1725 Computed Radiography

Computed Radiography

• The first commonly used digital system

• Work flow is very similar to film/screen work flow

• Can be used interchangeably with any radiographic imaging system

• CR creates a latent image and uses the IR to store electrons in high energy traps

How are patients radiographed for computed radiography

• Patient is radiographed exactly the same way as in conventional radiography.

• Patient is positioned using appropriate positioning techniques

• Cassette is place on the tabletop or within the bucky

• Patient is exposed using the proper combination of kVP mAs and distance

Film/Screen Flowchart

Circuit → Tube → Beam → Subject → IR → Chemicals → View Box

Digital Radiography Flowchart

Circuit → Tube → Beam → Subject → IR → Computer → Display Monitor

Computed Radiography Flowchart

Circuit → Tube → Beam → Subject → IR → Reader → ADC → Computer → Display Monitor

IP

Imaging Plate

PD

Photodiode

PMT

Photomultiplier Tube

PSL

Photostimulable Luminescence

PSP

Photostimulable Phosphor

SP

Storage Phosphor

SPS

Storage Phosphor Screen

Two stage image acquisition

• Latent image captured on PSP plate

  • Multiple images are allowed

• Image data extracted in reader

  • Analog light conversion

• Each plate is separate file in the computer

  • No matter how many images are on the plate

  • Even double exposure errors

Photostimulable Luminescence

• The property of a material where light is emitted in response to external stimulation

  • Can be immediate; also can emit light at a later time when exposed to a different light source

Photostimulable Phosphors

• Have the PSL property and are used to “store” the latent image in CR

• Made of barium fluorohalide

• Combined with europium as an activator

  • Responsible for the storage property of PSL

When x-ray interacts with barium fluorohalide atoms

They produce metastable electrons (Latent image occurs in the form of metastable electrons)

When metastable electrons return to ground state

Visible light is emitted

What stimulates returning to ground state

Exposing phosphor to intense infrared light from a laser

Why do SPS appear white

Because the small PSP particles scatter light (turbid)

SPS are

• Mechanically stable

• Electrostatically protected

• Fashioned to optimize the intensity of stimulated light

CR Imaging Plate

• Houses the PSP screen

• Rugged cassette

• CR can be substituted for screen film radiography and used with any x-ray system

• Contains lead backing that reduces backscatter x-rays

  • Improves contrast resolution of the image receptor

Imaging plate barcode label

• Cassette contains a barcode label on the cassette or the imaging plate viewed through a window on the cassette

• Label enables technologist to match the image information with the patient identifying barcode on the exam request

PSL is similar to what

Optically stimulated luminescence (OSL)

Optically stimulated Luminescence (OSL)

• One of the main radiation monitors

• Light emitted with OSL crystal is illuminated

Thermoluminescent Dosimetry (TLD)

• One of the main radiation monitors

• Light emitted when TLD crystal is heated

Light stimulation-emission process Expose

• The first of a sequence of events that results in an x-ray induced image forming signal

• X-ray beam Exposes PSP

• Energy transferred results in excitation of electrons

• ~50% of electrons return to ground state immediately = prompt emission of light

• Remaining electrons return to ground state over time

  • Causes image ot fade

  • CR signal loss is objectionable after approx 8 hours

Light stimulation-emission process Stimulate

• Stimulation of the latent image results from the interaction of a infrared laser beam with the photostimulable phosphor

  • Diameter of the laser beam determines spatial resolution of the CR system

• As laser intensity increases, intensity of emitted signal (Image) increases

• As laer beam penetrates, it spreads

  • Spread increases with PSP thickness

What does Laser stand for

Light Amplification by Stimulated Emission of Radiation

The laser

• Creates and amplifies a narrow intense beam of coherent light

• Requires a constant power source to prevent output fluctuations

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

Light Stimulation- Emission Process Read

• The light signal emitted after stimulation is detected and measured

• Laser beam causes electrons to return to ground state

  • Emission of shorter wavelength light in blue region of visible spectrum

  • Some signal is lost

    • Scattering of emitted light

    • Collection efficiency of photodetector

      • Photodiode are the light detector for CR

Light Stimulation-Emission Process Erase

• Final step

• Before plate reuse, any residual metastable electron are moved to the ground state by an intense light

• The stimulation cycle does not return ALL electrons to ground state

• If residual image remained causes ghosting

• Any residual latent image is removed by flooding the phosphor with very intense white light from fluorescent lamps

Light Stimulation-Emission Process Erase Continued

• The process of reading the image returns most of electrons to a lower energy state

  • Reading effectively removes the image from the plate

• Imaging plates are extremely sensitive to scatter radiation

  • Plates should be erased to prevent a buildup of background signal

• Plates should be run at least once a week under an erase cycle to remove background radiation and scatter

• 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 erasure is unknown

• The CR PSP is sensitive to background radiation and can become fogged

The CR reader Mechanical features

• CR inserted

• IP is removed and fitted in a precision drive mechanism

Slow scan

The precision drive mechanism moved IP constant but slow along the long axis

Fast Scan

A deflection device (rotated polygon or oscillating mirror) deflects the laser beam back and for across IP

Optical features of the CR reader

• Laser

• Beam shaping optics

• Light collecting optics

• Optical filters

• Photodetector

Laser

• The source of stimulating light

• Requires a constant power source to prevent output fluctuations

• Small laser beam diameter = high spatial resolution

Beam Shaping Optics

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

• The more angles the beam, the more elliptical the shape of the beam

• If this change in the beam shape were ignored, output of the screed would differ from the middle to the edges

• Result would be differing spatial resolution and inconsistent output signal depending of the position and angle of the laser beam

• To correct, beam is “shaped” by special optics that keep the beam size, shape, and speed largely independent of the beam position

Light collecting optics and optical filters

• Emitted light (signal) from IP is channeled into a funnel like fiber optic collection assembly

• Light is filtered before photodetection

  • Removed long wavelength stimulated emission light (noise)

  • Emitted light from the IP is the signal, stimulating light is the noise

    • Improves signal to noise ratio

    • Improves contrast resolution

Photodetector

• The light is directed to the photomultiplier/charged coupled device that converts the light to an electronic signal

• That electronic signal is digitized by an analog to digital converter (ADC)

The CR Reader Computer Control

• Output of the photodetector is a time varying analog signal that is transmitted to a computer system

• Sampling - Time between samples

• Quantization - Value of each sample

• Sampling and quantization are the tow processes of analog to digital conversion (ADC)

Spatial Resolution

How resolved your image is, how clear the image is, ability to distinguish parts from one another, how clear an edge is

Contrast Resolution

Visual differences from the shading, differences in all the shades of grey

Noise

A problem, comes in every piece of equipment because we have to convert. The goal is to reduce the amount of noise. It will affect the differences in the greys, it will add or detract from your image

Artifacts

Anything added to the image that’s not supposed to be there (ex: A scratch)

Dynamic Range

The number of grey shades that an imaging system can reproduce

Wide latitude

More leeway, allows for a bigger margin of error

Imaging characteristics

• CR has large dynamic range and wide latitude

  • A 14 bit CR image has 16,384 grey levels

  • More tissue densities on the digital radiograph are seen in CR giving the appearance of more detail

Image receptor response function

• Spatial resolution: Refers to the amount of detail present in any image

• For example, an AP knee radiograph typically does not show soft tissue structures on the lateral aspects of the distal femur or proximal tibia or fibula

• An AP knee digital image shows not only the soft tissue but also the edge of the skin. This is due to the wider dynamic recording range and does not mean that there is additional detail

Image noise

• Principle source is scatter radiation no matter what image receptor used

Patient radiation dose

• CR is faster IR compared to film screen systems

• CR should be performed at lower techniques than film screen radiography

• Remember that increasing kVp will allow a decrease in mAs needed and will decrease exposure to the patient. Both the mAs that is necessary and the appropriate kVp value helps keep the dose ALARA