Int J CARS (2007) 1:265–273DOI 10.1007/s11548-006-0062-4

ORIGINAL ARTICLE

A new cone-beam computed tomography systemfor dental applications with innovative 3D software

Alessandro Pasini · F. Casali · D. Bianconi ·A. Rossi · M. Bontempi

Received: 8 August 2006 / Accepted: 28 November 2006 / Published online: 12 January 2007© CARS 2007

AbstractObjective Cone beam computed tomography (CBCT)is an important image technique for oral surgery (den-toalveolar surgery and dental implantology) and max-illofacial applications. This technique requires compactsized scanners with a relatively low radiation dosage,which makes them suitable for imaging of the craniofa-cial region. This article aims to present the concept andthe preliminary findings obtained with the prototype ofa new CBCT scanner with dedicated 3D software, spe-cifically designed for dental imaging.Methods The prototype implements an X-ray tube witha nominal focal spot of 0.5 mm operating at 70–100 kVpand 1–4 mA. The detector is a 6 in. image intensifiercoupled with a digital CCD camera. Dosimetry was per-formed on a RANDO anthropomorphic phantom usingBeryllium Oxide thermo-luminescent dosimeters posi-tioned in the phantom in the following site: eyes, thyroid,skin (lips, cheeks, back of the neck), brain, mandible,maxilla and parotid glands. Doses were measured us-ing four configurations, changing the field-of-view (4′′and 6′′) and acquisition time (10 and 20 s) of the CBCT.Acquisitions were performed with different parame-ters regarding the x-ray tube, pixel size and acquisi-

A. Pasini (B) · F. Casali · D. Bianconi · A. RossiDepartment of Physics, University of Bologna,viale Berti Pichat 6/2, Bologna, Italye-mail: alessandro.pasini@bo.infn.it

A. Pasini · D. Bianconi · A. RossiNECTAR Imaging srl Imola (BO),via Bicocca 14/c, Imola (BO), Italy

M. BontempiCEFLA Dental Group Imola (BO),via Selice provinciale 23/a, Imola (BO), Italy

tion geometries to evaluate image quality in relationto modulation transfer function (MTF), noise andgeometric accuracy.Results The prototype was able to acquire a completemaxillofacial scan in 10–15 s. The CT reconstructionalgorithm delivered images that were judged to havehigh quality, allowing for precise volume rendering. Theradiation dose was determined to be 1–1.5 times that ofthe dose applied during conventional dental panoramicstudies.Conclusion Preliminary studies using the CBCT pro-totype indicate that this device provides images withacceptable diagnostic content at a relatively low radia-tion dosage, if compared to systems currently availableon the market.

Keywords Computed tomography · X-ray ·Computed tomography · Cone beam · Dosimetry ·Image reconstruction · 3D · Maxillofacial radiology ·Panoramic radiography

Introduction

Cone beam computed tomography (CBCT) has becomean important imaging technique for oral and maxillo-facial applications [1–5]. Compared with the standardmedical fan-beam CT, cost and dose are lower [6–12].

In this short technical communication the authorspresent a prototype CBCT scanner and its dedicated 3Dsoftware for dental imaging. Empirical approaches wereused to determine the optimal dose in relation to the im-age quality. Furthermore, the prototype was analyzedin relation to all parameters employed in the CBCTscanner; namely: acquisition, mechanical design, recon-struction algorithm, and 3D visualization. The time for

266 Int J CARS (2007) 1:265–273

the exam was reduced to 10 s to minimize patientmotion artifacts. A new customized reconstruction algo-rithm was developed to maximize the digital acquisi-tion system performances in terms of use and outputimage quality. The visualization and analysis softwarewas designed both to create a powerful tool for noviceusers and also with advanced functions for expert users.The software is able to render CT data either with MIP(maximum intensity projection) or iso-surface, to createtransversal, coronal and sagittal projections (orthogonalviews), arbitrary slices, to measure angles and distances,to print reports, and to both import and export DICOMfiles and other standard graphic formats.

Materials and methods

The prototype is composed of: an X-ray tube with anominal focal spot of 0.5 mm, operating in the energyrange of 70–100 kVp and from 1–4 mA. The detector is a6 in. image intensifier coupled with a digital charge-cou-pled device (CCD) camera having a 1,000 × 1,000 pixelmatrix, and providing a bit depth of ADC = 12 bits. TheX-ray scintillator is Cesium Iodide (CsI). A mechani-cal axis can rotate the source and detector at differentspeeds. The PC interface of the detector is ‘Camera-Link.’, and additional connections were needed for thesystem to adjust CCD and X-ray tube parameters.

Doses were evaluated using a RANDO anthropo-morphic phantom (a human skull covered by equiva-lent soft tissue material) with slots for dosimetric probes(Fig. 1). Forty-eight Beryllium Oxide (BeO) thermo-luminescent dosimeters (TLD) were employed for eachtest. These were positioned in the phantom in slots cho-sen for their dosimetric interest; namely: eyes, thyroid,skin (lips, cheeks, back of the neck), brain, mandible,

maxilla and parotid glands. An additional 10 dosimeterswere used to assess natural ground checking.

Dosimeters were characterized individually andselected from a total of 106 having a reproducibilityof ±10% in terms of sensitivity. Tests were first per-formed on commercial CT and CBCT scanners (GELightSpeed 16 and NEWTOM 9000) and on a digi-tal dental panoramic machine (PLANMECA Promax).Medical personnel were asked to consider the phan-tom as a regular patient while using the usual protocolsfor dental and maxillofacial image acquisitions. Com-parison between different scanners and their acquisi-tion technologies (image intensifier, flat panel, fan beamdetector, . . .) is very difficult. Conventional CT-scanners,for example, were developed for total body exams andonly some particular protocols can be chosen to per-form craniofacial analysis following the manufacturersuggestion. The usual method for adjusting the dose lev-els to compensate for a particular anatomical part is tomodify the tube current and/or the rotation time of thegantry to change the mAs value (tube current × time).On modern CT scanners, tube settings can cause thecurrent to vary from around 60 to over 450 mA. For thisreason, CT manufacturers can pre-program their scan-ners with a wide range of protocols for different exami-nation types. Parameters like tube energy, tube current,slice thickness, helical pitch, gantry speed are gener-ally set for an ‘average’ patient to permit the operatorto change the protocols on a patient-by-patient basis.These particular techniques enable the operator to limitmain CT artifacts and choose the image noise level ofthe tomographic reconstruction. In any case, the detec-tion system of these scanners does not allow one toreduce the dose and at the same time maintain goodimage quality, like intensified detectors do. For exam-ple, reducing the mAs the noise increases according tothe formula: 1√

mAs, thus if the mAs is reduced by a

Fig. 1 a RANDO phantom used for dose estimations indicating dosimeter positions, b the same phantom during the positioning fordose measurements in the NewTom 9000 scanner

Int J CARS (2007) 1:265–273 267

factor of 2, then noise should increase by a factor of 1.414(40% increase!).

Even if this study would be a general overview of thedose estimation between the new prototype and somestate-of-the-art scanners, we must point out that theposition of the phantom and the dosimeters can havea significant impact on the final result, mainly on theperipheral organs.

The prototype was tested and the dosimetry resultswere compared with those obtained using commercialsystems. For the CBCT prototype, dose was evaluated infour configurations, changing the field-of-view (4′′ vs. 6′′)and acquisition time (10 vs. 20 s). The geometric param-eters selected were: source-detector distance: 90.0 ±0.5 cm, source-object distance: 64.0±0.5 cm, and object-detector distance: 26.0 ± 0.5 cm.

Numerous acquisitions were performed using differ-ent parameters for the X-ray tube, pixel size and acqui-sition geometries to evaluate image quality in relation tomodulation transfer function (MTF), noise and geomet-ric accuracy. We used the 6′′ field of view for all trials.Physical properties were measured on projected imagesand reconstructed volume in terms of spatial resolutionand distortion. The line spread function (LSF) and themodulation transfer function (MTF) were determinedfrom radiographies of an edge according to the methodproposed by Boone JM and Siebert [13]. Radiographsof an edge were produced using a test object with athin layer of lead, inclined at approximately 45◦. Nosmoothing was applied in the raw data.

The release version of the software used for construct-ing the prototype system software was C++ with Micro-soft� C++.NET 2003 compiler. Open-source librarieswere used for 3 D visualization and proprietary algo-rithms for image processing. The mathematical algo-rithms were first tested either in MATLAB� or IDL�.

The reconstruction algorithm was developed in houseto provide complete control of the process and toensure the possibility for further improvements, such asparallel reconstruction. Experimental reconstructionswere performed on a HPxw6000 dual processor (In-tel Xeon 3.2 G), 3 GB of RAM, 2 hard disks (mirror0–230 GB). Images were saved in raw format (16 bitsor 32 bits), without compression. Important functionswere developed as DLLs to facilitate updates andmodifications.

Results

Presently, most of the CT systems are developed forradiologists, with few implantology sites having theirown CBCT system. The target is to create an easy-to-usesystem (software and hardware) for dentists with afriendly graphic user interface (GUI), which also in-cludes advanced features for expert users (Fig. 2). Thesoftware we have developed is able to explore volumet-ric data from a CBCT exam: it is possible to surf thereconstructed volume in three directions to make vir-tual cuts and emphasize details. The software currentlyallows four kinds of visualizations:

• Ortho-slice: sagittal, coronal, transversal views and3D (Fig. 2);

• M.I.P.: maximum intensity projection (Fig. 3);• Iso-surface: by choosing a threshold value;• Rx-CT: it generates two virtual radiographies where

the user can choose six slices in different positions(Fig. 3). The user can rotate the whole volume inall directions to obtain the best position for themeasurements.

Fig. 2 Screenshots of ‘DentalPlan’: orthoslice view (axial, sagittal, coronal and 3 D) of the reconstructed phantom in two differentvideo layouts

268 Int J CARS (2007) 1:265–273

Fig. 3 a Rx-CT view: colors indicate axial slice on the bottom of the window; b Maximum intensity projection (M.I.P.) view

Fig. 4 a GIZMO tool: the user can rotate the volume in all directions to define the cut plane of the axial slices; b Line tool to measuredistances and angles, it is possible to take measurements in all 2 D views

Fig. 5 a Users can create panoramic views from the volume: starting from an axial slice, the user can follow the best dental path; bVolume section of a narrow path along the user-defined trajectory; c Virtual reconstruction of a standard panoramic view from the sameuser-defined trajectory

Standard tools such as zoom, angles and distancemeasurements, label and marker 3 D are alreadyimplemented in the software package (Fig. 4). To facil-itate generation of the final report, a useful print-toolcan print all virtually created views. Panoramic radiog-raphy is a common diagnostic exam for dentists, this isthe main reason we have developed an automatic toolthat provides distortion-free panoramic images (Fig. 5).

A study was conducted to measure the resolution ofthe system. If we consider the field of view (FOV) of thesystem and the CCD camera we can conclude the nom-inal resolution of the detector is 3.3 and 1.66 lp/mm inbinned mode. Taking into account the magnification ofthe system (M = 1.38), the resulting nominal resolutionis 4.57 lp/mm for a 1,000 × 1,000 image and 2.29 lp/mmfor a 500 × 500 image.

Int J CARS (2007) 1:265–273 269

Radiography (1000x1000 pixels)

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

1,00

0 0,5 1 1,5 2 2,5 3 3,5

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MT

F (

%)

Fig. 6 MTF curve of a projected image

The MTF of the detection system is the product ofthe components MTF (scintillator layer, image inten-sifier, optical coupling, CCD, . . .). In Fig. 6 is shownthe pre-sampling MTF of a projected image (imagesize = 1,000 × 1,000 pixels). The resolution power ofa reconstructed slice in binning mode (2 × 2) was, in-stead, approximately 1.5 lp/mm in the 6′′ FOV at a MTFof 10%. The MTF in the vertical direction is slightlybetter than in the horizontal direction.

Circularity and geometric measures were calculatedusing an aluminum tube (Fig. 7).

Circularity is defined by the formula:

4π × area/(perimeter)2

Circles circularity is 1, instead all other shapes arecharacterized by circularity <1. A free software packagecalled ImageJ (NIH, USA) was used to measure circu-larity. The value we calculated from the perimeter andarea was 0.9999.

Using the same software we have calculated the innerand outer diameter of the aluminum test tube: the cal-culated value was 250 pixels for the inner diameter and260 for the outer diameter (Figs. 8, 9).

A study was also performed to compare our proto-type system and a commercial CT scanner in terms ofdose [14]. GE LightSpeed 16 with DentalScan protocolshowed the highest amount of dose (up to 21 mSv forskin), the NEWTOM 9000 had doses up to 3.7 mSv andPromax up to 0.45 mSv (Table 1, Fig. 10). The prototypeis positioned between NEWTOM 9000 and the Promaxwith a maximal dose up to 0.5 mSv for skin (Fig. 6).Doses to critical organs (eyes and thyroid) are low forall machines of course proportionally to the medicalfan beam CT scanner using regular protocols: 1.4 mSvfor GE LightSpeed 16, 0.6 mSv for NEWTOM 9000,0.04 mSv for Promax and 0.06 for our prototype.

Discussion

The CBCT prototype can acquire a complete maxillofa-cial scan in 10 s. It is possible to improve the image qual-ity by increasing the acquisition time (and the dose). Ourexperimental results suggest a time that ranges between10 and 15 s, as an optimal compromise between expo-sure time and dose. The prototype algorithm used forthe CBCT reconstruction delivers good image quality.The precision of the reconstructed volume is confirmedby a number of geometric measurements made withphantoms.

The comparison of absorbed doses shows that theCBCT prototype is similar to dental panoramic radiol-ogy, which has requirements that are considerably lowerthan medical CT imaging.

The reason for such a wide difference of absorbeddose is that medical CT scanners and their exposureprotocols are designed for different purposes, which

Fig. 7 CT reconstructionof the test tube

270 Int J CARS (2007) 1:265–273

Fig. 8 Inner diameter of the test tube. The beginning of the border was calculated using a threshold in the profile (median value in theraising edge)

Fig. 9 Outer diameter of the test tube. The tuber border is 9 pixels

include the need for soft tissue contrast that is not re-quired for dental implant planning. The GE LightSpeed16 can perform examinations of any part of the body,the QR NEWTOM 9000 is designed for maxillofacialimaging only (in this regard is similar to the CBCT proto-type), the Planmeca Promax does 2D examinations only.The prototype unit is optimized instead for dental appli-cations where 3D is important. A comparison with the

literature concerning doses from commercial CT-scan-ners, shows consistency between the present study andprior reports, both qualitatively and quantitatively [15].It’s clearly visible and measurable that image noise ofthis CBCT is higher than that of conventional helical CT[16]. This is mainly due to two facts: the diffused radia-tion because of the collimation and the image intensifierdetector.

Int J CARS (2007) 1:265–273 271

Fig. 10 a Doses estimated in commercial scanner for nine positions; b plots of the doses of our prototype compared with NewTOM9000 and Planmeca ProMax

Dose reduction, low cost and improved image qual-ity are the three most important drivers of the expo-nential increase of interest in the CBCT technique. TheCBCT prototype and its new software confirmed thevalue of 3D in diagnostic imaging: the third dimen-sion changes the way the CT data is analyzed. It isyet undefined if dentists and patients will benefit fromthe higher diagnostic content. It is well-know that cone

beam images provide undistorted views of the jaws. Onthe contrary, conventional panoramic images are bothmagnified and distorted. On the other hand, the adop-tion of CBCT, as substitute of conventional dental imag-ing, bears the risk of increasing the dose imparted to thepatient. This is the reason why dose reduction was oneof the main guidelines during the development of theCBCT prototype.

272 Int J CARS (2007) 1:265–273

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This study gives a picture of the correlation amongCT exams and absorbed dose. Lots of work remains tobe done, especially in defining a standard protocol tomeasure the dose in CBCT scanners. Other recent stud-ies are aiming to measure and compare dosimetry andimage quality [17,18]. In this respect, specific units andprotocols should be validated with non-arbitrary studies.

In the near future, several manufacturers may intro-duce new imaging systems designed for dental andmaxillofacial investigations. A standard measurementprotocol will avoid speculation on such a sensitive mat-ter as the absorbed dose and the diagnostic accuracy.

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