Int Poster J Dent Oral Med 2000, Vol 2 No 4, Poster 55
New Aspects concerning X-ray Protection - Dose Reduction versus Diagnostic Information?
Authors: Catrin Segerath, Thomas Stamm, Bernhard Renger, Ulrike Ehmer, Walter L. Heindel
Department of Orthodontics and Institute for Clinical Radiology, University of Muenster, Germany
123. Jahrestagung der DGZMK und Ihrer Akademie Praxis und Wissenschaft,
50. Jahrestagung der Arbeitsgemeinschaft Kieferchirurgie,
21. Jahrestagung des Arbeitskreises Oralpathologie und Oralmedizin,
Maritim Congress Centrum Bonn
DGZMK-Poster-Award 2001 for the best poster in 2000
Dysmorphological facial structures result from pathophysiological changes of the boney and bordering cranial structures. For this reason DELAIRE and other authors [1, 2, 3, 4] emphasize the importance of the arcitectonical and structural analysis of the entire scull (fig.3). PRECIOUS and MILES  find it indispensable to x-ray the patient for the complete scull analysis using the following parameters: 9mA/s / 80 kV (screen -film combination class 400), distance of 1,5 m (fig.1).
|Fig. 1: Conventional lateral cephalogram with collimation of the entire scull necessary for the Delaire analysis.
||Fig. 3: Structural and arcitectonical components of the DELAIRE analysis.
The aim of this study was to develop a method for a valid analysis of the cranial and facial scull structures using digital radiography (DR) with reduced exposure and conventional collimation of the facial scull.
Material and Methods
At the Dental Center of the University of Muenster lateral cephalograms for orthodontic diagnosis are taken using digital luminescence radiography with a format of 24x30 as well as conventional screen- film combinations. After the signals have been processed the digital lateral cephalogram can be analyzed in DICOM-format with 10 bit data depth (1024 shades of gray) and a local resolution of 1568x1968 pixel (6,2 pixel per mm) (fig.2). The permanent picture is stored locally on a hard disk as well as on a storage film.
|Fig. 2: Digital lateral cephalogram with conventional collimation of the facial scull.
2.1 Methology concerning X-ray Reduction
Using the exposure parameters for the present screen-film systems class 200 and 400 - in consideration of the linear correlation of the intensity of the photo-stimulated luminescence and the dose - it is possible to reduce the dose with sensor-technique software (fig. 4a and 4b). The dose was reduced step wise and the quality of the images evaluated independently by two radiology employees and two orthodontists. The parameters contrast, spatial resolution, collimation, positioning as well as artifacts in the image were taken into consideration. As a result of the observation that despite collimation close to the anode the entire scull remains visible, a collimation technique with a 0,5 mm lead shield close to the patient was developed (fig.7a and b). The region between cervical spine and collarbone is routinely protected with a thyroid shield (fig.7a and b).
|Fig. 4a: Dose values in gray shades dependant on the dose variation.
|Fig. 4b: The storage plate shows a linear correlation between the intensity of the photo-stimulated luminescence.
|Fig. 7a: Collimation close to the object with 0,5 mm lead shield. Thyroid shield.
||Fig. 7b: Result using collimation close to the object.
2.2 Reproducibility of the DELAIRE analysis
100 images were randomly chosen from 400 digital lateral cephalograms and were used to evaluate the intra-individual reproducibility of the landmarks proposed by DELAIRE. The digital lateral cephalograms were evaluated twice at different times (4 week interval) using the NIH-image 1.62 program on a G3-work station. The landmarks were reduced to x- and y-coordinates and the distance between the coordinates of the examinations T1 and T2 were registered as an error radius in mm. Mean values and standard deviations were calculated for the statistical analysis, the WILCOXON test was used for related random tests. The tested landmarks are listed in figure 6.
|Fig. 6: Chosen landmarks.
3.1 Dose Reduction
The stepwise dose reduction is presented in the central figure. Under consideration of the adjustment technique proposed by PRECIOUS and MILES  we reduced the dose by 80% using the method introduced.
3.2 Reproducibility of the DELAIRE Analysis
The distribution of the various distances is shown in figure 5. The average difference of the error radius between the landmarks within the collimated area can be compared to that between the points in the collimated and uncollimated areas. There were no significant differences between the point coordinates of the examinations T1 and T2.
|Fig. 5: Error radius of the examined landmarks in mm. Mean values and standard deviations of various points: BR 3,1 ± 1,9; CLP 1,9 ± 0,9; CP 2,0 ± 1,1; ENA 1,7 ± 0,8; M 2,0 ± 1,0; MET 2,2 ± 0,9; OD 2,5 ± 1,8; PTI 1,8 ± 0,7.
Discussion and Conclusions
The linearity between the intensity of the photo-stimulated luminescence and the dose using storage phosphor plates allows the reduction of the x-ray dose according to orthodontic needs, which is not possible using conventional film-screen systems. The method of collimation close to the object with a thyroid shield presents an active protection of the patient from x-ray scattering. The computer-protected sensor technique is able to amplify different areas of the storage plates to different standards according to the need. In the uncollimated areas the scattered x-rays are sufficient to attain an image of the scull for a valid measurement. The minimal dose proposed by PRECIOUS and MILES  was reduced by far using the method described, without reducing the information of the image. Therefore for the first time the fully collimated scull x-rays have become dispensable for orthodontic diagnosis (fig.8).
|Fig. 8: Digital lateral cephalogram with collimation near the focus. Software protected sensor technology allows the different amplification of various image areas. The complete collimation has therefore become dispensable.
- DELAIRE J: L'analyse architecturale et structurale craniofaciale (de profil); principes theoriques; quelques examples d'emploi en chirurgie maxillofaciale. Rev Stomatol 79: 1, 1978.
- DELAIRE J, SCHENDEL SA, TULASNE JF: An architectural and structural craniofacial analysis: a new lateral cephalometric analysis. Oral Surg 52: 226, 1981.
- PRECIUS DS, DELAIRE J: Correction of anterior mandibular vertical excess: the functional genioplasty. Oral Surg 59: 229, 1985.
- DELAIRE J, PRECIUS DS: Avoidance of the use of vomerine mucosa in primary surgical management of velopalatine clefts. Oral Surg 60: 589, 1985.
- PRECIUS DS, MILES DA: The lateral craniofacial cephalometric radiograph. J Oral Maxillofac Surg 45: 737-8, 1987.
This Poster was submitted on 29.07.00 by
ZA Catrin Segerath.
ZA Catrin Segerath
Poliklinik für Kieferorthopädie
Waldeyer Straße 30
D - 48149 Münster