unit iii creating the image chapter 25 digital radiography

Unit III

Creating the Image

Chapter 25

Digital Radiography

Objectives

• Describe various digital radiography image receptor and detector systems

• Explain critical elements used in the different digital radiography systems

• Discuss limitations inherent in currently available digital radiography systems

Objectives

• Describe how the digital radiography histogram is acquired

• Describe how the display algorithm is applied to collected data

Objectives

• Explain why digital radiography systems have greater latitude than conventional film-screen radiography systems

• Analyze elements of digital radiography systems

Objectives

• Discuss what makes them prone to violation of ALARA radiation protection concepts

• Explain the causes of sever digital radiography artifact problems

Historical Development

• Fuji Systems– 1980s

• Today’s Systems– Several manufacturers

Indirect Photostimulable Phosphor Imaging Plate Systems

• Photostimulable imaging plates

• Latent image production

• Image acquisition

• Reading digital radiography data

Photostimulable Imaging Plates

• Photostimulable phosphor– PSP

• Imaging plate– IP

Common Phosphors

• Europium activated barium fluorohalides– Chemical formulas

• BaFBr:Eu• BaFI:Eu

K-edge attenuation

• Best between 35 – 50 keV– 35 keV: average energy of 80 kVp beam

• More exposure needed if applied kVp is outside of this range

Scatter Radiation

• PSPs absorb more low energy radiation than radiographic film– More sensitive to scatter both before and

after exposure than radiographic film

Latent Image Production

• Electron pattern is stored in active layer of exposed IP

• Fluorohalides absorb beam through photoelectric interactions– Energy transferred to photoelectrons– Several photoelectrons liberated– More electrons freed by photoelectrons

Latent Image Production

• Liberated electrons have extra energy

• Fluoresce - or- get trapped by fluorohalide to create holes

Hole Formation

• Fluorohalide crystals trap half of the liberated electrons

• Europium sites contain electron holes– This is the actual latent image

Important Note!

• The latent image will lose about 25 percent of its energy in 8 hours, so it is important to process the cassette shortly after exposure

Image Acquisition

• IP cassettes– Also know as filmless cassettes– Can be used tabletop or with a grid

• Rules of positioning remain the same

• Wider latitude when compared to film/screen radiography

Radiographic Technical Factor Selection

“It is the responsibility of the radiographer to select proper technique; chronic overexposure should be avoided.”

• Ethical principles

• ALARA concept

Reading Digital Radiography Data

• Trapped electrons are freed– IP is scanned by finely focused neon-

helium laser beam in a raster pattern

• Electrons return to lower energy state– Emit blue-purple light

• Light captured by Photomultiplier (PM) tubes

• PM tubes convert light to analog electronic signal

• Analog electronic signal sent to analog to digital converter (ADC)

• ADC sends digital data to computer for additional processing

• IP erased via exposure to intense light

• Two types of IP processing– Point by point readout– Line by line readout

• Plate throughput– 30 – 200 plates per hour

• Throughput and spatial resolution can be improved by using dual-sided PSP

• Self contained units– House plates and reader within upright

bucky or table

• PM tubes output signal– Infinite range of values must be digitized

• Converted to limited, discrete values

– Automatically adjusted• Optimizes handling during digitization

– Pixel depth

Pixel Depth

• Determines number of density values– Affects density and contrast of system

• Controlled by ADC– 10 bit (210 = 1024)– 12 bit (212 = 4096)– 16 bit (216 = 65,536)

Pixel Size

• Inversely related to spatial resolution

• Sampling frequency– Expressed as pixels/mm

• Dependent on:– Matrix– Image receptor size

Image File Size

• Affected by:– Pixel size– Matrix– Bit depth

Preprocessing

• Communicates to the system:– What part– Orientation of the part– Number of projections per plate

Analog to Digital Conversion

• System locates raw data

• Samples

• Quantitize

• Determine average value

Exposure Data Recognition (EDR)

• Fuji systems’ method of locating the raw data– Automatic

• Adjusts the latitude and sensitivity for the image

– Semiautomatic• Adjusts the sensitivity, but not the latitude

Exposure Data Recognition (EDR)

• Fuji systems’ method of locating the raw data– Fixed

• Does not adjust sensitivity or latitude

Multiple Projections on One IP

• Scanning projection pattern– “The beam and part should be centered

within each pattern, and collimation should be parallel and equidistant from the edges of the imaging plate.”

• Automatic mode– Used when

collimation is parallel/equidistant and the central ray and part are centered

• Semiautomatic mode– Can be used when collimation is not

parallel/equidistant

• Fixed mode– Requires use of proper technical factors

Histogram

• Graphic representation of pixels and signal intensities present in image

Look Up Table Data

• Contains standard contrast, speed and latitude for given exam

• Appropriate part and projection selected by radiographer prior to processing

Look Up Table Data

• True patient image information is determined– Automatically rescaled– Algorithms used for processing

Histogram Adjustment

• Image processing in proper range of exposure– Yields consistent gray scale regardless of

technique

• Outside of appropriate range– System cannot compensate

Image Reprocessing

• Raw data– Stored by CR system

workstation

Gradation Curves

• Contrast requirements

• Similar to DlogE curves of different types of radiographic film

• Scale of contrast or the slope of the DlogE curve can be adjusted

– Window width

Spatial Frequency Processing

• Affects image sharpness– Edge enhancement

• Unsharp mask technique• Low-pass filter• High spatial frequency signal remains• High spatial frequency signal is amplified and

added back into the image

• Affects image sharpness– Edge enhancement

• Increases noise resulting in lower quality images

• Lower contrast and higher base fog levels

Computed Radiography Image Quality– Fuji System

• Each manufacturer has their own system

• Basic concepts are similar

CR Image Quality—Fuji System

• S number– Inversely related to the amount of

exposure to the image receptor– Properly exposed IP should have S

number of 150-250– S number 200 ~ 1mR exposure

• Higher S number indicates overexposure

• Increased latitude compared to film/screen radiography

• Linear response– No Dmax

– Computer can bring densities into visual range despite overexposure

Toleration of Overexposure

• Radiographers professional and ethical responsibility– Minimize patient dose– ALARA concept

Image Acquisition Elements

• Sensitivity

• Data clipping

• Spatial frequency processing– Edge enhancement– Image blurring

• Look up table adjustments

Image Acquisition Elements

• Histogram equalization

• Collimator edge identification

• Image stitching

• Grid use

Data Clipping

• Clinically irrelevant data is not included in image display– Dependent upon the part and projection

• Edge enhancement

• Image blurring

Look-up Table Adjustments

• Adjustment similar to changing DlogE curve of the image receptor

Histogram Equalization

• Example– Normal chest x-ray– Bone enhanced histogram image– Soft tissue histogram image

• Possibilities endless– ACR standard procedure

Collimator Edge Identification

• Algorithm that detects edges of exposure vs. nonexposure

• Can sometimes be triggered by prosthetics or implants

Image Stitching

• Overlapping exposures

• Verified registration marks

• Combine several images into one

Grid Use

• Digital systems are more sensitive to scatter radiation

• Grids should be used more often

• Radiography of the chest– > 24-26 cm should use grid

Overexposure

• Overexposure > 2X– Results in enough scatter to degrade

Underexposure

• Quantum mottle/reticulation

Direct Exposure Imaging Systems

• Direct selenium flat panel imaging plate systems

• Indirect silicon flat panel imaging plate systems

Direct Selenium Flat Panel Imaging Plate Systems

• Amorphous selenium directly converts ionization from x-rays into electronic signal

• Electronic signal received by thin film transistors (TFTs) and sent to computer

Indirect Silicon Flat Panel Imaging Plate Systems

• Amorphous silicon combined with scintillator

• Scintillator or intensifying screen converts x-rays to light

• Amorphous silicon acts as photodiode– Converts light to electronic signal– TFTs send signal to computer

Thin Film Transistors (TFTs)

• Array or matrix of pixel detectors

Charged Coupled Devices (CCD)

• Photodetector typically used with a screen scintillator

• Requires optical coupling by lenses or fiber optics

• Electric signal from CCD sent to computer

DICOM Standard

• System of computer software standards

• Allows different digital imaging software to understand each other

Computed Radiography Artifacts

• Acquisition artifacts

• Post acquisition artifacts

• Display artifacts

Acquisition Artifacts

• Phantom images

• Scratches

• Light spots

• Dropout

• Fogging

• Quantum mottle (reticulation)

• Heat blur

Heat Blur

Post Acquisition Artifacts

• Algorithm artifacts

• Dropout artifacts

• Laser film transport artifacts

• Histogram error

• Nonparallel collimation

Display Artifacts

• Density/brightness window level adjustments

• Contrast window width adjustments

• Image enhancement artifacts

unit iii creating the image chapter 25 digital radiography

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