Evaluation of Physical Properties of Different Digital Intraoral Sensors

Benefits and limitations of digital intraoral radiography have already been presented.³ The use of digital radiographic systems is on the rise, and most owners of these systems are satisfied with the equipment while experiencing increased productivity.⁴

When faced with the decision as to which intraoral digital technology should be implemented in practice, the average practitioner faces a dilemma: sorting through the various claims by different manufacturers touting their sensors. It should be noted that digital sensor configuration is not standardized across vendors, thus imaging specifications, optical properties, and physical design differ among different brands.

Direct digital sensors can be either wired to a computer or connected through a wireless network. There are two main technologies on which direct sensors are based: charged couple device (CCD); and complementary metal-oxide semiconductor (CMOS). The advantages and disadvantages of these technologies have been previously described.^5-8 The CMOS technology is currently implemented by several leading manufacturers in their latest products. As part of the transition from traditional film to digital imaging, several CMOS digital intraoral sensors have been evaluated at Case Western Reserve University, School of Dental Medicine (CWRU). The evaluation included various parameters related to the physical design of the sensors (vide infra). The physical design of the sensors often is neglected in the decision process for several reasons: the practitioners usually are unaware of the impact of physical design features on a sensor’s performance; physical features are not always advertised or disclosed by manufacturers; the lack of information does not allow comparison of the digital sensor with standard film. The purpose of this paper is to present the results of the evaluation of the physical design of eight CMOS digital intraoral sensors.

The following CMOS intraoral sensors with all available sizes from various vendors were provided for the purpose of the evaluation (Figure 1): XDR (Cyber Medical Imaging, www.xdrradiology.com); RVG 6100 (Carestream Dental LLC [distributor of Kodak products in US], www.carestreamdental.com); Platinum® (DEXIS LLC, www.dexis.com); CDR Elite (Schick Technologies Inc., www.schickbysirona.com); ProSensor™ (Planmeca, www.planmecausa.com); EVA (ImageWorks, www.imageworkscorporation.com); XIOS Plus (Sirona, www.sirona.com); and GXS-700™ (Gendex Dental Systems, www.gendex.com). The size 1 EVA sensor was not evaluated in this paper. It should also be noted that other companies, including Progeny Dental (Lincolnshire, IL, USA), Soredex (Tuusula, Finland), Instrumentarium (Tuusula, Finland), and Suni Medical (San Jose, CA, USA) were contacted but their sensors were not available.

The following parameters were evaluated:

• cable configuration – number of wires exiting the sensor and the diameter of the cable
• interface – the nature of the sensor’s interface with a computer system
• presence of back-scattering shield – some sensors use a back-scattering shield to reduce the back-scattered radiation from reaching the imaging (CMOS) chip
• thickness – the thickness of the sensor plate and its thickness at the point where the cable connects to the plate
• active sensor area – computed by dividing the imaging area tested by the overall sensor’s casing area

The authors also compared the sensor’s active area to a corresponding film (eg, a size 2 sensor was evaluated versus a size 2 film, a size 1 sensor versus a size 1 film, etc.). Kodak InSight F-Speed films (Carestream Dental) were used as the baseline for comparison of the dimensions of different sensor sizes. All companies involved in the testing with the exception of Schick Technologies, which does not advertise technical specifications of its sensors online,⁹ provided the authors with information regarding technical specifications of their sensors.

Since it was not possible to disassemble the sensors and observe the internal structure, digital radiographs were taken to evaluate active imaging area, back-scattering shield, and number of wires inside each sensor’s cable. Patterson Eaglesoft v15 (Patterson Dental, www.pattersondental.com) was used to display images acquired from a size 2 Planmeca ProSensor that served to acquire the digital images on all other sensors, including another tested Planmeca ProSensor.

A 15-inch MacBookPro (Apple Inc., www.apple.com) with Core 2 Duo processor and 4 GB of RAM was used for acquiring images of the sensors. The MacBook Pro was running a 32-bit version of Windows 7 Professional (Microsoft Corp., www.microsoft.com) with latest updates running under Boot Camp. Boot Camp enables Mac computers to run Windows as if they were a PC.

Two x-ray machines were used for acquiring the images. The Intrex VSK (Keystone X-Ray, Doylestown, PA, USA) intraoral x-ray machine was used because it allowed higher kVp settings up to 90 kVp, which enabled imaging of sensors equipped with a back-scattering shield to allow counting the number of wires inside the cable. The default exposure settings for molar teeth (70 kVp, 10 mA, and 0.25 sec. exposure time) were used. The same images were used to identify the number of wires present in the sensor’s cable.

Planmeca Intra intraoral x-ray machine was used for imaging the active area of a sensor using a mesh phantom with digital exposure (63 kVp, 8 mA, and 0.050 sec.) settings. The mesh was placed on the outer aspect of the sensor. Image J software v1.45s (National Institutes of Health, USA) was used for image analysis. The “Analyze>Set Scale” tool in Image J was used to set a distance of 1 mm between the two edges of the 1 mm² square on the mesh phantom image, after zooming in to 300% for better accuracy. Once pixel size calibration was completed, Image J automatically calculated the width and height of the image in millimeters based on the width and height dimension in pixels for the image.

Some of the tested sensors have beveled corners, leading to some loss of the total active surface area of the sensor used to record information about teeth and other bony structures. The polygon selection tool in Image J was used to trace the outline of the active area on the mesh image after image calibration was done using the same step as above. This provided an objective benchmark of the actual active area.

An Absolute Digimatic digital caliper (Mitutoyo, www.mitutoyo.com) was used for measuring the sensors’ casing and the cables. The caliper was checked for calibration before doing physical measurements. Both caliper ends were brought into contact, the reset button was pressed, and a zero reading was obtained. The measurements were done using the tips of the caliper as instructed by the manufacturer. Because sensor cables are flexible and compressible, the caliper tips were lightly pressed against the cable in order to avoid deformation.

The properties of the sensors are summarized in Table 1. The columns list different sensors, and the rows list properties for each specific sensor. Advertised information collected from vendors brochures’ are listed as “Ad” in the table to distinguish the advertised information from the data collected through the evaluation process.

Most vendors offer sensors in sizes 2 and 1. Kodak, Schick, and Planmeca provide sensors in size 0 as well. Dexis is the only company that offers only a single size sensor. For the purpose of this study, the authors considered the Platinum sensor to be in the size 2 category, since the sensor is used for taking radiographs for posterior teeth and for bitewing.

The cable of each sensor has a different number of wires running inside to transmit the signal from the sensor to the computer. The lowest number of wires was seen with the ProSensor with two wires only exiting the sensor, while the highest number of wires was seen in both the RVG 6100 and EVA sensors with 12 wires. The other sensors that were tested have four wires except for CDR Elite and XDR with five wires each.

Cable diameter represents the thickness of the cable. The thinnest cable was 2.7 mm for the ProSensor, whereas the thickest cable was 3.8 mm for the RVG 6100.

The RVG 6100 has the most effective back-scattering shield with no image of the internal structure depicted at 70 kVp. The images obtained for XDR and EVA did not provide evidence that a back-scattering shield is present at all, since the images appeared very dark (radiolucent or low density), indicating most of the x-ray beam was able to pass through these sensors. All other sensors’ images provided evidence for the existence of a back-scattering shield.

Without accounting for the added cable width at the back of a sensor, the thinnest sensor was the EVA at 5 mm, while the thickest sensor was the Platinum with 8.5 mm. When the connecting cable thickness is taken into consideration, the thinnest was the ProSensor at 11.3 mm and the thickest was the RVG 6100 at 14 mm.

The total active area of the sensor represents the image size the practitioner will obtain when using a digital sensor. Corners beveling may lead to reduction of the active imaging area (Figure 2). For size 2 sensors, both the CDR Elite and the EVA had no active imaging area lost due to beveling. The highest surface area loss around the corners due to beveling was noted for the Platinum and the GXS-700 sensors with loss of 47.3 mm² (Figure 2).

For size 1 sensors, all except the XDR and GXS-700 had no active surface area loss related to corner beveling. The highest loss was recorded for the GXS-700 sensor, at 20.5 mm².

It should be noted that these numbers should be looked at as part of the total active imaging area of a sensor when comparing different sensors.

For size 2 sensors, the largest imaging area measured was 940 mm² for the CDR Elite sensor, whereas the smallest was 802 mm² for Platinum (one should keep in mind that Platinum comes in a single size). These measurements account for imaging area lost at the corners of the sensor due to beveling. For size 1 sensors, the largest imaging area was 645 mm² for the ProSensor, while the smallest area measured was 575 mm² for GXS-700. For size 0 sensors, the largest imaging area was 469 mm² for the ProSensor and the lowest was 361 mm² for RVG 6100.

As a reference, the active imaging area for the size 2 Kodak Ultraspeed film is 1235 mm^2, for size 1 it is 960 mm^2, and for size 0 it is 770 mm².

The surface utilization ratio was computed by dividing the total active imaging area by the total gross area of the sensors. Table 1 shows that most size 2 sensors have a utilization ratio between 65.4% and 73.9%; the highest utilization ratio was 73.9% for the Platinum sensor, whereas the lowest was 65.4% for the XIOS Plus sensor. For size 1 sensors, the highest utilization ratio was 64.6% for the XDR sensor, and the lowest was 60.7% for the CDR Elite sensor. For size 0 sensors, the range was between 58.6% for the ProSensor and 53.7% for the RVG 6100 sensor.

The active area of an F-Speed size 2 film has dimensions of 40.5 mm x 30.5 mm, totaling 1235 mm². All size 2 digital sensors have much smaller active area in comparison. Dividing the dimensions of the active area of a sensor by the active area of a film gives an estimate on how much smaller the image provided by the sensor is in percentage terms; the larger the percentage the better, because more information can be recorded.

The largest imaging area compared to the F-Speed film for size 2 sensors was 76.1% for the CDR Elite, whereas the smallest was 64.9% for the Platinum. For size 1 sensors, the largest ratio was 67.2% for the ProSensor, whereas the smallest was 59.9% for the GXS-700 sensor. For size 0, the largest ratio was 60.9% for the ProSensor, and the smallest was 46.9% for the RVG 6100 sensor.

Most sensors connect to the computer via a USB (Universal Serial Bus) interface either directly or via an interface box. The following sensors connect directly to the computer via a USB 2 interface: XDR, Platinum, and GXS-700. Other sensors require an adaptor (an interface box) in order to be connected via a USB port: CDR Elite, ProSensor, EVA, and XIOS Plus. The RVG 6100 has the interface box integrated in the cable. Two sensors, ProSensor and XIOS Plus, offer network connectivity via the Ethernet port beside USB connectivity.

In this paper, all sensors were plugged into a USB 2 port. All sensors displayed the radiographs on the monitor screen within 2 to 3 seconds after exposure.

Digital x-ray technology provides a significant improvement in dental imaging. Moreover, it has become clear that there are differences between different sensors available in the marketplace. The results of this study revealed that these details are not always available through the material provided by the manufacturers and are often not advertised. These differences are discussed below:

Similar to traditional films, sensors are manufactured in different sizes to accommodate different areas of the mouth. Most companies have size 1 sensors in addition to the larger size 2. Few also offer size 0 sensors (Carestream Dental, Schick, and Planmeca). Dexis is the only company that has a single size sensor that is supposed to provide a solution for situations covered by other companies with a size 1 and size 2.

Wired CMOS sensors are connected to the computer via a cable. According to many sales representatives from different companies, the main reason why sensors fail is because of damage to the cable either near the sensor’s end or near the computer’s end. There is no published literature to confirm that this indeed is the number one reason for failure.

Some authors state that the cable is “easily damaged and it may interfere with sensor placement.”¹⁰ Furthermore, the cable interferes with teeth closure in occlusion while taking bitewings, so imaging of the alveolar crest may be a challenge.¹⁰ It is clear that it is preferable to have a sensor with a cable that has the thinnest profile possible. The thinnest cable was measured in the ProSensor (diameter = 2.7 mm), and the thickest was connected to the RVG 6100 (diameter = 3.8 mm).

The CDR Elite sensor is the only sensor that provides a cable that can be easily replaced. The connection between the cable and the sensor is proprietary and has two screws at the back of the sensor that can be removed to allow installation of a new cable.

Damage to a wire inside the cable would lead to malfunction or render the sensor inoperable until the cable is replaced. In addition, a fewer number of wires have the potential to allow thicker and more durable protective sheets without affecting the overall thickness of the cable; the ProSensor has only two wires in the cable and therefore it is not surprising that it has the thinnest cable diameter. A fewer number of wires could also increase the durability of the sensor since there is less chance that one of the wires would be damaged.

The back-scattering shield is a piece of thin metal placed at the back of the sensor behind the sensor chip. Its main function is to reduce back-scattered radiation from reaching the imaging (CMOS) in the same way that the thin lead foil behind a conventional film acts to avoid fogging. The lead foil also prevents unwanted radiation from reaching the patient11-13; similarly, one manufacturer (Dexis) claims in its website that “a protective shield affixed to the interior wall of the rear housing protects the patient by preventing x-ray back scattering.”¹⁴

All sensors provided a back-scattering shield except for the XDR and EVA sensors. It should be noted, however, that back-scattering shields are not standardized and it seems that there is a range of effectiveness. The results suggest that the RVG 6100 has the most effective shield; however, it is not clear what the clinical implication of the shield’s effectiveness is, and whether the patient indeed benefits from a significant reduction in radiation when a more effective shield is present.

When a sensor is exposed to radiation the signal initially is analog and not digital. To convert the analog signal to a digital signal, an analog-to-digital converter (abbreviated ADC, A/D, or A to D) circuit is required. The ADC circuitry is either integrated into the printed circuit board (PCB) of the sensor allowing direct connection of the sensor to the computer, or the ADC is placed outside the sensor in an interface box located between the sensor and the computer.

Three sensors in this study featured a direct connection (XDR, Platinum, and GXS-700). The RVG 6100 is the only sensor that has the interface box integrated into the cable, while for all other sensors that feature an interface box the cable connecting the sensor to the interface box can be detached. Two manufacturers (Planmeca and Sirona) offer an alternative to the USB connection, using an interface box that can be connected to the network via Ethernet. Sirona’s Ethernet interface box is fixed on the wall (Figure 3) and it is the single interface box that can accept concomitantly both size 1 and size 2 sensors, avoiding the need to unplug and plug when switching sizes. Similar to all other USB interface boxes, Sirona’s USB interface box requires switching sensors when different sizes are to be used.

Planmeca uses a magnetic interface to plug the sensor into the interface box as a safety feature similar to the MagSafe™ connector available on Apple laptops; the cord safely disconnects from the socket when pulled (Figure 4). The way the sensors connect to the ACD and the computer (presence of ACD, magnetic versus rigid connectors, and USB versus network connections) address different needs in different settings. The practitioners will have to make their own decision regarding which type of connection will be the most beneficial in the setting of their practice.

In conventional imaging, radiographic film represents the active area that ultimately will capture the x-ray photons and later display the radiographic information. The film is placed inside a protective barrier that adds additional dimensions to the film as inactive area; the overall area is the total surface of the film.

In digital radiography, the intraoral sensor uses a CMOS layer that captures the photons. This layer is the active area of the sensor. The CMOS layer is placed inside a plastic casing to protect the components of the sensor. The size of the casing defines the total surface area of the sensor. The larger the active area, the larger the field of view, allowing the practitioner to visualize more structures.

The ratio between the active to total surface area of the sensor determines the efficiency of surface utilization. A high ratio denotes that there are less useless margins around the CMOS sensor chip. For size 2 sensors, about 25% of the overall margins of the sensor are unused. For size 1 sensors, the ratio is lower compared to the size 2, and for size 0 sensors the ratio further lowers, leading to very limited field of view.

This ratio compares a sensor’s active area to the traditional same size F-speed film’s active area. Sensors have smaller active area dimensions compared with traditional film.⁵ Most size 2 sensors fell in the range between 70.2% and 76%, meaning that switching to digital sensors from conventional films will lead to a smaller field of view for digital radiographs. Digital sensors, therefore, have a smaller field of view compared to traditional film of similar size and this can lead to missing structures such as the distal aspect of the canine, distal aspect of the last erupted tooth, or the crest bone height. It should also be noted that the rigidity of the digital sensors compared to the regular film leads to an increase in placement errors in the molar area and vertical angulation errors in the anterior segments that lead to missing incisal edges of teeth and increased number of retakes when compared to dental film.¹⁵ Cone-cuts and difficulties with bitewings resulting in missing structures and patient discomfort, have also been reported.^16,17 It is clear that the rigidity of the sensors and the resulting placement challenges enhances the importance of a larger active area compared to regular film.

The Platinum sensor has the lowest ratio (64.9%) due to its overall smaller active area dimensions and due to cutting of the corners (beveling). CDR Elite size 2 sensor has the highest ratio due to its overall larger dimensions of the active area and lack of beveling.

Digital sensors are much thicker than conventional radiographic films. The thicker the sensor plate the harder it is to place it inside the patient’s mouth without causing discomfort.^18,19 Placing the sensor in patients who have smaller jaws or anatomical variations like tori can be challenging. The thickest sensor was the Platinum; the thinnest was the EVA sensor. Taking the placement of the cable at the back of the sensor into consideration while measuring a sensor’s thickness, the RVG 6100 is the bulkiest and the ProSensor the thinnest. Despite the intuitive value of these results, it is not clear how much the differences in the thickness of different sensors affect the comfort of the patients. The literature shows mixed results regarding patient comfort when digital sensors are compared with film and phosphor plates.^20,21

The information presented in this study can be helpful for practitioners to make an informed decision when purchasing a digital intraoral sensor. While the discussion considered objective aspects of digital sensors’ configuration, there remain several considerations that may need further study. These include:

• patient comfort using different sensors to clarify what is the clinical significance of different sensor designs and dimensions

• the clinical significance of the back-scatter shields and the actual radiation that is transmitted to the patient by each sensor

In conclusion, practitioners should be aware of the advantages and limitations of transitioning from conventional film to digital radiography; the data presented here should be a useful starting point in making decisions. It needs to be recognized that different systems have different direct costs. For example, Dexis tries to provide a comprehensive solution with only one sensor; other manufacturers charge separately for the ACD interface boxes. In addition, long-term costs of ownership such as warranty, replacement of the sensor, cable, and/or the ACD interface box are not standard among suppliers and a careful evaluation of these costs should be part of the purchasing process.

The authors would like to thank Douglas Yoon, DDS, from Cyber Medical Imaging for his help providing the mesh phantom used for testing the active area of the intraoral sensors.

The authors have no conflict of interest.

Wisam Al-Rawi, BDS, MSc, MS Assistant Professor, University of Michigan, School of Dentistry, Ann Arbor, Michigan

Sorin Teich, DMD, MBA Associate Professor, Assistant Dean of Clinical Operations, Case Western Reserve University, School of Dental Medicine, Cleveland, Ohio

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