|BEST ESSAY AWARD - UNDERGRADUATE CATEGORY
|Year : 2008 | Volume
| Issue : 2 | Page : 77-80
Conventional dental radiography vs. Advanced dental imageology
Kirthana Devaji Rao
CRI, Meenakshi Ammal Dental College, Chennai, India
Kirthana Devaji Rao
CRI, Meenakshi Ammal Dental College, Chennai
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Rao KD. Conventional dental radiography vs. Advanced dental imageology. J Indian Acad Oral Med Radiol 2008;20:77-80
Radiographs are an important adjunct to providing oral health care for the total patient. Historically, radiographic images have been produced using film-based systems. However, in recent years, with the arrival of new technologies, many practitioners have begun to incorporate digital radiographic imaging into their practices.
| Radiation Physics|| |
Matter, occurring in three states in nature is anything that occupies space and has inertia and mass, and can exert force, or can be acted upon by force. Atoms are the fundamental units of elements, which are divisions of matter. Since atoms cannot be observed directly, they are described by various models, of which the most accepted is the Neils Bohr model. He described an atom to consist of a central nucleus with protons and neutrons, and electrons revolving around them in various orbits.
Radiation is transmission of energy through space and matter, which may be particulate or electromagnetic. X-rays belong to electromagnetic radiation where the movement of energy through space is a combination of electric and magnetic fields.
| Conventional Radiography|| |
Conventional dental radiology involves an X-ray machine producing X-rays and an image receptor such as an X-ray film.
The conventional X-ray machine consists of the X-ray tube, which is made of the anode and cathode, the power supply and timer. Production of X-rays is primarily by Bremsstrahlung and sometimes by characteristic radiation. These differ by the point at which the high energy electron strikes to be radiated as a photon. The factors controlling the X-ray beam are tube current, tube voltage, exposure time, filtration and collimation. X-rays interact with matter by coherent and Compton scattering or photoelectric absorption.
X-ray film has an emulsion made of silver halide crystals and sulfur compounds and a base made up of polyester polyethelene terephthalate, which is protected by an overcoat. X-ray films are available in various sizes for use in different areas. Films used for periapical view come in three sizes, which can be chosen according to the area of usage. The other films available are for occlusal view, panoramic view and cephalometric view, in order of increasing size.
Another important accessory to the X-ray film in extraoral radiography is the intensifying screen, which creates an image receptor ten to sixty times more sensitive than the film alone. It consists of a phosphor layer, a base made of polyester plastic and a protective coat. Use of grids, made of alternating lead strips and radiolucent material aid in altering the clarity of the image, and also in reducing the scattered radiation exiting a subject that reaches the film.
Accuracy of radiographs
The factors affecting the quality of an image are image size and shape distortion, which have to be minimal to view the accurate size and shape of the object. Accuracy in dental radiology is influenced by the technique used and the angulation.
There are two techniques used for oral radiography:
- Bisecting angle technique also called the short cone technique and
- Paralleling technique, which is the right angle or long cone technique.
The variation between these two techniques is the position of the central beam in relation to the X-ray film and the long axis of the tooth. Bisecting angle technique is easy to use, but has a major disadvantage of distortion, which is minimized in the paralleling technique by use of film holders and directing the central ray perpendicular to the X-ray film and the long axis of the tooth. The bisecting angle technique works on the Cieszynski's rule, which says that two triangles are equal when they share a complete side and have two equal angles.
Angulation of the tube head is another important consideration influencing the accuracy of radiographs. The horizontal angulation should always be oriented at right angles to the buccal or facial surface of the teeth, or it may lead to overlapping. The vertical angulation of the tube head varies for every tooth and even for the maxilla and mandible. Improper vertical angulation may lead to elongation or shortening of images.
Radiation can cause various complications, by affecting the oral mucous membrane, taste buds, salivary glands, teeth and bone as far as the oral cavity is concerned. When it comes to the rest of the body it can cause acute radiation syndrome, gastrointestinal, cardiovascular and central nervous system syndromes. Radiation can affect the fetus and embryo also. It has carcinogenic and mutagenic potential.
National council on radiation protection and measurements and International commission on radiological protection have established guidelines to limit the exposure to radiation by both occupationally exposed individuals and the public. Dose specifications have been given for bone marrow, thyroid, gonad etc. Exposure and dose can be reduced by considering factors like patient selection, use of intraoral high speed image receptor, intensifying screens and calibrating the focal spot-to-film distance, collimation, filtration and use of leaded aprons and collars.
Disadvantages of conventional radiography
Two dimensional representation: Since conventional radiographs are two-dimensional representations of three-dimensional objects, it is difficult to localize a particular area with a single radiograph. To overcome this, the object localization technique can be used where two radiographs are taken by shifting the X-ray tube and the same side lingual opposite side buccal rule is used to identify the position of the object.
Laborious processing: The main disadvantage of conventional radiographs is that the film has to be processed in a developing and fixing solution. This requires separate equipment. Film processing can be manual or automatic. Manual is time consuming whereas automatic is quick and is more accurate as it has an inbuilt timer. Processing of an X-ray film can be done only in a dark room under specific lighting and temperature. The solutions need to be replenished or they may lead to faulty radiographs.
Faulty radiographs: Faulty radiographs, a major drawback in conventional radiography may be because of faulty technique, or the solutions used. The common problems are light or dark radiographs, insufficient contrast, film fog, dark spots or lines, artifacts, light spots, yellow or brown stains, blurring, partial images and emulsion peel. They may occur due to defective processing, exposure, equipment, processing solutions, technique, etc.
Bitewing or interproximal radiographs are used to view maxillary and mandibular teeth simultaneously. They may be vertical or horizontal, based on the film placement.
Occlusal radiographs are used to view the whole set of teeth and supporting structures in a transverse plane. They are cross sectional, anterior or lateral for the maxilla and mandible.
Orthopantamograph (OPG) shows the maxilla, mandible and other structures in a panoramic view. The angulation varies accordingly and OPG requires separate equipment for exposure.
Extra oral radiographs are used to view the entire skull, using different techniques, focusing on the area of interest. The various views are, lateral cephalometric or lateral skull projection, submentovertex projection, Water's projection, posteroanterior skull or cephalometric projection, reverse Towne's projection and mandibular oblique lateral projection, which may focus on the mandibular ramus or body.
| Digital Radiography|| |
Since the 1970s, advancements in imaging technology have made dental digital radiographic imaging a reality in oral and maxillofacial radiology. Dental digital radiographic imaging includes all non-film-based methodologies and is often referred to as computed dental radiography, direct dental radiography, or simply as digital radiography.
In the late 1980s, the Trophy Corporation introduced the first digital radiography system to the dental profession, which was a Charged Coupled Device based system called Radiovisiography. Following this, the Complementary-Metal-Oxide-Semiconductor detectors and photostimulable phosphor technology, were introduced, which have the advantage of manipulation using computer software, but without an increase in the information available for diagnosis. What digital imaging allows is the alteration on how the information is displayed.
To produce a radiographic digital image, a source of ionizing radiation, an image detector, a computer, and a monitor to display the image are required. As with conventional dental radiography, the detector is positioned in the mouth with a holding device similar to the types used to hold the film in place. X-rays pass through the dental structures carrying information to the detector, first sensing it and then capturing it. This information is then transformed or converted from an analog format (continuous data) to a digital format (discrete data) that can be read by the computer. The image can then be displayed on the computer monitor using up to 256 shades of gray. The image is made up of individual picture elements or pixels.
The analog to digital converter measures the amount of radiation registered in the sensor and converts it to digital form by assigning a number using a binary system where each binary digit (bit) is represented by a numeral zero or one. These bits are combined into an eight-bit word, or byte, that allows a maximum combination of 256 (2 to the exponent 8) gray levels.
Indirect digital images are made from radiographs acquired from conventional techniques. It involves digitization of the image using a scanner or a digital picture of the image.
Digital radiography includes scanography, computed tomography, cone beam radiography, magnetic resonance imaging, nuclear imaging and ulrasonography.
Scanography uses a narrowly collimated, fan shaped beam of radiation to scan an area of interest, producing an image on a moving film. Image contrast is greater because collimation of the X-ray beam reduces the scattered radiation. Scanography may be linear or rotational.
Computed tomography (CT) produces an axial cross sectional image using a narrowly collimated moving beam of X-rays. A scintillation crystal detects the remnant radiation of this beam, and the resulting analog signal is fed into a computer, digitalized and analyzed by a mathematical algorithm, to reconstruct the data as an axial tomographic image. The recent advances in computed tomography are ultrafast CT, spiral CT and xenon CT.
- Ultrafast CT is ten times faster than conventional CT. It is not of much use in dentistry.
- Spiral CT, in which the gantry with the X-ray tube and detectors revolve around the patient on a moving table, is more advantageous because of the reduced examination time and radiation dose compared to conventional CT.
- Xenon CT, in which inhalation of stable xenon gas is used as a CT contrast medium, is used mainly for blood flow measurements.
CT is commonly used for assessment of trauma and malignancies of maxillofacial complex, implant planning, and of late to locate complicated root canals.
Cone beam radiography
This uses a round or rectangular cone shaped X-ray beam centered on a two-dimensional X-ray sensor to scan a 360 o rotation about a patient's head. This is less expensive than CT.
Magnetic resonance imaging
Magnetic resonance imaging (MRI) uses nonionizing radiation from the radiofrequency band of the electromagnetic spectrum, in contrast to the other techniques described above. Here, the patient is placed in a large magnet, which induces a relatively strong external magnetic field, which causes the atoms in the body to align themselves with the magnetic field, which is released as energy from the body after the application of radiofrequency signals. This is detected and used to construct the magnetic resonance image by the computer. MRI is used in the diagnosis of soft tissue lesions because of the high contrast sensitivity to tissue differences.
| Nuclear Imaging|| |
Nuclear imaging is based on the radiotracer method, which assumes that radioactive atoms or molecules behave in the same manner as their stable counterparts because they are chemically indistinguishable. Radiotracers, used in quantities far below amounts lethal to cells, allow measurement of tissue function in vivo and provide an early marker of disease through measurement of biochemical change. Of the many isotopes used, like I 131 , Ga 67 and Se 74 , the most commonly used is Tc 99m . The tracers are imaged by a stationary Anger camera More Details or Gamma scintillation camera, which is capable of producing a flat plane image of an area or organ. Rotating the same camera 360 o with specialized ring detectors makes the principle of single photon emission computed tomography (SPECT). The collected data are used to construct multiplanar images of the area of study. The most recent development over SPECT in the field of nuclear medicine is PET - Positron emission computed tomography, which relies on Positron emitting radionuclides generated in a cyclotron. The commonly used radionuclides in PET are 11 C, 13 N, 15 O and 18 F. PET has sensitivity hundred times that of a gamma camera. Nuclear imaging is used in tumour analysis and detection of recurrence.
| Ultrasonography|| |
Ultrasonography is based on the principle of reflection of sound beam depending on the acoustic impedance of the tissue, which has a characteristic internal echo pattern of that particular tissue. The echo patterns delineate different tissues and also correlate with the pathologic changes in the tissues. It is used to diagnose lesions involving the temporomandibular joint, muscles, space infections and mainly for oral submucous fibrosis.
| Advantages and Disadvantages of Digital Imaging|| |
- One of the biggest advantages of digital imaging is the ability of the operator to post-process the image, which allows the operator to manipulate the pixel shades to correct image density and contrast, as well as perform other processing functions that could result in improved diagnosis and fewer repeated examinations.
- With the advent of electronic record systems, images can be stored in the computer memory and easily retrieved on the same computer screen and can be saved indefinitely or be printed on paper or film if necessary.
- All digital imaging systems can be networked into practice management software programs facilitating integration of data.
- With networks, the images can be viewed in more than one room and can be used in conjunction with pictures obtained with an optical camera to enhance the patients' understanding of treatment.
- Digital imaging allows the electronic transmission of images to third-party providers, referring dentists, consultants and insurance carriers via a modem.
- Digital imaging is also environment friendly since it does not require chemical processing. It is well known that used film processing chemicals contaminate the water supply system with harmful metals such as the silver found in used fixer solution.
- Radiation dose reduction is also a benefit derived from the use of digital systems.
- The initial cost can be high depending on the system used, the number of detectors purchased, etc.
- Competency using the software can take time to master depending on the level of computer literacy of team members.
- The detectors, as well as the phosphor plates, cannot be sterilized or autoclaved and in some cases CCD/CMOS detectors pose positioning limitations because of their size and rigidity. This is not the case with phosphor plates; however, if a patient has a small mouth, the plates cannot be bent because they will become permanently damaged.
- Finally, since digital imaging in dentistry is not standardized, professionals are unable to exchange information without going through an intermediary process.
| References|| |
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|3.||Walker A, Horner K, Czajka J, Shearer AC, Wilson NH. North Western Medical Physics Department, Christie Hospital and Holt Radium Institute, Manchester, UK Quantitative assessment of a new dental imaging system. Br J Radiol 1991;64:529-36. [PUBMED] |
|4.||Gijbels F, Jacobs R, Sanderink G, De Smet E, Nowak B, Van Dam J, et al. A comparison of the effective dose from scanography with periapical radiography. Dentomaxillofac Radiol 2002;31:159-63. [PUBMED] [FULLTEXT]|
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