Transcript on Digital - El Camino College

Review
CR & DIGITAL IMAGING (1)
2012 – RT 244 wk 15
References: Bushong Vol 9
www.sprawls.org/resources/DIGRAD/classroom.htm
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Objectives

Digital imaging review

Review CR fundamentals
2
Digital Imaging
Image acquisition that produces an
electronic image that can be viewed and
manipulated on a computer.
 Examples?

3
Methods to Digitize an Image
•
•
•
•
1. Film Digitizer
2. Video Camera (vidicon or plumbicon)
3. Computed Radiography
4. Direct Radiography
– PACS
– DICOM
4
Digital
DDR
CR
Radiography
Direct
Capture
Indirect
Capture
Direct-to-Digital
Radiography
(DDR)-Selenium
Computed
Radiography
(CR) - PSL
Direct-to-Digital
Radiography
Silicon Scint.
Laser
Scanning
Digitizers
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Computer Language
• Computers operate on the Binary Number
System
• It has only two digits, 0 and 1
• Computers function by converting all data
into binary values.
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Byte
Represents one character, digit, or value.
 A bit describes the smallest unit of
measure 0 or 1 – computers ultimately
understand only 0 or 1
 Byte are 8 bits
 A kilobyte represents 1024 bytes,
megabyte is 1 million bytes, gigabyte is
approximately 1 billion bytes

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Alphabet in
Binary
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What is a Pixel?
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Basics of Digital Images
• digital
images are a
(matrix) of
pixel (picture
element)
values
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Pixel
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Computed
Radiography
Fundamentals
of
Computerized
Radiography
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CR SYSTEM COMPONENTS
What are the CR system
components?
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CR SYSTEM COMPONENTS
CASSETTES (phosphor plates)
 ID STATION
 IMAGE PREVIEW (QC)
STATION
 DIGITIZER
 VIEWING STATION

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Imaging Plate (IP)

Contained in a cassette

Handled the same as S/F cassettes

Processed more like daylight processor
with no chemicals

IP has lead backing to reduce scatter
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CR – PSP plate
• photostimulable phosphor (PSP) plate
• Exit photons energizes the PSP plate
• The energy is stored in traps on plate
(latent image)
• PLATE scanned in CR READER
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Imaging Plate Construction

A thin sheet of plastic

IP’s have several layers
◦ A protective layer. This is a very thin, tough,
clear plastic that protects the phosphor layer
◦ A phosphor or active layer. This is a layer of
photostimulable phosphor that “traps”
electrons during exposure
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Active Layer - Crystals
• The materials that make up the PSP plate
are from the barium fluorohalide family.
• Barium fluorohalide, chlorohalide, or
bromohalide crystals. The most common
crystal uses is barium fluorohalide with
europium
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Acquiring the Image
What is the correct order?
• Violet light is captured by PMT – is amplified and
converted into a digital signal
• cassette is put into the reader, the imaging plate is
extracted
• light is sent to the analog to digital converter (ADC). To
convert light to binary.
• e- return to ground state, visible light is emitted
• remnant beam interacts with electrons in the barium
fluorohalide crystals
• imaging plate is scanned with a helium laser beam or
solid-state laser diodes
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Acquiring the Image
• The remnant beam interacts with electrons in
the barium fluorohalide crystals. This
interaction stimulates, or gives energy to,
electrons in the crystals, allowing them to
enter the conductive layer, where they are
trapped in an area of the crystal known as the
color or phosphor center.
• This trapped signal will remain for hours,
even days, although deterioration begins
almost immediately. IR should be processed
as soon as possible.
• The trapped signal is never completely lost.
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Imaging Plate Construction

A reflective layer. This is a layer that
sends light in a forward direction when
released in the cassette reader. This layer
may be black to reduce the spread of
stimulating light and the escape of emitted
light. Some detail is lost in this process.
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IP Construction
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Cross section of a PSP screen
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Needle PSP increase the absorption of xrays and limit the spread of light emission
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IP Design

Designed to optimize the intensity of light
release. (CE)

Enhance the absorption of x-rays (DQE)

Limit the spread of light emission for
more detail.
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Photostimulable Luminescence
• When the cassette is put into the reader,
the imaging plate is extracted and
scanned with a helium laser beam or, in
more recent systems, solid-state laser
diodes. This beam, about 100μm wide
with a wavelength of 633 nm (or 670 to
690 nm for solid state), scans the plate
with red light in a raster pattern and gives
energy to the trapped electrons.
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X-ray interaction with a PSP screen
1
X-ray interactions with the
screen phosphors causes
an e- to excited
2
When e- return to ground
state visible light is emitted
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CR Phosphor Plates
ABSORPTION
EMISSION
LASER STIMULATION
X-RAY
ELECTRON
TRAP
ELECTRON
TRAP
LIGHT
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CR Reader – PSP plate

Stimulates the matrix of trapped E- by a
RED OR ULTRAVIOLET laser light

Trapped E- energy is released in a form of
VIOLET/BLUE light

Violet light is captured by PMT – is
amplified and converted into a digital
signal
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Producing a PSL signal

50% of the excited e- return to ground
state immediately, resulting in light
(VIOLET/BLUE) emission.
Slow scan = plate
 Fast scan = laser

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How CR works

Released light is captured by a PMT (photo
multiplier tube). An ultrasensitive
photomultiplier tube or CCD (charged
couple device)

PSP light is amplified by the PMT or CCD

This light is sent to the analog to digital
converter (ADC). To convert light to
binary.
34
Sequence of CR imaging
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Processing of digital images can be used to
change most image characteristics.
• Three possibilities include
processing methods to:
• Adjust and optimize the
image contrast
characteristics
• LUT & Processing
Algorithms
• Reduce image noise
• Increase visibility of detail
• Some type of digital
image processing is used
with most of the medical
imaging modalities.
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Brightness & Contrast
• Optimum kVp & mAs has changed for digital
• kVp changes for SUBJECT contrast – not
image contrast
• mAs does not influence DENSITY the
same as it did with F/S
• The image is POSTPROCESSED – with
changing PROCESSING ALGORITHMS
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• digital processing
methods that can be
used to adjust the
contrast
characteristics of an
image.
•Look Up Table (LUT) processing
•Windowing
•Are used in digital radiography as well
as with many of the other imaging
modalities.
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DIGITAL “DENSITY”
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Dynamic Range
•
•
•
•
•
The range of exposure values to which the
image receptor will respond.
The greater the range of values that a
receptor will respond to the greater the
dynamic range.
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Characteristic curve
of radiographic film
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Widow level & width
Same photons at the image receptor
Image is post processed – changing
Brightness and contrast of image appearance
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• The ability to window is a
valuable feature of all digital
images.
• Windowing is the process of
selecting some segment of
the total pixel value range
• and then displaying the pixel
values within that segment
over the full brightness
(shades of gray) range from
white to black.
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windowing
• Important point...Contrast will be visible only for
the pixel values that are within the selected
window.
• All pixel values that are either below or above
the window will be all white or all black and
display no contrast.
• The person controlling the display can adjust
both the center and the width of the
window. The combination of these two
parameters determine the range of pixel values
that will be displayed with contrast in the image.
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advantages of windowing?
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Detective Quantum Efficiency
DQE
• An indicator of the potential “speed class”
or dose level required to acquire an
optimal image.
• The DQE performance is obtained by
comparing the image noise of a detector
with that expected for an “ideal” detector
having the same signal-response
characteristics.
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Exposure Latitude
•
•
•
•
It is the optimal exposure range
relative to the “ideal” exposure
that produces a quality image at
an appropriate patient dose.
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Why do digital systems have
significantly greater latitude?
• Linear response give the imaging plates
greater latitude
• Area receiving little radiation can be
enhanced by the computer
• Higher densities can be separated and
brought down to the visible density ranges
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Exposure Latitude
• The analog receptor exposure latitude
ranges from approximately
• 30% underexposed
• to 50% overexposed relative to
• the “ideal” exposure level.
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Exposure Latitude
The digital image receptor
• exposure latitude ranges from
• approximately
• 50% underexposed
• to 100% over exposure
• relative to the “ideal” exposure level.
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Exposure Indicators
• Imaging plates get a signal from the
exposure they receive
• The value of the signal is calculated from
the region identified as the anatomy of
interest
• The signal for the plate is an average of all
signals given to the plate
Note
It is important to note that just because a
• digital imaging system has the capacity to
• produce an image from gross underexposure
• or gross overexposure it does not equate to
• greater exposure latitude.
• The reason the system is capable of producing
an image when significant exposure errors occur
is through a process called automatic rescaling.
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• In a digital system, underexposure of
• 50% or greater will result in a mottled
• image.
• In a digital system, overexposure
• greater than 200% of the ideal will result
• in loss of image contrast.
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Darker
Lighter
Histogram showing pixel values in an image. The
pixel values in gray are on the horizontal with the
total number for each on the vertical.
Screen / Film Imaging = self regulating
2 mAs
Under exposed
6 mAs
Correct exposure
24 mAs
Over exposed
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CR imaging results in 10,000 shades of gray
Fixed kVp exposures
mAs =
0.5
S = 357
mAs =
1.0
S = 175
mAs = 2.0
S = 86
mAs = 5.0
S = 35 71
LUT
•
•
•
•
Look Up Table (LUT)
Each anatomic area has a LUT
Used to adjust contrast and density
Other terms that may be used for this
– Contrast rescaling
– Contrast processing
– Gradation processing
– Tone scaling
LUT
• The image data from the histogram is
rescaled for application of the LUT
• The LUT maps the adjusted data through
a “S” curve that is similar to an H & D
curve
• The result is an image that has the correct
contrast and brightness (density)
Characteristic curve &
histogram
Underexposed
Overexposed
Just right!
LOOK UP TABLE (LUT)
Linear LUT
Black
Saturation
White
Saturation
Black Shirt
Facial Tones
* No Detail in Black Areas
* High Contrast
* Only Detail in White
Areas can be seen
* No Detail in White Areas
* Low Contrast
* Only Detail in Black
Areas can be seen
Digital Images – Bit Depth
• Pixel values can be any bit depth (values
from 0 to 1023)
• Bit depth = # or gray shades available for
image display
• Image contrast can be manipulated to
stretched or contracted to alter the displayed
contrast.
• Typically use “window width” and “window
level” to alter displayed contrast and
brightness
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Display Bit Depth
1 bit
6 bit
8 bit
2 shades
shades
64 shades
256
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BIT Depth
• The number of gray shades available for
image display.
Number of gray shades is 2n .
• Where n is the number of bits available for
each pixel
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Digital - Grayscale
Bit depth.
Number of gray shades available for display
• 8 bit 256
• 10 bit 1024
• 12 bit 4096
• 14 bit 16384
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CR Image Quality
Pixels, Field of View, Image receptor
 Sampling frequency, Quantization,
Nyquest frequency
 Noise
 Magnification
 Image compression (lossless vs lossee)


Questions?
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