From Volume 41, Issue 2, March 2014 | Pages 126-134
Quality assurance (QA) is essential in dental radiography. Digital radiography is becoming more common in dentistry, so it is important that appropriate QA tests are carried out on the digital equipment, including the viewing monitor. The aim of this article is to outline the tests that can be carried out in dental practice.
The provision of diagnostic radiographs, whilst keeping patient radiation dose as low as reasonably possible, is a cornerstone of good clinical practice. The National Radiological Protection Board's Guidance Notes for Dental Practitioners on the Safe Use of X-Ray Equipment document states that essential procedures tested within a quality assurance (QA) programme should include:1
A QA programme should be comprehensive but affordable and straightforward to undertake. Exact details of the tests, their outcomes and who is responsible for them should be included as part of a dental practice's radiation protection file.
Most articles in the dental literature detailing how these procedures are undertaken are orientated towards conventional dental radiography, with very few addressing digital dental systems.2,3,4 As technology advances, more dental practices are installing digital dental radiography systems. Approximately 25% of practices are now using digital radiography systems,5 which means that previously essential QA tests, involving film and film processing, are redundant and need replacing with tests more relevant to digital detectors. In addition, display monitors need to be included in the QA programme.
Although the need for digital dental radiography QA testing is recognized, there is little in the literature on how actually to perform these tests. The Institute of Physics and Engineering in Medicine (IPEM) have produced a document suggesting the tests that should be carried out for dental radiography and image display devices.6 This paper will focus on the QA tests for digital intra-oral and panoramic systems, which can be carried out within the dental practice setting. It is not the purpose of this article to cover the other aspects of QA that are not affected by the change from conventional to digital imaging.
There are essentially two types of digital detectors in use, namely photostimulable phosphor plates (PSPs) and solid-state detectors (SSDs).
PSPs are composed of a polyester base with a phosphor layer (europium activated barium fluorohalide) on one side. After the PSP is exposed it is scanned in a reader before the image is displayed on a monitor. The phosphor layer is relatively delicate and any damage to it can degrade the image significantly.
SSDs contain solid state materials such as amorphous silicon or amorphous selenium in their construction.7 The detectors contain either a charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS). The intra-oral systems generally have a flexible cable connecting the detector directly to the PC, although wireless systems are available.
The Institute of Physics and Engineering in Medicine (IPEM) has divided the quality assurance tests into two categories, according to the level of expertise required to undertake them:
The IPEM further classifies QA tests according to priority:
The tests are undertaken at different time intervals. The recommended frequency for each test is given as a range, eg 1–3 months. This is related to the clinical workload. Thus, the higher the radiographic workload, the shorter the frequency between testing.
The tests may have a corresponding remedial and suspension level. These are levels of equipment performance above which corrective action or termination of use must be undertaken. Equipment can still be used whilst remedial action is taken. If a test result shows performance at a suspensory level, use of the equipment should be stopped.
All equipment involved in medical exposures must be subject to a QA programme.8 The Ionizing Radiations Regulations 1999 (IRR99) stipulate that overall responsibility for quality assurance testing falls upon the employer.9 The radiation protection supervisor (RPS) is a suitably trained member of staff appointed by the employer who helps to ensure compliance with IRR and the local rules.8 The QA tests should be carried out by suitably trained members of staff and it is essential that formal records of all the tests are maintained and checked.1 A Medical Physics Expert (MPE) is a state registered clinical scientist with corporate membership of the IPEM,8 who will give advice on a number of matters including equipment QA.
Each PSP ideally should have a marker on it so it can be identified if there are faults (Figure 1). PSPs should be stored in a dust-free environment. However, the PSPs should be visually checked for scratches and dust particles. If dust particles or debris is seen on the PSP, it should be cleaned according to the manufacturer's instructions. The IPEM recommend the condition of the PSPs is checked monthly, although it is recommended by the authors that this test is carried out more frequently, particularly in a high workload practice.
Additionally, the receiving tray (if applicable) of the PSP reader should be wiped and cleaned on a daily basis.
On a monthly basis (or earlier if visible scratches are seen on the surface of the PSP), the PSP should be exposed to see if there are significant scratches. The PSP should be placed on a flat surface, at a set distance from the end of the spacer cone, and given a short exposure. The PSP should be read and then graded using a subjective scale. An example of such a system is shown below, but other grading systems have been suggested.10,11
PSPs falling into Category 2 should be removed from circulation and disposed of appropriately.
The casing should be inspected on a monthly basis for cracks/damage. The cable should also be inspected for damage (Figure 4).
If there is any damage to the casing, the SSD should be exposed in the same way as described for the PSPs to ensure that there is no damage to the enclosed detector. If damaged, the SSD should be taken out of clinical use. In many cases, damage to the underlying detector or the cable will require the purchase of a new SSD.
On a monthly basis, images (produced using either a PSP or a SSD) should be compared with a good quality reference image. If there is a visual deterioration in image quality, corrective action should be taken.
This test is necessary to ensure satisfactory image quality and should be performed on a 1–3 monthly basis. The method for the test is that previously described, ie the image receptor is placed at a fixed distance from the spacer cone and exposed to a short exposure. The image is inspected for areas of non-uniformity (lines or rectangular areas) and other artefacts (such as scratches in the case of PSPs). Any lines or rectangles seen on the image should be investigated. If there are gross areas of non-uniformity, the image receptor should not be used until the issue is resolved (Figures 5 and 6).
This particular test is relevant to both conventional and digital imaging and is included here as the test may not be well known to general dental practitioners. These two tests are carried out together and are necessary to ensure that there are no regions of non-uniformity or artefacts on the image, and also to check that the panoramic exposure is entirely within the area of the image receptor and that there is no disruption in the motion of the mechanical components.
A 1 mm thick strip of copper approximately 2 cm wide will need to be purchased in order to perform the test. This acts as a filter to attenuate the x-ray beam. The test is performed with the image receptor in the carriage on the unit and should be carried out after installation of the unit. This result acts as the reference image to which subsequent tests can be compared. To carry out the test, the 1mm copper strip is taped over the beam collimator, taking care to avoid sticking it to any moving parts. A panoramic exposure is carried out using a standard adult programme and the subsequent image examined.
For the reproducibility and uniformity part of the test, the ‘density’ of the image should be compared with that of the reference image. There should be no areas of non-uniformity. It is important to recognize that the higher density band in the midline of the image is due to a programmed increase in exposure, which is required to penetrate the cervical spine (Figure 7).
The image should be visually checked to confirm that the radiation field is contained within the edges of the image receptor, with no overlap. Additionally, there should be no signs of additional vertical banding which would suggest a fault in the unit's rotational motion (Figure 7).
Images obtained as part of the initial commissioning tests should be archived electronically for future reference. Routine testing images should be archived if significant artefacts are observed. This allows any faults to be seen by engineers/Medical Physics staff to allow for their correction.
The Royal College of Radiologists provide guidance on the specification of image display devices. The images being displayed are for diagnostic purposes and therefore the screen resolution needs to be at least 1280 x 1024 (~1.3 megapixels). The maximum luminance (the amount of light produced from the display device) should be at least 170 cd/m2 and the luminance contrast ratio should be at least 250:1.12 Discussion with the Medical Physics expert before purchase would be useful to ensure appropriate equipment is ordered and is fit for purpose.
There are four level A, priority 1 tests recommended by the IPEM. These are:
The monitor should be kept clean using appropriate cleaning materials. There is a choice of test patterns that can be used to check monitor condition. The first is the Society of Motion Pictures and Television Engineers known as SMPTE and the second is the Technical Group 18 QC known as TG18-QC. These images should be viewed full-screen for all tests. Examples of these test patterns are shown in Figure 8. Whichever test pattern is used, the 5% detail superimposed on the 0% square and the 95% detail superimposed on the 100% square should be visible (Figure 9). If they are not visible, the monitor settings should be adjusted until they become visible.
A photometer is suggested to measure the luminance within the 100% (white) and the 0% (black) squares (Figure 10). If the ratio of white:black is below 250 it should be investigated. This test requires the purchase of a photometer, which is an expensive purchase that could deter practices from carrying out this test. It may be more appropriate, if possible, to include this test within the service level agreement within the local medical physics service.
Distance calibration could be carried out using a metal ruler with the measurement scale etched on the surface. The metal ruler is placed on the detector and exposed using exposure factors that allow the scale on the ruler to be visualized (Figure 11). The dental digital software distance measuring tool is then used to measure a known distance, as large as possible on the ruler.
A second exposure with the ruler placed at 90° to the original position should also be carried out and measurements made. This ensures that measurements have been carried out both in the horizontal and vertical planes. The IPEM suggest changes more than +/- 5 mm need investigation. The authors suggest differences between the measured and actual distance greater than 4% would require remedial action.
Angle measurement could be carried out easily using the same ruler if the end of the ruler has been cut at a right angle. A radiograph of the end of the ruler is acquired and the angle at the end of the ruler measured using the angle measurement tool in the software. The IPEM suggest any changes that vary by more than +/-3° need investigation.
The high and low contrast resolution patterns on the SMPTE or TG18-QC patterns should be examined, making sure that resolution at the centre and periphery is similar to that of the baseline image (Figure 12). All the patterns should be resolvable.
There are several other level B tests suggested, including the limiting spatial resolution of intra-oral systems. Elsewhere in the IPEM Report 91 the low contrast sensitivity and limiting spatial resolution testing are suggested for other direct digital radiography systems (SSD systems) and computed radiography systems (PSP systems). These are designated level A tests which should be carried out every 4–6 months.
These tests require the purchase of a suitable test object. One test object, or phantom, called the Tor Den Digital produced by Leeds Test Objects is designed for use on intra-oral, panoramic and cephalometric units (Figure 13). It contains a lead test pattern for measuring both the spatial resolution (using groups of lead lines separated by air gaps of different frequencies) and the low contrast resolution (using four low contrast details of different sizes ranging from 1.0–2.5 mm diameter).
This test is easily carried out for intra-oral radiography. The phantom containing the image receptor (PSP or SSD) is placed over the open end of the spacer cone and an exposure is made (Figure 14). The image is viewed and the highest frequency of lead lines and air gaps is recorded. This measures the spatial resolution and the manufacturer suggests this should be at least 5 line pairs per millimetre. The result measured at baseline should be referred to during routine testing and any significant changes would necessitate remedial action. The smallest diameter low contrast detail is also recorded, which measures the low contrast resolution. For intra-oral systems, all the low contrast details should be distinguished from the background (Figure 15).
The same tests can also be carried out on panoramic and cephalometric units and require the use of a tripod to hold the phantom in the required position (Figure 16). The test on the panoramic unit is technically more difficult to carry out and positioning of the test object in the focal plane is very important.
Quality assurance is essential and there is a legal obligation for it to be carried out. With the advent of digital systems, new tests need to be incorporated into the QA programme to include the digital x-ray equipment and the viewing monitors. A summary of the level A QA tests for digital systems is shown in Table 1.
| QA check | Frequency | Priority | Remedial level | Suspension level | |
|---|---|---|---|---|---|
| Condition of digital receptors | Monthly | 1 | Damage to cable or detector or PSP | - | |
| Image quality | Monthly | 1 | Visible deterioration compared with reference image | - | |
| Image uniformity | 1–3 monthly | 1 | Lines or rectangles apparent | Gross non-uniformity | |
| Low contrast sensitivity* | 4–6 monthly | 2 | For the Tor Den Digital investigate any deviation from the reference image | ||
| Limiting spatial resolution** | 4–6 monthly | 2 | |||
| Panoramic radiography | Reproducibility and uniformity | 1–3 monthly | 2 | Significant visible difference to baseline | – |
| Beam alignment and synchronization of exposure with tube motion | 1–3 monthly | 2 | Edge of beam not visible on image | - | |
| Image display monitor | Condition | Daily–weekly | 1 | If 5% and 95% details not visible | - |
| Greyscale | 3 monthly | 1 | Ratio white to black <250 | ||
| Distance and angle calibration | 3 monthly | 1 | +/- 5 mm |
- | |
| Resolution | 3 monthly | 1 | Investigate any deviation from baseline | - | |
