Computed Radiography (CR) PDF

COMPUTED RADIOGRAPHY  OBJECTIVES 1. Understand the origin of CR 2. List & describe the different image forming elements in a digital system 3. State the differences between a direct and indirect digital system 4. Explain the principal of CR technology 5. Name and explai...

COMPUTED RADIOGRAPHY  OBJECTIVES 1. Understand the origin of CR 2. List & describe the different image forming elements in a digital system 3. State the differences between a direct and indirect digital system 4. Explain the principal of CR technology 5. Name and explain the functions of the basic components in a CR reader 6. Name and explain the function of the layers in a CR plate 7. Describe the relevant storage features of a CR plate 8. Interpret/analyze the histogram of a CR image 9. Explain the meaning of exposure index value in CR 10. To describe the QC procedures in CR 11. To understand the basic function of a CR reader 12. To understand the specific characteristics of the laser beam in a CR reader 13. To explain the formation of the latent image in CR 14. To explain the processing of the CR image 15. To list advantages/disadvantages of CR 2  videos Digital imaging https://www.youtube.com/watch?v=uHLNqySs-dk&feature=emb_rel_end CR Plate Cleaning demonstration: https://www.youtube.com/watch?v=Ms1bPiDzIzI CR Cassette: https://www.youtube.com/watch?v=4diemjHe4D4 Identifying a CR Kodak Carestream cassette: https://www.youtube.com/watch?v=hKM0R7gPNP8 CR Plate (PSP) https://www.youtube.com/watch?v=qOIL96D0A9k DR System: https://www.youtube.com/watch?v=YzV1kovMjkI&app=desktop Spatial Resolution https://www.youtube.com/watch?v=jRvKMQBIWLo CR Latent Image Computed Radiography - YouTube CR Image Receptor - YouTube 3  DIGITAL RADIOGRAPHY CR DR COMPUTED RADIOGRAPHY DIRECT RADIOGRAPHY Cassette based system Cassette-less based system INDIRECT INDIRECT DIRECT CONVERSION CONVERSION CONVERSION SCINTILLATOR (CsI) & a-Si SCINTILLATOR (PSP) & PHOTOMULTIPLIER A-SELENIUM SCINTILLATOR CsI) & CCD 4 Fuji invented CR in the late 1970s  Applications supported by CR: o All general Radiography examination. Applications not supported by CR: o Angiography Use of Grids o Use grid as usual for structures over 10 cm in thickness. o Use moving grids whenever possible o Use a grid latitude of 103 lines/inch or higher to eliminate the Moire Artefact if stationary grids are used 6 Moire Artefact  AGFA CR Components The cassette identification Station in the AGFA Unit RF ID tablet: o The patient demographics and examination information are entered in a TAG memory chip embedded in the cassette o The cassette identification is performed by a no-touch via built-in antenna card located inside the reader Computer, Monitor, keyboard & mouse 8 The ID Station in FUJI, Carestream and other CR Systems use a Barcode reader 9  The CR Cassette Durable, plastic, lightweight material Aluminum back to absorb backscatter Inside of the cassette is lined with felt material to minimize electrostatic build up, dust or mechanical stress on cassette 10  The Photostimulable Plate (PSP) Re-usable Photostimulable Storage Phosphor (PSP) plates Sensitive to X-rays, but also to light after exposure. Therefore, they should not be exposed to white light once exposed 11  The PSP layers Protective layer: plastic layer that protects the phosphor layer Phosphor layer: 1 mm to 2.50 mm thick Phosphor compound made of BaFl halide doped with Europium2 crystals Reflective layer: It sends light in a forward direction when light is released by the IP Conductive layer: decreases static electricity Support layer: It gives the IP plate some rigidity Backscatter layer: It absorbs backscatter 12  CHARACTERISTICS OF THE PSP mechanically stable, electrostatically protected, and fashioned to optimize the intensity of light emitted A thin phosphor layer, to improve spatial resolution o Usually, 1 mm thick phosphor crystals in the phosphor layer are shaped as linear filaments (needle like shape) o enhances the absorption and conversion of x-rays (DQE) o limits the light spread; improving resolution highly sensitive therefore wide dynamic range PSP plates must be erased every morning to eliminate background radiation on the PSP that has accumulated over night 13  Luminescence It is the emission of light from a substance (phosphor) when bombarded by radiation or other stimuli (biochemical reaction, electric current or visible light). 14  Two types of Luminescence Fluorescence: (immediate emission) ▪ It is the type of luminescence that is produced only during the period of irradiation and will stop immediately after the termination of exposure, i.e., smaller than 10-8 sec. after exposure. ▪ In Radiology, fluorescence is emitted by image intensifiers (Radioscopy), DR detectors that use scintillation technology, Computerized Tomography (CT) detectors and Nuclear Medicine (NM) cameras. Phosphorescence: (delayed emission) ▪ This type of luminescence continues to glow after the end of the exposure, i.e. greater than 10-8 sec. up to a few sec. ▪ Also called: Afterglow, Screen Lag and Persistence. ▪ Phosphorescence is undesirable for radioscopic screens: it gives a continuous double image on the TV monitor in radioscopy. ▪ An Application of phosphorescence is Computed Radiography (CR) 15 When x-ray photons (radiation exposure) hit the imaging plate 9phosphor crystals), light emission occurs immediately (50%). The other 50% of the energy transferred to the imaging plate is stored in the plate until it is released by an external source 16  Photostimulable Luminescence (PSL) is the release of stored energy in a phosphor by external stimulation to produce a luminescent (light) signal 17  The PSP Phosphor Crystals The phosphor crystals are made of Barium Fluoro Halide (Ba++FHa-:Eu2+) doped with Europium. o The Europium acts as an activator enhancing the storage property of the crystal o It creates a “luminescence-center” in the crystal called the “F-center” o The “F-center” becomes ionized by the incident radiation and is the site where energy (trapped electron) is stored. 18  THE LATENT IMAGE FORMATION IN CR When the PSP is exposed to X-rays, electrons in the PSP Barium Fluoro Halide crystals are excited causing them to move to a higher orbit. Approximately 50% of these electrons return to their ground state immediately, resulting in prompt emission of light, the remaining electrons are trapped in “F-Centers” (latent image) and will naturally & very slowly return to original state. This is called Delayed emission. The trapped electrons are released by an external source (laser) found inside the reader. 19  THE LATENT IMAGE FORMATION IN CR The CR plate should be processed immediately after exposure because the latent image fades over time. image degradation occurs almost immediately post exposure o After one-hour post exposure, 24% of the image is lost o an image remains on the IP for a period of 24 hours o The integrity of the image is questionable if processed 8 hours post exposure The OTIMROEPMQ’s SoP recommend that an exposed CR plate be processed within the first 2 hours following exposure To reveal the latent image, the PSP is scanned line by line using a red laser beam thus, releasing the trapped electrons in the form of light. The intensity of the emitted light is proportional to the number of X-ray photons absorbed by the PSP. 20  Not all unstable electrons are returned to the ground state when stimulated by the laser inside the reader. Some electrons remain trapped. Erasure of the plate after scanning the plate is erased after the latent image is processed by exposing it to an intense fluorescent white light The erasing time (exposure duration to the white light) vary according to the exposure intensity the plate receives “GHOSTING” are artefacts that may occur due to the partial retention of the latent image if plates are not erased properly after they are scanned (read) 21  The CR Reader Sections Unloading section Erasing section Loading section MULTI-READER SINGLE-READER 22  Processing the Imaging Plate LASER The CR cassette is inserted into the CR reader, the PSP is removed and is fitted to a precision drive mechanism. This drive mechanism moves the PSP constantly, yet PMT slowly (“slow scan”) along the long axis of the IP. Fiber Small fluctuations in velocity can result in banding optic assembly artifacts, so the motor drive must be absolutely constant. during this time, an infrared Helium-Neon laser scans the plate in a raster pattern, (“fast scan”) thus stimulating the electrons trapped in the unstable state. As the electrons become stable, they release light that is captured by a photomultiplier (PMT). 23 23  LASER PMT Fiber optic assembly 24 banding artifact 25  A Photomultiplier is a device that converts LIGHT photons into an electrical signal. It uses dynodes which is an electrode in a vacuum tube that serves as an electron multiplier through secondary emission. 26  IMAGE FORMING ELEMENTS IN CR Capture element is the Photo Stimulable Plate (PSP) Coupling element is an assembly of fiber optics used to direct the emitted light from the PSP to the collecting element Collection element is the Photomultiplier 27  Mechanical features of the Reader Slow and constant scanning Velocity fluctuations can cause banding artefacts 28  The laser Fast scanning of the PSP by the laser in a raster pattern A beam deflecting mirror moves the laser beam rapidly back and forth across the PSP to stimulate the phosphors As PSP moves thu’ the reader, the laser scans across the PSP multiple times in a process known as translation A set of optic lenses is used to control the shape, size, speed and intensity of the laser beam An assembly of fiber optics is used in some to direct the emitted light from the IP plate to a PMT Another set of optic filters is used to filter the stimulation light just before photodetection occurs 29  The Laser Characteristics The finer the laser beam, the higher the spatial resolution o The laser beam is 100μm in diameter; wavelength of 633 nm The light emitted by the laser beam is: o Monochromatic o in the infrared region – single wavelength (line spectrum emission) The light signal emitted by the IP is: o polychromatic & in the blue region o proportional to the intensity of the laser beam The laser beam is much more intense than the emitted light The optical filter at the PMT blocks the laser beam light & only allows the light emitted by the PSP 30  LASER EMITS A SINGLE WAVELENGTH LIGHT EMISSION - MONOCHROMATIC PSP EMITS A CONTINUOUS LIGHT EMISSION - POLYCHROMATIC  MULTIPLE EXPOSURES ON SAME CR CASSETTE more than one exposure on same cassette is possible in CR The CR system applies automatic masking and image division Specific rules must be respected to ensure the proper functioning of auto masking Auto masking is applied only if the collimation borders are detected by the system 32  In CR, accuracy in automatic masking depends on 1- One or more sides of a rectangle may lie outside the cassette border 33 In CR, accuracy in automatic masking depends on 2- The rectangle may be rotated with respect to the cassette borders. 34  In CR, accuracy in automatic masking depends on 3- The rectangle should include the center of the collimated cassette part. In the example below automatic collimation is not possible as the collimation area does not include the center of the collimated cassette part 35 automatic masking failure 36  The EIV in the AGFA CR unit is called the Log Mean Value (LgM) o Acceptable range o 2.0 to 2.3 Name of processing algorithms used by AGFA o AGFA: MUSICA (MUlti-Scale Image Contrast Amplification) 37 AGFA’s Image Processing Algorithms Multi-Scale Image Contrast Amplification MUSICA 38  AGFA ALGORITHMS - “MUSICA” MUSI - Enhances contrast on all scales Contrast Enhances contrast of small detail structures, including edges Edge - Enhancing edge contrast will also enhance noise and may contrast cause artefacts Latitude - Reduces scale of contrast in regions of long scale of contrast to reduction emphasize medium and small structures Noise - Attenuates contrast, thus reducing noise regions where noise is Reduction more prominent 39 MUSI – Contrast: Enhances contrast on all scales 40 program analyzes image in multiple spatial frequency ranges and optimizes display for both soft tissues and skeletal details 41 42  Edge Enhancement: Enhances small scale details, including edges 43 Edge Enhancement 44 Artefact caused by Over Edge enhancement 45  Latitude Reduction Latitude Reduction Reduces scale of contrast in regions of long scale of contrast to emphasize medium and small-scale details. Improves visualization of soft tissue 46  QC Program for a CR system Frequency Tasks Responsibility System inspection for physical defects Physical inspection of display devices Daily Radiographer erasure of IPs before 1st exposure of the day is taken Verification of system interface/network Verification of displayed images Phantom image quality control testing Weekly Radiographer - Image quality - Artefacts Inspect and clean image receptors Monthly Review image rejection rate Radiographer QC review for ‘out-of-tolerance’ issues Evaluate image quality and patient dose Acceptance tests to re-establish baseline value Review for: Semi-annually/ - Patient exposure trends Medical physicist Annually - Retake activity - QC records - Service history 47 Visual SC-35, Cassette & PSP - check for dust 1x/W Inspection p.45 PSP(under UV light) SC-35, Cleaning Method: 1x/M p.45 Manufacturer’s recommendation At least 200 sensitivity (relative speed) SC-35, Speed RS of 100 requires an exposure of p.23 1.28 mR to produce an optical density of 1.2 ID Inside & outside SC-35, Front Al Equiv. 1.2 mm Al (Use the Al/mm chart) p.73 CR advantages: Compatible with already existing equipment Drawbacks of CR lower spatial resolution than film and DR cassettes must be erased daily before use 49  The Processing Area Function - To standardize & maintain the image processing procedures in order to handle and process image receptors safely & efficiently It can be: o Centralized o Spread out in the department CR READER AREA In cassette-less systems such as DR the processing is automatic CONTROL PANEL and within the imaging system AREA 50  Dry laser Film Printer It is used to print on a film (hard copy) the digital image. It uses a laser system with a low power modulated near-infrared laser beam to record the latent image on a specialized behenate silver crystal film. The behenate Silver Crystals turn black when stimulated by heat. The developing process is known as Photothermography. Film sizes available are for printing 14”x17”, 10”x12” and 8”x 10” 51  Dry laser Film Printer A dry laser printer is an important hardware component promoting workflow efficiency. The main advantages include: a) ease of operation b) operation in daylight c) Handles multiple film sizes d) serves multiple modalities e) minimized maintenance 52  THE FILING ROOM Filing room tasks: o CD copying. Exams are copied on CDs for patients. o Uploading external cases to the PACS o Hard copy films are digitized and entered in the PACS o Management of images related to research o Ensure that the distribution of daily cases to designated radiologists for reporting is kept up to date 53