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Flashcards in digital imaging Deck (17):

how do pixels work?

A “number” is assigned to each pixel, depending on how many x rays struck the pixel area on the image receptor....


what was the first dental digital imagin sensor?

RadioVisioGraphy (RVG) sensor – the first dental digital imaging sensor


has digital imaging been commonplace in medical setting for some time?



how does digital imaging work?

Obtaining a digital radiographic image involves exposure of a special image receptor to an x ray beam, conversion of the signal received by the image receptor into digital information, conversion of this information into a grayscale image that looks like a conventional radiographic image, followed by display of the image on a computer display. The digital image receptor is used instead of a conventional film packet or film cassette, which were used in the era of “film-based imaging”. With some digital systems, you are able to view images on the computer screen almost immediately after the x ray exposure is completed. The process of digitral radiography has similarities to using a digital camera.

After they are produced, digital images can be adjusted by altering various parameters using software. Digital images can be saved or stored on a computer hard drive or other media, and retrieved for later viewing. Single digital images can also be electronically transferred.


what are the three basic types of dental digital radiography systems?

1. Charge-coupled device (CCD)
2. Complementary Metal Oxide Semiconductor (CMOS) sensors
2. Photostimulable storage phosphor (PSP) plate


how do charge-coupled device (CCD) sensors work?

The software maps out a thin sheet of pure silicone into a two-dimensional array of “pixels” or “picture elements”. Each pixel acts as a small capacitor that can store an electrical charge when exposed to x rays. The charge is proportional to the x ray exposure the pixel receives. That is, the more x rays that strike a particular pixel, the greater the electrical charge stored.

The charge on each pixel is amplified and converted by the computer that transfers it to a graphics program, and displays the information as a grayscale image on a monitor or screen. Basically, each pixel is assigned a number corresponding to the electrical charge is received, and this “number” determines how light or dark the pixel appears in the final image. The image appears on the screen within a few seconds. The original RVG system is one example of a unit employing a CCD sensor.

The electrons go from valence to conduction band. When that happens the electrons are unstable and they drop back down to the valence back quickly. But right after exposure the program will record the number of electrons or the electrical charge in that area which tells it how to record in the pixel. If there are a lot of electrons the pixel will be darker

CMOS sensors work basically the same as this.


how do complementary metal oxide semiconductors (CMOS) work?

Complementary metal oxide semiconductor (CMOS) sensors are similar to CCD sensors in that they are silicon-based. The manner in which data from each pixel is obtained and converted into a visible image is slightly different, but from a practical standpoint, use of CCD and CMOS sensors is the same. Both generally require a physical connection (wire) to the computer unit, and both allow almost instantaneous display of the image.

Some manufacturers sell “wireless” versions of CCD/CMOS sensors that employ radiofrequency transmission from the sensor to the processing unit (computer), but these are more expensive and the sensors are slightly more bulky than the “wired” variety.


how do photostimulable storage phosphor (PSP) plates?

higher energy band that the electrons may be pushed up to but they will drop back down to the valence band.

The valence band electrons get energized to the F center and they accumulate in there.

Scanning laser takes the electrons out of the F center and when this happens the electrons drop back down and release a photon of energy that corresponds to light which corresponds to a certain grey level in a pixel.

So there is an exposure, latent image and then the laser beam readout.

A second type of imaging receptor uses a material that “stores” the energy of the x ray beam. After the imaging plate (“storage plate”) is exposed, it is removed from the mouth and placed into a device that scans the “phosphor” with a laser beam. This scanning releases the stored energy of the x rays and causes the release of visible light from the storage plate in a pattern that duplicates the pattern of the x ray beam that struck the sensor. The pattern of light output is then converted into a grayscale image by the computer. The darkness or lightness in any area of the final image is dependent on the amount of light emitted from that area, which is dependent on how many x rays struck it during exposure. Since the plate must first be exposed and then placed into the scanner, obtaining the image takes a little longer than using CCD or CMOS systems.

Depending on the particular system, it takes several seconds after the plate is placed into the scanner before the image can be seen; the more imaging plates exposed at a given time, the greater the time before the scanning can be completed, and all of the images viewed. The time required before an image produced using a PSP system can be viewed is therefore greater than with a CCD system. A PSP system, however, is still usually faster than using film, at least if you are taking a small number of radiographs at a given time. If you are taking a large number of radiographs, the PSP system is probably not much faster than using film-based imaging.



• ability to distinguish different densities (shades of grey)
• computer monitor can show ~ 240 different shades of grey
• humans can visualize up to 60 shades at a time; this ability decreases to ~ 30 shades or less if room is well illuminated (this is why it is good to look at radiographs in a darker room)



ability to distinguish fine detail
• measured in line pairs per millimeter

• for digital imaging, dependent on pixel size (the smaller, the better)
• pixel size of CCD / CMOS sensors ~ 20 microns
• size of silver grain in film ~ 8 microns

FILM - can provide resolution of at least 20 line pair per millimeter
DIGITAL SYSTEMS - can provide resolution of at least 7 line pairs per millimeter (some systems are better, and some are close to that of film)
film can go up to about 20 line pairs per mm.

Regardless, the resolution provided by virtually all dental digital systems is still sufficient for the diagnostic purposes that we require – the unaided human eye can only detect about 8 lp/mm.

generally you don’t get as high of spatial resolution in digital systems than you do in film imaging.



This is the ability to record
a wide range of different x ray exposures. Film, and CCD and CMOS sensors, have a limited range, in that they are incapable of recording sufficient information to create any kind of useful image if the exposure is very low or very high. PSP plates have the advantage of having much greater exposure latitude; this means that a visible image may be possible to obtain even with very low or very high exposures by “post-processing” or “adjusting” an image produced by a PSP imaging plate through adjustment of brightness and contrast.

• CCD/CMOS similar to film (can be extended using digital enhancement of contrast and brightness)
• PSP sensors have much greater latitude (able to record a wide range of densities)

at low exposures if you gave x-ray over a period of exposure ranges you had a little more give and it wouldn’t get much darker until you got to a certain point where if you continued to expose, it would get dark and couldn’t get darker, it would be saturated.
The digital phosphor plates, it is linear.
If the exposure is too low for the CCD then you won’t be able to see anything because it will be too light. If you leave it on too long it will be dark. You need to be in the range.



• sensitivity to x radiation
• current PSP systems allow dose reduction of
~ 50% compared to F-speed (Insight) film • CCD and CMOS systems slightly less sensitive
than PSP, but overall reduction in radiation is similar for all digital sensors

using more sensitive films you can decrease the exposure time to get an acceptable image for diagnosis.

This refers the sensitivity of the sensor to x radiation. Most PSP systems have a sensitivity that is about twice that of the F-speed dental films that we are currently using. CCD and CMOS systems are less sensitive than PSP systems, but the difference is minimal in practical terms.

The bottom line is – digital image receptors are generally much more sensitive than film, and images can be obtained with far less radiation compared to using film. This means the dose to the patient should be significantly reduced when using digital imaging!


What post-exposure adjustments can be made?

This is dependent on the software used, but some of the more common adjustments are:
 Adjustment of brightness and contrast
 Magnification
 Increased illumination of small areas
 Reverse polarity
 Colourization
In addition to these, some software allows obtaining linear measurements on images.


what are the 6 advantages of digital imaging?

1. Instantaneous, or at least more quickly obtained, images compared to using film. This is a big advantage, particularly if you use a digital system when doing procedures such as endodontics.
2. Elimination of film, processing chemicals, processors or processing tanks. This will save some long-term operating and capital expenses. Elimination of processing chemicals is also “environment-friendly”.
3. Decrease in radiation dose to patient. Manufacturers sometimes make tremendous and at times unrealistic claims, but there is no doubt dose is lower with digital systems compared to using film. Dose reduction should be close to 50% compared to what was required for “E-speed” film.
4. Once obtained, the image can be adjusted (“post-processed”) using software to alter contrast or brightness, to brighten a portion, to make linear measurements, etc. There is evidence that adjustment of contrast, brightness, etc. may increase the diagnostic information available. One study showed that dentists actually preferred an “adjusted” image to the “original”.
5. “Salvaging” a poor image. If the “original” digital image is too light or too dark, adjusting it may make it acceptable. Most digital systems are at least “8 bit”, and thus can provide at least 28 (256) shades of gray (the Carestream sensors are “12 bit” and can provide 4096 shades of gray); the computer screen/display can only show about 64 shades at a time. This provides much flexibility to change the grayscale of the image. Remember, the human eye can only distinguish no more than about 60 shades of grey at a time.
6. Once obtained, the image can be stored and retrieved in the patient’s computer file, or it can be electronically transferred. It is possible, for example, to obtain a digital image in your office, transfer it electronically to a specialist in another office or city for an opinion, and receive a response very quickly.


what are 11 disadvantages to the digital imaging?

1. Some CCD/CMOS systems have a small sensor size. This may result in more retakes if the anatomic area of concern is missed or not enough of it is recorded.
2. Most CCD/CMOS type sensors have a wire attached to them, connecting them to the computer. The presence of the wire sometimes makes using the system a little more cumbersome compared to film or PSP plates. This concern is avoided with the “wireless” systems.
3. CCD/CMOS sensors are bulky compared to film and may be difficult to place adequately in small mouths or in patients who poorly tolerate foreign intraoral devices. In comparison, most PSP plates are very much like dental film packets, and do not need a wire or other physical connection to a computer or other hardware.
4. Sensors and plates cannot be heat sterilized, and barriers must be used.
5. CCD/CMOS sensors can be damaged if dropped, and care must be taken to avoid physically damaging them.
6. PSP plates are prone to minor physical damage (e.g. cracks, scratches) that may render the image diagnostically less acceptable or unacceptable.
7. There is the potential for altering images by exporting them into a graphics program and performing various manipulations; e.g. adding “caries”. It may be difficult to determine if an image has been altered. There is, therefore, a potential for fraud, although is probably not as great a concern as some people originally though it to be.
8. Image quality, specifically spatial resolution, of digitally obtained images has historically been not quite as good as that obtained with film. Digital images in the past often did not “look” as good as film-based images, but the majority of evidence shows that these digital images were quite acceptable for what we need them for. In the end, commentaries on image resolution and quality may be moot, since the unaided human eye can only appreciate 8 lp/mm. The quality of the digital image is dependent on several factors, including the computer screen/monitor. Overall, image quality with most digital systems is not a concern now; for example, the spatial resolution of our Carestream sensors is reported to be virtually the same as, or very close to, that of film.
9. Electronic image transfer for the purposes of consultation, patient referral, etc. is possible. However, confidentiality and privacy regulations MUST be followed (e.g. patient is not identified in an email or an attachment, particularly if the communcation contains any information about their history).
10. Digital image receptors and software from different manufacturers have not always been compatible. There has been great improvement in this area, and there are far fewer concerns than was the case several years ago.
11. Cost. Digital systems still cost a lot, but their advantages may reduce the concern about price. Also, with digital systems, many of the ongoing costs of producing film- based images (film, processing chemicals, film mounts, etc.) are eliminated.

One other potential disadvantage is the information may be lost if image printed to "hard copy" medium..

Print on good quality photographic paper using high print resolution - reasonable results if original digital image is of good quality...


what is direct digital radiography, with is indirect digital radiography?

• direct acquisition of image
• conventional image scanned


what does the research say about digital images compared to film?

Digital images compare favourably with film for diagnostic purposes.

“Adjusted” or “post processed” images may be better; most observers prefer “adjusted” images.