1. Review CR & DIGITAL IMAGING (1) 2012 – RT 244 wk 15
References: Bushong Vol 9

2. Objectives
Digital imaging review
Review CR fundamentals

3. Digital Imaging
Image acquisition that produces an electronic image that can be viewed and manipulated on a computer.
Examples?

4. Methods to Digitize an Image
1. Film Digitizer
2. Video Camera (vidicon or plumbicon)
3. Computed Radiography
4. Direct Radiography
PACS
DICOM

5. DDR CR Digital Radiography Direct Capture Indirect Capture Computed
(CR) - PSL
Direct-to-Digital
Radiography
(DDR)-Selenium
Direct-to-Digital
Radiography
Silicon Scint.
Laser
Scanning
Digitizers

7. 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.

9. 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

10. Alphabet in Binary

11. What is a Pixel?

12. Basics of Digital Images
digital images are a (matrix) of pixel (picture element) values

13. Pixel

15. Computed Radiography
Fundamentals of
Computerized
Radiography

16. What are the CR system components?

17. CR SYSTEM COMPONENTS CASSETTES (phosphor plates) ID STATION
IMAGE PREVIEW (QC) STATION
DIGITIZER
VIEWING STATION

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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.

24. 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.

25. IP Construction

26. Cross section of a PSP screen

27. Needle PSP increase the absorption of x-rays and limit the spread of light emission

28. 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.

29. 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.

30. X-ray interaction with a PSP screen1
X-ray interactions with the screen phosphors causes an e- to excited 2
When e- return to ground
state visible light is emitted

31. CR Phosphor Plates ABSORPTION EMISSION X-RAY LIGHT LASER STIMULATION
ELECTRON
TRAP
ELECTRON
TRAP
X-RAY
LIGHT

32. 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

33. 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

34. 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.

35. Sequence of CR imaging

36. 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.

37. 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

40. Look Up Table (LUT) processing Windowing
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.

42. DIGITAL “DENSITY”

43. 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.

44. Characteristic curve
of radiographic film

48. Widow level & width Same photons at the image receptor
Image is post processed – changing
Brightness and contrast of image appearance

49. 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.

50. 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.

51. advantages of windowing?

56. 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.

61. Exposure Latitude It is the optimal exposure range
relative to the “ideal” exposure
that produces a quality image at
an appropriate patient dose.

62. 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

63. Exposure Latitude
The analog receptor exposure latitude ranges from approximately
30% underexposed
to 50% overexposed relative to
the “ideal” exposure level.

64. Exposure Latitude The digital image receptor
exposure latitude ranges from
approximately
50% underexposed
to 100% over exposure
relative to the “ideal” exposure level.

66. 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

67. 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.

68. 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.

69. 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.

70. Screen / Film Imaging = self regulating
2 mAs
Under exposed
6 mAs
Correct exposure
24 mAs
Over exposed

71. CR imaging results in 10,000 shades of gray Fixed kVp exposures
mAs = 1.0
S = 175
mAs = 0.5
S = 357
mAs = 2.0
S = 86
mAs = 5.0
S = 35

72. 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

73. 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)

74. Characteristic curve & histogram

75. Underexposed

76. Overexposed

77. Just right!

78. LOOK UP TABLE (LUT) Black Saturation White Saturation Linear LUT
Black Shirt
One more concept in the image processor that is critical. It’s called the LUT. It stands for lookup table. The situation here is that if we take this picture and we say “Let’s take a look at how much of the image is black and how much is white and how much is gray”. This is a little fancy curve, it’s called a histogram. What you see is all the blacks are down here. This is a lot of her shirt down here. And this is the gray area. In fact, these are most of the facial tones. If you want to reproduce this image exactly as it was taken on the image intensifier, you want the whole thing to be linear. Every time you put this energy in, you want the exact out. Again, like anything else, I can play with this lookup table and say “Forget about the blacks. Everything that’s below this curve let’s make black. Because what I really want to do is I want to see the shades of gray in this white area”. Or I can do the reverse. I can say “Forget about all the white stuff. I want to see the shades of gray in here”. Now, why would anybody in their right mind want to do this? Easy. Radiography is a game of shades of gray. That’s all the radiologist is doing. He wants to see a little bit of a calcification here, a little bit of air in the lung here. It’s always a game of the shades of gray. So, the more you can take his area of interest and stretch it out for him, he’s going to be a happy camper. And he’s willing to compromise that the other areas are going to be bad. Look at this original image here. You’d say “That’s the image I’d want”. Can you tell me what kind of sweater she’s got on. Look at the ribbing. So, we might say, we’re not interested in her face and her hair, but we’re interested in the shades of gray in the vest, or in the cardiac or mediastinum in the medical world. So, the point here is that by varying lookup tables, we can actually also vary the ability to see varying shades of gray. This image processor can play games with low pass, high pass, or lookup tables to create images that are just spectacular, or garbage if you happen to set it up wrong.
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

79. 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

80. Display Bit Depth 1 bit 6 bit 8 bit
2 shades shades shades

81. 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

82. Digital - Grayscale Bit depth.
Number of gray shades available for display
8 bit 256
10 bit 1024
12 bit 4096
14 bit 16384

86. CR Image Quality Pixels, Field of View, Image receptor
Sampling frequency, Quantization, Nyquest frequency
Noise
Magnification
Image compression (lossless vs lossee)
Questions?