Introduction toPositron Emission Tomography

with your host, Terry Oakes

Positron Annihilationγ#1 γ#2

180 o

T.R.OakesUniv. WI-Madison

Positron Annihilation

180 o

positronemittingatom

neighboringatom

Positron range:1-10 mm

Gamma-Ray range:10 mm - 8

positronannihilation

γ#2γ#1

T.R.OakesUniv. WI-Madison

Positron Emission Tomography

detector #2

detector #1

γ#2

γ#1

detectorring

T.R.OakesUniv. WI-Madison

Positron Emission Tomography

detector #1

γ#2

γ#1

detector #2

T.R.OakesUniv. WI-Madison

Positron Emission Tomography

T.R.OakesUniv. WI-Madison

Positron Emission Tomography

T.R.OakesUniv. WI-Madison

Positron Emission Tomography

T.R.OakesUniv. WI-Madison

P.E.T. measures Concentration of Radioactivity

1) Gamma-rays escape from body:External detection possible.

2) Two gamma rays emittedat 180 when a positronannihilates:The annihilation occuredsomewhere between the two detectors.

o

3) Regions with greaterradioactivity levelsproduce more LORs:Concentration of radioactivitycan be measured and quantified.

T.R.OakesUniv. WI-Madison

P.E.T. yields Quantitative results. ( µCi/cc tissue )

Corrections must be applied to raw data:1) Dead-time, Random events2) Calibration3) Normalization4) Scatter-correction5) Attenuation correction6) Reconstruction

T.R.OakesUniv. WI-Madison

Corrections to Sinograms color scale

raw sinogram(20 cm cylinder)

trim(remove edges)

Scatter-Correct

mash(combine angular views)

Normalized,without Scatter-Correction

Normalize

T.R.OakesUniv. WI-Madison

Effect of Corrections on Images

Uniform20 cm cylinder

color scaleT.R.Oakes

Univ. WI-Madison

Scatter-corrected

Normalized

Attenuation-corrected

X

X

X

X

X

X

X

X

X

CalibrationConverting

“counts/pixel”to

“microCi / cc tissue”

detectorring

knownradioactivityconcentration

Measure response ofthe PET scanner to aknown concentrationof radioactivity.

Obtain Calibration Factor

applicable for all scans.( CF ~ (µCi/cc)/counts )

T.R.OakesUniv. WI-Madison

Normalizationdetector

ring

T.R.OakesUniv. WI-Madison

Normalization

Normalization produces auniform response to thesame level of radioactivity.

Individual detectors responddifferently to the same levelof radioactivity.

T.R.OakesUniv. WI-Madison

Attenuation CorrectionGamma-ray attenuation proportional to:1) density of object2) pathlength through object

Attenuation can be corrected with a density-weighted map of the object.

gamma-rayattenuated

rotatingradioactiveline-source

Attenuation causes a non-uniform loss of events; more gamma rays passing through the middle of the object are lost.

T.R.OakesUniv. WI-Madison

Scatter CorrectionScatter tends to increase the concentration measured toward the center of the object.

Scattered gamma rays causerecorded events to be mispositioned.

gamma-ray scattered scattered

events

unscattered(true) events

num

ber o

f eve

nts

T.R.OakesUniv. WI-Madison

radial bin

PET Data Acquistion

raw sinogram

Radioactive sourcein PET scanner

T.R.OakesUniv. WI-Madison

P.E.T. data are acquired as discrete Lines of Response (LORs)

radial binsradial bins

radial bins

Angular View #1(theta = 0)

Angular View #2(theta = 45)

Angular View #3(theta = 90)

T.R.OakesUniv. WI-Madison

Angular View #1(theta = 0)Three Angular Views

Angular View #2(theta = 45)

T.R.OakesUniv. WI-Madison

Angular View #3(theta = 90)

Angular View #2(theta = 45)

T.R.OakesUniv. WI-Madison

Angular View #3(theta = 90)

Angular View #1(theta = 0)

angu

lar

view

sSinogram

#1

#2

#3

radial bins

Image Reconstruction

raw sinogram Reconstructed Image

it’s magic!

T.R.OakesUniv. WI-Madison