digital imaging chapters 1, 4-7 carter. chapter 1 conventional radiography film/screen system light...
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Digital ImagingDigital ImagingDigital ImagingDigital Imaging
CHAPTERS 1, 4-7CHAPTERS 1, 4-7
CARTERCARTER
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CHAPTER 1• Conventional radiography• Film/screen system• Light exposes film• Film processed with chemicals• Film taken to radiologist for
interpretation
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Let’s compare rooms for
• Conventional (Film)• CR• DR
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Let’s compare image formation/acquisition
• Film
• CR
• DR
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Let’s compare image processing
• Film
• CR
• DR
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CR- Chapter 4IP layers
• Protective • Phosphor/active• Reflective• Conductive• Color• Support• Backing• barcode
• Protects phosphor• PSP-barium fluorohalide• Sends light forward when
released in the reader• Absorbs/reduces static
electricity• Absorb stimulating light
(laser)/reflects emitted light• Protects the back of
cassette• Match image with patient
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Reading the IP• Red laser light scans in a raster
pattern at 2 eV • Laser scans multiple times as IP
moves through reader =translation
• Light produced –detected by photomultiplier
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DIGITIZING• PHOSPHOR STORAGE CENTER IS
SCANNED• RELEASED ELECTRON ENTER DIGITIZER
DIVIDES THE ANALOG SIGNAL INTO SQUARES (MATRIX).
• EACH SQUARE IS ASSIGNED A NUMBER BASED ON THE BRIGHTNESS OF THE SQUARE
• SQUARE IS CALLED A PIXEL
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SPATIAL RESOLUTION• Film screen = 10 line pairs per mm• CR =2.55 to 5 line pairs per mm
(lp/mm)• Less detail in CR but more tissue
densities seen given the appearance of better detail
• Wider dynamic recording range
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speed• Film – determined by size and
layers of crystals and phosphors• CR – amount of photostimulable
luminescence given off = 100 film speed screen (approx)
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EXPOSURE –CASSETTE BASED-chapter 5
• What is the relationship between selecting the correct body part and computer interpretation of the image?
• Too much kVp (above 120) and too little (below 45) can over excite or produce too little excitation of the phosphors
• Does the pixel size of a 2000 x 2000 matrix change when using an 8 X10 vs
a 14 x 17 CRcassette? • How does the change in pixel size impact
resolution?
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MOIRE PATTERN• Grid lines from a stationary grid
can cause a wavy artifact known as a moire pattern. The grid lines and the scanning laser run parallel
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Exposure Indicators• Exposure indicator number• Fuji, Philips, Konica – S number-
indirect relationship• Kodak –Exposure index (EI)-direct
relationship• Page 88 tables 5-1 and 5-2
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HISTOGRAM• Graphic representation of the
numerical tone (grays/blacks/whites) of an x-ray exposure
• More when we move on to Chapter 7
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OTHER ARTIFACTS• Plate artifacts
– Adhesive tape residue– cracks
• Plate reader artifacts– Line patterns– Plate reader loads multiple IP in one cassette
• Image processing artifacts– Incorrect erasure (ghosting)– moire
• Printer artifacts– White lines
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CASSETTELESS -Chapter 6
• Direct and indirect• DR plate (amorphous selenium)
or(a-Se)-converts radiation into an electrical signal –CCD or silicon detectors
• Signal stored in a Thin film transmitter (TFT)
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INDIRECT DIRECT• Two step process• X-rays convert to
light• Light converted
to electrical signal
• Sent to TFT
• X-rays converted directly to an electrical signal
• Sent to TFT
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DQE• Detective Quantum Efficiency• Ability to convert an x-ray signal
into a useful image• Of the following-which do you think
has the most efficient DQE?• Film or CR or indirect capture DR or
direct capture DR?
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Digital Radiographic Digital Radiographic Image Processing and Image Processing and
ManipulationManipulation
Digital Radiographic Digital Radiographic Image Processing and Image Processing and
ManipulationManipulation
Chapter 7Chapter 7
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CR image *sampling (*conversion from analog to
digital)• Histogram• X axis = amount of
exposure(number of grays)
• Y axis = number of pixels for each exposure
• Low kVp= wider histogram????
• High kVp = narrower histogram???
• Histogram is anatomy specific
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histogramhistogramhistogramhistogram
http://http://bloggingradiography.blogspot.cobloggingradiography.blogspot.co
m/2007/08/anatomy-of-m/2007/08/anatomy-of-histogram.htmlhistogram.html
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NYQUIST THEOREM• When sampling(converting from analog to
digital) a signal, the sampling frequency must be greater than the bandwidth of the input signal
• http://images.google.com/imgres?imgurl=http://www.cs.cf.ac.uk/Dave/Multimedia/nyquist.gif&imgrefurl=http://www.cs.cf.ac.uk/Dave/Multimedia/node149.html&h=420&w=529&sz=6&hl=en&start=10&um=1&tbnid=zyDOLJNUH6mM8M:&tbnh=105&tbnw=132&prev=/images%3Fq%3Dnyquist%
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Sample? Try this at home
• http://www.pcguide.com/art/soundSampleRate-c.html
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ALIASING• When the spatial frequency is
greater than the Nyquist frequency and the sampling occurs less than twice per cycle, information is lost causing a moire effect.
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Automatic Rescaling• Fixing the image• Why is this dangerous?
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LUT and Windowing• Automatic rescaling to achieve
appropriate contrast on an image
• http://www.sprawls.org/resources/DIGPROCESS/module.htm#13
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LATITUDE• Find the percentage the exposure
can be greater or less than before it impacts the image in CR
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IMAGE MANIPULATION cont.
• Window- how light or dark an image should be
• Level-contrast• Background removal or shuttering
– Removing the unexposed borders or to blacken the white borders
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MTF• Modular Transfer function• Reproducing the spatial resolution
of an object as a diagnostic image• 100% of the spatial resolution of the
object can never be perfectly reproduced – even with DR and CR – why? -See Bushong – pgs 451-454
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acronyms1. PSP2. CRT3. ADC4. IP5. CR6. DR7. PACS8. SNR9. CNR
10. CCD11. FOV12. LUT13. DICOM14. RIS15. HIS16. TFT17. DQE
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1. Photostimulable phosphor-europian activated barium fluorohalide
2. Cathode-ray tube or computer monitor3. Analog to digital converter4. Imaging plate5. computed6. /digital radiography7. Picture archiving communication systems
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8. SIGNAL TO NOISE RATIO PG 410 –BUSHONG The higher the signal the less the noise.
9. CONTRAST TO NOISE RATIO can be manipulated until noise
becomes too apparent-limited by the SNR 10. Charge coupled device- crystal silicon- small, replaces
vidicon in fluoro, device used in DR
11. Field of View –how much of the patient is imaged in the matrix
12. Look up Table – plotting grays on a scale
13. Digital imaging and communications in medicine -blending PACS and other imaging modalities
14. Radiology Information system15. Hospital Information system
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16.Thin Film Transmitters- indirect and direct conversion detector in DR. Stores electronic charge (from converted light) before computer processes it.
17.Detective Quantum Efficiency converting x-ray intensities into a radiographic image