Lecture5 B PET

7/23/2019 Lecture5 B PET

1/26

How it works: Positron Emission

Radioactive decay unstable atomic nuclei due to too

many protons relative to thenumber of neutrons

decays to stable form byconverting a proton to a neutron

ejects a 'positron' to conserveelectric charge

positron annihilates with anelectron, releasing two anti-colinear high-energy photons

np

np

n

p

np n

pn

p

np

p

p

np n

p

np

n

p n np

np

n

p

np n

pn

p

np

n

p

np n

p

np

n

p n

~2 mm

18F 18O

~180 deg

E = mc2

= 511 keV

!+

e-


2/26

Types of Photons

X-ray photons

gamma (!) ray photons (Greek letters for

radiation from nuclear decay processes)

annihilation photons

all can have the same energy


3/26

Molecular Imaging: Glu Metabolism

FDG-6-PO4is trapped and is a

good marker for glucose

metabolic rates*

glucose

glucose 6-phosphate

pyruvate lactate

gylcolysis(anaerobic,inefficient)

TCA(oxidative,efficient)

HOCH2

H18

F

H

OH HHO

H

OH

H

radioactive

fluorine

O

[18F]fluorodeoxyglucose (FDG)

what

we

see

FDG

FDG 6-

phosphateX


4/26

How it works: Scintillation

high energy

511 keV photon

optical photons (~ 1eV)

scintillator(e.g. BGO Denseyet transparent)

current

pulse foreach UVphoton

detected

photomultipliertubes (PMTs)gain of ~ 106


5/26

Scintillators used in PET Scanners

new

technology

very

short

somewhat

lower than

LSO

very highmore

expensiv

e

GSO

new

technology

very

short

highhighmore

expensiv

e

LSO

workhorselonghighestlowestexpensiv

e

BGO

Hygroscop

ic

longlowesthighestcheap

(relativel

y)

NaI(Tl)

CommentsDecay

time (s)determine

s

deadtime

and

randoms

Effective

Densitydetermines

scanner

sensitivity

Effectivenumber

of scintillation

photons @ 511

keV determinesenergy and spatial

resolution

CostMateri

al


6/26

How it works: Timing coincidence

"t < 10 ns?

detector A

detector B

record

positron

decayevent

scanner

FOV

#++ e-

annihilation


7/26

Typical PET Scanner Detector Ring


8/26

Anatomy: PET gantry

Detector+ PMT

assem-blies


9/26

Typical PET Image

Lung cancer example: Very obvious!

Elevated uptake of FDG (related to metabolism)


10/26

What is Attenuation?The single most important physical effect in PET imaging:

The number of detected photons is significantly reduced compared tothe number of positron decays in a spatially-dependent manner

For PET it is due to Compton scatter out of the detector ring

For CT it is a combination of Compton scatter and photoelectricabsorption

one 511 keV photon

scattered out of scannerone 511 keV photon absorbed

scanner

patient


11/26

PET image without

attenuation correction

Simulation of the Effects of notPerforming

Attenuation Correction of PET Emission Image

True PET image

(simulation of abdomen)

profile

Enhanced 'skin'

Reduced interior

(even neg.!)

Locally

increased

contrast


12/26

Effects of Attenuation: Patient Study

PET: without

attenuation correctionPET: with attenuation

correction (accurate)

CT image (accurate)

Enhanced

skin uptake

reducedmediastinal

uptake

Non-uniform

liver

'hot' lungs


13/26

Attenuation Correction Transmission scanning with an external photon source is used

for attenuation correction of the emission scan The fraction absorbed in a transmission scan, along the same

line of response (LOR) can be used to correct the emission scandata

The transmission scan can also be used to form a 'transmission'or 'attenuation' image

$

sy

x

same line of response(LOR) L(s,")

Emission scan (EM) Transmission (TX)

tracer uptake tissue density

photon source

rotation

t

f(x,y)

FOVscanner


14/26

PET Transmission imaging

(annihilation photon imaging)

Using 3-point coincidences, we can reject TX scatter

(x,y)is measured at needed value of 511 keV

near-side detectors, however, suffer from deadtime due to high

countrates, so we have to limit the source strength (particularly in 3D)

(x,y)

orbiting

68Ge/68Gasource

PET

scanner

near-side

detectors

511 keVannihilation

photon


15/26

And, if you have PET/CT scanner: X-ray TX

Photon flux is very high, so very low noise Greatly improved contrast at lower photon energies.

Scatter and beam-hardening can introduce bias.

(x,y,E)is measured as an weighted average from ~30-120 keV, so (x,y,511keV) must be calculated, potentially introducing bias

(x,y)

orbiting X-ray tube and

detectorassembly

X-ray

detectors

30130 keV

X-ray photon


16/26

X-ray and Annihilation Photon Transmission Imaging

Linear attenuation coeffcient at511 keV

Not a physical quantity

SlowFast

NoisyLow noisePET Transmission(511 keV)X-ray (~30-120 keV)

Energy spectra


17/26

Quantitative errors in measurement

Lost (attenuated)event

Scattered coincidenceevent

Random coincidenceevent

incorrectly determined LORs

Comptonscatter

no LOR


18/26

3D versus 2D PET imaging

2D Emission Scan 3D Emission Scan

detectedabsorbedby septa

detected

detected

! fewer true, scattered, andrandom coincidences

!more true, scattered, andrandom coincidences

septa


19/26

Effect of random coincidence

corrections in 2D and 3D

2D Emission Scan 3D Emission Scan

FOV for random

coincidences

FOV for truecoincidences


20/26

Noise Equivalent Counts or NEC

NEC ~ 'Effective' count ratePrompt coincidences are what the scanner sees: P=T+S+R

but true coincidences are what we want: T=P-S-R, which addsnoise

so overall we have: SNR

2! NEC =

T

1+S/T+R/T

In this measured

example, at 10 kBq/cc(about 6 mCi) thescanner's count rate forcoincidences will be~450 kcps, but the

effective count rate(aka NEC) will be only~75 kcps


21/26

NEC comparisons Major arguing point for some vendors

Determined partly by detector type, detector and scannergeometry, acquisition mode, and front-end electronics

Important, but not sole factor for image quality

0

20

40

60

80

100

120

140

0 5 10 15 20 25 30 35 40

kcps

Range for whole-body scansPeak NEC rates


22/26

Partial Volume Effect

Apparent SUV drops with volume

Also effected by image smoothing

Final ImageFillable spheres


23/26

PET Resolution Losses

Simulation study with

typical imaging protocols

Limits quantitation in

oncology imaging,important for following

therapy if size changes

true tracer uptake reconstructed values(scanner resolution + smoothing of noisy data)


24/26

Time of Flight (TOF) PET/CT

Uses difference in photon detection times toguess at tracer emission point

without timing info, emission point could be

anywhere along line

c = 3x1010cm/s, so "t = 600 ps ~ "d = 10 cm

in resolution

"d = c "t/2

timingresolutionuncertainty

best guess about

location (d)

#++ e-

annihilation


25/26

Philips Gemini TF

PET scanner LYSO : 4 x 4 x 22 mm3

28,338 crystals, 420 PMTs 70-cm bore, 18-cm axial FOV

CT scanner

Brilliance 16-slice

Installation at U.Penn Nov 05Validation and research patient imaging

Nov 05 Apr 06 50 patients

Beta testing and upgrade to production release softwareMay 06 Jun 06 40 patients (to date)


26/26

Heavy-weight patient study

13 mCi

2 hr post-inj3 min/bed

MIP

Colon cancer

119 kgBMI = 46.5

non-TOF

Improvement in lesion detectability with TOF

TOFLDCT

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