Keywords: Computed radiography; CR; digital radiography; direct digital radiography; DR; film-screen radiography; flat panel detectors; FSR, MTF,DQE, fill factor, pixel pitch, PSP plates
|How to cite this article:|
Verma B S, Indrajit I K. Advent of digital radiography : Part 1. Indian J Radiol Imaging 2008;18:113-6
| Introduction|| |
Analog versus digital
In analog systems, a variable is measured on a continuous scale with an infinite number of possible values. In digital systems, however, measurements can only have a limited number of discrete values.  Illustratively, analog systems can be represented by an escalator ride where a person can be at any position from top to the bottom. Digital systems an be represented by a staircase where one can be only at a limited number of discrete positions.
Rapid advancement in the field of medical imaging has been possible due to the use of computers as they can process digital data very fast and efficiently. However, nature uses analog system including signals generated in diagnostic imaging. The human eye-brain system can handle analog signals very effectively. To use computers in medical imaging, analog data first need to be converted to digital data for processing and then converted back to analog images for viewing and interpretation.  This is done by analog-to-digital converters (ADC) and digital-to-analog converters (DAC), respectively. Most of the imaging devices in a radiology department, e.g. ultrasound, CT, MRI, DSA, etc., already use digital imaging technology.
| Radiography|| |
Radiography is recording of information about an object using X-ray transmission. The intensity of X-rays is nearly uniform before entering an object being radiographed. After passing through the object, the spatial distribution of transmitted X-ray intensities carries all the radiographic information about the object. This information can be detected by means of something that is sensitive to radiation. Conventionally this is done by film-screen radiography (FSR). It can also be done by some digital detectors. When digital detectors are used to capture this information, the process is termed as digital radiography.
Simplified definitions of some frequently used terms
As we have seen above, the spatial distribution of transmitted X-ray intensities carries all the radiographic information about the object. How faithfully and accurately this information is recorded is called the modulation transfer function (MTF). Thus, equipment with higher MTF will provide better spatial resolution. The efficiency with which this radiation information is captured is known as detective quantum efficiency (DQE). , Detectors with higher DQE will require less radiation dose than the detectors with lower DQE for similar image quality or signal-to-noise ratio (SNR).  Alternatively higher DQE detectors will provide better SNR for the same radiation dose.  Both MTF and DQE are depicted in the form of a graph as a function of frequency or spatial resolution in line pairs/mm (lp/mm). Both are higher at low resolution and decrease with increasing spatial resolution.  Most of the technical literature describes DQE at a spatial resolution of 0 lp/mm. Both DQE and MTF are higher in better detectors. DQE is a better and more comprehensive measure of the detector quality.
A digital detector has a large number of picture elements or pixels. All pixels are square in shape and "pixel size" is the length of one side in µm (micrometer). The distance between the centers of two adjacent pixels is known as the "pixel pitch". As the distance between adjacent pixels is usually negligible, pixel pitch and pixel size are usually equal. Pixel size is a measure of limiting resolution, which is variously described as pixel size/pixel pitch in µm, pixels/mm, and lp/mm. Thus, a detector with a 200-µm pixel size may have a limiting spatial resolution depicted as 05 pixels/mm or 2.5 lp/mm. 
All the parts of a digital detector being exposed to radiation may or may not be able to convert X-rays into electrical signal. The area of the detector that is sensitive to X-rays in relation to the total detector area is known as the "fill factor".  Detectors with higher fill factors are more efficient users of absorbed radiation.
| Conventional Radiography|| |
In FSR, the absorbed X-rays are first converted into light by a pair of intensifying screens. Film sandwiched between these screens records a latent image that becomes visible after chemical processing. During the more than 100 years of its use, conventional radiography has been found to be very useful. Intensifying screens, introduced over 60 years ago and rare earth screens in recent years, have greatly reduced the radiation dose required for producing good quality images. Advancements in FSR technology have almost reached the limit of possible improvements. Only a completely new technology will be able to provide substantial advantage over the current FSR techniques. The advantages and limitations of FSR are listed in [Table - 1].
| Digital Radiography Systems|| |
A digital detector replaces film and screens in digital radiography. There are two basic types of digital radiography systems depending upon the types of detectors used to capture radiographic information: 
- Computed radiography (CR) systems use a photo-stimulable phosphor (PSP) plate enclosed in a light tight cassette.  CR utilizes a two-stage process with the image capture and image readout done separately.
- Direct Digital Radiography (DR) systems use detectors that have a combined image capture and image readout process. ,
About two-thirds of patients visiting radiology departments are referred for plain radiography.  It is inevitable that conventional FSR will sooner or later be replaced by Digital Radiography due to the numerous advantages and electronic compatibility of the latter.
The advantages and limitations of the digital radiography systems (both CR and DR) are listed in [Table - 2].
| Computed Radiography Systems|| |
CR cassettes use PSP plates in place of film and screens. These plates are coated with europium-activated barium fluoro-halide (BaFX: Eu 2+).  The halide used may be bromide, iodide, or a combination of both. CR cassettes are used just like conventional cassettes on normal radiographic equipment and are available in similar sizes.
X-ray information is stored in PSP imaging plates as electrons, in semi-stable higher energy states, in sinks or "F" centers. The number of such trapped electrons is directly proportional to the absorbed X-ray dose. The imaging plate comes out or is exposed by opening the CR cassette within the CR reader. Image information is acquired by scanning the plate by a laser beam [Figure - 1]. Red laser light excites these trapped electrons during scanning. Electrons eject from the higher energy sinks and come down to the base level. They emit a higher energy blue light during this process. This light is captured by a light guide, converted into electrical signals, amplified, digitized and used to form the image. The imaging plate is ready for reuse after exposure to white light. 
Patient information and cassette ID needs to be linked in a CR system [Figure - 2], as there is no direct electrical connection between the CR reader and the cassette. A bar code reader or a chip embedded on the CR cassette is used for this purpose. The PSP imaging plates may be flexible or rigid. The base used in these plates may be opaque or translucent. Due to different types of CR cassette designs and image readers available, all cassettes from the same vendor may not be compatible with all readers. Some of the CR plate readers can process one plate while holding multiple cassettes in a queue. This "drop and go" feature helps improve workflow.
Dual-side readout is available in some systems using PSP plates with translucent bases.  These systems use laser scanning from one side but capture light from both sides of the plate, increasing the DQE by 50 to 100%.  The spatial resolution of the CR images depends on the laser spot size, PSP plate characteristics (like packing density and thickness of the phosphor layer) and the sampling rate of the emitted light. Diffusion of the scanning laser light as well as the emitted light leads to some loss of spatial resolution. It is possible to achieve a resolution of 5-10 pixel/mm in general purpose CR cassettes. A resolution of 20 pixel/mm is available in most CR systems approved for mammography.
The time taken for scanning a PSP plate depends on the plate size, resolution desired, dual/single side readout and varies from 40 to 90 s. Some newly introduced systems use line scanning techniques, reducing the image read time to 20-30 s or even less. The advantages and limitations of CR systems are listed in [Table - 3].
| Direct Digital Radiography Systems|| |
To increase the workflow, it is important to avoid handling of the cassette, which is used in both, FSR and CR. This became possible with the availability of a new class of detectors, that were able to combine the processes of image capture and image readout, "without user intervention".  Details of direct digital radiography systems and the effect of digital radiography technology on the projection radiography workflow will be covered in Part II of this article.
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B S Verma
Department of Radiodiagnosis and Imaging, Army Hospital (Research and Referral), Delhi Cantt. - 110 010
Source of Support: None, Conflict of Interest: None
[Figure - 1], [Figure - 2]
[Table - 1], [Table - 2], [Table - 3]