Both Computed Radiography (CR) and Direct Capture or Digital Radiography (DR) are digital process. Both CT & DR are comprised of digital imaged. To help distinguish the processes, CR is often referred to as Cassette-Based radiography and DR is referred to as Cassette-Less radiography. However the most common term for cassette-lest radiography is Digital Radiography and can add confusion in the classification of CR and DR systems.
Types of Digital Radiography:
The CR system uses the typical general diagnostic x-ray tube and table, but an imaging plate (IP) replaces the film-screen inside the cassette. Like film-screen, CR systems can be used in an existing bucky making the conversion to their use relatively easy in comparison to DR systems. The Cassette-based IP consists of a base or support layer, a phosphor layer, and a protective emulsion layer. The IP records the latent image in a similar way that a latent image is formed on the x-ray film when struck by the x-rays passing through the patient. Typically there is a photo stimulable phosphor layer (PSP) that has a unique ability to "trap" electrons when excited by an x-ray photon. The CR cassettes need not be light-tight because the PSP screen inside is not light sensitive. In fact, it can even be opened briefly in the light without the loss of the latent image because it does not contain light-sensitive film. Also unlike film, the IP can be used repeatedly. Imaging plates are available in standard sizes.
As mentioned before, the PSP screen is the actual image receptor (IR). The PSP screen has a layer of barium fluorohalide that can be either granular or needle-shaped. The more desirable needle-like phosphors have better x-ray absorption and less light diffusion. Just under the barium fluorohalide layer is a reflective layer that helps to direct emitted light up toward the CR reader. Below the reflective layer is the base, behind that is an antistatic layer and then the lead foil to absorb backscatter. Over the top of the barium fluorohalide is a protective layer.
When the barium fluorohalide phosphors absorb x-ray energy, electrons are released in one of two ways. One group of electrons initiate immediate luminescence. The other group becomes trapped within the phosphor's halogen ions, forming a "color center." When the phosphors are exposed to a monochromatic laser light source they emit polychromatic light, termed photostimulated luminescence (PSL). The PSP screen can store its latent image for several hours; however, after about 8 hours noticeable image fading will occur.
After exposure, the IP is placed into the CR reader, where the PSP screen is automatically removed. The latent image on the PSP screen is changed to a manifest image as it is moved at a constant speed and scanned by a narrow high-intensity helium-neon or solid state laser to obtain the pixel data. The laser causes excitation of the electrons storing the latent image resulting in the electrons to emit a light equal to the intensity of the latent image. The challenge associated with exciting an electron is to making sure that the (wavelength) light given off by the excited electron is different from that laser light (wavelength) used to excite it. The longer, red, 430-550 nm wavelength light from the newer solid-state lasers is unlikely to interfere with the light being emitted by the PSPs (bluish-purple, blue-green, etc.).
This appropriate wavelength is there, causing photostimulated luminescence (PSL) to occur during the excited state of electrons. An optical filter is used that permits transmission of the PSL, but attenuates the laser light; this filter is mounted in front of the photomultiplier tube (PMT). The PMT or photodiode (PD) is used to detect the PSL and convert it to electrical signals. The electrical energy is sent to an analog-to-digital converter (ADC) where it becomes the digital image. The digitized images can also be manipulated in post-processing, electronically transmitted, and stored. The PSP screen is then erased by a bright light inside the reader, reloaded into the IP, and returned ready for the next exposure.
One corner of the IP's rear panel has a memory chip with an identification number to match with a number in the Radiology Information System (RIS) for patient information. After the exposure is made, the IP and the patient's information is linked at the CR reader workstation. Often information can be linked using a bar code reader or by manual input on the keyboard at the cassette reader; therefore the usual lead blocker found on conventional film cassettes is not present.
Digital radiography does not use IPs or a traditional x-ray table; it is hard-wired to the image processing system and is therefore filmless. Detectors are used, instead of an intensifying screen (analog) or PSP screen (CR), to detect the x-ray signal and the sensors are permanently installed inside a rigid protective housing. DR provides an immediate display of the image on a workstation monitor, much like the workstation at the CR reader. The DR workstation allows image preview and post-processing.
There are two types of cassette-less systems; direct capture or conversion and indirect capture or conversion.
Direct Conversion
In direct conversion flat panel detector systems, x-ray energy is absorbed and immediately converted to an electrical signal in a single layer of material called a scintillator, typically made of a-Se. This electrical signal (electron) is then stored in the thin-film transistor (TFT) detectors which are photosensitive arrays made up of small pixel elements. The number of TFTs is equal to the number of image pixels. After temporary storage, the TFT acts like a switch and transfers the electron signal to the integrated circuits on the outer edges of the matrix of detectors that control the line scanning sequence, readout, amplification, and analog-to-digital conversion. When the TFT switches close after transferring to the integrated circuits, the electric charge information discharges. It takes less than one second for approximately one million pixels can be read and converted to a digital image.
Indirect Conversion
Indirect conversion also uses TFT technology. Indirect conversion is a two step process: (1) x-ray photons are converted to light photons, and (2) light photons are converted to an electrical signal. In step one, x-rays strike and are absorbed by the scintillation layer and are converted to light photons. In step two, a photosensitive array made of small pixels converts light into electrical charges. Each pixel contains a photodetector (such as amorphous silicon photodiode array, TFT array, or a charge coupled device) that absorbs light from the scintillator and generates electrical charges.
A thin-film transistor (TFT) is composed of glass with amorphous selenium (in fluid form rather than crystalline) on both sides. Scintillation light must be able to pass through the detector material to reach the ADC. Because silicon is opaque, glass is the most commonly used interface because it is highly transparent and compatible with the system. Since glass is not a semiconductor like silicon, a thin film of amorphous selenium (a-Se) is deposited on top and bottom—the transistors are made using this thin layer: hence, the name "thin-film transistor."The electrical signal is transmitted to the ADC.
The amorphous selenium detector uses thin films of silicon integrated with arrays of photodiodes. These photodiodes are coated with crystalline cesium iodide (CsI) scintillator or a rare-earth scintillator. When the scintillator is struck by x-rays, visible light proportionate to the incoming x-rays is emitted. Light photons are then converted to electrical charges by the photodiode arrays.
With the charged couple device (CCD), x-ray photons interact with a scintillation material such as photostimulable phosphors and convert them into light photons. Each CCD chip within the flat-panel detector has many pixel electronics on its photosensitive silicon surface. As CsI or GdOS scintillation falls upon each pixel, electrons are liberated and built up within the pixel—the greater the light intensity, thegreater the number of electrons. The electronic signal is then transmitted to the ADC for digitization. CCD's are common in digital cameras.
Detective Quantum Efficiency
How efficiently a digital imaging system converts the x-ray input signal into a useful output image is known as detective quantity efficiency (DQE). Compared to film-screen systems, CR systems have a wider DQE latitude, which implies that CR has the ability to convert incoming x-rays into "useful" output over a much wider range of exposure than can be accommodated with film-screen systems. In other words, CR records all of the phosphor output. Systems with higher DQE can produce higher quality images at lower dose. Both indirect and direct DR conversion has higher DQE than CR. In particular, DR direct conversion increases DQE the most because it does not have the light conversion step and consequently no light spread. There is no light to interfere with the recorded signal output which means that less dose is required than for CR. In addition, higher quality images are produced.
Currently there are two types of technology being used in digital fluoroscopy (DF) systems. First is the more common analog-to-digital conversion system. This system uses a conventional image intensifier and a television system to produce a traditional analog or visible image. This visible image from the output side of the image intensifier is then recorded by a high-resolution video camera and converted to a digital format.
A second type of DF system utilizes the latest technology consisting of direct signal conversion, commonly referred to as "flat detectors." Currently, this type of technology is used primarily in cardiac and vascular applications. With this newer system, the digital detector replaces the image intensifier, the video camera, and the digital conversion system.
Advantages of the flat detector technology include the following:
