Joint CI-JAI advanced accelerator lecture series Imaging and detectors for medical physics
Lecture 4: Radionuclides
Dr Barbara Camanzi [email protected]
Course layout
Day AM 09.30 – 11.00 PM 15.30 – 17.00 Week 1 6th June Lecture 1: Introduction to Lecture 2: Detectors for medical imaging medical imaging 7th June Lecture 3: X-ray imaging 8th June Tutorial Week 2 13th June Lecture 4: Radionuclides 14th June Lecture 5: Gamma Lecture 6: SPECT cameras 16th June Lecture 7: PET Week 3 22nd June Tutorial
Page 2/55 Books & references
1. N Barrie Smith & A Webb Introduction to Medical Imaging Cambridge University Press 2. Edited by M A Flower Webb’s Physics of Medical Imaging CRC Press 3. A Del Guerra Ionizing Radiation Detectors for Medical Imaging World Scientific 4. W R Leo Techniques for Nuclear and Particle Physics Experiments Springer-Verlag Nuclides live charts − http://www.nndc.bnl.gov/nudat2/ − https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html
Page 3/55 Nuclear medicine imaging
• Imaging of radioactive decay products of a radiopharmaceutical (radiotracer) introduced into the body → emission imaging (as opposed to X-ray imaging = transmission imaging) • Spatial distribution depends on how radiopharmaceutical interacts with tissues in the body • Administration of radiopharmaceutical: 1. Intravenous injection into bloodstream 2. Inhalation into lungs 3. Subcutaneous administration 4. Oral administration Page 4/55 Nuclear medicine imaging techniques • SPECT = Single Photon Emission CT = Single Photon Emission Computed Tomography • PET = Positron Emission Tomography • Planar scintigraphy
Page 5/55 Nuclide notation
Nucleus Notation • Formed of nucleons Element X
neutron A
Z X • Nucleons: A = mass number = number of – proton = particle with positive protons + neutrons charge Z = atomic number = number – neutron = particle with zero of protons charge
Isotopes of an element = nuclides with same number of protons (same 푍) but different number of neutrons (different 퐴) Page 6/55 Forces within the nucleus
coulomb
strong neutron proton strong
strong • In stable nuclei forces are well balanced • In unstable nuclei there are too many neutrons or protons → forces are not balanced → nucleus is prone to undergo nuclear rearrangement and decay • Line of stability – For low 푍: 푁 ≈ 푍 – For high 푍: 푁 ≈ 1.5 × 푍 – No stable nuclei for Z > 82 (Lead)
Page 7/55 Radioactivity
• Intrinsic property of unstable nuclei that have too many neutrons or protons → unstable nuclei emit particles or 훾-rays to become more stable • Definitions: 1. Radionuclide = nuclide that is unstable and undergoes radioactive decay 2. Radioisotope = radioactive isotope 3. Radioactive disintegration or decay = spontaneous change in nucleus composition with associated emission of energy to reach a more stable state 4. Radiotracer = radiopharmaceutical
Page 8/55
Radioactive decay law
• Number of radioactive atoms in a sample decreases with time: 푑푁 = −휆푡 푑푡 • 푁 푡 = number of atoms left at given time 푡 decreases exponentially: 푁 푡 = 푁0exp −휆푡 푁0 = number of atoms at 푡 = 0 휆 s−1 = decay constant exp −휆푡 = decay factor
Page 9/55 Decay constant
• Probability that any individual radioactive atom will undergo decay per unit time • Statistical definition → average rate of decay • Exercise: Q: If 휆 = 0.01 s−1 on average how many atoms undergo radioactive decay per unit time?
Page 10/55 (Radio)activity 푸
Ref. 1 – Chapter 3.2 and Ref. 2 – Chapter 5.4.1 • (Radio)activity 푄 = number of disintegrations per s = rate of change of number 푁 of radioactive nuclei 푑푁 푄 = − = 휆푁 푑푡 • Units for 푄: 1. SI unit = Bequerel (Bq) 1 Bq = 1 disintegration per second 2. Curies (Ci) = named after Pierre Curie and defined as number of disintegrations per second from 1 gramme of 226푅푎 1 Ci = 3.7 × 1010 disintegrations per second = 3.7 × 1010 Bq
Page 11/55 (Radio)activity 푸 decay law
• (Radio)activity 푄 decreases with time too • Exercise: Q: Determine the (radio)activity 푄 decay law
Page 12/55 Half-life
• Half-life 휏1/2 = time required for 푄 to drop to half (50%) of its initial value → 휏1/2 is independent of 푁
• Exercise: Calculate relation between 휏1/2 and 휆 and express 푄 as function of 휏1/2
Page 13/55 Atomic half-lives
푍 (protons) 푍 = 푁 Half-life (seconds) > 1e+15 1e-01 1e+10 1e-02 1e+07 1e-03 1e+05 1e-04 1e+04 1e-05 1e+03 1e-06 1e+02 1e-07 1e+01 1e-15 1e+00 < 1e-15 unknown
Courtesy Piero Posocco (Imperial College) 푁 (neutrons) Page 14/55 Biological and effective half-life
• In many cases excretion of radiotracer from tissue follows an exponential decay law → biological half-life 휏1/2,푏𝑖표 used to characterise the decay → 휏1/2,푏𝑖표 gives a measure of how long radiotracer remains in the body
• Effective half-life 휏1/2,푒푓푓 given by: 휏1/2 ∙ 휏1/2,푏𝑖표 휏1/2,푒푓푓 = 휏1/2 + 휏1/2,푏𝑖표
→ 휏1/2,푒푓푓 always less than the shorter between 휏1/2 and 휏1/2,푏𝑖표
Page 15/55 Exercise
• Q: Two patients undergo nuclear medicine scans. One receives a dose of radiotracer A with 휏1/2 = 6 h and the other a dose of radiotracer B with 휏1/2 = 24 h. If dose of radiotracer A is 3 × dose of radiotracer B and 휏1/2,푏𝑖표 of A is 6 h and of B 12 h, at what time the radioactivity in the body of the two patients is the same?
Page 16/55 Radioactive decay modes
Ref. 2 – Chapter 5.4.3 • 훼+2 decay • 훽− decay • 훽+ decay • Electron Capture (EC) • Isomeric transitions – Radiative 훼+2, 훽− and 훽+ decays – Radiative EC • Internal conversion (IC)
Page 17/55 휶+ퟐ decay
• High A radionuclide emits 훼-particle = helium nucleus = +2 charge • Most energy distributed between: 1. Daughter nucleus = recoil energy 2. 훼-particle = kinetic energy = 4÷8 MeV → travels few mm in tissue • If nucleus left in excited state → de-excitation is through emission of g-rays • Not use in medical imaging (shallow penetration in tissue) but as sealed X- or g-rays sources in therapy
Page 18/55 휷− decay
• Neutron-rich radionuclide ejects Example 훽− particle = e− = −1 charge in 훽− 14 decay 14 − the process: 6퐶 7푁 + 훽 + 휈 + 퐸 푛 → 푝 + e− + n 퐸 = shared randomly between n and • Three-body decay → energy kinetic energy of 훽− spectrum of e− = continuum up to a maximum 14 퐶 − • 푍 → 푍 + 1, 퐴 and atomic weight 6 훽 decay remain the same • e− penetration in tissue < 2 mm 14 → not used in medical imaging 7푁
Page 19/55 Radiative 휷− decay (휷−, 휸)
• If following 훽− decay daughter nuclide Example ∗ is produced in excited state 푋 → 훽− 133 133 ∗ γ 133 prompt de-excitation to more stable 54푋푒 55퐶푠 55퐶푠 state through emission of g rays 133 g decays • 푍 → 푍 + 1, 퐴 and atomic weight 54푋푒 remain the same 133 54푋푒 • Typical energy of g rays = 50÷500 keV 0.38 MeV → useful for imaging • Disadvantage: patient still exposed to 0.16 MeV − 훽− decays 훽 particle → dose 0.08 MeV 0 MeV 133 55퐶푠
Page 20/55 휷+ decay
• Proton-rich or neutron deficient Example radionuclide ejects 훽+-particle = e+ = 훽+ 15 decay 15 + +1 charge in the process: 6푂 7푁 + 훽 + 휈 + 퐸 푝 → 푛 + e+ + 휈 퐸 = shared randomly between 푣 and • Three-body decay → energy spectrum kinetic energy of 훽+ + of e = continuum up to a maximum 푚푎푥 Average kinetic energy 퐸 + ≅ 퐸 + /3 • 푍 → 푍 − 1, 퐴 and atomic weight 훽 훽 15 remain the same 8푂 + 1.022 MeV • e travels ~1 mm in tissue → comes + to rest → combines with atomic e− → 훽 decay
2 back-to-back 511 keV g-rays 푚푎푥 퐸 + = 1.7 MeV • If daughter nuclide is produced in 훽 15 excited state → de-excitation is 7푁 through emission of g-rays
Page 21/55
Electron Capture (EC) and radiative Electron Capture (EC, 휸)
• In proton-rich radionuclide inner Example − orbital (K-shell) e = closer to 퐸퐶 125 − 125 ∗ γ 125 nucleus, captured within nucleus: 53퐼 + 푒 52푇푒 + 휈 + 퐸 52푇푒 푝 + e− → 푛 + 휈 + 퐸 125 • 푍 → 푍 − 1 53퐼 • e− from outer orbital fills vacancy → emission of X-ray characteristic EC of daughter nuclide = can be useful for imaging if high enough 퐸 0.035 MeV 125푇푒∗ • The higher 푍 the closer to the 52 훾 nucleus are the e− shells → probability of EC increases with 푍 125 52푇푒 • If daughter nuclide is produced in excited state 푋∗ → de-excitation is through emission of g-rays Page 22/55
Feynman diagrams for 휷−, 휷+ decays and EC
훽− 퐴 퐴 − 푍푋 푍+1푌 + e + 푣푒
훽+ 퐴 퐴 + 푍푋 푍−1푌 + e + 푣푒
퐸퐶 퐴 − 퐴 푍푋 + e 푍−1푌 + 푣푒
Page 23/55 Courtesy Piero Posocco (Imperial College) 휷 emitters
(휷−, 휸) emitters1
Nuclide Half-life 퐸훽 (MeV) 퐸훾 (keV) 60퐶표 5.27 yrs 0.096 1173, 1332 131퐼 8.04 days 0.192 364 133푋푒 5.24 days 0.101 81 137퐶푠 30.00 yrs 0.173 662 휷+ emitters 푚푎푥 + Nuclide Half-life (min) 퐸훽+ (MeV) 훽 푟푎푛푔푒 in water (cm) 11퐶 20.30 0.961 0.103 13푁 10.00 1.190 0.132 15푂 2.07 1.720 0.201 18퐹 110.00 0.635 0.064 1Only dominant 훽− and 훾 emissions shown Page 24/55 EC and (EC, 휸) radionuclides
EC radionuclides
Nuclide Half-life 푬푿−풓풂풚 (keV) 125퐼 60.1 days ~30 201푇푙 3.04 days ~70 (EC, 휸) radionuclides
Nuclide Half-life 푬휸 (keV) 57퐶표 270 days 122, 136 67퐺푎 78.3 h 93, 185 111퐼푛 2.83 days 171, 245 123퐼 13.2 h 159
Page 25/55 Isomeric transitions (IT) and metastable states
• Excited state in which daughter Example nuclide can be produced called 99 푚 isomeric state • 푇푐 most common example of metastable isotope used in nuclear • Sometimes radiative decays from medicine isomeric state to ground state are called isomeric transition • Decay chain:
• Isomeric transitions can take from 훽− γ 99푀표 99푇푐푚 99푇푐 fractions of seconds (short-lived states) to many years (long-lived Half-life for 훽− decay = 66 h states) Half-life for isomeric transition = 6 h • Long-lived isomeric states are 퐴 푚 called metastable states 푍푋
Page 26/55 Internal Conversion (IC)
• 훾-ray emitted in isomeric transition interacts with atomic e− → e− is ejected = conversion electron • Interaction is usually with K-shell e− as they are closest to nucleus • Conversion e− has kinetic energy 퐸: 퐸 = 퐸훾 − 퐸푏𝑖푛푑𝑖푛푔 • e− from outer shell fills vacancy → characteristic X-ray emitted • X-ray emitted can interact with other outer shell e− − − → e get ejected if 퐸푋−푟푎푦 > 퐸푏𝑖푛푑𝑖푛푔 = Auger e
Page 27/55
Radioactive decay table
푍 (protons)
Stable EC, 훽+ 훽− 훼 P N Unknown
Courtesy Piero Posocco (Imperial College)
푁 (neutrons) Page 28/55 Production of radionuclides
Ref. 2 – Chapter 5.4.2 • Man-made production: 1. Neutron capture = neutron activation 2. Nuclear fission 3. Charged-particle bombardment 4. Radioactive decay
• Naturally-occurring radionuclides
Page 29/55 Man-made production technologies • Nuclear reactors: 1. Neutron capture = nuclear absorption 2. Nuclear fission • Accelerators: 1. Charged-particle bombardment • Radionuclide generators: 1. Radioactive decay
Page 30/55 Neutron capture / nuclear absorption • Radionuclides produced when neutron absorbed by atomic nucleus 푛푒푢푡푟표푛 + 푛푢푐푙푒푢푠 → 푟푎푑𝑖표푛푢푐푙𝑖푑푒
• For neutron to be captured 퐸푛 needs to be low in the range 0.03÷100 eV = thermal neutrons • Radionuclides produced predominantly neutron rich → decay mainly by 훽− • Production system: 1. Nuclear reactor: creates thermal neutrons 2. Target: placed inside field of thermal neutrons
Page 31/55 Neutron capture reaction chain
• Neutron capture leaves nucleus excited → de-excitation via emission of 훾-ray: 푛 + 퐴푋 → 퐴+1푋 + 훾
Notation: 퐴푋 푛, 훾 퐴+1푋 • Radionuclide produced = isotope of target material = same 푍 but 퐴 + 1 → very difficult to separate → low purity and activity • Exception that can be easily separated: 125퐼 from decay of 125푋푒 with half-life 17 h: EC 표푟 훽+ 124푋푒 푛, 훾 125푋푒 125퐼
Page 32/55 Nuclear fission
• Nuclear fission process: 1. Heavy nuclei (232푇ℎ, 235푈, 237푈, 239푃푢 and others with 퐴 > 92) are irradiated with thermal neutrons = neutron bombardment → absorb neutrons → become unstable 2. Unstable nuclei undergo fission = break up into two lighter nuclei of approximately similar atomic weight • Fission-produced nuclides have 28 < 퐴 < 65 • Radionuclides produced predominantly neutron rich → decay mainly by 훽− • Fission products can be separated chemically with high specificity → high quality radiopharmaceuticals
Page 33/55 Nuclear reactor
• Main components: 1. Fuel cells: contain fissionable material
2. Moderator: commonly graphite or 퐷2푂 surrounding fuel cells = slows down neutrons 3. Control rods: commonly boron exposing or shielding fuel cells = heavy neutron absorbers 4. Ports in reactor core: allow samples to be inserted for irradiation with Courtesy Piero Posocco (Imperial College) neutrons Fission of 235푈 or heavily enriched • Position of fuel cells and control 235푈 giving: rods determine rate of chain 1. Fission products reaction 2. Thermal neutrons → can be used to create radionuclide by neutron capture
Page 34/55 Reactor-produced radionuclides Nuclear absorption
Radionuclide Production reaction 푬휸 (keV) Half-life 흈 (Barn) 51퐶푟 50퐶푟 푛, 훾 51퐶푟 320 27.7 days 15.8 59퐹푒 58퐹푒 푛, 훾 59퐹푒 1099 44.5 days 1.28 99푀표 98푀표 푛, 훾 99푀표 740 66.02 h 0.13 131퐼 130푇푒 푛, 훾 131푇푒 → 131퐼 364 8.04 days 0.29 Nuclear fission
Radionuclide 푬휸 (keV) Half-life Fission yield (%) 99푀표 740 66.02 h 6.1 131퐼 364 8.05 days 2.9 133푋푒 81 5.27 days 6.5 137퐶푠 662 30 yrs 5.9
Page 35/55 Charged-particle bombardment
• Radionuclides produced through interaction of charged particles (퐻±, 퐷+, 3퐻푒2+, 4퐻푒2+) with nuclei of stable atoms 푐ℎ푎푟푔푒푑 푝푎푟푡𝑖푐푙푒 + 푛푢푐푙푒푢푠 → 푟푎푑𝑖표푛푢푐푙𝑖푑푒 • Radionuclides produced predominantly neutron deficient → decay by 훽+ or EC • Production system:
1. Accelerator: kinetic 퐸푐ℎ푎푟푔푒푑 푝푎푟푡𝑖푐푙푒 needs to be high enough to overcome nucleus (+) electrostatic repulsion 2. Target
Page 36/55 Accelerators
• Two basic types used for medical imaging: 1. Cyclotron → most commonly used and usually located near hospitals due to radionuclide short half-lives 2. Linear accelerator
Top view
Target
B field
푞퐵 Cyclotron frequency = 푓 = 2휋푚 Page 37/55 Path of +ve ion in cyclotron
Magnetic field into page
+ve ion +ve Ex(x) source
AC volts
-ve x Dees = vacuum chambers Courtesy Piero Posocco (Imperial College) Page 38/55 Path of +ve ion in cyclotron
Magnetic field into page
+ve ion +ve Ex(x) source
AC volts
-ve x Dees = vacuum chambers Courtesy Piero Posocco (Imperial College) Page 39/55 Path of +ve ion in cyclotron
Magnetic field into page
+ve ion +ve Ex(x) source
AC volts
-ve x Dees = vacuum chambers Courtesy Piero Posocco (Imperial College) Page 40/55 Compact biomedical cyclotron
Power supplies and Retractable shields Target support unit
Courtesy Piero Posocco (Imperial College) Page 41/55 Accelerator-produced radionuclides
Radionuclide Principal 휸-ray Half-life Production reaction 1 푬휸 (keV) 11퐶 511 20.4 min 14푁 푝, 훼 11퐶 13푁 511 9.96 min 13퐶 푝, 푛 13푁 15푂 511 2.07 min 15푁 푝, 푛 15푂 18퐹 511 109.7 min 18푂 푝, 푛 18퐹 67퐺푎 93, 184, 300 78.3 h 68푍푛 푝, 2푛 67퐺푎 111퐼푛 171, 245 67.9 h 112퐶푑 푝, 2푛 111퐼푛 120퐼 511 81 min 127퐼 푝, 8푛 120푋푒 → 120퐼 123퐼 159 13.2 h 112푇푒 푝, 2푛 123퐼 127퐼 푝, 5푛 123푋푒 → 123퐼 124퐼 511 4.2 days 124푇푒 푝, 푛 124퐼 201푇푙 68÷80.3 73 h 203푇푙 푝, 3푛 201푃푏 → 201푇푙 1511 keV 훾-rays come from 훽+ decay
Page 42/55 Radioactive decay
• Radioactive decay of parent radionuclide can lead to: 1. Unstable nuclide = radioactive nuclide = daughter radionuclide 2. Stable nuclide • 푍 of radionuclide daughter depends on decay type • Good radionuclides for medical imaging: 1. Daughter is short-lived and has 푍 different from parent → can be easily separated 2. Parent has sufficiently long half-life for production, processing and shipment
Page 43/55 Radionuclide generator
• The generator: 1. Receives in input radionuclides produced from nuclear reactors or accelerators 2. Contains: a) Chemical separation system of daughter radionuclide from parent radionuclide: chromatographic techniques most common b) Extraction system • Main features: 1. Portable → provides local supply of short-lived radionuclides without a nearby accelerator or nuclear reactor 2. Daughter product replenished continuously by decay of parent → can be extracted repeatedly Page 44/55
Generator-produced radionuclides Parent Parent Mode of Daughter Daughter Daughter Daughter P half-life decay D decay half-life 휸 푬휸 (keV) P → D mode 62푍푛 9.1 h 훽+ 62퐶푢 훽+ 9.8 min 511 EC EC 1173 68퐺푒 280 days EC 68퐺푎 훽+ 68 min 511 EC 1080 81푅푏 4.7 h EC 81퐾푟 IT 13 s 190 82푆푟 25 days EC 82푅푏 EC 76 s 777 훽+ 511 99푀표 66.02 h 훽− 99푇푐푚 IT 6.02 h 140 113푆푛 115.1 days EC 113퐼푛푚 IT 1.66 h 392 195퐻푔푚 40 h IT 195퐴푢푚 IT 30.6 s 262 EC
Page 45/55 ퟗퟗ푴풐 − ퟗퟗ푻풄풎 generator
Ref. 1 – Chapters 3.4 and 3.5 • 99푇푐푚 most common radioisotope used in nuclear medicine: 99푀표 99푇푐푚 99푇푐 + 140 keV 훾 퐻푎푙푓−푙𝑖푓푒=66 ℎ 퐻푎푙푓−푙𝑖푓푒=6.02 ℎ
• Also called a Molly or Cow • Typically used for one week • 99푀표 bound to alumina column − as molybdate ion ( 푁퐻4 2푀표푂4 ) 99 푚 • 푇푐 : – Chemically different → not bound to column → eluted from column with 5÷25 ml saline – 75÷85% of available 99푇푐푚 extracted
Page 46/55 Equation for number of ퟗퟗ푻풄풎 atoms produced with generator
휆1 휆2 99푀표 99푇푐푚 99푇푐 푁1 푁2 푁3
99 • Number 푁1 of 푀표 atoms decreases with time from 푁0 due to decay: −휆1푡 푁1 = 푁0푒
99 99 푚 • Number 푁3 of 푇푐 atoms increases with time due to decay of 푇푐
99 푚 • Number 푁2 of 푇푐 atoms has two components = one decreases with time due to 99푇푐푚 own decay, other increases with time due to 99 푀표 decay → first order differential equation for 푁2: 푑푁 푑푁 2 = 휆 푁 − 휆 푁 → 2 + 휆 푁 = 휆 푁 푑푡 1 1 2 2 푑푡 2 2 1 1
With boundary condition: 푁2 = 0 at 푡 = 0
Page 47/55 Solution of first order differential equation for 푵ퟐ
• Solution of first order differential equation for 푁2 made of two terms:
−휆2푡 −휆1푡 푁2 = 퐶푒 + 퐷푒 −휆2푡 1. Homogeneous: 푁2 = 퐶푒 −휆1푡 2. Particular: 푁2 = 퐷푒 휆 푁 • From boundary condition → 퐶 = − 1 0 휆2−휆1 휆 푁 • Solving particular solution for 퐷 → 퐷 = 1 0 휆2−휆1 • Final solution of first order differential equation for 푁2: 휆1푁0 휆1푁0 −휆2푡 −휆1푡 푁2 = − 푒 + 푒 휆2 − 휆1 휆2 − 휆1
휆1푁0 −휆1푡 −휆2푡 푁2 = 푒 − 푒 휆2 − 휆1
Page 48/55 Radioactivity 푸 of ퟗퟗ푻풄풎 produced with the generator • Radioactivity 푄 of 99푇푐푚 produced with the generator given by: 푄 = 휆2푁2 • Using solution for 푁2 the radioactivity 푄 is finally given by: 휆1푁0 휆1휆2푁0 −휆1푡 −휆2푡 −휆1푡 −휆2푡 푄 = 휆2 푒 − 푒 = 푒 − 푒 휆2 − 휆1 휆2 − 휆1 99 푁0 = number of 푀표 at 푡 = 0 99 ln 2 ln 2 −1 휆1 = 푀표 decay constant = 1 = = 0.0105 h 휏1/2 66 99 푚 ln 2 ln 2 −1 휆2 = 푇푐 decay constant = 2 = = 0.116 h 휏1/2 6 • Radioactivity proportional to difference of two exponentials = one governing increase in 99푇푐푚 due to 99푀표 decay and other decrease in 99푇푐푚 due to its decay
Page 49/55 Naturally-occurring radionuclides • Very long-lived elements • Mainly very heavy elements
Nuclide Abundance (%) Half-life (yrs) 40퐾 0.01 1.26 × 109 87 10 푅푏 27.8 4.88 × 10 232푇ℎ 100 1.40 × 1010 235푈 0.7 7.04 × 108 238푈 99.3 4.46 × 109
• → Not useful for imaging
Page 50/55 Choice of radionuclides for imaging Ref. 2 – Chapter 3.4.4 • Desirable physical characteristics of radionuclides for nuclear medicine imaging: 1. Physical half-life: a. Long enough to allow: 1) Preparation of radiopharmaceuticals 2) Completion of imaging procedures b. Short enough to ensure dose to patient and staff is minimised 2. Decay via isomeric transition = produces 훾 rays with: a. High photon yield → good counting statistics
b. Suitable 퐸훾 3. Absence of particulate emission (훼 or 훽 particles) → no unnecessary dose to patients Page 51/55 Emitted photon energy
• Emitted photon energy critical and chosen as “compromise” for various reasons:
1. High enough 퐸훾 so that: a. Photon is able to efficiently escape the body b. Photopeak is easily separated from scattered radiation
2. Low enough 퐸훾 so that: a. Detection efficiency is still good b. Do not penetrate thin collimator septa → thickness of collimator septa not too big c. Photons are not too difficult to shield and to handle
Page 52/55 Commonly used radionuclides for imaging Nuclide Decay Product 푬 (keV) Half-life Imaging system Comment mode 11퐶 훽+ 훾 511 20 min PET 13푁 훽+ 훾 511 10 min PET 15푂 훽+ 훾 511 2 min PET 18퐹 훽+ 훾 511 110 min PET ~80% of all PET imaging (FDG) 67퐺푎 EC 훾 93, 185, 300 3.3 days g-camera, SPECT 82푅푏 훽+ 훾 511 1.25 min PET 99푇푐푚 IT 훾 140 6.0 h g-camera, SPECT > 80% of all nuclear medicine imaging 111퐼푛 EC 훾 172, 247 2.8 days g-camera, SPECT Used for longer term studies 123퐼 EC 훾 159 13 h g-camera, SPECT 201푇푙 EC X-ray 68÷80 3.0 days g-camera, SPECT
Page 53/55 Radiopharmaceuticals
Ref. 2 – Chapter 5.4.5 • Radiopharmaceutical = radioactive compound (biomolecule or drug) of suitable quality to be safely administered to Courtesy Piero Posocco (Imperial College) humans for diagnosis, therapy or research • Radiopharmaceutical composition: 1. Usually radionuclide + pharmaceutical compound 2. Some exceptions: a. No associated pharmaceutical compound, for ex. 133푋푒 gas b. Pharmaceutical component = counter ion, for ex. 푁푎퐼 Page 54/55 Radiopharmaceutical chemistry and biology Ref. 2 – Chapters 5.4.5, 5.4.6 5.4.7 and 5.4.8 • Radiolabelling = “attach” the radionuclide to the pharmacological compound • Distribution of radiopharmaceutical within living system depends on various factors including: 1. 3D structure and size of the molecule 2. Blood flow • Quality control: 1. Biological purity: toxicity, sterility and apyrogenicity 2. Radiopharmaceutical purity: radionuclide, radiochemical and chemical purity
Page 55/55 Choice of radiopharmaceuticals for imaging • Characteristics of radiopharmaceuticals for nuclear medicine imaging: 1. Accumulation / rate of uptake or clearance of radiopharmaceutical should be related to a physiologic, biochemical or molecular process, target or function 2. No pharmacological or toxicological effects on system / organ under study → concentration usually subpharmacological (micromolar to nanomolar) 3. High uptake in target tissue compared with non-target tissue = specificity → lower required dose + increase image contrast 4. Half-life appropriate for the duration of the study 5. Easily synthesised or labelled 6. Sufficiently long shelf life before and after radiolabelling 7. Be of required pharmaceutical quality
Page 56/55 Some common radiopharmaceuticals Compound Nuclide Measurement Example of clinical use
Ammonia 13푁 Myocardial perfusion Coronary artery disease Fluorodeoxyglucose 18퐹 Glucose metabolism Cancer, neurological disorders (FDG) and myocardial diseases Gallium citrate 67퐺푎 Sequestered in tumours Tumour localization 99푇푐푚-methylene 99푇푐푚 Bone metabolism Metastatic spread of cancer diphosphonate (MDP) Sestamibi, Tetrofosmin 99푇푐푚 Myocardial perfusion Coronary artery disease MAG3, DTPA 99푇푐푚 Renal function Kidney disease HMPAO, EDC 99푇푐푚 Cerebral blood flow Neurologic disorders Labelled white blood 111퐼푛 Sites of infection Detecting inflammation cells Sodium Iodide 131퐼 Thyroid function Thyroid disease
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