1. Computed Radiography

2. Objectives Historical perspectives of computerized imaging
S/F vs CR vs DDR imaging
Basics of CR image capture
CR imaging equipment
Advantages of CR imaging
Radiation Protection

3. Digital Imaging
Image acquisition that produces an electronic image that can be viewed and manipulated on a computer.
Analog-to-digital converters.

4. Development of Digital Imaging
The second major milestone in medical imaging the invention of CT.
Began the coupling of the computers and imaging.
Godfrey Hounsfield in the 1970’s.

5. First-generation CT unit: dedicated head scanner (Photograph taken at Roentgen Museum, Lennep, Germany.)

6. Digital Imaging Techniques
Are used in:
Computed Tomography
Magnetic Resonance Imaging
Diagnostic Ultrasound
Computerized Radiography
Digital Radiography
Digital Fluoroscopy

7. CR & DR imaging
Was limited until sufficient computer technology became available to process the large quantities of data generated
Clinical use began in the 1980’s (CR)
1990’s (DDR)

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

9. Computed Radiography Terms
PSL = photostimulable luminescence
PSP = photostimulable phosphor
SPS = storage phosphor screen
IP = imaging plate
SP = storage phosphor
PMT = photomultiplier tube
PD = photodiode
S/F = screen-film

10. Computed Radiography
Fundamentals of
Computerized
Radiography

12. CR IR Cassette-based digital radiography
In S/F the intensifying screens contain phosphor that emits light in response to x-ray interaction.
In CR, the response to x-ray interaction is trapped photon energy (e-) on the PSP plate.

13. Conventional radiography latent image formation

14. CR latent image formation

15. What Is Digital Imaging?
Digital imaging is the acquisition of images to a computer rather than directly to film.
FOR MANY YEARS WE HAVE BEEN PUTTING IMAGINES ONFILM RATHER THAN INTO A FILM

16. IMAGE CREATION SAME RADIOGRAPHY EQUIPMENT USED
THE DIFFERENCE IS HOW IT IS CAPTURED
STORED
VIEWED
And POST -PROCESSED

17. PSP cassette exposed by conventional x-ray equipment.
General Overview CR
PSP cassette exposed by
conventional x-ray equipment.
Latent image generated as a matrix of trapped electrons in the PSP plate.

18. CR BASICS
Eliminates the need for film as a recording, storage & viewing medium.
PSP Plate – receiver
Archive Manager – storage
Monitor - Viewing

19. 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

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

21. ID STATION

22. CR Readers
AKA
CR Processors

23. Computed Radiographic Readers
Agfa
Fuji
Kodak

25. 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

26. 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

27. 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

28. 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

29. 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
The Conductuve layer is where they are trapped in an area of the crystal known as the color or phosphor center

30. Acquiring the Image cont
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

31. 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.

32. Imaging Plate Construction
Conductive layer. This is a layer of material that absorbs and reduces static electricity
A color layer. Newer plates may contain a color layer, located between the active layer and the support, that absorbs the stimulating light but reflects emitted light.
A support layer. This is a semirigid material that gives the imaging sheet some strength.
A backing layer. This is a soft polymer that protects the back of the cassette.

33. Cross section of a PSP screen

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

35. 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.

36. 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.

37. 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

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

39. 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

40. Sequence of CR imaging

42. 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.

43. PMT (photomultiplier tube)
The intensity (brightness) of the light – correlates to the density on the image
So lots of light will correlate to what size number & what color on the image?
The digital numeric values are stored in matrix form called pixels for display on a cathode ray tube (CRT) or printed on film

44. ERASING THE SCREEN ~50% of trapped electrons released during “read”
After image is recorded
Plate is erased with high intensity white light and re-used
Erasing should be done after every exposure or at minimum every 24 hours to avoid ghosting on future images

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

46. Pixels
Digital images are made of discrete picture elements, arranged in a matrix. The size of the image is described in the binary number system
Modern imaging systems are at least 1024 x 1024
4096 x 4096 is being developed for digital radiography

47. Matrix = each cell corresponds to a specific location on the image

48. Pixel

49. Pixels

50. 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

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

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

53. Computed Radiography
As the plate is scanned “read” data is collected at a specific frequency = Sampling frequency
Spatial resolution determined by sampling frequency
With some systems, the smaller IR’s have higher sampling frequencies & more pixels per mm = more spatial resolution

54. Signal Loss (reducing image quality) Signal-to-Noise Ratio
Principle source of noise on the image is scatter radiation.
Scattering of emitted light off screen.
The efficiency of the photomultiplier tubes (PMTs) and photodiodes (PDs)

55. Improving Signal-to-Noise Ratio
Optical filter is used to prevent the longer-wavelength laser light affecting the image formation.
Increasing exposure to IR. Newer CR systems are better at reducing noise at lower exposure levels.

56. Technique Selections
What are some factors that technologist must take into consideration when selecting a technique?
What regulates that technologists to select appropriate techniques in F/S? CR & DR ?

57. Screen / Film Self regulating system
The receptor speed and film H&D curve defined the proper exposure
Achieving optimal OD guaranteed appropriate exposure to the patient
Over exposed = dark image
Correct exposure = correct OD
Under exposed = light image

58. Characteristic curve
of radiographic film

59. Characteristic Curve
Sometimes called the H&D curve for Hurter and Driffield, who first described the relationship
Describes the relationship between OD and exposure
Exposure changes near the toe or shoulder result in very little OD changes

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

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

62. S/F fundamental principle
Keep receptor exposure constant for given receptor response
The receptor exposure level (mR) depends on
Receptor “speed”
100 speed ~ 2 mR (to IR for appropriate OD)
200 speed ~1 mR
400 speed ~ 0.5 mR

63. Speed Class
For CR imaging the speed is determined by the image quality required by the Radiologists or programmed by the service engineer
Dependent on exposure to the PSP plate.
200 RS = highest quality images.
Increasing RS will reduce contrast resolution and patient dose.
S numbers, Index numbers, Sensitivity numbers, Exposure index
Each imaging system is unique

64. Dose Implications
Images nearly always look better at higher exposures.
F/S system on average used a 400 RS combination
CR looks best at 200 RS.
If you were a manufacture how would you set up your system for image quality? vs ALARA?

66. 80 KVP 5 30 5 15
100
200
500

67. Dose Implications
Huge dynamic range means nearly impossible to overexpose
Then the COMPUTER corrects majority of exposure errors
Therefore almost ANY technique can be used on the patient –
The computer will fix it.

68. POST PROCESSING
Part Selection

69. Workstation Menu

70. ALGORITHM – a set of mathematical values used to solve a problem or find an average
HISTOGRAM – a bar graph depicting the density distribution (in numerical values) of the imaging plate

71. Wrong Algorithm

72. 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.

73. Histogram Analysis
A histogram is a plot of gray scale value vs. the frequency of occurrence
# pixels of the gray value in the image
A graph that displays signal value
x-axis related to amount of exposure
y-axis displays number of pixels for each exposure
Series of peaks and valleys
Pattern varies for each body part

74. Statistical plots of the frequency of occurrence of each pixel's value

75. Initial Image Processing
Automated exposure field edge detection
Eliminates signals outside collimation margins
If margin not detected, extraneous data included in the histogram.

76. EDR Exposure Data Recognition
When laser scans it is looking for area of plate that has exposure
Some read from center out and look for two sides of collimation
Works best when image centered

77. 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

78. Exposure Values
Each system has range of values for appropriate exposure for part
The range used by vendor is very broad
Each facility should develop its own exposure range taking into account
Radiologist preference
ALARA

79. S numbers, Index numbers, Sensitivity numbers, Exposure index
The total signal is not a measure of the dose to the patient but indicates how much radiation was absorbed by the plate
A 1 mR exposure will give
Fuji S# 200
Kodak/Carestream EI 2,000
Agfa 200 speed lgm reference value for site

80. EXPOSURE VALUES Exposure indicator S number inverse to exposure
Plates sensitive to 0.1 mR – 100 mR
“S” number for Fuji
S number inverse to exposure
S=2 (100 mR), S=200 (1 mR)
Carestream/Kodak uses exposure index – direct relationship
2000 (1 mR), 3000 (10 mR)

81. Using Exposure Numbers
Fuji, if appropriate # is 200 then
At 400, too light, double mAs for 200
At 100, too dark, half mAs for 200
Kodak, if appropriate # is 1800
At 1500, too light, double mAs for 1800
At 2100, to dark, half mAs for 1800

82. Exposure Numbers
The exposure numbers can only be used if all other parameters are correct
Centering to plate
Collimation
Position over AEC, look at mAs readout to determine if poor positioning caused light or dark image

83. Machine shut down and closed collimator when turned back on
Machine shut down and closed collimator when turned back on. S# 12,361 lat CXR
S# 8,357
S# 12,361 lat CXR
S# 8,357

84. Same technique, different centering and collimation

85. 􀂡2 on 24 X 30
􀁹Technique adjusted
2 on 24 X 30
􀁹Same technique
􀁹Rescaling error.

86. Histogram Analysis Collimation is very important
If plate reader cannot find collimated edges then all the exposure on plate will be included in the histogram
Histogram from plate is compared to body part histogram stored in computer

87. Characteristic curve & histogram

88. Histogram with H&D curve

89. Underexposed

90. Overexposed

91. Just right!

92. Changes to Histogram Hip prosthesis Line caused from dirt collected
in a CR Reader

93. Higher kVp
Smaller signal difference
Narrower data range
Photons to IR

94. Lower kVp Larger signal difference Wider data range
More photons will be absorbed

95. kVp vs. Data Width Very difficult concept for radiographer
Focused on exposure vs. appearance
High kVp = longer scale = wider
Low kVp = shorter scale = narrower
Change focus to underlying physics
High kVp = less differential attenuation
Smaller signal difference

96. LUT Look Up Table (LUT) Each anatomic area has a LUT or Algorithm
Used to adjust contrast and density
Other terms that may be used for this
Contrast rescaling
Contrast processing
Gradation processing
Tone scaling

97. 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)

98. Automatic rescaling
Mapping grayscale to “pixel values of interest” to achieve specific display levels.
Critical elements
Peaks
Troughs
Width
Locates VOI (values of interest)
Exposure index determination

99. LUT
1. is unprocessed, 2. algorithm finds anatomy, 3. finished

100. Myths for CR #1 & 2 2. . kVp – myth: digital is kVp driven.
1. mAs – myth: digital is mAs driven
Truth:
2. . kVp – myth: digital is kVp driven.

101. Contrast What determines contrast ?
Which factors have more impact on contrast than kVp for all systems?
Anatomical structure
Contrast media
Grid utilization
Grid vs. non-grid
Grid efficiency
Processing algorithm

102. TECHNIQUE CONISDERATIONS
kVp energy dependant
Now COMPUTER controls CONTRAST
Higher kVp to stimulate electron traps
kVp range for CR

103. Using Higher kVp with Digital
There is a limit ! !
If higher kVp is used to limit dose.
Remember basic physics.
Higher kVp – more transmission
Lower kVp – more photoelectric
Too low mAs can cause quantum mottle regardless of kVp used

105. kVp Selection Anatomical contrast
Inherent Δ attenuation due to structure
Grid utilization
Yes, No, Efficiency
Ratio, frequency, lead content.
Contrast improvement
Default processing algorithm

106. kVp Selection Determined by anatomy and grid
Adults: 􀂃 Optimal range 60 – 120
􀁹 Pediatrics < 100 lbs: Optimal range
May use higher kVp than with S/F.
Helps limit dose increase
Narrows acquired data range.

107. kVp ranges for CR Infant extremities 50 - 60 kVp
Adult extremities kVp
Bucky extremities 75 – 90 kV
AP spine kVp
Lateral spine 85 – 100 kVp
Chest 110 – 130 kVp
Skull 80 – 90 kVp
Only 1 kVp is not reccomended

108. Another set of suggested kVp ranges.
Distal Extremities = 65 – 75
Ped. extremities =
KUB =
IVU = 70 – 80
BE =
Grid extremities =
L-spine/Pelvis = 85 – 90
Chest / grid = 110 – 130
Ped. chest NG 70 – 80
Ribs = 80 – 90
T-spine = 90 – 100

109. Collimation
Proper collimation is the best way to enhance your CR image. Why?
What is shuttering?
Is it the same as collimating?

110. Myths for CR #3
Collimation – myth: you cannot collimate.
Truth:

111. Shuttering

112. If radiologist object. Apply back border.

113. Collimation Collimation Less water irradiated Less scatter produced
Improves contrast
Reduces effective dose
Reduces automatic rescaling errors

115. Exposure Field Recognition If the exposure field is not recognized the entire plate is used in image construction

121. Myths for CR #4 Grid – myth: cannot use grids and don’t need them.
Truth:

122. Grid vs. Non-grid

123. Myths for CR #5
5. SID – myth: magnification doesn’t occur with digital so SID is unimportant.
Truth:

124. Myths for CR #6
Speed class – myth: it is a 200 speed class, you need to double your mAs and increase your kVp by 10.
Truth:

125. Myth for CR #7 & 8
7. Fog – myth: digital systems can’t be fogged by scatter or background radiation.
Truth:
Myth: fluorescent lights fog PSP plates.

126. ADVANTAGE OF CR/DR vs FS
Rapid storage
retrieval of images NO LOST FILMS!
PACS (storage management)
Teleradiology - long distance transmission of image information
Economic advantage - at least in the long run?

127. ADVANTAGE OF CR/DR
Can optimize image quality by manipulating digital data to improve visualization of anatomy and pathology AFTER EXPOSURE TO PATIENT

128. Image Preprocessing Data recognition is very important
Two on one imaging? Can it be done? Is it good practice?
Carter pg. 87

129. Improves Exam Times ?

130. High resolution with digital imaging

131. NEW IMAGE
towel that was used to help in positioning a child
CR is MORE sensitive to
ARTIFACTS

132. Questions ?