Basics of Nuclear Contents Medicine - unibas.chBasics of Nuclear Medicine Principles of Medical Imaging Prof. Dr. PhilippeCattin MIAC, University of Basel Oct 24th, 2016 Principles

Basics of NuclearMedicine

Principles of Medical Imaging

Prof. Dr. Philippe Cattin

MIAC, University of Basel

Oct 24th, 2016

Oct 24th, 2016Principles of Medical Imaging

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Prof. Dr. Philippe Cattin: Basics of Nuclear Medicine

Contents

Abstract

1 Introduction

Transmission versus Emission Imaging

Transmission versus Emission Imaging (2)

Transmission versus Emission Imaging (3)

2 Nuclear Parameters and Decay Schemes

Nuclear Structure

Isotopes, Isotones and Isobars

Nuclear Decay Rate

Nuclear Decay Rate (2)

Measurement

Modes of Decay

Alpha Decay

Beta Minus Decay

Positive Beta Decay

Spectrum of Beta Decay

Gamma Decay

Gamma Decay (2)

Radiation Properties

3 Radionuclide Production

Radionuclide Production

Cyclotron Production

Cyclotron Production (2)

Reactor Production

Generator Production

Common Radioisotopes

4 The Gamma Camera

Gamma Camera

Gamma Camera (2)

Positional and Pulse Height Analysis

Animated Schematic of the Gamma Camera


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(2)


Abstract

In this chapter...

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Introduction


(4)Transmission versus EmissionImaging

Transmission Imaging: planar X-ray, Fluoroscope, CT

Radiation position and strength (spectrum, energy) is known

(6.1)

with known, we measure and infer the tissue absorption factor .

Fig. 6.1: Principle of X-ray transmission imaging

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Introduction

(5)


Transmission versus EmissionImaging (2)

Position of the radiation source and its strength is unknown

Energy is known (mono-energetic)

Fig. 6.2: Principle of nuclear medicine

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Introduction

(6)


Transmission versus EmissionImaging (3)

Transmission Imaging (X-ray based methods)

We measure tissue dependent attenuation coefficients

→ anatomical information

Emission Imaging (Scintigraphy, SPECT, PET)

We measure concentrations of injected radio-

pharmaceutics → corresponds to ???

Hundreds of different radio-pharmaceutics have beendesigned to visualise various physiological e.g.functional processes → Functional imaging

Fig. 6.3:

Fig. 6.4: Typical PET

acquisition of a brain

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Nuclear Parameters andDecay Schemes


(8)Nuclear Structure

A nucleus of any atom consists of a mixture of protonsand neutrons. A simple nuclear model therefore consistsof

protons

neutrons

making a total of nucleons. Each element can thus bewritten in terms of these three components

(6.2)

where is the atomic number, the number ofneutrons and the atomic mass ( ).

The atomic number is redundant, as the element name defines the value of , the mass number can thus beused as the single identifier, e.g.

(6.3)

Fig. 6.5: Nucleus with three

electrons (green) neutrons,

(red) protons, (blue) electrons

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Nuclear Parameters and Decay Schemes

(9)


Isotopes, Isotones and Isobars

Nuclides with a constant proton number

but with a varying neutron number

are called Isotopes.

Nuclides with a constant neutron

number but with a varying proton

number are call Isotones.

Nuclides having a constant mass but

varying and are called Isobars.

Not all isotopes are stable. forexample has a half-life of .

Fig. 6.6: Example isotopes, isotone and

isobars for three atoms

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(10)


Nuclear Decay Rate

Decay of an unstable nuclide is a statistical process mathematically expressed, asthe rate of transformation of nuclei changing per unit time , yielding

(6.4)

where is the decay constant. The negative sign indicates that the number ofnuclei decreases with each event. The solution of this differential equation is

(6.5)

where is the number of nuclei at time and is the radionuclide'sparticular decay rate.

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(11)


Nuclear Decay Rate (2)

A more commonly used parameter as thedecay constant is the half-life . Given a

sample of a particular radionuclide, thehalf-life is the time taken for half theradionuclide's atoms to decay. The half-life isrelated to the decay constant as follows

(6.6)

Together with Eq 6.5 this yields

(6.7)

Isotope Half-life Clinical use

Vascular imaging

Lung ventilation

Cardiac imaging

Universal imaging

Imaging sepsis

In vitro analysis

Marker source

Tab. 6.1: Physical half-lifes of common

clinical nuclides

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(12)


Measurement

Radioactivity is equal to the

number of disintegrations per second.

(6.8)

where is measured in → Bequerel[http://en.wikipedia.org/wiki/Bequerel] with

(6.9)

The intensity of radiation incident on adetector at range from a radioactivesource is

(6.10)

where is the energy of each photon.

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(13)


Modes of Decay

There are three basic → modes of decay [http://en.wikipedia.org

/wiki/Nuclear_decay#Decay_modes_in_table_form]

→ Alpha decay [http://en.wikipedia.org/wiki/Alpha_decay] ( )

→ Beta decay [http://en.wikipedia.org/wiki/Beta_decay] ( and )

→ Gamma decay [http://en.wikipedia.org/wiki/Gamma_ray] ( )

Other more complex modes of decay exist but are not covered in this lecture.

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(14)


Alpha Decay

Principle:

The alpha decay involves an alpha particle (Heliumnucleus) and causes the greatest loss of energy froman unstable nucleus, since it looses two neutrons andtwo protons.

(6.11)

Clinical value:

Alpha decay is rarely used in clinical work.Fig. 6.7: Alpha decay

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(15)


Beta Minus Decay

For those unstable nuclei with an excess ofneutrons a negative particle (electron) isproduced by neutron decay forming a proton

(6.12)

Since the mass number does not change theparent and daughter nuclei are isobars, but asthe proton number increases the elementchanges from to .

The emitted photon has an energy of .

Clinical Value:

Very few emitters are used as they cause ahigh patient radiation dose.

Fig. 6.8: Beta minus decay

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(16)


Positive Beta Decay

A positive beta particle (Positron) is producedby proton decay in the nucleus

(6.13)

Since the mass number does not change theparent and daughter nuclei are isobars, but asthe proton number decreases the elementchanges from to .

The positron (anti-matter) undergoes mutualannihilation with a nearby electron thatresults in two opposed photons with each.

Clinical Value:

Positron emission tomography (PET) imagingrelies on the decay which produces opposedphotons ( -radiation).

Fig. 6.9: Beta plus decay

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(17)


Spectrum of Beta Decay

The random nature of the energy loss throughthe neutrino and anti-neutrino makes theenergy spectrum of -decay continuous, Fig6.10.

Fig. 6.10: Kinetic energy spectrum of

beta decay ranges from 0 to maximum

energy (in the range of ) that

depends on the nuclear states of the

participating nuclei

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(18)


Gamma Decay

Gamma radiation in the range of is ideal forimaging, since lower energies undergo tissue absorptionand higher energies are not seen (absorbed) by thedetector material in the gamma camera.

The photon emitted in the previously shown decayschemes came from short lived (pico-seconds) excitednuclear states, others can last for relatively long periods(up to hours) and are called metastable states.

(6.14)

As no other decay process ( is involved they impart a

low radiation dose to the patient.

Clinical Value:

Gamma radiation in the range of is optimalas it has low tissue absorption and can be well detectedby the gamma camera.

Fig. 6.11: Gamma decay

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(19)


Gamma Decay (2)

Molybdenum-99 ( ) has a long half-life and can be easily transported over longdistances to hospitals, where its decay product Technetium-99m ( ) with ashort half-life is → extracted [http://en.wikipedia.org/wiki/Technetium-99m_generator] and usedin a variety of nuclear medicine diagnostic procedures.

Fig. 6.12: Molybdenum decays to a

metastable state that then decays with a pure

gamma photon to a stable state.

Fig. 6.13: Atom model

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(20)


Radiation Properties

Property Alpha Beta Gamma

Type High speed electrons Electromagnetic

Range in air

Range in tissue

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(22)Radionuclide Production

Three methods exist for producing nuclear medicine radionuclides

Bombardment of stable elements with charged beams (cyclotron) → requires a

considerable amount of electrical power and is thus costly

1.

Irradiation of stable elements with neutrons (in a nuclear reactor) → cheaper2.

Generator production3.

Both charged beams and nuclear reactors can provide parent isotopes that decayto give short half-life daughters which may be removed or eluted from time totime.

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(23)


Cyclotron Production

Short explanation of the → Cyclotron Principle[http://en.wikipedia.org/wiki/Cyclotron]

The charged particles are injected near the

centre of the magnetic field

Electron gun similar to the design in an

X-ray cathode tube

Light gases (hydrogen, deuterium or

helium) that were ionised an electron

beam or radio-frequency field

1.

The particles accelerate only when passing

through the gap between the electrodes

2.

The perpendicular magnetic field, combined

with the increasing energy of the particles,

forces the particles onto a spiral path

3.

Charged particles, extracted from the Cyclotron,can be accelerated to sufficiently high energiesso that when they collide with target materialsnuclear reactions are induced.

Fig. 6.14: Design of a clinical cyclotron

Fig. 6.15: An electron beam in a

perpendicular magnetic field ionises a

gas and makes it glow

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(24)


Cyclotron Production (2)

The small cyclotrons available at hospitalscommonly use negative ion acceleration ().

The stripping foil removes the electrons

leaving a proton beam ( ) that hits thetarget material.

Depending on the target material differentradionuclides can be produced.

In particular can be produced whenhitting a target. is a positron emitteroften used in PET imaging.

Since the target and the producedradionuclei are mostly different elementsthey can be separated chemically.

Fig. 6.16: Particle path through a cyclotron

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(25)


Reactor Production

Neutron bombardment of stable elements in a nuclear reactor produceradionuclides by two different reactions

Neutron capture: the nucleus accepts an additional neutron. The

nucleus then has an excess of neutrons and decays as

Fission: the nucleus accepts an additional neutron, becomes unstable

and splits into two smaller nuclei

A complex fission sequence yields for example that can bechemically separated and is a very common generator for (99-Technetium).

Canadian → nuclear reactor shutdown [http://en.wikinews.org

/wiki/Canadian_nuclear_reactor_shutdown_causes_worldwide_medical_isotope_shortage]

(heavy water leak) causes worldwide medical isotope shortage.The reactor produces of the international supply of medicalisotopes.

Fig. 6.17:

Simplified

design of a

small nuclear

reactor used

for

radionuclides

production.

The enriched

fuel

is held in

graphite

blocks which

act as

moderator to

slow down

the neutrons

to allow

neutron

capture. The

carmium

control rods

regulate the

reaction

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(26)


Generator Production

Cyclotrons and nuclear reactors can produce long-lived (days) parent isotopes, e.g.( ) shown in Fig 6.12, that can be easily shipped. As the parent decays, theactivity of the daughter rises ( ).

Fig. 6.18: The parent isotopes slowly decays. From time to time the clinically useful daughter

element can be chemically separated (eluted)

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(27)


Common Radioisotopes

Radioisotope Half-life EnergyPhoton Energy

SPECT

PET

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The Gamma Camera


(29)Gamma Camera

The first → Gamma Camera[http://en.wikipedia.org/wiki/Gamma_camera]

was developed by → Hal Anger[http://en.wikipedia.org/wiki/Hal_Anger] in1957 and is thus often referred to asan Anger Camera.

Gamma radiation is absorbed by

the sodium iodide scintillator

and converted into ultraviolet

photons

The closely attached

photomultiplier tubes amplify

the incident photons

Fig. 6.19: Basic design of the gamma camera

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The Gamma Camera

(30)


Gamma Camera (2)

Mature technology (desiged

by → Hal Anger in 1957

[http://en.wikipedia.org

/wiki/Hal_Anger])

The Photo-Multiplier Tubes

(PMT) are directly coupled

with the continuous NaI(TI)

crystal

Spatial resolution

at FWHM

Energy resolution at

FWHM

Large area is

typical

Simple and cost-effectiveFig. 6.20: Image of a gamma camera (Anger camera)

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The Gamma Camera

(31)


Positional and Pulse Height Analysis

Position Extraction:

When gamma photon ( -photon) hitsthe scintillator crystal a flash of lightwith multiple ultraviolate photons isproduced.

The → Photomultiplier tubes[http://en.wikipedia.org/wiki/Photomultiplier]

(PMT) then amplify these photons.

As the photons are spread overmultiple PMTs their relativestrengths can be used for a moreaccurate estimation of incident

location on the scintillator crystal.

Pulse Height:

The cumulative PMT strength isused to estimate and differentiatethe energy of the incident -photon.

Fig. 6.21:

Fig. 6.22: Photomultiplier design

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The Gamma Camera

(32)


Animated Schematic of the GammaCamera

Fig. 6.23: Animated schematic of the gamma-camera physics and its main constituents.

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Basics of Nuclear Contents Medicine - unibas.chBasics of Nuclear Medicine Principles of Medical Imaging Prof. Dr. PhilippeCattin MIAC, University of Basel Oct 24th, 2016 Principles