Chapter 4 Photostimulable Phosphor (PSP) image Capture
Describe Erasing the Image?
-Plates should be run through an erase cycle regularly to remove background radiation and scatter -Erasure mode allows the surface of imaging plate to be erased without scanning for an image -Systems automatically erase the plate by flooding it with light to remove any electrons still trapped after the initial plate reading -Cassettes should be erased before use if the last time of exposure is unknown
What are some common CR image acquisition Errors?
-Artifacts can detect and degrade images: -Imaging plate artifacts -Plate reader artifacts -Image processing artifacts -printer artifacts -Operator errors
What are Imaging Plate Artifacts?
-As the imagin plate ages, it becomes prone to cracks form the action of removing and replacing the imaging plate within the reader -cracks in the imaging plate appear as radiopaque or radiolucent areas on the image -The imaging plate must be replaced when cracks occur in clinically unseal areas -adhesive tape used to secure lead markers to the cassette can leave residue on the imaging plate -Of static exists because of low humidity, hair can cling in to the imaging plate -Backscatter created by the x-ray photons transmitted through the back of the cassette can cause dark line artifacts -areas of the lead cooling of the cassette that are worn or cracked allow scatter to image those weak area. Proper collimation and regular cassette inspection helps to eliminate this problem
Describe Contrast Resolution?
-Because of the greater number of available shades of gray (bit depth) that can be displayed, the difference in resolution is difficult to discern -More tissue densities on the digital radiograph are seen giving the appearance of more detail -EX: fat made on a lateral elbow are difficult to see and are important for the diagnosis of elbow fractures -Ina digital image, the fat pads are easily seen because of the ability to display more shades of gray -One can visualize more tissues of varying densities
Describe using the laser to read the imaging plate?
-Cassette is put into the reader -Imaging plate is extracted -Plate is scanned with a laser -Laser beam is about 100 um wide with a wavelength of 633nm or 670 to 690nm
Describe Grid Frequency?
-Grid frequency refers to the number of grid liner per centimeter or lines per inch -The higher the frequency of the more lines per inch, the finer the grid lines in the image and the less they interfere with the image -typical grid frequency is between 80 and 152 lines per inch -The closer the grid frequency is to the laser scanning frequency, the greater the likelihood of frequency hormonics as matching and the more likely the risk of moire effects.
Describe Cassette and imaging plate?
-Cassette contains a window with a barcode label or barcode sticker on the cassette -Identifies the imaging plate -Enables technologists to match the image information with the patient-identifying barcode on the exam request -For each new exam, check the following: -Patient identifying barcode and the barcode label on the cassette must be scanned and connected to the patent position or exam menu -Cassette alsi is labeled: -with stickers indicating the top and left side of the cassette or -With a label on the back of the cassette indicating the top and right series of the patient -Stickers serve to orient the cassette to the top of the patient and the patients right side so that the image orientation is in line with the computer algorithm
What are Phostostimulable Phosphor Image Capture?
-Digital radiographs were first being produced by scanning films -In 1983, fuji developed an alternative system. Single-use film was replaced by a re-usable phosphor plate that could be scanned without the use of chemicals to develop the picture. -If physical images were desired, they could created from the digital images using a special printer -PSP systems are often referred to as computed radiography or CR -This term is vague and is easily confused with computed tomography (CT) -PSP will be used in this course for clarity, but you may see CR used in the clinical setting, other texts, and the ARRT exam
What is Operator Error?
-If the cassette is exposed with the back of a cassette toward the source the result is an image with a white grid type pattern and white areas that correspond to the internal hardware. -Care should be taken to expose only the tube side of the cassette
How do PSPs capture images?
-In CR remnant beam interacts with electrons in the barium fluorohalide crystals contained within the imaging plate -Interaction stimulates, or gives energy to, electrons in the crystals -Electrons enter into the conduction band, where they are trapped in a area of the crystal known as the F center of phosphor center -Electron energy is increased: they were at the valence band (ground state) now in the conduction band (high energy state) -If electrons trapped in proportion to absorbed radiation
Describe the speed/sensitivity to x-rays?
-In conventional radiography, speed is determined by the size and layers of crystals in the film and screen -In CR. speed is not exactly the same because there is no intensifying screen or film -The phosphors emit light according to the width and intensity of the laser beam as it scans the plate, the size of the phosphors and the thickness of the phosphor layer -CR systems are typically configured with a relative speed that is roughly equivalent to a 200 speed film/screem system -In CR, most cassettes have the same speed, however, there are special extremity or chest cassettes that produce greater resolution -These are typically 100 relative speed -Great care must be taken when converting to a CR system form a film/screen system to adjust technical factors to reflect the new speed?
Describe Digitizing the signal?
-In the process of digitizing the light sound, each phosphor storage center is scanned, and the electrons from the PMT enter a digitizer (ADC) that divides the analog image into squares (matrix) and assigns each square in the matrix a number based on the brightness of the square -Pixel location and depth -Analog to digital converted (A to D converter of ADC)
Describe Technical Factors: KVP and MAS Selection?
-Kilovoltage Peak selection -Kilovoltage peak, milliampere second, and distance are chosen in exactly the sam manner as for conventional or film/screen radiography -Kilovoltage peak must be chosen for appropriate penetration/attenuation -In the early days of CR, kilovoltage peak minimum values were set at about 70. This is no longer true or necessary
Describe Kilovoltage Peak Selection?
-Kilovoltage peak values now range from around 45 to 120. It is not recommended that kilovoltage peak values less than 45 or greater than 120 be used because these values may be inconsistent and may produce too little or too much excitation of the phosphor -The K-edge of phosphor imagingg plates range from 30 to 50 Kev so that exposure ranges of 60 to 110 kip are optimum -Remember, the process of attenuation of the x-ray beam is exactly the same as in conventional film/screen, PSP, pr FPD radiography -It takes the same kilovoltage peak to pentrate the abdomen with CR systems as it dose with in film/screen or FPD systems -It is vital that the proper balance between patient dose and image contrast be achieved
Describe the Laser?
-Laser stands for Light Amplification by Stimulated Emission of Radiation -Laser creates and amplifies a narrow, intense beam of coherent light -"Coherent" means all photons have the same frequency and direction of travel -Atoms or molecules of a crystal such as a gas, liquid, or substance such as ruby or garnet are excited so that they emit light at a constant frequency, amplitude, phase -Electrical potential (V) across tube excites electrons -Emits a light photon when ground state -Photon stimulates adjoining atoms to emit coherent -Reflect energy back and forth as atoms bombard each other -Stimulate the lower energy atoms to emit secondary photons at the same frequency and in phase as the bombarding atom -mirror at output end of tube is semi reflective, allowing some of the laser light through -laser requires a constant power source to prevent output fluctuations -laser beam passes through beam shaping optics to an optimal mirror, which directs the laser beam to the surface of the imaging plate
What is the PSP Imaging Plate?
-Looks like a film/screen cassette and compatible with most existing Bucky trays and holders -No intensifying screens are used
Describe Grid Selection?
-Moire pattern: a wavy interference pattern as artifact caused by grid lines running parallell to the laser scanning motion -The oscillating motion of a moving grid in the Bucky blurs the grid lines and eliminate the interference -Because of the ability of CR imaging plates to record a very high number of x-ray photons, the use of a grid is much more critical than in film/screen radiography -Appropriate selection of stationary grids reduces this interference as well grid selection factors are frequency, ratio, focus and size
What is Grid Focus?
-Most grids chosen by radiography departments are either parallel or focused -Parallel grids are less critical to beam centering but should not be used at distances less than 48 inches -Focused grids consist of lead strips angled to coincide with the diversion of the x-ray beam and must be used within specific SIDs using a precisely centered beam.
What are Phosphorimager?
-PSP plates are scanned in a phosphorimager -aka a scanner or reader -The plate ie extracted from the case and scanned with a laser beam -The scanning releases the image as blue light which is captured electronically and digitized -Plates are erased with white light
Name and describe the different layers of the Imaging Plate?
-Protective layer- a very thin, tough, clear plastic or carbon fiber for protection of the phosphor layer -Phosphor, or active layer -A layer of photostimulable phosphor that "traps" electrons during exposure -usually made of phosphor from the barium fluorohalide family -may also contain dye that differentially absorbs the stimulating light to prevent as much spread as possible -Reflective Layer- a layer that sends light in a forward direction when released in the cassette reader -May be black to reduce the spread of stimulating light and the escape of emitted light (non-reflective) -Conductive layer- a layer of material such as felt that will absorb and reduce static electricity -Support layer- a semi rigid material that gives the imaging sheet some strength (flexible imaging plates) Could also be the base support layer (aluminum) in rigid imaging plates -Backing layer- a soft polymer that protects the back of the cassette
Describe using the laser to read the imaging plate 2?
-Reader scans the plate with red light in a zigzag, or raster pattern -Laser gives energy to the trapped electrons -red laser light is emitted at approximately 2ev, which is necessary to energize the trapped electrons -extra energy allows the trapped electrons to escape the active layer, where they emit visible blue light at an energy of 3ev as they relax into lower energy levels -beam deflector move the laser beam rapidly back and forth across the imaging plate to stimulate the phosphor -mirrors are used to ensure that the beam is positioned consistently -Because the type of phosphor material is the imaging plate has an effect on the amount of energy required, the laser and the imaging plate are designed to work together -Imaging plate moves through the reader -Laser scans across the imaging plate multiple times -imaging plate moves relative to laser-slow scan or transition -Scanning produces 1 was of light intensity information detected by a photomultiplier -Photomultiplier amplifies the light and sends it to a digitizer -The light photons are converted to electrons, typically by a photodiode -the light collection optics direct the released phosphor energy to an optical filter and then to the photodetectos -Although there will ba variances between manufacturers, the typical throughput is 50 cassettes per hour -Some manufacturers claim up to 150 cassette per hour, but an average hospital department workflow, 50 cassettes per hour is much more realistic
What is Spatial Resolution?
-Spatial resolution is the amount of detail present in any image -Phosphor layer thickness and pixel size determines resolution in PSP -The thinner the phosphor layer, the higher the resolution -Film/screem radiography resolution at its best is 10 line pairs per millimeter (10 lp/mm) -PSP resolution is 2.55 to 5 lp/mm, resulting in less detail -Factors affecting PSP resolution include laser beam spot size, translation speed, sampling frequency, and the laser beam sweep in point beam readers
What are Plate Reader Artifacts?
-The intermittent appearance of extraneous line patterns can be caused by problems in the electronics of the plate reader -Reader electronics may have to be replaced to remedy this problem -Horizontal white lines may be caused by dot on the light guide in the plate reader. Service personnel need to clean the light guide -If the plate reader loads multiple imaging plates in a single cassette, only one of the plates will usually be extracted leaving the other to be exposed multiple times -The result is similar to a conventional film/screen double exposed cassette -Incorrect erasure settings result in a residual image left in the imaging plate before the next exposure -Results vary depending on how much residual image is left and where it is located -orientation of a grid so that the grid lines are parallel to the laser scan lines at the plate reader results in the moire pattern error. Grids should be high frequency, and the grid lines should run perpendicular to the laser scan liner of the plate reader.
Describe Grid Ratio?
-The relationship between the height of the lead strips and the space between the lead strips is known as grid ratio. -The higher the ratio, the more scatter radiation is absorbed -The higher the ratio, the more critical the positioning is, such that high grid ratio is not the appropriate choice for mobile radiography -A grid ratio of 6:1 would be proper for mobile radiography, a 12:1 grid ratio would be appropriate for departmental grids that are more stable, and less likely to be inaccurately positioned, causing a grid cutoff error
What is Grid Size?
-The size of the grid matters in CR exams -The smaller the cassette being used, the higher the sampling rate will be -When using cassettes that 10X12 inches or smaller, it is important to select a high frequency grid to eliminate scatter that will interfere with quality image interpretation by the computer algorithm -Remember that the CR imaging plate is able to records a wider range of exposure, including scatter
Describe acquiring and forming the image?
-This trapped signal retains diagnostic quality for several hours, but can remain visible for days or weeks -In this state, it is considered a latent image
Describe Imaging Plate Selection?
-Use of the smallest imaging plate possible for each exam results in the best detail -The algorithms the computer used to process the image are set for specific imaging plate sizes. Optimized for common anatomy/plate combinations -Matrix size (pixel size/spacing) is defined by the reader when scanning and is either based in a fixed sampling frequency or the aiming plate size -A 2000 X 2000 matrix on a 8X10 inch cassette results in much smaller pixel size, thereby increasing resolution -Appropriate image plate selection for the exam also eliminates scatter outside the initial collimation and increases image resolution -In addition, the image size on hard copy and soft copy is affected by cassette selection
Describe Collimation?
-When an exposure is made and radiation enters the patient, the larger the volume of tissue being irradiated, and the greater the kilovoltage peak used, the more likely the production of Compton interactions or scatter -While the use of a grid decreases the amount of scatter that reaches the Ir, properly used calculation reduces the area of irradiation and reduces the volume of tissue in which scatter can be created -Results in increased contrast due to reduced fog
