Chapter 10: Digital Image Receptors PDF

Chapter 10 Digital Image Receptors Copyright © 2020 by Elsevier Inc.  Objectives  Describe the design of computed radiography detectors  Describe the design of direct radiography detectors  Explain the process of...

Chapter 10 Digital Image Receptors Copyright © 2020 by Elsevier Inc.  Objectives  Describe the design of computed radiography detectors  Describe the design of direct radiography detectors  Explain the process of image acquisition using computed radiography detectors  Explain the process of image acquisition using the three general types of direct radiography detectors  Explain the process of image extraction and processing for computed radiography and direct radiography systems  Describe digital image display and postprocessing functions  Explain the use of exposure indicators for computed radiography systems and dose-area product for direct radiography systems  Identify quality control tests and test patterns used with digital systems  Describe the Picture Archiving and Communication System, including its role, principal systems, and challenges Copyright © 2016 by Elsevier Inc. 2  Introduction  Digital  Radiography is the last of the medical imaging modalities to make this transition to digital technology.  With the advancement in technology, it is important for radiographers to learn and stay updated on new concepts and practices Copyright © 2020 by Elsevier Inc. 3  Digital Receptors  Digital imaging is not a new concept.  Computed tomography (CT), sonography, nuclear medicine, and magnetic resonance imaging (MRI) have been digital for some time.  Digital receptor type  Computed radiography (CR) Systems that use storage phosphors to temporarily store energy representing the image signal  Direct radiography (DR) Systems that have detectors that directly capture and read out an electronic image signal Copyright © 2020 by Elsevier Inc. 4  Detector Types: Computed Radiography Systems (CR)  Can be integrated with existing radiographic equipment.  Automatic exposure control (AEC) must be recalibrated.  Preprogrammed techniques in the anatomically programmed radiography (APR) and technique charts must be adjusted. Copyright © 2020 by Elsevier Inc. 5 Computed Radiography Systems (CR)  Primary parts of a cassette-based system:  Cassette  Photostimulable phosphor (PSP) plate  Plate reader  Computer workstation Copyright © 2020 by Elsevier Inc. 6 Computed Radiography Systems (CR)  Cassette  A container for the PSP plate  Made of lightweight plastic  Lined with felt material  Backed with a sheet of aluminum Copyright © 2020 by Elsevier Inc. 7 Computed Radiography Systems (CR)  PSP plate layers:  Protective layer  Phosphor layer  Reflective layer  Conductive layer  Color layer (in some newer plates)  Support layer  Backing layer Copyright © 2020 by Elsevier Inc. 8  Phosphor Layer Arrangement A turbid type phosphor A structured phosphor layer. layer with columnar phosphor crystals Copyright © 2016 by Elsevier Inc. 9 Computed Radiography Systems (CR)  Plate response to x-ray exposure  X-rays ionize phosphor atoms and ~50% of electrons are trapped in the conduction band as a latent image.  The quantity and distribution of the liberated electrons is proportional to the x-ray exposure received in each particular area of the plate.  During processing, the energy of trapped electrons is released by exposure to a laser. Copyright © 2020 by Elsevier Inc. 10 Computed Radiography Systems (CR)  General reader design  Drive mechanism moves the plate through the laser scanning process (in some the laser moves)  Optical system is made up of a laser, beam-shaping optics, light-collecting optics, and optical filters, to project and guide a precisely controlled laser  A photodetector senses the light released during scanning  Analog-to-digital converter (ADC) converts light to an electronic signal  Computer processes and displays the image Copyright © 2020 by Elsevier Inc. 11  Extraction and Processing for CR  Image extraction: slow scan/fast scan Copyright © 2020 by Elsevier Inc. 12  Extraction and Processing for CR  As the electrons, liberated by the laser, return to their shells, they release excess energy as light that is directed to the photodetector via a fiber- optic bundle or a solid, light-conducting material.  The photodetector then amplifies the light energy and converts it to an electronic signal.  This signal is then passed through an analog-to- digital converter (ADC) where it is digitized. Copyright © 2020 by Elsevier Inc. 13  Extraction and Processing for CR  In digitizing the analog signal, it is divided into a matrix.  The size of the matrix determines the resolution: the larger the matrix, the greater the number of pixels and the greater the resolution.  The scanning of the plate results in a continuous signal being sent to the photodetector and onto the ADC for sampling and quantization. Copyright © 2020 by Elsevier Inc. 14  CR Sampling  Sampling Frequency: the frequency at which a data sample is acquired from the detector and is expressed as sampling pitch.  The closer the samples are to each other, the greater the sampling frequency.  Sampling pitch: describes how digital detectors sample the x- ray exposure.  They do so discretely—that is, at specific locations separated by specific intervals.  For CR systems, the sampling pitch is the distance between laser beam positions during processing of the plate, and for DR systems it is the distance between adjacent detector elements (DELs).  The distance between the center of one pixel and the center of an adjacent pixel is called pixel pitch and is measured in microns. Copyright © 2016 by Elsevier Inc. 15 CR Sampling Frequency & Pitch Copyright © 2016 by Elsevier Inc. 16  Extraction and Processing for CR  The image is digitized by both location (spatial resolution) and intensity (grayscale) of each part of the signal.  Grayscale is assigned during the process of digitizing the image.  Bit depth is the available grayscale.  The number of photons detected within a given pixel determines the shade of gray it displays.  The computer of the CR reader puts the image data through a series of steps to create the displayed image. Copyright © 2020 by Elsevier Inc. 17 Computed Radiography (CR) Systems Copyright © 2016 by Elsevier Inc. 18  Direct Radiography Systems (DR)  DR systems  Image-forming radiation is captured and transferred to a computer from the detector array for viewing at the control panel.  DR categories  Indirect capture  Direct capture Copyright © 2020 by Elsevier Inc. 19  Direct Radiography Systems (DR)  Charge-coupled device (CCD), x- ray scintillator, and optics  Indirect capture  Tiling: a process in which several CCD detectors abut to create one larger detector Copyright © 2020 by Elsevier Inc. 20  Direct Radiography Systems (DR)  Scintillator with cesium iodide or gadolinium oxysulfide as the phosphor, photodetectors, and a thin-film transistor (TFT) array  Indirect capture Copyright © 2020 by Elsevier Inc. 21  Direct Radiography Systems (DR)  Indirect capture methods cause a loss of resolution.  The direct capture method avoids this problem by not using a scintillator.  Photoconductor and TFT array Copyright © 2020 by Elsevier Inc. 22  Image Acquisition, Extraction and Processing, and Display  Acquisition  Histogram: a graphic representation of the data set  Histogram analysis A priori model Neural analysis model Copyright © 2020 by Elsevier Inc. 23  Extraction and Processing for DR  Image extraction for DR systems  There are two indirect methods and one direct method.  One indirect method uses a cesium iodide phosphor plate as the scintillator coupled to a CCD by a fiber- optic bundle or optical lenses. The x-ray energy is absorbed by the scintillator and converted to light. The light is transmitted to the CCD and an electronic signal is created. This analog signal is passed through an ADC where it is digitized. Copyright © 2020 by Elsevier Inc. 24  Extraction and Processing for DR  The other indirect method uses either a cesium iodide or a gadolinium oxysulfide scintillator coupled to a photodetector and a TFT.  The panel is configured as a network of pixels with each pixel containing a photodetector and TFT.  The x-rays are absorbed by the scintillator and converted to light.  The light is absorbed by the photodetectors and converted to an electronic signal that is collected by the detector elements (DELs) and then digitized by an ADC. Copyright © 2020 by Elsevier Inc. 25  Extraction and Processing for DR  The direct method uses an amorphous selenium photoconductor and a TFT array.  Before exposure, an electric field is applied via the bias electrode across the surface of the amorphous selenium layer.  During exposure, x-rays are absorbed by the amorphous selenium and electric charges are created in proportion to received x-ray exposure.  The charges are stored in storage capacitors attached to TFTs where they are amplified and converted to digital code. Copyright © 2020 by Elsevier Inc. 26  Extraction and Processing for DR  From this point all three DR systems go through the same basic image-forming steps previously described:  A histogram is created and analyzed.  The exposure field is recognized and the histogram analysis occurs.  Automatic rescaling takes place.  However, with these systems only the detector pixels that were exposed contribute to the image. Copyright © 2020 by Elsevier Inc. 27  Display: Film vs. Digital Receptor  The display of a digital image illustrates the most significant difference between digital detectors and film.  Exposure latitude  Film latitude Copyright © 2020 by Elsevier Inc. 28  Display  With digital receptors, the response to exposure is linear and the range of exposures is very wide (dynamic range).  Ultimately this means that digital receptors can respond to exposure levels much lower and much higher than film and are processed to display them as visible shades of gray.  The result is that more anatomic information can be captured and displayed. Copyright © 2020 by Elsevier Inc. 29  Display  Display workstation guidelines apply to primary display workstations.  Among these guidelines are the following:  Maximum luminance levels of at least 171 cd/m2  Contrast response requirements that meet American Association of Physicists in Medicine (AAPM) Task Group 18 requirements  A minimum of 8-bit luminance resolution  Minimal veiling glare  Minimizing reflections from, and levels of, ambient light sources Copyright © 2020 by Elsevier Inc. 30  Display  Post-processing functions are computer software operations available to the radiographer and radiologist that allow manual manipulation of the displayed image.  The windowing-leveling function allows the radiographer to expand any region of the grayscale to one that can be seen and differentiated.  Caution: Overuse of these functions can negatively alter the digital image data set. Copyright © 2020 by Elsevier Inc. 31  Using Digital Receptors  Exposure indicators: in CR systems, the exposure indicator value represents the exposure level to the PSP plate and the values are vendor specific.  Fuji, Philips, and Konica use sensitivity (S) numbers and the value is inversely related to the exposure to the plate (Philips also has an EI value and S is not equal to EI).  Carestream (Kodak) uses exposure index (EI) numbers and the value is directly related to the exposure to the plate and the changes are logarithmic expressions.  Agfa uses log mean (lgM) numbers and the value is directly related to exposure to the plate and changes are also logarithmic expressions. Copyright © 2020 by Elsevier Inc. 32  Using Digital Receptors: CR  Use exposure indicator values as a guide for optimum technique.  If the value is within the acceptable range, then post- processing functions will not degrade the image.  If the exposure is outside the acceptable range, the post-processing functions will not correct for improper receptor exposure and may result in noisy or suboptimal images.  For CR systems, histogram analysis is the basis for determining the exposure indicator value. Copyright © 2020 by Elsevier Inc. 33  Using Digital Receptors: DR  Use dose area product (DAP) as an indicator of exposure.  DAP is a measure of exposure in air measured by a DAP meter embedded in the collimator.  The DAP value depends on the exposure factors and field size and is expressed in centigray-meter squared (cGY-m2).  DAP reflects both the dose to the patient and the total volume of tissue being irradiated. Copyright © 2020 by Elsevier Inc. 34  Using Digital Receptors  Standardization of exposure values  Exposure index (EI): represents the exposure at the detector relevant to the region being imaged and is defined by the signal-to-noise ratio (SNR)  Target exposure index (EIT): the target reference exposure obtained from a properly exposed image receptor  Deviation index (DI): a measure of the deviation of the EI from projection-specific EIT values. Copyright © 2020 by Elsevier Inc. 35  Using Digital Receptors  Detective quantum efficiency (DQE): an expression of the radiation exposure level that is required to produce an optimal image.  DQE is a measurement of the efficiency of an image receptor in converting the x-ray exposure it receives to a quality radiographic image.  The higher the DQE of a system, the lower the radiation exposure required to produce a quality image, thereby decreasing patient exposure.  DQE “predicts” patient dose. Copyright © 2020 by Elsevier Inc. 36  Using Digital Receptors  Image noise  Any undesirable fluctuation in image brightness  Electronic components of digital imaging systems contribute undesirable noise Copyright © 2020 by Elsevier Inc. 37  Using Digital Receptors  Spatial resolution: equal to one half the Nyquist frequency  Nyquist frequency: the highest spatial frequency that a digital detector can record; determined by the sampling frequency of CR systems and the DEL spacing of DR systems  Measures of spatial resolution  Limiting spatial resolution (LSR): detector’s ability to resolve small structures  Modulation transfer function (MTF): system’s ability to preserve signal contrast Copyright © 2020 by Elsevier Inc. 38  Using Digital Receptors  The primary factor influencing contrast with digital is the lookup table (LUT).  LUTs are histograms of luminance values used as a reference to evaluate the input intensities and assign predetermined grayscale values Copyright © 2020 by Elsevier Inc. 39  Using Digital Receptors  Rescaling: the adjusting of the image by the computer program to present an image of predetermined image brightness Copyright © 2020 by Elsevier Inc. 40  Using Digital Receptors  Digital receptors are much more sensitive to scatter radiation and low-energy radiation in general; increasing kVp increases the opportunities for scatter production.  There are two ways to effectively control scatter radiation’s effect on the image: collimation and grid use. Copyright © 2020 by Elsevier Inc. 41  Using Digital Receptors  Recognize image processing errors that can degrade image clarity.  Pay attention to the exposure indicator values as an indicator of proper exposure.  Review factors that may result in a histogram analysis error.  Evaluate proper positioning and tube-part-receptor alignment. Copyright © 2020 by Elsevier Inc. 42 Quality Assurance and Quality Control  Digital imaging quality control focuses on the display monitors and viewing environment.  AAPM: Report No. OR-03  Some tests may be performed by a QC radiographer.  Additional tests are performed annually by a qualified medical physicist. Copyright © 2020 by Elsevier Inc. 43  Quality Assurance vs. Quality Control  Quality Assurance: refers to the professionals who operate the equipment and their relationships and interactions (i.e., assigning responsibility, evaluation of care, action to improve care, etc.)  Quality Control: tests/checks that must be performed at specific intervals to ensure continued safe equipment operation and performance Copyright © 2016 by Elsevier Inc. 44  Daily QC  Overall visual assessment  TG18-QC test pattern  Test looks at general image quality and for the presence of artifacts Copyright © 2020 by Elsevier Inc. 45  Monthly or Quarterly QC  Geometric distortion  TG18-QC test pattern  Looks for a variation in the shape of the displayed image from the original image Copyright © 2020 by Elsevier Inc. 46  Monthly or Quarterly QC  Reflection  TG18-AD test pattern  Evaluates the ambient light contribution to the light reflected by the display monitor Copyright © 2020 by Elsevier Inc. 47  Monthly or Quarterly QC  Luminance response  TG18-LN01, TG18- LN08, TG18-LN18, and TG18-CT test patterns  Assesses the displayed luminance values versus the input values from the display system Copyright © 2020 by Elsevier Inc. 48  Monthly or Quarterly QC  Luminance dependencies  TG18-UNL10 (A) and TG18-UNL80 (B) test patterns  This test evaluates the image for nonuniformity and effects of viewing at different angles Copyright © 2020 by Elsevier Inc. 49  Monthly or Quarterly QC  Resolution  TG18-QC and TG18- CX test patterns  Assesses the system’s ability to display images of different parts of an image with high fidelity TG18-CX Test Pattern Copyright © 2020 by Elsevier Inc. 50  Monthly or Quarterly QC  Plate maintenance schedule  Cleaned and inspected every 3 months or as needed  Erased every 48 hours if unused  Processing codes  Identical processing codes should be used for all digital systems within a facility to ensure the consistency of image appearance.  Printing digital images to film  Dynamic range is sacrificed  This film is more sensitive to heat and moisture Copyright © 2020 by Elsevier Inc. 51 Picture Archiving and Communications Systems  PACS  An electronic network for communication between the image acquisition modalities, display stations, and storage  Digital Imaging and Communication in Medicine (DICOM)  A common language used by different systems for communication Copyright © 2020 by Elsevier Inc. 52 Picture Archiving and Communications Systems  A PACS system consists of the following:  Acquisition (imaging modalities)  Display (viewing and workstations)  Storage (archive server) Copyright © 2020 by Elsevier Inc. 53 Picture Archiving and Communications Systems  Role of PACS: to allow for the display and storage of medical images  Through teleradiology, image files can be accessed throughout the facility or even by clients outside the facility.  Information management systems such as the Radiology Information System (RIS) handle textual and other information portions stored on the PACS. Copyright © 2020 by Elsevier Inc. 54 Picture Archiving and Communications Systems  Display station:  A desktop computer that allows for the retrieval and viewing of medical images from the PACS  Station quality and function varies  General viewing: a 1-megapixel (Mp) (1280  1024 pixel) monitor is sufficient.  General interpretation/quality control: a 2-Mp (1600  1200 pixel) monitor is necessary.  Interpretation of digital mammograms: a 5-Mp (2048 × 2560 pixel) monitor is necessary. Copyright © 2020 by Elsevier Inc. 55 Picture Archiving and Communications Systems  Display station software varies.  General viewing: a very basic package allowing minimal adjustment may be all that is available.  Quality control and reading: these stations have greater function and capability, such as more advanced image manipulation (windowing and leveling), annotation, patient demographic information, cropping, and magnification (zoom).  Technologists may have access to different functions protected by login and password. Copyright © 2020 by Elsevier Inc. 56 Picture Archiving and Communications Systems  One particular challenge has been expansion and maintenance of PACS systems, access to and storage of data over decades, not to mention changes in vendors.  One latest solution is the trend towards vendor neutral archives (VNAs).  One of the biggest challenges of a PACS is storage. Copyright © 2020 by Elsevier Inc. 57 Picture Archiving and Communications Systems  The archiving component of PACS  Image manager: the component that handles the workflow of the system moving images back and forth between viewing stations and storage  Storage: the component that archives the data on a storage medium, such as magnetic tape or optical disk Copyright © 2020 by Elsevier Inc. 58 Picture Archiving and Communications Systems  Storage is usually classified as follows:  Online: data are stored on magnetic hard drives with access times in milliseconds and transfer times in the range of tens and hundreds of megabytes per second.  Nearline: a tape or jukebox uses robotic arms to retrieve the tapes automatically and insert them into a drive to read or write data. This type can access data within 60 seconds and is able to transfer data at a few megabytes per second.  Offline: a removable tape or optical media will be stored on a shelf in a catalog and retrieved manually. Copyright © 2020 by Elsevier Inc. 59