Radioactive radiation in medicine March,2010. S.Dolanski Babić

Plan 1. Radioactive isotopes 2. Interaction ionizing radiation with tissue 3. Equipments in nuclear medicine

Radioactive isotopes Radioactive isotopes (radionuclides) of some elements ( 14 C, 15 N, 133 I,...) have unstable nuclei, which decay in the process of transmutation to more stable nuclei. These new formed nuclei are usually in the excited state and relax to the ground state by emission of radioactive radiation: in the form of particles ( ,  ) or photons (  ). 1. Radioactive isotopes

Radioactivity is a spontaneous transmutation of one atom into another atom with the emission of radiation. Types of ionizing radiation

The half-life is the time taken for half of the atoms of a radioactive substance to decay. activity – the number of decays in 1 s the half-life time radioactive decay constant

 -emiters are mostly used in diagnostics; such radionuclides are applicable for the diagnostics if the energies of emitted photons are lower than 200 keV; then the probability of interaction with tissue atoms is very low. 1. Radioactive isotopes Technetium – 99m Metastable isotopes are convenient, because the life time of nucleus in excited state is rather long. 99m Tc is mostly used because: - it is pure  -emiter, it has not added beta radiation - energy is not to big for detection and protection - the half-time of 6 hours is enough for diagnostics procedures - it might be added with chemical bonds for many others molecules - it is not expensive

Radioactive isotopes Radioactive isotopes are often accumulated in specific tissues inside the body. The detectors collect  radiation coming from the patient; the image represents the distribution of radionuclides in the body Image resolution in methods of nuclear medicine is 10 to 100 times lower than for structural techniques (CT, MRI), but it is possible to obtain a functional image. 1. Radioactive isotopes

Production of radioisotopes In diagnostics we need the radioisotopes which are not present in the nature. They are produced in different ways: 1. in nuclear reactors by bombardment of stable nuclei with slow neutrons; we can get the radioactive isotopes of elements which are present in traces in our body: 2. in fission process we use the products of fragmentation of large nuclei caused by neutrons: for medical use we can get radioactive iodine and molybdenum. spleenbloodpancreassource of 99m 43 Tc 1. Radioactive isotopes

Production of radioisotopes 3. in accelerators by bombardment of stable nuclei with high energy positive particles: radionuclides for bones and spinal chord 1. Radioactive isotopes Accelerators: cyclotron, linear accelerator, betatron

Methods which use  -metastable isotopes 1. Labeling method in one metabolite the stable isotope is substituted by radioactive one (egz. ) metabolites are transfered into the body in different ways and inside the body they follow the metabolic pathways from collected photons, it is possible to reconstruct: spatial distribution of radionuclides to follow their path through the body – temporal events the change in distribution in organ with time – spatially- temporal events we can monitor the function of an organ by measuring the temporal change of activity from the radionuclide deposited in that organ 1. Radioactive isotopes

2. Method of isotopic dilution the measured activity of the sample of body fluid decreases with time; activity is defined as the number of decays in 1 s in a certain volume of extracted blood, radioactive iron of known activity is inserted instead of stable isotope - the blood is transfered back into the circulation system; after a while the same volume is extracted again and the activity measured again by such measurements it is possible to determine the volume of blood and other body fluids, concentrations of ions, blood flow rate, the amount of iron in blood 1. Radioactive isotopes

Specific binding of radionuclides radionuclide is covalently bound to the carrier molecule which by its terminus can be bound to specific membrane receptor of tumor cell this method enables the accurate localization of radionuclide and better image of particular part of the body 1. Radioactive isotopes

Dynamic properties of radioisotopes The time interval of the presence of radionuclide in the body must be long enough for the examination, but not too long in order to avoid the unnecessary irradiation of the patient. This time interval is determined by two parameters: 1. half-life (T ½ ) which depends on radioactive nuclei: 2. biological half-life, the interval in which the half of initial quantity of radionuclide is eliminated from the body (T b ) the presence of radionuclide in the body is measured by effective time interval (T ef ) Effective time interval is determined by faster process 1. Radioactive isotopes

Radionuclides used in medicine americium years cesium years cobalt days iodine days iodine hours iodine days iron hours manganese days molybdenum hours technetium – 99m 6.02 hours 1. Radioactive isotopes Half-life

Interaction ionizing radiation with tissue 2. Interaction ionizing radiation with tissue temporal scaleconsequences sphysical processes s – schemical processes, the changes of molecular structure s schemical-biological processes, the changes of macromolecular structure 1 s – 10 yearsbiological consequences in the body several generationsmutations and genetic disorder

1. photoelectric effect - production of excited cations and primary electrons - in relaxation of cations, secondary X photons are emitted which are absorbed in tissue while high speed electrons can induce new excitations and ionizations Interaction X and  radiations with tissue 2. Interaction ionizing radiation with tissue

2. Compton effect - production of excited cations, primary electrons and scattered photons - scattered photons exit the body and electrons can induce new excitations and ionizations 2. Interaction ionizing radiation with tissue

Interaction X and  radiations with tissue 2. Interaction ionizing radiation with tissue about 70% of energy of primary electrons is spent on ejecting of secondary electrons after slowing down, electrons become caught into electronic cloud of atom and anions are produced photons and electrons with lower energies induce the excitations of molecules, cleavage of covalent bonds and formation of free radicals they are chemically reactive, tend to bind on macromolecules and change their structure and consequently their function by radiolysis of water H and OH radicals are generated

electrons (   particles ) with high energies are used for the irradiation of surface tumors at the begining they spend their energy by Brehmsstrahlung creating secondary photons electrons which are slowed down ionize the tissue, generating secondary electrons and secondary photons electrons are not moving straight due to frequent collisions with atoms and their range in tissue is short Interaction  - particles with tissue 2. Interaction ionizing radiation with tissue

Interaction heavy positive particles with tissue cations and  – particles they act as moving source of electric field of high energy passing in the vicinity of an atom they “pull out” orbital electron; this electron induces new ionizations  -particles create a high number of ionic pairs (cations+electrons) and ionization is most intensive at the end of track they transfer into neutral helium atoms by subsequent addition of two electrons 2. Interaction ionizing radiation with tissue

The radiation damage Nonstochastic effects the probability for damage depends on dose there is treshold dose somatic effects (appear only in irradiated person) - development of radiation sickness 2. Interaction ionizing radiation with tissue Stochastic effects the probability for damage depends on dose the damage appears for any dose genetic effects (transfered to next generations) and development of cancer

– – tecnique which depicts the distribution of radioactivity within an organ or within the whole body Radioisotope scanning – scintigraphy These are types of equipments in use: 1.Linear scanner 2.Gamma camera 3. SPECT (single photon emission computed tomography) 4. PET (positron emission tomography) 3. Equipments in nuclear medicine

- the scanner is moving above the body and collects  -photons in each position Linear Scanner -consists of three parts: 1.the detecting device: scintillation detector,collimator and photomultiplier tube; 2.the amplifier; 3.the recording apparatus -collimators can have different shapes, according to the way of scanning - tumor tissue – cold zone - the measurement takes a long time 3. Equipments in nuclear medicine E photons E electrons U - voltage analyzer discards the electric impulses arrising from scattered  photons

Scintigraphy (gamma camera) 3. Equipments in nuclear medicine

-a number of collimators is arranged at the surface of huge monocrystal of NaI - in photomultiplier tube the photon energy is transformed into voltage impulses which are processed in the detection unit - the image is reconstructed from the parameters obtained only from primary photons the measurement takes 1-2 minutes Technetium is convenient because it emitts photons of 143 keV, which practically do not interact with the tissue. by collecting the photons coming from different angles it is possible to obtain 3D image of the object. 3. Equipments in nuclear medicine Scintigraphy (gamma camera)

SPECT (Single Photon Emission Tomography) - movable gamma camera can rotate around the body - in each position it creates a planar distribution of radionuclides - using computer we get the reconstruction of the image of particular layers - it is possible to see the structures which are hidden behind the other tissues 3. Equipments in nuclear medicine

PET (Positron Emission Tomography) radioisotopes of light elements ( 11 C, 13 N, 15 O, 18 F) are produced in cyclotron in vicinity of diagnostic room positron emiters are incorporated in metabolic substances; they will be deposited specifically in particular organs emitted positron will annihilate with one of nearby electrons generating 2  photons: e + + e - 2  photons are detected simultaneously in pairs of opposite detectors and the site of annihilation is calculated positron's path in tissue is less than 1 cm and depends on energy 3. Equipments in nuclear medicine

it is useful for functional diagnostics binding of drugs on brain receptors – dynamical studies application in neurology and psychiatry it is often combined with other tomography methods (MRI; CT) 3. Equipments in nuclear medicine

Literature: 1. J.Brnjas-Kraljević, Struktura materije i dijagnostičke metode, Medicinska naklada, Zagreb, C. Guy and D.ffytche: The Principles of Medical Imaging, Imperial College Press, London,