What is radiographic quality assurance?

Radiographic quality assurance is a program of regular physical testing designed to detect changes in a radiographic system before they can negatively affect diagnostic performance. No matter what image receptor is used, this program should include periodical testing of the x-ray system in order to see that tubehead stability, collimation, kVp, half-value layer, exposure time, output reproducibility, etc. are within the tolerances required by state regulatory agencies. Once it is ascertained that the x-ray unit is functioning properly, the display device should be tested to determine that it is functioning optimally.  After this is done, the entire system, including the image receptor, can be evaluated using a suitable test object or phantom. A series of test exposures is made using a well-defined and reproducible geometry. The exposure time is adjusted until an optimal image is obtained. A baseline radiographic image is then produced and saved as a reference. Subsequently, “follow-up” images are produced at regular intervals under identical conditions. The follow-up images are compared to the baseline image and corrective actions are taken if significant changes are noted. The images are saved for later reference, and records are maintained of any image parameters that are measured and any changes or repairs that are made.

What is required for quality assurance testing of digital systems?

Quality assurance testing of digital systems requires (1) a computer monitor adjusted to display a high-quality radiographic image and viewed under optimal conditions, (2) a suitable phantom or test object to be used for the assessment of image quality, (3) a procedure providing verification that the radiographic technique being used yields the maximum diagnostic information at an acceptable level of dose, (4) the exposure of a baseline image to serve as a reference for subsequent images, (5) the exposure of follow-up radiographs at regular intervals, and (6) a system of record keeping for purposes of documentation.

How can we verify that the monitor is functioning properly?

Since digital radiographs are viewed on a computer monitor, it should be verified that the monitor is viewed under optimal conditions. The monitor should be tested to verify that it is functioning optimally and the brightness and contrast are properly adjusted.

Digital radiographs should be viewed with the center of the display positioned slightly below eye level. Subdued lighting should be used and every effort should be made to eliminate reflections from extraneous sources of light such as room lights or light-boxes.   

The monitor should be checked periodically using the Society for Motion Picture and Television Engineers (SMPTE) Medical Diagnostic Imaging Test Pattern or the equivalent (Gray, 1992). The overall image should be inspected to insure the absence of gross artifacts such as blurring or bleeding of bright display areas into dark areas. If the SMPTE pattern is used, all the provided gray levels should be visible and both the 5% and the 95% areas should be seen as distinct from the adjacent 0% and 100% areas. Brightness and contrast should be adjusted until these conditions are met.

What is a quality assurance phantom?

A quality assurance phantom is a device containing test objects suitable for assessing low-contrast detectability and spatial resolution as well as a step-wedge or some other suitable object covering the relevant range of radiographic attenuation. The phantom should be exposed using the source-receptor distance used clinically and the test objects should be positioned at the same distance from the x-ray source and image receptor as the relevant anatomy. A radiographic phantom makes possible a comprehensive evaluation of image quality. It allows the dentist to discriminate between sensors of varying quality and between images exposed with different levels of radiation.

Why is a quality assurance phantom necessary?

Simple reference objects such as step-wedges or extracted teeth, which may be useful in the evaluation of film-based systems, are not sufficient for the assessment of digital radiographs.

This is primarily due to the fact that the software used in most digital systems compensates for under- and overexposures. With film, underexposed and overexposed radiographs will appear too light and too dark, respectively. In the case of digital radiographs, a visually acceptable image can be produced over a wide range of exposures. While the radiographs may appear to be satisfactory, they may be severely underexposed or overexposed.

What are the effects of underexposure or overexposure?

With underexposure, quantum noise can compromise contrast-detail perceptibility, potentially obscuring diagnostic information. Overexposure creates unnecessary patient risk and can result in loss of diagnostic information in the thinnest portions of the anatomy if “burn-out” occurs. The quality control phantom should be able to detect these effects should they take place. 

What is Exposure Optimization?

Exposure optimization is a procedure which employs a radiographic phantom to determine the exposure factors (kVp, mA, and exposure time) with the lowest radiation dose that produce images of the highest diagnostic performance. Quantitative measures of image quality such as spatial resolution, dynamic range and contrast detail detection can used to determine the optimal exposure settings for a specific x-ray unit and x-ray receptor.

Why should you perform Exposure Optimization?

Each x-ray unit produces x-rays of different energy spectra. Each x-ray receptor (film, CCD/CMOS sensors, or photostimulable phosphor image plates) respond differently to the incident x-ray energy. Therefore it is necessary to determine the optimum exposure for each x-ray unit and x-ray receptor that is used to acquire diagnostic radiographic images. Based on the exposure optimization, radiographic technique charts can be developed that indicate appropriate settings on the x-ray unit for a specific anatomical area and will ensure the least amount of radiation exposure to produce high quality diagnostic radiographic images.

How is exposure optimization performed?

Exposure optimization is a process of trial and error but it is a necessary step before longitudinal monitoring begins. The table below lists pertinent data obtained for a representative source/sensor combination using the DDQA phantom. The doses in the first column were measured at the tip of the position indicating device.  The contrast-detail wells are of two types. One has a constant depth with changing diameter and the other has a constant diameter and changing depth.  The gray levels associated with each step of the wedge have been tallied (columns labeled Step 1 through Step 7) as well as the number of distinct steps. 

It will be noted that the set of gray values representing the step-wedge are virtually identical in all the exposures from 0.63 to 1.57 mGy. This demonstrates rather dramatically the futility of using a step-wedge for determining the exposure time required for a digital image.  Similar results will be obtained with extracted teeth, coins, key chains, cell phones and other common objects used for “calibration” by well-meaning but untrained “technicians” and “service engineers”. Many digital systems have a very wide latitude and, in the case of the present system an exposure of  2.52 mGy can be reached before “burn-out” occurs.  At that level only 6 steps can be visualized.  At the lower end of the scale, an exposure of 0.75 mGy leads to sufficient quantum noise to reduce the number of visible contrast-detail wells. An exposure of 1.01 mGy would be recommended as the baseline for this system, there being no demonstrable advantage in using higher exposures. This number is below the Achievable Dose of 1.2 mGy recommended by the NCRP. The properties of the detector make it possible to increase the number of visible wells even further but doing so would exceed the Diagnostic Reference Level of 1.6 mGy.

What is the optimal dose?

The optimal dose for a specific radiographic examination can be defined as the minimum radiation dose required to be delivered by an x-ray imaging system to produce an image that is of adequate quality for the intended purpose. This requires that every component of the digital imaging chain including the diagnostic display device, x-ray source, and x-ray sensor/receptor be working appropriately.

How do you measure the radiation dose to the patient?

A measure of radiation dose at the skin-entrance plane in milligray (mGy) can be made using an electronic device like the RaySafe ThinX Intra or RaySafe Solo Dent. These devices can also measure kVp, mA, exposure time, and half value layer.

What is a Diagnostic Reference Level (DRL)?

The idea of a Diagnostic Reference Level (DRL) was first introduced in 1996 by the International Commission on Radiological Protection (ICRP Publication 73, 1996). The DRL allows a dental practices to compare their radiation dose data to aggregated dose data collected on a local, regional or national level. Their recommendation was that if any radiographic procedure within a practice consistently exceeded the relevant diagnostic reference level, a review of procedures and equipment should be undertaken to determine whether the procedure has been adequately optimized. If not, measures aimed at reduction of the doses should be taken. Diagnostic Reference Levels are not the suggested or an ideal dose for a particular procedure or an absolute upper limit for dose. They should be used as part of a quality assurance program to ensure that radiation doses used in a dental practice are consistent with other dental practices for the same radiographic procedures. The DRL for a specific radiographic examination is defined as the 75th percentile from the aggregated dose data. The National Council on Radiation Protection (NCRP) has established the diagnostic reference level for intra-oral periapical and bitewing radiography as 1.6 mGy (183 mR).

Reference Levels and Achievable Doses in Medical and Dental Imaging: Recommendations for the United States. National Council on Radiation Protection and Measurement (NCRP) Report 172, 2012.

What is the Achievable Dose (AD)?

For those dental practices which are currently below the DRL, the Achievable Dose can serve as goal for further reducing the radiation dose to the patient. The Achievable Dose for a specific radiographic examination is defined as the 50th percentile from the aggregated dose data. It should be stressed, however that clinically appropriate image quality and dose must always be considered. If the dose can be lowered while maintaining adequate clinical image quality, then the dose should be lowered. It is NOT appropriate to sacrifice clinical image quality in order to lower patient dose. The National Council on Radiation Protection (NCRP) has established the Achievable Dose for intra-oral periapical and bitewing radiography as 1.2 mGy (137 mR).

Reference Levels and Achievable Doses in Medical and Dental Imaging: Recommendations for the United States. National Council on Radiation Protection and Measurement (NCRP) Report 172, 2012.

What records should be kept of QA testing?

A permanent  record should be kept of baseline and follow-up data. The date of the test and the name of the technician performing the test procedure should be included.   The nominal kilovoltage and milliamperage as well as the baseline exposure time should be noted as well as the parameters that are measured.  A separate log should be maintained for every x-ray unit/sensor combination and, in the case of a practice using PSP plates, every x-ray unit/scanner combination. 

What are the benefits of QA testing?

The purpose of any radiographic quality assurance program is to provide images with the highest diagnostic quality and the lowest radiation risk to the patient and operator. Recently, a study was performed to determine if the initial establishment of baseline exposures for digital dental radiography would help to accomplish this objective in a private office environment. Twelve representative dental offices using digital radiography were visited.  Six offices used Schick sensors and six offices used Dexis sensors.  The exposure techniques routinely used for adult molar bitewing exposures were evaluated for entrance dose and image quality using a radiation monitor and phantom containing a step wedge, resolution pattern, and two rows of contrast-detail wells. Using the protocol for baseline assessment described previously, new exposures were determined and recommended for future use.  The new entrance doses and image quality parameters were compared to the original values.  The following graph shows the frequency distribution of entrance dose in all the offices using a representative sensor-type included in the study:

The wide range of doses before quality assurance reflects the hit-and-miss approach to technique factors so often found in dental offices. Selecting technique factors in a systematic way based on an objective assessment of image quality resulted in a significant overall reduction in the mean entrance exposure as well as a narrowing in the range of exposures over the entire population of sensors. Although the dose was generally decreased, in no case was image quality compromised. In those cases where the dose was increased, the image quality improved.  Similar results were obtained for the three other sensor types. These results show that the initial implementation of quality assurance procedures eliminates the uncertainty in choosing technique factors. Continued longitudinal testing should assure that high quality images continue to be produced at the lowest achievable level of dose.


Gray, J.E. (1992). “Use of the SMPTE test pattern in picture archiving and communication systems,” J. Digit. Imaging 5:54-58.

Mah, P., McDavid,W.D., and Dove, S.B.(2011) “Quality assurance phantom for digital dental imaging,” Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endod 112: 632-639.

NCRP (2012). National Council on Radiation Protection and Measurements. Reference levels and achievable doses in medical and dental  imaging: recommendations for the united states, NCRP Report No. 172 (National Council on Radiation Protection and Measurements, Bethesda, Maryland).

Udupa, H., Mah, P., Dove, S.B., and McDavid, W.D. (2013) “Evaluation of image quality parameters of representative intraoral digital radiographic systems,”  Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endod 116: 774-783.

Walker, T.F., Mah,P., Dove, S.B., and McDavid, W.D. (2014) “Digital intraoral Quality Assurance and Control in Private Practice”, General Dentistry  September/October 2014.