Digital Radiography and PACS By Professor Stelmark

THE ACCELERATION to all-digital imaging continues because it provides several significant advantages over screen-film radiography. Screen-film radiographic images require chemical processing, time that can delay completion of the examination. After an image has been obtained on film, little can be done to enhance the information content. When the examination is complete, images are available in the form of hard copy film that must be catalogued, transported, and stored for future review. Furthermore, such images can be viewed only in a single place at one time.

Unlike CR, DR is hard-wired to the image processing system and is cassetteless. In DR detectors, the materials used for detecting the x-ray signal and the sensors are permanently enclosed inside a rigid protective housing. Thin-film transistor (TFT) detector arrays may be used in both direct- and indirect-conversion detectors.

Direct Conversion In direct conversion, x-ray photons are absorbed by the coating material and immediately converted into an electrical signal. The DR plate has a radiation- conversion material or photoconductor, typically made of a-Se. This material absorbs x-rays and converts them to electrons, which are stored in the TFT detectors. The thin-film transistor (TFT) is a photosensitive array made up of small (about 100 to 200μm) pixels. Each pixel contains a photodiode that absorbs the electrons and generates electrical charges.

Indirect Conversion Indirect-conversion detectors are similar to direct detectors in that they use TFT technology. Unlike direct conversion, indirect conversion is a two-step process: x-ray photons are converted to light, and then the light photons are converted to an electrical signal. A scintillator converts x-rays into visible light. That light is then converted into an electric charge by photodetectors such as amorphous silicon photodiode arrays or charge-coupled devices (CCDs).

X-ray photons striking the dielectric receptor are absorbed by a scintillation layer in the imaging plate that converts the incident x-ray photon energy to light. A photosensitive array, made up of small (about 100 to 200μm) pixels, converts the light into electrical charges. Each pixel contains a photodiode that absorbs the light from the scintillator and generates electrical charges.

Amorphous Silicon Detector This type of flat-panel sensor uses thin films of silicon integrated with arrays of photodiodes. These photodiodes are coated with a crystalline cesium iodide (CsI) scintillator or a rare-earth scintillator (terbium-doped gadolinium dioxide sulfide). When these scintillators are struck by x-rays, visible light is emitted proportionate to the incident x-ray energy. The light photons are then converted into an electric charge by the photodiode arrays. Unlike the selenium-based system used for direct conversion, this type of indirect-conversion detector technology requires a two-step process for x-ray detection. The scintillator converts the x-ray beams into visible light, and light is then converted into an electric charge by photodetectors, such as amorphous silicon photodiodes.

Cesium Iodide Detector The oldest indirect-conversion DR system is based on charge-coupled devices (CCDs). X-ray photons interact with a scintillation material, such as photostimulable phosphors, and this signal is coupled, or linked, by lenses or fiberoptics that 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 4cm2) CCDs that 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- digital converter. Even though CCD-based detectors require optical coupling and image size reduction, they are both widely available and relatively low cost

Developed by NASA, complementary metal oxide silicon (CMOS) systems use specialized pixel sensors that, when struck with x-ray photons, convert the x-rays into light photons and store them in capacitors

Detective Quantum Efficiency How efficiently a system converts the x-ray input signal into a useful output image is known as detective quantity efficiency (DQE). DQE is a measurement of the percentage of x-rays that is absorbed when they hit the detector. The linear, wide-latitude input/output characteristic of CR systems relative to screen/film systems leads to a wider DQE latitude for CR, 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 screen/film systems. In other words, CR records all of the phosphor output. Systems with higher quantum efficiency can produce higher quality images at lower dose.

Both indirect and direct DR capture technology has increased DQE over CR. However, DR direct capture technology, because it does not have the light conversion step and consequently no light spread, increases DQE the most. There is no light to blur the recorded signal output; less dose is required than for CR; and higher quality images are produced. Newer CMOS indirect DR capture systems may be equal to direct image acquisition because of the crystal light tubes, which also prevent light spread.

Dynamic Range The dynamic range of the digital imaging system refers to the ability of the detector to capture accurately the range of photon intensities that exit the patient. Compared with film-screen detectors, digital IRs have much larger exposure latitude (wide dynamic range). In practical terms, this wide dynamic range means that a small degree of underexposure or overexposure would still result in acceptable image quality. This characteristic of digital receptors is advantageous in situations where automatic exposure control (AEC) is not normally available, such as in portable radiography.

Signal-to-Noise Ratio Signal-to-noise ratio (SNR) is a method of describing the strength of the radiation exposure compared with the amount of noise apparent in a digital image. Image noise is a concern with any electronic digital image. Because the photon intensities are converted to an electronic signal that is digitized by the ADC, the term signal refers to the strength or amount of radiation exposure captured by the IR to create the image. Increasing the SNR improves the quality of the digital image. Increasing the SNR means that the strength of the signal is high compared with the amount of noise, and therefore image quality is improved. Decreasing the SNR means there is increased noise compared with the strength of the signal, and therefore the quality of the radiographic image is degraded. Quantum noise results when there are too few x-ray photons captured by the IR to create the latent image. In addition to quantum noise, sources of noise include the electronics that capture, process, and display the digital image.

Detector Size Detector size is critical. Detectors must be large enough to cover the entire area to be imaged and small enough to be practical. For chest x-rays, the detector field needs to be at least 17 × 17 inches so that both lengthwise and crosswise examinations are possible. Special examinations such as leg length and scoliosis series may require dedicated detectors.

Spatial Resolution Depending on the detector’s physical characteristics, spatial resolution can vary a great deal. Spatial resolution of a-Se for direct detectors and CsI 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. Digital images can be processed to alter apparent image sharpness; however, excessive processing can lead to an increase in perceived noise. The best resolution will be achieved by using the appropriate technical factors and materials.

Pixel Size and Matrix Size The amount of resolution in an image is determined by the size of the pixels and the spacing between them, or pixel pitch. 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. Typically, 2000 pixels/row are adequate for most diagnostic examinations.

The Picture Archiving and Communication System (PACS) is an integral part of the digital radiology imaging department. PACS is an electronic network for communication between the image acquisition modalities, display stations, and storage. For these different systems to communicate with each other, a common language is necessary. The Digital Imaging and Communications in Medicine (DICOM) standard was first formulated in 1983 for this purpose.

P—Picture: the digital medical image(s) A—Archiving: the “electronic” storage of the images C—Communication: the routing (retrieval/sending) and displaying of the images S—System: the specialized computer network that manages the complete system