UCL CHEM2601 Imaging L3-4 (Nuclear Imaging & Radiochemistry)

Chem 2601/2011

Molecular Imaging Lecture 3 and 4: Introduction to Nuclear imaging and Radiochemistry

Dr. Erik Årstad, KLB room 2.11 ([email protected])

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Overview (lecture 3 and 4):

1) The principles of Nuclear imaging

2) Nuclear imaging techniques

3) Instrumentation

4) Introduction to radioactivity

5) Production of radionuclides

6) Radiochemistry

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Principles of Nuclear Imaging (PET and SPECT)

O

HO

HO

OH

HO

18F

Tracer, e.g. [18F]FDG

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Nuclear Imaging techniques: 1) Positron emission tomography (PET)

2) Single Photon Emission Computed Tomography (SPECT)

3) Autoradiography

- Positron: the antimatter equivalent of an electron

- Positrons are emitted from certain radioactive substances

- Positrons and electrons annihilates to produce two gamma rays

Positron

Electron

Gamma ray (511 KeV)

Gamma ray (511 KeV)

- A chemical is labelled with a radioactive isotope (positron emitter)

- Positrons annihilate in surrounding tissue

- The resulting gamma rays are emitted from the subject

Generates 3D maps of radioactivity concentration - tomographic

Tracer labelled with gamma (single photon) emitting radionuclide

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Autoradiography (imaging in vitro):

Contact exposure of radioactive samples (e.g. 20 μm tissue section on X-ray film) Lower resolution than fluorescence microscopy, but quantitative Requires low energy beta emission

Burton et al. (2009), TOXICOLOGICAL SCIENCES, 111(1): 131–139. http://www.nationaldiagnostics.com

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Instrumentation: 1) Detector principles

2) Principle of PET scanners

3) Principle of SPECT scanners

Detector physics and image analysis: From gamma rays to 3D image

Scintillation detector: Converts gamma rays to light

Photomultiplier tube (PMT): Converts light to electricity and amplifies signal

Outgoing amplified signal

PET camera: how does it work?

detector 1

detector 2

coincidence window

time (ns)

SPECT camera: how does it work?

No coincidence – detectors are fitted with collimators to filter gamma radiation

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Properties of nuclear imaging techniques: (1) Resolution (time and space) +

(2) Sensitivity +++

(3) Selectivity +++

(4) Quantification +++

(5) Tissue penetration +++

(6) Invasiveness +++ = non-invasive

(7) Structural information +

(8) Functional information +++

Nuclear imaging enables non-invasive quantitative imaging of biological processes in vivo

SPECT vs. PET

SPECT PET Resolution 12-15mm 4-7mm Sensitivity

(gamma detection) 0.03% 3.0%

dual radionuclides yes no Radionuclides Gamma emitters

half-life > 6 hours Positron emitters

half-life < 2h Sensitivity

(Target concentration)

10-13 molar 10-14 molar

Cost $$ $$$ (£500-2000/scan)

Radioactivity: 1) Introduction to radioactivity

2) Ionizing radiation

3) Half-life and radioactive decay

4) Specific activity

5) Attenuation

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What is radioactivity?

A brief introduction to Radioactivity:

1895: Roentgen discovered X-rays 1896: Henri Becquerel discovered rays from uranium

1897: Marie Curie named the rays ‘radioactivity’ 1898: Marie and Pierre Curie discovered Polonium and Radium 2012: > 2500 radioactive nuclides are known!

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Definitions: A nuclide (from nucleus) is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z and its number of neutrons N. A radionuclide is any radioactive nuclide. Isotopes are atoms from the same element (i.e. same proton number) but different number of neutrons.

18F- Mass number = N + Z

Proton (Z) number

Charge

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Radioactivity is defined as the process in which unstable atomic nuclei spontaneously emit ionizing radiation

Types of ionizing radiation:

Alpha particles = He nucleus

Beta particles = electron

Positrons = antimatter of electrons

Gamma rays = highly energetic photons

E = mc2

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The units of radioactivity: Historical units: Ci (curie) = 3.7 × 1010 disintegrations per second (= 1 g of 226Ra) mCi = 37 x 106 disintegrations per second

SI units: Bq (Becquerel) = 1 disintegration per second KBq = 1 x 103 Bq MBq = 1 x 106 Bq GBq = 1 x 109 Bq Conversion factor: 1 mCi = 37 MBq

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Ionizing radiation and energy The energy of ionizing radiation is measured in electron volts (eV) Units: KeV = 1000 eV or MeV = 1000,000 eV For particles it is the kinetic energy (typically 100 KeV to 1 MeV) For gamma rays it is the energy of the photon NB: Gamma energies for SPECT imaging ~ 100-300 KeV For PET the gamma rays are always 511 KeV (the combined mass of an electron and a positron = 1.022 MeV)

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At = A0 x e − λt

t1/2 = ln 2 / λ (ln = natural logarithm, ln 2 = 0.693)

Where n equals number of whole half-lives: At = A0(1/2)n

Relationship between half-life, time and radioactivity

The activity of a radioactive sample at any time is:

The half-life (t1/2) of a radionuclide is determined by its decay constant lambda (λ):

Where A0 is the activity at time zero and e = natural constant (2.718)

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Half-life and radioactive decay over time

Question: Carbon-11 has a half-life of 20 min. The synthesis of a tracer takes 40 min and it takes another 20 min to analyse the product before injection to a subject. The radiochemical yield is 20%. How much of the initial activity is available for injection?

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Specific activity: Activity / Mass = Bq / µmol

Direct correlation between half-life and maximum specific activity: t1/2 = ln 2 / λ λ is the probability of radioactive decay:

Low λ = long t1/2 High λ = short t1/2 The shorter the half-life the higher the maximum specific activity

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Samples exclusively made up of molecules containing the radioactive nuclide are carrier-free (c.f.). Samples without addition of non-radioactive carrier but containing naturally occurring isotopic dilutions are non-carrier-added (n.c.a.). Samples diluted with non-labelled molecules are carrier-added (c.a.).

Specific activity – important terms:

Radiation properties: Attenuation

Range of a couple of cm in air – stopped by a sheet of paper

Range of mm to cm in tissue – stopped by a sheet of aluminium

long range – requires thick lead for shielding

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Attenuation: effect of matter

http://en.wikibooks.org/wiki/Basic_Physics_of_Nuclear_Medicine/Attenuation_of_Gamma-Rays

Gamma ray intensity I0 Gamma ray intensity Ix

0 X

Δ I = I0 – Ix, where Δ I is proportional to Z3 Doubling the atomic number leads to 8 fold increase in attenuation!

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Attenuation: effect of matter AND energy

http://en.wikibooks.org/wiki/Basic_Physics_of_Nuclear_Medicine/Attenuation_of_Gamma-Rays

Half value (in cm) for gamma rays:

Biological tissues: “Different stopping power of radiation” e.g. lungs vs. bones

Attenuation of positrons in tissue:

NB: this defines the maximum theoretical resolution of PET!

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Nuclear imaging is quantitative, because: Radioactive decay is determined by the half-life Radioactive decay is unaffected by the environment The interactions of ionizing radiation with matter follows clear physical rules and can be accounted for

Question: What would happen if a subject is injected with a PET tracer but scanned with SPECT camera? What would happen if a subject is injected with a SPECT tracer but scanned with PET camera?

Radiochemistry: 1) Production of radionuclides

2) Labelling with 11C

3) Labelling with 18F

4) Labelling with 123I

5) Labelling with 3H

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Radiochemistry and production of radionuclides Examples for PET: 11C (t1/2 20.4 min) and 18F (t1/2 110 min) Example for SPECT: 123I (t1/2 13.1 h) Example for autoradiography: 3H (t1/2 12 years)

Reaction: Product: half-life: Decay mode:

16O (p,α) 13N 10 min β+ (positron)

14N (p,α) 11C 20 min β+ (positron) 14N (d,n) 15O 2 min β+ (positron) 18O (p,n) 18F 110 min β+ (positron) 124Te(p,2n) 123I 13.1 h γ (gamma) 99mTc 6.01 h γ (gamma) 68Ga 68 min β+ (positron)

82Rb 1.26 min β+ (positron)

Generator based radionuclides:

Production of radionuclides with a cyclotron

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Production of radionuclides: formation of 3H

6Li + n 4He + 3H (half-life 12 years, beta emitter)

Nuclear reactor provides neutron flux:

All radionuclides for biomedical research are either produced by a cyclotron, or in nuclear reactors (directly or indirectly).

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Radiochemistry – general principles: Fast reactions

High yields

Reliable and reproducible reactions

Few side products

Simple purification Introduce the radionuclide as late in the synthesis

as possible!

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Radiochemistry – important definitions: Radiochemical yield (r.c.y): the efficiency of a labelling reaction measured as the proportion of radioactivity that has been transferred from a reagent to a product. Radiochemical yield can be decay corrected or non-decay corrected: Decay-corrected: the amount of activity in the product is corrected for the decay that has occured during the synthesis before calculation of radiochemical yield Non-decay-corrected: there is no correction for decay, so the radiochemical yield is simply the amount of radioactivity in the product divided on the amount of radioactivity in the reagent.

Radiochemical yield - example: Carbon-11 has a half-life of 20 min. You start the synthesis with 2 GBq of 11CO2. After 40 min you obtain 200 MBq of a tracer. The non-decay corrected radiochemical yield is: 0.2 GBq/2 GBq = 10% The decay-corrected radiochemical yield is: Decay correction: 0.2 GBq/(0.5 x 0.5) = 0.8 GBq 0.8 GBq/2 GBq = 40%

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Radiochemistry: labelling with carbon-11 Advantages: Enables isotopic labelling, i.e. replacement of 12C with 11C in the molecule. The biological fate of the molecule is unchanged

Very versatile labelling chemistry – it’s carbon! Disadvantages: Short half-life (20.4 min) Only available on sites with a in-house cyclotron Alternative for biochemical applications: 14C (half-life 5730 years, beta emitter)

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Radiochemistry: Labelling with carbon-11

14N (p,α) 11C

Cyclotron:

H2 11C H4

O2 11C O2

I2 11C H3I

ROH (alcohols) 11C H3OR

RNH2 (amines) 11C H3NRH

ArOH (phenols) 11C H3OAr

Sn2 reactions:

RSH (thiols) 11C H3SR

RCONH2 (amides) RC(O)NH11C H3

RMgX , SOCl2 (Grignard reagents)

R11C (O)Cl

R’NH2 (amines)

R11C (O)NHR’

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Radiochemistry: Labelling with carbon-11 [11C ]Methionine: [11C ]Methionine

[11C ]Way 100635:

11C H3I

Base

11C O2

H2N C

HS

O

OHH2N C

H311CS

O

OH

MgBr CO

OMgBrCO

Cl SOCl2

ON

NN

C

N

O

H3C

RNHR’

[11C ]WAY 100635

Typical specific activities: 40-200 GBq/µmol

Sources of 12C: atmospheric CO2

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Radiochemistry: labelling with fluorine-18 Advantages: Near ideal half-life (110 min) and low positron energy

Small size makes it suitable for labelling of small molecules Fluoride can reduce metabolism of tracers Tracers can be transported for imaging at centres without a cyclotron Disadvantages: Limited reactivity, need to protect OH and NH groups!

Limited chemistry – fluoride is the most electronegative of all elements

18F – Nucleophilic aliphatic substitutions

18F-

Leaving group (triflate)

Acetate (protecting group)

OAcO

AcO OAc

OAc

18F

OAcO

AcO

O

OAc

OAcSO2CF3

OHO

HO

OH

18FOH

HCl (aq)

[18F]FDG:

[18F]FDG 45

[18F]Fluoride – Nucleophilic aliphatic substitutions

N

N

O

OO

DMTrO

18F

DMBn

MeCN, K2CO3, Kryptofix 100 ºC, 10 min,

Deprotection

38% RCY

N

NH

O

OO

HO

18F

[18F]FLT 13% RCY

Grierson and Shields, Nuclear Medicine & Biology 2000, Vol. 27; 143–156

Protecting group

Protecting group

18F-

Leaving group = nosylate O2N

SO

OO

R

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[18F]Fluoride – Nucleophilic aliphatic substitutions

[18F]Fallypride

TsOHN

OMe

OMe

O

N

18F-

Leaving group: tosylate (TsO)

H3C

SO

OO

R

18FHN

OMe

OMe

O

N

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[18F]Fluoride – fluoroalkylation

18F-

Leaving group: tosylate (TsO)

18FOTs

[18F]Fluoroethyltosylate - reacts similarly to 11C H3I

HONH2

O-

O

ONH2

O-

O

18F

TsOOTs

Base (deprotonates the phenol)

[18F]FET (fluoroethyltyrosine)

H3C

SO

OO

R

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Radiochemistry: labelling with iodine (123,124,125,131I) Advantages: Easy labelling chemistry Many isotopes available (autoradiography, SPECT and PET)

Range of half-lives from 13 h to 60 days

Disadvantages: Limited metabolic stability

Large size (similar to benzene!)

Limited labelling chemistry

Direct labelling of peptides/proteins with iodine:

123I- “ 123I+ “ Oxidant

HO

O

NHR

HN R

OTyrosine residue

HO

O

NHR

HN R

O

123I

Advantages: High yields Simple chemistry Disadvantages: Low metabolic stability No control of labelling positions (in large proteins)

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Labelling of trialkyl tin compounds with iodine (aryl and alkene):

Advantages: Site specific labelling Good yields Disadvantages: Only suitable for small molecules (or indirect labelling)

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123I

OH

OMe O

HN

N

SnMe3

OH

OMe O

HN

N

123I-, H2O2

Dilute acid

[123I]IBZM

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Radiochemistry: labelling with tritium (3H or T) Advantages: Can label almost any organic compound

Ideal for isotopic labelling – replacing H with tritium

Widely used basic research Disadvantages: Only possible in specialised labs (custom service)

Only suitable for in vitro applications

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N

N

T2, Pd/C

N

N

T

T

Labelling with 3H (also known as tritium - T):

Reduction of alkenes, alkynes and other saturated bonds with 3H2 gas:

Maximum specific activity for labelling with 3H ~ 2-3 GBq/µmol

Alkylation with C3H3I – equivalent to 11C H3I and suitable for the same reactions!

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Exercise 3: How would you label this molecule?

Nuclear imaging – strengths and weaknesses: + Excellent sensitivity (picomolar range ) + Deep tissue penetration + Allows absolute quantification - Limited resolution (time and space)

- Expensive

- Involves ionizing radiation

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Synopsis: PET and SPECT principles

PET - Positron emitters - short half-life (11C, 18F) < 2 h - Detects annihilation gamma rays - Coincidence detection for location - Higher resolution - Better for quantification - More expensive

SPECT - Gamma emitters - Longer half-life (123I) > 6 h - Detects gammas emitted directly - Collimators for location - Can use multiple radionuclides - Lower resolution - Lower cost

Synopsis: autoradiography Detects beta particles by X-ray film. Suitable radionuclides have long half-lives – 2 weeks to many years! The best results are achieved with low energy beta emitters, as energetic particles are not fully stopped by the imaging medium.

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Synopsis: Radioactivity

Radioactive decay:

-There main modes of decay are: alpha, beta, positron and gamma emission

- Radioactivity is measured in Bq = 1 disintegration per second

- Activity is defined by the half-life and the number of a radionuclide

- Specific Activity is the activity per mass (in Bq / µmol)

Interactions of ionizing radiation with matter:

-Each type of radiation interacts with matter in a unique way

- Particles are rapidly stopped by matter and travel only short distances

- Attenuation of gamma rays depends strongly on the gamma energy and atomic number of the absorbing matter

NB: Radioactivity can readily be quantified as the activity level is unaffected by the environment and the interactions of ionizing radiation with matter can be accounted for.

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Synopsis: Radiochemistry

Production of radionuclides:

Most PET radionuclides are produced with particle bombardment in a cyclotron

Some radionuclides are produced in high flux nuclear reactors

Radiochemistry:

Reactions should be fast, efficient and reliable

Radionuclides typically introduced late in the synthesis

Chemistry of carbon-11, fluorine-18, iodine-123 and tritium

Carbon-11 converted to CH3I or CO2

Carbon-11 labelling typically with methylation or Grignard reactions

Fluorine-18 typically introduced by Sn2 nucleophilic reactions

Iodine-123 introduced by addition to tyrosine or reaction with trialkytin groups

Tritium typically introduced by reduction of unsaturated bonds

Learning outcomes - you should understand: - The principles of radioactivity - The decay modes - The interaction of radiation with matter - The principles for imaging with PET, SPECT and autoradiography - How the decay mode, half-life and energy range of the radiation effect the suitability for imaging -The advantages and disadvantages of nuclear imaging

- The chemistry of common radionuclides for PET, SPECT and autoradiography

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Assessment – you should be able to apply your knowledge of radioactivity and nuclear imaging to explain underlying principles, solve practical problems and provide rationale explanations related to: - The principles of radioactivity - The decay modes - The interaction of radiation with matter - The principles for imaging with PET, SPECT and autoradiography - How the decay mode, half-life and energy range of the radiation effect the suitability for imaging -The advantages and disadvantages of nuclear imaging

- Labelling reactions with common radionuclides for PET, SPECT and autoradiography

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Useful websites and background reading: Radioactivity: http://en.wikipedia.org/wiki/Radioactive_decay Interaction of gamma rays with matter: http://en.wikibooks.org/wiki/Basic_Physics_of_Nuclear_Medicine/Attenuation_of_Gamma-Rays Sensitivity of nuclear imaging: John V. Fragiono, Journal of Clinical Oncology, 26(24): 4012-4021 Autoradiography: http://www.nationaldiagnostics.com Burton et al. (2009), TOXICOLOGICAL SCIENCES, 111(1): 131–139. Langstrom et al. (2007), Mol Imaging Biol, 9(4): 161-175. Charon et al. (1998), Nuclear Medicine & Biology, 25:699–704.

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