chapter 4 -- photostimuable phosphor image capture (CR system)

cassette-based systems

the cassette is simply a light-proof container that protects an imaging plate from light and handling (damage)

computed radiography (CR)

a type of indirect capture digital radiographic system

• detector + reader

cassette

• durable, lightweight plastic

• backed by aluminum or lead to absorb backscatter x-ray photons

• antistatic material (usually felt) protects against static electricity build-up, dust collection, and mechanical damage

imaging plate

• records the image on a thin sheet of plastic known as the imaging plate

  • photostimuable imaging plates (PSP)

• constructed of several layers: protective layer, phosphor layer, light reflective layer, conductive layer, support layer, light shielding layer, backing layer, and barcode label

imaging plate construction

• protective layer:

  • thin, tough, clear plastic protects the phosphor layer

• phosphor layer or active layer (most important layer)

  • contains photostimuable phosphor that “traps” electrons during exposure

  • most common — barium fluorohalide family

  • may also contain a dye

• barcode label or sticker

  • for each new examination, the patient-identifying barcode and the barcode label on the cassette must be scanned and connected to the patient position of examination menu

• colored mark or sticker — indicates the appropriate orientation of the cassette in relation to the patient

  • orientation affects image display

acquiring and forming the image

• the patient is radiographed exactly the same way as in conventional radiography

• differences lie in how the exposure is recorded

• In PSP, the remnant beam interacts with electrons in the barium fluorohalide crystals contained within the imaging plate

  • interactions gives energy to electrons in the crystals, trapping them in an area of the crystal known as the color of phosphor center

  • this trapped signal remains for hours or even days and is never completely lost

    • latent image

  • deterioration begins almost immediately

  • small amount of exposure remains trapped; imaging plates are never completely erased

    • residual trapped electrons are so few that they do not interfere with subsequent exposure

    • build-up is problematic

remnant beam

the remaining x-ray beam that completely passes through the patient’s body and hits the image receptor

the reader

two types of PSP readers:

• point scan

• line scan

after exposure, the cassette is fed into a reader that removes the imaging plate and scans it with a laser to release the stores electrons

there are two scan directions within:

• fast scan direction

• slow scan direction

point scan readers

• have an optical stage, a scanning laser beam, translation mechanics, light pick-up guides, a photomultiplier, a signal transformer/amplifier, and an analog-to-digital converter (ADC)

• at any point in time, only a single laser point radiates the image plate

line scan readers

• based on simultaneous stimulation of the imaging plate one line at a time

• acquisition of the photostimulated luminescence (PSL) occurs with a charge-coupled device (CCD) linear array photodetector

• the scanning module contains several linear laser units and optical light collection lenses

• the system requires a lens array to focus each laser beam to a corresponding point on the CCD array

photostimulated luminescence (PSL)

refers to the emission of light from the phosphor layer after stimulation by the relevant light source

fast scan direction

• the movement of the laser across the imaging plate

• also known as “scan”

slow scan direction

• the movement of the imaging plate through the reader

• also known as “translation or subscan direction”

the laser

a laser creates and amplifies a narrow, intense beam of coherent light

• light amplification by stimulated emission of radiation

requires a constant power source to prevent output fluctuations

passes through beam-shaping optics to an optical mirror, which directs the laser beam to the surface of the imaging plate

using the laser to read the imaging plate

• the cassette is put into the reader, and the plate is extracted

• the plate is scanned with a helium laser beam or solid-state laser diodes

• the reader scans the plate with red light in zigzag or raster, pattern

• the laser gives energy to the trapped electrons

• the red laser light is emitted at ~2 eV, which is necessary to energize the trapped electrons

• the extra energy allows the trapped electrons to escape the active layer, where they emit visible blue light at an energy of 3 eV as they relax into lower energy levels

• the imaging plate moves through the reader or a scanning mechanism moves across the stationary imaging plate

• the laser scans across the imaging plate multiple times

• the scanning process produces lines of light intensity information detected by a photodetector

• the photodetector amplifies the light and sends it to an ADC

• translation speed of the plate must be coordinated with the scan direction of the laser or the spacing of the scan lines will be affected

• a beam deflector moves the laser beam rapidly back and forth across the imaging plate to stimulate the phosphors

  • mirrors are used to ensure that the beam is positioned consistently

• because the type of phosphor material in the imaging plate has an effect on the amount of energy required, the laser and the imaging plate should be designed to work together

• the typical throughput is 50 cassettes per hour

  • some manufacturers claim up to 150 cassettes per hour, but based on average hospital department workflow, 50 cassettes per hour is much more realistic

• the light collection optics direct the released phosphor energy to an optical filter and then to the photodetector

analog

represents changing values as continuously variable physical quantities

digitalizing the signal

• the assignment of a numeric value to each light photon - discrete values

• the scanning process results in the conversions of the light emitted from the storage phosphor into an electrical signal

• the electrical signal is sampled and digitalized to represent a specific location within the image matrix an displays as a specific brightness

• the typical number of pixels in a matrix range from about 512 × 512 to 1024 × 1024 and can be as large as 2500 × 2500

• the image is digitalized by positioning (spatial location) and by intensity (gray level)

matrix

a group of squares that make up the image formation

pixel

also called picture element; each square of the matrix

• the more __ there are, the greater the image resolution for a fixed field of view

erasing the image

• the image reading process returns most of the electrons to a lower energy state, effectively removing the image from the plate

• imaging plates are extremely sensitive to scatter radiation

  • plates should be run at least once a week under an erase cycle to remove built-up background radiation and scatter

• erasure mode

• systems erase the plate by flooding it with light to remove any trapped electrons left after the initial plate reading

• cassettes should be erased before use if the last erasure time is unknown

erasure mode

allows the plate surface to be scanned without recoding the generated signal

preprocessing, processing, and forwarding the image

• after the imaging plate has been read, the signal is sent to the computer, where it is preprocessed

• the data then goes to a monitor that displays the image

• the technologist can do the following:

  • review the image

  • manipulate it if necessary (postprocessing)

  • send it to the quality control station and ultimately to the picture archiving and communications systems (PACS)

• all systems use a region of interest (ROI) to define the area where the part to be examined is recognized and the exposure outside the region of interest is subtracted

  • each vendor has a specific tool for different situations, such as neck, breasts, hips, or pediatrics, in which the anatomy requires some special recognition

part selection

• selecting the proper body part and position is important for the proper conversion to take place

  • if the improper part or position is selected, the image may be processed incorrectly and fail to display properly

• it is not acceptable to select a body part of position different from that actually being performed to improve the quality of the displayed image

• if the proper exam/part selection results in a suboptimal image, service personnel should be notified to correct this issue

• improper menu selections may lead to overexposure of the patient and to repeated exams

technical factors

• kVp, mAs, and distance are chosen in exactly the same manner as for F/S radiography

  • kVp was selected for penetration and tissue type

  • mAs was selected according to the number of photons requires for the body part to be imaged

• attenuation of the x-ray beam is exactly the same as in F/S radiography

• the proper balance between patient dose and part penetration is vital

kilovoltage peak selection

• digital image contrast is no longer dependent on kVp

• higher kVp values can be used, allowing for lower mAs values

• the dynamic range of digital receptors and exposure latitude are much higher/wider/larger than F/S and inherently produces a wide variety of gray values

• contrast is determined by computer processing

equipment selection

• imaging plate selection

  • two important factors should be considered when selecting the PSP imaging cassette:

    • type

    • size

• grid selection

  • because PSP systems are more sensitive to low levels of radiation, the use of grids is much more critical than in F/S radiography

  • grid selection factors include frequency, ratio, and focus

  • more lead, higher patient dose

imaging plate selection

• typically, two types of imaging plates:

  • standard

  • high resolution

    • cassettes should be marked on the outside to indicate high-resolution imaging plates

    • contain a thinner phosphor layer

      • the thinner layer reduces light spread yielding greater image sharpness

    • are limited to smaller cassette sizes and are most often used for extremities and mammography

• PSP digital images are displayed in a matrix of pixels, and the pixel size is an important factor in determining the resolution of the displayed image

• appropriate image plate selection eliminates scatter outside the initial collimation by reducing the amount of imaging plate not being exposed

  • this increases contrast resolution

grid selection

• grid lines that are projected onto the imaging plate when using a stationary grid can interfere with the image

• this results in a Moire pattern:

  • a wavy artifact caused by grid lines running parallel to the laser scanning motion

    • grid feeding direction is important

  • missing data from digital system is another reason to cause this artifact

side/position markers

• conventional lead markers should be used the same way they were used in F/S systems

  • electronic markers can be easily used to mark images with left and right side markers or other position or text markers after the exposure has been made

• marking the patient exam at the time of exposure identifies the patient’s side and the technologist performing the exam

  • legal issue

artifacts

defined as any undesirable objects on the processed image other than those caused by scatter and fog

• examples:

  • adhesive tape used to secure lead markers to the cassette can leave residue on the imaging plate

  • if static exists because of low humidity, hair can cling to the imaging plate

  • backscatter — dark lines

  • worm, cracked areas of the cassette lead coating allow scatter to image these weak areas

  • horizontal white lines — caused by direct on the light guide in the reader

    • service needs to clean the light guide

  • loading multiple imaging plates in a single cassette — only 1 plate usually extracted, leaving the other to be exposed multiple times

  • insufficient erasure settings — residual image left in PSP plate before the next exposure

    • also occurs if the erasure lamp needs repair

  • results vary depending on how much residual image is left and where it is located

  • orientation of a stationary grid relative to the direction of the laser scanner is critical to reduce the likelihood of the Moire pattern error

    • the grid lines must be perpendicular to the laser scan direction

    • moving grids generally do not demonstrate the moire effect because the grid lines are blurred out

• vendors have identified specific grid frequencies for their equipment to prevent the artifact

• proper collimation and regular cassette inspection help to eliminate these problems

operator errors

• underexposure procedures quantum mottle

• overexposure reduces subject contrast

• proper technical factors are critical for:

  • patient dose

  • image quality