Physics of PET-CT
David S. Graff PhD

The Short, Rich Life of Positronium
What is PET
In this instance, the celebration is of
discoveries about antimatter that enable
contemporary scientists, engineers, and
physicians to make everyday use of
antimatter in ways that would have been
Radiation
regarded as impossibly exotic only a few
decades ago.
The sculpture, built in the Michigan shops by
,\"$-+.#$&. Jens Zorn, has an abstract quality for the
risks
23 general viewer; however people familiar with the
ens Zorn relevant science will recognize it as a
bronze,
now lo-
stylized expression of the creation and
Randall destruction of the simplest atom of antimatter.
West Hall. Positrons and electrons are antiparticles that
pril 5, 1999. are strongly attracted to one another.
Electrons are common constituents of ordi-
nary matter, while positrons are created only
under special circumstances of atomic and
PET Tracers
nuclear interactions. Once created, the
positron inevitably finds itself in the
company of electrons, and a typical fate for
the positron is to couple with one of those
electrons in a mutually encircling,
ever-decreasing orbit. This two-body system
T he sculpture “The Short, Rich Life
of Positronium” commemorates the
is called “positronium” and resembles an
ordinary atom in many ways. During its
How a
fundamental research on antimatter done at short lifetime (only a few millionths of a
the University of Michigan by Arthur Rich second), positronium emits light as its orbit
(1937-1990), his students and colleagues. shrinks. Finally, the positron and electron
Its creation was made possible by the annihilate one another as they convert into
scanner works
Physics Department, by the University’s a pair of high-energy gamma-rays in the
Administration, and by the friends and fam- purest-known example of Einstein’s famous
ily of Arthur Rich. It provides a permanent, relationship of mass to energy: E=mc2.
visual reminder to the University Michigan physicists, (notably Arthur Rich,
community and visitors that scientists at later with David Gidley, and their students
Michigan have made (and continue to make) and collaborators) have studied the
important discoveries. formation and annihilation of positronium
17 News of Michigan Physics
Corrections

The Short, Rich Life of Positronium
What is PET
In this instance, the celebration is of
discoveries about antimatter that enable
contemporary scientists, engineers, and
Radiation physicians to make everyday use of
antimatter in ways that would have been
risks regarded as impossibly exotic only a few
decades ago.
The sculpture, built in the Michigan shops by
. Jens Zorn, has an abstract quality for the
general viewer; however people familiar with the
PET Tracers
relevant science will recognize it as a
n
,
-
stylized expression of the creation and
ll destruction of the simplest atom of antimatter.
l. Positrons and electrons are antiparticles that
9. are strongly attracted to one another.
Electrons are common constituents of ordi- How a
nary matter, while positrons are created only
scanner works
under special circumstances of atomic and
nuclear interactions. Once created, the
positron inevitably finds itself in the
company of electrons, and a typical fate for
the positron is to couple with one of those
electrons in a mutually encircling,
ever-decreasing orbit. This two-body system Corrections
T he sculpture “The Short, Rich Life
of Positronium” commemorates the
fundamental research on antimatter done at
is called “positronium” and resembles an
ordinary atom in many ways. During its
short lifetime (only a few millionths of a
the University of Michigan by Arthur Rich second), positronium emits light as its orbit
(1937-1990), his students and colleagues. shrinks. Finally, the positron and electron
Its creation was made possible by the annihilate one another as they convert into
Physics Department, by the University’s a pair of high-energy gamma-rays in the
Administration, and by the friends and fam- purest-known example of Einstein’s famous
ily of Arthur Rich. It provides a permanent, relationship of mass to energy: E=mc2.

What is PET
P
E
T

What is PET?
Positron Emission Tomography

What is PET?
Positron Emission Tomography

Tomography
From CT: tomography requires
lines of sight over all angles
Lines of sight 360º coverage

Tomography
From CT: reconstruct with
Filtered Back Projection

Tomography requires sampling along known
lines of sight through patients
Emission
Positron

What is PET?
Positron Emission Tomography

In Transmission tomography, we know the
location of the source and the detector, so
we can reconstruct a line of sight
Line of Sight

In Emission Tomography, source is
diffused through patient. How to
construct a line of sight?

Tomography requires sampling along known
lines of sight through patients
Emission tomography is challenging because
source position (and hence line of sight) is
difficult to find
Positron

What is PET?
Positron Emission Tomography

A Positron is the antimatter
partner of an electron
Mass: 511 keV Mass: 511 keV
Charge: –1.6 10-19C Charge: +1.6 10-19C

Positron decay makes
back-to-back photons
1 keV
= 51 11
hν 5
eV y: 0 =+
51 1k – rg :
e ge
En ar m:
h/λ
: hν
= 511 h entu
gy : 0
er ge λ=
- + C m
Mo
n r
E a : h/
Ch men tum
Mo TOTAL
Energy: 2mc2 = 1022 keV
Charge: 0
Momentum: 0

Professor Arthur Rich holds a special place in the hearts o
community. His untimely death in 1990 prompted the
The short, rich life of positronium remember his contributions with sculpture.
Jens Zorn PhD, University of Michigan of Positroniu
The Short, Rich Life
In this instan
discoveries abou
contemporary sc
physicians to m
antimatter in wa
regarded as imp
decades ago.
The sculpture, bu
!\"#$%\"&'()$*+,\"$-+.#$&. Jens Zorn, has a
/&0+('&1+23 general viewer; how
sculpture by Jens Zorn relevant science
1998, welded bronze,
64”x38\"x20\" is now lo-
stylized express
cated between Randall destruction of the s
Laboratory and West Hall. Positrons and elec
It was installed April 5, 1999. are strongly att
Electrons are com
nary matter, while
under special circ
nuclear interact
positron inevita
company of electr
the positron is to
electrons in a
ever-decreasing o
T he sculpture “The Short, Rich Life
of Positronium” commemorates the
fundamental research on antimatter done at
is called “positro
ordinary atom in
short lifetime (on
the University of Michigan by Arthur Rich second), positroni
(1937-1990), his students and colleagues. shrinks. Finally,
Its creation was made possible by the

Coincidence detection
determines line of sight

Coincidence detection
determines line of sight

Tomography requires sampling along known lines
of sight through patients
Emission tomography is challenging because source
position (and hence line of sight) is difficult to find
Positron annihilates into back-to-back photons.
Line of sight is between two detectors.
Allows Tomography

physicians to make everyday use of
antimatter in ways that would have been
regarded as impossibly exotic only a few
$&.
Positron annihilates into back-to-back photons
decades ago.
The sculpture, built in the Michigan shops by
Jens Zorn, has an abstract quality for the
Simultaneous detection yields line of sight
general viewer; however people familiar with the
orn relevant science will recognize it as a
ze,
lo-
stylized expression of the creation and
dall destruction of the simplest atom of antimatter.
Enables efficient emission tomography
Hall. Positrons and electrons are antiparticles that
999. are strongly attracted to one another.
Electrons are common constituents of ordi-
nary matter, while positrons are created only
under special circumstances of atomic and
nuclear interactions. Once created, the
positron inevitably finds itself in the
company of electrons, and a typical fate for
the positron is to couple with one of those
electrons in a mutually encircling,
ever-decreasing orbit. This two-body system
T he sculpture “The Short, Rich Life
of Positronium” commemorates the
fundamental research on antimatter done at
is called “positronium” and resembles an
ordinary atom in many ways. During its
short lifetime (only a few millionths of a
the University of Michigan by Arthur Rich second), positronium emits light as its orbit
(1937-1990), his students and colleagues. shrinks. Finally, the positron and electron
Its creation was made possible by the annihilate one another as they convert into
Physics Department, by the University’s a pair of high-energy gamma-rays in the
Administration, and by the friends and fam- purest-known example of Einstein’s famous
ily of Arthur Rich. It provides a permanent, relationship of mass to energy: E=mc2.
visual reminder to the University Michigan physicists, (notably Arthur Rich,
community and visitors that scientists at later with David Gidley, and their students
Michigan have made (and continue to make) and collaborators) have studied the
important discoveries. formation and annihilation of positronium
17 News of Michigan Physics

What is PET
Radiation
risks
PET Tracers
How a
scanner works
Corrections

300× more
lead needed to
block PET
photons
Typical
diagnostic scan 511 keV

d PET/CT Shielding 6
only used
511 keV Lead
dose
q
How much protection would be
afforded by a lead vest?
se rate
similar
ositron- FIG. 1. Plot of lead broad beam transmission factors as a function of lead
e high- thickness.

Staff doses from PET
can be high
33 mSv/hr 30 µSv/hr
4 mSv/hr
Typical dose 1.5 mSv/scan Typical dose 0.2 mSv/scan
2 lbs ALARA<50 mSv/yr ALARA<5 mSv/yr

PET materials are more
risky Ci for Ci
6
Exposure rate constant
4
R/Ci/hr/cm2
2
Thallium 201 0
Technetium 99m
Fluorine 18

The majority of the patient dose
comes from the CT scan
PET
0.7 rem / scan
CT
1.8 rem / scan
Ref: Beyer T, Mueller SP, Brix G et al.
Radiation exposure during combined
whole-body FGD-PET/CT imaging. 51st
Annual Meeting, Society of Nuclear
Medicine, June 22, 2004. Abstract 1331.

Positron annihilates into back-to-back photons
Simultaneous detection yields line of sight
Enables efficient emission tomography
PET uses penetrating photons (TVL 16 mm Pb)
Potential serious dose from injection and patient
positioning

What is PET
Radiation
risks
PET Tracers
How a
scanner works
Corrections

Type of decay depends
on isotope Too Big
Number of Protons
to ns
y pro
m an ns
Too ut ro
e
n yn
o ma
To
Number of Neutrons

Positrons are created
with β+ radioactive decay
n+
p0
+

Isotopes used in PET
Number of Protons
Number of Neutrons

PET Isotopes have to be
manufactured in a cyclotroron

Production of 18F
Number of Protons
Number of Neutrons

Nearly all PET
scans use FDG
as a tracer
Shows Glucose use
and hence general
metabolic activity

FDG traces glucose usage.
• Actively brought • Actively brought
into active cell into active cell
• Phosphorilized • Phosphorilized
• Metabolized • Decay
• Metabolized

Normal PET scan

These PET scan images superimposed on MRI
scan images shows a healthy medical student's
brain function when performing arithmetical tasks at
the same time as being exposed to irrelevant
speech. Some brain areas are found significantly
modulated: (A) Shows bilateral decreases in the
auditory cortex. (B) Shows an increased activity in
the left posterior parietal cortex. Courtesy
and © Karolinska Institute and Hospital,
PET Cognitive Neurophysiology, Sweden.

NonHodgkin’s Lymphoma

41 year old female with a history of breast cancer, status post lumpectomy and
axillary node dissection, and radiation treatment was referred for a restaging
PET/CT examination. Metastatic breast cancer with ovarian metastases

Lymphoma - 56-year-old male

Positron annihilates into back-to-back photons
Simultaneous detection yields line of sight
Enables efficient emission tomography
PET uses penetrating photons (TVL 16 mm Pb)
Potential serious dose from injection and patient
positioning
Most scans use FDG as a tracer
Produced offsite in a cyclotron
Traces glucose uptake

What is PET
Radiation
risks
How a
PET Tracers scanner works
Corrections

How PET works
A tracer isotope decays and
emits back-to-back photons
Two detectors light up at the
same time
We infer the position of the
tracer somewhere along the
line of sight
More lines of sight localize the
tracer

New OSEM
reconstruction
algorithm handles
noise better

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• Given two competing PET systems, which
would you recommend?
• The one with better contrast.
• The one with less noise.

Positron annihilates into back-to-back photons
Simultaneous detection yields line of sight
Enables efficient emission tomography
PET uses penetrating photons (TVL 16 mm Pb)
Potential serious dose from injection and patient
positioning
Most scans use FDG as a tracer
Produced offsite in a cyclotron
Traces glucose uptake
Pixelated scintillator absorbs photon and emits light
Light detected by photomultiplier tube
Computer model of patient iteratively modified until
statistically matches scanner data (OSEM)

What is PET
Radiation
risks
PET Tracers
How a
Corrections
scanner works

Photons from deep
tissue may attenuate

CT measures attenuation
Measure PET and CT at
the same time

Including
attenuation

Scatter can add noise.
One of the two photons
scatters in the patient
A “wrong” detector lights up
The wrong line of sight is
inferred
Incorrect information adds noise to image

Too much activity can
add noise. Randoms
Two decays occur at almost
the same time
The wrong detectors light up
at the same time
The wrong line of sight is
inferred
Incorrect information adds noise to image

Time of flight PET can
decrease noise
It takes more time for the
photon to travel to the far
detector. (Speed of light:
1 ft/0.0000000001 sec)
Measure the time difference
between detectors
Deduce where along line of Δt = 1.2 ± 0.3 ns
sight event came from.
More information reduces noise

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Positron annihilates into back-to-back photons
Simultaneous detection yields line of sight
Enables efficient emission tomography
PET uses penetrating photons (TVL 16 mm Pb)
Potential serious dose from injection and patient
positioning
Most scans use FDG as a tracer
Produced offsite in a cyclotron
Traces glucose uptake
Pixelated scintillator absorbs photon and emits light
Light detected by photomultiplier tube
Computer model of patient iteratively modified until
statistically matches scanner data (OSEM)
Noise from Scatter, Randoms
Randoms can increase noise with high activity
Time of flight can reduce noise for large patients

physicians to make everyday use of
antimatter in ways that would have been
regarded as impossibly exotic only a few
$&.
Positron annihilates into back-to-back photons
decades ago.
The sculpture, built in the Michigan shops by
Jens Zorn, has an abstract quality for the
Simultaneous detection yields line of sight
general viewer; however people familiar with the
orn relevant science will recognize it as a
ze,
lo-
stylized expression of the creation and
dall destruction of the simplest atom of antimatter.
Enables efficient emission tomography
Hall. Positrons and electrons are antiparticles that
999. are strongly attracted to one another.
Electrons are common constituents of ordi-
nary matter, while positrons are created only
under special circumstances of atomic and
nuclear interactions. Once created, the
positron inevitably finds itself in the
PET uses penetrating photons (TVL 16 mm Pb)
company of electrons, and a typical fate for
the positron is to couple with one of those
electrons in a mutually encircling,
Potential serious dose from injection and patient
ever-decreasing orbit. This two-body system
T he sculpture “The Short, Rich Life
of Positronium” commemorates the
fundamental research on antimatter done at
is called “positronium” and resembles an
ordinary atom in many ways. During its
short lifetime (only a few millionths of a
positioning
the University of Michigan by Arthur Rich second), positronium emits light as its orbit
(1937-1990), his students and colleagues. shrinks. Finally, the positron and electron
Its creation was made possible by the annihilate one another as they convert into
Physics Department, by the University’s a pair of high-energy gamma-rays in the
Administration, and by the friends and fam- purest-known example of Einstein’s famous
Most scans use FDG as a tracer
ily of Arthur Rich. It provides a permanent, relationship of mass to energy: E=mc2.
visual reminder to the University Michigan physicists, (notably Arthur Rich,
community and visitors that scientists at later with David Gidley, and their students
Michigan have made (and continue to make) and collaborators) have studied the
important discoveries.
Produced offsite in a cyclotron
formation and annihilation of positronium
Traces glucose uptake
17 News of Michigan Physics
Pixelated scintillator absorbs photon and emits light
Light detected by photomultiplier tube
Computer model of patient iteratively modified until
statistically matches scanner data (OSEM)
Noise from Scatter, Randoms
Randoms can increase noise with high activity
Time of flight can reduce noise for large patients