Essentials of Digital Imaging

Transcription

1 Essentials of Digital Imaging Module 2 Transcript 2016 ASRT. All rights reserved.

2 Essentials of Digital Imaging Module 2 Processing 1. ASRT Animation 2. Welcome Welcome to Essentials of Digital Imaging Module 2 Processing. 3. License Agreement 4. Objectives After completing this module, you will be able to: Describe how photostimulable phosphor image receptors extract data. Discuss the analysis of image data extracted from image receptors. Identify and describe the most common exposure indicators for image detectors. Explain how automatic rescaling affects image quality. 5. Introduction In digital radiography, a receptor serves the same function as film. It receives the remnant x-ray beam after the beam passes through the patient. When electronic detector elements in the receptor absorb x- rays from the remnant beam, an electrical signal is produced. The electrical signal from the detector elements is an analog signal. The analog signal must be converted into digital data that can be used for image display, archiving, and transfer. 6. PSP Image Receptors Technologists work with 2 types of photostimulable phosphor, or PSP, image receptors. The cassettes for cassette-based PSPs are handled by technologists. The technologist identifies the cassette in the computer so that the plate can be inserted into the reader to extract the image data. In a cassette-less system, the reader is incorporated into the equipment. The plate stays in an upright or table Bucky and the technologists never actually touch it. The technologist can move the entire mechanism, but never physically inserts a plate into a reader. The data is extracted in both systems by using a flying-spot, line or a dual-sided scanner. Following exposure of a PSP plate, the plate must be scanned to extract the image data. These pictures show both sides of a PSP plate outside the cassette. The technologist doesn t routinely touch the imaging PSP plate. The front, or white side shown on the left, contains the photostimulable phosphors and holds the latent image until processing occurs in the image reader. The back side is black and has two bar-code stickers. The cassette containing the PSP is read immediately following the exposure. 7. PSP Plate Bar Code The bar-code sticker is an extremely important part of cassette-based digital radiography. Each PSP has a unique code that is associated with an individual patient and a specific exam. After scanning the PSP s bar code, the software of the plate-reading device associates the patient s information with a specific image. The scanned PSP is also associated with the processing code for a specific exam. Scanning the bar code is extremely important to ensure that the image links the patient to the correct exam. It doesn t matter when the bar code is scanned with the cassette-based digital imaging system. The PSP bar code may be scanned before or after an exam is performed. However, be careful when scanning multiple bar codes before an exam because a processing error occurs when a bar code isn t used for the designated scanned exam. For example, if a plate is scanned for a PA chest image but is used for a lateral chest radiograph, the image won t display properly because of incorrect processing codes. 8. PSP Plate Readers A technologist s workstation with bar-code scanning hardware and software may be located in an examination room, behind the control panel or in a centrally located area of the imaging department. The

3 department manager makes this decision based on current workflow, space requirements, distance from the main department and economics. In the picture on this slide, the bar-code scanning hardware and software, along with the plate reading device, are located near the control panel. There are varying possibilities in the configuration of a PSP plate reader. In some, one section is the reading area and another is the plate erasure area and the plate moves through the reader; however, not all readers transport the plate through the device. In some cassette-based readers, the cassette is stationary and the equipment moves around the plate as the plate is read. 9. PSP Image Extraction When an image is extracted at the phosphor crystal level an x-ray beam ionizes the crystal. The x-ray beam exposes the pixel area and the x-ray photons are absorbed. The resulting energy produces an electrical signal. Electrons are knocked out of their normal valence band and move up to the conduction band. As the electrons move across the band, some fall into the electron trap, or F-trap, and others drop back to the valence band. The PSP plate produces light when exposed to the x-ray beam, along with storing the electrons that the x-ray beam dislodges. The electrons left in the F trap form the latent image on the PSP receptor. At this time, a laser light strikes the plate and the F-trap releasing electrons, which in turn produce the photostimulated luminescence used to create the image. Often electrons are left behind in the F-trap and this is the reason why plates must be erased after each exposure. Exposing the plate to very bright white light releases the electrons left in the F-trap back to the valence band so the plate is ready to use again. 10. Light Guide A rotating polygonal mirror uses a point scanner to direct the laser beam to the PSP receptor. The rotating mirror allows the laser beam to scan across the PSP faster and with greater precision than a mechanical arm. The light guide assembly collects the light and directs it toward a device that converts the light photons into an electrical signal, such as a photomultiplier tube, photodiode, or a CCD. This signal is then sent to the analog-to-digital converter, or ADC, where it is converted to digital information. The light guide assembly is a single piece of highly engineered acrylic resin approximately 7 mm thick at its end. The device works much like fiber optic technology to move information from the PSP to the photomultiplier tube. The light the PSP emits during the image extraction process is not exceptionally bright. This is the reason most of the photons must be collected and then converted in the photomultiplier tube. This is also why routinely cleaning the light guide assembly is important to make a PSP plate reader operate efficiently. 11. The Scanning Process The mechanics of scanning a plate occur in one of two ways, depending on the manufacturer. The plate is either scanned with a flying spot or a line scan. The scanning process involves either moving the plate past the laser light or the laser moving across a stationary plate. Newer, dual-sided scanners read light from both sides of the plate. 12. Flying-spot Scanner When using a flying-spot scanner, the technologist inserts the plate and it moves through the reader. As the plate moves, the laser beam scans the entire receptor. Plates can be scanned at low-frequency or high-frequency sampling rates. It is possible to manipulate sampling frequency, depending on the equipment's manufacturer. Sometimes it's controlled by choosing the proper cassette. Other times, the reader may control the rates. No matter the option, technologists must understand that changing sampling frequency affects the spatial resolution of the PSP digital image. The dual-head flying-spot scanner reads dual-sided PSP plates. This configuration offers the benefit of reading data from both sides of the plate at the same time. Dual-sided reading increases the detective quantum efficiency (DQE) of the system and reduces image noise.

4 13. Line Scanner A cassette-less PSP system is more likely to use a line scanning process. In this system, one of two things occurs to extract data from the plate. The plate is either pulled underneath the linear scanner or the plate remains stationary and the laser scanner moves across the plate. The scanning process and data extraction are the same regardless of whether a system is cassettebased or cassette-less. The laser beam scans across the plate, which causes the electrons elevated to F- traps by the x-ray beam to drop back down into their normal orbit. When the electrons return to their normal orbit, they emit light. The light guide collects the light and directs it into a device that converts the light photons into an electrical signal. The device can be a photomultiplier tube, photodiode, or a CCD. The electrical signal is sent to an ADC where the signal is sampled and assigned a discrete numerical value. 14. PSP Receptor Erasure Not all the trapped electrons are released when the laser scans the plate. Because some electrons remain, it's essential that the plate is erased before its next use. It's also important to erase the PSP if you don t know the last time the plate was used, because the plate is sensitive to background radiation. Remember, these plates trap and store any energy from radiation that strikes them. The trapped electrons represent fog, and fog degrades images. An erasure lamp mechanism s high intensity light erases the PSP plate. Although it's not a terribly sophisticated device, when the bulbs are defective or dirty the device can t completely erase plates and subsequent images are of poor quality. 15. Knowledge Check 16. Knowledge Check 17. PSP Reader Configuration PSP plate readers come in a variety of configurations, depending on the manufacturer, and may be found in various places throughout the imaging department. PSP readers are more likely to be centrally located because of their high cost and need for periodic maintenance. But it s not unusual to find a PSP reader in an individual examination room if the location is distant from the main department. Although some equipment is designed to handle a single cassette, other PSP readers allow cassettes to be stacked so that multiple cassettes can be placed in a reader simultaneously, and the reader processes the plates in order. This feature is very convenient for a busy department with centrally located PSP readers. There are numerous bar code scanning devices and PSP plate reader options from a variety of manufacturers such as Fuji, Carestream, and Konica. No one configuration is recognized as better than the other. Each device looks and operates differently on the outside but the devices are very similar when it comes to internal function. For example, all plate readers require PSP cassettes to be inserted in the reader in a certain way. The insertion requirement is based on how the reader opens the cassette to pull out the plate and extract the image.

5 18. PSP Image Extraction Steps The steps a PSP plate reader takes during image extraction occur regardless of whether the plate is inside a cassette that a technologist manually loads into a reader or whether the plate is permanently installed inside a radiographic table. Once the plate is prepared for image extraction, a laser beam strikes the PSP plate and interacts with the phosphors located in the plate s phosphor layer. The energy trapped in the phosphor layer, or the latent image, is released in the form of light during this interaction. The light guide collects the light photons. The photons are then sent to a photomultiplier tube, photodiode, or CCD and then to the ADC, where the photons are converted into a digital signal. Any remaining energy on the plate is removed during the erasure process when exposing the plate to bright white light removes the remaining electrons. 19. Flat-panel Image Receptors The flat-panel detector often is used in cassette-less equipment, although there also are cassette-based models. The flat-panel image receptor contains a thin-film transistor or TFT with individual detector elements that collect the image. Flat-panel detectors can be either indirect or direct capture. As you can see from this illustration, the indirect detector uses a scintillator, usually cesium iodide (CsI) or gadolinium oxysulfide (Gd2O2S). The scintillator converts the x-rays to light and the amorphous silicon (asi) photodiode converts the light to electrons, that are then collected by the thin film transistor. The signal is sent to the ADC. The other indirect system also uses a scintillator to convert the x-rays to light. The light is directed to a CCD or CMOS where it is converted into electrons and sent to the ADC. The direct method of image capture, doesn t convert the x-rays to light. In this receptor, the x-rays strike an amorphous selenium (ase) photoconductor and are directly converted to electrical charges that are collected by the thin-film transistor and sent to the ADC. 20. Cassette-less PSP vs. Flat-panel Image Capture Cassette-less PSP systems or flat-panel receptors don t require all of the same steps during image capture and speed image extraction time. The technologist doesn t load a cassette into a plate reader, scan the bar code, or wait in line for image extraction. Instead of identifying the patient by scanning a bar code, the technologist must select the patient from a work list generated by the hospital information system or HIS, or radiology information system or RIS. Technologists also can key in patient information during an emergency situation, although this isn t recommended on a routine basis because it increases the chance of data entry errors.. Once the patient and exam are identified in the exam work list, the exposure is made and the image is extracted in a matter of seconds.

6 21. Thin-film Transistor The thin-film transistor is an important piece of equipment, regardless of whether it's used in a scintillator or a nonscintillator detector. Each square in the matrix is known as a detector element, or DEL. Each DEL is a pixel, or picture element. An image is created on a flat-panel detector over the thin film transistors. The DEL collects electrons that are extracted from the detector assembly. The electrons are converted into a digital value by an ADC. That process creates the image that displays on the monitor. DEL size controls the recorded detail, or spatial resolution, for the flat-panel device. The technologist can t change the size of the DEL, which is fixed for that piece of equipment. During the exposure, electrons are collected across the area of the image receptor that's exposed to radiation. Shortly after the exposure, the DELs read out the electrons in a sequential pattern that matches their location within the detector matrix. As the electrons are removed from the thin-film transistor array, they re sent to the ADC, which relays the digital signal to the computer. Once the DELs are read, the flatpanel detector refreshes and is ready for another exposure. 22. Flat-panel Detector Image Extraction The steps required for a flat-panel image receptor to extract an image from the detector elements that capture the image data begins with the exposure. Following exposure, the image data from each DEL is sent to the analog-to-digital converter in a sequential pattern based on where the DEL is located in a detector matrix. The ADC relays this information to a computer that reconstructs the image. As soon as the image data is sent to the ADC, the DEL is refreshed, or erased, and ready for a new exposure. 23. Knowledge Check 24. Knowledge Check 25. CCD and CMOS Image Extraction The charge-coupled device, or CCD, and complementary metal oxide semiconductor, or CMOS, need a scintillator to convert x-ray photons into light. When the light strikes the CCD, electrons are released by the CCD silicon in proportion to the light striking it. This process creates packets of charged electrons which are moved one row at a time by varying the voltage of adjacent rows. A CMOS, which is a chip with p-type (positive) and n-type (negative) semiconductor transistors, is another technology that converts light into electrical charges. Semiconductors are used in many computer memory systems. The charge in the CMOS is read row by row, and then, as with a CCD, the signal is sent to the ADC. 26. PSP Exposure Field Recognition To extract image data from a PSP image receptor a computer analyzes the image data that the ADC receives from the PSP plate. A key step is the computer recognizing collimation or the exposure field borders so it can reject any scatter or off focus radiation outside the borders. The plate reader scans the entire plate, so it scans both the exposed and unexposed areas. If the computer analysis fails to recognize an exposure field border, or collimation, it can create an image processing error. In other words, the computer looks at the entire plate, if the data analysis includes all the information outside the collimation border it can result in poor image quality. When the exposure field borders, or collimation borders, are recognized everything outside of the black should be clear. The fingers in these images represent two similarly placed exposure fields. However, notice how dark the area is where the fingers are located and all the gray area outside of the exposure field borders. In this instance, the exposure fields weren t recognized and a rescaling error occurred when the analysis incorporated the extra exposure outside the collimation field. 27. Flat Panel Exposure Field Recognition

7 To extract image data from an analysis of flat-panel receptor DEL values, much like the PSP receptor, the flat-panel receptor must identify the exposure field. The receptor recognizes the exposure field by the number of electrons present in the DEL. Unlike the PSP system, however, which looks at the entire plate, the flat-panel receptor only analyzes the exposed area inside the collimation borders. The exposure level controls the number of electrons, or electrical charges, that the DEL collects. A portion of the analysis involves looking for the uniform edge of the exposure field, represented by the difference between values found in adjacent DELs. The edge is recognized as a collimation border when the difference between the DELs indicates a welldefined boundary on the image receptor. When the exposure field is identified, the exposure outside that border is not included in creating the displayed image. 28. Histogram Formation Each digital image consists of a range of data values, from low to high exposures, depending on the technique and subject factors. Histograms graphically represent a collection of exposure values extracted from the receptor. The x axis, which is the horizontal line, indicates receptor exposure, and the y axis and vertical lines display the number of pixels for each exposure. The sampling frequency for PSP plates or the detector elements for flat-panel receptors affects the number of pixels in the matrix. The small red line, with an assortment of blue bars underneath it, represents digital values matching points on the line. Remember, the ADC must quantize, or convert, the continuous stream of electrons into unique digital values. The histogram provides a tally of the digital signal values extracted from the receptor. The pattern of values or histogram varies for each anatomical part. 29. Generating a Histogram A histogram is created for the PSP receptor beginning when the PSP plate is pulled into the reader. A finely-tuned laser beam strikes the plate and releases light that is sent through a light guide to the photomultiplier tube. Next, the analog signal from the photomultiplier tube is transmitted to an ADC where the signal is converted to unique digital values. The resulting histogram is the value distribution of the quantized data. Two formats can extract and analyze the data that create the final digital image viewed on a monitor. The first format is called a priori histogram analysis, and the second is the neural histogram analysis. 30. A Priori Histogram Analysis A priori histogram analysis involves comparing the exposure data set to a single standardized exposure data set for a matching examination. The standardized values for each examination were derived during experiments that used similar subjects under ideal conditions. There are 3 varieties of a priori histogram analysis; type 1, type 2, and type Type 1 Histogram Analysis The type 1 histogram analysis requires 2 specific values. The first represents the greatest attenuator, bone, which signifies the minimum value in the histogram. The other specific value represents the area outside the patient where unattenuated radiation strikes the receptor. The values of interest, or VOI, that is, the exposure values that are manipulated to form the image, occur between these 2 values. This ankle is a classic image created using a type 1 histogram. The exposure distributions range from completely white, representing the maximum attenuation by bone, to the dark areas past the skin line where unattenuated radiation hit the receptor. 32. Type 2 Histogram Analysis

8 The type 2 histogram analysis displays the values of interest from the maximum attenuator up through the maximum value of the main histogram. But in this case, the skin line doesn t represent the maximum value. Type 2 histograms are used for torso, spine, skull, and pelvic examinations. A type 2 histogram analysis was used to process this image of the abdomen. The maximum attenuator is the spine and has the brightest displayed intensity; the least attenuating structure appears darker on the display because it receives the most radiation. In this particular case, a skin line is not important, so the anchoring point for the maximum intensity is represented by the least attenuating structure. 33. Type 3 Histogram Analysis The type 3 histogram is unique because it takes into account a significant attenuating object, such as a prosthetic hip or a stomach full of barium. The attenuating object adds an additional section to an otherwise normally distributed histogram. On the histogram shown here, the far left peak represents the values for barium or metal. To accurately display the image, we must exclude that area from the values of interest and show the maximum attenuator as bone and the least-attenuating object as bowel gas, lung tissue, or some other structure. The least-attenuating structure represents the high end of our values of interest. If the high-attenuation object, such as barium, is included in the values of interest, it affects image processing. This image shows barium and air in the stomach. When the image is processed by the computer, the barium should not be included as a value of interest or the image won t display properly. 34. Neural Histogram Analysis The neural histogram analysis differs from the a priori analysis in that predefined data is matched with the exposure data extracted from the receptor. With this type of analysis, the image data is compared to 2 or more predefined histograms. The predefined histogram that most closely matches the data extracted from the image receptor is used to process the image for display. A series of 4 pediatric chest radiographs can result in 4 histograms that are similar in shape but differ based on the anatomy included in the collimated field. Below each histogram displayed here is the anatomy included in the collimated field that produced each graph. Notice how the varying pixel values of the digital images translate into differently shaped neural histograms. 35. Knowledge Check 36. Knowledge Check 37. Histogram summary Each anatomical part generates a different histogram, and the computer stores the appropriate histogram for each part. The analysis compares the minimum and maximum values in the image histogram to a standardized stored histogram and its pixel values for that type of examination. If the captured values don t match the standardized values, the captured values are rescaled to match the standardized ones as closely as possible to display an acceptable image. The process of matching the captured image values to standardized values is called automatic rescaling. 38. Automatic Rescaling The values of interest used to create the image are predetermined based on the reference histogram. Automatic rescaling makes data output and image display consistent even when there are errors in exposure technique. At the bottom of the graph, the white line is a reference histogram for a specific body part when the correct exposure is delivered to an image receptor. The histogram in black simulates the data collected when an image receptor is overexposed resulting in higher exposure values.

9 39. Automatic Rescaling Exposure is measured in mr on the graph s horizontal axis. The vertical axis represents the signal strength for each exposure level. The oblique line demonstrates the receptor s response to the specific units of exposure on the horizontal axis. On the right is a graph that shows the data output presented to a technologist on a digital display monitor. Reference lines indicate where the white proper exposure data appears in the exposure range. The raw data value peak on the display monitor is indicated where the white exposure reference line intersects with the oblique line representing the receptor luminescence levels. Follow the tracing for the properly exposed histogram to see the tracings for an overexposure. First, notice that the black line tracing for the overexposure falls within the image receptor s exposure and response range. Now there is a data range for the proper exposure and overexposure plotted on the raw digital output scale of values from 0 to 1,023. The overexposure plot follows the same slope as the proper exposure plot, but at a higher amplitude, or height. The program automatically rescales the image when it detects this gap between expected and received amplitude. An automatic adjustment of the overexposure data plot, which brings the overexposure data and histogram in line with the proper exposure histogram. The image viewed on the display monitor doesn t look overexposed, but in fact, the brightness and gray-scale levels appear as expected. 40. Automatic Rescaling The radiograph of a hand on the left is the result of excessive exposure to the receptor. This image wasn t rescaled. The image on the right displays the same image with rescaling. The image now is acceptable. In essence, rescaling created an acceptable image, despite overexposure to the receptor. Rescaling the image pixel values to display properly doesn t prevent overexposure to the patient. The technologist has no visual cue that overexposure occurred. With an analog system, a technologist would have seen the image on the left as it came out of the processor and the excessive density on the image would be a visual cue to repeat the exam. 41. Dose Creep The term dose creep refers to the potential to gradually increase patient exposure over time. It can occur when a technologist lacks visual feedback showing that additional radiation is being used to produce the images. Technologists are responsible for maintaining standards within a department to limit patient exposure, and must look to sources other than displayed images to detect dose creep. 42. Look-up Table Automatic rescaling of the histogram compensates for overexposure and underexposure. The final step before display is to set the gray scale, or contrast, for the image. Contrast is set by using the look-up table or LUT. The graph of the receptor response to x-ray exposure demonstrates a linear response, which results in a low-contrast or gray image. The LUT can change the linear representation to a curve. The LUT changes the numerical values of the pixels to the contrast levels that are best for each anatomical part. For example, the matrix of pixel values that would display as low contrast, after the LUT is applied, change from 40 to 20 and from 60 to 80, resulting in higher contrast than the original. Automatic rescaling and the LUT work best if the image is acquired with the correct technique factors, collimation, and source-to-image receptor distance for the anatomical part. 43. PSP Exposure Indicators Exposure indicators, or EIs, are used with several types of PSP image receptors. Most digital systems provide an EI that technologists can use to determine the approximate level of radiation exposure to a receptor since there are no visual cues to show over or under exposure.

10 A histogram analysis of the distribution of radiation intensities is used to calculate an EI. Remember that the histogram represents a frequency distribution of discrete values within an image. The EI value is not absolute; the value represents an approximation of the dose a receptor receives. 44. Calculating EI with Histograms This ankle image is an example of using a type 1 histogram to help detect bony structures within the image and the outermost edge, or skin line, of a patient. The labels for bone and skin line on this type 1 histogram represent the endpoints of the values of interest. The EI is calculated using a mathematical analysis. A mathematical analysis determines the EI based on the distribution of radiation intensities between those two values. Any changes in the values of bone and skin line results in a change to the EI. If the histogram is shifted to the right, more exposure was used, so the EI would reflect a high exposure value. Here s an example of the 2 points used to calculate the EI of a type 2 histogram. The type 2 histogram is useful when a skin line isn t present in the image field. Notice that point 2 on this histogram is in a different position than it was on the type 1 analysis. The values of interest for a type 2 histogram analysis are different than the values of interest for a type 1 histogram analysis. Therefore, when a type 1 image is processed as a type 2 image, you get an error or the incorrect EI. This example of a type 3 histogram includes an attenuator which in this case is barium. The attenuator is represented by the large peak on the left of the histogram. Notice that the number 1 is located to the right of the attenuating object. If a type 3 histogram analysis wasn t used, the number 1 would include the low level exposure in the histogram analysis. This low level of exposure would have indicated to the computer that the image was underexposed and caused an error in the EI calculation. The inaccurate calculation also could make the image display incorrectly. The exposure indicator must be calculated over specific values of interest or it will produce a calculation error. 45. Manufacturer-specific PSP Exposure Indicators Manufacturers use different names for their exposure indicators and slightly different parameters. The variation in EIs can be confusing and makes it difficult to compare systems. The American Association of Physicists in Medicine, or AAPM, has made a concerted effort to standardize EIs. Fuji and Konica use a sensitivity number to represent the EI which is called an S number. With S-number EIs, when exposure to the receptor increases, S-number indicator values decrease. Philips uses an exposure index abbreviated EI that is inversely proportional to exposure. As the exposure to the receptor increases, the EI value decreases. Carestream, formerly known as Kodak, uses an exposure indicator known as the exposure index, and also is abbreviated by the manufacturer as EI. With the exposure indicator, as exposure to the receptor increases, EI values increase. 46. Flat-panel Detector Exposure Indicators Flat-panel exposure indicators vary by manufacturer. Phillips uses a value called the EI. The higher the EI value, the higher the dose a receptor receives. Siemens uses a value called the EXI. The EXI value directly relates to the exposure level the receptor receives. Essentially, increasing the exposure increases the EXI value. Canon uses a value known as the REX. The REX value also directly relates to the exposure level that the receptor receives, which means increasing the exposure increases the REX value. 47. Exposure Indicator Guidelines The AAPM has developed a standard exposure indicator that can be used with all digital radiographic imaging systems. It is intended to display radiation exposure to a receptor in order to obtain high quality images with acceptable patient dose. The term used for the exposure indicator is deviation index, or DI.

11 The DI will indicate whether or not the correct technique was used to acquire the image. A deviation index, or DI, of 0 is an ideal exposure, but the target range is anywhere between -0.5 to A DI of +1.0 to +3.0 indicates overexposure. If the anatomy of interest is too dark, consider repeating the image. Any DI greater than +3.0 suggests excessive radiation exposure to the patient. Again, a repeat radiograph should only be performed if the anatomy of interest is too dark. Underexposure is represented by a DI of or less. The radiologist reading the image will determine is a repeat exam is necessary. If the DI falls below -3.0, a repeat radiograph is required. 48. Exposure Indicator Miscalculation The EI is calculated based on an analysis of how values are distributed within the histogram. An abnormality in that distribution can result in an EI calculation error. It also may cause the image to display incorrectly, either as underexposed, overexposed, or of poor contrast. For example, the EI could be miscalculated if the collimation margins, or exposure fields, aren t recognized. In this case, values outside the collimated borders are included in the distribution used to create the EI and caused an error in image display. An unexpected exposure variation recorded on the receptor, that is, an unexpected attenuator, can change how values are distributed in the histogram. The result can alter the EI and produce a rescaling error that distorts the displayed appearance of the image. 49. Attenuating Material An attenuating material, such as lead, is an unexpected object that could potentially cause a miscalculation of the EI, as well as alter the appearance of the image. To display a normal chest radiograph the histogram needs to include values for the unexpected attenuator. A prosthetic device can impact the distribution of pixel intensities in a histogram. The area of the prosthesis in the histogram represents an underexposed area of the image and is included in the calculation of the EI. The end result is that the EI shows a decrease in receptor exposure when in fact the image was properly exposed. The addition of metal into the image data set alters the acquired image histogram compared with the standard histogram of a knee without a prothesis. Although not shown above, the result is an image that does not display with correct brightness and contrast and a miscalculation of the EI. The EI indicates the image was underexposed. In the radiograph shown above, the image brightness and contrast has been corrected to prevent an additional exam from being performed. 50. PSP Exposure Indicator Ranges Optimal EI ranges for digital systems are best set by the radiologist, who can evaluate the level of noise within a set of images. Radiologists determine the acceptable level of noise to set up the EI levels for each department. This process is similar to the way images are evaluated in the analog environment to tailor images to the preferences of the radiologists responsible for image interpretation. A radiologist s preference may drive the threshold for acceptable vs excessive image noise for a department. 51. Dose Area Product Meter Currently, there are no standards established for using a dose area product, or DAP, meter. The DAP meter is a device that might be linked to the x-ray unit. It measures the actual patient entrance skin dose with accurately calibrated equipment. Although the DAP meter helps determine entrance skin dose, it doesn t indicate the amount of exposure delivered to the image receptor. The DAP meter calculates the dose by determining the entrance dose in centigray multiplied by the field area, or exposure field size, used to create the image. The meter is located in the collimator box and is commonly used with cassette-less systems. It has a detector interface with the generator. Because the DAP meter measures the entrance skin exposure delivered to the patient, DAP readouts can be considered part of a patient's image record.

12 52. Flat-panel Detector Dose Area Product Reducing the exposure field size affects the entrance skin exposure to the patient as measured by a DAP meter. The problem with the DAP meter is that the readout depends on collimated beam size, which can vary according to the examination or the facility. Changing the collimated field results in different DAP readout values. DAP meters require frequent calibration, and defining a proper DAP value also is a problem. Patient exposure level doesn't equal an appropriate receptor exposure. 53.Exposure Indicator When technologists view digital images, they can t rely on the appearance of the image for visual feedback regarding exposure to the patient. However, given the EI of the digital image receptor, technologists can determine the accuracy of technical factors used to expose the receptor. The EI is a way to let the technologist know whether the correct exposure technique was used and, therefore, is a valuable tool to evaluate radiation protection quality control. If department EIs are chronically high, it indicates possible overexposure of patients and poor technique selection. Carefully monitoring EI values helps to ensure that radiation exposure to the patient is as low as reasonably achievable. But, remember that EIs are not measurements of actual patient dose. 54. Knowledge Check 55. Knowledge Check 56. Conclusion This concludes Essentials of Digital Imaging Module 2 Processing. You should now be able to: Describe how photostimulable phosphor image receptors extract data. Discuss the analysis of image data extracted from image receptors. Identify and describe the most common exposure indicators for image detectors. Explain how automatic rescaling affects image quality. 57. References AAPM Report No An exposure indicator for digital radiography. July American Association of Physicists in Medicine website. Accessed February 14, Bushong SC. Radiologic Science for Technologists: Physics, Biology, and Protection. 10th ed. St Louis, MO: Mosby Elsevier; Carlton RR, Adler AM. Principles of Radiographic Imaging: An Art and A Science. 5th ed. Clifton Park, NY: Thomson Delmar Learning; Carroll QB. Radiography in the Digital Age: Physics, Exposure, Radiation Biology. Springfield, IL: Charles C Thomas; Carter CE, Vealé BL. Digital Radiography and PACS. St Louis, MO; Mosby Elsevier; Practice Standards for Medical Imaging and Radiation Therapy. Radiography practice standards. June 19, American Society of Radiologic Technologists website. Accessed February 14, Seeram E. Digital Radiography: An Introduction for Technologists. Clifton Park, NY: Delmar Cengage Learning; Strategic Document. Version , April 11, Digital Imaging and Communications in Medicine website. Accessed February 14, 2013.