Introduction
The purposes of this study were to examine the accuracy and the head positioning effects on measurements of anterior tooth length using 3-dimensional (3D) and conventional dental panoramic radiography and to investigate whether 3D panoramic radiography is suitable for the evaluation of anterior tooth length.
Methods
A simulated human head was radiographed at 4, 8, and 12 mm displaced positions, and at 5°, 10°, and 15° tilted positions from the standard head position using 3D and conventional panoramic radiography, and also using cone-beam computed tomography. Anterior tooth lengths were measured on the panoramic and cone-beam computed tomography images. The values for the standard head position in the panoramic radiographs were defined as the standard values. Measurement error was defined as the standard value minus the cone-beam computed tomography value on each panoramic radiograph. The head position ratio of the measurement value to the standard value at each head position was calculated.
Results
Measurement errors for the 3D panoramic radiographs were significantly smaller than those for the conventional panoramic radiographs. In the 3D panoramic radiographs, the head position ratios at the 4, 8, and 12 mm displaced positions and at the 5° tilted position were within ±5% of the standard value.
Conclusions
Three-dimensional panoramic radiography is suitable for the quantitative evaluation of anterior tooth length with high accuracy.
Highlights
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Accuracy of anterior tooth length using 3D dental panoramic radiography is described.
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Measurement errors were smaller with 3D panoramic than with conventional radiographs.
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Three-dimensional panoramic radiography is suitable for quantitative evaluation of anterior tooth length.
Quantitative evaluation of tooth length is important for the observation of external apical root resorption, which is a frequent undesirable complication of orthodontic treatment. Conventional periapical and panoramic radiographs have been routinely used for the evaluation of tooth length. Periapical radiographs may reflect interradiographer variability, however, because they are difficult to standardize. The panoramic radiograph does not reflect interradiographer variability but does have several other disadvantages including nonconstant magnification, image distortion, and a narrow image layer. Recently, cone-beam computed tomography (CBCT) has been shown to be reliable for evaluation of tooth lengths. However, CBCT must be used with extreme caution to avoid additional unnecessary radiation exposure. Repeated CBCT examinations, which are necessary for the measurement of tooth length during orthodontic treatment, are not recommended because of the increased radiation exposure. As a result, conventional radiographs have been used for evaluating tooth lengths despite the several disadvantages.
Three-dimensional (3D) panoramic images can be obtained using a new panoramic radiography with tomosynthesis and 3D mapping technique. The 3D panoramic radiograph seems to be suitable for quantitative measurements because it provides a large image layer with no image magnification. Because the 3D equipment has a larger image layer, it has been reported that measurement errors with the 3D panoramic radiograph are small, and head positioning has less influence on the measurements. In the conventional panoramic radiograph, which is the projected image, the projected size differs from the actual size because of varying anteroposterior inclinations. A previous study reported that if a vertical object is inclined toward the film, its projected length will decrease. Thus, it is difficult to evaluate tooth lengths using conventional panoramic radiographs because of the narrow image layer and the inclination of the teeth in the anterior tooth region. It would be useful to compare tooth lengths obtained using 3D panoramic radiographs with measurements obtained using conventional panoramic radiographs. If tooth lengths can be measured in the same way as CBCT using 3D panoramic radiography, a routine element of examination in orthodontic practice, this information will be helpful in the diagnosis and treatment of orthodontic patients.
The purposes of this study were (1) to examine the accuracy of anterior tooth lengths obtained using 3D and conventional dental panoramic radiography, (2) to examine the effect of varying head positions on radiographs made using the 2 panoramic radiographs, and (3) to investigate whether 3D panoramic radiography is suitable for the evaluation of anterior tooth length.
Material and methods
A simulated or phantom human head (dental x-ray head phantom; Kyoto Kagaku, Kyoto, Japan) was used as the subject. The phantom is composed of soft tissue and hard tissue equivalent media with x-ray absorption rates equivalent to those of the human body.
Images of the subject were recorded using a lateral cephalometric scanner (CX-150W; Asahi Roentgen Industry, Kyoto, Japan), and both 3D and conventional dental panoramic radiography equipment (QR master-P; Telesystems, Osaka, Japan; and AUTO1000; Asahi Roentgen Industry, respectively). A 2.0-mm thick occlusal bite was placed in the anterior teeth of the phantom. The distances of the x-ray source and the detector were 965 mm in the 3D equipment and 916 mm in the conventional panoramic radiography equipment. In the 3D panoramic radiography equipment, x-ray tube voltage and current were 80 kVp and 4 mA, respectively. The voltage and current were 70 kVp and 6 mA, respectively, in the conventional panoramic radiography equipment. The phantom was positioned with the Frankfort horizontal plane parallel to the floor, the midsagittal plane perpendicular to the floor, and the maxillary left canine cusp tip aligned with the canine light guide. This position was defined as the standard head position. Using a graduated seat (Manual Positioner; SIGMA KOKI, Tokyo, Japan) ( Fig 1 ) that accurately adjusts the position of the phantom’s head, the phantom was radiographed at the standard and various head positions by an author (M.M.) 5 times with minimum intervals of 1 day. The various head positions were 4, 8, and 12 mm displaced forward; 4, 8, and 12 mm displaced backward; 4, 8, and 12 mm displaced right; 4, 8, and 12 mm displaced left; 5°, 10°, and 15° tilted upward; and 5°, 10°, and 15° tilted downward from the standard head position ( Fig 2 ). For examination of the tilted head, the maxillary left canine cusp tip was aligned with the canine light guide after tilting the head positions. Images of the phantom were also recorded using a CBCT scanner (Alphard-3030; Asahi Roentgen Industry). The x-ray tube voltage and current were 80 kVp and 5.0 mA, respectively. Slice thickness was 0.3 mm with a pixel size of 0.3 mm.
