November/December 2005, Volume 1, Issue 2
Published by AEGIS Communications
Important Considerations When Evaluating Digital
V. Kim Kutsch, DMD
Digital radiography is rapidly becoming the standard of care in dentistry. As the adoption of this technology becomes widespread, it is important to differentiate and understand the unique qualities of available systems.
While almost all manufacturers make substantial claims regarding the image quality and radiation dose-reduction capabilities of their pbodyicular products, few offer verifiable proof of these claims. For example, many companies make claims about the specification known as “line pairs per millimeter” (lp/mm) as evidence of image quality. I have learned that not only does this specification have little bearing on image quality, but that higher lp/mm values—which are typically considered “good”—can actually mean poor clinical image quality and require a significant increase in radiation dosage.
It is a common belief that all digital radiography systems reduce radiation exposure when compared to film.1 This is not the case. The amount of radiation required varies widely from system to system, and some of the better-known systems require as much, or more, than ultra high-speed film in order to obtain a suitable image. When considering digital radiography, the dental professional has the responsibility to consider the true dose reduction—meaning, the ALARA principle (As Low As Reasonably Achievable)—as much as any other factor.
The purpose of this bodyicle is to familiarize dentists with specific features and advantages of digital radiography. I will provide necessary and understandable information to give the reader the insight necessary to identify an excellent system, one capable of delivering consistent high-quality images with greatly reduced radiation exposure.
Naturally, every digital radiography manufacturer claims to have the best system. Some use terms like “megapixels”, “dynamic range”, “up to 90% less radiation”, and “lp/mm” to support their marketing claims. Too few companies back up these claims by allowing the dentist to experience live imaging prior to a purchase commitment (i.e., the try before you buy scenario). Are the claims valid? What do they mean? Which ones are important and which ones aren’t? Which ones make a clinical difference?
At first glance, 1p/mm would seem to be a good indicator of system quality, but it is not. In a recent study conducted at the University of Louisville, researchers discovered that lp/mm has little bearing on the ability of a system to resolve anatomy. For example, 2 systems—1 measured as having a specification of 9 lp/mm and 1 having 20 lp/mm—were both able to resolve to as small as .1 mm. This level of resolution is clearly suitable for proper diagnosis under virtually any condition. Hence, the lp/mm specification is not a true indicator of image quality as it relates to practical clinical suitability.
More importantly, some systems with higher lp/mm specifications require additional radiation. While I will be the first to agree that dental radiographs are very safe under most situations, I strongly believe that as dental professionals, we have the responsibility to keep exposure levels as low as possible. When evaluating new digital radiography systems for purchase, I believe typical system dose requirements are an extremely important selection criteria.
ALARA: As Important as Image Quality, but Often Forgotten
ALARA is not simply an acronym, but a principle, a minimally invasive mindset, and a culture of professional excellence. Recent studies have demonstrated that dental radiographs can contribute to low birth-weight babies,2 and I care about my patients, even the ones who haven’t been born yet. The Food and Drug Administration (FDA) and American Dental Association (ADA) also care:
“Once a decision to obtain radiographs is made, it is the dentist’s responsibility to follow the ALARA Principle (As Low As Reasonably Achievable) to minimize the patient’s exposure to radiation. [An example] of good radiology technique [is] use of the fastest image receptor compatible with the diagnostic task.”3
The concept of ALARA is not new to radiology. In 1977, the Nuclear Regulatory Commission (NRC) began pushing for radiation standards that lowered the dose to patients and occupational workers. As a result, the Office of Standards of the NRC published NUREG-0267: Principles and Practices for Keeping Occupational Radiation Exposures at Medical Institutions As Low As Reasonably Achievable. The acronym ALARA remained as the document’s impact on the radiology community.
In 1994, the ALARA document became a pbody of Title 10 of the Code of Federal Regulations (10CFR35.20), which is binding on all institutions as a NRC regulation. Consequently, the ALARA principle should be practiced as a matter of mandate of federal code. When a dentist practices radiography according to the ALARA principle, it is not only mandated, but it is also respectful to the patient.4
As dental radiologists, we have a responsibility to select the fastest image receptor possible. That means digital radiography,1 and not just any digital radiography, but a system that significantly reduces radiation dose compared to high-speed film.5
How to Evaluate Digital Radiography Systems
As mentioned previously, many of the specification claims touted by digital radiography companies can be misleading. There are characteristics that are basic to all digital image detectors.6 Following is an outline of some of these specifications and an attempt to demystify them.
Line Pairs Per Millimeter (lp/mm). In my opinion, this value can be very misleading. This metric is often referenced and marketed as a measure of clinical image quality, but in reality, it is not. Some companies quote “theoretical resolution”, which is even more misleading. My best advice: ignore this number.
Dynamic Range (also known as Gray-scale). Every sensor has an inherent amount of signal that each pixel can capture. The more information that a pixel can record before it becomes “full” and cannot record any more, the better the dynamic range. This would be a very valuable comparative specification if this information was reported more often. Unfortunately, most companies report the number of bits of the Analog-to-Digital (ADC) conversion. This is simply the computer device that slices up the information from the sensor. It is an easy component to manipulate and, if reported, is not a true representation of any comparative quality.
Number of Pixels. Not a valuable comparator.
Pixel Size. Sensors with a smaller pixel size also have higher spatial resolution. The 2 typically go hand-in-hand. But as pixel size decreases, the amount of noise-to-signal increases. So, while not universally true, a good rule of thumb is that larger pixels (around 40 µm) provide a better noise-to-signal ratio than those of smaller-pixel sensors.7
Active Area and Sensor Dimensions. These 2 numbers are very valuable.8 From them, you can easily determine the percentage of sensor area that is actually capturing an image. Look for sensors with 67% or higher active area.
2. Live Demonstration Tips
The best indication of a digital radiography system’s capabilities is the live in-office demonstration. Immediately rule out any company that is not able to allow you to take live images prior to a purchase commitment. And be careful; some 30-day trials are purchase commitments.
Take images of yourself and your staff. One company brought their equipment in for a demonstration, but limited my testing to a cadaver mandible. I told them, “No thank you, I rarely treat skeletons!” It still amazes me how many of our colleagues will buy a digital radiography system that they have never used or tested. Instead, they base their decisions only on prepared images provided by the manufacturer. No manufacturer is going to show anything less than their best image, but that may not be representative of typical clinical images with their system.
You must have a “live demo” if you are considering a digital radiography system. It is the best policy when you are purchasing any high-tech device. Here’s what to look for during the “live” in-office demonstration.
Image Quality. Use your own judgment; you know what a good radiograph looks like. If you’re new to digital radiography, the images may look a little “flat” to you. This is normal. Ask the sales representative to explain the various image enhancement tools. Evaluate the system based on images you have taken with your own x-ray head. That’s the one you will be using with the system.
Exposure Settings. Not all systems are created equal. Settings should be much lower than those for comparable film images, even if you use high-speed film. If they are not, I recommend you keep looking; there are systems available that use significantly less radiation.
Positioning. Some companies offer a great variety of positioning devices; others seem to have forgotten that they are needed. Who makes the positioners? Are they comfortable? Well-designed? Do they work? I have found that the companies that cover these types of details seem to have the rest of their act together.
Multiple Images and Retakes. Take a few images of a full-mouth series. How many times did you need to touch the keyboard between images? Zero should be the answer. If a retake is required, you should be able to easily compare the old image to the new one before you discard the old.
Local Service. Who provides on-site service, and how close are they? Call a couple of colleagues in your area who have digital radiography systems and ask about their experiences. If the sales representative can’t refer you to a colleague in your area with their system, that should be a concern.
Warranty. Not all warranties are created equal, either. A multi-year warranty that covers manufacturer’s defects is only 1 consideration. Ask if visible damage is covered. Find out how a software problem gets fixed. How much will it cost for a service contract the 5 years after the included warranty expires?
Digital radiography offers the best images available today and significantly reduces radiation exposure to the patient. Numerous bodyicles have previously been written identifying all of the additional advantages of digital radiography. Clinical and business results are well-documented. When making your selection, remember the ALARA principle, and select a system after a successful live demonstration in your own office.
1. Kiefer H, Lambrecht JT, Roth J. Dose exposure from analog and digital full mouth radiography and panoramic radiography. Schweiz Monatsschr Zahnmed. 2004;114(7):687-93.
2. Hujoel PP, Bollen AM, Noonan CJ, et al. Antepbodyum dental radiography and infant low birth weight. JAMA. 2004;291(16): 1987-93.
3. ADA, U.S. FDA. Selection of patients for dental radiographic examinations. 2004 Available at: http://www.fda.gov/cdrh/radhlth/adaxray-1.html.
4. Joseph N Jr. Principles of Patient Radiation Protection and ALARA. Radiographic Imaging CEU Source, LLC. 2004. Available at: http://www.radiographicceu.com/bodyicle10.html.
5. Farman AG, Farman TT. Digital intra-oral radiography ‘04. AADMRT Newsletter. Louisville, KY: University of Louisville; Spring 2004.
6. Farman AG. Fundamentals of image acquisition and processing in the digital era. Orthod Craniofac Res. 2003. 6 Suppl 1:17-22.
7. Weitzel K, Frosini D. When digital cameras need large pixels. Optoelectronics World. Tulsa, OK: PennWell Corp; March 1999.
8. Blendl C, Stengel C, Zdunczyk S. A comparative study of analog and digital intraoral x-ray image detector systems. Rofo. 2000; 172(6): 534-41.
About the Author
V. Kim Kutsch, DMD