Digital radiography, also known as direct digital radiography, uses x-ray–sensitive plates that directly capture data during the patient examination, immediately transferring it to a computer system without the use of an intermediate cassette as is the case with CR. Commonly referred to as plates, these flat panel detectors use a combination of amorphous silicon detectors with cesium or gadolinium scintillators that convert X-ray to light which is ultimately translated by thin film transistors into digital data (Fig. 2-38). This technology is significantly more expensive than CR technology, but the images are of the highest quality and are seamlessly sent to a computer display. These systems are popular in dedicated imaging facilities and hospitals with high workloads.
Digital detectors allow implementation of a fully digital picture archiving and communication system, in which images are stored digitally and are available anytime. Image distribution in hospitals can now be achieved electronically by means of web-based technology with no risk of losing images. Other advantages of digital radiography include higher patient throughput, increased dose efficiency, and the greater dynamic range of digital detectors with possible reduction of radiation exposure to the patient. The future of radiography will be digital, and it behooves radiologists to be familiar with the technical principles, image quality criteria, and radiation exposure issues associated with the various digital radiography systems that are currently available.
Digital radiography is performed by a system consisting of the following functional components:
- A digital image receptor
- A digital image processing unit
- An image management system
- Image and data storage devices
- Interface to a patient information system
- A communications network
- A display device with viewer operated controls
The physical principles of digital radiography do not differ much from those of screen-film radiography. However, in contrast to screen-film radiography, in which the film serves as both detector and storage medium, digital detectors are used only to generate the digital image, which is then stored on a digital medium. Digital imaging comprises four separate steps: generation, processing, archiving, and presentation of the image.
The digital detector is exposed to x-rays generated by a standard tube. Ultimately, the energy absorbed by the detector must be transformed into electrical charges, which are then recorded, digitized, and quantified into a gray scale that represents the amount of x-ray energy deposited at each digitization locus in the resultant digital image. After sampling, postprocessing software is needed for organizing the raw data into a clinically meaningful image.
After final image generation, images are sent to a digitized storage archive. A digital header file containing patient demographic information is linked to each image. Although it is possible to print digital images as hard-copy film, the advantages of digital radiography are not realized completely unless images are viewed digitally on a computer workstation. Digital images can be manipulated during viewing with functions like panning, zooming, inverting the gray scale, measuring distance and angle, and windowing. Image distribution over local area networks is possible. Digital images and associated reports can be linked to a digital patient record for enhanced access to diagnostic data.
Direct conversion requires a photoconductor that converts x-ray photons into electrical charges by setting electrons free. Typical photoconductor materials include amorphous selenium, lead iodide, lead oxide, thallium bromide, and gadolinium compounds. The most commonly used element is selenium.
All of these elements have a high intrinsic spatial resolution. As a result, the pixel size, matrix, and spatial resolution of direct conversion detectors are not limited by the detector material itself, but only by the recording and readout devices used.
Selenium-based direct conversion DR systems are equipped with either a selenium drum or a flat-panel detector. In the former case, a rotating selenium-dotted drum, which has a positive electrical surface charge, is exposed to x-rays. During exposure, a charge pattern proportional to that of the incident x-rays is generated on the drum surface and is recorded during rotation by an analog-to-digital converter. Several clinical studies have confirmed that selenium drum detectors provide good image quality that is superior to that provided by screen-film or CR systems. However, because of their mechanical design, selenium drum detectors are dedicated thorax stand systems with no mobility at all.
A CCD is a light-sensitive sensor for recording images that consists of an integrated circuit containing an array of linked or coupled capacitors. X-ray energy is converted into light by a scintillator such as Tl-doped cesium iodide. The amount of light emitted is then recorded by the CCD, and the light is converted into electrical charges. Because the detector area cannot be larger than the CCD chip, it is necessary to combine several chips to create larger detector areas.
CCDs can be used for radiography as part of either a lens-coupled CCD system or a slot-scan CCD system. In lens-coupled CCD systems, an array consisting of several CCD chips forms a detector area similar to that of a flat-panel detector. Optical lenses are needed to reduce the area of the projected light to fit the CCD array. One drawback of the lens system is a decrease in the number of photons reaching the CCD, resulting in a lower signal-to-noise ratio and relatively low quantum efficiency.
Digital images consist of picture elements, or pixels. The two-dimensional collection of pixels in the image is called the matrix, which is usually expressed as length (in pixels) by width (in pixels). Maximum achievable spatial resolution (Nyquist frequency, given in cycles per millimeter) is defined by pixel size and spacing. The smaller the pixel size (or the larger the matrix), the higher the maximum achievable spatial resolution.
The overall detector size determines if the detector is suitable for all clinical applications. Larger detector areas are needed for chest imaging than for imaging of the extremities. In cassette-based systems, different sizes are available.
New storage phosphors and scanning systems are being investigated for use in Computed Radiography (CR). These phosphors are structured, since their crystals are grown in a needle shape, and are coated on a glass or aluminum substrate without any binding material between the crystals. This technique offers tighter phosphor packing and reduced pixel size, resulting in DQE values that are as high as those for indirect conversion flat-panel detector systems. In addition, images are scanned line by line with this system, resulting in shorter scanning times. Line scanners could also read out each pixel of a line for a longer time if scanning time is kept constant compared with that of a flying-spot scanner, which results in a higher signal being produced by the emitted light. Initial clinical studies in chest radiography with this system have shown equal quality with a state-of-the-art unstructured CR system with the exposure lowered to 50%.
The important advantage of digital imaging is cost and access. The hospitals save money from lower film cost, reduced requirement for storage space, and lesser staff required to run the services and archiving sections. The images are instantly available for distribution to the clinical services without the time and physical effort needed to retrieve film packets and reviewing previous imaging on a patient is much easier.
Spatial resolution was limited in earlier versions of CR but newer versions have overcome this problem. Flat panel CR is another technological advancement. The yield of electrons is five times as compared with CR and it gives a superior image quality and dose efficiency.
Solid state flat panel DR provides better quality than CR or SFR and at the same time requires a lower radiation dose. These are composed of x ray detector material superimposed on micro circuit array. The indirect version of this technology exhibits a much better signal to noise ratio. A portable version has also been devised. The direct DR version, amorphous selenium replaces the photo sensors. It is very useful for imaging of extremities and shows the trabecular bone pattern very well. The clinical utility of these recent developments is still under evaluation but it is probable that the overwhelming advantages offered by these newer modalities will lead to their widespread use.