Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
JIAOMR REVIEW ARTICLE Comparison of Spiral Computed Tomography and Cone-Beam Computed Tomography Comparison of Spiral Computed Tomography and Cone-Beam Computed Tomography Alexander Maninagat Luke, Krishna Prasad Shetty, SV Satish, Krishnarao Kilaru ABSTRACT Imaging is a highly dependent technology in the clinical assessment of a patient. With respect to the dental field, imaging has travelled a long way from the conventional radiographs to modern techniques like the computed tomography (CT) and the cone-beam computed tomography (CBCT). This article describes the evolution made in the CT in the field of dental imaging and attempts to compare the CT with the more advanced CBCT which is fast gaining popularity in the field of dental imaging. Keywords: Radiology, Cone-beam computed tomography, Spiral. How to cite this article: Luke AM, Shetty KP, Satish SV, Kilaru K. Comparison of Spiral Computed Tomography and Cone-Beam Computed Tomography. J Indian Aca Oral Med Radiol 2013;25(3):173-177. Source of support: Nil Conflict of interest: None INTRODUCTION Imaging is an important diagnostic adjunt to the clinical assessment of the dental patient. The introduction of panoramic radiograph in the 1960s and its widespread adoption throughout the 1970 and 1980s herald major programs in dental radiology.1 Nothing has captured the dental profession’s imagination in the past few years like the introduction of cone-beam volumetric imaging (CBVI), sometimes referred to as cone beam computerized tomography (CBCT) or cone-beam volumetric tomography (CBVT).2 Dental radiology has long played an exciting and critical diagnostic role in dentistry. This began with discovery of X-rays by Roentgen.3 Intraoral and extraoral procedures, used individually or in combination, suffer from same inherent limitations of all planar two-dimensional (2D) projections magnification, distortion, superimposition and misrepresentation of structures. Efforts were made toward three-dimensional (3D) radiographic imaging and for the first time, practitioners had access to X-ray devices that could generate narrow cross-sectional images, usually perpendicular to the long axis of the human body, hence, the term computed axial tomography or CAT scan. Computed tomography acquisition has subsequently been refined to incorporate a helical or spiral synchronous motion, fan-shaped beam, and multiple detector acquisition (MDCT), which enables fast scan times that provide highquality images that can be integrated to produce a volumetric dataset. Image acquisition process in CBCT differs from that of traditional medical computerized tomography (CT) scanners in that the patient is not usually supine, the image gathered is in a voxel (volume element) format, the X-ray dose absorbed by the patient is substantially lower, appointment availability is much easier, and it is less expensive.4-6 In 1972, the independent findings of Hounsfield and Cormack revolutionized diagnostic imaging with the invention of the CT scanner.7,8 Willi Kalender (1970), who is credited with the invention prefers the term spiral scan CT.9 An early volumetric CT predecessor of CBCT, the dynamic spatial reconstructor, was developed in the late 1970s by the Biodynamic Research Unit at the Mayo Clinic.10 CBCT provided an alternate method of cross-section image production to fan-beam CT using a comparatively less-expensive radiation detector than conventional CT. CBCT to dentistry first occurred in 1995. Italian coinventors, Attilio Tacconi and Piero Mozzo, developed a CBCT system for the maxillofacial region and was designed and produced by Quantitative Radiology, Inc. of Verona, Italy. The NewTom DVT 9000 became the first commercial CBCT unit, and was first introduced in Europe in 1999; its design was similar to conventional CT. The purpose of this article is to describe the important differences between fan-beam CT and CBCT technologies regarding beam geometry, image acquisition, image detection, image reconstruction, radiation dose and their application in different fields of dentistry. EVOLUTIONS IN CT The introduction of CT heralded 2 firsts in medical diagnostic imaging: (1) data acquisition and image display were both digital in nature, with images viewed on special monitors and (2) a series of single, discrete, contiguous crosssectional image slices of complete body sections in parallel to the image acquisition geometry were generated instead of planar projection images, which resulted in the superimposition of anatomic structures.11 A fan-beam CT scanner images a region of interest (ROI) using an X-ray tube rigidly linked to a detector located on the other side of the patients. Together the tube and the detector initially scan across, but in later generations around, patients on a fulcrum, sweeping a narrow X-ray beam through one thin slice at a time. Contiguous image slices are acquired from multiple X-ray projections around the object. The Journal of Indian Academy of Oral Medicine and Radiology, July-September 2013;25(3):173-177 173 Alexander Maninagat Luke et al A B C D Figs 1A to D: Four basic scanning methods or systems: (A) first generation, (B) second generation, (C) third generation, and (D) fourth generation reconstruction of the transmitted X-ray attenuation data by specific software algorithms produces adjacent image slices of the individual’s imaged volume.12 This concept is better understood if the human body (part thereof under examination) is made up of a stack of contiguous transverse slices. Each scan aims to determine the composition of one transverse section. This slice or section can be imagined as composed of discrete blocks of tissue known as volume elements or voxels.11 The first generation of CT scanner in the early 1970s incorporated a pencil-like X-ray beam and single detector that acquired data in a translate-rotate motion (Fig. 1A): The X-ray source and detector gradually scanned across patients in a linear movement (translation) and then, after a secondary rotational movement of the apparatus, the translation was performed again. After completing a 360 rotation, supine patients were translated slightly through the apparatus on a gantry such that a new section was exposed. In 1972, second-generation scanners were introduced that incorporated various efficiencies in image geometry acquisition. Although the motion remained a combination of translation followed by rotation, the shape of the X-ray beam changed from a narrow pencil beam to fan shaped, with 174 gradual widening of the fan (Fig. 1B). The number of the detector elements also increased from a single to a linear array of detectors. Scan times, however, remained long, partly because of the use of stationary anodes. In 1976, the third generation of CT scanners, known as rotate-rotate, was introduced. The linear array of detectors was modified to an arc and their number increased to several hundred. In addition, the X-ray beam shape was wide and divergent enough to cover the arc (Fig. 1C). Finally the linear motion of the X-ray tube was abandoned and a circular motion was adopted with a fulcrum centered through the middle of patients. During a single scan, the X-ray tube/arc-shaped detector array combination continuously rotated around the patients while the patients were translated on the gantry. This design facilitated the incorporation of rotating anodes, which resulted in higher generator output and reduced scan times. In fourth-generation scanners, introduced in 1978, multiple detectors (up to a few thousand) are used and form a stationary ring around the patients (Fig. 1D). In this design, only the X-ray tube rotates around the patients inside the detector ring. During scanning, a wide fan-shaped beam exposes only a portion of the detectors, which is different for the various projections. JIAOMR Comparison of Spiral Computed Tomography and Cone-Beam Computed Tomography The 4 generations of CT design incorporated innovations designed to increase efficiency and reduce single-image scan times; however, the resulting data set was a compilation of single scans interposed by a stop/start action necessary for table translation. In addition, extra time was required for hardware adjustments during scanning (e.g. cable unwinding). Both factors contributed in slowing down the overall acquisition process and resulted in increased examination time.13 The introduction of the spiral or helical CT acquisition in 1989 that image data would finally become isotropic. Unlike in sequential CT, the technique involves the continuous acquisition of projection data through a 3D volume of tissue by the continuous rotation of the X-ray tube and detectors and simultaneous translation of the patients through the gantry opening.14 The X-ray tube continues to travel on a circular arc (Fig. 2); however, relative to moving patients, it seems that it follows a spiral or helical trajectory.11 The spiral motion of CT acquisition has been facilitated by mechanical innovations, such as slip ring technology, which prevents the winding and unwinding of cables and electrical advances, including increased heat capacity of the X-ray generators. Different reconstruction algorithms (interpolation algorithms) were developed that provided single-plane reconstructions despite the fact that the projections were acquired in a spiral rather than planar motion.13 The most recent technological advance in CT acquisition occurred in 1998 with the incorporation of multiple rows of detectors (initially 4) instead of a single row. MDCT has revolutionized the versatility of CT data with the partitioning of the information received from the incident beam into multiple small segments.15 Current detector configurations range from 16 to 64 and now 256 and even 512 channel systems. The geometric configuration and acquisition mechanics for the CBCT technique are theoretically simple.16-19 Four technological developments converged to facilitate the construction of affordable CBCT units small enough to be used in the dental office: the introduction of X-ray detectors capable of rapid acquisition of multiple basis images, the development of suitably resilient X-ray generators, the evolution of suitable image acquisition and integration algorithms,20-22 and the availability of computers powerful enough to process the enormous amount of acquired image data. Fast data acquisition of the maxillofacial region with a single revolution, shorter examination time, and the fairly simple and inexpensive detection system are obvious advantages of CBCT that have propelled the technology for clinical applications in dentistry. Is CBCT different to medical CT? Yes, CBCT is different in the following ways, Shape of beam: In CBCT the shape of the X-ray beam is divergent, like a cone, hence the name. In medical CT, the shape of the beam is flat, like a fan. Patient posture and movement of X-ray tube: In CBCT, the patient is usually sitting up in a dedicated chair, although with some machines the patient is standing up, while with one particular machine, the patient is lying down. Once the patient is in position, the X-ray tube head then revolves around the patient’s head in a single 360º rotation. In medical CT, the patient lies supine on the CT table. The table then moves the patient into the CT scanner. Depending on the type of CT scanner (Fig. 3), the table may move slowly as the X-ray exposure takes place, or the table stays static as Fig. 2: Trajectory and the axis of rotation of the X-ray source and the detector around the object Fig. 3: Path of the motion of the X-ray source, detectors and the patient Journal of Indian Academy of Oral Medicine and Radiology, July-September 2013;25(3):173-177 175 Alexander Maninagat Luke et al the X-rays are generated. A modern medical CT scanner (multislice spiral CT) can scan a whole person in just a few minutes. Software processing: In CBCT the initial raw data (image dataset) from one scan has to be processed immediately to allow reconstruction into different planes for viewing. This usually takes a few minutes or sometimes quicker depending on the volume of data. Thereafter, depending on the patient’s treatment needs and the dentist’s treatment plan, the image data can be further processed, e.g. to help with implant planning, construction of surgical drill guide, image-guided surgery. These specialized applications require additional resources. Medical CT datasets can be processed in a similar way too because both CBCT and medical CT have their raw data in a standard universal format known as DICOM. Radiation dose CBCT scans involve much less radiation dose compared to medical CT because a smaller volume of tissue is irradiated. CBCT machines also use additional means to reduce dose, such as pulsing the X-ray beam (instead of continuous emission of X-rays), 180º scan (instead of 360º). Field of view (FOV) and image resolution one of the advantages of CBCT is that the irradiated volume, or field of view, can be as small as 4 × 4 cm. At this size, up to two adjacent molars can be included in their entirety. At this small volume size, the image voxel size can be as small as 0.076 × 0.076 × 0.076 mm. In CBCT, the voxels are isotropic (equal lengths in all three dimensions), whereas in medical CT the voxels are anisotropic rectangular blocks, up to 2 mm long. The smaller the voxel size the better the image resolution and CBCT is better than medical CT. DISADVANTAGES OF CBCT Artefacts: Is defined as a visualized structure in the reconstructed data that is not present in the object under investigation. The following artefacts are reported extinction, beam hardening, partial volume, ring and motion (misalignment). Noise: Two sorts of noise are considered in the reconstruction images: (a) additive noise (b) photon-count noise. CBCT machines for dose reduction reasons are operated at mill amperes that are one order of magnitude below than those of medical CT machines. Thus, the signal-to-noise ratio is much lower than CT. A high noise is present in CBCT images. CBCT is not able to demonstrate nor differentiate soft tissues and soft tissue lesions. CONCLUSION CBCT, an emerging technology, is sure to make prominent inroads into imaging in dentistry. It is a unique technique that allows 3D visualization of dental tissues there by permitting accurate diagnosis and treatment planning for all 176 specialties of dentistry. CBCT makes clinical decision making easier and more precise, patient treatment decisions more accurate and visualization of the X-ray data more meaningful. Dentistry is moving away from ‘radiographic interpretation’ and into ‘disease visualization,’ and it could not have come at a better time. CBCT is capable of providing accurate, submillimeter resolution images at shorter scan times, lower dose, and lower costs. Increasing availability of this technology provides an imaging from diagnosis to image guidance of operative and surgical procedures. REFERENCES 1. Scarfe WC, Farman AG. What is Cone beam CT and How does it work? Dent Clin N Am 2008;52:707-730. 2. Ziegler CM, Woertche R, Brief J, Hassfeld S. Clinical indications for digital volume tomography in oral and maxillofacial surgery. Dentomaxillofac Radiol 2002;31:126-130. 3. Rontgen wc_on a new kind of rays. Science. 1896 Feb 14;3(59): 227-231. 4. Chau AC, Fung K. Comparison of radiation dose for implant imaging using conventional spiral tomography, computed tomography, and cone-beam computed tomography. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2009;107:559-565. 5. Dula K, Mini R, van der Stelt PF, Sanderink GC, Schneeberger P, Buser D. Comparative dose measurements by spiral tomography for preimplant diagnosis: the Scanora machine versus the Cranex Tome radiography unit. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2001;91:735-742. 6. Hirsch E, Wolf U, Heinicke F, Silva MA. Dosimetry of the cone beam computed tomography veraviewepocs 3D compared with the 3D Accuitomo in different fields of view. Dentomaxillofac Radiol 2008;37:268-273. 7. Hounsfield GN. Nobel lecture, 8 December 1979. Computed medical imaging. J Radiol 1980;61:459-468. 8. Cormack AM. Early two-dimensional reconstruction (CT scanning) and recent topics stemming from it. Nobel lecture, December 8, 1979. J Comput Assist Tomogr 1980;4:658-664. 9. Kalender WA. Technical foundations of spiral CT. Semin Ultrasound CT MR 1994;15(2):81-89. 10. Robb RA, Lent AH, Gilbert BK, et al. The dynamic spatial reconstructor: a computed tomography system for high-speed simultaneous scanning of multiple cross sections of the heart. J Med Syst 1980;4:253-288. 11. Kalender WA. Computed tomography. Fundamentals, system technology, image quality, applications. 2nd ed. Erlangen (Germany): Puplicis Corporate Publishing; 2005. 12. Bushberg JT, Siebert JA, Leidholdt EM, et al. The essential physics of medical imaging. Baltimore (MD): Williams and Wilkins 1993. 13. Mahesh M. Search for isotropic resolution in CT from conventional through multiple-row detector. Radiographics 2002;22: 949-962. 14. Kalender WA, Seissler W, Klotz E, et al. Spiral volumetric CTwith single breath hold technique, continuous transport and continuous scanner rotation. Radiology 1990;176:181-183. 15. Saini S. Multi-detector row CT: principles and practice for abdominal applications. Radiology 2004;233:323-327. 16. Sukovic P. Cone beam computed tomography in craniofacial imaging. Orthod Craniofac Res 2003;6(Suppl 1):31-36. JIAOMR Comparison of Spiral Computed Tomography and Cone-Beam Computed Tomography 17. Hayakawa Y, Sano T, Sukovic P, et al. Cone beam computed tomography—a paradigm shift for clinical dentistry. Nippon Dental Review 2005;65:125-132. 18. Farman AG, Levato CM, Scarfe WC. A primer on cone beam computed tomography. Inside Dent 2007;3:90-93. 19. Scarfe WC, Farman AG. Cone beam computed tomography: a paradigm shift for clinical dentistry. Australasian Dental Practice 2007;102-110. 20. Feldkamp LA, Davis LC, Kress JW. Practical cone-beam algorithm. J Opt Soc Am A 1984;1:612-619. 21. Wischmann HA, Luijendijk HA, Meulenbrugge HJ, et al. Correction of amplifier nonlinearity, offset, gain, temporal artifacts, and defects for flat-panel digital imaging devices. In: Antonuk LE, Yaffe MJ, editors. Medical imaging 2002: physics of medical imaging-proceedings of SPIE. San Diego (CA): SPIE 2002;4682:427-437. 22. Grangeat P. Mathematical framework of cone beam 3D reconstruction via the first derivate of the radon transform. In: Herman GT, Louis AK, Natterer F, editors. Mathematical methods in tomography. Berlin: Springer; 1991;66-97. ABOUT THE AUTHORS Alexander Maninagat Luke (Corresponding Author) Radiologist, Department of Oral Medicine and Radiology, College of Dentistry, Ajman University of Science and Technology, UAE, Phone: 00971555171094, e-mail: [email protected] Krishna Prasad Shetty Professor, Department of Conservative Dentistry and Endodontics Navodaya Dental College and Hospital, Raichur, Karnataka, India SV Satish Professor, Department of Conservative Dentistry and Endodontics Navodaya Dental College and Hospital, Raichur, Karnataka, India Krishnarao Kilaru Assistant Professor, Department of Restorative Dental Sciences, King Khalid University, Saudi Arabia Journal of Indian Academy of Oral Medicine and Radiology, July-September 2013;25(3):173-177 177