Cone Beam Computed Tomography: A Clinician’s Guide to 3D Imaging Prashant P Jaju
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History of Cone Beam Computed TomographyChapter 1

Sushma P Jaju
 
INTRODUCTION
Radiography (conventional X-ray imaging) is the recording of a shadow image of an optically opaque object using penetrating radiation and a recording medium. In 1895, Wilhelm Conrad Röntgen records the first radiography soon after he discovered one of the penetrating radiations, the X-ray (Fig. 1.1).
In conventional X-ray imaging an object is irradiated by photons generated from an X-ray source and the transmitted photons are registered on a photographic plate. Since the X-ray attenuation of the irradiated object is proportional to its electron density, the X-ray intensity transmitted after traversing a region of lower density (e.g. muscle) is greater than the X-ray intensity transmitted after traversing a region of higher density (e.g. bone). In other words, the gray level of the image recorded is inversely proportional to the attenuation of the object in the ray path. This gives contrast and thus imaging of the object as it has been applied for diagnostic purpose.1
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Figure 1.1: Father of radiology, Sir Wilhelm Conrad Röntgen
This clinical method of imaging has however, some drawbacks. The three-dimensional (3D) structure of an object is collapsed onto a two-dimensional (2D) film of detector, which causes the loss of depth information. Furthermore, conventional X-ray images produce a 2D image of the anatomy only perpendicular to the X-ray beam (Fig. 1.2).
All these obstacles found its solution in the computed tomography (CT). The CT refers to the cross-sectional imaging of an object from its projection data slice by slice. The origin of the word ‘tomography’ is from the Greek word ‘tomos’ meaning ‘slice’ or ‘section’ and ‘graphe’ meaning ‘drawing’. It is a diagnostic procedure in which a large series of cross-sectional (2D) X-ray images or tomographic slides are used to generate 3D images of the internals of an object noninvasively. Generally, the principle of CT consists of measuring the spatial distribution of a physical quantity of an object to be examined from different directions and to compute superposition free images from these data. The mathematical basis for CT was first discovered by Radon in 1917.1
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Figure 1.2: Intraoral periapical radiograph
2However, it was not until 1972 that the first CT scanner was invented for which GN Hounsfield and Allan McCormack received the Nobel Prize. Since then, many improvements have been made in scanner technology as well as in the algorithms used for CT reconstruction. Cone beam computed tomography (CBCT) presents as a separate evolutionary arm to CT imaging. An early volumetric CT predecessor of CBCT, the dynamic spatial reconstructor, was developed in the late 1970s by the Biodynamics Research Unit at the Mayo Clinic. Initial interest focused primarily on applications in angiography in which soft-tissue resolution could be sacrificed in favor of high temporal and spatial resolving capabilities, CBCT prototypes based on C-arms were demonstrated as early as 1983. CBCT provided an alternate method of cross-sectional image production to fan beam CT using a comparatively less-expensive radiation detector than conventional CT. The technology transfer of CBCT to dentistry first occurred in 1995. Italian co-inventors, Attilio Tacconi and Piero Mozzo, developed a CBCT system for the maxillofacial region that was designed and produced by QR, Inc of Verona, Italy. This unit, the NewTom DVT 9000 became the first commercial CBCT unit marketed specifically to the dental market, initially introduced in Europe in 1999.2
 
DEVELOPMENT OF CBCT MACHINE
In the late 80's the diffusion of the dental implantology imposed to dentists the use of tomographic radiological examinations to program with good accuracy surgical treatments. Dental tomographs were realized to produce images of jaws cross sections by means of synchronous movement of the X-ray source and the radiographic film. Moreover, special software were created on CT machines to supply images useful to dentists (Figs 1.3 and 1.4).
Nevertheless, the poor quality of images from conventional tomographs did not allow to obtain the required information, mainly from a quantitative point of view and the CT machines were (nowadays are) predominantly dedicated to serious pathologies and were not easily available for implantology.
In those years, we held courses about radiological techniques at the University of Verona and some dentists asked us whether it would be possible to realize small CT dedicated to dentists, which were easy to use, suitable for small radiological or dental practices, and much less expensive than conventional CT.
This is how the idea of realizing a cone beam tomography was born. The capability of such a system to obtain in a unique rotation all the data necessary to reconstruct a whole volume allows to realize an unsophisticated hhardare system composed by a C-arm with a low power X-ray source and a X-ray imaging detector. The C-arm rotates around the patient's head in about 60 seconds and frames are captured every degree. The X-ray detector was an image intensifier coupled with a charge-coupled device (CCD) camera. A personal computer (PC) was used to control the scan and the data acquisition and also to reconstruct the 3D volume. With the PCs available at the time of the reconstruction of a volume of 512 × 512 × 20 voxel took many hours of calculations. In the late 90's, with the arrival of 100 MHz-CPU PC, it became possible to bring to the market the first CBCT. The marketing decided to call it New Tom (Figs 1.5A and B).
The new radiological machine quickly obtained a great success, as for the first time it became easily possible for dentists to have 3D representations of the jaws, with axial and transaxial views, and also to obtain precise measurements of distances and thickness. In such a way the treatment planning became more accurate and reliable.
In the next years, the device had many important improvements, the image intensifier was replaced by a flat-panel detector and faster electronics allowed to scan the volume of interest in less than 30 seconds. Also the reconstruction time decreased until few minutes. Nowadays, CBCT is considered a standard in the field of dental radiology.
Although with little more than a decade of application in dentistry, CBCT has revolutionized oral and maxillofacial imaging.
The unit scanned the entire maxillofacial region with a volume of 15 cm × 15 cm and a complete 360° rotation for data acquisition. The detector was an image intensifier coupled with a solid-state CCD camera.
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Figure 1.3: Attilio Tacconi
3
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Figure 1.4: Pierluigi Mozzo
At the same time, a limited volume CBCT system, scanning a cylindrical volume with a diameter of 4 cm, was under development. This prototype, called ortho-CT, was created by Arai and co-workers (1999) and based on the Scanora stand (Soredex Corporation Helsinki, Finland) with the patient in a sitting position during the examination. In 2000, actual construction of this CBCT device was transferred to J Morita MFG Corporation (Kyoto, Japan) where the 3DX limited CBCT was made ready for commercial use. In 2002, it was introduced to the European market under the name 3D Accuitomo and a year later, in March 2003, a 3D Accuitomo unit was installed at the Clinic of Oral and Maxillofacial Radiology, Public Dental Health, Göteborg, Sweden. It was equipped with an image intensifier connected to a CCD camera as detector. The X-ray field size was 3 cm × 4 cm in the rotation center and the reproduced volume thus corresponded to a cylinder of 3 cm in height and 4 cm in diameter. In August 2005, the 3D Accuitomo was replaced by a newer version, 3D Accuitomo flat-panel detector (FPD) in which a FPD is used instead of an image intensifier and a CCD camera. In this model, it is possible to choose between an X-ray field size of either 4 cm × 4 cm or 6 cm x 6 cm with reproduced volumes corresponding in size. In both versions the X-ray exposure is continuous. Commercially available CBCT systems for dentomaxillofacial imaging include the CB Mercury and CB Throne (Hitachi Medical, Kashiwa-shi, Chiba-ken, Japan), 3D Accuitomo products (J Morita manufacturing, Kyoto, Japan) and i-CAT (Xoran technologies).
Ann Arbor, Mich and Imaging Sciences International, Hatfield, PA.
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Figures 1.5A and B: NewTom DVT 9000
Similar systems designed for point-of-service head and neck imaging have also recently become available (MiniCAT, Xoran Technologies; 3D Accuitomo and 3D Accuitomo 170, J Morita Manufacturing; ILUMA cone beam CT, IMTEC, Ardmore, Okla and GE Healthcare, Chalfont Saint Giles, UK).
 
CONCLUSION
Dental radiology has moved a step forward with the introduction of CBCT. The CBCT imaging has given dental fraternity a diagnostic tool, which is capable of providing accurate diagnosis and treatment planning.
REFERENCES
  1. Miracle AC, Mukherji SK. Conebeam CT of the head and neck, part 2: clinical applications. AJNR Am J Neuroradiol. 2009;30(7):1285-92.
  1. Angelopoulos C, Scarfe WC, Farman AG. A comparison of maxillofacial CBCT and medical CT. Atlas Oral Maxillofacial Surg Clin North Am. 2012;20(1):1-17.