| Abstract|| |
This review reveals the possibility of using the newest technology of cone beam CT for acquiring three-dimensional images of periapical lesions and, mandibular canal. It has a higher efficiency in X-ray use, inhent quickness in volumetric data aquisition, and potential for reducing the cost of CT. It requires only a single scan to capture the entire object with a cone of X-rays with reduced exposure time. The high quality and affordable cone beam CT is ideally suited for diagnosis, treatment planning and treatment monitoring in endodontics especially when surgical approach is chosen. Three dimensional anatomic and pathological observations improve the surgical performance.
Keywords: Periapical lesions, CBCT, Imaging
|How to cite this article:|
Govila S, Gundappa M. Cone beam computed tomography - an overview. J Conserv Dent 2007;10:53-8
| Introduction|| |
Various imaging modalities have been used in dentistry but none could give entirely satisfactory results. Diagnosis, treatment planning and treatment monitoring in endodontics depend to a very large extent on results from radiographic examination. The complex anatomy in respect to the teeth themselves as well as their surrounding structures may render those tasks difficult. The new technology of cone beam CT for acquiring three dimensional images hold promises for improvement in all those areas, particularly for the detection of periapical lesions and mandibular canal as it can display the image in all its three dimensions and remove disturbing anatomical structures to make it possible to evaluate each root and its closest surroundings in detail. They also provide images taken at different points in time that are similar in geometry and contrast making it possible to evaluate differences occurring in the fourth dimension time  .
| Evolution of Cone Beam CT|| |
The 1 st generation C.T. was invented in 1967 by Sir Godfrey N.Hounsfield, an engineer, in 1967. In this device a single detector element was used to capture a beam of X-ray. Both detector and source rotated in a single degree that is why they were also called as "pencil" beam scanner or translaterotate. They were designed to scan the head only. 
The 2 nd Generation CT system was introduced in 1975, these, were also known as hybrids machines. They used more than one detector and a small fan beam; however, the image quality was poorer due to patient motion caused by the amount of time required to take the scan  [Figure 1].
The 3 rd generation CT Scanner appeared in 1976 using fluoroscopic systems, light image intensifier and TV camera. They used a single detector element to capture a beam of X-ray. Both detector and source rotated in a single degree thus they were also called as "Pencil beam" scanner or translaterotate  [Figure 1].
The 4 th Generation C.T. scanners replaced the arc shaped detector with an entire circle of detectors. In this design the X-ray tube rotates around the patient while the detector stays stationary  [Figure 1].
Recent advances in CT include multi-row electrons and spiral scanning which incorporates moving table with rotating X-ray tube with the net effect that X-ray tube describes a helical path around the patient  [Figure 2].
Computed tomography scanners have been an active area of research due to the requirements such as contrast and special resolution.
The first Cone Beam CT (CBCT) scanner was built by Robles RA in 1982 for angiography purpose  . Cone beam CT scanner utilizes a two dimensional detector which allows for a single rotation of the gentry to generate a scan compared to conventional CT scanners whose multiple slices must be stacked to obtain a complete image.
The CBCT machine called as NewTom-9000 has been developed recently in Italy by Quantitative Radiology, Verona, Italy. But it is bulky, takes 75 seconds of scan time and effective exposure of 36 seconds.
The new CBCT machine 3D Accuitomo-XYZ slice view Tomograph developed by J. Morita Mfg. Corporation, Japan is more compact (about 400kg), comparatively inexpensive and small enough to be used in dental offices, emergency rooms and intensive care units without any special adjustments.
| Technical characteristics of cone beam CT|| |
This article would mainly discuss the features of 3D Accuitomo machine developed by J. Morita, Japan. This new technology produces highly detailed three dimensional imaging volume data using a cylinder 40 mm diameter and 30 mm high. Three dimensional views Axial, Coronal, and Saggital allow observation of the designated part. It is possible to re-slice the image at any angle for optimum viewing. The 3D Accuitomo uses tube voltage of 60-80 kV, 1-10 mA of tube current and has 0.5 x 0.5 mm of focal spot. The exposure time is 18 seconds or less. The slice width is from 0.125 mm to 2mm  . 3D Accuitomo uses dedicated software, i-dixel, to freely set slice angles. Other major features include, display of histogram, patient direction, density measurement, distance and angle measurement. 
| Advantages|| |
1. Precise Identification and detection of Periapical Lesions
Cone beam CT imaging technique can be applied in the diagnosis and treatment planning of dental and maxillofacial structures with particular reference to "Periapical lesions". Special application of software allows attainment of 3D images perpendicular to Dental arch to obtain "cross- section or "slices" of the periapical lesion in the same geometric conditions. It can differentiate the lesion from the surrounding bone by evaluating the bone thickness and differentiation between cancellous and cortical bone in all dimensions. 
This allows assessment of amount of bone removal to access the periapical lesion thus making the procedure more predictable.
2. Detection of Mandibular Canal
Cone beam CT imaging provides excellent information not available from dental radiographs for treatment planning in apical surgery, implant placement of mandibular premolars and molars by detecting the mandibular canal which can be in a close proximity to the lesion or root apex. The exact position of the inferior alveolar neurovascular bundle can be seen in cross section with a high predictability. This is helpful when the bundle is not surrounded by ossified canal [Figure 3].
3. A complete 3D Reconstruction and Display from any angle
CBCT clearly depicts impacted teeth and foreign bodies in maxillary sinus and as well as cystic walls. The articular interrelation could be easily demonstrated due to its saggital and coronal images which are quickly reformatted following the long axis of the condyle  [Figure 3].
After completing the primary reconstruction all the secondary reconstruction like cross sections, panoramic or 3D images can be evaluated.
4. Patient Radiation dose five times lower than Normal CT
As the exposure time is approximately 18 seconds, i.e. 1/7 th the amount compared to the conventional medical CT  .
5. Easy Patient Positioning
The patient is positioned comfortably and the laser positioning system aids the operator. 
Scout images enable even more accurate positioning. This can be explained with the absence of artifacts due to the misalignment of the slices deriving from the motion of the patient. , [Figure 4].
6. Accurate three dimensional image
Gives an accurate three dimensional image. Moreover there is no overlapping of profile dose in the single slices which leads to reduced irradiation  .
7. Excellent Resolution
Accurate measurements can be obtained in all display modes, with reference to all the possible special orientation of the reconstructed images. All images can be printed on a 1: 1 scale. The accuracy of measurements is one of the main advantage of the system , .
8. Accurate underlining of Mandibular Canal & Easy to install
The resulting canal information can subsequently be available in axial and transaxial sections. The three dimensional information and better visualization of the bone structure, as well as the bone relationship with anatomic structures such as the maxillary sinus and the mandibular canal  . It installs automatically into the optimum position to capture an accurate image  (5).
| Disadvantages|| |
The only disadvantage is its cost. But considering the enormous benefits this cost effect can be overlooked since the ease of the machine makes it suitable to be used also in minor diagnostic procedures. 
| Conclusion|| |
Over the past few decades, various imaging modalities such as conventional radiography, digital radiography, ultrasound and MRI have been used in the diagnosis of periapical lesions. But none of them give entirely satisfactory result. Computed tomography provides 3D image, high spatial resolution smaller size lower acquisition cost and maintenance making it a natural fit for detomaxillofacial imaging, better visualization of bone structures and mandibular canal and for detection of periapical pathology. Thus to conclude the 3D computed tomography technology has made the dento - facial radiology a privileged field that has driven growth of this existing technology out of R&D infancy into commercial maturity  In short, Cone Beam CT is ideally suited for high quality and affordable CT Scanning of Head & Neck in Detomaxillofacial & for Periapical lesions.[Figure 5]
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Department of Conservative Dentistry and Endodontics, Saraswati Dental College & Hospital, 233, Tiwarganj, Faizabad Road, Juggour (via Chinhat), Lucknow - 227105 Uttar Pradesh
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]