Abstract:
A quality assurance phantom for intraoral digital dental imaging and related method. The quality assurance phantom measures three different physical properties of an imaging system: dynamic range, contrast detectability, and spatial resolution. The phantom comprises a dynamic range portion having a plurality of steps, each step of the plurality of steps having a different thickness from the other steps; a contrast detail portion having a uniform thickness and a plurality of wells formed therein; a spatial resolution portion having a plurality of line sets, each line set of the plurality of line sets having different line spacing from the other line sets; an attenuating body having uniform thickness positioned between the source and the contrast detail portion and the spatial resolution portion; and a lead mass adjacent to the dynamic range portion. 
     According to the method of the present invention, a series of images is created and analyzed, either manually or automatically with a computer, to determine a baseline quality assurance exposure (BQAE). At subsequent monitoring intervals therefrom, an image is created using the same exposure parameters as the BQAE and compared to the baseline image to ascertain changes.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application No. 61/255,293, filed Oct. 27, 2009 and entitled “Quality Assurance Phantom for Digital Dental Imaging,” which is incorporated by reference herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to intraoral dental imaging. More specifically, the invention is a quality assurance phantom for intraoral digital dental imaging and related method. 
     2. Description of the Related Art 
     The degree of servicing dental radiographic equipment varies from office to office and institution to institution. The level of monitoring radiology equipment varies from one jurisdiction to another and range from unknown, annually, every five years, and, in some instances, only when a problem is encountered. As a result, some radiographic imaging systems can be significantly out of tolerance while still in clinical use. The objective of any quality assurance program is to ensure accurate diagnosis and to ensure that doses are kept as low as reasonably achievable (ALARA). This requires a system of regular monitoring procedures to ensure that the various components of the imaging system function within the manufacturer&#39;s recommended tolerances. 
     Most states and regulating bodies have guidelines which state that regular quality assurance of all dental 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. 
     Any intra-oral digital imaging system comprises essentially three components: an x-ray source; a digital image acquisition component (e.g., solid-state sensor or PSP plate and scanner; and an image display component (e.g., a computer or monitor). Each of these components needs to be regularly monitored for performance and function as part of the quality assurance program. 
     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. 
     One dental radiographic phantom was designed primarily for conventional x-ray film and is commercially available from Fluke Biomedical, formerly Medi-Nuclear Corporation, as the CDRH Dental Image Quality Test Tool. The phantom was developed as a joint collaboration between the Food and Drug Administration (FDA) Center for Devices and Radiological Health (CDRH) and Conference of Radiation Control Program Directors. The phantom consists of a wooden cradle (to hold the test tool body), built-in slots (for attenuation filters), a film slot, an exposure chamber holder, and a mounting screw for use with a tripod. The test tool comes with an aluminum step wedge that is designed for evaluating darkroom fog and consistency testing. The step wedge has two slots: one for exposing a film pack and one for evaluating darkroom fog. The film slot also ensures easy, reproducible placement of the film for consistent imaging. The phantom is designed specifically for testing the functionality of dental x-ray units and provides a means of evaluating half-value layer, determining kVp, and assessing overall image quality, including x-ray film processing. The test tool also contains a human tooth to simulate a clinical image. 
     A second phantom is the Quart Dental phantom, which is designed to monitor high-contrast and low-contrast special resolution. In addition, a Unfors Mult-O-Meter external detector can be inserted into the phantom to measure kVp, dose and exposure time. 
     A third phantom is the CD Dent phantom, which is comprised of a three-millimeter aluminum sheet with one hundred cylindrical holes. The CD Dent phantom is designed to optimize the radiation dose and image quality. 
     A fourth phantom is the DIQUAD analyzer, a hexagonally-shaped device comprising an optically-stimulated light dosimeter to measure dose, and several metal filters and a mesh target to assess image quality. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a radiographic phantom and associated method for quality assurance in intraoral digital dental imaging. The quality assurance phantom measures three different physical properties of an imaging system: dynamic range, contrast detectability, and spatial resolution. The phantom comprises a dynamic range portion having a plurality of steps, each step of the plurality of steps having a different thickness from the other steps; a contrast detail portion having a uniform thickness and a plurality of wells formed therein; a spatial resolution portion having a plurality of line sets, each line set of the plurality of line sets having different line widths and line spacing from the other line sets; an attenuating body having uniform thickness positioned between the source and the contrast detail portion and the spatial resolution portion; and a lead mass adjacent to the dynamic range portion. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is an isometric view of a preferred embodiment of the present invention in use with an x-ray source. 
         FIG. 2  is a top elevation of the preferred embodiment shown in  FIG. 1 . 
         FIG. 3  is a bottom elevation of the preferred embodiment. 
         FIG. 4  is a sectional view of the preferred embodiment through line  4 - 4  of  FIG. 2  and  FIG. 3 . 
         FIG. 5  is a top elevation of the image acquisition assembly of the preferred embodiment. 
         FIG. 6  is a side sectional elevation through line  6 - 6  of  FIG. 5 . 
         FIG. 7  is a side sectional elevation through line  7 - 7  of  FIG. 5 . 
         FIG. 8  is a side sectional elevation through line  8 - 8  of  FIG. 5 . 
         FIG. 9  is an initial baseline worksheet that can be used with the method of the present invention. 
         FIG. 10  is a quality assurance record that can be used with the method of the present invention. 
         FIG. 11  is an image created using the preferred embodiment of the apparatus. 
         FIG. 12  is exemplary image data showing attenuation of energy propagated through line  12 - 12  of  FIG. 11 . 
         FIG. 13  is exemplary image data showing attenuation of energy propagated through line  13 - 13  of  FIG. 11 . 
         FIG. 14  is exemplary image data showing attenuation of energy propagated through line  14 - 14  of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1-4  show a preferred embodiment  20  of the present invention in use with a digital imaging sensor  16  and position indicating device (PID)  18  that is attached to an x-ray source (not shown). Referring to  FIGS. 1 &amp; 2 , the embodiment  20  comprises a generally-square horizontal acrylic platform  22  having a top surface  24  that is elevated relative to a support surface with acrylic rails  26  positioned at opposing sides of the platform  22 . The rails  26  are fastened to the platform  22  using screws  28 . 
     Four acrylic vertical spacing members  30  are fastened to the top surface  24  of the platform  22  and extend vertically from the top surface  24  at right angles. The spacing members  30  are identically shaped and angled relative to the centerline  34  of the platform  22  by approximately fifty degrees. The spacing members  30  are oriented around the center of the platform  22  to define an image acquisition area  32 . Each spacing member  30  comprises a horizontal top surface  36 , an angled surface  38  relative to the top surface  36 , and a vertical surface  40 . The PID  18  contacts and rests on the top surfaces  36  of the spacing members  30 . 
     As shown in  FIG. 2 , longitudinal slots  44  extend through the top platform  22  in line with the centerline  34 . An image acquisition assembly  46  is positioned substantially within the image acquisition area  32 . An attenuating body  33  of aluminum alloy 1100 is positioned between the PID  18  ( FIG. 1 ) and the image acquisition assembly  46  and fastened to the platform  22 . 
       FIG. 3  is a bottom elevation of the preferred embodiment showing the bottom surface  48  of the platform  22 . Screws  50  fasten the spacing members  30 . Two acrylic clamping members  52  are engaged with and hold the sensor  16  stationary relative to the image acquisition assembly  46 . The clamping members  52  are urged toward the image acquisition assembly  46  by compression springs  54 , which exert an expansive force against the rails  26  and the clamping members  52 . 
       FIG. 4  is a sectional view through line  4 - 4  of  FIGS. 2 &amp; 3 , and more fully shows operation of the clamping members  52 . Each clamping member  52  has an angled proximal surface  56  that engages with and holds the sensor  16  in place directly below the image acquisition assembly  46 . The distal and proximal ends of the compression spring  54  are positioned in spring holes  58  formed in the proximal sidewalls  60  of the rails  26  and the distal sidewalls  62  of the clamping members  52 . As the clamping members  52  are moved closer to the rails  26 , the force of the spring  54  increases, urging the clamping members  52  inward. 
     A knurled thumb screw  64  extends through each of the longitudinal slots  44  in the platform  22  and engages the clamping members  52  through bolt holes  66 . Tightening the thumb screws  64  holds the clamping members  52  stationary against the bottom surface  48  of the platform  22 , while loosening the screws  64  allows slidable movement of the clamping members  52  toward and away from the image acquisition area  32 . 
     Each clamping member  52  comprises a raised portion  68  fitted within a depression  70  formed in the bottom surface of the platform  22 . The raised portions  68  define distal shoulders  72  and proximal shoulders  74 . Inward movement of the clamping members  52  is limited by ultimate contact of the proximal shoulder  74  with the proximal sidewall of the depression  70 . Lateral movement of the clamping member  52  is limited by ultimate contact with the two lateral surfaces of the depression  70  formed in the bottom surface of the platform  22 . 
       FIG. 5  shows a top elevation of the image acquisition assembly  46  of the preferred embodiment  20 , which comprises an aluminum dynamic range portion  76  having a plurality of steps  78   a - 78   e  of varying thicknesses, a contrast detail portion  80  of a piece  82  of acrylic plastic of uniform thickness with first group  84   a - 84   f  and a second group  86   a - 86   f  of cylindrical wells formed in the top surface  88  thereof, and a spatial resolution portion  90  having a plurality of high-contrast line sets  92   a - 92   p  with gradually increasing spatial frequency encompassing the range of frequencies encountered in dental intra-oral radiography. 
     The dynamic range portion  76  comprises five steps  78   a - 78   e  of varying thickness of aluminum and a lead mass  94  positioned through a cutout  96  formed in the acrylic plastic  82 . In the preferred embodiment, the first step  78   a  is a first thickness; the second step  78   b  is a second thickness that is greater than the first thickness; the third step  78   c  is a third thickness that is greater than the second thickness; the fourth step  78   d  is a fourth thickness that is greater than the third thickness, and the fifth step  24   e  is a fifth thickness that is greater than the fourth thickness. The steps  78   a - 78   e  are integrally formed from a block of aluminum alloy 1100. The dynamic range portion  76  further comprises a lead mass  94  that has a thickness equal to the fifth thickness and is positioned adjacent to the fifth step  78   e . Adjacent to the first step  78   a  is an empty volume  98 . 
     The varying thicknesses of the dynamic range portion  76  cause varying degrees of attenuation of the energy as it propagates from the source, through the image acquisition assembly  46 , and to the sensor  16 . These varying degrees of attenuation simulate the range of contrast encountered in dental intraoral radiography. 
     More specifically, energy from the PID  18  is not attenuated as it passes through the empty volume  98 . At the other end of the dynamic range portion  76 , the lead mass  94  causes complete, or almost complete, attenuation because of the inability of x-rays to propagate through the material. The first through fifth steps  78   a - 78   e  cause varying levels of attenuation that are functions of the corresponding thicknesses. 
     The spatial resolution portion  90  provides for a high-contrast resolution evaluation of the imaging system, and comprises a spatial resolution pattern of sixteen line sets  92   a - 92   p  formed of a gold foil on a background. The line sets  92   a - 92   p  range from five lines per millimeter for the first line set  92   a  to twenty lines per millimeter at the sixteenth line set  92   p . As the energy from the source propagates through the spatial resolution portion  90 , some of the line sets will be discretely discernable while some of the line sets will appear solid, depending on the resolving capabilities of the sensor/source system. The results may be subjectively analyzed with the naked eye or objectively analyzed with image analysis software to remove operator biases such as eyesight, fatigue, viewing conditions, and other factors. 
     The contrast detail portion  80  provides a low contrast detectability pattern that comprises a first group  84   a - 84   f  of six cylindrical wells of identical depth but of varying diameters. In the preferred embodiment, the diameters of the first group of wells range from approximately one-half millimeters to approximately four-and-a-half millimeters. The contrast detail portion  80  also comprises a second group  86   a - 86   f  of cylindrical wells, each well having an identical diameter, but varying depths within the second group  86   a - 86   f.    
     Use of the preferred embodiment is initially described with reference to  FIGS. 1-4 . The sensor  16  and PID  18  to be analyzed are positioned relative to the embodiment  20 . Specifically, the PID  18  is positioned on the horizontal surfaces  36  of the spacing members  30 , which provides a fixed, reproducible distance from the image acquisition assembly  46  during each assessment of the quality assurance process, regardless of when the process is performed. In addition, placement of the PID  18  onto the spacing members  30  assures propagation of x-rays normal to the image acquisition area  32 . 
     Referring specifically to  FIGS. 3-4 , the thumb screws  64  are loosened to allow the clamping members  52  to slide relative to the platform  22 . The sensor  16  is placed between the clamping members  52  adjacent to and directly under the image acquisition assembly  46 . Angled surfaces  56  of the clamping members  52  contact the sensor  16  to hold it stationary, while the compression springs  54  provide an inward force to immobilize the sensor  16  relative to the embodiment  20 . Thereafter, thumb screws  64  may be tightened to prevent inadvertent movement of the sensor  16  as it is maintained against the bottom surface  48  of the platform  22 . 
     When testing multiple sensors of the same design, the invention provides for quick exchange of the sensors by loosening only one of the thumb screws  64 , sliding the corresponding clamping member  52  outward to release the sensor  16 , inserting the next sensor, and sliding the clamping member  52  inward and retightening the thumbscrew  64 . This negates the need to realign the active area of a different sensor of identical make with the image acquisition area and provides for more rapid assessment. 
     Assessing a sensor/source system with the present invention is a two-step process. First, if the specific sensor/source combination has never been assessed, a technician performs an initial baseline assessment, as will be described infra. 
     Thereafter, a longitudinal assessment is performed at a predetermined monitoring interval, which may be at a predetermined time-interval or after the occurrence of a particular event (e.g., the sensor is inadvertently dropped or bitten by a patient) that brings operational reliability into question. 
     To perform an initial baseline assessment, the technician places the sensor directly under the image acquisition assembly  46  and rests the PID  18  on the horizontal surfaces  36  of the spacing members  30 . Thereafter, the kVp and mA of the source are adjusted to the settings that will be used for clinical exposures. Starting with the lowest possible exposure time, a series of digital images is acquired and saved at incrementally-increasing exposure times. When exposure times are too short, the resulting images are underexposed, and the different density levels are indistinct in the areas  106 ,  102   e , and  102   e  of  FIG. 11 . In contrast, when exposure times are too high, the resulting images are overexposed, and the different density levels are indistinct in the areas of  104 ,  102   a , and  102   b  of  FIG. 11 . 
     Between these two extremes will be a set of images in which all the different density levels will be discernable in the image. The group of images is identified in which all seven density levels—that is, zero density of the empty volume  98 , the first through fifth steps  78   a - 78   e  of aluminum alloy, and the lead mass  94 —are clearly discernable. 
     Analyzing the set of images, the number of discernable line sets A-P and the number of areas  110   a - 110   f ,  112   a - 112   f  are determined for each image of the set of images. The lowest exposure time in which the highest line pairs per millimeter and maximum number of wells  110   a - 110   f ,  112   a - 112   f  can be identified is the Baseline Quality Assurance Exposure (BQAE). 
     The final step of initial baseline assessment is recording the baseline image as well as the characteristics of the BQAE for future comparison.  FIG. 9  discloses an exemplary worksheet that may be used by the technician to record the characteristics of the BQAE. 
     At each monitoring interval, a longitudinal quality assurance assessment may be performed using a new digital image acquired using the sensor and source settings determined by and recorded during the initial baseline assessment. The image is acquired with the exact same exposure settings as the BQAE. 
     After the image is acquired, the image can be compared to the baseline image for any change in dynamic range, spatial resolution, and contrast/detail resolution. A change in one or more of these characteristics indicates that corrective action should be taken with respect to the sensor/source system, and alerts the technician that the characteristics of images taken since the BQAE may have changed. 
       FIG. 10  discloses an exemplary worksheet that may be used by the technician when performing longitudinal quality assessment. 
       FIG. 11  is an image  100  created with the preferred embodiment  20  of the invention. The image  100  comprises areas of varying brightness corresponding to exposure to energy from the source, with brighter (i.e., whiter) areas having been exposed to a lesser amount of energy and darker areas having exposed to a greater amount of energy. Specifically, areas  102   a - 102   e  correspond to the location of the first through fifth steps  78   a - 78   e  of the dynamic range portion  76  in  FIG. 5 . Area  104  corresponds to the empty volume  98 . Area  106  corresponds to the position of the lead mass  94  in  FIG. 5 . Area  108  corresponds to the spatial resolution portion  90  in  FIG. 5 . Areas  110   a - 110   f  correspond to the positions of the first group of wells  84   a - 84   f . Areas  112   a - 112   f  correspond to the positions of the second group of wells  86   a - 86   f  in  FIG. 5 . 
       FIG. 12  is a line graph resulting from image analysis of line  12 - 12  of  FIG. 11  plotting position against intensity. Specifically, a first section  200  of the graph corresponds to the energy received at area A of  FIG. 11 , which has five lines per millimeter; a second section  202  of the graph corresponds to the energy received at area B of  FIG. 11 , which has six lines per millimeter; a third section  204  of the graph corresponds to the energy received are area C of  FIG. 11 , which has seven lines per millimeter; a fourth section  206  of the graph corresponds to the energy received at area D of  FIG. 11 , which has eight lines per millimeter; and a fifth section  208  of the graph corresponds to the energy received at area E of  FIG. 11 , which has nine lines per millimeter. The greatest number of lines per millimeter that comprises five distinct peaks and four distinct troughs is the line set resolution of that image, which in the present case is the fourth section  206  corresponding to the fourth line set  92   d  of eight lines per millimeter. 
       FIG. 13  is an exemplary image data showing attenuation of energy propagated through line  13 - 13  of  FIG. 11 . Section  300  corresponds to well  110   f . Section  302  corresponds to well  110   e . Section  304  corresponds to well  110   d . Section  306  corresponds to well  110   c . Section  308  corresponds to well  110   b . Section  310  corresponds to well  110   a.    
       FIG. 14  is an exemplary image data showing attenuation of energy propagated through line  14 - 14  of  FIG. 11 . Section  400  corresponds to area  106 . Section  402  corresponds to area  102   e . Section  404  corresponds to area  102   d . Section  406  corresponds to area  102   c . Section  408  corresponds to area  102   b . Section  410  corresponds to area  102   a . Section  412  corresponds to area  104 . 
     The present invention is described in terms of preferred embodiment in which a specific system and method are described. Those skilled in the art will recognize that alternative embodiments of such system, and alternative applications of the method, can be used in carrying out the present invention. Other aspects and advantages of the present invention may be obtained from a study of this disclosure and the drawings, along with the appended claims. Moreover, the recited order of the steps of the method described herein is not meant to limit the order in which those steps may be performed.