Radiographic imaging - consistent!

May 1, 2000
Film stacks up nicely to digital counterparts, particularly when it comes to radiation reduction and diagnostic details.

Part II

Editor`s Note: The first segment of this point/counterpoint series appeared in the April 2000 issue.

Film stacks up nicely to digital counterparts, particularly when it comes to radiation reduction and diagnostic details.

Thomas Schiff, DMD, and

Jack Hadley, DDS

Mike Maroon`s enthusiasm for direct digital radiography is certainly clear. It`s also understandable in many ways. Technology is exciting. It gets people "jazzed."

But that doesn`t mean it`s always a wise investment. Driving a minivan hardly gives the same thrill as tooling around in a sports car. Does that mean the average family should buy a Ferrari? Probably not - even if it did come equipped with color-coordinated child seats!

It`s the same with direct digital radiography. Sure, it`s new and exciting. But let`s not let the thrill of expensive new technology cloud the facts. Both film and direct digital offer benefits. The key is to compare the two technologies fairly.

Let`s take an example from Dr. Maroon`s article last month. In it, he states that "the main problem with film is that there are too many variables." However, direct digital radiography is also subject to variables. Mistakes are mistakes, regardless of the imaging modality.

Here at University of the Pacific, we often consult for dental practices. We see many patients on a referral basis. When some of these patients come to us, their image records are digital X-rays. In many cases, the quality of these X-rays is terrible. In fact, among referral images, the percentage of direct digital images that are unusable is far higher than the percentage of truly bad film images.

This tells us several things. First, mastering direct digital takes time, attention, and skill. It`s true, as Dr. Maroon points out, that direct digital system software offers tools like icons and menus that help users manipulate files and images. However, the imaging process itself is far from foolproof. Dentists still must control variables like sensor placement.

The only fair approach is to compare apples to apples. Dentists have to compare quality films to quality direct digital images. We have a professional responsibility to learn the imaging technology we adopt, regardless of modality. We have a responsibility to our patients to put processes in place that ensure quality images, eliminate re-takes, and attain the highest standard of care. Switching to sloppy direct digital imaging will not solve sloppy film processing. Instead, let`s serve the profession`s best interests by achieving quality images no matter what medium we use.

Another statement Dr. Maroon makes is that direct digital reduces radiation by as much as 90 percent, compared to film. This statement is potentially misleading, because it compares direct digital imaging to D-Speed film. We agree that reducing radiation exposure is an important goal for dental practices. But let`s not forget that the simplest way to achieve this is to switch to a faster film. High quality films are available that will let dentists reduce radiation dosages immediately (without converting to a completely new imaging technology). A related point is that when dentists make mistakes with digital, the only option is to re-take the X-ray. Needless to say, this can quickly cancel any potential reduction in radiation exposure. Given the fact that, in our experience, more practices are making irreparable errors with digital than with film imaging, the actual reduction of radiation is probably less than the theoretical reduction.

Studies, including some conducted here, show that the latest generation of high-speed films is comparable to D-Speed films in terms of image quality. We use this type of film at our university, with excellent results.

Compared to Kodak Ektaspeed Plus™ film, direct digital radiography still allows a lower radiation dose. But it is not a 90 percent reduction. It is closer to 50 percent. A move to the new Kodak InSight™ F-speed film will reduce the exposure even further.

Furthermore, as first noted above, this reduction is effectively cancelled out when dentists have to re-take digital X-rays. In our experience, re-takes might be necessary for a number of reasons. It is important to note that, although the outside dimensions of a digital sensor are the same as the outside dimensions of a piece of film, the actual area that can be imaged is smaller. This can (and does) lead to mistakes. The sensor has to be carefully centered, or the image will "miss" the tooth apices.

Another potential problem is that the average computer printer is not capable of producing a diagnostic-quality hardcopy of a digital image. This crops up with our patient referrals as well. A potential solution would be to transmit the digital image to us in electronic form. We expect to see more digital image-sharing in the future. Today, it is not always practical or possible. Instead, we again have to re-image the patient, which in turn increases his or her total radiation exposure.

Dr. Maroon also suggests that radiation reduction allows dental practices to market themselves to patients concerned about X-ray exposure. We agree that dentists can use this issue to attract patients. However, the question remains: why not switch to a quality Kodak Ektaspeed Plus film or Kodak InSight F-Speed film and publicize that? The upfront costs are much less, while the marketing benefits would be the same.

Another comparison that must be made more fairly is that of diagnostic efficacy.

Dr. Maroon lists many digital radiography system bells and whistles, such as image enhancement, image enlargement and colorization.

It is true that direct digital systems offer these functions. However, it is not true that these types of enhancements lead to more accurate diagnostics and treatments.

This is a key distinction. The diagnostic information captured by a X-ray film is at least as complete as that captured by digital X-ray. At our university, we use both direct digital and film imaging systems. We teach our students to diagnose using both modalities. If improved diagnostics were possible using direct digital, we would abandon film immediately. Yet we continue to use film. Why? Because it is at least as good a diagnostic tool as our direct digital systems. Dr. Maroon`s statement that "things show up on digital images that just can`t be seen on film" is simply not true. A quality X-ray film will present as much information as the best digital X-ray.

One example of image enhancement that Dr. Maroon offers is the ability to enlarge digital images on a computer screen. We can use this example to illustrate our point. Enlarging an image makes it bigger. That`s all. It does not add diagnostic information to the image, any more than looking at a film through a magnifying glass would add diagnostic information to a conventional X-ray.

Enlarging a digital image on a computer screen could even have the opposite effect. With standard computer monitors, an image can only be enlarged so far before it starts to degrade. It is possible to purchase specialized computer screens that display images at higher resolutions. Many hospital radiology departments use this type of equipment. However, these monitors are expensive. Buying one would add many thousands of dollars to the cost of a direct digital radiography system.

A related misconception about direct digital is reflected in Dr. Maroon`s statement that direct digital supports "tremendous dynamic range as compared to film." It is impossible for this statement to be correct. Consider, first, the meaning of "black" and "white" in an X-ray image. Pure black would represent the portion of an image where enough radiation was detected to convert all the silver to silver halide. Pure white would represent the portion where no radiation was detected. This is simply physics. A piece of film cannot misrepresent total exposure, or zero exposure. So how could a direct digital image produce "blacker blacks" or "whiter whites"? It`s not physically possible.

That leaves the grays (the portions of the image where some radiation was detected. Here it is again important to note the facts. Film is an analog medium, both for receiving X-ray radiation and for displaying it. Its presentation of grays is continuous tone. Digital images, on the other hand, present grays in discreet increments. This is the definition of digital: the information is numerical. The higher the number of increments, the closer the system approximates continuous tone. For this reason, it is never accurate to say that direct digital provides "better" definition of intermediary gray tones. We can agree that today`s direct digital can get very close to film (so close that to the human eye the difference might not be significant). But it would be inaccurate to say direct digital is "better."

In addition, displaying direct digital images can be problematic from a grayscale standpoint. Again, this is a matter of computer monitor capabilities. Most conventional computer monitors can display only 256 shades of gray. That is far less than the continuous tone "display" achieved by a piece of film! Buying a more expensive computer monitor, of course, would help solve this discrepancy, but this is not a financially viable option for most dental practices.

Another of Dr. Maroon`s arguments for direct digital is also questionable. It is not necessarily true that direct digital systems "will actually save you money." For instance, to compute the true costs of a direct digital system, dentists must include factors such as service and maintenance contracts, software upgrades and periodic replacement of sensors. Nor do direct digital systems let dentists completely replace "disposables" (as Dr. Maroon mentions, sensors must be covered by disposable barrier films and wiped with disinfectant after each use). These are both disposables that add incremental costs to direct digital systems.

It would be interesting for Dr. Maroon to provide an actual breakdown of his direct digital system costs, so that they could be accurately compared to the costs of purchasing and maintaining a film system. If all the costs of a direct digital system are included, it`s likely that direct digital will prove more expensive than film imaging, not less.

(And incidentally, while some direct digital systems can be installed without changing X-ray tubes, some brands require the practice to install new X-ray equipment.)

The other counterpoints to Dr. Maroon`s opening arguments for direct digital are less central, but nonetheless worth mentioning:

• Measurement and calibration of digital images is not as error-proof as Dr. Maroon might seem to imply. A foreshortened image is a foreshortened image, regardless of how it was captured. Again, we need to compare quality film to quality digital images to make accurate judgements.

• Some of the advantages that Dr. Maroon sees in digital can also be accomplished by scanning film images. For example, a scanned film image can be integrated into a patient report to be e-mailed or faxed to an insurer. So this is not an argument for direct digital capture per se.

As we stated in our first article in this series, we believe that the future for direct digital imaging is very bright. We believe that, over the next decade or so, the sensors will become thinner. This will address questions about patient comfort. The actual imaging area of the sensor will become larger. This will help prevent errors in sensor placement. Improvements in printing technology will make it possible to produce diagnostic-quality hardcopy of images that were captured digitally.

However, today the gold standard for diagnostic imaging in dentistry is still film. It is an inexpensive, time-tested way to generate quality images. The image record is extremely stable, lasting for years and years (unlike with digital systems, where accessing the record is impossible if the computer breaks, or if a software upgrade renders older files obsolete).

And last but not least, the diagnostic information of a quality film X-ray is at least as complete as that of any digital modality.

So, if direct digital systems had come first, and a manufacturer offered us a technology with the advantages of film, we believe our answer would have to be: "Film? Yes!"

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