The Quality of Image and Radiation Risk in mammography Carlo Maccia Medical Physicist CAATS 43 Bd du Maréchal Joffre – Bourg-La-Reine – FRANCE XI. National.

The Quality of Image The Quality of Image and Radiation Risk and Radiation Risk in mammographyin mammography

Carlo MacciaCarlo MacciaMedical PhysicistMedical Physicist

CAATS 43 Bd du Maréchal Joffre – Bourg-La-Reine – FRANCE

XI. National Turkish Medical Physics Congress 14-18 November 2007 - Antalya

IntroductionIntroduction

Subject matter : mammography (scope is Subject matter : mammography (scope is breast cancer screening)breast cancer screening)

The physics of the imaging systemThe physics of the imaging system How to maintain the image quality while How to maintain the image quality while

complying with dose requirementscomplying with dose requirements Main features of quality controlMain features of quality control

ContentsContents

Introduction to the physics of mammographyIntroduction to the physics of mammography Important physical parametersImportant physical parameters The mammographic X-ray tubeThe mammographic X-ray tube The focal spot sizeThe focal spot size The high voltage generatorThe high voltage generator The anti-scatter grid The anti-scatter grid The Automatic Exposure ControlThe Automatic Exposure Control The dosimetryThe dosimetry Quality controlQuality control

Introduction to the physics of Introduction to the physics of mammographymammography

X-ray mammography is the most reliable X-ray mammography is the most reliable method of detecting breast cancermethod of detecting breast cancer

It is the method of choice for the Breast It is the method of choice for the Breast Screening Program in a variety of Screening Program in a variety of developed countriesdeveloped countries

In order In order to obtain to obtain high quality high quality mammogramsmammograms at an acceptable breast at an acceptable breast dose, dose, it is essentialit is essential to use the correct to use the correct equipmentequipment

Main components of the mammographic Main components of the mammographic imaging systemimaging system

A mammographic X-ray tubeA mammographic X-ray tube A device for compressing the breastA device for compressing the breast An anti-scatter gridAn anti-scatter grid A mammographic image receptor (film, A mammographic image receptor (film,

photostimulable plate, flat panel)photostimulable plate, flat panel) An automatic Exposure Control SystemAn automatic Exposure Control System

Main variables of the mammographic Main variables of the mammographic imaging systemimaging system

ContrastContrast: capability of the system to make visible : capability of the system to make visible small differences in soft tissue densitysmall differences in soft tissue density

SharpnessSharpness: capability of the system to make visible : capability of the system to make visible small details small details (calcifications (calcifications down to 0.1 mmdown to 0.1 mm))

DoseDose: the female breast is a very radiosensitive : the female breast is a very radiosensitive organ and there is a risk of carcinogenesis organ and there is a risk of carcinogenesis associated with the technique associated with the technique

NoiseNoise: determines how far the dose can be reduced : determines how far the dose can be reduced given the task of identifying a particular object given the task of identifying a particular object against the backgroundagainst the background

The contrastThe contrast

Linear attenuation coefficientsLinear attenuation coefficients for different for different types of breast tissue types of breast tissue are similar in magnitudeare similar in magnitude and the soft tissue contrast can be quite small and the soft tissue contrast can be quite small

The contrast must be made The contrast must be made as high as possibleas high as possible by by imaging with a low photon energy (hence imaging with a low photon energy (hence increasing breast dose)increasing breast dose)

In practice, to avoid a high breast dose, a In practice, to avoid a high breast dose, a compromisecompromise must be made between the must be made between the requirements of low dose and high contrastrequirements of low dose and high contrast

Variation of contrast with photon Variation of contrast with photon energyenergy

10 20 30 40 50 Energy (keV)

1.0

0.1

0.01

0.001

Ca5 (PO4)3 OH Calcification

of 0.1mm

Glandular tissue of 1mm

Con

tras

t

•The contrast decreasesby a factor of 6 between15 and 30 keV

•The glandular tissuecontrast falls below 0.1for energies above 27

keV

Contributors to the total unsharpness in Contributors to the total unsharpness in the imagethe image

ReceptorReceptor unsharpness: unsharpness: ((screen-film combination) screen-film combination) can be as small as 0.1 - 0.15 mm (full width at half can be as small as 0.1 - 0.15 mm (full width at half maximum of the point response function) with a maximum of the point response function) with a limiting value as high as 20 line pairs per mmlimiting value as high as 20 line pairs per mm

Geometric unsharpnessGeometric unsharpness: focal spot size and imaging : focal spot size and imaging geometry must be chosen so that the overall geometry must be chosen so that the overall unsharpness reflects the performance capability of the unsharpness reflects the performance capability of the screenscreen

Patient movementPatient movement

The breast doseThe breast dose

Dose decreases rapidly with depth in tissue due to Dose decreases rapidly with depth in tissue due to the low energy X-ray spectrum usedthe low energy X-ray spectrum used

Relevant quantityRelevant quantity: : The The average glandular doseaverage glandular dose ((AGDAGD) related to the tissues which are believed to ) related to the tissues which are believed to be the most sensitive to radiation-induced be the most sensitive to radiation-induced carcinogenesiscarcinogenesis

The breast doseThe breast dose

The breast doseThe breast dose is affected by: is affected by: the breast composition and thicknessthe breast composition and thickness the photon energythe photon energy the sensitivity of the image receptorthe sensitivity of the image receptor

The breast composition The breast composition has a significant has a significant influenceinfluence on the dose on the dose

The area of the compressed breast The area of the compressed breast has a small has a small influenceinfluence on the dose on the dose

the mean path of the photons < breast dimensionsthe mean path of the photons < breast dimensions majority of the interactions are photoelectricmajority of the interactions are photoelectric

Variation of mean glandular dose with Variation of mean glandular dose with photon energyphoton energy

10 20 30 40 (keV)

20

10

2

1

0.2

8 cm

Mea

n G

lan

du

lar

Dos

e (a

rb. U

nit

s) •The figure demonstrates the rapid increase in dosewith decreasing photon energyand increasing breast thickness

•For the 8 cm thick breast thereis a dose increase of a factor of 30between photon energies of 17.5 and 30 keV

•At 20 keV there is a dose increase of a factor of 17 betweenthicknesses of 2 an 8 cm

2 cm

Contributors to the image noiseContributors to the image noise

1) the quantum mottle 1) the quantum mottle

2) the properties of the image receptor 2) the properties of the image receptor

3) the film development and display systems 3) the film development and display systems

N.B. :N.B. : both quantum mottle and film granularity both quantum mottle and film granularity contribute significantly to the total image noise for contribute significantly to the total image noise for screen-film-mammography screen-film-mammography

ContentsContents


Contradictory objectives for the spectrum Contradictory objectives for the spectrum of a mammographic X-ray tubeof a mammographic X-ray tube

The The ideal ideal X-ray spectrumX-ray spectrum for mammography for mammography is a compromise between :is a compromise between :

to achieve a high contrast and high signal to to achieve a high contrast and high signal to noise ratio (noise ratio (low photon energylow photon energy))

to keep the breast dose ALARA (to keep the breast dose ALARA (high high photon energyphoton energy))

The X-ray spectrum in mammographyThe X-ray spectrum in mammography

In a practice using a screen-In a practice using a screen-film, it may not be possible to film, it may not be possible to vary the SNR because the film vary the SNR because the film may become over or under-may become over or under-exposedexposed

The figure gives the The figure gives the conventional mammographic conventional mammographic spectrum produced by a Mo spectrum produced by a Mo target and a Mo filtertarget and a Mo filter 10 15 20 25 30

15

10

5

Energy (keV)

Nu

mb

er o

f p

hot

ons

(arb

itra

ry n

orm

alis

atio

n)

X-ray spectrum at 30 kV for an X-ray tube with a Mo target and a 0.03 mm Mo filter

MMain features of the X-ray spectrum in ain features of the X-ray spectrum in mammography mammography

Characteristic X-ray lines at 17.4 and 19.6 keV Characteristic X-ray lines at 17.4 and 19.6 keV and the heavy attenuation above 20 keV (position and the heavy attenuation above 20 keV (position of the of the MoMo K-edge) K-edge)

Reasonably close to the energies optimal for Reasonably close to the energies optimal for imaging breast of small to medium thicknessimaging breast of small to medium thickness

A higher energy spectrum is obtained by replacing A higher energy spectrum is obtained by replacing the the MoMo filter with a material of higher atomic filter with a material of higher atomic number with its K-edge at a higher energy (number with its K-edge at a higher energy (RhRh, , PdPd))

WW can also be used as target material can also be used as target material

Options for an optimum X-ray Options for an optimum X-ray spectrum in mammographyspectrum in mammography

Several scientific works have demonstrated that Several scientific works have demonstrated that contrast is better for the Mo/Mo target/filter contrast is better for the Mo/Mo target/filter combinationscombinations

This advantage decreases with increasing breast This advantage decreases with increasing breast thicknessthickness

Using Rh/Rh for target/filter combination brings a Using Rh/Rh for target/filter combination brings a substantial dose saving for bigger breasts while substantial dose saving for bigger breasts while keeping an acceptable contrast levelkeeping an acceptable contrast level

Options for an optimum X-ray Options for an optimum X-ray spectrum in mammographyspectrum in mammography

Focal spot size and imaging geometryFocal spot size and imaging geometry:: The overall unsharpness The overall unsharpness UU in the mammographic image in the mammographic image

can be estimated by combining the receptor and can be estimated by combining the receptor and geometric unsharpness geometric unsharpness

U = ([ fU = ([ f22(m-1)(m-1)22 + F + F2 2 ]]1/21/2) / m) / m (equation 1) (equation 1)

where:where:

ff: effective focal spot size: effective focal spot size

mm: magnification: magnification

FF: receptor unsharpness: receptor unsharpness

Variation of the overall unsharpness with the Variation of the overall unsharpness with the image magnification and focal spotimage magnification and focal spot

1.0 1.5 2.0Magnification

0.15

0.10

0.05

0.8

Ove

rall

un

shar

pn

ess

(mm

) •For a receptor unsharpness of 0.1 mm

•Magnification can only improve unsharpnesssignificantly if the focal spot is small enough

•If the focal spot is too large, magnification will increase the unsharpness

0.4

0.2

0.1

0.01

ContentsContents


The focal spot sizeThe focal spot size

Ideally, for the screening unit a single-focus X-ray Ideally, for the screening unit a single-focus X-ray tube with a 0.3 focal stube with a 0.3 focal spot is recommendedpot is recommended

For general mammography purposes, a dual focus For general mammography purposes, a dual focus X-ray tube with an additional fine focus (0.1) to X-ray tube with an additional fine focus (0.1) to be used for magnification techniques exclusively be used for magnification techniques exclusively is requiredis required

The size of the focal spot The size of the focal spot should be verifiedshould be verified ( (star star pattern, slit camera or pinhole methodpattern, slit camera or pinhole method) ) yearly or yearly or when resolution decays rapidlywhen resolution decays rapidly

Target/filter combinationTarget/filter combination

The window of the X-ray tube should be beryllium The window of the X-ray tube should be beryllium (not glass) with a maximum thickness of 1 mm(not glass) with a maximum thickness of 1 mm

The typical target/filter combinations nowadays The typical target/filter combinations nowadays available are: available are:

Mo + 30 Mo + 30 m Mom Mo Mo + 25 Mo + 25 m Mom Mo W + 60 W + 60 m Mom Mo W + 50 W + 50 m Rhm Rh W + 40 W + 40 m Pdm Pd Rh + 25 Rh + 25 m Rhm Rh

X-ray tube filtrationX-ray tube filtration

Total permanent filtrationTotal permanent filtration 0.5 mm of Al or 0.5 mm of Al or

0.03 mm of Mo (recommended by 0.03 mm of Mo (recommended by ICRP 34)ICRP 34)

The The beam qualitybeam quality is defined by the HVL is defined by the HVL

A better index of the A better index of the beam qualitybeam quality is the total is the total

filtration which can be related to the HVL using filtration which can be related to the HVL using

published datapublished data

State-of-the-art specifications State-of-the-art specifications for screen-film mammographyfor screen-film mammography

A nearly constant potential waveform with a ripple A nearly constant potential waveform with a ripple not greater than that produced by a 6-pulse not greater than that produced by a 6-pulse rectification systemrectification system

The tube voltage range should be The tube voltage range should be 25 - 35 kV25 - 35 kV The tube current The tube current should be at least 100 mA on should be at least 100 mA on

broad focus and 50 mA on fine focusbroad focus and 50 mA on fine focus.. The range of tube current exposure time product The range of tube current exposure time product

(mAs) (mAs) should be at least 5 - 800 mAsshould be at least 5 - 800 mAs It should be possible to repeat exposures at the It should be possible to repeat exposures at the

highest loadings at intervals highest loadings at intervals < 30 seconds< 30 seconds

ContentsContents

Introduction to the physics of mammographyIntroduction to the physics of mammography Important physical parametersImportant physical parameters The mammographic X-ray tubeThe mammographic X-ray tube The focal spot sizeThe focal spot size The high voltage generatorThe high voltage generator The anti-scatter gridThe anti-scatter grid The Automatic Exposure ControlThe Automatic Exposure Control The dosimetryThe dosimetry Quality controlQuality control

Why an anti-scatter grid ?Why an anti-scatter grid ?

Effects of scatter may significantly degrade the Effects of scatter may significantly degrade the contrast of the image and the need forcontrast of the image and the need for an efficient an efficient anti-scatter anti-scatter devicedevice is evident is evident

The effect is quantified by the :The effect is quantified by the :

Contrast Degradation Factor (CDF)Contrast Degradation Factor (CDF) : : CDF=1/(1+S/P)CDF=1/(1+S/P)

where: where: S/P :S/P : ratio of the scattered to primary ratio of the scattered to primary radiation amountsradiation amounts

Calculated values of CDF: 0.76 and 0.48 for breast Calculated values of CDF: 0.76 and 0.48 for breast thickness of 2 and 8 cm respectively thickness of 2 and 8 cm respectively [Dance et al.][Dance et al.]

The anti-scatter gridThe anti-scatter grid

Two types of anti-scatter grids available:Two types of anti-scatter grids available: stationary gridstationary grid: : with high line density (e.g. 80 lines/cm) with high line density (e.g. 80 lines/cm)

and an aluminium interspace materialand an aluminium interspace material moving gridmoving grid:: with about 30 lines/cm with paper or with about 30 lines/cm with paper or

cotton fiber interspacecotton fiber interspace

The performance of the anti-scatter grid can be The performance of the anti-scatter grid can be expressed in termsexpressed in terms of of the contrast improvementthe contrast improvement (CIF)(CIF) and and Bucky factorsBucky factors (BF)(BF)

The anti-scatter grid: performance The anti-scatter grid: performance indexesindexes

The The CIFCIF relates the contrast with the grid to that without relates the contrast with the grid to that without the grid while the grid while

The The BFBF gives the increase in dose associated with the use gives the increase in dose associated with the use of gridof grid

CIF and BF values for the Philips moving gridCIF and BF values for the Philips moving grid

Breast Thickness (cm)

CIF BF

2 1.25 1.68

4 1.38 1.85

6 1.54 2.06

8 1.68 2.24

Automatic exposure control device Automatic exposure control device (AEC)(AEC)

The system should produce a stable optical The system should produce a stable optical density (OD density (OD variation of less than variation of less than 0.2 0.2 ) in spite ) in spite of a wide range of mAsof a wide range of mAs

Hence the system should be fitted with an AEC Hence the system should be fitted with an AEC located after the film receptor to allow for quite located after the film receptor to allow for quite different breast characteristicsdifferent breast characteristics

The detector should be movable to cover The detector should be movable to cover different anatomical sites on the breast and the different anatomical sites on the breast and the system should be adaptable to at least three film-system should be adaptable to at least three film-screen combinationsscreen combinations

ContentsContents


Breast dosimetry in screen-film Breast dosimetry in screen-film mammographymammography

There exists a small risk of radiation induced There exists a small risk of radiation induced cancer associated with X-ray examination of the cancer associated with X-ray examination of the breast breast

Achieving an image quality at the lowest possible Achieving an image quality at the lowest possible dose is therefore requireddose is therefore required

Hence breast dosimetry Hence breast dosimetry The Average Glandular Dose (The Average Glandular Dose (AGDAGD) is the ) is the

dosimetry quantity generally recommended for risk dosimetry quantity generally recommended for risk assessmentassessment

Dosimetry quantitiesDosimetry quantities

The The AGDAGD cannot be measured directly cannot be measured directly but it but it is derived from (ESAK) measurements with is derived from (ESAK) measurements with the standard phantom for the actual technique the standard phantom for the actual technique set-up of the mammographic equipmentset-up of the mammographic equipment

The Entrance Surface Air Kerma (The Entrance Surface Air Kerma (ESAKESAK) ) free-in-air (i.e. without backscatter) has free-in-air (i.e. without backscatter) has become the most frequent used quantity for become the most frequent used quantity for patient dosimetry in mammographypatient dosimetry in mammography

For other purposes (compliance with For other purposes (compliance with reference dose level) one may refer to ESD reference dose level) one may refer to ESD which includes backscatterwhich includes backscatter

Dosimetry quantitiesDosimetry quantities

ESAK can be determined by:ESAK can be determined by: a TLD dosemeter calibrated in terms of air kerma free-in-air a TLD dosemeter calibrated in terms of air kerma free-in-air

at a HVL as close as possible to 0.4 mm Al at a HVL as close as possible to 0.4 mm Al with a standard with a standard phantomphantom

a TLD dosemeter calibrated in terms of air kerma free-in-air a TLD dosemeter calibrated in terms of air kerma free-in-air at a HVL as close as possible to 0.4 mm Al at a HVL as close as possible to 0.4 mm Al stuck to the stuck to the patient skinpatient skin (appropriate backsactter factor should be applied (appropriate backsactter factor should be applied to Entrance Surface Dose measured with the TLD to express to Entrance Surface Dose measured with the TLD to express ESAK)ESAK)

NoteNote : due to low kV used the TLD is seen on the image : due to low kV used the TLD is seen on the image a radiation dosemeter with a dynamic range covering at least a radiation dosemeter with a dynamic range covering at least

0.5 to 100 mGy (better than 0.5 to 100 mGy (better than 10% accuracy) 10% accuracy)

How have IQ and dose standards been How have IQ and dose standards been developed in European guidelines ?developed in European guidelines ?

Digital should not be worse than film-screen Digital should not be worse than film-screen systemssystems

IQ : Contrast detail measurements using CDMAM IQ : Contrast detail measurements using CDMAM test objecttest object

Dose : breasts simulated with PMMADose : breasts simulated with PMMA

Anatomy of a normal breastAnatomy of a normal breast

The female breast The female breast is a complex organis a complex organ

It is important to It is important to know tissues at know tissues at risk for tumor risk for tumor inductioninduction

Incident air-kerma and conversion Incident air-kerma and conversion factorfactor

Experimental Experimental set-up for the set-up for the measurement measurement of the Half of the Half Value Layer Value Layer (HVL) (HVL)

AGD = K.g.c.sAGD = K.g.c.s where :where :

k =k = Entrance Surface Air Kerma Entrance Surface Air Kerma g =g = incident air-kerma conversion factor for incident air-kerma conversion factor for

breast thicknesses (50% water, 50% breast thicknesses (50% water, 50% fat) fat)

c =c = glandularity factor glandularity factor s =s = x-ray spectrum correction factorx-ray spectrum correction factor

Average Glandular DoseAverage Glandular Dose




ContentsContents


Why Quality Control ?Why Quality Control ?

BSS requires Quality Assurance for medical BSS requires Quality Assurance for medical exposuresexposures

Principles established by WHO, (ICRP for dose), Principles established by WHO, (ICRP for dose), guidelines prepared by EC, PAHO,…guidelines prepared by EC, PAHO,…

A Quality Control program should ensure:A Quality Control program should ensure: The best image qualityThe best image quality With the least dose to the breastWith the least dose to the breast Hence regular check of important parametersHence regular check of important parameters

Parameters to be considered by a QC Parameters to be considered by a QC program (1)program (1)

XX-Ray generation and control-Ray generation and control Focal Spot sizeFocal Spot size ((star pattern, slit camera, pinholestar pattern, slit camera, pinhole))

Tube voltage Tube voltage ((reproducibility, accuracy, HVLreproducibility, accuracy, HVL))

AEC system AEC system ((kV and object thickness kV and object thickness

compensation, compensation, OD OD control, short term control, short term reproducibilityreproducibility......))

Compression Compression ((compression force, compression plate compression force, compression plate

alignmentalignment)) Bucky and image receptorBucky and image receptor

Anti Scatter grid Anti Scatter grid ((grid system factorgrid system factor))

Screen-FilmScreen-Film ((inter-cassette sensitivity, screen/film inter-cassette sensitivity, screen/film contactcontact))

Film ProcessingBase line (temperature, processing time)

Film and processor (sensitometry)

Darkroom (safelights, light leakage, film

hopper,.….)

Film ProcessingViewing Box (brightness, homogeneity)

Environment


System PropertiesReference Dose (entrance surface dose)

Image Quality (spatial resolution,

image contrast, noise

threshold contrast

visibility, exposure time)


SummarySummary

To achieve the best image quality while To achieve the best image quality while keeping the breast dose at the ALARA level keeping the breast dose at the ALARA level is the final goal to be reached when is the final goal to be reached when consistently using a film-screen or digital consistently using a film-screen or digital mammography equipment.mammography equipment.

Implementing a well defined QC protocol Implementing a well defined QC protocol can effectively contribute to the achievement can effectively contribute to the achievement of such a goal.of such a goal.

The Quality of Image and Radiation Risk in mammography Carlo Maccia Medical Physicist CAATS 43 Bd du Maréchal Joffre – Bourg-La-Reine – FRANCE XI. National.

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