The development of the radiography over the past few decades has transformed the imaging of radiology. The digital radiography systems are mainly defined and categorized into CR (Computed Radiography) and DR (Direct Digital radiography) systems, which are followed by a quick description of the systems of CR. The benefits and drawbacks of the FSR (Film Screen Radiography), the digital radiography systems, and the different CR systems are generally listed in the tabular form.
Digital radiography systems have a tendency to use a plate of photostimulable phosphor, which is generally enclosed in a cassette. In terms of CR, the image acquisition happens in a two-stage process, where first the image is captured and then reading of the image is done separately. The DR systems make use of detectors, which has a single process, basically combines the two-above process in one go. The DR systems are also known as DDR or DDR systems by the random vendors.
Direct Digital Radiographic Systems
Cassettes have emerged to be a vital component in terms of both FSR (Film – screen radiography) and CR. To improve workflow and avoid the usage of cassettes, detectors of a new class were first manufactured and then combined with this process of image capturing and then image reading. This then formed an evolutionary basis for the DR systems.
There are majorly 4 different types of digital radiology systems used for radiology reporting, which are available depending on the types of detectors used:
FPD (Flat Panel Detector)
The DR systems based on FPD are known to be amongst the popular ones. Here there is a usage of TFT (Thin Film Transistors), which are made up of amorphous silicon. The Silicon semiconductor sheets are generally used with detector elements, which are square with 70-200 on each of the sides, in a glass substrate. Each element in the FPD has a switching transistor and a capacitor. Then there are drain lines and Gait, which are connected to each of the transistors and the capacitors, which enable active charges (Readouts) from each element of the detectors separately. On the TFT matrix, the X-ray converter material is used to make a detector of a flat panel.
There are majorly two types of flat panel detectors, which majorly depend on the materials and the methods used to convert X-rays into the electrical signals. These are:
Direct X-Ray Conversion Type of Directors
These detectors use (a-Se) and amorphous Selenium photoconductor. These are seen sandwiched between the two electrodes, where very high voltage is applied. When the X-rays are seen falling on the layers, holes and electrons are then directly produced in a proportional value to the number of X-rays which are absorbed. This high voltage is then seen separating the holes and the electrons so that the signals are not spread out. The direct technology of conversion has been majorly derived from the numerous experiences which are gained during the use of the selenium drums in the photocopier machines and xeroradiography.
Hence, this makes the direct FPD’s way more popular in the mammography than in the routine radiography.
Indirect X-Ray Conversion Type of Directors
Here, first, the X-rays are converted to light in a phosphor, which is then detected by the photodiodes and the arrays of TFT. The most commonly used phosphor material is the Thallium – doped Cesium Iodide, which is formed in a needle-shaped crystal-like structure. The lights which are produced by the X-rays in the crystals is then channeled by the reflections of the internal and does not tend to spread, which eventually maintains a great resolution of speed.
2D or ‘Area’ CCD Array-Based Systems
In the DR systems, X-rays are first absorbed and then converted into the visible light forms in phosphors. This light is then seen getting channelized by means of prisms, lenses, mirrors, etc. and are coupled properly to match the small light-sensitive array of CCDs. The CCD arrays are generally very small of about 2-5 cm and have tiny detectors, which are 10-2-um; in such a case, demagnification is a must.
Such detectors are considered to be a little bulky but are known to be a little less costly than the FPDs. An example of such a system can be Xplorer from the Dynamics of the Image generally has a pixel size of 108 ums, and the area of the image tends to be 43 x 43 cm. There are vendors out there who had introduced the CMOS (Complementary Metal Oxide Semiconductors) systems based in place of CCD.
Slot- Scanning Types
These are the systems that make use of a very narrow beam of fan that tends to move across the anatomical regions. There are generally two of the aligned collimators of the moving slits, on either of the sides of the patient, are majorly used in these systems. This helps in then preventing the scatter radiations from reaching out to the detectors. Here, the usage of the radiographic grid isn’t very important, as it ends up reducing the dose of radiation. These systems are seen using a narrow CCD array with a few detector rows to scan the anatomy of the patient. There is a technique that is commonly used – TDI (Time Delay Integration) is generally used to transfer the information from one of the detector rows in the CCD to the next few rows as the gantry is seen passing over the parts of the body.
Hence, the formation of the image about a body’s section gets reinforced, which ends up increasing the SNR. This system very commonly works similar to that of a scanogram in the CT. Here there are somewhat; gantry moves when the patient is stationary. These long body anatomies can easily be covered continuously by these systems. One of the systems, Statscan by Lodox, can easily cover the entire body in a whopping 13 seconds. These units are then taken through a different view without actually making the patient move. The very long exposure times do require a high capacity of the X-ray tubes and generators.