Dental Imaging Consultants

What Is Dental Imaging Quality Assurance?

Overview

Most states and regulatory bodies have guidelines which state that regular quality assurance of all radiographic equipment be performed. Similar guidelines have been advocated by the American Academy of Oral and Maxillofacial Radiology and the American Dental Association. This means regular testing to detect equipment malfunctions, planned monitoring and scheduled maintenance to produce consistent diagnostic radiographic images. All dental facilities using x-ray equipment, from a simple intraoral dental unit to an advanced three dimensional imaging system such as cone beam computed tomography, will benefit from adopting a quality assurance program. Simply stated: quality assurance entails the consistent production of x-ray images of high quality in order to provide the maximum amount of diagnostic information at a minimal radiation exposure to the patient.

There are essentially three components involved with any intra-oral digital imaging system: the intra-oral x-ray generator, the digital image acquisition device (solid-state sensor or PSP plate and scanner), and the image display device (computer/monitor). Each of these components needs to be regularly monitored for performance and function as part of the quality assurance program.

X-ray Generator

The intra-oral x-ray generator should be tested for x-ray filtration, half-value layer, x-ray beam collimation and alignment, and tube head stability as part of the initial equipment acceptance testing. The x-ray generator may be evaluated periodically by measuring the x-ray output in milliroentgens (mR) or ┬ÁSv, operating potential in kilovolts (kV) and exposure time in milliseconds (ms). Probably the simplest and most effective quality assurance assessment should be made by measuring the tube output at periodic intervals under the same geometry. This quantity can be compared from year to year to help establish consistency in equipment performance. There are several commercial products available such as the Unfors ThinX RAD, Barracuda Piranha and RadCal RAPIDOSE which can be used to test the x-ray generator. Most states have developed guidelines for the periodic assessment of the x-ray generator.

Image Acquisition Device

The digital image acquisition component can be evaluated either qualitatively or quantitatively using a radiographic phantom designed to produce a digital image containing information related to fundamental imaging characteristics. These include spatial resolution, contrast resolution or dynamic range, contrast/detail resolution, field uniformity, saturation, and signal to noise response.

The Dental Digital Intra-Oral Phantom provides users with a unique comprehensive test object designed for routine reproducible performance evaluation for dental digital intra-oral systems. The phantom contains 4 components:

  1. an aluminum step wedge of sufficient thickness to simulate the range of subject contrast encountered in dental intraoral radiography,
  2. a piece of polymethyl methacrylate plastic of uniform thickness with two series of cylindrical wells; one of constant diameter and varying depths and another series of cylindrical wells of gradually diminishing diameter and uniform depth to provide low contrast detectability patterns,
  3. a high contrast pattern of line pairs with gradually increasing spatial frequency encompassing the range of frequencies encountered in dental intra-oral radiography, and
  4. a piece of aluminum material of uniform thickness overlying areas (2) and (3) to attenuate the x-ray photons to the middle portion of the range determined by the stepwedge.

The imaging area of the intra-oral phantom is approximately 31 x 41 mm (suitable for any #2 size imaging device). To stabilize the intra-oral phantom the device has been incorporated into a large acrylic base with supporting beams to allow for clearance of the sensor and on the top aspect there are four stops for the beam indicating device of the x-ray source to rest on. These 4 plastic rest tabs allow the beam indicating device to rest securely and ensure a parallel alignment of the x-ray source and sensor for both round and rectangular beam indicating devices. These rest tabs also establish a source to receptor distance equivalent to that which is used for clinical imaging with the use of Dentsply Rinn or similar positioning devices.

Phantom From Top Phantom From Front

Using the DDQA phantom initial baseline exposures can be made to determine the optimum performance for the combination of x-ray generator and x-ray sensor. Once this image has been acquired and stored, subsequent assessments may be made by acquiring an image of the DDQA phantom with the same exposure factors and determining if any change has occurred in the contrast/detail, spatial resolution or dynamic range over time. While most x-ray generators and sensors are very stable over time, any change in their performance could dramatically affect diagnostic performance or increase the radiation dose to the patient. We recommend that a regular interval (quarterly, semi-annually or annually) be established for each dental practice depending on usage and operator interaction.

Image Display Device

The image display component can be evaluated by displaying a standard digital image such as the Society for Motion Picture and Television Engineers (SMPTE) Medical Diagnostic Imaging Test Pattern. The contrast and brightness should be adjusted to optimize contrast at the lowest and highest luminance of the image. Optimal viewing conditions including a quiet, darkened room with proper digital background masking of the screen so that the majority of the light from the display is from the digital image. The overall SMPTE image appearance should be inspected to assure the absence of gross artifacts such as blurring or bleeding of bright display areas into dark areas or aliasing of spatial resolution patterns. As a dynamic range test, both the 5% and 95% areas should be seen as distinct from the respective adjacent 0% and 100% areas. Digital Imaging Consultants provides this image free of charge from this web site in order to perform the quality assurance of the image display component. No recommendations have been made by states for the interval for assessing the image display device. While most display devices are very stable over time, any adjustment by the user to optimize the image display for other than radiographic interpretation my dramatically affect diagnostic performance. We recommend that a regular interval (daily, weekly, or monthly) be established for each dental practice depending on usage and operator interaction.