A study of the zone of sharpness of three panoramic x-ray machines and the effect of screen speed on the sharpness zone

Mapping of a multilayer panoramic radiography device

OBJECTIVE

To map the shape, location, and thickness of the focal trough of a panoramic radiography device with a multilayer imaging program.

METHODS

An acrylic plate (148 × 148 × 3 mm) containing 1156 holes distributed in a matrix of 34 × 34 rows was placed in the OP300 Maxio at the levels of the maxilla and mandible. 20 metal spheres (3.5 mm in diameter) were placed on the holes of the plate under 15 different arrangements and panoramic images were acquired for each arrangement at 66 kV, 8 mA, and an exposure time of 16 s. The resulting panoramic radiographs from the five image layers were exported, the horizontal and vertical dimensions of the metal spheres were measured in all images using the Image J software, and the magnification and distortion rates of the spheres were calculated. All metal spheres presenting a magnification rate lower than 30% in both vertical and horizontal dimensions and a distortion rate lower than 10% were considered to map the focal troughs of each of the five image layers.

RESULTS

All panoramic image layers had a curved shape ranging from 39° to 51° for both dental arches and varied in position and thickness. The anterior region of maxilla was anteriorly displaced when compared to the anterior region of the mandible for all layers. Image layers are thicker at the level of the mandible than those at the level of the maxilla; also, inner layers were thinner and outer layers were thicker.

CONCLUSION

All image layers in the studied panoramic radiography device had a curved shape and varied in position and thickness. The anterior region of maxilla was anteriorly displaced when compared to that of the mandible for all layers.

Development of panorama resolution phantom for comprehensive evaluation of the horizontal and vertical resolution of panoramic radiography

Panoramic radiography is the most commonly used equipment in the dental field, but there is no comprehensive standard about how to evaluate the spatial resolution of panoramic radiography. In this study, panorama resolution phantoms were developed for evaluation of horizontal and vertical resolution reflecting unique characteristics of panoramic radiography. Four horizontal resolution phantoms of staircase shape were designed to obtain images of horizontal lead line pairs in a 52 mm width. Four vertical resolution phantoms with vertical lead line pairs placed at an oblique angle were also designed. A phantom stand was made. Three machines were evaluated twice by two oromaxillofacial radiologists. The horizontal lead line pairs were readable over the entire measured area at the values of 1.88, 2.32, and 2.58 lp/mm for all machines. A readable area of horizontal 3.19 lp/mm was observed in the lingual side. In the vertical resolution phantoms, it was possible to read a narrower range. By using the panorama resolution phantoms and phantom stand, it was possible to evaluate the resolution of a wide buccolingual width in four significant areas. By evaluating the resolution of the vertical and horizontal compartments separately, it was possible to gain a better understanding of the obtained images.

Correlation between spatial resolution and ball distortion rate of panoramic radiography

BACKGROUND

The purpose of this study was to analyze the correlation between spatial resolution and ball distortion rate of panoramic radiography and to elucidate the minimum criterion for ball distortion rate, which is very relevant to clinical readability.

METHODS

Horizontal and vertical spatial resolution and ball distortion rates were calculated in the same position, such as the incisor, premolar, molar, and temporomandibular joint area with various object depths corresponding to 48 mm. Three devices were evaluated. A region showing spatial resolution above the reference standard was selected, and the ball distortion rate corresponding to the same region was divided into horizontal and vertical phantom groups. The mean and standard deviation of the obtained ball distortion rates were calculated. Student's t-test was used to statistically analyze the mean difference in ball distortion rates between vertical and horizontal phantom groups.

RESULTS

In all devices, the horizontal line pair phantom, but not the vertical line pair phantom, was readable in all areas measured at the line pair value of at least 1.88 lp/mm. The line pair value tended to be higher toward the center and lower toward the outside. The ball distortion rate tended to decrease closer to the center and increased further away. In addition, ball distortion rates could not be measured at some areas as they were not recognized as balls due to the high degree of distortion at the outermost and innermost sides. The number of balls satisfying the reference value using the horizontal line pair phantom was 102 (mean of ball distortion rates, 20.98; standard deviation, 15.25). The number of balls satisfying the reference value using the vertical line pair phantom was 49 (mean of ball distortion rates, 16.33; standard deviation, 14.25). However, mean ball distortion rate was not significantly different between the two groups.

CONCLUSIONS

Image layer of panoramic radiography could be evaluated by the spatial resolution using horizontal and vertical line pair phantoms and by assessing ball distortion rates through a ball-type panorama phantom. A ball distortion rate of 20% could be used as a threshold to evaluate the image layer of panoramic radiography.

Development of a new ball-type phantom for evaluation of the image layer of panoramic radiography

Purpose

This study proposes a new ball-type phantom for evaluation of the image layer of panoramic radiography.

Materials and Methods

The arch shape of an acrylic resin phantom was derived from average data on the lower dental arch in Korean adult males. Metal balls with a 2-mm diameter were placed along the center line of the phantom at a 4-mm mesiodistal interval. Additional metal balls were placed along the 22 arch-shaped lines that ran parallel to the center line at 2-mm buccolingual intervals. The height of each ball in the horizontal plane was spaced by 2.5 mm, and consequently, the balls appeared oblique when viewed from the side. The resulting phantom was named the Panorama phantom. The distortion rate of the balls in the acquired image was measured by automatically calculating the difference between the vertical and horizontal length using MATLAB®. Image layer boundaries were obtained by applying various distortion rate thresholds.

Results

Most areas containing metal balls (91.5%) were included in the image layer with a 50% distortion rate threshold. When a 5% distortion rate threshold was applied, the image layer was formed with a small buccolingual width along the arch-shaped center line. However, it was medially located in the temporomandibular joint region.

Conclusion

The Panorama phantom could be used to evaluate the image layer of panoramic radiography, including all mesiodistal areas with large buccolingual width.

[Measurement of mesiodistal root angulation for panoramic images and the effect of buccolingual root angulation]

Panoramic images were taken from an anatomically correct phantom with three different buccolingual angulations for each tooth (Original, 10 degrees buccal root torque, and 10 degrees lingual root torque). The true mesiodistal angulation of each tooth was determined with a three-dimensional coordinate measurement machine. Each tooth had at least one angle measurement that was statistically different from the other mesiodistal angles with different buccolingual orientations. Roots with buccal root orientations were projected more distally than they were in reality; and the roots lingually positioned were projected more mesially. The canine and premolars in both arches were most affected and the phenomenon was more pronounced in the maxilla than the mandible. Buccolingual orientation changes did not affect the root angulation expression on the incisor area.

Technical note. Modulation transfer function analysis of a newly revised rotational panoramic machine

OBJECTIVES

To determine the modulation transfer function (MTF) and noise equivalent passband (NE) values for a newly revised rotational panoramic X-ray machine, the PC-1000 (Panoramic Corp., Fort Wayne, Indiana, USA).

METHODS

Images of a 10 microns test slit were taken at various locations along the X-ray beam projection path using a Lanex Regular/T-Mat G image receptor. Line spread functions were obtained at specific beam paths by scanning slit images with a microdensitometer.

RESULTS

MTF values were highest around the central plane of the image layer, with a maximum near the centre of 0.25 at 4 cycles/mm. The NE values near the central plane of image layer were 1.4 cycles/mm. MTF and NE values in the anterior region were low, 0.2 and 1.2, respectively. The width of the image layer was narrower in the anterior and wider in the posterior segments. Rapid decreases in MTF and NE values were found on the X-ray tube side compared with the receptor side of the central plane. Using a spatial frequency of 0.25 MTF the shape of the image layer was coincident with that determined visually.

CONCLUSION

On the basis of the MTF and NE values the image resolution produced by this machine is considered acceptable for panoramic dental radiography.

Effect of head positioning in panoramic radiography on vertical measurements: an in vitro study

OBJECTIVE

To assess whether it is possible to make accurate vertical measurements of the jaws from panoramic radiographs.

METHODS

Five dry skulls were shifted 5 mm forwards and backwards and tilted 5 degrees up and down in the sagittal plane. Panoramic radiographs of each skull were obtained in nine different positions. In the maxilla, three reference lines were used and vertical measurements were made at the distal surfaces of the first premolar and first molar and in the midline. In the mandible, measurements were made at the distal surfaces of the first premolar and first molar, and at the mental foramen on both sides, and in the midline. The points and lines were marked manually and the radiographs were digitized, magnified and measured.

RESULTS

Sagittal shifting and tilting had only a slight effect on measurements in the mandible. Sagittal tilting of the head had the greatest effect on all the measurements made from the line between the articular eminences to the alveolar crest, as well as measurements in the maxillary midline.

CONCLUSIONS

The line between the articular eminences is unsuitable as a reference line for measurements of the tooth-bearing areas. A slight misalignment of the head does not significantly affect the vertical measurements in the mandible or of the posterior maxilla if the reference lines are in the same vertical plane as the teeth.

Dose reduction in dental radiography

All exposures to ionizing radiation for medical and dental purposes carry risks to both the patient and staff. It is now accepted practice that all exposures should be kept as low as reasonably practicable to minimize these risks. The various methods currently available for dose limitation in intraoral and panoramic radiography and their effects on image quality are considered. In intraoral radiography the use of fast (E-speed) film and rectangular collimation offer dose reductions of approximately 50 per cent and 60 per cent respectively. Constant potential X-ray units, longer focus-to-skin distances and rare-earth filtration will permit further dose reductions. In panoramic radiography the radiation dose to the patients can be reduced by up to one-eighth by combining the use of constant potential X-ray units, rare-earth intensifying screens and rare-earth filtration.

The focal trough of the Autopan panoramic dental x-ray machine

The focal trough of the Autopan machine was evaluated with a multiline test object. The width of the focal trough varied from 5.5 mm anteriorly to about 23 mm in the midramus area with 1.7 line pairs per millimeter of resolution. The focal trough covers most tooth position areas. The machine is capable of producing clinically acceptable radiographs in most clinic populations.

Comparison of image layer location among panoramic machines of the same manufacturer

The central plane of the image layer was located in several panoramic machines of the same manufacturer by making radiographs of a spherical test object placed in different positions. A round image indicated that the test object was at the central plane. The inconsistency of the location of the central plane in machines tested indicates the need for establishing a quality assurance test to verify image-layer location before patient exposure.

Comparison of focal trough dimensions and form by resolution measurements in panoramic radiography

Panoramic focal trough dimensions affect both the number of patient structures seen in radiographs and the margin of error possible when positioning a patient to take a radiograph. This study compares the dimensions, or size, and position, or form, of the focal troughs of four machines by resolution measurements. There is a continued need for technicians to position the patient's anterior teeth accurately in the zones of sharpness of these machines, but the widths of the sharpness zones show an increase in dimensions over older panoramic machines.

Resolution of several screen-film combinations in rotational panoramic radiography

Data for screen-film combinations suitable for panoramic radiography were obtained from the Eastman Kodak Company, Rochester, New York. Image resolution was calculated at various object depths with the use of system data for the Panorex 2 unit (S. S. White Company, Holmdel, New Jersey). An experimental test was performed in which clinicians ranked the various combinations with respect to diagnostic quality. The rankings by the observers generally reflected the resolution of the screen-film combinations.

The central plane of the image layer determined experimentally in various rotational panoramic x-ray machines

The location of the image layer with respect to how a patient is positioned in a rotational panoramic machine is very critical. At present, no simple method exists for routine testing of panoramic machines to verify that the image layer is where the manufacturer intended it to be. The purpose of this experiment was to locate the central plane of the image layer with respect to a reference point on various machines currently on the market. The data obtained from this study can be used for further evaluations of the consistency in the location of the image layer, first with the existing device and later with a test device that would be adaptable to these machines and serve as a quality-control device in assuring that the machines are functioning properly.