Screen-Film and Digital Mammography
for our patients…The Image Still Matters!
Bob Pizzutiello, MS, FAAPM, FACMP
Upstate Medical Physics
Handouts at http://www.upstatemp.com/
The Team Approach
Physician
Medical
Physicist
Technologist
Which image would you prefer?
Outline
Image Characteristics
SFM - FFDM
Technical Tips
•
•
•
•
•
To optimize
Image Quality
and
Radiation Dose
CNR - SNR
Contrast
Blur
Noise
Artifacts
Dose
SFM = Screen-film Mammography
FFDM = Full Field Digital Mammography
•
Screen-film Mammography
• Film is used for image
• Acquisition
• Display
• Storage
•
Digital Mammography
• Separate devices used for
• Acquisition
• Display
• Storage
Image Characteristics
•
•
•
•
•
Contrast
Blur
Noise
Artifacts
Dose
Contrast
A - Black Background
B - Dense Glandular Tissue
C - Fatty tissue
Higher Contrast
Lower Contrast
Radiographic Contrast
• Difference in OD between areas of interest on the
mammogram
• Determined by Subject Contrast and
Film Contrast
Optical Density
• X-ray exposure incident on the
screen is converted into
shades of gray (film)
• Areas that receive the
Maximum radiation exposure
turn film black
• Areas that receive the
Minimum radiation exposure
makes film clear (appears
white)
Log E
Black background
Dense
Fatty
Optical Density
Log E
Subject Contrast
Optical Density
Ratio of number of
x-ray photons in the
aerial image
corresponding to
different regions of
the breast
Log E
Film Contrast
Optical Density
• Translates different
amounts of screen
light to different film
Optical Density
• Slope of D-Log E
curve
Log E
Film Contrast and Compression
• Images of thicker breasts have lower contrast
• Hence, compression improves contrast
• Compression also moves objects closer to the
image receptor (less blur)
• Compression reduces dose, since less
radiation is required to image a thinner breast
• Compression also improves imaging by
improving separation of overlapping structures
70mm
40mm
A
B
10 mm lesion
Film
C
D
Film Contrast and Latitude
The Tradeoff
• Latitude:
the range of film exposures for a readable
image
– Not too light
– Not too dark
Optical Density
Range of
acceptable
optical density
Log E
Range of
acceptable
exposure
Film Contrast and Latitude
The Tradeoff
• High Film Contrast inherently means
Low Latitude
• SF imaging is limited by these
characteristics of the film
Contrast in FFDM
• Display contrast can be adjusted as high as
desirable.
• Small Window Width (WW) = large display contrast
• Image display can be adjusted multiple times to
view different areas of the breast (e.g., fatty and
dense regions) by adjusting the Window Level
(WL)
• Since there is essentially no limit on display
contrast, subject contrast is less important, so
beam energy can be higher
• Shorter exposure times
• Lower AGD
SFM Contrast Summary
• Subject Contrast creates differences in
x-ray absorption
• Film Contrast amplifies those differences
• Need adequate Radiographic Contrast to
visualize abnormalities
• High Contrast means low latitude
• Darker (regions of) mammograms have better
contrast - and conversely
• Compression improves contrast
• Challenge: balance film characteristics to
produce a diagnostic quality image
Image Blur
Image Blur
• Contact imaging
• Motion
– Exposure time
– Compression
• Focal Spot
– Size
– Object
Location
Same for SFM and FFDM
B
A
Objects that are closer
to the image receptor
have less blur and are
not magnified
B
A
B
A
Magnification
X-ray tube
•
•
•
•
•
•
•
Motion
Exposure time
Compression Compressed
breast
Increase kVp
Focal Spot
Size
Object Location
Film
X-ray target
b
a
Compression
paddle
c
Magnification
Stand
Film
e
d
Blur region
Noise
Noise
• Any undesired variation in image
background
• Most commonly caused by insufficient
number of x-ray photons used to create the
image
• Noise limits visibility of low-contrast objects
• We could produce images with much lower
radiation dose, but
they would have more noise
• Same for SFM and FFDM, but since
contrast is not an issue for FFDM, want
more signal to detector
Low and High Noise Images
Low and High Noise Images
Artifacts
Minus density (white) artifacts
–
–
–
–
–
–
Objects that absorb x-rays or light
Dust (very common)
Eye glasses
Nose
Fingerprints
Screen scratches or stains
Plus density (black) artifacts
–
–
–
–
Additional exposure of film granules
Light leaks
Static
Pressure from rollers
Technical Tip #1
• Measure optical density in several
locations with a spot densitometer to
determine quantitatively if a
mammogram is underexposed.
• If the readings in the densest
glandular area of the film are
consistently less than 1.0, the
mammogram is underexposed.
• Increase the density control (not kVp)
to make the image darker.
Example
Note that actual values may vary with the film used
Fatty Tissue
Region
Dense Tissue
Region
Mammogram
Optical
Film
Optical
Film
exposure level Density Contrast Density Contrast
Under-exposed
1.10
3.5
0.8
2.8
Appropriately
exposed
2.00
4.5
1.05
3.5
How do we maintain consistent
image quality ?
Technical Tip #2
• Goal: consistent background density
• When Phantom Image BD and DD vary
(and mAs remains constant),
• Obtain phantom images at the next
higher or lower density control setting
needed to achieve the desired BD.
• Post the control booth (or alter the default
AEC settings) to use this density control
until further notice.
• Sign and date the note.
How can we help the
radiologists to see abnormalities
in a mammogram ?
Technical Tip #3
• To assure optimal perception of well
exposed SF mammograms, use viewboxes
with sufficient brightness (luminance) to
permit the observer to operate in day
vision, even when viewing darker
mammograms.
• ACR recommends that mammography
viewboxes produce at least 3,000 cd/m2.
• Many facilities now use viewboxes that
produce > 5,000 cd/m2 to optimally
visualize darker mammograms.
Technical Tip #4
• For both SF and Digital mammography,
• To minimize glare, maintain subdued
lighting in the reading room and mask
each mammogram using fully exposed
mammography film, sliding masking
panels or “hand held tunnel viewers”.
• Technologists should use similar
viewboxes (and eventually monitors) to
assess overall image quality.
What should I
do when the
exposure time
seems long ?
Technical Tip #5
• If exposure times exceed about 2
seconds, increase the kVp but do not
change the density control (aka the “2second rule” ),
• Listen to the exposure beep. When in
doubt...
• Divide the post-exposure mAs by the mA
to obtain the exposure time.
• To do this, you must know the mA (for
large and small focal spots) of each unit
in your department or practice.
Contact Mammography
is 26 kVp, 225 mAs too long?
Unit 1
Unit 2
Unit 3
100 mA
80 mA
150 mA
225 mAs
2.25 sec
225 mAs
2.8 sec
225 mAs
1.5 sec
Hogge, J. P. et al. Radiographics 1999;19:503-522
Copyright ©Radiological Society of North America, 1999
Image Quality
• Screen-Film Mammo
• ACR Phantom Scores
• Optical Density & Contrast
• FFDM
• ACR Phantom Scores
• Pass/fail requirements differ by vendor
• Signal-to-Noise Ratio (SNR)
• Contrast-to-Noise Ratio (CNR)
Image Quality
ACR Phantom Imaging
GE & Fuji
Lorad &
Siemens
Fibers
4
5
Specks
3
4
Masses
3
4
Courtesy Eric Berns, PhD
Selenia 7.4 Test Procedure
• It is important for performing this test that the
kVp and Filter selection is done at a thickness
of 4.5 cm.
• Using the alternate test procedure guarantees
the correct AEC behavior for the test because:
• Selenia Auto-kV and Auto-Filter modes, kV and filter
selections are strictly controlled by compression
thickness.
• The mAs value is determined by a short preexposure in all AEC modes.
• Following the steps in the alternate test procedure
is clinically equivalent to acquiring the phantom
image at 4.5 cm compression thickness using AutoFilter or Auto-kV.
Image Quality
Fuji
FCRm
Manual Techniques
GE
2000D,
DS,
Essential
Hologic
Selenia
Siemens
Novation
Sort of…
Sort of…
X
X
X
Clinical Technique
X
CNR
X
X
Courtesy Eric Berns, PhD
“What exactly is CNR and SNR”?
• Signal-to-Noise Ratio (SNR)
• Variation in background for uniform exposure
• Signal = average pixel value
• Noise = standard deviation
• SNR =
Average Signal Value – DC offset
Standard Deviation
• SNR must be > 40
Noise and SD ?
SD = 2.3
SD = 3.4
SD = 5.1
CNR
• Contrast-to-Noise Ratio (CNR)
• Similar to Density Difference (Film)
• Contrast = Difference in pixel values (bkg-disc)
• Noise = standard deviation
• CNR =
Mean(bkg) – Mean (disc)
Standard Deviation (bkg)
GE 2000D
•ACR Phantom Imaging
•Manual technique
(Mo/Mo, 26 kVp, 125 mAs)
•Score the processed image
• Acquisition workstation
• Each monitor of the RWS
• Laser imager
Courtesy Eric Berns, PhD
GE 2000D
•Contrast-to-Noise Test
(CNR)
•To examine consistency of
CNR ratio measured over time
•Use the raw image
•+ 20% of baseline
Background ROI
Mass ROI
CNR = (Meanbackground - Meanmass)/SDbackground
Courtesy Eric Berns, PhD
CNR & MTF (DS, Essential)
Courtesy John Sandrik
Courtesy John Sandrik
Courtesy
John Sandrik
Fuji FCRm
•Contrast-to-Noise Test (CNR)
•To examine consistency of CNR measured over time
•Use 4 cm acrylic & 0.2 mm Al
•Manual technique (Mo/Mo, 26 kVp, 125 mAs)
•Calculate CNR using software
•+ 20% of baseline
Courtesy Eric Berns, PhD
Rememebr this from Kodak?
• Recommendations
forwas updated
Service Bulletin
– Starter – now 3 oz per gallon,
across the board
– New replenishment rates
– Be sure your service people follow these
• Kodak MinR Guide
– Processor QC before-after service
– Checklist available
– http://www.kodak.com/global/plugins/acrobat
/en/health/pdf/prod/Mammo/M3-103.pdf
Summary
Image Characteristics
• Contrast
• Blur
• Noise
• Artifacts
• Dose
Technical Tips
To optimize
Image Quality
and
Radiation Dose
CNR - SNR
Thank you to the RMA!
For many years of service to
the mammography
community.
Now for some
well-deserved rest…
http://www.upstatemp.com
click Education & Training
Look for a link “RMA May 1, 2010”
http://www.upstatemp.com/