Dose Calibrator Quality Control in Nuclear Medicine

 Lecture no :1 Nuclear Imaging

Contents

1.      - Introduction to radionuclide imaging

2.      Atomic structure

3.      Radioactivity

Lecture no 1

1-introduction to nuclear imaging:

·      In nuclear imaging, we are using radionuclides to be injected into the patient for imaging or therapy purposes. 

·      The radionuclide is targeted toward the organ using a pharmaceutical that is designed based on the type of nuclear medicine procedure.

·      The radionuclide is preferable to emit only gamma radiation for imaging and alpha or beta particles for therapy purposes/Why?

o   Gamma radiation has a higher penetration power than radiation particles, so it can reach the detector (gamma camera or PET) for image formation.

o   Gamma radiation deposits less energy inside the patient in comparison with radiation particles, so  gamma photons have Low LET (linear energy transfer)

o   Radiation particles deposit all their energy in a short distance, so they can be used for treatment as the radiation energy destroys the cancer cells and spares the normal cells.

o   Alpha particles have high LET, and Beta particle has a moderate LET.

2-Atomic structure:

Any material is composed of atoms. Each atom has a nucleus surrounded by electrons moving in shells. The central mass of an atom is in the nucleus, as the electron mass is negligible against the mass of protons or neutrons.

1-    The nucleus:

1- The nuclear diameter is around 1000x smaller than the atomic diameter. The nuclear diameter is approximately 1x10^-14m. The nucleus contains nucleons (protons and neutrons). The net charge of the nucleus is positive.

1-Protons (P):

                           A proton contains three quarks’ particles. two up quarks and one down quarks. The up quark has a +2/3 charge, while -1/3 is the charge of the down quark, so the net charge of the proton is +1 = (+2/3)+(2/3)+(-1/3)

 

 

2-Neutrons (N):

A Neutron contains three quarks’ particles. Two down quarks and one up quark. The up quark has a +2/3 charge, while -1/3 is the charge of the down quark, so the net charge of the neutron is 0 = (+2/3) +(-1/3) +(-1/3).

 

The relative mass of a proton or neutron is +1.

3-Nuclear Force:

•       The protons have the same charge, so repulsion electrostatic force exists in the nucleus. this repulsion force should be balanced by the strong nuclear force to keep nuclear stability.

•       Nuclear stability arises from the special arrangement of nucleons to keep the nucleus stable. The balance between the number of protons and neutrons is mandatory to keep the nuclear binding force greater than the repulsion force.

•       The nuclear force arises from the attraction force of the different charges of quarks between the neutrons and protons.

•       If there is an unbalance in the number of protons to the neutrons, it causes nuclear instability. The unstable nucleus will convert proton to neutron or neutron to proton to reach nuclear stability.

•       Strong nuclear force: There is a strong force of attraction at distances between nucleons of <10^-15 m which changes to a repulsive force at <10^-16 m. The nucleons are kept apart at a distance of ~ 5 x 10-16 m, the distance at which there is the greatest attraction.

•       Electrostatic force: this is the force of repulsion between protons. At distances of 10^-15 to 10^-16 m the strong attractive interaction (strong nuclear force) is much greater than the repulsive electrostatic force and the nucleus is held together.

Atomic number and mass number:

·       Atomic number (Z)is the number of protons or electrons of the atom

·       Atomic number is responsible for the chemical properties of the material, so the materials with the same Z ,can’t  be separated chemically

·       Cyclotron: materials produced can be separated chemically as ,all have different Z.

·       Mass number (A) is the number of protons and neutrons of the nucleus

·       Mass number is responsible for the physical properties of the material (shape, density,…)

·       Nuclear Reactor:  materials produced can’t be separated chemically as all have the same Z.

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4-Ionization & Excitation:

•       Ionized atom → if one of its electrons has been completely removed → ion pair

"electron + positive ion"

•       Excited atom → if an electron is raised from one shell to a farther one with the

absorption of energy → The atom has more energy than normal.

When it falls back → energy is re-emitted as a single 'packet' of energy or light photon.

3-Radioactivity

 

Why atom is radioactive?

 The unbalance between the number of protons to neurons causes increasing the repulsion force more than the nuclear binding force. the improper nucleon arrangement increases the electrostatic repulsion which causes nuclear instability.

Why atom is emitting radiation?

 The atom emits radiation to reach stability by keeping the balance between the number of neutrons and protons. If an atom has too many or too few neutrons and does not lie upon the "line of stability", it becomes unstable and decays to a more stable form..

How does the atom reach the balance between the protons and neutrons?

When a nuclide undergoes radioactive decay, it breaks down to fall into a lower energy state expending the excess energy as radiation. The radioactivity released can be:

1.     Alpha particles decay:

·       If the atom has an excess number of protons and neutrons Z >83, it is called a heavy atom (C region in the figure). This radioactive heavy atom tends to be stable by losing the extra protons and neutrons to achieve the balance for nuclear stability.

·       The heavy parent atom emits an alpha particle (Helium particle ₂He⁴) to be converted into a daughter atom.

·       Alpha particle has 2 neutrons and 2 protons. The emission of an alpha particle causes a reduction in the number of nucleons of parents by 4.

·       The mass number (A) is the summation of the number of nucleons, so A is reduced by 4.

An alpha particle is a helium nucleus.

•       It has a relative charge of +2.

•       Alpha particle is a charged particle that interacts with atoms around. This interaction causes ionization (knocking out electrons from the atoms) of the surrounding atoms and losing alpha particle energy in a short traveling distance. Its penetration power is the lowest among the three types of particles and can be blocked by a piece of paper or a few cm (1-10 cm) of air.

•       Its ionizing power is the highest among the three types of particles ( High LET).

•       Alpha particles can be used for cancer treatment because of the following:

1-    High Linear Energy transfer (LET). It means high energy loss per unit of distance traveled inside the medium. This energy causes damage to the DNA of cancer cells through the ionization process. DNA damage initiates the apoptosis process (Cell Death).

2-    A short range of alpha particle cause sparing of normal cells from radiation exposure, so the treatment is highly localized.

 

 

1.     Beta particles:

1-B^- decay:

·       If the atom (Area B in the figure) has an excess number of neutrons (n>p), it converts n to p to achieve the required n/p balance for nuclear stability. It releases negative electrons (B-) and antineutrino.

·       The atomic number increase by 1, while the mass number remains the same.

·       It has a relative charge of -1.

·       Not all beta particles have the same energy, but they are produced in a range of energies.

·       The average energy of beta particles is one-third the maximum energy.

·       The beta particles traveled a certain distance based on their energy and the density of the material.

·        The maximum distance traveled by beta particles is called the range. the range is directly proportional to beta energy and inversely proportional to material density.

·       Its penetration power is in the middle among the three types of particles and can be blocked by a thin sheet of aluminum.

·       Its ionizing power is in the middle among the three types of particles.

·       It is used in thyroid cancer treatment using the I-131 beta emitter.

 2-B⁺ (Positron) decay:

·       If the atom (Area A in the figure) has an excess number of protons (p>n), it converts p to n to achieve the required n/p balance for nuclear stability. It releases positive electrons (positron B+) and neutrino.

·       It has a relative charge of +1.

·       B⁺ collides with an electron around (few mm ) to annihilate two gamma photons 180⁰ to each other with an energy of 511 Kev.

·       The two gamma photons are detected by a Positron emission tomography PET scanner.

·       F-18 is an example of a positron emitter labeled with glucose to form FDG(fluor deoxy glucose) .

·       FDG is highly used for whole-body imaging using PET-CT.

 

·       The atomic number decrease by 1, while the mass number remains the same.

 

 

1.     Electronic capture

·       it happens for atoms that are highly rich in protons (high Z). this decay method competes for the B+ decay

·        

·       I-123 is an example of electronic capture decay, which emits 28 Kev X-rays and 160 Kev gamma.

·       The atomic number decreases by 1, while the mass number remains the same.

 

 

4-Isomeric transition

·       The atom converts from an excited state (metastable state) to a stable state without a change in atomic or mass number.

·       The parent and daughter are called isomers. that’s why we are calling this decay method an isomeric transition.

Two isomers mean the same A and Z. The only difference is the energy state.

5-Gamma Rays

·       Gamma photons are produced following alpha or beta decays i. e I-131.

·        I-131 is used for cancer thyroid treatment using Beta particles.

·        I-131 gamma photons are used for thyroid imaging using a gamma camera.

·       Gamma rays can be produced from the isomeric transition

·       Gamma photon can give all its energy to an electron in the electronic shell to be knocked out (internal conversion).

·       It does not have a charge.

·       Its penetration power is the highest among the three types of particles and can be blocked by several cm of lead.

Its ionizing power is the lowest among the three types of particles (Low LET).

Definitions

·      Activity

o   The activity is not determined by the number of unstable atoms but by the number of transformations from instability to stability status per second (decay rate).

o   One disintegration per second is considered as one decay.

o   The unit of activity is Bq which is one decay per second.

o   Your body contains around 2000 Bq .

o   The administrated activity in MBq = 10^6 Bq

o   The generator activity in GBq = 10^9 Bq .

o   The old unit is Ci ,mci = 37 MBq.

 

·      The measured count rate and Activity

o   The measured count rate (CPS) is in direct proportionality with the activity  & and number or mass of radioactive atoms in the sample.

o   The measured CPS is not representing the activity as not all radiations are detected by the detector.

·      Physical Half-life

o   The half-life (t1/2) of a radionuclide is the time taken for its activity to decay to half of its original value .

o   The half-life (t1/2) is characteristic of each radioactive material Cant be changed,Can’t be affected by electricity, heat, or chemicals

·      Exponential decay

o   The fundamental law of radioactive decay states that the activity of a radioactive sample decreases by equal fractions (percentages) in equal intervals of time. This is referred to as the exponential law.

·      Effective half life

o   If the radionuclide is stored in a bottle, its activity decays with its physical half-life, tphy .

o   If the radiopharmaceutical is administered to a person, the radioactivity in specific tissues, an organ or the whole body decreases because of the simultaneous effects of radioactive decay and metabolic turn over and excretion Tbío

4-Producing radioisotopes:

 There are three methods of radionuclide production.

1-Cyclotron

2-Nuclear Reactor

3-Radionuclide generator

 

 

 

1.     Cyclotron:

1.     The cyclotron is used to produce radionuclides by accelerating the proton to a higher level of energy to be ready for O-18 bombardment.

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2.    The cyclotron is composed of two electrodes (D1 and D2) connected with an alternating electric field which can alternate the electrode potential from +ve to -ve or reverse.

3.    the two electrodes (D1 and D2) are inside an external magnetic field .

4.    Once charged particle is inserted in the middle of two electrodes, it would be affected by the electric force to be moved from +ve D1 to the -ve D2 electrode.

5.    The charged particle reaches the surface of the electrode D2 so it has to be affected by an external magnetic field which changes the direction of a charged particle to move in a circular path toward the surface of the electrode D2.

6.    By changing the electrode potential from -ve to +ve, the charged particle will move to -ve electrode D1 to reach the electrode surface.

7.    By the effect of the external magnetic field, the charged particle moves in a circular path toward the surface of electrode D1.

8.    Each cycle of the charged particle will increase the radius of circular path and the energy of the charged particle.

9.    Once the charged particle reaches the required energy , it is directed to O-18 to bombard for radionuclide F-18 production . \

10. 

11. Radionuclides produced in a cyclotron can be obtained carrier-free. They can be separated chemically from the original stable nuclides, as they have different atomic numbers and so different chemical properties.

 

 

2.    Nuclear Reactor

·      Addition of neutrons by nuclear reactor

 

·      

 

·      Radionuclides produced in a nuclear reactor cannot be separated from the original stable nuclides, as they have the same atomic number and so the same chemical properties.

·      Radioactive fission products

·      As the molybdenum is different chemically from the other products, it can be separated and prepared in a very pure form.

 

 

·      U-235 is bombarded by neutrons which causes fission to the atom to form Ba-142, Kr-91 and three neutrons

·      The three neutrons are going to another fission process.

·      A giant heat energy is released from this fission reaction, so water is used for cooling,

·      Using water as a moderator to slow down the energy of neutron for better interaction.

·      To control the fission reaction, Cadmium rods are used as an absorbing material for the neutrons to stop the reaction.

 

 

3 Radionuclide generator:

·      Molybdenum-99 is produced as a fission product in a nuclear reactor.

·      The molybdenum (Mo-2) is based on Alumina (Al2o3 +3).

·      The molybdenum has a half-life 67 hrs and decays to 99mTc which has 6 hrs half-life.

·      The molybdenum work as a parent which form 99mTc (daughter) .

·      the molybdenum can be used to produce a certain amount of 99mTc on daily basis (cow machine)

·      every 24hrs after elution, the 99mTc reach the highest activity and be ready for milking.

·      The relation between the parent (Mo-99) and daughter (99mTc) is a transient equilibrium, as the parent has longer half life than the daughter

·      

·      Mechanism of elution process:

o   As shown in the picture, there are two openings with a needle (U-shape) passing through the molybdenum column to trach to the second opening.

o   The first opening is used to put the saline which pass through the molybdenum column to bind with the 99mTc.

o   The alumina prevents the Mo-99 to be washed out as there is a strong bond between alumina +3 and Mo-2 .

o   While 99mTc is loosely bound as it’s charge is -1 which cause  easily binding with Na+ cl-(Saline).

o   The saline has to be pushed to the second opening , that’s why we are using an evacuated vial to withdraw the saline to be collected in an evacuated shielded vial.

o   

o   This process is called the milking or elution process.

o   The eluate should be free 99mTc , where there is no alumina or Mo-99.

o   The presence of alumina will affect on labeling efficiency of 99mTc with pharmaceutics .

o   The presence of Mo-99 will affect on the labeling efficiency , increase patient dose ( beta emitter) and reduce the image quality as Mo-99 emt gamma photon with energy exceeds 700 Kev .

o   The 700 Kev cause high scatter and reduce the image contrast. 

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