Copyright 2006 by Thomson Delmar Learning ALL RIGHTS
Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED.
Unit III Creating the Image Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED.
Chapter 25 Digital Radiography Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED.
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 Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 4
Objectives • Describe how the digital radiography histogram is acquired • Describe how the display algorithm is applied to collected data Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 5
Objectives • Explain why digital radiography systems have greater latitude than conventional film-screen radiography systems • Analyze elements of digital radiography systems Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 6
Objectives • Discuss what makes them prone to violation of ALARA radiation protection concepts • Explain the causes of sever digital radiography artifact problems Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 7
Historical Development • Fuji Systems – 1980 s • Today’s Systems – Several manufacturers Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 8
Indirect Photostimulable Phosphor Imaging Plate Systems • • Photostimulable imaging plates Latent image production Image acquisition Reading digital radiography data Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 9
Photostimulable Imaging Plates • Photostimulable phosphor – PSP • Imaging plate – IP Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 10
Common Phosphors • Europium activated barium fluorohalides – Chemical formulas • Ba. FBr: Eu • Ba. FI: Eu Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 11
K-edge attenuation • Best between 35 – 50 ke. V – 35 ke. V: average energy of 80 k. Vp beam • More exposure needed if applied k. Vp is outside of this range Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 12
Scatter Radiation • PSPs absorb more low energy radiation than radiographic film – More sensitive to scatter both before and after exposure than radiographic film Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 13
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 Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 14
Latent Image Production • Liberated electrons have extra energy • Fluoresce - or- get trapped by fluorohalide to create holes Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 15
Hole Formation • Fluorohalide crystals trap half of the liberated electrons • Europium sites contain electron holes – This is the actual latent image Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 16
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 Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 17
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 Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 18
Radiographic Technical Factor Selection “It is the responsibility of the radiographer to select proper technique; chronic overexposure should be avoided. ” • Ethical principles • ALARA concept Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 19
Reading Digital Radiography Data • Trapped electrons are freed – IP is scanned by finely focused neonhelium laser beam in a raster pattern • Electrons return to lower energy state – Emit blue-purple light • Light captured by Photomultiplier (PM) tubes Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 20
Reading Digital Radiography Data • 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 Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 21
Reading Digital Radiography Data Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 22
Reading Digital Radiography Data • Two types of IP processing – Point by point readout – Line by line readout Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 23
Reading Digital Radiography Data • 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 Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 24
Reading Digital Radiography Data • PM tubes output signal – Infinite range of values must be digitized • Converted to limited, discrete values – Automatically adjusted • Optimizes handling during digitization – Pixel depth Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 25
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) Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 26
Pixel Size • Inversely related to spatial resolution • Sampling frequency – Expressed as pixels/mm • Dependent on: – Matrix – Image receptor size Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 27
Image File Size • Affected by: – Pixel size – Matrix – Bit depth Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 28
Preprocessing • Communicates to the system: – What part – Orientation of the part – Number of projections per plate Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 29
Analog to Digital Conversion • • System locates raw data Samples Quantitize Determine average value Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 30
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 Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 31
Exposure Data Recognition (EDR) • Fuji systems’ method of locating the raw data – Fixed • Does not adjust sensitivity or latitude Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 32
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. ” Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 33
Multiple Projections on One IP • Automatic mode – Used when collimation is parallel/equidistant and the central ray and part are centered Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 34
Multiple Projections on One IP • Semiautomatic mode – Can be used when collimation is not parallel/equidistant Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 35
Multiple Projections on One IP • Fixed mode – Requires use of proper technical factors Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 36
Histogram • Graphic representation of pixels and signal intensities present in image Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 37
Look Up Table Data • Contains standard contrast, speed and latitude for given exam • Appropriate part and projection selected by radiographer prior to processing Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 38
Look Up Table Data • True patient image information is determined – Automatically rescaled – Algorithms used for processing Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 39
Histogram Adjustment • Image processing in proper range of exposure – Yields consistent gray scale regardless of technique • Outside of appropriate range – System cannot compensate Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 40
Image Reprocessing • Raw data – Stored by CR system workstation Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 41
Gradation Curves • Contrast requirements • Similar to Dlog. E curves of different types of radiographic film • Scale of contrast or the slope of the Dlog. E curve can be adjusted – Window width Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 42
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 Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 43
Spatial Frequency Processing • Affects image sharpness – Edge enhancement • Increases noise resulting in lower quality images • Lower contrast and higher base fog levels Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 44
Computed Radiography Image Quality– Fuji System • Each manufacturer has their own system • Basic concepts are similar Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 45
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 ~ 1 m. R exposure • Higher S number indicates overexposure Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 46
CR Image Quality—Fuji System • Increased latitude compared to film/screen radiography Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 47
CR Image Quality—Fuji System • Linear response – No Dmax – Computer can bring densities into visual range despite overexposure Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 48
Toleration of Overexposure • Radiographers professional and ethical responsibility – Minimize patient dose – ALARA concept Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 49
Image Acquisition Elements • Sensitivity • Data clipping • Spatial frequency processing – Edge enhancement – Image blurring • Look up table adjustments Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 50
Image Acquisition Elements • • Histogram equalization Collimator edge identification Image stitching Grid use Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 51
Data Clipping • Clinically irrelevant data is not included in image display – Dependent upon the part and projection Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 52
Spatial Frequency Processing • Edge enhancement • Image blurring Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 53
Look-up Table Adjustments • Adjustment similar to changing Dlog. E curve of the image receptor Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 54
Histogram Equalization • Example – Normal chest x-ray – Bone enhanced histogram image – Soft tissue histogram image • Possibilities endless – ACR standard procedure Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 55
Collimator Edge Identification • Algorithm that detects edges of exposure vs. nonexposure • Can sometimes be triggered by prosthetics or implants Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 56
Image Stitching • Overlapping exposures • Verified registration marks • Combine several images into one Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 57
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 Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 58
Overexposure • Overexposure > 2 X – Results in enough scatter to degrade image Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 59
Underexposure • Quantum mottle/reticulation Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 60
Direct Exposure Imaging Systems • Direct selenium flat panel imaging plate systems • Indirect silicon flat panel imaging plate systems Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 61
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 Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 62
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 Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 63
Thin Film Transistors (TFTs) • Array or matrix of pixel detectors Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 64
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 Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 65
DICOM Standard • System of computer software standards • Allows different digital imaging software to understand each other Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 66
Computed Radiography Artifacts • Acquisition artifacts • Post acquisition artifacts • Display artifacts Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 67
Acquisition Artifacts • • Phantom images Scratches Light spots Dropout Fogging Quantum mottle (reticulation) Heat blur Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 68
Heat Blur Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 69
Post Acquisition Artifacts • • • Algorithm artifacts Dropout artifacts Laser film transport artifacts Histogram error Nonparallel collimation Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 70
Display Artifacts • Density/brightness window level adjustments • Contrast window width adjustments • Image enhancement artifacts Copyright © 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 71
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