Download RAD 216 ADVANCED IMAGING MODALITIES DIGITAL IMAGING

RAD 216 ADVANCED IMAGING
MODALITIES
BASICS OF DIGITAL IMAGING
DIGITAL IMAGING
Computer-assisted imaging has been around since
the 70’s and found application in such imaging
modalities such as computed tomography (CT)
and MRI.
DIGITAL IMAGING
Conventional radiography involves recording a
projected image onto a film recording medium.
Using this method, the film (radiograph) is at once
the recording, processing and display medium.
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DIGITAL IMAGING
Computer-assisted imaging separates the
recording, processing and display elements. This
separation gives imaging far more flexibility.
Each element can also be improved
independently.
DIGITAL IMAGING
• COMPUTED RADIOGRAPHY
• DIGITAL RADIOGRAPHY
– DIRECT CAPTURE
– INDIRECT CAPTURE
COMPUTED RADIOGRAPHY
In order to have a meaningful discussion of
computed radiography, we should take a few
moments to review the basic steps in digital
imaging process.
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COMPUTED RADIOGRAPHY
LATENT IMAGE
IMAGE DISPLAY
SCAN LATENT IMAGE
DAC
ADC
IMAGE PROCESSING
PACS
COMPUTED RADIOGRAPHY
An imaging plate consisting of “photostimulable”
phosphors (e.g., BaFI:Eu or BaFBr:Eu) captures a
latent image by means of photoelectric interaction.
PHOTOSTIMULABLE IMAGING PLATE (IP)
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COMPUTED RADIOGRAPHY
The next step involves scanning the latent image.
A laser scans the plate in a “raster” pattern. Laser
interaction with the plate at points where x-ray
interaction occurred will release light. That light is
detected and converted into an amplified
electrical signal using a photomultiplier device
(PM).
DIGITIZATION
COMPUTED RADIOGRAPHY
The amplified electrical signals are then sent to
device called an “analog-to-digital” converter.
What this does is convert the electrical signal
into a form that a computer can process. This
means transforming a continuous data set into a
discrete data set consisting of 1’s and 0’s.
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COMPUTED RADIOGRAPHY
Once the latent image has been digitized, the data
is processed by a computer. The computer uses a
mathematical technique or “algorithm” using an
image profile that has been entered into its
memory. This profile is called a “histogram.”
HISTOGRAM
COMPUTED RADIOGRAPHY
The processed image – still in digital form – must
be converted back into a form that can be seen by
the human eye. This is accomplished using a
device called a “digital-to-analog” converter.
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COMPUTED RADIOGRAPHY
Provided that the correct histogram was applied
and that the image was exposed correctly, the
resulting image should be displayed showing the
proper level of image brightness and contrast.
IMAGE DISPLAY
The resulting image is displayed on a video display
monitor. This is accomplished by arraying the screen
into a series of rows and columns consisting of
discrete squares or rectangles. These tiny boxes are
called “pixels,” or picture elements. The entire array
is called a “matrix.”
EVALUATING IMAGE QUALITY
Because of the increased exposure latitude of
CR imaging systems, we need a quantitative
method of determining quality. Also, we need
an indirect method of determining the amount
of radiation received by the patient in
generating an image.
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EXPOSURE LATITUDE: FILM
EXPOSURE LATITUDE: DIGITAL IMAGING
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EXPOSURE INDICES
Each of the major manufacturers of computed
radiographic equipment has developed a method
to indirectly measure exposure received by the
imaging plate and are reviewed on the next slide.
Note: this is not a perfect correlation.
EXPOSURE INDICES
Exposure Index Numbers Relative to Imaging
Plate Exposure
Agfa LgM
Speed Class
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Fuji S Value
Kodak Exposure
Index
mR
100
2.95
2.15
1.45
20
200
2,000 20,000
2750
2,000
1,250
10
1
0.1
0.01
EXPOSURE INDICES
Acceptable Exposure Ranges
Manufacturer
Exposure Range
Agfa
2.05-2.35 (log M) logarithmic median
Fuji
200-300 (S-value) sensitivity range
Kodak
1850-2150 exposure index
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DEVIATION INDICES
= 10 ∗ log
DEVIATION INDICES (ACTION
LEVELS)
DI
ACTION RANGE
> +3.0
Excessive patient radiation exposure. Repeat
only if relevant anatomy is clipped or “burned
out.” Require immediate management followup.
+1.0 to +3.0
Overexposure: repeat only if relevant anatomy
is clipped or “burned out.”
-0.5 to +0.5
Target Range
<1.0
Underexposure: Consult radiologist for repeat.
<-3.0
Repeat
From Understanding Exposure and Deviation Indices, by D. Leong,
and M. Butler (AAPM, 2011)
PACS
Transmission of image data from the technologist’s
workstation to the radiologist’s workstation is
achieved by means of a high-speed network called
“Picture Archiving and Communication System.”
Permanent storage of image data is also achieved
by use of this network.
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DIGITAL RADIOGRAPHY
Another method of capturing and processing
images does not involve the use of imaging
plates, but a built-in system of electronics. In
this fashion, a separate image reading device is
not needed.
DIGITAL RADIOGRAPHY
• Amorphous silicon (a-Si) arrays
• Amorphous selenium (a-Se) arrays
INDIRECT CAPTURE: AMORPHOUS SILICON
DETECTOR ARRAYS
This type of system uses a cesium iodide (CsI)
scintillator and silicon (a-Si) diode array. The CsI
converts x-rays to light. The a-Si converts the
light into an electrical impulse (signal) that is
then stored in a matrix of thin-film transistors
(TFT’s).
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AMORPHOUS SILICON ARRAY
AMORPHOUS SILICON ARRAY
DIRECT CAPTURE: AMORPHOUS
SELENIUM ARRAY
This system directly converts x-rays into electrical
signals using a thin layer of amorphous selenium
(a-Se). The interaction with selenium by x-rays
produces so-called “ion hole pairs” that charge
thin-film transistors.
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AMORPHOUS SELENIUM ARRAY
DIGITAL RADIOGRAPHY
Regardless of DR system used, the data contained in
each TFT is systematically read and converted into
digital form using the ADC, then processed. The
image data is converted back into analog form using
a DAC, then displayed on a computer screen.
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ADVANTAGES OF DIGITAL IMAGING
•
•
•
•
•
INCREASED CONTRAST RESOLUTION
FASTER TURNAROUND TIME
SMALLER SPACE REQUIREMENTS
POTENTIAL DOSE REDUCTION
GREATER EXPOSURE LATITUDE
EXPOSURE LATITUDE
DISADVANTAGES OF DIGITAL IMAGING
• LOWER SPATIAL RESOLUTION (CR) COMPARED
WITH FILM-SCREEN IMAGING
• STARTUP COSTS
• LAX STANDARDS INCREASE DOSE
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COMPARING SPATIAL RESOLUTION
• DIGITAL RADIOGRAPHY
– AMORPHOUS SELENIUM ARRAYS (a-Se)
– AMMORPHOUS SILICON ARRAYS (a-Si)
• FILM-SCREEN RADIOGRAPHY
• COMPUTED RADIOGRAPHY
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