A 3D IMAGING SYSTEM FOR DENTAL IMAGING BASED ON DIGITAL TOMOSYNTHESIS AND CONE BEAM CT C. Badea, Z. Kolitsi, N. Pallikarakis Department of Medical Physics, School of Medicine, University of Patras, Patras, Greece
[email protected] Abstract: The use of cone beam projections for tomographic reconstruction using CBCT has triggered the interest of many investigators. Tomographic systems for dental applications based on CBCT and using isocentric rotational fluoroscopic machines has already been proposed. Collecting, however, projection data over 3600 as required by CBCT is not always desirable due to radiation protection considerations. Limited arc methods for tomographic reconstruction such as Digital Tomosynthesis (DTS), could have been used prior to dental surgery. In this paper, we propose the use of both DTS and CBCT reconstruction methods as an integrated solution for providing tomographic data in dental applications. A system for 3D imaging based on DTS-CBCT techniques is described and tested on simulated and actual data. Such a system may be implemented on commercial available hardware components appropriately selected for optimization of performance. Introduction Both conventional and Computed Tomography (CT) are important for the diagnosis and treatment planning of dental and maxillo-facial structures. CT is used for 3D reconstruction and provides good image quality at the expense however of a significantly high radiation dose. A solution to this problem is partially given by the use of Cone Beam CT (CBCT) for maxillo-facial imaging , which can provide image quality of sufficient quality for the specific diagnostic needs at significantly reduced absorbed radiation dose [1]. CBCT involves sampling with isocentric rotation over 3600 and was implemented using Isocentric Rotational Fluoroscopic (IRF) systems. Collecting projection data over 3600 may not be always necessary, as tomograms of appropriate image quality may be provided by using a limited angle method such as Digital Tomosynthesis (DTS)[2]. In this method, the projection data is acquired in cone beam geometry, at selected intervals and over a limited arc and is then used to reconstruct, post priori, planes of various orientations. Filtered DTS (FDTS) used in conjunction with IRF units, was shown to be a limited angle equivalent of CBCT, where only a subset of the projection data is used for reconstruction [3].
Furthermore an experimental analysis of image quality characteristics, as a function of the size of the reconstruction arc, has demonstrated that in between 400 (DTS domain) and 3600 (CBCT domain), FDTS with an extended arc may be used for a 3D image reconstruction of adequate image quality depending on the requirements of the clinical application. Therefore, a combined DTS-CBCT approach presents a potential for optimisation of image quality and dose for any given clinical application. This paper establishes the feasibility of such a DTSCBCT system and its benefits for combined diagnostic imaging and maxillofacial planning in dentistry. Materials and Methods The proposed system is mainly based on the existing DTS clinical prototype system presented in detail elsewhere [4]. The imaging process as shown by figure 1, can be divided into three basic steps: (i) capture of the projection images, (ii) their preprocessing, (iii) the tomographic reconstruction. A set of projection data can be acquired over a chosen arc of at least 400 using an IRF system, such as a motorized C-arm. Such systems also offers the advantage of user selectable sampling planes according to the requirements of the clinical case. In figure 1, 2 such possible trajectories are presented. Some of the available C-arm systems allow sampling over a restricted arc (e.g. 2050 for the Philips BV25) while others (e.g. Swemac FluoroScan Premier), allow a complete rotation of 3600. Whatever the situation and given that dental imaging concerns high contrast structures, tomographic reconstructions using projections taken over 1800 could be sufficient to offer optimized image quality. Indeed, further increase of the sampling arc will not further enhance neither the spatial resolution nor the morphological information content of high contrast objects [3]. Projection images are indexed in terms of the gantry angle at which they are taken. The C-arm system interfaces to the digital imaging chain through an electronic control device that controls the motion of the gantry and registers the gantry angle. The analog signal from the camera is fed to the frame grabber and digitized to 512x512x8 bit matrix.
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Figure 1: A flowchart of DTS-CBCT system. Preprocessing of the projection images, prior to their use for reconstruction, is applied in order to correct for: (i) pincushion and distortion due to earth magnetic field; (ii) system inhomogeneities and sampling errors [4]. A high-pass filter is subsequently applied to the corrected projection images, using a Hamming window. DTS reconstruction is based on the Multiple Projections Algorithm (MPA) [3] and utilizes an appropriate set of projection images, over a 40° arc to synthesize user selected tomographic slices of various orientations through the object. The proposed system also benefits from a 3D localization method [6] for accurate positioning and measurements in planning dental implants. The method involves simultaneous reference to projection and tomographic data allowing the user to verify perception of spatial relationships and ensuring a high degree of accuracy; localisation accuracy and precision of the system using a DTS prototype imaging system is equal to 1.3 mm and 0.4 mm respectively. In order to evaluate the proposed system, both simulated and real data were used. Simulated data is particularly useful in studying specific effects and is free of distortion and inaccuracies inherent to radiographic units. Simulated projections were generated with the use of a software data generator for radiographic imaging investigations [7]. Projections were created using an anatomical phantom reconstructed from 512x512 CT slices of 1mm thickness taken from the Visible Human data base.
Slices numbered from 1006 to 1336 and representing the patient head were stacked to form a volume and were subsequently subjected to a simulated isocentric irradiation process, in a cone beam set-up similar to C-arm gantry moving in a coronal plane having the Source to Isocenter Distance (SID) equal to 30 cm and the Source to Image Intensifier Distance (SIID) equal to 70 cm. Simulated projection images were thus obtained and used for tomographic reconstruction. To further test the feasibility of implementation of the system on existing imaging systems, a set of actual radiographic data of an anatomic skull was obtained by sampling with an angiographic unit having (SID=67.5 cm) and (SIID=23.5 cm). The sampling was performed every 20 over a 400 arc. The projections were corrected for pincushion distortions prior to being used for reconstructions. Results Reconstruction results are displayed in figures 2 and 3, respectively. Figure 2 (a,b,c) show 2D tomograms of various orientations obtained using FDTS applied over a 1800 reconstruction arc. Moreover, a 3D reconstruction of the jaws is shown by figure 2 (d). The 3D rendering was obtained by computing an isosurface at the level of bones. Streaking artifacts caused by metal fillings are visible both in tomograms and the 3D reconstruction. DTS tomograms of 2 axial planes distanced at 1 cm, obtained from the real data set and using only a 400 arc are presented in figure 3.
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Figure 2: Tomographic reconstruction using extended arc filtered digital tomosynthesis corresponding to (a) coronal, (b) sagital and (c) axial planes; (d) 3D reconstruction of the jaws.
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Figure 3: Fig.3 DTS tomograms of axial planes at 1cm distance. Discussion The possibility to exploit both a limited arc reconstruction techniques (DTS, FDTS) and full arc reconstruction (CBCT) permits optimization of the imaging technique and customization to each clinical case. Therefore. In some cases, a sampling arc as little as 400 provide all necessary data while reducing the exposure of the patient. Moreover, by sampling over an arc of around 180o and using extended arc FDTS, accurate 3D information can be obtained for maxillofacial planning. 3D reconstruction and visualization of jaw structures is also important for treatment planning in cases of multiple implants. Given the short reconstruction times and the multiple functionalities of the system, the entire process chain from patient data acquisition to the required dental intervention could be a few minutes. Conclusions The investigations presented here reveals the potential to apply both CBCT and DTS techniques in dental imaging by using the same sampling set-up based on an isocentric fluoroscopic system. A main advantage refers to the reduction of the exposure to x-ray by allowing limited arc reconstruction methods such as DTS, FDTS. These two techniques could be integrated in one imaging system which may be implemented on commercial available hardware components selected for optimization of performance. Further work will concentrate on the application of the present system on patients as well as on artifact reduction caused by metal fillings. References: [1] Mozzo P, Procacci C, Tacconi A, Tinazzi P, Bergamo A. (1998): ‘A new volumetric CT machine for dental imaging based on the cone-beam technique: preliminary results’, Eur. Radiology 9, pp 1558-1564. [2] Kolitsi Z, Panayiotakis G, Anastassopoulos V, Scodras A, Pallikarakis N. (1992): ‘A multiple
projection algorithm for digital tomosynthesis’, Med. Phys. 19 pp. 1045-50. [3] Badea C, Kolitsi Z, Pallikarakis N. ‘Image Quality in extended arc filtered digital tomosynthesis’, accepted for publication in Acta Radiologica, Oct. 2000. [4] Kolitsi Z, Yoldassis N, Siozos T, Pallikarakis N. ‘Volume Imaging in fluoroscopy: a clinical prototype system based on generalised digital tomosynthesis technique’, Acta Radiologica 1996; 37, pp.741-8. [5] Fahrig R, Moreau M, Holdsworth DW. (1997): ‘Three-dimensional computed tomographic reconstruction using a C-arm Mounted XRII; Correction of Image Intensifier Distortion’, Med.Phys. 24, pp.1097106. [6] Messaris G, Kolitsi Z, Badea C, Pallikarakis N. (1999): ‘Three-dimensional localisation based on projectional and tomographic image correlation: an application for digital tomosynthesis’, Med. Eng.& Phys. 21, pp. 101-9. [7] Lazos D, Kolitsi Z, Pallikarakis N. (2000): ‘A software data generator for radiographic investigations’, IEEE Trans Inf Technol Biomed. 4, pp.76-9.
