1. DIGITAL EQUIPMENT
MAY 2008

2. TERMINOLOGY REVIEW
ARRT CONTENT SPECS
2008

3. ARRT SPECS - DIGITAL Image Receptors digital image characteristics
spatial resolution
sampling frequency
DEL (detector element size)
receptor size and matrix size
image signal (exposure related)
quantum mottle
SNR (signal to noise ratio) or
CNR (contrast to noise ratio)
Digital Systems
electronic collimation
grayscale rendition or look-up table (LUT)
edge enhancement/
noise suppression
contrast enhancement
system malfunctions (e.g., ghost image, banding, erasure, dead pixels, readout problems, printer distortion)

4. ARRT SPECS - DIGITAL Image Display Image Acquisition and Readout
viewing conditions (i.e., luminance,ambient lighting
spatial resolution
contrast resolution/dynamic range
DICOM gray scale function
window level and width function
Image Acquisition and Readout
PSP (photo-stimulable phosphor)
flat panel detectors
(direct and indirect)
Noise
Acceptable Range of Exposure
Exposure Indicator Determination
Gross Exposure Error
Image Degradation (mottle, light or dark, low contrast)

5. ARRT SPECS - DIGITAL Recognition of Malfunctions
Digital Image Receptor Systems
Digital artifacts
(grid lines, Moiré effect or aliasing)
maintenance (e.g., detector fog)
( non-uniformity, erasure)

6. ARRT SPECS - DIGITAL PACS
HIS (hospital information system) - work list
RIS (radiology information system)
DICOM
Workflow (inappropriate documentation, lost images, mismatched images, corrupt data)
windowing and leveling

8. Review of Digital Radiography and PACS

9. Key Terms Computed radiography
DICOM (digital imaging and communications in medicine)
Digital imaging
Digital radiography
Direct capture DR
Indirect capture DR
PACS
Teleradiology

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

11. Image Acquisition and Readout
PSP (photo-stimulable phosphor)
flat panel detectors
(direct and indirect)

12. Computed Radiography Uses storage phosphor plates
Uses existing equipment
Requires special cassettes
Requires a special cassette reader
Uses a computer workstation and viewing station and a printer

13. Computed Radiography
Storage phosphor plates are similar to intensifying screens.
Imaging plate stores x-ray energy for
an extended time.
Process was first introduced in the United States by Fuji Medical Systems of Japan in 1983.
First system used a phosphor storage plate, a reader, and a laser printer.

14. Imaging Plate Construction
Image recorded on a thin sheet of plastic known as the imaging plate
Consists of several layers:

15. Cassette and Imaging Plate
Cassette contains a window with a barcode label or barcode sticker on the cassette.
Label enables technologist to match the image information with the patient-identifying barcode on the exam request.

16. Using the Laser to Read the Imaging Plate
The light collection optics direct the released phosphor energy to an optical filter and then to the photodetector.
Although there will be variances between manufacturers, the typical throughput is 50 cassettes per hour.
Some manufacturers claim up to 150 cassettes per hour, but based on average hospital department workflow, 50 cassettes per hour is much more realistic.

17. Process up to 101 cassettes an hour.
• Handle 16 cassettes at one time:
up to 8 queued for processing,
and 8 erased and ready for new
imaging studies.
• Cassette is ready to reuse in
40 seconds.*
• Review an image in 34 seconds
at a Kodak DirectView remote
operations panel.*
• “Drop-and go” workflow virtually

18. Based on proven DirectView CR 850 system design
·Process up to x 43 cm plates an hour
·Small footprint size of 25 x 29 inch (63.5 x 73.6 cm

19. Digital Radiography Cassetteless system
Uses a flat panel detector or charge-coupled device (CCD) hard-wired to computer
Requires new installation of room or retrofit

20. Digital Radiography DR is hard-wired. DR is cassetteless.
Detectors are permanently enclosed inside a rigid protective housing.
Thin-film transistor (TFT) detector arrays may be used in direct- and indirect-conversion detectors.

21. Digital Radiography Two types of digital radiography
Indirect capture DR
Machine absorbs x-rays and converts them to light.
CCD or thin-film transistor (TFT) converts light to electric signals.
Computer processes electric signals.
Images are viewed on computer monitor.

22. Digital Radiography Direct capture DR Photoconductor absorbs x-rays.
TFT collects signal.
Electrical signal is sent to computer for processing.
Image is viewed on computer screen.

23. Digital Radiography
DR used CCD technology developed by the military and then used TFT arrays shortly after.
CCD and TFT technology developed and continues to develop in parallel.
No one technology has proved to be better than the other.

24. Flat-Panel Detectors Consist of a photoconductor Amorphous selenium
Holds a charge on its surface that can then be read out by a TFT

25. Direct Conversion X-ray photons are absorbed by the coating material.
Photons are immediately converted into an electrical signal.
The DR plate has a radiation-conversion material or scintillator.

26. Direct Conversion DR Scintillator
Typically made of amorphous selenium
Absorbs x-rays and converts them to visible photons
Converts photons to electrical charges
Charges stored in the TFT detectors

27. Indirect Conversion
Similar to direct detectors in that the TFT technology is also used
Two-step process:
X-ray photons are converted to light.
Light photons are converted to an electrical signal.
A scintillator converts x-rays into visible light.
Light is then converted into an electrical charge by photodetectors such as amorphous silicon photodiode arrays or charge-coupled devices, or CCDs.

28. Indirect Conversion
More than a million pixels can be read and converted to a composite digital image in under a second.

29. Comparison of Film to CR and DR
For conventional x-ray film and computed radiography (CR), a traditional x-ray room with a table and wall Bucky is required.
For DR, a detector replaces the Bucky apparatus in the table and wall stand.
Conventional and CR efficiency ratings are about the same.
DR is much more efficient, and image is available immediately.

30. Comparison of Film to CR and DR
A storage phosphor plate is placed inside of CR cassette.
Most storage phosphor plates are made of a barium fluorohalide.
When x-rays strike the photosensitive phosphor, some light is given off.
Some of the photon energy is deposited within the phosphor particles to create the latent image.
The phosphor plate is then fed through the CR reader.

31. Comparison of CR and DR CR, continued
Focused laser light is scanned over the plate, causing the electrons to return to their original state, emitting light in the process.
This light is picked up by a photomultiplier tube and converted into an electrical signal.
The electrical signal is then sent through an analog-to-digital converter to produce a digital image that can then be sent to the technologist review station.

32. Comparison of CR & DR DR No cassettes are required.
The image acquisition device is built into the table and/or wall stand or is enclosed in a portable device.
Two distinct image acquisition methods are indirect capture and direct capture.
Indirect capture is similar to CR in that the x-ray energy stimulates a scintillator, which gives off light that is detected and turned into an electrical signal.
With direct capture, the x-ray energy is detected by a photoconductor that converts it directly to a digital electrical signal.

33. Amorphous Silicon Detector
The light photons are then converted into an electric charge by the photodiode arrays.
Unlike the selenium-based system used for direct conversion, this type of indirect-conversion detector technology requires a two-step process for x-ray detection.
The scintillator converts the x-ray beams into visible light, and light is then converted into an electrical charge by photodetectors, such as amorphous silicon photodiodes

34. Cesium Iodide Detectors
A newer type of amorphous silicon detector uses a cesium iodide scintillator.
The scintillator is made by growing very thin crystalline needles (5 µm wide) that work as light-directing tubes, much like fiber optics.
This allows greater detection of x-rays, and because there is almost no light spread, there is much greater resolution.

35. Cesium Iodide Detectors
These needles absorb the x-ray photons and convert their energy into light, channeling it to the amorphous silicon photodiode array.
As the light hits the array, the charge on each of the photodiodes decreases in proportion to the light received.

36. Charge-Coupled Devices
The oldest indirect-conversion DR system is based on CCDs.
X-ray photons interact with a scintillation material, such as photostimulable phosphors, and this signal is coupled or linked by lenses or fiber optics, which act like cameras.

37. Charge-Coupled Devices
These cameras reduce the size of the projected visible light image and transfer the image to one or more small (2 to 4 cm2) CCDs, which convert the light into an electrical charge.
This charge is stored in a sequential pattern and released line by line and sent to an analog-to-digital converter.

38. Charge-Coupled Devices
Even though CCD-based detectors require optical coupling and image size reduction, they are widely available and relatively low in cost.

39. Summary
There are two types of cassetteless digital imaging systems: direct and indirect.
Direct sensors are TFT arrays of amorphous silicon coated with amorphous selenium.
Direct sensors absorb x-ray photons and immediately convert them to an electrical signal.

40. Summary
Indirect-conversion detectors use a scintillator that converts x-rays into visible light, which is then converted into an electrical charge.
CCDs act as miniature cameras that convert light produced by x-ray interaction with photostimulable phosphors into an electrical charge.

41. Image Display viewing conditions (i.e., luminance,ambient lighting
DICOM gray scale function
window level and width function
spatial resolution
contrast resolution/dynamic range

42. MONITOR RESOLUTION DICOM gray scale function
window level and width function

43. Depending on modalities such as CT, CR, MRI, resolution requirements can range
from 1.3 megapixels to 5 megapixels.
Generally, 3 megapixel and higher class displays are used for softcopy interpretation.
Where higher accuracy and a subtle reproduction of grayscale are critical in applications such as
mammography imaging, 5 megapixel resolution is required.  

44. viewing conditions luminance,ambient lighting

45. DICOM gray scale function window level and width function
A photometer to a monitor screen in a check of the monitor's conformance with the DICOM Grayscale Standard Display Function.

46. DICOM gray scale function window level and width function

52. Grayscale or color monitors
Digital Systems
electronic collimation
grayscale rendition or look-up table (LUT)
edge enhancement/
noise suppression
contrast enhancement

53. Detective Quantum Efficiency
How efficiently a system converts the x-ray input signal into a useful output image is known as detective quantum efficiency, or DQE.
DQE is a measurement of the percentage of x-rays that are absorbed when they hit the detector.

54. Detective Quantum Efficiency
In other words, CR records all of the phosphor output. Systems with higher quantum efficiency can produce higher-quality images at a lower dose.
Indirect and direct DR capture technology has increased DQE over CR.
However, DR direct capture technology, because it does not have the light conversion step and consequently no light spread, increases DQE the most.

55. Image Display
spatial resolution contrast resolution/dynamic range

56. Spatial Resolution
Spatial resolution refers to the amount of detail present in any image.
Phosphor layer thickness and pixel size determines resolution in CR.
The thinner the phosphor layer is, the higher resolution.
Film/screen radiography resolution at its best is limited to 10 line pairs per millimeter (lp/mm).
CR resolution is 2.55 lp/mm to 5 lp/mm, resulting in less detail.

57. Spatial Resolution
CR dynamic range, or the number of recorded densities, is much higher, and lack of detail is difficult to discern.
More tissue densities on the digital radiograph are seen, giving the appearance of more detail.

58. SPATIAL RESOLUTION

59. Spatial Resolution determined by:
􀁹 Pixel size.
CR- sampling frequency
DR – DEL size
􀁹 There are relationships between
Pixel size
Receptor size
Matrix size
􀁹 pixel size = larger matrix
􀁹 receptor size = larger matrix
Spatial resolution is not related the amount of exposure

60. Spatial Resolution
knee radiograph typically does not show soft tissue structures.
A digital image shows not only the soft tissue but also the edge of the skin. This is due to the wider dynamic recording range and does not mean that there is additional detail.

61. Spatial Resolution
Depending on the physical characteristics of the detector, spatial resolution can vary a great deal.
Spatial resolution of amorphous selenium for direct detectors and cesium iodide for indirect detectors is higher than CR detectors but lower than film/screen radiography.

63. Spatial Resolution
Excessive image processing, in an effort to alter image sharpness, can lead to excessive noise.
Digital images can be processed to alter apparent image sharpness; however, excessive processing can lead to an increase in perceived noise.
The best resolution is achieved by using the appropriate technical factors and materials.

64. Speed
In conventional radiography, speed is determined by the size and layers of crystals in the film and screen.
In CR, speed is not exactly the same because there is no intensifying screen or film.
The phosphors emit light according to the width and intensity of the laser beam as it scans the plate, resulting in a relative speed that is roughly equivalent to a 200-speed film/screen system.

65. Speed
CR system speeds are a reflection of the amount of photostimulable luminescence given off by the imaging plate while being scanned by the laser.
For example, Fuji Medical Systems reports that a 1-mR exposure at 80 kVp and a source-to-image distance of 72 inches will result in a luminescence value of 200, hence the speed number.

66. Speed
In CR, most cassettes have the same speed; however, there are special extremity or chest cassettes that produce greater resolution.
These are typically 100 relative speed.
Great care must be taken when converting to a CR system from a film/screen system to adjust technical factors to reflect the new speed.

67. Exposure Latitude or Dynamic Range
Conventional radiography
Based on the characteristic response of the film, which is nonlinear.
Radiographic contrast is primarily controlled by kilovoltage peak.
Optical density on film is primarily controlled by milliampere-second setting.

68. CR Cassettes
Because so many more densities are recorded in CR (wide dynamic range), images appear more detailed.
Because energy stored in the imaging plate is lost over time, imaging plates should be read as quickly as possible to avoid image information loss.
Imaging plates are erased by exposing them to bright light such as fluorescent light.

69. Exposure Latitude or Dynamic Range
CR and DR
Contain a detector that can respond in a linear manner.
Exposure latitude is wide, allowing the single detector to be sensitive to a wide range of exposures.
Kilovoltage peak still influences subject contrast, but radiographic contrast is primarily controlled by an image processing look-up table. LUT
Milliampere-second setting has more control over image noise, whereas density is controlled by image-processing algorithms.

70. Density
.25 TO -2.5
The straight line of the H&D curve

71. Optical Density
A numerical value indicating the degree of blackening on the film.
(average OD seen on a radiograph = 1.2 Range is 0.21 – 2.5)
# of photons coming through film = OD # # of photons hitting film
OD=
1 = = = =

75. Why do digital systems have significantly greater latitude?
Linear response give the imaging plates greater latitude
Area receving little radiation can be enhanced by the computer
Higer densities can be separated and brought down to the visibile density ranges

76. 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.

78. 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.

79. Look-Up Table The look-up table (LUT) is a reference histogram.
LUT is used as a cross-reference to transform the raw information.
LUT is used to correct values.
LUT has a mapping function:
All pixels are changed to a new gray value.
Image will have appropriate appearance in brightness and contrast.
LUT is provided for every anatomic part.

80. Look-Up Table LUT can be graphed as follows:
Plotting the original values ranging from 0 to 255 on the horizontal axis
Plotting new values, also ranging from 0 to 255 on the vertical axis
Contrast can be increased or decreased by changing the slope of this graph.
Brightness (density) can be increased or decreased by moving the line up or down the y-axis.

81. Histogram Analysis
It is important to choose the correct anatomic region on the menu before exposing the patient.
Raw data used to form the histogram are compared with a “normal” histogram of the same body part by the computer.

82. Image Receptors digital image characteristics spatial resolution
sampling frequency
DEL (detector element size)
receptor size and matrix size
image signal (exposure related)
quantum mottle
SNR (signal to noise ratio) or
CNR (contrast to noise ratio)

83. Matrix size is determined by . . .
􀁹 Receptor size (Field of View: FOV)
􀁹 Pixel size
CR - Sampling frequency
DR - DEL size (Dector ELement)

84. DIGITAL: MATRIX SIZE The number of rows and columns of
pixels in the image representation.
7 X 7

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

89. Digitizing the Signal
So, how bright a pixel is determines where it will be located in the matrix in conjunction with the amount of gray level or bit depth.
Some CR systems have bit depths of 10 or 12, resulting in more shades of gray.
Each pixel can have a gray level between 0 (20) and 4096 (212). The gray level will be a factor in determining the quality of the image.

90. Summary
Pixel and matrix size are important in determining the amount of resolution and the size of the image to be stored in the PACS system. In TFT technology, pixel and matrix size are determined by the amount of area available to “fill” with photons.

91. ARRT SPECS - DIGITAL PACS
HIS (hospital information system) - work list
RIS (radiology information system)
DICOM
Workflow (inappropriate documentation, lost images, mismatched images, corrupt data)
windowing and leveling

93. Picture Archival and Communication Systems
Networked group of computers, servers, and archives to store digital images
Can accept any image that is in DICOM format
Serves as the file room, reading room, duplicator, and courier
Provides image access to multiple users at the same time, on-demand images, electronic annotations of images, and specialty image processing

94. PACS

95. PACS Uses Made up of different components Reading stations
Physician review stations
Web access
Technologist quality control stations
Administrative stations
Archive systems
Multiple interfaces to other hospital and radiology systems

96. DICOM
stands for digital imaging and communications in medicine, and it is a universally accepted standard for exchanging medical images between networked medical devices.

97. HIS RIS
The HIS holds the patient’s full medical information from hospital billing to the inpatient ordering system.
The RIS holds all radiology-specific patient data from the patient scheduling information to the radiologist’s dictated and transcribed report.

98. HIS – RIS INTERFACE

99. Image Acquisition and Readout
PSP (photo-stimulable phosphor)
flat panel detectors
(direct and indirect)
Noise
Acceptable Range of Exposure
Exposure Indicator Determination
Gross Exposure Error
Image Degradation (mottle, light or dark, low contrast)

100. Exposure Indicators
The amount of light given off by the imaging plate is a result of the radiation exposure that the plate has received.
The light is converted into a signal that is used to calculate the exposure indicator number, which is a different number from one vendor to another.

101. Exposure Indicators
The base exposure indicator number for all systems designates the middle of the detector operating range.
For Fuji, Phillips, and Konica systems, the exposure indicator is known as the S, or sensitivity, number.
The S number is the amount of luminescence emitted at 1 mR at 80 kVp, and it has a value of 200.

102. Exposure Indicators
The higher the S number with these systems, the lower the exposure.
For example, an S number of 400 is half the exposure of an S number of 200, and an S number of 100 is twice the exposure of an S number of 200.

103. Exposure Indicators
The numbers have an inverse relationship to the amount of exposure so that each change of 200 results in a change in exposure by a factor of 2.
Kodak uses exposure index, or EI, as the exposure indicator.
A 1 mR exposure at 80 kVp combined with aluminum/copper filtration yields an EI number of 2000.

104. Exposure Indicators
An EI number plus 300 (EI + 300) is equal to a doubling of exposure, and an EI number of minus 300 (EI − 300) is equal to a halving of exposure.
The numbers for the Kodak system have a direct relationship to the amount of exposure so that each change of 300 results in change in exposure by a factor of 2.

105. Exposure Indicators
This is based on logarithms, only instead of using 0.3 (as is used in conventional radiographic characteristic curves) as a change by a factor of 2, the larger number 300 is used.
This is also a direct relationship; the higher the EI, the higher the exposure.

106. Exposure Indicators
The term for exposure indicator in an Agfa system is the lgM, or logarithm of the median exposure.
An exposure of 20 µGy at 75 kVp with copper filtration yields an lgM number of 2.6.
Each step of 0.3 above or below 2.6 equals an exposure factor of 2.
An lgM of 2.9 equals twice the exposure of 2.6 lgM, and an lgM of 2.3 equals an exposure half that of 2.6.
The relationship between exposure and lgM is direct

107. Image Receptors digital image characteristics spatial resolution
sampling frequency
DEL (detector element size)
receptor size and matrix size
image signal (exposure related)
quantum mottle
SNR (signal to noise ratio) or
CNR (contrast to noise ratio)

108. Stiching an image

109. Portrait vs landscape mode

110. Edge enhancement & post processing

111. Enhanced Visualization Image Processing
Kodak
Takes image diagnostic quality to a new level
Increases latitude while preserving contrast
Process decreases windowing and leveling
Virtually eliminates detail loss in dense tissues

112. Grid Selection
Digital images are displayed in tiny rows of picture elements or pixels.
Grid lines that are projected on the imaging plate when using a stationary grid can interfere with the image, resulting in a wavy artifact known as a moiré pattern.
This pattern occurs because the grid lines and the scanning laser are not parallel.

113. Collimation
Although the use of a grid decreases the amount of scatter that exits the patient from affecting latent image formation, properly used collimation reduces the area of irradiation and reduces the volume of tissue in which scatter can be created.

114. Collimation
This results in increased contrast because of the reduction of scatter as fog and reduces the amount of grid cleanup necessary for increased resolution.
Through postexposure image manipulation known as shuttering, a black background can be added around the original collimation edges, virtually eliminating the distracting white or clear areas.

115. Collimation
However, this technique is not a replacement for proper preexposure collimation.
Shuttering is an image aesthetic only and does not change the amount or angles of scatter created.
There is no substitute for appropriate collimation, for collimation reduces patient dose.

116. Automatic Data Recognition
Collimation is automatically recognized, and a complete histogram analysis occurs.
Good collimation practices are critical because overcollimation or undercollimation leads to data recognition errors that affect the histogram.

117. Mis registration – needs reprocessing

118. Common CR Image Acquisition Errors
As with film screen, artifacts can detract and degrade images.
Imaging plate artifacts
Plate reader artifacts
Image processing artifacts
Printer artifacts
Operator errors

119. Imaging Plate Artifacts
As the imaging plate ages, it becomes prone to cracks from the action of removing and replacing the imaging plate within the reader.
Cracks in the imaging plate appear as areas of lucency on the image.
The imaging plate must be replaced when cracks occur in clinically useful areas.

120. Imaging Plate Artifacts
Adhesive tape used to secure lead markers to the cassette can leave residue on the imaging plate.
If static exists because of low humidity, hair can cling to the imaging plate.

121. Imaging Plate Artifacts
Backscatter created by x-ray photons transmitted through the back of the cassette can cause dark line artifacts.
Areas of the lead coating of the cassette that are worn or cracked allow scatter to image these weak areas. Proper collimation and regular cassette inspection helps to eliminate this problem.

122. Plate Reader Artifacts
The intermittent appearance of extraneous line patterns can be caused by problems in the electronics of the plate reader.
Reader electronics may have to be replaced to remedy this problem.

123. Plate Reader Artifacts
Incorrect erasure settings result in a residual image left in the imaging plate before the next exposure.
Results vary depending on how much residual image is left and where it is located.
Orientation of a grid so that the grid lines are parallel to the laser scan lines of the plate reader results in the moiré pattern error. Grids should be high frequency, and the grid lines should run perpendicular to the laser scan lines of the plate reader.

124. Operator Errors
Insufficient collimation results in unattenuated radiation striking the imaging plate.
The resulting histogram is changed so that it is outside the normal exposure indicator range for the body part selected.
Using the smallest imaging plate possible and proper collimation, especially on small or thin patients, eliminates this error.

125. Operator Errors
If the cassette is exposed with the back of a cassette toward the source, the result is an image with a white grid-type pattern and white areas that correspond to the hinges.
Care should be taken to expose only the tube side of the cassette.