Computed Radiography: Physics and Technology PDF

Chapter 3: COMPUTED RADIOGRAPHY: PHYSICS AND TECHNOLOGY By: Jacquiline M. Gealon, RRT  Introduction In review, CR is a digital radiographic imaging modality, whereby a digital detector is used to capture X-rays transmitted through the patient. CR is based on the phenomenon of pho...

Chapter 3: COMPUTED RADIOGRAPHY: PHYSICS AND TECHNOLOGY By: Jacquiline M. Gealon, RRT  Introduction In review, CR is a digital radiographic imaging modality, whereby a digital detector is used to capture X-rays transmitted through the patient. CR is based on the phenomenon of photostimulable luminescence which is exhibited by photostimulable phosphors. The CR digital detector is made of a photostimulable phosphor which, when struck by X-rays, creates a latent image. The latent image is rendered visible when the detector is scanned by a laser beam to produce light (photostimulable luminescence) that is subsequently converted into electrical signals. Introduction These signals are digitized and processed by a digital computer that produces the CR image, using special digital image processing algorithms.  Terms Synonymous with CR These include photostimulable luminescence (PSL), storage phosphor radiography (SPR), digital luminescence radiography (DLR), photostimulable storage phosphor (PSP) radiography, and digital storage phosphor (DSP) radiography  A Brief History of CR The history of CR is linked to photostimulable phosphors and the phenomenon of photostimulable luminescence and can be traced back to the 1600s with the discovery of the Bolognese stone (glowing stone) in Italy. Later, in the 1800s, Becquerel worked on the notion of de-excitation of atoms by optical means. It was in 1983 that commercialization of CR imaging systems for use in diagnostic radiology began, when Fujifilm (Tokyo, Japan) introduced their FCR-101 unit.  The CR Imaging System It is clear that the imaging process consists of four distinct steps: image acquisition, image plate scanning and erasure, image processing, and image display.  Image Acquisition Refers to X-ray exposure of the storage phosphor plate cassette or imaging plate (IP). Image acquisition also refers to the mechanism of X-ray interaction with the phosphor to produce a latent image and subsequent scanning of the IP by a laser beam to produce photostimulable lumines- cence (PSL). This read-out process consists of essentially laser scanning, detection, and conversion of the PSL and digitization of the signal from the analog-to-digital converter (ADC). The IP is subsequently erased and can be used again for the next examination.  Image Processing Refers to the use of several digital operations for pre-processing and post- processing of the CR image data. While pre-processing deals with shading corrections, pattern recognition, and exposure field recognition, post-processing of the CR image data on the other hand refers to contrast enhancement (image grayscale processing) and edge enhancement, a technique which is based on frequency processing Additionally, energy subtraction imaging can be done in CR using a special algorithm to create separate images of bone and soft tissue  Image Display, Storage, and Communications After images have been processed, they are displayed for viewing and interpretation. In a Picture Archiving and Communication Systems (PACS) environment, the technologist determines and assesses the overall image quality of the image and subsequently sends the image to the PACS. In CR, images are displayed on a computer workstation. either a cathode-ray tube (CRT) display monitor or an active-matrix liquid crystal display (LCD) device.  Image Display, Storage, and Communications The workstation also allows the technologist to do any post-processing on images before they are communicated to the PACS for storage. Image storage and archiving in a CR-PACS environment include the use of magnetic tapes and disks, magneto-optical disks, optical disks, and digital videodisks (DVDs). Finally, communication of the CR image to the PACS is accomplished via computer networks.  Basic Physics of CR Image Formation The CR imaging process, that is, the image acquisition in particular, consists of a three-step cycle of image formation The IP consists of a photostimulable storage phosphor (PSP) layered on a base to provide support. Photostimulable phosphors have the property of creating and storing a latent image, when exposed to X- rays. To render the latent image visible, the PSP must be scanned by a laser beam of a specific wavelength.  Basic Physics of CR Image Formation Laser scanning produces a luminescence (light) that is proportional to the stored latent image. This luminescence is referred to as photostimulated luminescence (PSL). After laser scanning, the PSPIP is erased, by exposing it to a high- intensity light beam, to get rid of any residual latent image. Nature of PSPs The phosphors used in radiology must have certain physical characteristics to be useful in CR imaging. For example, phosphors should have good X-ray absorption efficiency and must be capable of being stimulated by a helium- neon (He-Ne) laser. Additionally, the luminescence light must be compatible with the photomultiplier tube (PMT) phosphor and the time for luminescence must be shorter than 1 μs Finally, these phosphors should be able to store the latent image for a number of hours without compromising the signal from the IP.  Nature of PSPs The phosphors that meet the above requirements and are used by several manufacturers are, in general, barium fluorohalide: europium (BaFX: Eu2+). The halide (X) can be chlorine (Cl), bromine (Br), or iodine (I) or a mixture of them The phosphor is usually doped with Eu2+ which acts as an activator to improve the efficiency of PSL.  Nature of PSPs As mentioned above, good X-ray absorption efficiency is one of the requirements of a PSP for CR imaging. Such efficiency depends on not only the kVp (X-ray energy) used but also the thickness of the phosphor used in the IP The X-ray absorption efficiency of BaFBr PSP compared with the X-ray absorption efficiency of the rare-earth phosphors, gadolinium oxysulfide (Gd2O2S) and cesium iodide (CsI), is an important point to note  Latent Image Formation and PSL X-ray exposure of the PSPIP creates a latent image, and laser scanning of the exposed IP produces PSL The information captured from the PSL is used to create the CR image  Latent Image Formation and PSL 1.When X-rays fall upon the PSPIP, the europium atoms are ionized by the radiation, and the electrons move from the valence band (ground state) to the conduction band (higher energy). 2.Electrons in the conduction band are free to travel to a so-called F-center. a. The number of trapped electrons is proportional to the absorbed radiation. 3.In addition to this mechanism, X-ray exposure of the PSPIP causes it to fluoresce (emits light when it is exposed to X-rays) for a very brief duration.  Latent Image Formation and PSL 5. To render the latent image visible, the PSPIP is taken to the CR reader/processor to be scanned by a laser beam. 6. While in the CR reader, the PSPIP is scanned systematically The laser light used must be capable of being absorbed by the “F-centers.” 7. This absorption causes the trapped electrons to move up to the conduction band, where they are free to return to the valence band, thus causing the Eu3+to return to the Eu2+ state. This transition of the electrons from a higher energy state to a lower energy state (ground state) results in an emission of bluish-purple light (~415 nm wavelength).  Latent Image Formation and PSL 8. The PSL from the IP is collected by a special light collection device and sent to a photomultplier tube that produces an electrical signal. Fading is a term that refers to the time it takes for the latent image to disappear. The latent image can last for several hours; however, it is important to read the exposed IP in a reasonable time, so as not to compromise the PSL signal. CR Technology  The CR Imaging Plate The digital detector used in CR imaging is the imaging plate (IP) The IP consists of the PSP layer on a base that provides support. In addition, the IP structure consists of two protective layers: an electroconductive layer and a light-shielding layer. One of these critical components is the photostimulable phosphor. The phosphor is mixed with an organic binder (e.g., polymer, such as polyester) and coated onto the support layer. Two layers coat the phosphor, a front and a back protective layer  The CR Imaging Plate The front protective layer must be constructed so that it provides durability during multiple uses. This layer must also allow light from the laser and the stimulated light to pass through it. The purpose of the electroconductive layer is to reduce any problems when the IP is transported in the CR reader (CR scanner or processor) and static electricity problems that may degrade image quality. The CR Imaging Plate Essentially, there are two types of IPs: a standard- resolution IP and a high-resolution IP. While standard-resolution IPs have thick phosphor layers and absorb more radiation, high- resolution IPs have thinner phosphor layers and provide sharper images compared to thick phosphors. In terms of radiographic speed, thick phosphor IPs have faster speeds than high-resolution IPs (slow speeds), similar to the cassettes used in conventional radiography The imaging plate size varies depending upon the manufacturer; however, 17′′ × 17′′ (43 × 43 cm), 17′′ × 14′′ (43 × 35 cm), 14′′ × 17′′ (35 × 43 cm), and 14′′ × 14′′ (35 × 35 cm) are not uncommon.  The CR Imaging Plate CR IPs are housed in cassettes similar to conventional film-screen cassettes, with the IP replacing the film, and there are no intensifying screens. The CR cassette is usually made of aluminum (Fujifilm) or aluminum honeycomb panel (Carestream). While the front of the cassette is radiolucent, the back of the cassette is designed with a lead backing to prevent backscatter radiation from getting to the IP. Backscatter will lead to image artifacts.  The IP Imaging Cycle One of the advantages of CR is that the IP can be used over and over again for several hundreds of exposures. This cycle basically consists of at least three steps: X-ray exposure of the IP, readout of the exposed IP, and erasure of the IP. In the first step, the “ready- to- use” IP is exposed to X-rays using radiographic exposure factors (kVp, mAs) suitable to the needs of the examination.  The IP Imaging Cycle The energy stored is directly proportional to the intensity of x-rays striking the phosphor. The exposed IP is then readout in the CR reader to render the latent image visible Finally, in the third step of the IP imaging cycle, the IP is erased using a high-intensity light  The CR Reader: Types The CR reader, or scanner, as it is sometimes referred to, is a machine for scanning the exposed IP to render the latent image visible. The purpose of the CR reader is to render the latent image stored on the exposed IP visible. The electrical signal generated as a result of scanning the IP is amplified and subsequently digitized. There are two types of CR systems: cassette-based systems and cassetteless systems.  Cassette-Based Systems These systems use individual IPs of different sizes analogous to cassette-based film-screen radiography. This means that the IP is in contact with various transport mechanisms that may in time result in plate/ phosphor damage  Cassetteless Systems These systems evolved to overcome some of the problems with cassette-based systems. Cassetteless CR systems incorporate a fixed stationary single IP that is encased in a special housing that forms a part of the unit. There is also no contact with the IP in the unit when it is read. The single fixed IP can accommodate various exposure sizes ranging from 17′′ × 17′′ and 14′′ × 14′′ to 10′′ × 12′′ and 8′′ × 10′′ or 10′′ × 8′′.  The CR Reader: Scanning Technologies Acquiring the image from the exposed IP can be accomplished by point-scan (P-S) CR readers or by line-scan CR readers Firstly, the IP is removed from the cassette and is placed on the transport mechanism for scanning by a laser beam. ⚬ While the movement of the IP is referred to as the “slow-scan” direction, the laser beam movement across the IP is called the “fast- scan” direction The laser beam is used to stimulate the trapped electrons (latent image) in the exposed IP. The laser stimulation of the IP causes it to emit light (by photostimulable luminescence (PSL) that is of a much different wavelength than the stimulating laser light.  The CR Reader: Scanning Technologies Secondly, the emitted light from the IP is optically filtered and collected by the light channeling guide or light collection optics as it is sometimes referred to as. This light (PSL) is then sent to the photodetector (a photomultiplier tube) or charge-coupled device (CCD) which converts the PSL into an electrical signal (analog signal) that is first amplified and subsequently digitized by the ADC. Finally, the analog signal from the photodetector is sent to the ADC for digitization Digitization involves both sampling the analog signal and quantization. Depending on the amplification, the ADC will produce 8–16 bits of quantization per pixel, providing discrete gray levels ranging from 2^8 to 2^16.  The CR Reader: Scanning Technologies CR systems use several linear laser sources, a lens system, and a linear array of CCD photodetectors. While the laser beam is collected and shaped by the lens system, to scan the IP line by line, the PSL from the IP is collected by the CCD linear photodetector array. As described above, the IP is scanned on one side to emit the PSL that is used to produce an image In this way, more signal is obtained to improve the signal-to-noise ratio and, hence, improve image quality. Additionally, a thicker phosphor layer can be used to increase the absorption of X-rays, hence improving system efficiency.  The CR Reader: Scanning Technologies Erasure of the IP is done in the CR reader by exposing it to a high-intensity light that is brighter than the stimulating laser light, to get rid of any residual signal left on the IP.  The CR Workstation The CR workstation provides the technologist with the opportunity to interact with the entire CR process by facilitating a number of important functions A typical CR workstation consists of the image processing computer and image display monitor, keyboard, and mouse. In addition, some workstations offer a barcode reader and a magnetic card reader While cathode-ray tube (CRT) monitors are used for some CR work- stations, liquid crystal display (LCD) monitors have become commonplace.  The CR Workstation The keyboard allows the technologist to input relevant information about the CR examination, and the mouse allows the selection of various operations. Some monitors utilize touch-screen technology to enable the technologist to communicate with the software. Finally, the barcode reader enables the registration of the various IPs used.  Computer Networking and CR Once images are acquired and displayed for viewing by the technologist, they must be assessed for image quality and QA before they are sent to the PACS. Additionally, the CR system is interfaced to the hospital information system (HIS) and the radiology information system (RIS). These components, that is, the CR unit (CR reader and workstation), the HIS/RIS, and the PACS, should be fully integrated to communicate with each other. While DICOM facilitates the communication of images in the digital radiology environment, the Health Level-7 (HL-7) standard addresses the communication of textual data within the information systems environment.  Digital Image Processing in CR Image pcessing operations, such as image contrast and brightness control as well as spatial frequency filtering while images can be enhanced for sharp- ness and blurred for the purpose of meeting the viewing needs of the observer, were reviewed. Image processing in CR can be discussed in terms of pre-processing and post- processing operations both of which are intended to enhance the visual appearance of the image displayed Pre-processing operations are also referred to as acquisition processing. Pre-processing in CR is essential to correct the raw digital data collected from the IP and the CR reader that may have imperfections One important pre-processing method in CR is exposure field recognition also referred to as exposure data recognition (Fujifilm Medical Systems) and segmentation (Carestream).  Post-processing Operations There are several types of post-processing algorithms for use in CR. These in general include contrast enhancement; spatial frequency or edge enhance- ment; multi-scale, multifrequency enhancement; and dual-energy and disease-specific processing. ⚬ Contrast enhancement is also referred to as contrast scaling, and various manufacturers use different terms to refer to it. The purpose of contrast enhancement is to optimize the image contrast and density to enhance diagnostic interpretation of the image. ⚬ Another common post-processing operation is edge enhancement or spatial frequency processing. These algorithms are intended to adjust or control the sharpness or detail of an image by adjusting the frequency components of the image.  Exposure Control in CR It is important that technologists realize that the information content of an image depends primarily on the radiation dose used. ⚬ When the exposure is low, the film is underexposed and the image is light and is not acceptable. ⚬ When the exposure is high, the film is overexposed and the image is dark and is not acceptable Underexposure of the film results in noisy images while overexposure will result in high doses to the patient. The above problems are solved by a CR imaging system since the IP has wider exposure latitude than film  Exposure Control in CR If the exposure is too low or too high, the image quality is still acceptable due to the ability of the CR system to perform digital image processing to adjust the image quality to match the image quality that would be produced by the optimum exposure. A low exposure (underexposure) will produce high noise (that can be detected by the radiologist), while a high exposure (overexposure) will produce very good images, compared to the optimum image produced by the optimum exposure (appropriate exposure). The fundamental problem with high exposures is that of increased radiation dose to the patient. As noted by Dr. A Seibert, “because of the negative feedback due to underexposures, a predictable and unfortunate use of higher exposures, ‘dose creep’, is a typical occurrence.  Exposure Indicators An exposure indicator is a numerical parameter used to monitor the radiation exposure to the IP in CR imaging. This relationship shows that S is inversely proportional to the exposure; hence a low exposure will result in a high S-number, and a high exposure will result in a low S-number. It should be noted that if the exposure to the IP is low, the PMT signal will be weak and must be increased. On the other hand, if the exposure to the IP is high, the PMT signal is strong and must be decreased to produce optimum image quality. Adjusting the PMT’s signal in this manner means that the sensitivity of the PMT will be set for the final scan of the IP. Image Quality Descriptors The image quality descriptors for a digital image such as a CR image include spatial resolution (detail), density resolution, noise, quantum detective efficiency (DQE), and artifacts. 1) Spatial Resolution The spatial resolution of a digital image is related to the size of the pixels in the image matrix. The smaller the pixel size, the better the spatial resolution of the image. 2) Density Resolution The density resolution of a digital image is linked to the bit depth, which is the range of gray levels per pixel An image with a bit depth of 8 will have 256 (28) shades of gray per pixel. In general, the greater the bit depth, the better the density resolution of the image. 3) Noise Noise on the other hand can be discussed in terms of electronic noise (system noise) and quantum noise (quantum mottle). The quantum noise is determined by the number of X-ray photons (often referred to as the signal, S) falling upon the detector to create the image. While low exposure factors (kVp and mAs) will produce few photons at the detector (less signal and more noise, N), higher exposure factors will generate more photons at the detector (more signal and less noise). Detective Quantum Efficiency The final descriptor of digital image quality is the detective quantum efficiency (DQE) The DQE is a measure of the efficiency and fidelity with which the detector can perform this task. The DQE for CR is much better than film-screen (F-S) image receptors. Image Artifacts Overview An artifact is “any false visual feature on a medical image that simulates tissue or obscures tissue” An earlier definition of an artifact is provided by Willis et al. who state that “an artifact is a feature in an image that masks or mimics a clinical feature” In general, CR artifacts arise from the image acquisition process and include operator errors and the image processing system as well Continuous Quality Improvement Overview The maintenance of equipment is an integral part of the daily activities in radiology since it is considered a component of a continuous quality improvement (CQI) program. The notion of CQI was developed by the Joint Commission on the Accreditation of Healthcare Organizations (JCAHO) in 1991. CQI ensures that every employee plays a role in ensuring a quality product. Quality Assurance Quality assurance (QA) is a term used to describe systems and procedures for assuring quality patient care. It deals specifically with quality assessment, continuing education, the usefulness of quality control procedures, and the assessment of outcomes. QA deals with the administrative aspects of patient care and quality outcomes. QualityControl Quality control, on the other hand, is a component of QA and refers specifically to the monitoring of important variables that affect image quality and radiation dose. QC deals with the technical aspects (rather than administrative aspects) of equipment performance. The purpose of the procedures and techniques of CQI, QA, and QC is threefold: to ensure optimum image quality for the purpose of enhancing diagnosis, to reduce the radiation dose to both patients and personnel, and to reduce costs to the institution. Such activities range from acceptance testing, routine performance, and error correction QualityControl While acceptance testing is the first major step in a QC program and it ensures that the equipment meets the specifications set by the manufacturers, routine performance involves performing the actual QC test on the equipment with varying degrees of frequency (annually, semiannually, monthly, weekly, or daily). Finally error correction ensures that equipment not meeting the performance criteria or tolerance limit established for specific QC tests must be replaced or repaired to meet tolerance limits.  Parameters for QC Monitoring in CR QC for CR has evolved from simple to more complex tests and test tools for use by radiology personnel to ensure that the CR equipment is working to ensure optimum image quality within the as low as reasonably achievable (ALARA) philosophy in radiation protection. The AAPM has recommended several testing procedures for CR QC (AAPM), using specific tools developed for CR QC. A few of these testing procedures, for example, include physical inspection of the IP, dark noise and uniformity, exposure indicator calibration, laser beam function, spatial accuracy, erasure thoroughness, aliasing/grid response, positioning, and collimation errors, to mention only a few. Examples of these tools include screen-contact wire mesh pattern, anti- scatter grid, calibrated ion chamber, densitometer for hard copy film evaluation (in cases where film is used), manufacturer-approved cleaning solutions and cloths, two metric-calibrated 30-cm steel rulers, and low- contrast phantom to mention only a few.  Tolerance Limits or Acceptance Criteria The AAPM has also established acceptance criteria for the recommended tests. They offer both qualitative and quantitative criteria. Thank you! Do you have any questions?