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

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 (14C, 15N, 133I,...) 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 ().


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.

tTeAA

/0

21activity – the number of decays in 1 s 21

2ln

T

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.


Technetium – 99m

Metastable isotopes are convenient, because the life time of nucleus in excited state is rather long.

99mTc 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.


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.

MonMoSenSeFenFeCrnCr 9942

9842

7534

7434

5926

5826

5124

5024 ,,,,

spleen blood pancreas source of 99m43Tc


Production of radioisotopes 3. in accelerators by bombardment of stable nuclei with high

energy positive particles:

FpHeOFenHeCrTmnpEr 189

32

168

5226

42

5024

16769

16768 ,2,,

radionuclides for bones and spinal chord


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

1412C C


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

teAA 0


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


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 (T½) 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 (Tb)

the presence of radionuclide in the body is measured by effective time interval (Tef)

b

bef TT

TTT

21

21

2

21

lnT

Effective time interval is determined by faster process


Radionuclides used in medicine

americium - 241 433 years

cesium - 137 30.1 years

cobalt - 58 71.3 days

iodine - 124 4.17 days

iodine - 130 12.4 hours

iodine - 131 8.041 days

iron - 52 8.3 hours

manganese - 52 5.63 days

molybdenum - 99 66.02 hours

technetium – 99m 6.02 hours


Half-life

Interaction ionizing radiation with tissue


temporal scale consequences

10-15 s physical processes

10-13 s – 10-8 s chemical processes, the changesof molecular structure

10-6 s - 10-3 s chemical-biological processes, thechanges of macromolecularstructure

1 s – 10 years biological consequences in thebody

several generations mutations 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. Compton effect

- production of excited cations, primary electrons and scattered photons

- scattered photons exit the body and electrons can induce new excitations and ionizations


Interaction X and radiations 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


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


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


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)


- the scanner is moving above the body and collects -photons in each position

Linear ScannerLinear 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


Ephotons Eelectrons U

- voltage analyzer discards the electric impulses arrising from scattered photons

Scintigraphy (gamma camera)


-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.


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


PET (Positron Emission Tomography)

http://www.sumanasinc.com/webcontent/animations/content/positronemissiontomography.html

radioisotopes of light elements (11C, 13N, 15O, 18F) 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


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)


Literature:

1. J.Brnjas-Kraljević, Struktura materije i dijagnostičke metode, Medicinska naklada, Zagreb, 2001.

2. C. Guy and D.ffytche: The Principles of Medical Imaging, Imperial College Press, London, 2005.

3. http://www.mef.hr

Documents

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