Download Digital Radiography

Digital Radiography
Or
Things they never tell you
when the equipment is
installed REV 11/2008
Computed
Radiography
Fundamentals
of
Computerized
Radiography
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
IMAGE READING
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
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
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
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.
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
Characteristic curve &
histogram
Underexposed
Overexposed
Just right!
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
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)
1. is unprocessed, 2. algorithm finds anatomy, 3. finished
LOOK UP TABLE (LUT)
Linear LUT
Black
Saturation
White
Saturation
Black Shirt
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
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
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
EXPOSURE VALUES
Exposure indicator
– 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)
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)
Exposure Values
Agfa
– lgM, log gradient mean
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
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
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
S# 47
S# 86
S# 16
S# 8,357
S# 12,361 lat CXR
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
Same technique, different centering
and collimation
S# 592
S# 664
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
Comparison of DR & CR
IMAGE CAPTURE
CR
– PSP – photostimulable phosphor plate
– REPLACES FILM IN THE CASSETTE
DR – NO CASSETTE – PHOTONS
– CAPTURED DIRECTLY
– ONTO A TRANSISTOR
– SENT DIRECTLY TO A MONITOR
DR
TFT, thin film
transistor
Used under silicon or
selenium
Collects charge then
sends out as signal to
computer
Releases e- line by
line
TFT
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
Reference detector
f-theta
lens
Cylindrical mirror
Beam splitter
Light channeling guide
Laser
Output Signal
Source
Beam deflector
ADC
Laser beam:
Scan direction
Plate translation:
Sub-scan direction
PMT
Amplifier
To image
processor
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
1. X-ray Exposure
Patient
5. Computed
Radiograph
une xposed
2.
PSP
4.
Image
Image
Image
Reader
Scaling
Recorder
detector
X-ray
system
3.
exposed
re-usable
phosphor
plate
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
ADVANTAGE OF CR/DR
CHANGES MADE TO IMAGE
AFTER THE EXPOSURE
CAN ELIMINATE THE NEED TO
REPEAT THE EXPOSURE
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?
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
“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
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
Histogram Analysis
A histogram is a plot of gray scale value
vs. the frequency of occurrence
(# pixels) of the gray value in the image
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
Histogram
Low attenuation
(e.g., lungs)
High attenuation
(e.g., mediastinum)
12,000
Frequency
10,000
8,000
6,000
4,000
2,000
0
0
200
400
600
800
1,000
Digital number
Adapted
from AAPM
TG10
The algorithm attempts to
distinguish among the parts of the
histogram which represent the
range of densities from bone to
soft tissue
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
Methods to Digitize an Image
1. Film Digitizer - Teleradiography system
(PACS, DICOM)
2. Video Camera (vidicon or plumbicon)
3. Computed Radiography
4. Direct Radiography
FILM DIGITIZER
ANALOG TO DIGITAL IMAGE
Conversion of conventional analog
films
to digital format for PACs and
teleradiology applications
with scanning laser digitizers
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
High displayed contrast – narrow window width
Low displayed contrast (stretched) – wide window width
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
standard image
edge sharpening
REPROCESSED
NO GRID
HAND ALGO
POSITIONING & PROPER
COLLIMATION ARE CRITICAL
TO GOOD IMAGING
OUTCOMES
Just like Phototiming, it can
magnify your mistakes
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
A
B
Digital imaging is not the end
all, cure all for imaging
problems.
It is still technologist dependent.