Corrections for physical effects in Positron Emission Mammography

Corrections for physical effects in

Positron Emission Mammography

Nuno C. FerreiraNuno C. FerreiraDep. Biofísica – IBILIDep. Biofísica – IBILI

Fac. Medicina – Univ. CoimbraFac. Medicina – Univ. Coimbra

PEM Workshop – Lisbon, 1-2 July 2002PEM Workshop – Lisbon, 1-2 July 2002

2002, 2002, IBILIIBILI, FMUC, FMUC

Why correct data for physical effects?

To improve To improve image qualityimage quality Image should represent the true activity distributionImage should represent the true activity distribution Corrections improve contrast and tumour detectability, Corrections improve contrast and tumour detectability,

remove artefactsremove artefacts

To preserve To preserve quantificationquantification Useful in comparative studies (e.g. follow-up to Useful in comparative studies (e.g. follow-up to

therapy)therapy)

ScatteredScatteredcoincidences coincidences componentcomponent

AttenuationAttenuationof radiationof radiation

Random Random coincidencescoincidencescomponentcomponent

Detector Detector efficiencyefficiency

effectseffects TrueTruecoincidencescoincidencescomponentcomponent

Data corrections are necessary... ...because the measured projections are not the same as ...because the measured projections are not the same as

the projections assumed during image reconstruction the projections assumed during image reconstruction

ObjectObject(uniform(uniformcylinder)cylinder)

projectionprojectionmeasuredmeasured

projectionprojectionassumedassumed

integral of the activity integral of the activity along the line or tube of along the line or tube of

responseresponse


True eventTrue event

Correct data for what physical effects?

ComptonCompton

RandomRandom

Event lost due to Event lost due to AttenuationAttenuation

Event lost dueEvent lost dueto sensitivityto sensitivity

Random coincidencesRandom coincidences

Compton ScatterCompton Scatter

AttenuationAttenuation

Sensitivity variationsSensitivity variations(Normalization correction)(Normalization correction)






Sensitivity variations Sensitivity variations (Normalization correction)(Normalization correction)

RandomRandom

DELAYEDcoincidences = Estimate(RANDOMS)

Randoms Correction Using Delayed Channel

Detector 2 + delay

delay

time

Detector 1

Detector 2

= TRUES + RANDOMSPROMPT

coincidences

Estimate(TRUES) = (TRUES+RANDOMS) – Estimate(RANDOMS)

PROMPTS DELAYED“TRUES” –=


Random coincidences correction To improve the estimation of random To improve the estimation of random

coincidences:coincidences:

Option 1: Averaging of estimations obtained with various Option 1: Averaging of estimations obtained with various delaysdelays

Option 2: Measurement of the single events: Option 2: Measurement of the single events:

Detector 1Detector 1

Detector 2Detector 2

SS11

SS22

RR1212

Width of Width of coincidence coincidence

windowwindow

RR1212 = 2T . S = 2T . S11 . S . S22

RandomsRandomsraterate

Singles rate Singles rate in detectors in detectors

1 and 21 and 2

Random coincidences correction Randoms: PEM vs. PETRandoms: PEM vs. PET

PEM has less activity in the FOV PEM has less activity in the FOV less randoms than PET less randoms than PET In PEM most activity is outside the FOV and there are no In PEM most activity is outside the FOV and there are no

end-shields end-shields larger % of randoms from outside the FOV larger % of randoms from outside the FOV

ClearPEM: ClearPEM: LuAP is fast LuAP is fast very short coinc. window very short coinc. window less randoms less randoms

PETDetectors

End-shields

PEMDetectors

BrainBrain

MyocardiumMyocardium

BladderBladder

LiverLiver

TorsoTorso

Activity outside the FOV

Source: Raylman et al., IEEE Trans Nucl Sci (2001), 48(3):913-923 Source: Raylman et al., IEEE Trans Nucl Sci (2001), 48(3):913-923

MyocardiumMyocardium(9%)(9%)

TorsoTorso(82%)(82%)

LiverLiver(7%)(7%) BladderBladder

(1%)(1%)

PEMPEMDetectorsDetectors

Contributions of different sources to the total random coincidences from outside

the FOV BrainBrain(1%)(1%)







ComptonCompton


Effect of scatter Main effect:Main effect:

Reduces image contrast (target-to-Reduces image contrast (target-to-background activity ratio)background activity ratio)

Affects tumour detectabilityAffects tumour detectability

Other effects:Other effects: Reduces quantification accuracyReduces quantification accuracy Slight reduction of the total number of useful Slight reduction of the total number of useful

events detectedevents detected Slight degradation of spatial image resolutionSlight degradation of spatial image resolution

Scatter correction PEM vs. PETPEM vs. PET

Relative to PET, scatter is less important in PEM...Relative to PET, scatter is less important in PEM... ...but influence of activity from outside the FOV is more ...but influence of activity from outside the FOV is more

importantimportant Activity is mostly outside the FOV (heart, chest)Activity is mostly outside the FOV (heart, chest) No side shielding in PEMNo side shielding in PEM

Correction methods:Correction methods: Proposed by other groups for PEM: Proposed by other groups for PEM:

Monte-Carlo simulation Monte-Carlo simulation [Holdsworth et al., IEEE TNS, February 2002][Holdsworth et al., IEEE TNS, February 2002] Convolution-subtraction (Bergström)Convolution-subtraction (Bergström) [Raylman et al., IEEE TNS, [Raylman et al., IEEE TNS,

June 2001]June 2001] Proposed by our group for PET: Proposed by our group for PET:

Hybrid energy-based correction Hybrid energy-based correction [Ferreira et al., Phys Med Biol, May [Ferreira et al., Phys Med Biol, May 2002]2002]

Advantage: takes scatter from outside the FOV into accountAdvantage: takes scatter from outside the FOV into account







Event lost due to Event lost due to AttenuationAttenuation

Attenuation correction

Attenuation is more important in PET than in PEMAttenuation is more important in PET than in PEM Attenuation needs to be measured in PET, but probably not in PEM Attenuation needs to be measured in PET, but probably not in PEM

attenuation coefficient @ 511keV is ~ constant for the breastattenuation coefficient @ 511keV is ~ constant for the breast use of consistency conditions may be sufficiently accurateuse of consistency conditions may be sufficiently accurate

The amount of attenuation is mainly dictated by the dimensions of the The amount of attenuation is mainly dictated by the dimensions of the human body. human body.

When a radiation beam When a radiation beam penetrates a material with penetrates a material with thickness x and linear thickness x and linear attenuation coefficient attenuation coefficient , it is , it is attenuated by a factor:attenuated by a factor:ee--xxFor tissue, For tissue, =0.096 cm=0.096 cm-1-1 @ @ 511 keV511 keV

x=15 cm x=15 cm 24% 24%x= 5 cm x= 5 cm 62% 62%

x=50 cm x=50 cm 1% 1%

% of true % of true (unscattered) (unscattered) coincidences coincidences

detected:detected:


Attenuation correction

Source: Meikle, Bailey et al., Source: Meikle, Bailey et al., J NucJ Nuc MedMed (1995) (1995); 36:1680-1688; 36:1680-1688

Effect of Attenuation Correction (PET)Effect of Attenuation Correction (PET)







Includes various Includes various corrections, mainly corrections, mainly dependent of the dependent of the scanner:scanner:

- geometrical effectsgeometrical effects

- intrinsic efficiencies intrinsic efficiencies of individual of individual detectorsdetectors

- interferences interferences between individual between individual detectorsdetectors

- deadtime deadtime

Results from bibliographySource: Raylman et al., IEEE Trans Nucl Sci (2001), 48(3):913-923 Source: Raylman et al., IEEE Trans Nucl Sci (2001), 48(3):913-923

FDGFDG

Water (Cold)Water (Cold)

NormalizedNormalizedRaw imageRaw image(no corrections)(no corrections)

NormalizedNormalized+ Random correction+ Random correction

NormalizedNormalized+ Random correction+ Random correction+ Scatter correction+ Scatter correction

Scatter Scatter phantophantom m

uncorrecteduncorrectedcorrectedcorrected

Results from bibliography (2)Source: Raylman et al., IEEE Trans Nucl Sci (2001), 48(3):913-923 Source: Raylman et al., IEEE Trans Nucl Sci (2001), 48(3):913-923

(?)(?)

BreastBreastphantophantom m

Raw imageRaw image(no corrections)(no corrections) NormalizedNormalized NormalizedNormalized

+ Random correction+ Random correctionNormalizedNormalized

+ Random correction+ Random correction+ Scatter correction+ Scatter correction


Summary Specificities of PEM Specificities of PEM

Corrections:Corrections: Randoms correctionRandoms correction

Less randoms than PETLess randoms than PET Larger % of randoms from Larger % of randoms from

outside the FOV than PEToutside the FOV than PET Same methods as in PET Same methods as in PET

Scatter correctionScatter correction Less scatter than PETLess scatter than PET More scatter from outside the More scatter from outside the

FOV than PETFOV than PET Hybrid energy-based methodsHybrid energy-based methods

Attenuation correctionAttenuation correction Less attenuation than PETLess attenuation than PET Attenuation in breast is more Attenuation in breast is more

homogeneous than in PEThomogeneous than in PET No transmission measurement No transmission measurement

(but attenuation correction (but attenuation correction seems feasible)seems feasible)

Normalization correctionNormalization correction Correction is scanner Correction is scanner

dependent, as in PET dependent, as in PET

Specificities Specificities of PEM:of PEM:

Large influence of Large influence of activity outside the activity outside the FOV FOV

Little activity in Little activity in the FOVthe FOV

Most activity is Most activity is outside the FOVoutside the FOV

Side-shielding is Side-shielding is not possiblenot possible

Small FOV, objectSmall FOV, object Less attenuationLess attenuation

High sensitivity High sensitivity Large solid angleLarge solid angle Less attenuationLess attenuation

High resolutionHigh resolution

Indications for Indications for PEM camera PEM camera design:design:

Randoms correctionRandoms correction Smallest possible Smallest possible

coinc. window (LuAP is coinc. window (LuAP is OK)OK)

Capability to count Capability to count singles/channel...singles/channel...

...or count randoms ...or count randoms with multiple delayswith multiple delays

Scatter correctionScatter correction Good energy Good energy

information (~256 information (~256 energy channels)energy channels)

List-mode (with energy List-mode (with energy information)information)

Normalization correctionNormalization correction Information about Information about

single events (for single events (for deadtime correction)deadtime correction)

Corrections for physical effects in Positron Emission Mammography

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