-volume CBCT system (3DX Accuitomo,
Morita, Kyoto, Japan) was used. This system is equipped with a 4-inch II tube. The water-filled phantom
was placed in the machine in the usual correct orientation of the patient’s head. A CBCT scan was performed
with a rotation of 3608 for data acquisition. The exposure factors were 70 and 80 kV, 3 mA, 17-second rotation time with no added filtration. The limits of the
imaging area consisted of a cylinder 30 mm in height
and 40 mm in diameter. The geometrical location of
the area to be imaged was determined by using the
included fluoroscopic view in which a test object was
positioned at the center of the area to be imaged.
Three-dimensional sectional images in the axial, frontal, and sagittal planes were reconstructed with a 1-mm
slice thickness and no slice interval. The gray-scale
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Table I. CT value of test objects
Material
Trabecular bone equivalent phantom
Cortical bone equivalent phantom
Aluminum
Resinous x-ray filter
Copper
CT value (HU)
330
795
2140
3750
over 4000
CT, computed tomography; HU, Hounsfield unit.
(bit depth) of the images was 8 bits. An axial image
demonstrating the center part of a test object was used
for evaluation. Using a software system (I-View,
Morita, Kyoto, Japan) with which the standard 3DX
Accuitomo is equipped, the level and width of the gray
scale value of the image was adjusted in the histogram
to enable optimal interpretation. At the same time, the
original fluoroscopic image of the 3608 scan, which was
required and used by the software for slice image reconstruction, was obtained in an audio-video interleaved
(avi) movie file format.
The quality of the resulting image was observed,
noting the contrast, noise, and effect of the presumed
II halation artifact. The effect of the artifact on the image
was evaluated by comparing images of a test object positioned at the center of the water-filled phantom and
near the lateral or frontal margins of the phantom. The
area of a test object was measured using image-editing
software (Adobe Photoshop CS, Adobe Systems Inc,
San Jose, CA). The area of the image affected by the artifact was calculated based on the area in which the test
object was positioned at the center of the water container
as a control. The affected area due to artifacts was observable grossly. Area measurements were performed
3 times by one radiologist and the average area was
used. A movie file of the fluoroscopic projection was
used to observe the appearance of presumed halation
from the II tube.
RESULTS
The CT values of the test object are shown in Table I.
Only the copper component demonstrated a higher CT
value than the reliable measurement range (from ÿ1000
to 4000). Axially sliced images of the test objects are
shown in Fig. 3. In images in which the test object was
positioned at the center of the water container, the exact
geometrical shape was clearly visible, except for the
copper test object, which was deformed by metal spray
artifacts. The images of the inner trabecular bone phantom were rather noisy and lower in contrast than were the
other objects.
In images where the test object was positioned near
the lateral margin of the water container, arch-shaped
defects or deformities of the image were observed on
the lateral side. When the test object was positioned
near the frontal margin, an arch-shaped defect appeared
on the frontal side. Fig. 4 shows gray intensity profiles of
the distorted image when a test object was positioned
near the lateral margin of the water container. Fig. 5
shows the relationship between the relative area affected
by artifact and the CT value of the objects. A negative
correlation between the affected area from artifacts
and the CT value of the objects was found. The affected
area was larger when the test object was positioned near
the frontal margin than near the lateral margin of the
phantom. The affected area from artifacts was larger
in images made using 80 kV than with 70 kV.
When a severe artifact appeared, presumed halation
from II was seen in the original fluoroscopic view of
the projection and the outline of the test object was
blurred. On the other hand, when no artifacts appeared,
presumed halation from II was not observed in the original fluoroscopic view.
The timing of the appearance of the presumed
halation was compared with a schematic drawing that
represented the geometric arrangement among the focus, the x-ray beam, the water-filled phantom, the object
to be imaged, and the II during the 3608 scan. As shown
in Fig. 6, when a test object was positioned near the lateral margin of the water container, halation occurred
twice in the fluoroscopic image. The first incidence of
presumed halation peaked when the scan reached 808/
3608 of rotation, and the second incidence of halation
peaked at 2608/3608 of rotation. The timing of the
appearance of halation corresponded with the geometric
arrangement whereby part of the x-ray beam reached
the II tube directly without passing through the waterfilled phantom.
DISCUSSION
The 3DX Accuitomo, a limited-volume CBCT
system, was developed by using the platform of the
SCANORA (Sordex Orion Corporation, Helsinki, Finland), a dento-maxillary multimodal tomographic system.1,8 Another limited-volume CBCT system known
as PSR9000N also inherited technology from another
dento-maxillary multimodal tomographic system, the
AZ3000 (Asahi Roentgen).6 The modified use of these
dental tomographic platforms enables the cross-sectional imaging of a small defined area in the jaws and
dental arches.8 The factors were taken into consideration while building the CBCT platforms including geometric anatomical templates of the jaws and dental
arches, a compact machine size to fit in dental offices
and clinics, the comparatively lower cost than conventional fan-beam CT imaging, an efficient low-dose
x-ray generating system, and an x-ray tube and detector
arrangement for the precise imaging of a limited area.
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Katsumata et al. 655
Fig. 3. Axial limited cone-beam CT images showing the variations of the materials of a test object, the position of a test object in
the water-filled phantom, and the x-ray kilovoltage used.
The CBCT system studied includes a fluoroscopic apparatus with II, which is necessary to generate precise
images of hard tissue structures and to reduce dose.
The most remarkable advantage of limited-volume
CBCT is in the minimization of radiation dose. This advantage was achieved by a highly precise combination
of the aforementioned elements.
Difficulty in the acquisition of reliable CT values
is the main disadvantage of low-dose CBCT imaging
resulting in poor soft tissue resolution. On the other
hand, this disadvantage might be helpful to reduce metal
and beam hardening artifacts, which are inherent in CT
imaging. In several reports, authors confirmed CBCT
using II demonstrated sufficient image quality for the
depiction of hard tissue structures.5,6,9-11
However, no phenomenon similar to the present II
presumed halation artifact has been reported in the literature that we have reviewed. This artifact appeared only
656 Katsumata et al.
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Fig. 4. Grey intensity profiles of a distorted image. An inner trabecular bone-equivalent phantom was positioned near the lateral
margin of the water container. An 80 kV, 3 mA x-ray was used.
Fig. 5. Relationship between the relative area affected by
artifact and the CT value of the test objects.
when the area to be imaged was positioned near the facial surface. In addition, this artifact will not appear in
CBCT systems using II which are designed to scan large
fields of view (FOV) whereby larger II tubes are used.
Some systems include the NewTom 9000 and G-3
(NIM srl, Verona, Italy)10 and CB MercuRay (Hitachi
Medico Technology, Chiba, Japan).11 When the FOV
is large enough to embrace the entire head, the amount
of x-ray passing the object is not changed—even when
seen from any of the 3608 of scanning direction. In
this design, the fluoroscopic unit can be easily adjusted
as to not generate halation from the II. In limitedvolume CBCT imaging, the size of the FOV is small
as compared to the entire head and the intensity of
Fig. 6. Schematic drawing representing the geometric
arrangement of the focus, the x-ray beam, the water-filled
phantom, the object to be imaged, and the II at the initial scanning position and at the time when halation occurs (left). Original fluoroscopic images (right).
transparent x-radiation fluctuates during the 3608 scan
(Fig. 6). When an incident occurs in which some part
of the x-ray beam reaches the fluorescent surface without passing through the patient’s head, presumed halation from II occurs. An II tube usually has a circular
fluorescent surface area, which is bigger than the
rectangular area used for CBCT imaging. Halation from
II is common in medical fluoroscopic examinations.
Ordinarily, fluoroscopic imaging systems are equipped
with a mechanism that enables the reduction of halation
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by adjusting the intensity of the x-ray beam or the
brightness gain of the II unit automatically or manually.
As shown in this study, we have determined the appearance of II presumed halation artifact is predictable
by the geometric arrangement of the area to be imaged.
Since this artifact has been recognized, it might be possible to improve limited-volume CBCT images by automatically reducing this artifact as in fluoroscopy. In
practice, a lower voltage or current setting of the x-ray
tube can reduce the influence of this artifact. However,
it is clear that insufficient x-ray intensity leads to reduced
image quality. Another possibility involving more radiation is a reduction in the sensitivity or brightness setting
of the II unit. The establishment of a reliable standard
diagnostic object-related scanning parameter setting for
low-dose limited-volume CBCT imaging is desirable.
Van Daatselaar et al.7 reported that in an experimental
limited-volume CBCT imaging study of a dried mandible using a charge-coupled device (CCD) x-ray detector,
a ‘‘bright band artifact’’ was observed in the edges of
imaging area. They mentioned that this bright band artifact was caused by the placement of radiopaque objects
outside of the reconstructed imaging area. In addition,
they suggested that the effect from this artifact could
be reduced when appropriate filter algorithms were
applied to the image reconstruction. The similarity between bright band and II halation artifact is that they
are both caused by data discontinuity in local CT reconstructions. Application of certain filter algorithms may
be useful to reduce II halation artifact as well. The II presumed halation artifact as observed in our experiment
was not reported in the above experimental CBCT imaging study using a CCD detector. Although it may increase cost, the use of a CCD or a thin film transistor
(TFT) flat-panel x-ray detector may be helpful to reduce
or eliminate this artifact.
This in vitro study demonstrates II presumed halation
artifacts might influence the depiction of osseous structures when an object is located near the lateral or frontal
margin of the face. The lateral placement of test objects
in this in vitro study simulated the position of the temporomandibular joint (TMJ) condyle and the anterior dental arch. Critical misdiagnosis in the assessment of
alveolar bone levels or cortical bone thickness and the
presence of osseous degenerative lesions in the TMJ
should be considered. Metal artifacts in conventional
fan-beam CT images are common. Metals producing artifacts are usually detectable; it is difficult to detect the
appearance of II halation artifacts in CBCT because
their cause is outside the imaged area. Further study
is necessary to elucidate the characteristics of these II
presumed halation artifacts.
Katsumata et al. 657
CONCLUSIONS
An artifact seen in images of limited-volume CBCT
scans is reported. It is suggested this artifact is caused
by halation from II. This II presumed halation artifact
in limited-volume CBCT might appear when a radiopaque object to be imaged is located near the surface
of the body. This artifact was found to be closely related
to the type of tissue or object (ie, CT value) and the x-ray
energy applied.
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Reprint requests:
Akitoshi Katsumata, DDS, PhD
Department of Oral Radiology
Asahi University School of Dentistry
1851-1 Hozumi, Mizuho-shi
Gifu pref., 501-0296, Japan
[email protected]