1. Things they never tell you when the equipment is installed REV 11/2008
Digital Radiography Or
Things they never tell you when the equipment is installed REV 11/2008

2. Computed Radiography
Fundamentals of
Computerized
Radiography

3. Imaging Plates Latitude and Contrast
Independent functions
Contrast is dependent on look-up table (LUT) and final image processing
Image appearance not as closely related to exposure factor selection as F/S was
Except when extremely out of range

4. IMAGE READING

5. 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

6. Histogram Identifies all densities on plate
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

7. Histogram Location Example of a chest histogram
X-ray energy and absorption effects
Low kVp, large differences (high contrast)
High kVp, small differences (low contrast)
Different energies superimposed
Different doses (mAs), same kVp

8. 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.

9. 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

10. Characteristic curve & histogram

11. Underexposed

12. Overexposed

13. Just right!

14. 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

15. 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)

16. 1. is unprocessed, 2. algorithm finds anatomy, 3. finished

17. 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

18. 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

19. 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 EI 2,000
Agfa 200 speed lgm reference value for site

20. 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)
Kodak uses exposure index
2000 (1 mR), 3000 (10 mR)

21. Exposure Values Agfa has ADC dose monitoring
Stored reference dose for each exam
Based on range
Compares image obtained to reference
Indicates significant deviation
Notes lgM, not patient dose, plate dose
Doubling dose increases lgM by 0.3 (log)

22. Exposure Values Agfa lgM, log gradient mean Speed class, 200 or 400
Double dose is 0.3 since it is log
Have stored reference dose for exams
Set for hospital, each image compared to it
Speed class, 200 or 400
Tells relative amount of light to be read from plate
Allows electronics to be optimized to this range for better image data set

23. Agfa Agfa uses a speed system
Here is a hand taken at 50, 200 and 400 speeds
The mAs was changed for each one
Lgm is the same for all

24. 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
Agfa, if appropriate # is Lgm 2.2
At 1.9, too light, double mAs for 2.2
At 2.5, to dark, half mAs for 2.2
Kodak, if appropriate # is 1800
At 1500, too light, double mAs for 1800
At 2100, to dark, half mAs for 1800

25. S# 47
S# 86
S# 16

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

27. 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

28. Same technique, different centering and collimation

29. 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

30. Comparison of DR & CR

31. IMAGE CAPTURE CR DR – NO CASSETTE – PHOTONS
PSP – photostimulable phosphor plate
REPLACES FILM IN THE CASSETTE
DR – NO CASSETTE – PHOTONS
CAPTURED DIRECTLY
ONTO A TRANSISTOR
SENT DIRECTLY TO A MONITOR

32. DR TFT, thin film transistor Used under silicon or selenium
Collects charge then sends out as signal to computer
Releases e- line by line

33. TFT

35. CR – PSP plate Stimulated by a RED LIGHT
Energy is RELEASED in a form of BLUE light
LIGHT captured by PMT –
changed to a digiial signal

37. How CR works
Released light is captured by a PMT (photo multiplier tube)
This light is sent as a digital signal to the computer
The intensity (brightness) of the light – correlates to the density on the image

39. Densities of the IMAGE
The light is proportional to amount of light received
digital values are then equivalent (not exactly the same) to a value of optical density (OD) from a film, at that location of the image

42. ADVANTAGE OF CR/DR AFTER THE EXPOSURE
CHANGES MADE TO IMAGE
AFTER THE EXPOSURE
CAN ELIMINATE THE NEED TO REPEAT THE EXPOSURE

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

44. CR/DR VS FILM/SCREEN
FILM these can not be modified once processed
If copied – lose quality
DR/CR – print from file – no loss of quality

45. “no fault” TECHNIQUES F/S: RT must choose technical factors
(mAs & kvp) to optimally visualize anatomic detail
CR: the selection of processing algorithms and anatomical regions controls how the acquired latent image is presented for display
HOW THE IMAGE LOOKS CAN BE ALTERED BY THE COMPUTER – EVEN WHEN “BAD” TECHNIQUES ARE SET

46. DR Initial expense high very low dose to pt –
image quality of 100s using a 400s technique
Therfore ¼ the dose needed to make the image

47. Histogram Analysis A histogram is a plot of gray scale value
vs. the frequency of occurrence
(# pixels) of the gray value in the image

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

50. Adapted from AAPM TG10

51. The algorithm attempts to distinguish among the parts of the histogram which represent the range of densities from bone to soft tissue

52. Histograms set for specific exams (body parts)
should produce digital images that are consistant (regardless of kVp or mAs used
Correct Algorithm (body part) must be selected prior to processing imaging plate

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

55. FILM DIGITIZER

56. ANALOG TO DIGITAL IMAGE
Conversion of conventional analog films
to digital format for PACs and teleradiology applications
with scanning laser digitizers

57. CONTRAST & DENSITY
Most digital systems are capable of 1024 shades of gray - but the human eye can see only about 30 shades of gray
The Optical Density and Contrast can be adjusted after the exposure by the Radiographer.
This is POST - PROCESSING

58. High displayed contrast – narrow window width

59. Low displayed contrast (stretched) – wide window width

60. Basics of Digital Images
Pixel values can be any bit depth (values from 0 to 1023)
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

61. standard image
edge sharpening

62. REPROCESSED
HAND ALGO
NO GRID

63. POSITIONING & PROPER COLLIMATION ARE CRITICAL TO GOOD IMAGING OUTCOMES
Just like Phototiming, it can magnify your mistakes

64. COLLIMATION CRITICAL
AS THE COMPUTER READS THE DENSITY VALUE OF EACH PIXEL – IT IS AVERAGED INTO THE TOTAL
CLOSE COLLIMATION = BETTER CONTRAST
BAD COLLIMATION = MORE GRAYS AND LESS DETAIL

65. A B

66. Digital imaging is not the end all, cure all for imaging problems.
It is still technologist dependent.
You must continue to think and apply everything learned in Imaging 101.
A computer is not an intelligent machine.
It can only perform as good as the information it was given.