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Digital Imaging
CHAPTERS 1, 4-7
CARTER
CHAPTER 1
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Conventional radiography
Film/screen system
Light exposes film
Film processed with chemicals
Film taken to radiologist for
interpretation
Let’s compare rooms for
• Conventional (Film)
• CR
• DR
Let’s compare image
formation/acquisition
• Film
• CR
• DR
Let’s compare image
processing
• Film
• CR
• DR
CR- Chapter 4
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IP layers
Protective
Phosphor/active
Reflective
Conductive
Color
Support
Backing
barcode
• Protects phosphor
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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
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
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
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
speed
• Film – determined by size and layers
of crystals and phosphors
• CR – amount of photostimulable
luminescence given off = 100 film
speed screen (approx)
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?
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
Exposure Indicators
• Exposure indicator number
• Fuji, Philips, Konica – S numberindirect relationship
• Kodak –Exposure index (EI)-direct
relationship
• Page 88 tables 5-1 and 5-2
HISTOGRAM
• Graphic representation of the
numerical tone (grays/blacks/whites)
of an x-ray exposure
• Important in CR
• Four modes of date recognition
– Automatic data recognition
– Semiautomatic mode
–
Multiple manual selection mode
– Fixed mode
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
CASSETTELESS Chapter 6
• Direct and indirect
• DR plate (amorphous selenium) or(aSe)-converts radiation into an
electrical signal –CCD or silicon
detectors
• Signal stored in a Thin film
transmitter (TFT)
INDIRECT
• Two step process
• X-rays convert to
light
• Light converted to
electrical signal
• Sent to TFT
DIRECT
• X-rays converted
directly to an
electrical signal
• Sent to TFT
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?
Digital Radiographic
Image Processing and
Manipulation
Chapter 7
CR image *sampling
(*conversion from analog to
digital)
• Histogram
• X axis = amount of
exposure read
• Y axis = number of
pixels for each
exposure
• Low kVp= wider
histogram
• High kVp = narrower
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Histogram is
anatomy specific
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=h
ttp://www.cs.cf.ac.uk/Dave/Multimedia/ny
quist.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=z
yDOLJNUH6mM8M:&tbnh=105&tbnw=132
&prev=/images%3Fq%3Dnyquist%
Sample? Try this at home
• http://www.pcguide.com/art/soundSa
mpleRate-c.html
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.
Automatic Rescaling
• Fixing the image
• Why is this dangerous?
LUT and Windowing
• Automatic rescaling to achieve
appropriate contrast on an image
• http://www.sprawls.org/resources/D
IGPROCESS/module.htm#13
LATITUDE
• Find the percentage the exposure
can be greater or less than before it
impacts the image in CR
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
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
acronyms
1.
2.
3.
4.
5.
6.
7.
8.
9.
PSP
CRT
ADC
IP
CR
DR
PACS
SNR
CNR
10.
CCD
11. FOV
12. LUT
13. DICOM
14. RIS
15. HIS
16. TFT
17. DQE
1.
2.
3.
4.
5.
6.
7.
Photostimulable phosphor-europian activated
barium fluorohalide
Cathode-ray tube or computer monitor
Analog to digital converter
Imaging plate
computed
/digital radiography
Picture archiving communication systems
8.
SIGNAL TO NOISE RATIO PG 410 –BUSHONG The higher the signal the
9.
CONTRAST TO NOISE RATIO can be manipulated until noise
less the noise.
becomes too apparent-limited by the SNR
10. Charge coupled device- crystal silicon- small, replaces vidicon in
11.
12.
13.
14.
15.
fluoro, device used in DR
Field of View –how much of the patient is imaged in the matrix
Look up Table – plotting grays on a scale
Digital imaging and communications in medicine -blending PACS
and other imaging modalities
Radiology Information system
Hospital Information system
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