Gunilla Tronje

Basic technical properties of a system for direct acquisition of digital intraoral radiographs

The Sens-A-Ray system for direct digital intraoral radiography may be used with any computer compatible with an IBM PC/AT. The system relies on a charge-coupled device designed for direct conversion of x-ray energy to an electronic signal. It is the first such device for direct acquisition of radiographs. Technical properties of charge-coupled device detectors when exposed to radiation energies in the range of x-rays used in dental radiography have been studied. Even in the absence of light or x-radiation there is a spontaneous generation of charge within a charge-coupled device detector that gives rise to a background signal, a dark current. It was found that the dark current is a linear function of exposure time. The dose response of the charge-coupled device detector was determined at nominal kilovoltages that range from 50 to 90 kVp. The dose response was shown to be a linear function of exposure. The functions for all kVp settings were practically identical. The charge-coupled device detector is more sensitive to x-radiation than conventional dental films and, consequently, its exposure range is more narrow. The signal-to-noise ratio was calculated from the digital radiographs used for the dose response test. The ratio is above 10 for exposures higher than about 2 microC/kg. The line spread function was determined from test radiographs of a 10 microns wide slit in a test object of 1.5 mm thick tantalum. After curve fitting, the line spread function could be expressed as the sum of a Gaussian and an exponential function. Presampling modulation transfer functions valid at the detector plane and at an object plane were calculated from fitted data on the line spread function. It is concluded that the Sens-A-Ray system has such technical properties that it may replace conventional film-based systems.

Image distortion in rotational panoramic radiography. III. Inclined objects.

Mathematical calculations have been performed to analyse how accurately the angle between objects, inclined in space, is reproduced on panoramic films. A marked tolerance against angle distortion was found. Angular measurements may be performed on correctly exposed panoramic films, and the values obtained are satisfactorily accurate for most clinical purposes.

Radiographic changes in the temporomandibular joint in patients with generalized osteoarthritis and rheumatoid arthritis.

This is the first study concerned with radiographic characteristics in patients with generalized osteoarthritis and signs and symptoms of temporomandibular joint involvement. For comparison, patients with rheumatoid arthritis and temporomandibular joint involvement were used. The patient material comprised 20 patients with generalized osteoarthritis (20 joints) and 21 patients with rheumatoid arthritis (21 joints). The radiographic methods were corrected sagittal tomography (hard tissue changes, joint space, and condylar position), frontal tomography (hard tissue changes), and individualized oblique lateral transcranial projections (condylar translation). Sixteen (80%) joints in the group of patients with generalized osteoarthritis and 15 (71%) joints in the group with rheumatoid arthritis revealed structural changes. The condyle was the predominant location. No radiographic criterion was pathognomonic for generalized osteoarthritis or rheumatoid arthritis. However, osteophytes, flattening of the condyle, or a reduced joint space was observed more often in joints with generalized osteoarthritis, whereas erosions in the condyle were more common in joints with rheumatoid arthritis. The radiographic findings in patients with generalized osteoarthritis are more similar to those seen in patients who have the common form of temporomandibular joint osteoarthritis than to those in patients with rheumatoid arthritis.

Resolution as defined by line spread and modulation transfer functions for four digital intraoral radiographic systems

Line spread functions for four commercially available systems for direct digital intraoral radiography were determined from images of a slit of negligible width. From the fitted line spread functions presampling modulation transfer functions were calculated. The four systems were the Sens-A-Ray (Regam Medical System AB, Sundsvall, Sweden), the VIXA/Visualix (Gendex, Chicago Ill.), the RVG (Trophy Radiologic, Paris, France), and the Flash Dent (Villa Sistemi Medicale srd, Buccinasco, Italy). Digital intraoral radiography is in a state of rapid development, and detectors as well as computer hardware and software are continually modified and improved resulting in successively changing system parameters. As this occurs the present work provides a method that may be used to determine comparable data on future systems.

Image distortion in rotational panoramic radiography. IV. Object morphology; outer contours

On films exposed with rotational panoramic radiography, distortion effects are often apparent. The distortion of the outer contour of three-dimensional model objects jas been analysed mathematically. The applied mathematical expressions for coordinate transformation between object and film were confirmed experimentally. The distortion of the outer contour is dependent on the basic form of the object; the more rounded the object is, the less marked is the distortion. It is concluded that for practical clinical purposes the correctly exposed panoramic film is reliable when the form of rounded objects is assessed.

Radiographic interproximal angulations : implications for rotational panoramic radiography

Axial radiographs were taken on 160 subjects at the screening clinic of the Dental School, University of Texas Health Science Center at San Antonio. Intermeatal and midsagittal lines, together with coordinate axes and polynomial curves that represent the average dental arch form, were overlaid on each radiograph. Coordinate references for interproximal tangents at 3154 contacts along the average dental arch form were digitized and angulations between the arch form and midsagittal plane calculated. Interproximal angulations at the average arch form were found to vary greatly but the average fluctuated around 90 degrees over the length of the arch. Optimal beam angulations were shown to be considerably different from central ray angulations of current panoramic machines, and although most machines demonstrated favorable interproximal beam angulation in the anterior region, discrepancy in the premolar region ranged from 15 degrees to over 40 degrees. Overall the OrthOralix SD (Gendex Dental Systems, S.r.l., Monza, Italy) orthogonal projection and PM 2002 CC (Planmeca Inc. Helsinki, Finland) deviate least from optimal interproximal angulation over most of the dental arch. However, the Planmeca gives a better angulation in the canine/premolar region, whereas the OP5/10 (Palomex Instrumentarium Corp., Helsinki, Finland) is comparable with the OrthOralix SD in the molar region.

Electronic system for digital acquisition of rotational panoramic radiographs

A prototype system for digital panoramic imaging of the maxillofacial complex has been developed. In this system x-ray film is replaced by an electronic sensor that delivers the image information to a computer for storage in digital format. The images, which are similar to conventional panoramic radiographs, are displayed on a high-resolution video monitor and may be stored on optical disk for future use. Hard-copy output is also available. The present prototype system has been installed on an Orthopantomograph model OP10 panoramic x-ray machine is programmed for operation with this machine, but in principle the system can be installed on any such device. The system may be incorporated into the design of future panoramic x-ray systems or may be used to retrofit panoramic x-ray systems now using photographic film to record the radiographic image. Greater sensitivity of electronic sensors should make possible a reduction of x-ray dose to the patient, compared with film-based systems.

Preliminary evaluation of a digital system for rotational panoramic radiography

A prototype system for direct digital panoramic radiography has been evaluated with respect to density, contrast, magnification, distortion, resolution, and overall image quality. Density and contrast depend on detector calibration and may be modified by the display system or by digital processing of the captured image. Variation of magnification in the horizontal and vertical dimensions gives rise to distortion phenomena that are identical to those encountered in film-based systems. Resolution in the vertical dimension is determined by the pitch of the detector elements. In the horizontal dimension, resolution is limited by the effective width of the detector elements. To evaluate the clinical acceptability of the images, radiologists and general practice residents were asked to assess the perceptibility of important radiographic landmarks in film-based and digital images of both a radiographic phantom and a patient. The digital system performed on a par with film in the representation of normal morphologic structures of the clinical human subject whereas more differences were apparent in the phantom images. The general practice residents consistently rated the digital images higher than their radiologist counterparts did. No consistent trends were found to indicate any inherent deficiencies of the digital system in the depiction of any one area. The results indicate the promise of direct digital acquisition as a method of panoramic imaging.

Dimensional reproduction in direct digital rotational panoramic radiography

A linear x-ray detector array can be used to perform rotational panoramic radiography provided a technique for data acquisition is used that emulates the dimensional reproduction of a conventional rotational panoramic radiographic system. This may be accomplished with the use of a variable integration time throughout the excursion. The required interval depends upon the scanning geometry, the selected image plane, and the size of pixels used in the digital image. In a prototype device developed around an Orthopantomograph Model OP10 with 0.225 mm square pixels, the integration times range from about 8 milliseconds to 28 milliseconds. An experimental test was performed by radiographing a number of steel spheres positioned at different object depths. Within the limits of experimental error, there was satisfactory agreement between the measured and theoretical magnification.

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.