Chapter 6 Cassetteless Equipment and Image Acquisition

Digital Radiography Digital Radiography Used to describe images recorded on an electronically readable device. DR includes computed radiography and direct or indirect methods of digital image capture. For the purposes of this chapter, DR will be used to describe direct and indirect capture digital radiography. DR is hard-wired. DR is cassetteless. Detectors are permanently enclosed inside a rigid protective housing.

Flat-panel Detectors Flat-panel detectors Consist of a photoconductor such as amorphous selenium (a-Se), which converts the x-ray photons directly into electrical signals. This category also includes silicon and CCD detectors.

Direct Capture Detectors Consist of a photoconductor Amorphous selenium (A-Se) Holds a charge on its surface that can then be read out by a TFT Process X-ray photons are absorbed by the material. Photons are immediately converted into an electrical signal. Electrical signal is detected by TFT array.

TFT Thin Film Transitor Is a thin layer of transistors on a substrate (support material). Will detect either light or electrons. A transistor will be located at each pixel; therefore the current/light that is emitted from each pixel can be smaller and can be switched on and off more quickly. In other words, each pixel contains a photodiode. Generates electrical charges. Absorbs light from scintillator.

Direct Conversion DR TFT A field-effect transistor (FET) or silicon TFT Isolates each pixel element Reacts like a switch to send the electrical charges to the image processor

Direct Conversion DR TFT A million-plus pixels can be read and converted to a composite digital image in less than 1 second. A line of TFT switches, each associated with a photodiode, allows electrical charge information to discharge when switches are closed. The information is discharged onto the data columns and is read out with dedicated electronics. Specialized silicon integrated circuits are connected along the edges of the detector matrix.

Indirect Conversion Similar to direct detectors in that the TFT technology is also used Two-step process: X-ray photons strike a scintillator that produces light The scintillation layer in the imaging plate is excited by x-ray photons, and the scintillator reacts by producing visible light. That light strikes the amorphous silicon (a-Se) allowing the conduction of electrons into the detector This visible light then strikes the a-Se that conducts electrons down into the detector directly below the area where the light struck. An FET or silicon TFT isolates each pixel element and reacts like a switch to send the electrical charges to the image processor.

Indirect Conversion

Amorphous Silicon Detector Uses thin films of silicon integrated with arrays of photodiodes These photodiodes are coated with a crystalline cesium iodide scintillator or a rare-earth scintillator (terbium-doped gadolinium dioxide sulfide). When these scintillators are struck by x-rays, visible light is emitted proportional to the incident x-ray energy. The light photons are then converted into an electric charge by the photodiode arrays.

Cesium Iodide Detectors A newer type of amorphous silicon (a-Se) detector uses a cesium iodide scintillator. The scintillator is made by growing very thin crystalline needles (5 µm wide) that work as light-directing tubes, much like fiberoptics. This allows greater detection of x-rays, and because there is almost no light spread, there is much greater resolution. These needles absorb the x-ray photons and convert their energy into light, channeling it to the a-Se photodiode array. As the light hits the array, the charge on each of the photodiodes decreases in proportion to the light received. Each photodiode represents a pixel, and the amount of charge required to recharge each photodiode is read electronically and converted to digital data. This process is very low noise and very fast (approximately 30 million pixels per second).

Charge-Coupled Devices The oldest indirect-conversion DR system is based on CCDs. X-ray photons interact with a scintillation material, such as photostimulable phosphors, and this signal is coupled or linked by lenses or fiberoptics, which act like cameras. These cameras reduce the size of the projected visible light image and transfer the image to one or more small (2 to 4 cm2) CCDs, which convert the light into an electrical charge. This charge is stored in a sequential pattern and released line by line and sent to an analog-to-digital converter. Even though CCD-based detectors require optical coupling and image size reduction, they are widely available and relatively low in cost.

Complementary Metal Oxide Silicon Complementary metal oxide silicon (CMOS) systems Uses specialized pixel sensors that, when struck with x-ray photons, convert the x-rays into light photons and store them in capacitors. Each pixel has its own amplifier, which is switched on and off by circuitry within the pixel, converting the light photons into electrical charges. Voltage from the amplifier is converted by an analog-to-digital converter also located within the pixel. This system is highly efficient and takes up less fill space than CCDs.

Detective Quantum Efficiency Detective Quantum Efficiency (DQE) How efficiently a system converts the x-ray input signal into a useful output image. DQE is used to measure the quality of a digital image by looking at the combination of the effects of noise and contrast on the imaging system. Noise, either quantum or electronic, cannot be avoided in digital imaging. The effect of noise is usually expressed as signal-to-noise ratio (SNR). Signal refers to the useful image information, and noise refers to information that is not useful to image formation. The more signal you have with less noise, the higher the quality of the image. In other words, high SNR or low system noise allows the capture of the most useful image information and consequently yields a higher quality image.

Detective Quantum Efficiency Contrast in digital imaging refers to the system’s ability to accurately reproduce an object’s actual contrast. Digital detectors have a wide dynamic range that allows the capture of a very wide range of signal intensities. In addition, these detectors are able to produce very high-contrast resolution, which allows for the display of thousands of shades of gray that can be enhanced with automatic contrast enhancement and window/leveling parameters. The goal is to achieve high DQE so that there exists the ability to image small, low-contrast objects.

Detective Quantum Efficiency DQE Comparison Indirect and direct DR capture technology has increased DQE over CR. DR direct capture technology has the highest DQE of all systems because it does not have the light conversion step and consequently no light spread. Newer CMOS indirect capture systems may be equal to DR image acquisition because of the crystal light tubes, which also prevent light spread.

Detector Size The actual physical size—length and width—of the x-ray detector is critical. It must be large enough to cover the entire area to be imaged and small enough to be practical.

Spatial Resolution Depending on the physical characteristics of the detector, spatial resolution can vary a great deal. Noise remains constant in any particular system; however, there comes a point at which the most signal that can be captured has been captured and no more is available to increase the SNR. As pixel size decreases, the amount of signal captured decreases so that the finer a matrix is, the lower the SNR will be at each pixel. It should be clear, then, that there is a balance between pixel size and noise that manufacturers have to work toward in order to achieve the best spatial resolution possible. Spatial resolution of amorphous selenium for direct detectors and cesium iodide for indirect detectors is higher than CR detectors but lower than film/screen radiography. Excessive image processing, in an effort to alter image sharpness, can lead to excessive noise.

Pixel Size and Matric Size The amount of resolution in an image is determined by the size of the pixels and the spacing between them, or pixel pitch. More and smaller pixels produce greater spatial resolution. Larger matrices combined with small pixel size will increase resolution, but it may not be practical to use large matrices. The larger the matrix, the larger the size of the image, and the greater the space needed for network transmission and picture archival and communication system (PACS) storage

Technical Factor and Equipment Selection Kilovoltage peak, milliampere-second, distance, collimation, and anatomic markers are the same for cassetteless systems as they are for cassette-based systems. Typically only one exposure is made at a time on the image receptor, but that does not mean that collimation is unnecessary. Collimation may be more critical with PSP-based receptors because of the collimation border/edge detection requirement than it is with TFT devices. When grids are used in any digital imaging system, there is always the possibility that the grid lines will interfere with the pixel rows, resulting in the moiré-pattern error. Grid interaction artifacts are not always easy to identify and can decrease image quality, so caution and proper selection of the grid is advised.

Potential Cassetteless Image Acquisition Errors Although the major cause of noise in any digital system is insufficient radiation striking the receptor, electronic noise can also be a factor. Any noise reduces image quality. The more time allowed for signal conversion, the more precise the pixel values. Incomplete charge transfer causes inaccuracies in pixel values in subsequent exposures, reducing image quality. Additionally, if exposures are taken in too-rapid sequences, there may not be enough time for each previous exposure to transfer the entire signal, resulting in what is known as electronic memory artifact.