Alfarabi college .. Radiology .. Level 7 Group one Dr . Islam kassem Digital X ray Level 7 Group one 20091109 200911027 200911354 200911634
Introduction The second phosphor based approach is a stimulable phosphor system, in which the phosphor contains traps for electronsFor decades now, unlike any major excited by incident x-rays. The latent imagemedical imaging methods such as formed by the trapped electrons is thenultrasound, nuclear medicine etc, all of brought out, in the form of a blue-lightwhich are digital, conventional x-ray image, by illuminating the phosphor, pointimaging remains a largely analog to point, with a red laser. Unlike thetechnology. intensifier system, the stimulable phosphor system cannot produce instant images, for the cassette must be carried to a laser scanner for readout by a photo-multiplier,Why digital? which performs the digitization. The third commercial digital system is based on using an amorphous seleniumMaking the transition from analog to photoconductive layer to convert x-raydigital could bring several advantages to x- photons directly to charge carriers. It usesray imaging. These would include an amorphous selenium photoconductorimprovement in contrast and other aspects sensitized by depositing charges on itsof image quality by means of image surface by a corona discharge, as inprocessing: radiological images could be xerography. After exposure to x-rays, thecompared more with those obtained from image resides as a charge distribution onother imaging modalities, electronic the a-Se surface, which is read outdistribution of images within hospitals electronically and then digitized. With thecould make remote access and archiving use of flat panel detectors in the a-Sepossible, highly qualified personnel could method the x-ray image is captured and isservice remote or poorly populated regions converted directly to a digital signal forfrom a central facility by means of display, processing, and storage. Even a"teleradiology"; and radiologists could use higher resolution can be achieved due tocomputers more effectively to help with the use of these flat panel detectors in the a-diagnosis. Se method.Digital radiography is currently practiced through the use of three commercialapproaches, two of which also depend on phosphor screens. The first phosphorbased approach is to digitize the signal from a video camera that is optically coupledto an x-ray image intensifier to provide an instant readout
The use of X-ray image receptors that produce a digital image is becoming increasingly important. Possible benefits include improved dynamic range and detective quantum efficiency, improved detectability for objects of low intrinsic contrast, andreduced radiation dose. The image can be available quickly. The displayis separated from the image capture so that processing and contrast adjustment are possible before the image is viewed. The availability of a digital image means ready input into PACS and opens up the possibility of computer-aided detection and classification of abnormality. Possible drawbacks of digital systems include high cost, limited high contrast resolution and the fact that their clinical value is sometimes not proven in comparison with conventional, analogue techniques. The high contrast resolution attainable with such systems is discussed and the problem of sampling limitations and aliasing considered. The properties and limitations of digital systems using computed radiography, caesium iodide plus CCDs and active matrix arrays with either caesium iodide or seleniumdetectors are demonstrated. Examples are given of digital systems for mammography and general radiography and their performance is demonstrated in terms of clinical assessment and measurements of the modulation transfer function and detective quantum efficiency. Digital radiography It 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. Instead of X-ray film, digital radiography uses a digital image capture device. The gives advantages of immediate image preview and availability; elimination of costly film processing steps; a wider dynamic range, which makes it more forgiving for over- and under-exposure; as well as the ability to apply special image processing techniques that enhance overall display of the image.
Dental radiographs :Provide essential information about oral health. They are an important partof a patient’s dental record. Some dental offices now use computers to helpcapture, store and transmit dental radiographs. Dental radiographs producedwith a special computer create digital images (computerized dentalradiographs) that can be displayed and enhanced on the computer monitor.When the digital radiograph is exposed, the image is transmitted to acomputer processor(with or without a cable) or, in the case of an imagingplate, the clinician removes the plate from the mouth and scans it with aspecial reader, similar to a compact disc player. Unlike conventional filmthat may take between three and five minutes to process, a digitalradiographic image generally can be viewed quickly on the computer screen.The image is displayed in a large format on the screen, in comparison withthe small films that are viewed on a light box.The clinician can use magnification to enhance specific problem areas of atooth, as well as alter brightness and contrast in the image. Viewing anenhanced dental radiograph on a computer screen can help a dentist bettersee a problem area.Digital X-Ray Machine and Camera SystemParts of a Digital X-Ray Machine:The digital X-Ray machine consists of an X-Ray tube and driver to source X-Ray. TheX-Ray passes through the patients body and the digital camera (located on the otherside of the patient) captures the resulting image. The main base station controls theX-Ray tube, analyzes the image and displays the image on the CRT.Digital Camera:The digital camera converts the received image into digital signal and transfers thedigitized image to the base station through a fiber optic link.
Main Base StationThe functions of the main base station are as follows: Drive and control the X-Ray tube Communicate with the digital camera system through Fiber Store the pictures on the hard disk for image retrieval and processing Interface to an operator console for overall system control and image manipulation Analyze and enhance the image stored on the hard disk and display the enhanced X-Ray image on a CRT monitorPrinciples :Conventional imagingConventional intra-oral radiographic film consists of silver halidegrains in a gelatine matrix. When this film is exposed to X-rayphotons the silver halide crystals are sensitized and are reduced toblack during the developing process. The film acts as both theradiation detector and the image display.With extra-oral films indirect action receptors are used to helprecord the image. This type of film is sensitive to light photonswhich are emitted by adjacent intensifying screens. Although thefilm is constructed of silver halide crystals these are primarilysensitive to light rather than X-rays. The use of intensifyingscreens reduces the dose and can be used where fine detail is notrequired.Digital imagingIn digital radiography, instead of the silver halide grain the imageis constructed using pixels or small light sensitive elements. Thesepixels can be a range of shades of grey depending on the exposure,and are arranged in grids and rows on the sensor, unlike therandom distribution of the crystals in standard film. However,
unlike film the sensors are only the radiation detector and theimage is displayed on a monitor.The signal that is produced by the sensor is an analogue signal, i.e.a voltage that varies as a function of time. The sensor is connectedto the computer and the signal is sampled at regular intervals. Theoutput of each pixel is quantified and converted to numbers by aframe grabber within the computer. The range of numbers isnormally from 0 to 256 with 0 representing black, 256 representingwhite and all others are shades of grey.The number of grey levels relates to contrast resolution and thesize of the pixels is related to spatial resolution. Together thesedetermine the overall resolution (i.e. the ability to distinguishbetween small objects close together) of the image. Resolution canalso be expressed in line pairs per millimetre. Most conventional Espeed films have a resolution of 20 LP/mm whereas with digitalimages the resolution ranges from 7–10 LP/mm. The reducedresolution should not interfere with clinical diagnosis.Image acquisition :There are two ways to acquire a digital image.Indirect acquisitionA digital image can be produced by scanning conventionalradiographs using a flatbed scanner and a transparency adaptor, orby using a charged coupled device camera instead of the flatbedscanner. This image can then be manipulated using softwarepackages or be passed on to a second party via a modem.
Direct digital imagingThere are two systems available, one produces the imageimmediately on the monitor post-exposure and is therefore calledDirect Imaging. The second has an intermediate phase, wherebythe 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 involvesthe use of a phosphor storage plate (PSP). This plate stores energyafter exposure to radiation and emits light when scanned by a laser.The scanner stimulates the phosphor plate and stores a record ofthe number of light photons detected.Loading of the scanners generally only requires subdued lightingas the plates are slightly sensitive to visible light. However, someproducts are more light sensitive than others. The lasers used arecentred around the 600-nm band and are usually of the helium-neon variety. Scanners, the size of a breadmaker, canaccommodate multiple image plates at any one time. The exactnumbers varies between manufacturers. There is a delay while theimage is ‘developed’ before it appears on the monitor. Up to eightbitewing radiographs take about 90 seconds and a panoramicimage can take approximately 3 minutes to be scanned. Again, thescan times do vary between manufacturers. Although the plate canstore energy for a number of days, information starts to be lostwithin minutes after exposure and it is advised to scan the platesquite quickly to optimize the image recovered. To fully remove thelatent image the plate should be exposed to high intensity light (asfound on viewing boxes).Image plates are available in exactly the same sizes asconventional film and come with disposable plastic barriers. Theyhave no wires attached and are reusable for thousands of
exposures, but do need careful handling to avoid surface damage.Current systems have a spatial resolution of 6–8 LP/mm.Direct sensor systems. The sensor for the radiation image isusually a Charge Coupled Device (CCD). It consists of siliconcrystals arranged in a lattice and converts light energy into anelectronic signal. This technology is widely used in video cameras.The sensor cannot store information and must be connected viafibre optic wires to the monitor, which can make the sensor bulkyand awkward to use.The greatest advantage of the direct sensor system is the gain intime. The image is directly projected onto the computer screen.Originally, the active areas of the sensors were smaller thanconventional film, which increased the incidence of ‘coning off’and required repeat exposures to capture all the desiredinformation. Recent innovations have produced sensorsapproaching or equal to standard film sizes.Extra-oral digital imaging :Extra-oral digital imaging is available using both systems.However, the larger CCD sensors are extremely expensive andusually requires the purchase of new X-ray generators, although a‘retro-fit’ system has been developed in the USA. Theseconstrictions effectively mean that the PSP method is the one mostcommonly used.Panoramic radiographyThe PSP method of panoramic digital imaging is very similar toconventional film. The film and intensifying screen are replaced bya storage phosphor plate. The plate is scanned after exposure,
which can take up to 3 minutes or longer depending on the productused. The resolution of these systems is greater than 4 LP/mm.Cephalometric radiographyNaslund et al. investigated the effect of dose reduction obtainedwith PSP on the identification of cephalometric landmarks andconcluded that dose reductions of up to 75 per cent did not effectthe localization of cephalometric landmarks.1 It is also worthnoting that with CCD sensors the image is acquired over 15seconds as the sensor and narrow X-ray beam move up the facialbones and could lead to an increase in the incidence of movementartefact.Advantages of digital imaging :Dose reductionDose reductions of up to 90 per cent compared to E-speed filmhave been reported by some authors in the diagnosis of caries.2Although some researchers do claim dose reductions comparedwith conventional extra-oral film, in practice the background noiserises to unacceptable levels. It is now accepted that there is noappreciable reduction compared with films used in conjunctionwith rare earth intensifying screens.Image manipulationThis is perhaps the greatest advantage of digital imaging overconventional film. It involves selecting the information of greatestdiagnostic 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 orunder exposure of the digital image. It has been shown thatcontrast enhancement of CCD devices were more accurate than E-speed film for detecting simulated caries under orthodontic bands.3Measurements. Digital callipers, rulers and protractors are some ofthe many tools available for image analysis. Many authors havereported on their application in cephalometric analysis.4,5 Theimages can also be superimposed onto each other and onto digitalphotographs.3-D reconstruction. This application can be theoretically used toreconstruct intra- and extra-oral images. The uses range fromprofiling root canals to visualizing facial fractures in all threedimensions.Filtration. The addition of filters to the airspace around the facecan clarify the soft tissue profile if the original soft tissue imagewas poor.TimeMuch time is gained especially with the CCD system where theimage is displayed at the chairside immediately post exposure.Although a lag time between scanning and the appearance of animage exists with the PSP method it is still substantially faster thanconventional developing processes in general use.StorageStorage was initially a problem before the development of DVDsand CD ROMs as three peri-apical images would fill a floppy disc.
However, now a CD ROM can hold over 30,000 images. Thismeans that images can be stored cheaply and indefinitely.TeleradiologyThe digital image file can be further reduced in size bycompression techniques, and sent via a modem and telephone lineto colleagues for review. This had the advantages of not losingradiographs in the post and saving time if an urgent appointment isrequired. The operator at the other end can also manipulate theimage if desired.Environmentally friendlyNo processing chemicals are used or disposed of. Both CCDsensors and the PSP plates are capable of being reused for manythousands of exposures. They can, however, become scratched anddamaged if not handled carefully.Disadvantages of digital imagingThe majority of the disadvantages are associated with the CCDsystem.CostCurrently, the cost of converting from intra-oral film to digitalimaging is approximately 6600 Euros. This initial outlay should beoffset against the time saved and the efficiency of storage of theimages.
Sensor dimensionsThese are still quite bulky for the CCD system and awkward toposition due to trailing fibre optic wires. The original problem ofsmall sensor active areas has been rectified and the same amountof information can be captured as conventional film.Cross-infection controlEach intra-oral sensor and plate must be covered by a plastic bag,and this bag is changed between patients. However, if they becomedirectly contaminated there is no way of sterilizing them and theyshould be discarded regardless of expense.MedicolegalConcerns have been raised in the past about the ability tomanipulate the images for fraudulent purposes. Manufacturers ofsoftware programmes have installed ‘audit trails’, which can trackdown and recover the original image. Many insurance companiesin the USA are accepting digital images as valid attachments whenthe claims are electronically claimed.