Sensors are the receptors for the digital image, instead of using traditional x-ray film. There are two types of digital image receptors or sensors. They include direct solid-state (Figure 3) and indirect photo-stimulable phosphor plates (PSP) that are similar to flexible radiographic film (Figure 4). The solid-state technology uses different semi-conductor-based detectors 1) CCD, 2) CMOS, and 3) flat panel.5,9
The solid-state intraoral receptors contain a charge-coupled device (CCD) with an electronic circuit inside the digital sensor (Figure 5). The silicon chip detector converts x-ray photons into an electrical charge. The electrons produced are deposited into small boxes called pixels. The pixels are the equivalent of silver halide crystals in traditional film. Pixels provide multiple shades of gray to allow better contrast than traditional x-ray film. When dental students and faculty at the University of Texas School of Dentistry at Houston evaluated the image quality and technical accuracy with photostimulable phosphor (PSP) plate and charge-coupled device (CCD), they found a significant difference in image quality and technical accuracy, especially angulation with the photostimulable plates than with the charge-coupled device.11
The complimentary metal-oxide semiconductor imaging chip (CMOS) is sensitive to light and provides small details in the x-ray image by isolating each pixel from its neighboring pixels. Similar to CCD, electron hole pairs are generated within the pixels in relation to the amount of x-ray energy that is absorbed. This charge is transferred to the transistor as small voltage. The scintillator screen inside the sensor is made of materials such as cesium iodide converting radiation into visible light and guides the light through a micro-columnar structure. Fiber optics transmit the light to the surface sensor allowing a high signal-to-noise ration resulting in detailed images. The CMOS provides higher visible resolution, higher contrast and lower noise, diagnoses is enhanced. Some manufacturers can display 16,000 shades of gray using a 14-bit analog-to-digital converter, allowing more variation in densities that can be seen by the dental provider. CMOS also provides an enhanced quantum efficiency, allowing more consistent images within a range of exposure settings. This dynamic range provides a broad range of exposures, since quality assurance can be an issue with digital imaging. The enhanced quantum efficiency of some sensors can compensate for radiation variances with exposure times. The CMOS technology has been used since the 1960s in many consumer products such as digital cameras, video cameras, fax machines, microscopes, and telescopes.1,5,6,7,9
Flat panel detectors are used with medical imaging and extraoral imaging units. The photoconductor is made of selenium for more efficient x-ray absorption. The intensifying screens consist of an amorphous silicon photodiode circuitry layer and a semi-conductor device called a thin film transistor (TFT). When x-ray photons reach the scintillator, visible light is emitted and then recorded by an array of photodiodes and converted to electrical charges. The detectors provide large matrix areas with smaller pixel sizes allowing for direct digital imaging of larger areas of the body. Flat panel detectors have the advantage of a short exposure time of 10 seconds or less. Their disadvantage is their large size that cannot be used intraorally.5,9
Solid-state sensors have a fiber optic cable or wireless sensor that are powered by batteries and transmits data via radio waves. These options allow for toggling between hardwired USB and wireless WiFi, where staff can move between treatment rooms without carrying the director sensors with the fiber optic cables. Manufacturers offer multiple fiber optic cable lengths, depending on the size of the dental practice treatment rooms (Figure 6). This type of imaging is called direct digital imaging, as the hard sensors do not require scanners.