Orthopantomogram
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An Orthopantomogram(OPG) also known as an "orthopantogram" or "panorex", is a panoramic scanning dental X-ray of the upper and lower jaw. It shows a two-dimensional view of a half-circle from ear to ear.
Equipment
Dental panoramic radiography equipment consists of a horizontal rotating arm which holds an X-ray source and a moving film mechanism (carrying a film) arranged at opposed extremities. The patient's skull sits between the X-ray generator and the film. The X-ray source is collimated toward the film, to give a beam shaped as a vertical blade having a width of 4-7mm when arriving on the film, after crossing the patient's skull. Also the height of that beam covers the mandibles and the maxilla regions. The arm moves and its movement may be described as a rotation around an instant center which shifts on a dedicated trajectory.
The manufacturers propose different solutions for moving the arm, trying to maintain constant distance between the teeth to the film and generator. Also those moving solutions try to project the teeth arch as orthogonally as possible. It is impossible to select an ideal movement as the anatomy varies very much from people to people. Finally a compromise is selected by each manufacturer and results in magnification factors which vary strongly along the film (15%-30%). The patient positioning is very critical in regard to both sharpness and distortions.
Forming the image
Normally, the person bites on a plastic spatula so that all the teeth, especially the crowns can be viewed individually. The whole orthopantomogram process takes about one minute.
Because the collimation, while rotating, the X-rays projects on the film only a limited portion of the anatomy, at every instant but, as the rotation progresses around the skull, the whole maxillo-facial block is scanned. While the arm rotates, the film moves in a such way that the projected partial skull image (limited by the beam section) scrolls over it and exposes it entirely. Because the beam travels across the skull, the partial image it projects on the film every instant contains all the anatomical elements it crossed in the skull, overlapped. Not all the overlapped element images projected on the film have the same magnification because the beam is divergent and the elements are at different distances from the generator focus. Also not all the element images move with the same velocity on the target film as some of them are far and other closer to the instant rotation center. The velocity of the film is controlled in such fashion to fit exactly the velocity of projection of the anatomical elements of the dental arch side which is closer to the film. Therefore they are recorded sharply while the elements in different places are recorded blurred as they scroll at different velocity.
The dental panoramic image suffers from important distortions because a vertical zoom and a horizontal zoom both varying differently along the image. The vertical and horizontal zooms are determined by the relative position of the recorded element versus film and generator. Closer to the generator means bigger vertical zoom. More, the horizontal zoom is also dependent on the relative position of the element to the focal path. Inside the focal path arch means bigger horizontal zoom and blurred, outside means smaller horizontal and blurred.
The result is an image showing sharply the section along the mandible arch and blurred the rest. For example, the more radio-opaque anatomical region, the cervical vertebras (neck), shows as a wide and blurred vertical pillar overlapping the front teeth. The path where the anatomical elements are recorded sharply is called "focal path".
Films
There are two kind of film moving mechanisms one using a sliding flat cassette which holds the film and another using a rotating cylinder which has the film wound around. There are two standard sizes for dental panoramic films 30cm x12cm (12"x 5") and 30cm x 15cm (12"x6"). The smaller size film involves 8% less of X-ray dosage.
Digital
Dental X-rays radiology moves from film technology (involving a chemical developing process) to Digital X-ray which is based on electronic sensors and computers. One of the principal advantages compared to film based systems is the much greater exposure latitude. This means much less repeats, which also reduces patient exposure to radiation. Lost xrays can also be reprinted. Other significant advantages include instantly viewable images, ability to enhance images, ability to email images to practitioners and clients, easy and reliable document handling, reduced X-ray exposure, no darkroom is required, no chemicals are used.
One particular type of digital system uses a Photostimulable Phosphor Plate (aka PSP - Phosphor Plate) in place of the film. After X-ray exposure the plate (sheet) is placed in a special scanner where the latent formed image is retrieved point by point and digitized, using a laser light scanning. The digitized images are stored and displayed on the computer screen. This method is half way between old film based technology and the current direct digital imaging technology. It is similar to the film process because it involves the same image support handling and differs because the chemical development process is replaced by the scanning process. This is not much faster that film processing and the resolution and sensitivity performances are contested. However it has the clear advantage to be able to fit with any pre-existing equipment without any modification because it replaces just the existing film.
Also some times the term "Digital X-rays" is used to designate the scanned film documents which further are handled by computers.
The other types of digital imaging technologies use electronic sensors. A majority of them first convert the X-rays in light (using a GdO2S or CsI layer) which is further captured using a CCD or a CMOS image sensor. Few of them use a hybrid arrangement which first convert the X-ray into electricity (using a CdTe layer) and then this electricity is captured as an image by a reading section based on CMOS technology.
In current state-of-the-art digital systems, the image quality is vastly superior to conventional film-based systems.
Historical milestones for Digital Panoramic Systems
1995 - DXIS, the world wide first dental digital panoramic X-rays system available on the market, introduced by Signet (France). DXIS targets to retrofit all the panoramic models.
1997 - SIDEXIS, of Siemens (currently Sirona, Germany) offered a digital option for Ortophos Plus panoramic unit, DigiPan of Trophy Radiology (France) offered a digital option for the OP100 panoramic made by Instrumentarium (Finland).
1998-2004 - many panoramic manufacturers offered their own digital system.
Example images
Diagnostic uses
OPGs are used by Dentists to provide information on:
- Impacted wisdom teeth
- Periodontal bone loss
- Finding the source of dental pain
- Assessment for the placement of dental implants
- Orthodontic assessment
The most common use is to determine the status of wisdom teeth.
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