Download Computerized & Digital Radiography

Computed
Radiography
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

Digital Imaging
• Image acquisition that produces an
electronic image that can be viewed and
manipulated on a computer.
• Analog-to-digital converters.
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.
First-generation CT unit: dedicated head scanner
(Photograph taken at Roentgen Museum, Lennep, Germany.)
Digital Imaging Techniques

Are used in:
 Computed
Tomography
 Magnetic Resonance Imaging
 Diagnostic Ultrasound
 Computerized Radiography
 Digital Radiography
 Digital Fluoroscopy
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)
Methods to Digitize an Image
• 1. Film Digitizer - Teleradiography system
(PACS, DICOM)
• 2. Video Camera (vidicon or plumbicon)
• 3. Computed Radiography
• 4. Direct Radiography
Computed Radiography Terms

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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
Computed
Radiography
Fundamentals
of
Computerized
Radiography
CR IR

Cassette-based digital radiography

In S/F the intensifying screens contain
phosphor that emits light in response to xray interaction.

In CR, the response to x-ray interaction is
trapped photon energy (e-) on the PSP
plate.
Conventional radiography latent
image formation
CR latent image formation
What Is Digital Imaging?
Digital
imaging is the
acquisition of images
to a computer rather
than directly to film.
15
IMAGE CREATION
SAME RADIOGRAPHY EQUIPMENT
USED
 THE DIFFERENCE IS HOW IT IS
CAPTURED
 STORED
 VIEWED
 And POST -PROCESSED

General Overview
CR
 PSP
cassette exposed by
conventional x-ray equipment.
 Latent
image generated as a
matrix of trapped electrons
in the PSP plate.
CR BASICS
• Eliminates the need for film as
a recording, storage & viewing
medium.
• PSP Plate – receiver
• Archive Manager – storage
• Monitor - Viewing
DDR
Digital
CR
Radiography
Direct
Capture
Indirect
Capture
Direct-to-Digital
Radiography
(DDR)-Selenium
Computed
Radiography
(CR) - PSL
Direct-to-Digital
Radiography
Silicon Scint.
Laser
Scanning
Digitizers
CR SYSTEM
COMPONENTS
CASSETTES (phosphor plates)
 ID STATION
 IMAGE PREVIEW (QC)
STATION
 DIGITIZER
 VIEWING STATION

ID STATION
CR Readers
AKA
CR Processors
Computed Radiographic Readers
Fuji
Agfa
Kodak
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
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
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
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
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
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
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.
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.
Cross section of a PSP screen
Needle PSP increase the absorption of xrays and limit the spread of light emission
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.
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.
X-ray interaction with a PSP screen
1
X-ray interactions with the
screen phosphors causes an eto excited
2
When e- return to ground
state visible light is emitted
CR Phosphor Plates
ABSORPTION
EMISSION
LASER STIMULATION
X-RAY
ELECTRON
TRAP
ELECTRON
TRAP
LIGHT
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
Sequence of CR imaging
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). 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.
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
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
Basics of Digital Images
• digital
images are a
(matrix) of
pixel (picture
element)
values
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
Matrix = each cell corresponds to a
specific location on the image
Pixel
Pixels
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
Digital - Grayscale
Bit depth.
 Number of gray shades

 available
for display
8 bit 256
 10 bit 1024
 12 bit 4096
 4 bit 16384

Display Bit Depth
1 bit
6 bit
8 bit
2 shades
64 shades
256 shades
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
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)
Improving Signal-to-Noise Ratio

Optical filter is used to prevent the longerwavelength laser light affecting the image
formation.

Increasing exposure to IR. Newer CR
systems are better at reducing noise at
lower exposure levels.
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 ?
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

Characteristic curve
of radiographic film
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

Screen / Film Imaging = self regulating
2 mAs
Under exposed
6 mAs
Correct exposure
24 mAs
Over exposed
CR imaging results in 10,000 shades of gray
Fixed kVp exposures
mAs = 0.5
S = 357
mAs = 1.0
S = 175
mAs = 2.0
S = 86
mAs = 5.0
S = 35
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

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
 200 RS = highest quality images.
plate.
 Increasing
RS will reduce contrast resolution and
patient dose.

S numbers, Index numbers, Sensitivity numbers,
Exposure index

Each imaging system is unique
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?
80 KVP
5
5
100
30
15
200
500
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.
POST PROCESSING
Part Selection
Workstation Menu

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
Wrong Algorithm
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
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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
Statistical plots of the
frequency of occurrence of
each pixel's value
Initial Image Processing

Automated exposure field edge
detection

Eliminates signals outside collimation
margins

If margin not detected, extraneous data
included in the histogram.
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

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

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
 ALARA
preference
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
EXPOSURE VALUES

Exposure indicator
sensitive to 0.1 mR – 100 mR
 “S” number for Fuji
 Plates

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)
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
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
S# 8,357
S# 12,361 lat CXR
Same technique, different centering
and collimation
S# 592
S# 664
􀁹2 on 24 X 30
􀁹Technique adjusted
2 on 24 X 30
􀁹Same technique
􀁹Rescaling error.
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
Histogram with H&D curve
Underexposed
Overexposed
Just right!
Changes to Histogram
Hip
prosthesis
Line caused
from dirt
collected
in a CR
Reader
Higher kVp
• Smaller signal difference
• Narrower data range
• Photons to IR
Lower kVp
• Larger signal difference
• Wider data range
• More photons will be absorbed
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
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
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)

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
LUT
1. is unprocessed, 2. algorithm finds anatomy, 3. finished
Myths for CR #1 & 2

1.
mAs – myth: digital is mAs driven
 Truth:

kVp – myth: digital is kVp
driven.
2. .
 Truth:
Contrast

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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
TECHNIQUE CONISDERATIONS

kVp energy dependant

Now COMPUTER controls CONTRAST

Higher kVp to stimulate electron traps
kVp range for CR 45 - 120
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

kVp Selection
Anatomical contrast
 Inherent Δ attenuation due to structure
 Grid utilization
 Yes, No, Efficiency

 Ratio,
frequency, lead content.
Contrast improvement
 Default processing algorithm

kVp Selection
Determined by anatomy and grid
 Adults: 􀂃 Optimal range 60 – 120
 􀂃 Pediatrics < 100 lbs: Optimal range 50 90
 May use higher kVp than with S/F.
 Helps limit dose increase
 Narrows acquired data range.

kVp ranges for CR




Infant extremities 50 60 kVp
Adult extremities 65 75 kVp
Bucky extremities 75
– 90 kV
AP spine 85 - 95 kVp




Lateral spine 85 –
100 kVp
Chest 110 – 130 kVp
Skull 80 – 90 kVp
Only 1 kVp is not
reccomended
Another set of suggested kVp
ranges.





Distal Extremities =
65 – 75
Ped. extremities = 50
- 65
KUB = 80 - 85
IVU = 70 – 80
BE = 110 -120






Grid extremities = 85
- 90
L-spine/Pelvis = 85 –
90
Chest / grid = 110 –
130
Ped. chest NG 70 –
80
Ribs = 80 – 90
T-spine = 90 – 100
Collimation

Proper collimation is the best way to
enhance your CR image. Why?

What is shuttering?

Is it the same as collimating?
Myths for CR #3
 Collimation
collimate.
Truth:
– myth: you cannot
Shuttering
If radiologist object.
Apply back border.
Collimation
Collimation
 Less water irradiated
 Less scatter produced
 Improves contrast
 Reduces effective dose
 Reduces automatic rescaling errors

Exposure Field Recognition
If the exposure field is not recognized the
entire plate is used in image construction
Myths for CR #4
– myth: cannot use grids and
don’t need them.
 Grid

Truth:
Grid vs. Non-grid
Myths for CR #5

SID – myth: magnification doesn’t
occur with digital so SID is
unimportant.
5.
Truth:
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:
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.
 Truth:
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?

ADVANTAGE OF CR/DR
 Can
optimize image quality by
manipulating digital data to
improve visualization of anatomy
and pathology AFTER
EXPOSURE TO PATIENT
Image Preprocessing

Data recognition is very important

Two on one imaging?
Is it good practice?

Carter pg. 87
Can it be done?
Improves Exam Times ?
High resolution with digital
imaging
• towel that was used
to help in
positioning a child
• CR is MORE
sensitive to
• ARTIFACTS
NEW IMAGE
Questions ?