Digital Radiography Review
By Professor Stelmark
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Computed Radiography
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Presently, an acceleration in the conversion from screen-film radiography (analog) to digital radiography (DR) is occurring. Digital imaging began with computed tomography (CT) and magnetic resonance imaging (MRI).
DR was introduced in 1981 by Fuji with the first commercial computed radiography (CR) imaging system. After many improvements that were made over the next decade, CR became clinically acceptable and today enjoys widespread use.
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To conduct a screen-film radiographic examination, one should first produce a paper trail of the study, then process the image with wet chemistry, and finally physically file the image after accepting that it is diagnostic. CR imaging eliminates some of these steps and can produce better medical images at lower patient dose.
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THE COMPUTED RADIOGRAPHY IMAGE RECEPTOR Many similarities have been observed between screen-film imaging and CR imaging. Both modalities use as the image receptor an x-ray sensitive plate that is encased in a protective cassette. The two techniques can be used interchangeably with any x-ray imaging system. Both carry a latent image, albeit in a different form, that must be made visible via processing.
Here, however, the similarities stop. In screen-film radiography, the radiographic intensifying screen is a scintillator that emits light in response to an x-ray interaction. In CR, the response to x-ray interaction is seen as trapped electrons in a higher-energy metastable state.
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Photostimulable Luminescence Some materials such as barium fluorohalide with europium (BaFBr:Eu or BaFI:Eu) emit some light promptly, in the way that a scintillator does following x-ray exposure. However, they also emit light some time later when exposed to a different light source. Such a process is called photostimulable luminescence (PSL).
The europium (Eu) is present in only very small amounts. It is an activator and is responsible for the storage property of the PSL. The activator is similar to the sensitivity center of a film emulsion because without it, there would be no latent image.
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Over time, these metastable electrons return to the ground state on their own. However, this return to the ground state can be accelerated or stimulated by exposing the phosphor to intense infrared light from a laser—hence the term photostimulable luminescence from a photostimulable phosphor (PSP).
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The PSP, barium fluorohalide is fashioned similarly to a radiographic intensifying screen. Because the latent image occurs in the form of metastable electrons, such screens are called storage phosphor screens (SPSs).
SPSs are mechanically stable, electrostatically protected, and fashioned to optimize the intensity of stimulated light. Some SPSs incorporate phosphors grown as linear filaments that enhance the absorption of x-rays and limit the spread of stimulated emission.
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Cross section of a photostimulable phosphor (PSP) screen
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Some storage phosphor screens (SPSs) incorporate phosphors grown as linear filaments that increase the absorption of x-rays and limit the spread of stimulated emission.
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Prof. Stelmark Review
Imaging Plate The PSP screen is housed in a rugged cassette and appears similar to a screen-film cassette . In this form as an image receptor, the PSP screen-film cassette is called an imaging plate (IP). The IP is handled in the same manner as a screen-film cassette; in fact, this is a principal advantage of CR. CR can be substituted for screen-film radiography and used with any x-ray imaging system. The PSP screen of the IP is not loaded and unloaded in a dark room. Rather, it is handled in the manner of a screen-film daylight loader.
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When an x-ray beam exposes a PSP, the energy transfer results in excitation of electrons into a metastable state. Approximately 50% of these electrons return to their ground state immediately, resulting in prompt emission of light, with wavelength λe.
The remaining metastable electrons return to the ground state over time. This causes the latent image to fade and requires that the IP must be read soon after exposure. CR signal loss is objectionable after approximately 8 hours.
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Prof. Stelmark Review
When the CR cassette is inserted into the CR reader, the IP is removed and is fitted to a precision drive mechanism. This drive mechanism moves the IP constantly, yet slowly (“slow scan”) along the long axis of the IP. Small fluctuations in velocity can result in banding artifacts, so the motor drive must be absolutely constant. While the IP is being transported in the slow scan direction, a deflection device such as a rotating polygon or an oscillating mirror deflects the laser beam back and forth across the IP. This is the fast scan mode.
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IMAGING CHARACTERISTICS Medical imaging with CR is not much different from that with screen-film imaging. A cassette is exposed with an existing x-ray imaging system to form a latent image. The cassette is inserted into an automatic processor (reader) and the latent image is made manifest.
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In CR and DR, it is not really a characteristic curve but rather an image receptor response function.
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However, the screen-film image can display only approximately 30 shades of gray on a viewbox. That is why radiographic technique is so critical in screenfilm imaging.
CR imaging is characterized by extremely wide latitude. Four decades of radiation exposure results in 10,000 gray levels, each of which can be evaluated visually by postprocessing. Proper radiographic technique and exposure are essential for screen-film radiography. Overexposure and underexposure result in unacceptable images. With CR, radiographic technique is not so critical because contrast does not change over four decades of radiation exposure.
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Prof. Stelmark Review
Prof. Stelmark Review
At this time, it should be emphasized that the conventional approach that “kVp controls contrast” and “mAs controls OD” does not hold for CR. Because CR image contrast is constant, regardless of radiation exposure, images can be made at higher kVp and lower mAs, resulting in additional reduction in patient radiation dose.
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The transition from screen-film radiography to CR brings several significant changes. Fewer repeat examinations should be needed because of the wide exposure latitude. Contrast resolution will be improved and patient dose may be reduced
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CR should be performed at lower techniques than screen-film radiography
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Prof. Stelmark Review
Digital Radiography
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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.
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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.
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Prof. Stelmark Review
Prof. Stelmark Review
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 radiationconversion 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.
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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).
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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.
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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.
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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 analogdigital converter. Even though CCD-based detectors require optical coupling and image size reduction, they are both widely available and relatively low cost
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Prof. Stelmark Review
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
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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.
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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.
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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.
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Prof. Stelmark Review
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.
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Prof. Stelmark Review
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.
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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.
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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.
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Prof. Stelmark Review
Digital Image Characteristic
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Digital Imaging In digital imaging, the latent image is stored as digital data and must be processed by the computer for viewing on a display monitor. Digital imaging can be accomplished by using a specialized image receptor that can produce a computerized radiographic image. Two types of digital radiographic systems are in use today: computed radiography (CR) and direct digital radiography (DR). Regardless of whether the imaging system is CR or DR, the computer can manipulate the radiographic image in various ways after the image has been created digitally.
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A unique characteristic of digital image receptors is their wide dynamic range. Dynamic range refers to the range of exposure intensities an image receptor can accurately detect; this means that moderately underexposed or overexposed images may still be of acceptable diagnostic quality. Because the image is constructed of digital data and is viewed on a display monitor, it can exhibit a wider range of brightness or densities. As a result, anatomic areas of widely different x-ray attenuation, such as soft tissues and bony structures, can be more easily visualized on a digital image.
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Digital images are composed of numeric data that can be easily manipulated by a computer. When displayed on a computer monitor, there is tremendous flexibility in terms of altering the brightness (density) and contrast of a digital image. The practical advantage of such capability is that, regardless of the original exposure technique factors (within reason), any anatomic structure can be independently and well visualized. Computers can also perform various postprocessing image manipulations to improve visibility of the anatomic region further.
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Prof. Stelmark Review
A digital image is recorded as a matrix or combination of rows and columns (array) of small, usually square, “picture elements” called pixels. The size of the pixel is measured in microns (0.001 mm). Each pixel is recorded as a single numeric value, which is represented as a single brightness level on a display monitor. The location of the pixel within the image matrix corresponds to an area within the patient or volume of tissue
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For a given anatomic area, or field of view (FOV), a matrix size of 1024 × 1024 has 1,048,576 individual pixels; a matrix size of 2048 × 2048 has 4,194,304 pixels. Digital image quality is improved with a larger matrix size that includes a greater number of smaller pixels Although image quality is improved for a larger matrix size and smaller pixels, computer processing time, network transmission time, and digital storage space increase as the matrix size increases.
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The numeric value assigned to each pixel is determined by the relative attenuation of x-rays passing through the corresponding volume of tissue. Pixels representing highly attenuating tissues, such as bone, are usually assigned a low value for higher brightness than pixels representing tissues of low x-ray attenuation.
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Prof. Stelmark Review
Prof. Stelmark Review
Each pixel also has a bit depth, or number of bits that determines the amount of precision in digitizing the analog signal and therefore the number of shades of gray that can be displayed in the image. Bit depth is determined by the analog-todigital converter that is an integral component of every digital imaging system. Because the binary system is used, bit depth is expressed as 2 to the power of n, or the number of bits (2n). A larger bit depth allows a greater number of shades of gray to be displayed on a computer monitor.
Binary digits are used to display the brightness level (shades of gray) of the digital image. The greater the number of bits, the greater the number of shades of gray, and the quality of the image is improved.
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A system that can digitize and display a greater number of shades of gray has better contrast resolution. An image with increased contrast resolution increases the visibility of recorded detail and the ability to distinguish among small anatomic areas of interest.
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A system that can digitize and display a greater number of pixels has better spatial resolution. An image with increased spatial resolution increases the visibility of recorded detail and the ability to resolve small structures.
Pixel density The number of pixels per unit area
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The distance measured from the center of a pixel to an adjacent pixel determines the pixel pitch or pixel spacing
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Increasing the pixel density and decreasing the pixel pitch increases spatial resolution. Decreasing pixel density and increasing pixel pitch decreases spatial resolution.
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Prof. Stelmark Review
Prof. Stelmark Review
An important performance characteristic of an ADC is the sampling frequency, which determines how often the analog signal is reproduced in its discrete digitized form. Increasing the sampling frequency of the analog signal increases the pixel density of the digital data and improves the spatial resolution of the digital image The closer the samples are to each other (increased sampling frequency), the smaller the sampling pitch, or distance between the sampling points Increased sampling frequency decreases the sampling pitch and results in smaller-sized pixels. The distance between the midpoint of one pixel to the midpoint of an adjacent pixel describes the pixel pitch. Spatial resolution is improved with an increased number of smaller pixels resulting in a more faithful digital representation of the acquired analog image.
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Prof. Stelmark Review
Another important ADC performance characteristic is degree of quantization or pixel bit depth, which controls the number of gray shades or contrast resolution of the image. During the process of quantization, each pixel, representing a brightness value, is assigned a numeric value. Quantization reflects the precision with which each sampled point is recorded. The pixel size and pitch determine the spatial resolution, and the pixel bit depth determines the system's ability to display a range of shades of gray to represent the anatomic tissues. Pixel bit depth is fixed by the choice of ADC, and CR systems manufactured with a greater pixel bit depth (i.e., 14-bit [214 can display 16,384 shades of gray]) improve the contrast resolution of the digital image.
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With digital systems, the computer creates a histogram of the data set. The histogram is a graph of the exposure received to the pixel elements and the prevalence of the exposures within the image. This created histogram is compared with a stored histogram model for that anatomic part; VOIs are identified, and the image is displayed.
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Lookup Tables Following histogram analysis, lookup tables provide a method of altering the image to change the display of the digital image in various ways. Because digital IRs have a linear exposure response and a very large dynamic range, raw data images exhibit low contrast and must be altered to improve visibility of anatomic structures. Lookup tables provide the means to alter the brightness and grayscale of the digital image using computer algorithms. They are also sometimes used to reverse or invert image grayscale. If the image is not altered, the graph would be a straight line. If the original image is altered, the original pixel values would be different in the processed image and the graph would no longer be a straight line but might resemble a characteristic curve for radiographic film
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Prof. Stelmark Review
The level of radiographic contrast desired in an image is determined by the composition of the anatomic tissue to be radiographed and the amount of information needed to visualize the tissue for an accurate diagnosis. For example, the level of contrast desired in a chest image is different from the level of contrast required in an image of an extremity.
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Radiographic contrast or image contrast is a term used in both digital and filmscreen imaging to describe the variations in brightness and density. In digital imaging, the number of different shades of gray that can be stored and displayed by a computer system is termed grayscale. Because the digital image is processed and reconstructed in the computer as digital data, its contrast can be altered. Radiographic film images are typically described by their scale of contrast or the range of densities visible. A film image with few densities but great differences among them is said to have high contrast. This is also described as short-scale contrast. A radiograph with a large number of densities but few differences among them is said to have low contrast. This is also described as long-scale contrast.
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Digital Image Manipulation
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Prof. Stelmark Review
Collimation and Partition If the x-ray exposure field is not properly collimated, sized, and positioned, exposure field recognition errors may occur. These can lead to histogram analysis errors because signal outside the exposure field is included in the histogram.
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Prof. Stelmark Review
Collimation of the projected area x-ray beam is important for patient radiation dose reduction and for improved image contrast in screen-film radiography. In DR, proper collimation has the added value of defining the image histogram. If improperly collimated, the histogram can be improperly analyzed.
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Digital image receptors normally can recognize even-numbered (i.e., two or four) x-ray exposure fields that are centered and cleanly collimated. Three on one and four on one are not recommended unless the unexposed portion is shielded.
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For the image histogram to be properly analyzed, each collimated field should consist of four distinct collimated margins. . The use of three collimated margins usually works, but when fewer than three are used, artifacts may result. If images are not collimated and centered, image receptor exposure will not be accurate and cannot be used for image quality evaluation.
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Prof. Stelmark Review
If multiple fields are projected onto a single IP, each must have clear, collimated edges and margins between each field. This process, called partitioning,
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Alignment Alignment of the exposure field on the IP is important in the same way and for the same reason as collimation.
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Preprocessing Before an image is prepared “for processing,” several manipulations of the output of an image receptor may be necessary to correct for potential artifacts. Such artifacts can occur because of dead pixels or dead rows or columns of pixels
Preprocessing of digital images is largely automatic.
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Preprocessing is designed to produce artifact-free digital images. In this regard, preprocessing provides electronic calibration to reduce pixel-to-pixel, row-to-row, and column-to-column response differences. The processes of pixel interpolation and noise correction are automatically applied with most systems.
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A single pixel or a single row or column normally will not interfere with diagnosis. However, many of these defects must be corrected. Correction algorithms specific to each type of digital image receptor use interpolation techniques to assign digital values to each dead pixel, row, or column. Interpolation is the mathematical process of assigning a value to a dead pixel based on the recorded values of adjacent pixels.
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Flatfielding is a software correction that is performed to equalize the response of each pixel to a uniform x-ray beam.
Exposure to a raw x-ray beam shows the heel effect on the image. B, Flatfielding corrects this defect and makes the image receptor response uniform.
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Computed radiography cassettes are highly sensitive to background radiation and scatter. If a CR cassette has not been used for several days, it should be inserted into the reader for re-erasure . The practice of leaving cassettes in a supposedly “radiation-safe” area in an x-ray room during an examination must be discouraged.
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Postprocessing
Postprocessing functions are computer software operations available to the radiographer and radiologist that allow manual manipulation of the displayed image. These functions allow the operator to adjust manually many presentation features of the image to enhance the diagnostic value.
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Background Removal or Shuttering Anytime a radiographic image is viewed, whether it is film/screen or digital, unexposed borders around the collimation edges allow excess light to enter the eye. Known as veil glare, this excess light causes oversensitization of a chemical within the eye called rhodopsin that results in temporary white light blindness. Although the eye recovers quickly enough so that the viewer recognizes only that the light is very bright, it is a great distraction that interferes with image reception by the eye. In film/screen radiography, black cardboard glare masks or special automatic collimation view boxes were sometimes used to lessen the effects of veil glare, but no technique has ever been entirely successful or convenient. In CR, automatic shuttering is used to blacken out the white collimation borders, effectively eliminating veil glare. Shuttering is a viewing technique only and should never be used to mask poor collimation practices.
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Electronic collimation has no effect on overall image quality or patient exposure.
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Prof. Stelmark Review
Image Orientation
Image orientation refers to the way anatomy is oriented on the imaging plate. The image reader has to be informed of the location of the patient’s head versus feet and right side versus left side. The image reader scans and reads the image from the leading edge of the imaging plate to the opposite end. The image is displayed exactly as it was read unless the reader is informed differently. Vendors mark the cassettes in different ways to help technologists orient the cassette in such a way that the image will be processed to display as expected. Fuji uses a tape-type orientation marker on the top and right side of the cassette. Kodak uses a sticker reminiscent of the film/screen cassette identification blocker. Some examinations, however, require unusual orientation of the cassette. In these cases, the reader must be informed of the orientation of the anatomy with respect to the reader. In DR, for which no cassette is used, the position of the part should correspond with the marked top and sides of the imaging plate.
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Image Stitching When anatomy or area of interest is too large to fit on one cassette, multiple images can be “stitched” together using specialized software programs. This process is called image stitching. In some cases special cassette holders are used and positioned vertically, corresponding to foot-to-hip or entire spine studies. Images are processed in computer programs that nearly seamlessly join the anatomy for display as one single image. This technique eliminates the need for large (36-inch) cassettes previously used in film/screen radiography.
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Prof. Stelmark Review
Prof. Stelmark Review
Image Annotation Many times, information other than standard identification must be added to the image. In screen/film radiography, time and date stickers, grease pencils, or permanent markers were used to indicate technical factors, time sequences, technologist identification, or position. The image annotation function allows selection of preset terms and/or manual text input and can be particularly useful when such additional information is necessary. (Function availability depends on the manufacturer.) The annotations overlay the image as bitmap images. Depending on how each system is set up, annotations may not transfer to PACS. Again, input of annotation for identification of the patient’s left or right side should never be used as a substitute for technologist’s anatomy markers.
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Prof. Stelmark Review
Magnification Two basic types of magnification techniques come standard with digital systems. One technique functions as a magnifying glass in the sense that a box placed over a small segment of anatomy on the main image shows a magnified version of the underlying anatomy. Both the size of the magnified area and the amount of magnification can be made larger or smaller. The other technique is a “zoom” technique that allows magnification of the entire image. The image can be enlarged enough so that only parts of it are visible on the screen, but the parts not visible can be reached through mouse navigation
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Prof. Stelmark Review
Subtraction is a technique that can remove superimposed structures so that the anatomic area of interest is more visible. Because the image is in a digital format, the computer can subtract selected brightness values to create an image without superimposed structures
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Black/white reversal is a postprocessing technique that reverses the grayscale from the original radiograph.
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Edge enhancement is a postprocessing technique that improves the visibility of small, high-contrast structures. Image noise may be slightly increased, however.
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Smoothing is a postprocessing technique that suppresses image noise (quantum noise). Spatial resolution is degraded, however.
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Windowing is a postprocessing technique that alters image brightness and contrast.
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Brightness Because the image is composed of numeric data, the brightness level displayed on the computer monitor can be easily altered to visualize the range of anatomic structures recorded. This adjustment is accomplished using the windowing function. The window level (or center) sets the midpoint of the range of brightness visible in the image. Changing the window level on the display monitor allows the image brightness to be increased or decreased throughout the entire range. When the range of brightness displayed is less than the maximum, the processed image presents only a subset of the total information contained within the computer
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Assume that pixel values from 0 to 2048 are used to represent the full range of digital image brightness levels. A high pixel value could represent a volume of tissue that attenuated fewer x-ray photons and is displayed as a decreased brightness level. Therefore, a low pixel value represents a volume of tissue that attenuates more x-ray photons and is displayed as increased brightness.
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Moving the window level up to a high pixel value increases visibility of the darker anatomic regions (e.g., lung fields) by increasing overall brightness on the display monitor. Conversely, to visualize better an anatomic region represented by a low pixel value, one would decrease the window level to decrease the brightness on the display monitor. Prof. Stelmark Review
A direct relationship exists between window level and image brightness on the display monitor. Increasing the window level increases the image brightness; decreasing the window level decreases the image brightness.
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Contrast Once the digital image is processed, radiographic contrast can be adjusted to vary visualization of the area of interest; this is necessary because the contrast resolution of the human eye is limited. The window width is a control that adjusts the radiographic contrast. Because the digital image can display shades of gray ranging from black to white, the display monitor can vary the range or number of shades of gray visible on the image to show the desired anatomy. Adjusting the range of shades of gray visible varies the image contrast. When the entire number of shades of gray are displayed (wide window width), the image has lower contrast; when a smaller number of shades of gray are displayed (narrow window width), the image has higher contrast
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Prof. Stelmark Review
Prof. Stelmark Review
A narrow (decreased) window width displays higher radiographic contrast, whereas a wider (increased) window width displays lower radiographic contrast.
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Fundamentals of PACS
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Picture Archiving and Communication Systems (PACS) As imaging departments move from film-based acquisition and archiving (hardcopy film and document storage) to digital acquisition and archiving (soft-copy storage), a complex computer network has been created to manage images. This network is called Picture Archiving and Communication Systems (PACS) and can be likened to a “virtual film library.” Images stored on digital media are housed in PACS archives.
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PACS is a sophisticated array of hardware and software that can connect all modalities with digital output (nuclear medicine, ultrasound, computed tomography, magnetic resonance imaging, angiography, mammography, and radiography). The acronym PACS can best be explained as follows:
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
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A PACS can accept any image that is in digital imaging and communications in medicine (DICOM) format, for which it is set up to receive, whether it is from cardiology, radiology, or pathology. A PACS serves as the fileroom, reading room, duplicator, and courier. It can provide image access to multiple users at the same time, on-demand images, electronic annotation of images, and specialty image processing.
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Prof. Stelmark Review
Prof. Stelmark Review
A PACS is often custom designed for a facility. The software is generally the same, but the components are arranged differently. Specific factors are involved in designing a PACS for an institution, such as the volume of patients, the number of areas where images are interpreted, the locations where images are viewed by physicians other than radiologists, and the money available for purchase.
Prof. Stelmark Review
The connection of various equipment types and modalities to a PACS is complex. Standards have been developed to ensure that all manufacturers and types of equipment are able to communicate and effectively transmit images and information. Current standards include DICOM (Digital Imaging Communications in Medicine) and HL7 (Health Care Level 7). Although standards may not always provide for an instantaneous functionality between devices, they do allow for resolution of connectivity problems.
Prof. Stelmark Review
DICOM is universally accepted industry standard for transferring radiologic images and their medical information between computers.
Prof. Stelmark Review
The HL-7 standard oversees most clinical and administrative data such as demographics, reports, claims, and orders. As with DICOM, HL-7 is composed of many parts and is used at many levels within various hospital systems. It is the standard generally used in communication between the hospital information system (HIS) and the radiology information system (RIS). The HIS holds the patient’s full medical information, from hospital billing to the inpatient ordering system. The RIS holds all radiology-specific patient data, from the patient scheduling information to the radiologist’s dictated and transcribed report.
Prof. Stelmark Review
Digital Imaging and Communications in Medicine (DICOM) is a communication standard for information sharing between PACS and imaging modalities. Health Level Seven standard (HL7) is a communication standard for medical information.
Prof. Stelmark Review
For optimum efficiency, the PACS should be integrated with the Radiology Information System (RIS) or the Hospital Information System (HIS). Because these information systems support the operations of an imaging department through exam scheduling, patient registration, report archiving, and film tracking, integration with PACS maintains integrity of patient data and records and promotes overall efficiency.
Prof. Stelmark Review
When a PACS is used, instead of hard-copy radiographs that must be processed, handled, viewed, transported, and stored, the soft-copy digital images are processed with the use of a computer, viewed on a monitor, and stored electronically. Most PACS use web browsers to enable easy access to the images by users from any location. Physicians may view these radiologic images from a personal computer at virtually any location, including their home.
Prof. Stelmark Review
ADVANTAGES OF PACS: • Elimination of less efficient traditional film libraries and their inherent problem of physical space requirements for hard-copy images
• Convenient search for and retrieval of images • Rapid (electronic) transfer of images within the hospital (e.g., clinics, operating rooms, treatment units) • Ease in consulting outside specialists—teleradiology. Teleradiography is the electronic transmission of diagnostic images from one location to another for purposes of interpretation and/or consultation.
• Simultaneous viewing of images at multiple locations • Elimination of misplaced, damaged, or missing films • Increase in efficiency of reporting exams with soft-copy images (compared with hard-copy images) • Reduction of the health and environmental impact associated with chemical processing, as a result of decreased Prof.use Stelmark Review
PACS fundamental parts: • Image acquisition • Display workstations •
Archive servers
Prof. Stelmark Review
Prof. Stelmark Review
Image Acquisition In modern radiology departments, most images are acquired in a digital format, meaning that the images are inherently digital and can be transferred via a computer network. Ultrasound, computed tomography (CT), magnetic resonance imaging (MRI), and nuclear medicine have been digital for many years and have been taking advantage of PACS far longer than general radiography has
Prof. Stelmark Review
Display Workstations A display workstation is any computer that a health care worker uses to view a digital image. It is the most interactive part of a PACS, and these workstations are used inside and outside of radiology. The display station receives images from the archive or from the various radiology modalities and presents them for viewing. The display workstation has PACS application software that allows the user to perform minor image-manipulation techniques to optimize the image being viewed. Some display stations have advanced software to perform more complex image-manipulation techniques
Prof. Stelmark Review
Prof. Stelmark Review
Common screen (display) resolution: • • • •
1280 × 1024 (1K) 1600 × 1200 (2K) 2048 × 1536 (3K) 2048 × 2560 (5K)
Prof. Stelmark Review
Display stations can be categorized by their primary use: primary reading stations for radiologists, review stations for referring physicians, technologist quality control (QC) stations where technologists review images, and image management stations for the file room personnel. Each of these workstations has one specific main purpose and is strategically located near the end-user of its designated purpose.
Prof. Stelmark Review
Mammography requires a 5K or 5-megapixel resolution to provide the viewing capacity needed. 2K monitor is used for CR and DR readings. 1K monitor is sufficient to view the images by a referring physician.
Prof. Stelmark Review
Radiologist Reading Stations The radiologist reading station is used by a radiologist when making a primary diagnosis. The reading station has the highest quality hardware, including the best monitor. The computer hardware meets the needs of the PACS vendor, but it will usually be very robust, requiring little downtime. The keyboard and mouse can be customized.
Prof. Stelmark Review
Archive Servers An archive server is the file room of the PACS. It is composed of a database server or image manager, short-term and long-term storage, and a computer that controls the PACS workflow, known as a workflow manager. The archive is the central part of the PACS and houses all of the historic data along with the current data being generated. In many institutions the archive serves as the central hub that receives all images before being released to the radiologists for interpretation.
Prof. Stelmark Review
Prof. Stelmark Review
Workflow Workflow is a term that can be used in any industry or in any organization. It simply means how a process is done, step by step. In radiology, we have always used the term workflow to describe how we complete an examination from order entry to transcribed report. The workflow in each radiology department is different because there are many variables.
Prof. Stelmark Review
Prof. Stelmark Review
Prof. Stelmark Review
Prof. Stelmark Review
The file room may also be responsible for correcting patient demographics. If images with incorrect demographics are sent to the archive, then it is difficult to pull those images the next time the patient comes in for an examination. The archive is a database and is only as good as the information that is put into it.
Prof. Stelmark Review
Navigation Functions Navigation functions are used to move through images, series, studies, and patients. The worklist is used to navigate through patients.
Prof. Stelmark Review
Hanging Protocols Once a patient has been selected from the worklist, the images load into the display software. In most PACSs, each user has the ability to set up custom hanging protocols. A hanging protocol is how a set of images will be displayed on the monitor. For example, when I select a CT examination, I want to view four images on each monitor, but when I view a CR image, I want to view one image on each monitor.
Prof. Stelmark Review
