Chapter 15

Radioactivity and Nuclear Transformation

Activity

• Number of radioactive atoms undergoing transformation per unit time

A = - dN/dt = N

• 1 Curie(Ci) = 3.71010 disintegration/sec

• 1 Becquerel(Bq) = 1 disintegration/sec

– SI unit

• Diagnostic range: 0.1 ~ 30mCi

• Therapeutic range: 300mCi

Decay Constant & Half Life

• Radioactive decay is random process

– Decay constant :

– A = - dN/dt = N N=N0e

-t N=N02

-t/Tp1/2

• Physical half life

– Tp1/2=0.693/

– Fraction of second ~ billions of year

– Hours or days for medical purpose

Radionuclide Tp1/2

Fluorine 18(18F) 110 min

Technetium99m(99mTc) 6.02 hr

Iodine123(123I) 13.27 hr

Indium(111In) 2.81 d

Thallium 201(201Tl) 3.04 d

Iodine131(131I) 8.02 d

Phosphorus 32(32P) 14.26 d

Iodine125(125I) 59.41 d

Cobalt 57(57Co) 271.79 d

Decay Equation

• Nt=N0e-t A=- dNt/dt =N0e

-t=A0e-t

– Activity also decay exponentially

Ex) Remain activity of 500uCi 111In after 2 days

– Half life = 2.81 days =0.693/2.81=0.246 day-1

– A=A0e-t=500e-(0.246)(2) =306(uCi)

Nuclear Transformation

• Radioactive Decay

– Radiation emission during spontaneous decay

– Until to reach stable nuclide

• Parent nuclide daughter nuclide

• Decay types

1. Alpha decay

2. Beta-minus emission

3. Beta-plus(positron) emission

4. Electron capture

5. Iosmeric transition

same molecular formula but different structural formulas

Alpha Decay

• ZAX Z-2

A-4Y-2 + 24He+2 + transition energy

– Increase in N/Z ratio

Ex) 86220Rn 84

216Po + 24He+2 + 6.4MeV

• Mostly with heavy nuclides (A>150)

– Followed by gamma emission and X-ray emission

• Heaviest and least penetrating

– 1cm/MeV in air, 100um in tissue

– Not penetrate dead layer of skin

– Not used in medical imaging

134/86=1.56 132/84=1.57

• ZAX Z+1

AY+ + - + - + transition energy

– - : identical to electron, discrete maximal energy

– - : antineutrino, neutral subatomic particle smaller

mass than electron, rarely interact with mater

– Increase in Z, no change in A (isobaric transitions)

Ex) 1532P 16

32S + -

• Radionuclide having

excess number of neutrons – High N/Z ratio

Beta-Minus(Negatron) Decay

17/15=1.13 16/16=1.00

Beta-Plus Decay

• ZAX Z-1

AY- + + + + transition energy

– + : positron, poly energetic spectrum,

• annihilation with electron, 180°-2 ray of 0.511MeV

– : neutrino, antiparticle with antineutrino

– Decrease Z by 1, no change in A (isobaric transitions)

Ex) 918F 8

18O + +

• Neutron poor radionuclide

– Low N/Z ratio

– Accelerator produced radionuclide

9/9=1.00 10/8=1.25

Electron Capture Decay

• Alternative to beta plus decay – For neutron poor radionuclide

– Energy difference btw nuclides lower than threshold to produce positron

• ZAX + e- Z-1

AY- + transition energy

– Capture orbital electron of K or L shell

– Followed by characteristic radiation from outer shell

• Using for imaging

– Decrease Z by 1, no change in A (isobaric transitions)

Ex) 81201Tl + e- 80

201Hg + energy 120/81=1.48 121/80=1.51

Thallium Mercury

Isomeric Transition

• Not directly into stable state

– Through excited state: meta state, isomeric state

– Half life: 10-12 sec ~600 years

• Metastate stable state + gamma ray

• ZAmX Z

AX + transition energy

– No change in atomic(proton) number

– No change in neutron number

– No change in mass number

Ex) Tc-99m

Decay Schemes

• Decay: unique characteristic of the nuclide

• Parent/daughter nuclide, mode of decay, energy level, radiation emission

Decay Scheme of Radon-220

• Alpha decay

• Tp1/2=55sec

• 86220Rn 84

216Po

• 1(0.07%)

– Followed by isomeric decay

• 2(99.93%)

– Directly to ground state

• Decay data table

Decay Scheme of 32P

• - decay

• Tp1/2=14.3 days

• Into stable 32S

• Emax=1.71MeV

• Pure beta emitter

• Using as therapeutic agent

Decay Scheme of 99Mo

• - decay

– Followed by isomeric decay

• Tp1/2=2.75 days

• Into stable 99Tc

– Intermediate 99mTc

• Decay data table

Decay Scheme of 99mTc

• Isomeric transition

• Tp1/2=6.02 hrs

• 140.5keV, 142.7keV

• Detection for imaging

Decay Scheme of 18F

• + decay(97%) +electron capture(3%)

• Followed by annihilation: 2 gamma rays

• Most widely using in PET

Chapter 16

Radionuclide Production, Radiopharmaceuticals, and Internal Dosimetry

Artificial Radioactivity

• Most of commonly used in medicne

• 1st artificial radioactivity

– 1934 by Irene Curie and Frederic Joliot

– -from Polonium Born & Aluminum

• Continuous positron emission

• Currently 2500 artificial radionuclides

– Radionuclides in medicine

• By particle acceleration(cyclotron),

• By nuclear reactor

• By radionuclide generator

Cyclotron

• Bombard stable target – Require high energy to

overcome repulsive force

of nucleus

• Accelerate charged particle –H+, 2H+, 4He2+, H-

–Few ~ several hundred MeV

Medical Cyclotron

A. Radiation shields

B. Power supply & control cabinet

C. Cyclotron assembly(10t)

E. Cyclotron assembly open

D. Retractable radiation shielding(14t)

G. Beam shaping magnets

F. One of four cyclotron dees

• Small, lower power(10~30Mev)

• Commonly accelerate H-

Nuclear Reaction

• Gallium-67 production

– Bombarding 20MeV proton into zinc-68

– Gallium-67 with emitting 2 neutrons

3068Zn (p,2n) 31

67Ga

• Iodine-123 production

53127I (p,5n) 54

123Xe 53123I (T1/2 =13.27 hr)

54

124Xe (p,2n) 55123Cs 54

123Xe 53123I

• Indium-111 production 47

109Ag (,2n) 49111In , 48

111Cd (p,n) 49111In (T1/2 =2.81d)

48112Cd (p,2n) 49

111In

EC

T1/2 2hr

EC or +

T1/2 ~1sec

EC

T1/2 2hr

Nuclear Reaction

• Cobalt-57 production: 2656Fe (d,n) 27

57Co (T1/2 =271.8d)

• Thallium-201 production

81

203Tl (p,3n) 82201Pb 81

201Tl (T1/2 =3.04d)

• Using hospital based cyclotron

– Short half life

– Fluorene-18: 818O (p,n) 9

18F (T1/2=110min)

– Nitrogen-13: 816O (p,) 7

13N (T1/2=10min)

– Oxygen-15: 714N (d,n)8

15O, 715N(p,n)8

15O (T1/2=2min)

– Carbon-11: 714N (p,) 6

11C (T1/2=20.4min)

EC or +

T1/2 ~ 9.4 hr

Nuclear Reactor

• Using neutron: uncharged

– Can penetrate nucleus without high energy

– 2 methods: Nuclear fission, neutron activation

• Nuclear fission

– Splitting of atom into two smaller nuclei

– Need energy to overcome nuclear binding energy

• By absorption of neutrons

– U-235: most widely using

U-235

• Wide range of fragment nuclide

– 200 radionuclide

– Btw Z=70 & Z=160

• Neutron rich products

– - decay

• Medical use

– Mo-99, I-131, Xe-133

– Carrier-free radioisotopes

One example fission

Nuclear Chain Reaction

• Continue fission

– By moderating neutron speed

– Absoring extra neutrons

• By control rod

• Heat for power generation

Neutron Activation

• Using neutron produced by fission

• Bombarding stable target material

• Capture neutron and emit ray – (n, ) reaction & mostly decay with - decay

– Phosphorus-32: 31P (n, ) 32P (T1/2=14.3 days)

– Chromium-51: 50Cr (n, ) 51Cr (T1/2=27.8 days)

• Difficult to separate chemically from target isotope – not carrier-freeLimit in concentration

Radionuclide Generator

• Holding parent nuclide that daughter can easily be separated

– Tc-99m: widely using in medicine (T1/2=6 hrs)

• Impractical to store weekly

– Store parent nuclide Mo-99 (T1/2=67 hrs)

• Chemically separate Tc-99m

– Different chemical characteristics

– Elution by eluant

Transient Equilibrium

• Daughter build up, parent decay

• Transient equilibrium: 23 hours

– Production rate=decay rate

– Separate every morning (~23hr)

• 85% elution yield

– Weekly store

Comparison

Characteristic Cyclotron Fission Neutron Activation

Generator

Particle p, d, n n By decay

Product Neutron poor Neutron excess

Neutron excess Neutron poor/ excess

Decay mode Positron emission, electron capture

Beta-minus Beta-minus Several modes

Carrier free? yes yes no yes

High specific activity

yes yes no yes

Relative cost high low low low

Nuclides 201Tl, 123I, 63Ga, 18F, 15O, 57Co,

99Mo, 131I, 133Xe

32P, 51Cr, 125I, 111In, 89Sr, 153Sm

99mTc, 82Rb, 68Ga

Radiopharmaceuticals

• Radionuclide + Pharmaceuticals Radiopharmaceuticals – By injecting into freeze-dried pharmaceuticals

• Nuclear medicine imaging – Tc-99m: most widely in nuclear medicine

– 123I, 67Ga, 111In, 133Xe,201Tl

• Clinical PET imaging – Positron emission pharmaceuticals

– 18F as fluorodeoxyglucose(FDG): 85%

– 11C, 13N, 15O, 68Ga 82Rb,

Ideal Radiopharmaceuticals

• Low radiation dose

– Few particulate emission, abundance of clinically useful photons(~140KeV)

• Compromise among patient attenuation, spatial resolution, detection efficiency

– Effective half-life

• Long enough for imaging, short enough to minimize dose

• High target/Non-target activity

– Clinically useful uptake and clearance: hot/cold spot

• Safety, Convenience, Cost-Effective

– Carrier free, shelf life….

Mechanism of Localization

• Compartmental Localization

– Into anatomical compartment for abnormal opening finding

• Xe-133 gas inhalation into lung

• Tc-99m labeled RBC into circulatory system

• Cell Sequestration

– RBC withdraw label with Tc-99m and damage

reinject

– Measure spleen’s ability to remove damaged RBC

격리

Mechanism of Localization

• Passive Diffusion – Disruption of Blood-Brain Barrier by trauma, neoplasm

• Normally block radiopharmaceuticals

• Metabolism – FDG: glucose analogue & follow glucose metabolism

• Active transport – Transport using energy against gradient

• Iodine in thyroid gland, Thallium in muscle

• Capillary blockage – Tc-99m-MAA block to assess pulmonary perfusion

Internal Dosimetry

• Radiopharmaceutical disometry

– By Medical Internal Radiation Dosimetry(MIRD)

– Radiation Dose Assessment Resource(RADAR)

– Based on Dosimetry model and eqautions