Panoramic Systems Research Articles

Quality control and patient dosimetry in dental cone beam CT

This paper presents the initial experience in performing quality control and patient dose measurements in a cone beam computed tomography (CT) scanner (ILUMA Ultra, IMTEC Imaging, USA) for oral and maxillofacial radiology. The X-ray tube and the generator were tested first, including the kVp accuracy and precision, and the half-value layer (HVL). The following tests specific for panoramic dental systems were also performed: tube output, beam size and beam alignment to the detector. The tests specific for CT included measurements of noise and CT numbers in water and in air, as well as the homogeneity of CT numbers. The most appropriate dose quantity was found to be the air kerma-area product (KAP) measured with a KAP-metre installed at the tube exit. KAP values were found to vary from 110 to 185 microGy m(2) for available adult protocols and to be 54 microGy m(2) for the paediatric protocol. The effective dose calculated with the software PCXMC (STUK, Finland) was 0.05 mSv for children and 0.09-0.16 mSv for adults.

SU‐FF‐T‐238: GafChromic XR‐QA Film in Testing Panoramic Dental Radiography

Purpose: The location and the field size of the incident X‐ray beam in panoramic dental radiography most often cannot be ascertain visually. These parameters are needed for quality control testing and dosimetry determination. To alleviate this problem, GafChromic XR‐QA film was tested on two panoramic systems. Method and Materials: For each system, the dose‐area product was computed with the length of a cross section image of the incident beam and the exposure measurement with a pencil ion chamber. The result was confirmed by direct analysis of a dose distribution on a film. Placement of the ion chamber was determined by these images. The dose‐area products were thus determined to be 713 cGy.mm2 and 721 cGy.mm2 . From these values, the effective doses were computed to be 5.2 μSv and 5.3 μSv respectively. Results and Conclusion: This version of radiochromic film has thus been demonstrated to be a useful complement to a pencil ion chamber in the testing of a panoramic radiography system.

GafChromic XR‐QA film in testing panoramic dental radiography

The location and field size of the incident X‐ray beam in panoramic dental radiography cannot be ascertained visually most of the time. However, these parameters are needed for quality control and dosimetry determination. To alleviate this problem, we tested GafChromic XR‐QA film on two panoramic systems. For each system, we used the length of a cross‐sectional image of the incident beam and the exposure measurement with a pencil ion chamber to compute the dose–area product. The result was confirmed by direct analysis of a dose distribution on a film. Placement of the ion chamber was determined by the latter images. The GafChromic XR‐QA version of radiochromic film has thus been demonstrated to usefully complement a pencil ion chamber in the testing of a panoramic radiography system.PACS numbers: 87.52.Df, 87.59Bh, 87.66.Cd, 87.66Jj

Panoramic radiographic images and the prediction of asymmetry

I read with interest the obviously painstaking and detailed study reported recently published in Dentomaxillofacial Radiology concerning the accuracy of the Orthophos Plus Program 1 in depicting measurements on a series of 30 dry skulls ideally placed and also deliberately misplaced within the cephalostat. 1 From the results, it is obvious that this representative of traditional panoramic radiographic systems should not be used to assess precise angular or linear measurements and by extrapolation should not be used to assess asymmetry of the jaws. However, a test on one panoramic system should not be generalized to all panoramic systems. It would have been more appropriate to specify the type of panoramic system and program tested both in the article title and in the conclusions. Lions and elephants may both be mammals; however, it would be inappropriate to describe the eating habits of lions based upon a study of elephants. Likewise, it is not possible to generalize findings on inherent distortion from one panoramic system to all. This is not a criticism of the research conducted by Laster et al 1 on Program 1 of the Orthophos Plus, but rather a call for more precision in conclusions to reflect the exact technology studied. Panoramic systems are as different from one another as say the bisecting and paralleling projections used for intraoral radiography. They should be treated as such. Panoramic systems have improved over the years due to better knowledge of human jaw form, from computercontrolled electronic motion and evolution in components. Hence, findings also can be expected to change between generations of systems. Furthermore, with the introduction of more sophisticated digital detectors, limited threedimensional information can now in some circumstances

Fitting conics to paracatadioptric projections of lines

The paracatadioptric camera is one of the most popular panoramic systems currently available in the market. It provides a wide field of view by combining a parabolic shaped mirror with a camera ind...

Innovative lens improves imaging for surveillance systems

Historically, the army, border-security agencies, and transportation-security planners have recognized the advantages of panoramic imaging systems that contain fields of view of up to 180◦ in their coverage areas. Such advantages include increased coverage with fewer cameras, simultaneous tracking of multiple targets, full horizon detection, and immersion capability. However, existing panoramic imagers like fisheye, mirror, and PAL (short for panoramic annular imager) have disadvantages related to the number of effective pixels (the basic unit for video screen images) used on their sensors. These disadvantages are due to blind zones and circular or annular (ring-like) image areas that are commonly called footprints. These drawbacks seriously reduce the use of panoramic imagers for surveillance. To solve these problems, we designed a new imager—the Panomorph lens—that incorporates the advantage of full-area coverage (with a 180◦ field of view) and the ability to increase resolution where needed. Thus, the Panomorph lens allows for a larger coverage zone and distributes its image more efficiently on the sensor.1, 2 Two important parameters for the Panomorph lens are the amount and location of the resolution. These parameters, introduced at the optical design stage, allow the Panomorph lens to provide a higher resolution in a defined zone than any other standard panoramic imager. By comparison, a fisheye lens on a standard NTSC camera (i.e. using the accepted television format in North America and Japan) requires up to six times more pixels on its sensor to produce the same resolution.2 For the security and surveillance of buildings such as indoor parking garages, corporate offices, retail stores, shipping ports, storage facilities, airport terminals, and residential homes, a camera is mounted to the ceiling with its lens facing down (see Figure 1, left). The most significant objects are located in the zone at the periphery of the lens, also called the green zone or zone of interest. The green zone is the most important part of the Figure 1. (left) A camera is mounted on the ceiling in an indoor surveillance application. (right) The graph shows the pixel-to-angle coverage, with the dashed line representing a linear pixel coverage.

Fitting conics to paracatadioptric projections of lines

The paracatadioptric camera is one of the most popular panoramic systems currently available in the market. It provides a wide field of view by combining a parabolic shaped mirror with a camera inducing an orthographic projection. Previous work proved that the paracatadioptric projection of a line is a conic curve, and that the sensor can be fully calibrated from the image of three or more lines. However, the estimation of the conic curves where the lines are projected is hard to accomplish because of the partial occlusion. In general only a small arc of the conic is visible in the image, and conventional conic fitting techniques are unable to accurately estimate the curve. The present work provides methods to overcome this problem. We show that in uncalibrated paracatadioptric views a set of conic curves is a set of line projections if and only if certain properties are verified. These properties are used to constrain the search space and correctly estimate the curves. The conic fitting is solved naturally by an eigensystem whenever the camera is skewless and the aspect ratio is known. For the general situation the line projections are estimated using non-linear optimization. The set of paracatadioptric lines is used in a geometric construction to determine the camera parameters and calibrate the system. We also propose an algorithm to estimate the conic locus corresponding to a line projection in a calibrated paracatadioptric image. It is proved that the set of all line projections is a hyperplane in the space of conic curves. Since the position of the hyperplane depends only on the sensor parameters, the accuracy of the estimation can be improved by constraining the search to conics lying in this subspace. We show that the fitting problem can be solved by an eigensystem, which leads to a robust and computationally efficient method for paracatadioptric line estimation.

Dosimetry of digital panoramic imaging. Part I: patient exposure

To measure patient radiation dose during panoramic exposure with various panoramic units for digital panoramic imaging. An anthropomorphic phantom was filled with thermoluminescent dosemeters (TLD 100) and exposed with five different digital panoramic units during ten consecutive exposures. Four machines were equipped with a direct digital CCD (charge coupled device) system, whereas one of the units used storage phosphor plates (indirect digital technique). The exposure settings recommended by the different manufacturers for the particular image and patient size were used: tube potential settings ranged between 64 kV and 74 kV, exposure times between 8.2 s and 19.0 s, at fuse current values between 4 mA and 7 mA. The effective radiation dose was calculated with inclusion of the salivary glands. Effective radiation doses ranged between 4.7 microSv and 14.9 microSv for one exposure. Salivary glands absorbed the most radiation for all panoramic units. When indirect and direct digital panoramic systems were compared, the effective dose of the indirect digital unit (8.1 microSv) could be found within the range of the effective doses for the direct digital units (4.7-14.9 microSv). A rather wide range of patient radiation doses can be found for digital panoramic units. There is a tendency for lower effective doses for digital compared with analogue panoramic units, reported in previous studies.

Accuracy of digital panoramic images displayed on monitor, glossy paper, and film for assessment of mandibular third molars

Objective The aim of this study was to compare the accuracy of 3 modalities of digital panoramic radiographs—monitor-displayed images and printed copies on glossy paper and on blue transparent film—for assessment of position and morphology of mandibular third molars. Study design 164 third molars were recorded with one of 2 digital panoramic systems (Digora and Orthophos Plus) and assessed by 4 observers on 3 radiographic modalities: monitor display, glossy paper, and transparent film. The assessments were compared with surgeons' findings at the time of the operation (“gold standard”). Results Overall, the observer variation was larger than the variation between methods. A detailed paired analysis revealed some differences between the modalities for some diagnostic categories, but these were few and inconsistent. Conclusion Printed images from the Kodak 1200 ink-jet printer on glossy paper and blue transparent film may be as accurate as the original monitor-displayed digital panoramic images from the Digora and Orthophos Plus systems for assessment of position and morphology of mandibular third molars.

A status report on digital imaging for dentistry

Digital radiographic systems for use in dental practice have been available for almost two decades, but internationally most dentists still use analog silver halide X-ray film. This article is a review of the presumed advantages digital radiographic systems have over conventional X-ray film with special emphasis on the costs and benefits of current intraoral and panoramic systems. It is concluded that more evaluations of time and motion for digital imaging in dental practice are needed. Moreover, evidence-based logic should be applied to the determination of the physical properties required by image detectors for the performance of the different diagnostic tasks that are needed when practicing dentistry.

Paediatric absorbed doses from rotational panoramic radiography.

To determine the paediatric doses in rotational panoramic radiography with film/screen and photostimulable phosphor receptors. A paediatric anthropomorphic head and neck phantom was used. Absorbed doses were measured for two panoramic systems, the Orthophos (Sirona Dental Systems, Bensheim, Germany) and the PM 2002 CC (Planmeca Oy, Helsinki, Finland), with and without programmable child settings, using both screen/film and photostimulable phosphor receptors. Absorbed doses to the eye ranged from 5 to 24 micro Gy. Doses to the dental arches with the Orthophos unit ranged from 50 to 555 micro Gy with the adult and from 27 to 436 micro Gy with the child program; using the PM 2002 CC unit, doses ranged from 56 to 1040 micro Gy using the adult settings, and from 60 to 890 micro Gy with the paediatric settings. The paediatric exposure settings reduced doses at most locations for both panoramic systems. The highest doses were measured near the rotational axes of the X-ray beam. Paediatric settings with the Orthophos P10 resulted in the dose reduction more than 50% to the thyroid but not with the PM 2002 CC. When lower kVcp or mA settings were used, absorbed doses were effectively reduced for all combinations of machines, programs and detectors. Specific program settings for children reduced the absorbed doses from panoramic radiography irrespective of the machine or receptor used.

Two-dimensional modulation transfer functions for rotational panoramic radiography

The two-dimensional modulation transfer function (MTF) provides a method for describing resolution in panoramic x-ray systems where the process of image formation differs in the horizontal and vertical dimensions. As an example, two-dimensional MTFs for the Philips OrthOralix SD machine were calculated from point spread functions (PSF) describing the various sources of unsharpness within the system. The PSF for the focal spot was measured using a pinhole camera. The PSF for the screen-film combination was calculated from data supplied by the manufacturer. The PSF for the motion blurring that is inherent in rotational panoramic radiography was derived from standard mathematical models. The three functions were convolved together to obtain the PSF of the entire system and the two-dimensional MTF was derived from the system PSF by Fourier transformation. The two-dimensional MTF provides a tool which should be useful in the future for the evaluation of existing systems, for the design of improved equipment, and for the implementation of image processing algorithms.

The effect of system parameters on resolution in rotational panoramic radiography. A mathematical analysis.

The unsharpness in rotational panoramic radiography is the combined effect of motion unsharpness, screen-film unsharpness and geometrical unsharpness. A computer program was written to calculate the noise equivalent passband (Ne) from the modulation transfer function (MTF) of each of the three sources of unsharpness in both the horizontal and vertical dimensions for a typical panoramic machine. Effects on system resolution resulting from changing the size of the focal spot, the screen-film combination, the width of the beam, and the effective projection radius were computed and the interaction of the various parameters was evaluated. Data of this type is essential for evaluation of system performance and for optimizing the operation of panoramic systems.

Image distortion in rotational panoramic radiography. VI. Distortion effects in sliding systems.

A mathematical model for calculating the form distortion in rotational panoramic radiographic systems with a sliding beam path and an elliptical form of the sharply depicted plane was developed. The distortion of a spherical model object was calculated for two different systems exemplifying properties of commercially available equipment. The spherical object was distorted toward an ovoid shape in the image. No marked deviations were found between this ovoid distortion and the ellipsoid distortion previously calculated for a theoretical system having a constant effective projection radius and a cylindrical form of the sharply depicted plane. Except for extremely displaced objects in the anterior region the form reproduction in sliding rotational panoramic systems seems to be satisfactory for clinical purposes.