Screen-Film and Digital Mammography

Bob Pizzutiello, MS, FAAPM, FACMP

Upstate Medical Physics

for our patients…The Image Still Matters!

Handouts at http://www.upstatemp.com/

Physician

Technologist

Medical Physicist

The Team Approach

Which image would you prefer?

Outline

• Contrast• Blur• Noise• Artifacts• Dose

Image Characteristics

SFM - FFDM To optimize Image Quality

and Radiation Dose

CNR - SNR

Technical Tips

SFM = Screen-film MammographyFFDM = 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

Log E

Op

tical Den

sity

• 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)

Black background

Dense

Fatty

Optical D

ensity

Log E

Subject Contrast

Ratio of number of x-ray photons in the aerial image corresponding to different regions of the breast

Log E

Optical D

ensity

Film Contrast

Log E

Optical D

ensity

• Translates different amounts of screen light to different film Optical Density

• Slope of D-Log E curve

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

70mm40mm

10 mm lesionA C

B DFilm

Film Contrast and LatitudeThe Tradeoff

• Latitude: the range of film exposures for a readable image – Not too light– Not too dark

Log E

Op

tical Den

sity

Range of acceptable exposure

Range of acceptable

optical density

Film Contrast and LatitudeThe 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

LocationA

B

Same for SFM and FFDM

A

B

AB

Objects that are closer to the image receptor have less blur and are not magnified

Magnification

• Motion• Exposure time• Compression• Increase kVp• Focal Spot• Size• Object

Location

Compressed breast

X-ray tube

Film

a b

c

e d

X-ray target

Compression paddle

MagnificationStand

Film

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

Plus density (black) artifacts– Additional exposure of film granules– Light leaks– Static – Pressure from rollers

Minus density (white) artifacts– Objects that absorb x-rays or

light– Dust (very common)– Eye glasses– Nose– Fingerprints– Screen scratches or stains

• 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.

Technical Tip #1

Fatty Tissue Region

Dense Tissue Region

Mammogram exposure level

Optical Density

Film Contrast

Optical Density

Film Contrast

Under-exposed 1.10 3.5 0.8 2.8

Appropriately exposed

2.00 4.5 1.05 3.5

ExampleNote that actual values may vary with the film used

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 “2-second 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

Copyright ©Radiological Society of North America, 1999

Hogge, J. P. et al. Radiographics 1999;19:503-522

• 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

Image Quality

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 pre-exposure 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 Auto-Filter or Auto-kV.

Image Quality

Fuji FCRm

GE 2000D,

DS, Essential

Hologic

SeleniaSiemens Novation

Manual Techniques X

Clinical Technique X Sort of… Sort of…

CNR X X X X

Courtesy Eric Berns, PhD

• Signal-to-Noise Ratio (SNR)

• Variation in background for uniform exposure

• Signal = average pixel value

• Noise = standard deviation

• SNR =

• SNR must be > 40

“What exactly is CNR and SNR”?

Average Signal Value – DC offsetStandard Deviation

Noise and SD ?

SD = 2.3

SD = 3.4

SD = 5.1

• Contrast-to-Noise Ratio (CNR)

• Similar to Density Difference (Film)

• Contrast = Difference in pixel values (bkg-disc)

• Noise = standard deviation

• CNR =

CNR

Mean(bkg) – Mean (disc)Standard Deviation (bkg)

•ACR Phantom Imaging

•Manual technique

(Mo/Mo, 26 kVp, 125 mAs)

•Score the processed image

• Acquisition workstation

• Each monitor of the RWS

• Laser imager

GE 2000D

Courtesy Eric Berns, PhD

•Contrast-to-Noise Test

(CNR)

•To examine consistency of

CNR ratio measured over time

•Use the raw image

•+ 20% of baseline

Background ROIBackground ROI Mass ROIMass ROI

CNR = (MeanCNR = (Meanbackgroundbackground - Mean - Meanmassmass)/SD)/SDbackgroundbackground

GE 2000D

Courtesy Eric Berns, PhD

CNR & MTF (DS, Essential)

Courtesy John Sandrik

Courtesy John Sandrik

Courtesy John Sandrik

•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

Fuji FCRm

Courtesy Eric Berns, PhD

Rememebr this from Kodak?

Service Bulletin was updated • Recommendations for– 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

• Contrast• Blur• Noise• Artifacts• Dose

Image Characteristics

To optimize Image Quality

and Radiation Dose

CNR - SNR

Technical Tips

Thank you to the RMA!For many years of

service to the mammography

community.

Now for some well-deserved rest…

http://www.upstatemp.comclick Education & Training

Look for a link “RMA May 1, 2010”

http://www.upstatemp.com/