Digital Radiography in Dentistry

Digital Radiography in Dentistry

Abstract
The aim of this article is to explain the basic principles of digital
radiography, and to discuss the intra- and extra-oral imaging systems
currently available. Digital radiography is a form of x-ray imaging, where digital
X-ray sensors are used instead of traditional photographic film. Advantages
include time efficiency through bypassing chemical processing and the ability to
digitally transfer and enhance images. Also less radiation can be used to produce
an image of similar contrast to conventional radiography. There are two main types
of digital sensors available. One is based on charge coupled device technology and
the other consists of phosphor storage plates. The advantages and disadvantages of
each are highlighted with particular attention to orthodontics and endodontics.

Introduction
Digital radiography has been used widely in medicine, but it was only in
the 1980s that the first intra-oral sensors were developed for use in
dentistry. Unfortunately, the early systems could not capture panoramic
and cephalometric images, and this made it impossible for surgeries to
abandon film processing and adopt digital technology. Recently, the
development of cost-effective intra- and extra-oral digital technology
coupled with an increase in computerization of practices has made
digital imaging a superior alternative in many respects to conventional
film imaging. The options for radiography include not only film, but

with recent technological advances, it is now possible to meet a

wide range of NDT inspection applications with digital solutions that

are reliable and cost effective.
Advantages of this type of system for the orthodontist and the patient
include the ability to gain cephalometric analysis and superimposition
quickly on the chairside computer, manipulation of images to aid
diagnosis, dose reductions, and the ease of storage. This article aims to
introduce the basic principles behind digital radiography and also to
discuss some problems, such as set-up costs and cross-infection control
issues that can affect the systems currently available .
The radiological examinations in dentistry may be classified as:
1.Intraoral - where the film or the sensor is placed in the mouth, the
purpose being to visualize a limited region
2. Extraoral where the film or the sensor is outside the mouth and the
purpose is to visualize a wide region.
In dentistry, extraoral imaging splits in:
Panoramic X-ray (aka "panorex" or "pano") showing a section, curved
following more or less mandible shape, of the whole maxillo-facial
block and the Cephalometric X-ray showing a projection, as parallel as
possible, of the whole skull.
DIGITAL RADIOGRAPHY – WHY?

1. Financial – Improved productivity due to significantly reduces exposure times
2. Safety – Lower exposure levels; focused beam operator hazards eliminated
3. Environmental – Chemical processes; disposable handling, consumption reduced
4. Time – Immediate feedback; data confidence
5. Process Improvement – Data archive; storage, transmittal, labor savings
Principles
Conventional imaging
Conventional intra-oral radiographic film consists of silver halide grains in a gelatine matrix. When this film is exposed to X-ray photons the silver halide crystals are sensitized and are reduced to black during the developing process. The film acts as both the radiation detector and the image display.
With extra-oral films indirect action receptors are used to help record the image. This type of film is sensitive to light photons which are emitted by adjacent intensifying screens. Although the film is constructed of silver halide crystals these are primarily sensitive to light rather than X-rays. The use of intensifying screens reduces the dose and can be used where fine detail is not required.
Digital imaging
In digital radiography, instead of the silver halide grain the image is constructed using pixels or small light sensitive elements. These pixels can be a range of shades of grey depending on the exposure, and are arranged in grids and rows on the sensor, unlike the random distribution of the crystals in standard film. However, unlike film the sensors are only the radiation detector and the image is displayed on a monitor.
Image acquisition
There are two ways to acquire a digital image.
Indirect acquisition
A digital image can be produced by scanning conventional radiographs using a flatbed scanner and a transparency adaptor, or by using a charged coupled device camera instead of the flatbed scanner. This image can then be manipulated using software packages or be passed on to a second party via a modem.
Direct digital imaging
There are two systems available, one produces the image immediately on the monitor post-exposure and is therefore called Direct Imaging. The second has an intermediate phase, whereby the image is produced on the monitor following scanning by laser. This is known as semi-direct imaging.
Semi-direct image plate systems. The image plate method involves the use of a phosphor storage plate (PSP). This plate stores energy after exposure to radiation and emits light when scanned by a laser. The scanner stimulates the phosphor plate and stores a record of the number of light photons detected.
Direct sensor systems. The sensor for the radiation image is usually a Charge Coupled Device (CCD). It consists of silicon crystals arranged in a lattice and converts light energy into an electronic signal. This technology is widely used in video cameras. The sensor cannot store information and must be connected via fibre optic wires to the monitor, which can make the sensor bulky and awkward to use.
Extra-oral digital imaging
Extra-oral digital imaging is available using both systems. However, the larger CCD sensors are extremely expensive and usually requires the purchase of new X-ray generators, although a ‘retro-fit’ system has been developed in the USA. These constrictions effectively mean that the PSP method is the one most commonly used.
Panoramic radiography
The PSP method of panoramic digital imaging is very similar to conventional film. The film and intensifying screen are replaced by a storage phosphor plate. The plate is scanned after exposure, which can take up to 3 minutes or longer depending on the product used. The resolution of these systems is greater than 4 LP/mm.
Cephalometric radiography
Naslund et al. investigated the effect of dose reduction obtained with PSP on the identification of cephalometric landmarks and concluded that dose reductions of up to 75 per cent did not effect the localization of cephalometric landmarks. It is also worth noting that with CCD sensors the image is acquired over 15 seconds as the sensor and narrow X-ray beam move up the facial bones and could lead to an increase in the incidence of movement artefact


TYPES OF DIGITAL RADIOGRAPHY

Film Digitization

Process whereby a radiograph is produced in the conventional manner on a normal sheet of industrial x-ray film, the film is then placed in a reader, the image is read and digitized for viewing and archiving on software.
Film digitization helps extract greater information from film, assists in long term archiving and allows remote analysis by networking.







Direct Radiography (DR)

Utilizing this process, the image is captured directly on the flat plate and the image is transmitted directly to the computer. No intermediate steps or additional processes are required to capture the image. Process provides a direct feed from panel to imaging workstation. Direct Digital Capture is suitable for applications
where medium and finer grain film is employed.




Types
•Amorphous – Silicon digital x-ray detector systems
o Capable of high resolution real time radiography
•Amorphous Selenium detector systems
o High resolution, but no real time
•CMOS Technology
o High resolution, but limited real time capability



Computed Radiography (CR)

Rather than utilizing conventional x-ray film to capture an image, computed radiography uses an imaging plate. This plate contains photo sensitive storage phosphors which retain the latent image. When the imaging plate is scanned with a laser beam in the digitizer, the latent image information is released as visible light. This light is captured and converted into a digital stream to compute the digital image. A key consideration in the use of flexible storage phosphor plates and CR systems is that any exposure source that can be used with conventional X-ray films can also be used with this filmless technology. The flexible storage phosphor imaging plates can be directly substituted for film. They can be used in the same film holders and cassettes as those used for film and can be used in applications requiring a flexible medium, such as bending them around a circumferential specimen. This compatibility with existing sources and cassettes makes a transition from traditional film radiography to CR a fairly uncomplicated and in expensive proposition. Computed radiography suitable for applications where coarse grain film is employed.


The Equipment
•A computer
•A mega pixel, high resolution monitor
•Flexible, phosphor screens (instead of film): The imaging plate is a flexible image sensor in which
bunches of very small crystals(grain size: about 5 μm to 25 microns) of photo- stimulable phosphor
•A laser scanner for the screens
•A radiation source




Working

First, phosphor screen is exposed to record an image. At this stage the image recorded by the screen is an invisible latent image. The next step is to process




General Block Diagram


phosphor screen through the reader and processing unit. In this unit the screen is scanned by a very small laser beam. When the laser beam strikes a screen it causes light to be produced(stimulation process). The light that is produced is proportional to the x-ray exposure. The result is that an image in the form of light is produced on the surface of the phosphor screen. The light detector measures the light and sends the data on to produce digitized image. As the surface of the phosphor screen is scanned by the laser beam, the analog data representing the brightness of the light is converted in to digital values for each pixel and stored in the computer memory as a digital image.

• Imaging System
This is the device which produces the primary image information. Examples of these devices include CT scanners, ultrasound machines, x-ray fluorography systems, MRI systems, gamma cameras, PET scanners and computed radiography systems. The device is often physically separate from the other components as in the CT scanner, but may also be mounted in the same cabinet as the other components - as is the case for ultrasound machines. Image information produced by the imaging system is fed to the image acquisition circuitry of the digital image processor. Connections from the digital image processor to the imaging system are generally also present, for controlling specific aspects of the operation of the imaging system, eg gantry movement of a CT scanner. These additional connections are not shown in figure 2 for reasons of clarity.
• Image Acquisition
This component is used to convert the analogue information produced by the imaging system so that it is coded in the form of binary numbers. The type of device used for this purpose is called an Analogue-to-Digital Converter. The image acquisition component may also include circuitry for manipulating the digitised data so as to correct for any aberrations in the image data. The type of device used for this purpose is called an Input Look-Up Table. Examples of this type of data manipulation include pre-processing functions on ultrasound machines and logarithmic transformation in digital fluorography systems.
• Image Display
This component is used to convert digital images into analogue form so that they are in a form which is suitable for display on, for instance, a video monitor. A Digital-to-Analogue Converter is used for this purpose. The image display component may also include circuitry for manipulating the displayed images so as to enhance their appearance. The type of device used for this purpose is called an Output Look-Up Table and examples of this type of data manipulation include post-processing functions on ultrasound machines and 'windowing' on CT scanners. Other forms of image processing provided by the image display component include image magnification and the capability of displaying a number of images on one screen. This component also allows for the annotation of displayed images with the patient name and details relevant to the patient's examination.




• Image Memory
This component typically consists of a volume of RAM which is sufficient for the storage of a number of images which are of current interest to the user.
• Image Storage
This component generally consists of magnetic disks of sufficient capacity to store large numbers of images which are not of current interest to the user and which may be transferred to image memory when required.
• Image ALU
This component consists of an ALU designed specifically for handling image data. It is generally used for relatively straight-forward calculations, such as image subtraction in DSA and the reduction of noise through image averaging.
• Array Processor
This component consists of circuitry designed for more complex manipulation of image data and at very higher speeds than the Image ALU. It typically consists of an additional CPU as well as specialised high speed data communication and storage circuitry. It may be viewed as a separate special-purpose computer whose design has traded a loss of operational flexibility for enhanced computational speed. This enhanced speed is provided by the capability of manipulating data in a parallel fashion as opposed to a sequential fashion (which is the approach used in general-purpose computing). This component is used, for example, for calculating Fast Fourier Transforms and for reconstruction calculations in cross-sectional imaging modalities, such as CT, SPECT and MRI.
• Image Data Bus
This component consists of a very high speed communication link designed specifically for image data.



Working Figure Representation

Digital Image Processor
Computers used for digital image processing generally consist of a number of specialised components in addition to those used in a general-purpose computer. These specialised components are required because of the very large amount of information contained in images and the consequent need for high capacity storage media as well as very high speed communication and data manipulation capabilities.
Digital image processing involves both the manipulation of image data and the analysis of such information. An example of image manipulation is the computer enhancement of images so that subtle features are displayed with greater clarity. An example of image analysis is the extraction of indices which express some functional aspect of an organ under investigation. Most medical imaging systems provide expensive image manipulation capabilities with a limited range of image analysis features. Systems for processing nuclear medicine images (including SPECT and PET) also provide extensive data analysis capabilities. This situation results because of the functional, in contrast to an anatomical, emphasis in nuclear medicine.

Features
The various tools in ImPlan are designed to be easy to use without compromising functionality. The extensive help section in the software gives a full overview with step-by-step diagrams explaining usage.
Line and Angle Measurements
Measure the Distance between any two points on the 2D images, or measure the Angle.






Draw Nerves
Trace the Alveolar Nerve in the panoramic view; it will show up in a highlighted color in all the 2D and 3D views.



This preoperative photo of tooth 3, (A), reveals no clinically apparent decay other than a small spot within the central fossa. In fact, decay could not be detected with an explorer.
Radiographic evaluation, (B), however, revealed an extensive region of demineralization within the dentin (arrows) of the mesial half of the tooth.
When a bur was used to remove the occlusal enamel overlying the decay, (C), a large hollow was found within the crown and it was discovered that a hole in the side of the tooth large enough to allow the tip of the explorer to pass was contiguous with this hollow.
After all of the decay had been removed, (D), the pulp chamber had been exposed and most of the mesial half of the crown was either missing or poorly supported.


Placing an Implant
Navigate to a suitable slice in the Panoramic or Cross Sectional View. Optionally use the Circle or Square Density tools to determine the bone density of the location you wish to place an implant. Click once using your left mouse button on the location where you want the implant to be placed.


Manipulating an Implant
To Move your implant, simply Drag it to its new desired location. To Rotate the implant, click on its center using your left mouse button to highlight it. Then Drag the axis line to rotate.


Advantages of digital imaging
Dose reduction
Dose reductions of up to 90 per cent compared to E-speed film have been reported by some authors in the diagnosis of caries. Although some researchers do claim dose reductions compared with conventional extra-oral film, in practice the background noise rises to unacceptable levels. It is now accepted that there is no appreciable reduction compared with films used in conjunction with rare earth intensifying screens.
Image manipulation
This is perhaps the greatest advantage of digital imaging over conventional film. It involves selecting the information of greatest diagnostic value and suppressing the rest. Manufacturers provide software programmes with many different processing tools, however some are more useful than others and these include:
Contrast enhancement. This can effectively compensate for over or under exposure of the digital image. It has been shown that contrast enhancement of CCD devices were more accurate than E-speed film for detecting simulated caries under orthodontic bands.
Measurements. Digital callipers, rulers and protractors are some of the many tools available for image analysis. Many authors have reported on their application in cephalometric analysis. The images can also be superimposed onto each other and onto digital photographs.
3-D reconstruction. This application can be theoretically used to reconstruct intra- and extra-oral images. The uses range from profiling root canals to visualizing facial fractures in all three dimensions.
Filtration. The addition of filters to the airspace around the face can clarify the soft tissue profile if the original soft tissue image was poor.
Time
Much time is gained especially with the CCD system where the image is displayed at the chairside immediately post exposure. Although a lag time between scanning and the appearance of an image exists with the PSP method it is still substantially faster than conventional developing processes in general use.
Storage
Storage was initially a problem before the development of DVDs and CD ROMs as three peri-apical images would fill a floppy disc. However, now a CD ROM can hold over 30,000 images. This means that images can be stored cheaply and indefinitely.
Teleradiology
The digital image file can be further reduced in size by compression techniques, and sent via a modem and telephone line to colleagues for review. This had the advantages of not losing radiographs in the post and saving time if an urgent appointment is required. The operator at the other end can also manipulate the image if desired.
Environmentally friendly
No processing chemicals are used or disposed of. Both CCD sensors and the PSP plates are capable of being reused for many thousands of exposures. They can, however, become scratched and damaged if not handled carefully.
Disadvantages of digital imaging
The majority of the disadvantages are associated with the CCD system.
Cost
Currently, the cost of converting from intra-oral film to digital imaging is approximately 6600 Euros. This initial outlay should be offset against the time saved and the efficiency of storage of the images.
Sensor dimensions
These are still quite bulky for the CCD system and awkward to position due to trailing fibre optic wires. The original problem of small sensor active areas has been rectified and the same amount of information can be captured as conventional film.
Cross-infection control
Each intra-oral sensor and plate must be covered by a plastic bag, and this bag is changed between patients. However, if they become directly contaminated there is no way of sterilizing them and they should be discarded regardless of expense.
Medicolegal
Concerns have been raised in the past about the ability to manipulate the images for fraudulent purposes. Manufacturers of software programmes have installed ‘audit trails’, which can track down and recover the original image. Many insurance companies in the USA are accepting digital images as valid attachments when the claims are electronically claimed.
Conclusions
The technology is now available to run a practice almost paper free. It is theoretically possible to store clinical notes, photographs, radiographs, and study models on disc, and refer or consult online. The future of digital imaging could include the testing and upgrade of X-ray equipment and software on-line. Research is also continuing into the development of a credit card sized ‘smart card’, which could carry a patient's medical and dental notes along with their radiographic images. It is important that advances in technology are accepted and the benefits that they produce utilized in order that clinical practice and patient care continue to improve.
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