Digital Radiographic Imaging: Technology for the Next Millenium

Part A - Comparing Receptor Systems

Robert P. Langlais DDS, MS, FACD, FICD, FRCD(C) and Dale A. Miles DDS, MS, FRCD(C)

It’s truly hard for the practicing dentist to begin to sort out the "hype" from the truth about digital x-ray imaging systems. What is digital radiographic imaging? What makes it different from film? What is the difference between a CCD and a CMOS receptor? Do I really care? Why are some systems "wireless"? And, how do they do that? How many line pairs/millimeter is good enough? What is a line pair anyway? Why do I need 4096 gray levels? What do you mean by image processing? I thought that was just the chemistry I use. Dentists ask all of these questions each time we talk to a dental group. Then there’s the ultimate question: how will digital imaging improve my practice and my patient’s care?

We will attempt to answer all these questions in this article. In this first part we’ll discuss hardware; that is, the sensors (receptors). In the second part we will present information on or "image processing" (the software tools), the part of the system which truly impacts patient care, and the part dentists are least familiar with.

What is a "digital" image?

A digital image is made up of a binary language of 0s and 1s arranged in rows and columns called an image matrix. Each cell of the image matrix is called a pixel (short for picture element). A typical matrix can be seen in Figure 1.

Figure 1 (rows and columns of pixels)

A typical small pixel size for a dental digital x-ray image is about 40 micrometers by 40 micrometers. A 40-50 µm pixel size will result in an image resolution of about 10-11 line pairs per millimeter (lp/mm) with a high definition monitor. Resolution is a measure of small objects placed close together. The higher the resolution of a system, the better the image may appear. In the best of all worlds, a dentist viewing an image on a monitor could potentially separate 8-10 thin pairs of lines per millimeter of space with the naked eye.

A line pair is a measure of the system’s ability to capture detail; that is resolution. Manufacturers claim anywhere from 6 to as much as 22 lp/mm. Does it really matter? In truth the human visual system can only resolve about 8 lp/mm at the best of times without magnification. So does 12 lp/mm or 20 lp/mm resolution really matter? The answer is NO! It doesn’t matter…all of the commercially available digital x-ray systems have sensors which can produce a diagnostically acceptable image.

What makes a digital image different from film?

More information for treatment decisions is available to the clinician in a digital image since the image can be subjected to electronic image processing such as making it lighter (density) or changing the gray scale (contrast) without making another exposure. The amount of image processing the dentist can do depends on the types of software "tools" installed in the individual system. Most software systems offer the same basic image processing package; however, some manufacturers offer additional image processing features. Several software packages may be required for the office digital imaging set-up. Many of the processing tools available from the manufacturer are the same as in any desktop publishing software such as Adobe Photoshop ^TM. This ability to study the image more precisely by employing image processing makes the digital image that much more valuable for patient care.

Additional benefits (and differences) over film are storage and electronic transmission of the image (teledentistry). Many of our films, when retrieved from the patient record years later, are brown or yellow or some other hue because of improper fixation in the processing chemicals. We’ve all experienced that phenomenon. Digital images stored on hard drives, CD ROMs, or eventually DVDs, will maintain their "archival quality" for decades. Kodak^TM guarantees the image fidelity of their recordable CD ROM material for 100 years! By the way, Kodak still wants to sell we dentists dental film, but its medical division is one of the leading retailers and manufacturers of CCD devices for camera and radiographic imaging. You should ask them why they are holding dentistry back!

The ability to send images over existing telephone lines (teleradiology) exists today. The insurance industry will eventually accept image files with claim submissions for claims adjudication. Digital images stored at dental school and other educational sites can now be accessed for dentists to review or educate themselves. Just type http://w3.dh.nagasaki-u.ac.jp/tf/atoz.html. in the URL (Universal Resource Locator) in your internet account and witness the wonderful cases with plain film, CT and even magnetic resonance images that they have "on-line"! Alternately, you can type nagasaki dental school in your browser (Netscape, Explorer, etc…).

What is the difference between a CCD and a CMOS receptor?

There are 2 basic methods of acquiring a digital x-ray image: indirect and direct. There are now also 3 silicon-based digital image receptors now to choose from including the CCD (charge-coupled device), the CMOS (complimentary metal oxide semiconductor) and the CID (Charge Induction Device).

Indirect Image Acquisition

The indirect method of image acquisition converts original analog images (dental x-rays or radiographs) to digital images usually by a scanning process using digitization. This is also done with a CCD device, but it is a linear detector just like your fax machine, not a rectangular dectector like the intraoral CCD. Once digitized the resultant image can be further processed like any other digital image. For this type of "digital" image a commercial scanner, capable of scanning 600 dpi, with a transparency adapter in the lid is required. Color slides may also be scanned in this fashion.

Direct Image Acquisition

The direct method of image acquisition requires a solid state detector made of silicon, the infamous "chip", such as a CCD or CMOS receptor, to capture the image directly. These receptors are currently available for the acquisition of intraoral, panoramic⁴, tomographic⁵ and cephalometric⁶ images. The digital system chosen should be capable of producing images in the formats used by the office. For example the general practitioner needing intraoral and panoramic images should select a system offering the capability for both.

CCD Vs. CMOS

All solid state detectors are "wired" to the computer and range in thickness from 3.2 (DMD’s MPDx) to 8.8mm (ProVision – DEXIS) . The detectors actual imaging area is smaller than their outer dimensions and usually smaller than conventional dental film. They vary in price from $5000-$7000 for a #2 size sensor. Compared to E-speed film, these receptors generally require about 50% less radiation. CCD and CMOS receptors have a wider exposure latitude than film, but much less than the PSP (photostimualble phosphor) plates (which we’ll discuss next). Inadequate exposure time results in increased noise in the digital image. Noise is analogous to the graininess in film-based images, but the cause in digital imaging is the collection of too few x-ray photons. However, unlike an "underexposed" x-ray film, a lot of the image information can be recovered in the digital image. The information is there, it just needs "enhancing". With a film image, the halide is washed off and unrecoverable.

The two types of solid state detectors available are the CCD and the CMOS sensors. Both were developed along with the transistor in the 1960’s, but neither was as commercially viable as the transistor back then. It took the advent of the computer to make CCD and CMOS technology more technically and commercially viable. CMOS chips are used in every computer, and their manufacturing process is very mature. Therefore they can be made cheaper than CCD chips but, as yet, have not been adequately tested for x-ray image capture. CMOS chips contain some RAM operation circuitry and a microprocessor on the same silicon chip (Figure 2).

Figure 2 – CMOS chip with components

Only the small area called "imager" is actually dedicated to image reception.

Therefore there are concerns by some that the noise level will be greater with CMOS sensors than with CCD, and that the use of some of the "chip real estate" or area leaves less sensor area available for image capture. This might lead to a less image information in an x-ray system. They appear to be more suited for commercial products such as digital cameras and video cameras which operate in bright light conditions. In the industry, there is as yet no consensus as to which sensor is better. Table 2 outlines the advantages/disadvantages of these sensors in terms of their technical specifications. Bold terms in the table indicate which detector has an advantage in that area.

Table 2 - Modified from Technical Issues in Digital Imaging: Dr.D.A. Miles

Note: Bold words indicate an advantage of the system

While the time from image capture to acquisition and display on the monitor is only a few seconds with CCDs, many systems require that the operator go to the monitor for one or several steps before exposing the next area. These steps include trying to adjust the density or contrast before deciding to re-expose. This occurs with some frequency especially with existing AC type machines with older timers. Sometimes the dentist must reorient the image; that is, flip or rotate it, as the detector cannot recognize or separate left from right or upper from lower images. Also the keyboard and mouse need to be covered with a barrier material like clear plastic wrap. This series of steps may take from one to several minutes per image. After using the detector, it must be disinfected with a high level or tuberculocidal agent. Special barrier envelopes are available for detectors which must also cover about 6-8 inches of the cord. CCD detectors are somewhat delicate and must be handled with care, as replacement is expensive. Detectors should preferably be used with DC type intraoral machines with 1/100sec exposure intervals for the shorter exposure times.

" Wired Vs. Wireless " How do they do that?

Unlike the solid state detectors, PSP (photostimulable phosphor) systems are "wireless". Three manufacturers offer imaging systems which use (PSP) plates; Soredex (Helsinki, Finland, Digora^TM), Digident (Israel, CD-Dent^TM) and Gendex (Milwaukee, WI, DenOptix^TM). These plates have no wire to the computer and resemble in every way intraoral film including size, thickness, rigidity and receptor placement. The cost is about $30-$50 per plate.

During the exposure, the phosphors in the plate enter into an excited state proportional to the amount of radiation exposure. However, unlike conventional screen phosphors used in panoramic systems, the PSPs do not immediately fluoresce but only store the image information. The plates must be placed into a scanner ($12-25,000) and excited by a laser which then causes them to fluoresce. The fluorescence is captured as an electronic wave form and converted to a digital image (analog to digital conversion) by the computer. The resultant digital image can then be viewed on a monitor in about 30 seconds to 5.5 minutes depending upon the systems. The time from capture to viewing is one to several minutes and varies with film size or number of films being scanned. The image resolution is from 6 lp/mm to 9 lp/mm depending on the product. PSP plates are not as sensitive to exposure time variations as film because of their wide exposure latitude. This latitude is even greater than the solid state systems. So, though they lack the resolution of film or CCD/CMOS systems, they compensate by having an extremely wide exposure latitude (wider gray scale). PSP plates also come in 5 in. X 12. in panoramic and 8 in. X 10 in.cephalometric sizes. The resolution of the image is 6 or 9 lp/mm for the two available systems.

The plates are exactly the same size as the corresponding intraoral film including the thickness (about 1.6 mm.). They are soft and pliant much like film. There is no need to go to the computer between exposures even to check for proper density as the plates will be properly exposed over a wide range of exposure times. However each plate must be shaken out of it's barrier pouch and scanned individually or as a group by the laser scanner and imported into the computer. Thus the total time to process each image can take several minutes. The infection control step is carried out after exposing all of the plates. Each is carefully shaken out of the barrier envelope onto a clean surface, wiped with a disinfectant and then fed into the laser scanner. After use, the plates are placed into new barrier envelopes for the next patient. PSP plates are not delicate and they can be replaced for a fraction of the cost of detectors. Plates may be the system of choice for use with older AC x-ray machines.