Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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 OOOOE Volume 101, Number 5 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 OOOOE May 2006 654 Katsumata et al. 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. OOOOE Volume 101, Number 5 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. OOOOE May 2006 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 OOOOE Volume 101, Number 5 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. REFERENCES 1. Arai Y, Tammisalo E, Iwai K, Hashimoto K, Shinoda K. Development of a compact computed tomographic apparatus for dental use. Dentomaxillofac Radiol 1999;28:245-8. 2. Terakado M, Hashimoto K, Arai Y, Honda K, Sekiwa T, Sato H. Diagnostic imaging with newly developed ortho super-high resolution computed tomography (Ortho-CT). Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2000;89:509-18. 3. Honda K, Larheim TA, Johannessen S, Arai Y, Shinoda K, Westesson PL. Ortho cubic super-high resolution computed tomography: a new radiographic technique with application to the temporomandibular joint. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2001;91:239-43. 4. Hashimoto K, Arai Y, Iwai K, Araki M, Kawashima S, Terakado M. A comparison of a new limited cone beam computed tomography machine for dental use with a multidetector row helical CT machine. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2003;95:371-7. 5. Naitoh M, Katsumata A, Mitsuya S, Kamemoto H, Ariji E. Measurement of mandibles with microfocus x-ray computerized tomography and compact computerized tomography for dental use. Int J Oral Maxillofac Implants 2004;19:239-46. 6. Kobayashi K, Shimoda S, Nakagawa Y, Yamamoto A. Accuracy in measurement of distance using limited cone-beam computerized tomography. Int J Oral Maxillofac Implants 2004;19: 228-31. 7. van Daatselaar AN, Dunn SM, Spoelder HJ, Germans DM, Renambot L, Bal HE, et al. Feasibility of local CT of dental tissues. Dentomaxillofac Radiol 2003;32:173-80. 8. Tammisalo E, Hallikainen D, Kanerva H, Tammisalo T. Conprehensive oral x-ray diagnosis: Scanora multimodel radiography. A preliminary description. Dentomaxillofac Radiol 1992;21:9-15. 9. Linsenmaier U, Rock C, Euler E, Wirth S, Brandl R, Kotsianos D, et al. Three-dimensional CT with a modified C-arm image intensifier: feasibility. Radiology 2002;224:286-92. 10. Schulze D, Heiland M, Blake F, Rother U, Schmelzle R. Evaluation of quality of reformatted images from two cone-beam computed tomographic systems. J Craniomaxillofac Surg 2005;33: 19-23. 11. Araki K, Maki K, Seki K, Sakamaki K, Harata Y, Sakaino R, et al. Characteristics of a newly developed dentomaxillofacial x-ray cone beam CT scanner (CB MercuRay): system configuration and physical properties. Dentomaxillofac Radiol 2004;33: 51-9. 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]