Radiology is concerned with the application of radiation to Radiology is concerned with the application of radiation to the human body for diagnostically and therapeutically purposes.the human body for diagnostically and therapeutically purposes.This requires an understanding of:This requires an understanding of:

• the basic nature of radiationthe basic nature of radiation

• interaction between radiation and matterinteraction between radiation and matter

• radiation detectionradiation detection

• biological effects of radiationbiological effects of radiation

to evaluate the advantages and disadvantages of the various to evaluate the advantages and disadvantages of the various medical applications of radiation and its limitations.medical applications of radiation and its limitations.

There are various kind of radiation which can be classified in There are various kind of radiation which can be classified in electromagnetic radiation (EM) and particle radiation (p). The electromagnetic radiation (EM) and particle radiation (p). The X-raysX-rays and and -rays-rays are part of the electromagnetic spectrum; both have a wavelength are part of the electromagnetic spectrum; both have a wavelength range between range between 1010-4 -4 and 10and 1011 nm nm, they differ only in their origin., they differ only in their origin.

Nature and Origin of RadiationNature and Origin of Radiation

When interacting with matter EM-radiation shows particle like behavior.When interacting with matter EM-radiation shows particle like behavior.

The 'particles' are called photons. The energy of the photon The 'particles' are called photons. The energy of the photon and the frequency and the frequency (or wavelength (or wavelength ) of the EM-radiation are ) of the EM-radiation are determined by the Planck constant h:determined by the Planck constant h:

h=6.62h=6.62-34-34 J J s = 4.12 s = 4.12 1010-21-21 MeV MeVss

The photon energy for X-rays and The photon energy for X-rays and -rays is in the eV to MeV range.-rays is in the eV to MeV range.

X-rays originate either from characteristic deexcitation X-rays originate either from characteristic deexcitation processes in the atoms (Kprocesses in the atoms (K, K, K transitions) (Characteristic X-rays). transitions) (Characteristic X-rays).

The photon energy corresponds to the difference in binding energy of The photon energy corresponds to the difference in binding energy of the electrons in the excited levels to the K-level.the electrons in the excited levels to the K-level.

X-rays also originate from energy loss of high energy X-rays also originate from energy loss of high energy charged particles (e.g. electrons) due to interaction with the charged particles (e.g. electrons) due to interaction with the atomic nucleus (atomic nucleus (bremsstrahlungbremsstrahlung))

The X-rays have a smooth energy spectrum, The X-rays have a smooth energy spectrum, due to the continues energy loss effects in the due to the continues energy loss effects in the Coulomb field of the nucleus.Coulomb field of the nucleus.

-rays have typically higher energies than X-rays. They -rays have typically higher energies than X-rays. They originate mainly from deexcitation processes within the nucleus. originate mainly from deexcitation processes within the nucleus. The nucleus of mass M is characterized by Z protons of mass The nucleus of mass M is characterized by Z protons of mass mp and N neutrons of mass mmp and N neutrons of mass mnn. The mass number A=Z+N.. The mass number A=Z+N.

example:example: 181888OO1010. . AA

ZZElementElementZZ

The binding energy BE of the nucleus is the The binding energy BE of the nucleus is the difference between the mass of the nucleus and the difference between the mass of the nucleus and the mass of the Z protons and N neutrons.mass of the Z protons and N neutrons.

The mass is often expressed in terms of The mass is often expressed in terms of amu (atomic mass units) which is defined byamu (atomic mass units) which is defined by

As higher the binding energy as more stable is the As higher the binding energy as more stable is the nucleus. The binding energy is often calculated in terms of nucleus. The binding energy is often calculated in terms of the mass excess:the mass excess:

As higher the binding energy as more stable is the As higher the binding energy as more stable is the nucleus. The binding energy is often calculated in terms of nucleus. The binding energy is often calculated in terms of the mass excess:the mass excess:

EXAMPLE binding energy for EXAMPLE binding energy for 1818OO

Nuclei with equal Z but different A, N are called Nuclei with equal Z but different A, N are called isotopesisotopes

the three stable oxygen isotopes are:the three stable oxygen isotopes are: 161688OO88, , 1717

88OO99, , and and 181888OO1010

Nuclei with equal N but different A, Z are called Nuclei with equal N but different A, Z are called isotonesisotones

three N=10 isotones are: three N=10 isotones are: 171777NN1010, , 1818

88OO1010, and , and 191999FF1010

Nuclei with equal A but different Z, N are called Nuclei with equal A but different Z, N are called isobarsisobars

three A=18 isobars are: three A=18 isobars are: 181877NN1111, , 1818

88OO1010, and , and 181899FF99

All these nuclei have different binding energies All these nuclei have different binding energies because they differ in the number of protons and neutrons because they differ in the number of protons and neutrons from each other.from each other.

The nucleus can be in higher excitation if it rotates The nucleus can be in higher excitation if it rotates (rotational energy), if it vibrates ((rotational energy), if it vibrates (vibrationalvibrational energyenergy), if the single ), if the single particles are in higher quantum mechanically allowed states particles are in higher quantum mechanically allowed states (single particle excitation).(single particle excitation).

-emission-emission occurs by deexcitation of a high excitation occurs by deexcitation of a high excitation level of the nucleus to the ground state. The energy difference level of the nucleus to the ground state. The energy difference between the two excited states corresponds to the energy of between the two excited states corresponds to the energy of the the -radiation.-radiation.

Particle radiation is typically induced by decay processes of the Particle radiation is typically induced by decay processes of the nucleus. In these decay processes an internal reorganization of the nucleus. In these decay processes an internal reorganization of the nucleons takes place by which a more energeticallynucleons takes place by which a more energetically favorable state can favorable state can be reached (minimum of mass, maximum in binding energy).be reached (minimum of mass, maximum in binding energy).

In In -decay -decay processes a neutronprocesses a neutron is converted is converted into a proton by electroninto a proton by electron emissionemission ( (---decay-decay), or a ), or a proton is converted in proton is converted in a neutron by positron emissiona neutron by positron emission ( (++-decay-decay):):

-radiation are electrons which are emitted in the decay -radiation are electrons which are emitted in the decay process with a certain kinetic energy which originates from the process with a certain kinetic energy which originates from the energy difference between the decaying nucleus (energy difference between the decaying nucleus (mothermother) and the ) and the decay product (decay product (daughterdaughter). As part of the energy is distributed to a ). As part of the energy is distributed to a third particle (third particle (neutrinoneutrino) the ) the -spectrum of a particular decay -spectrum of a particular decay process is a smooth distribution.process is a smooth distribution.

The released energy is The released energy is translated into the kinetic energy of translated into the kinetic energy of the emitted a-particle and the heavy the emitted a-particle and the heavy recoil nucleus.recoil nucleus.

In In -decay-decay processes the nucleus reduces his mass by emitting a processes the nucleus reduces his mass by emitting a 4422HeHe22 (helium) nucleus ( (helium) nucleus (-particle) to reach a less massive state.-particle) to reach a less massive state.

-decay occurs in particular for heavy massive nuclei. The kinetic -decay occurs in particular for heavy massive nuclei. The kinetic energy of the emitted energy of the emitted a a particles is determined by the mass of the mother and particles is determined by the mass of the mother and daughter system.daughter system.

Neutron decayNeutron decay occurs either as consequence of a preceding occurs either as consequence of a preceding -decay-decay, , ((-delayed neutron decay-delayed neutron decay) or as a result of a reaction of fission ) or as a result of a reaction of fission process.process.

In most cases concerned with medical applications neutrons are In most cases concerned with medical applications neutrons are originated in fission, the splitting of a heavy nucleus in two approximately originated in fission, the splitting of a heavy nucleus in two approximately equally massed smaller nuclei.equally massed smaller nuclei.

oror

Natural Decay LawNatural Decay Law

The rate of the decay process is determined by the activity The rate of the decay process is determined by the activity A A (number of decay processes per second) of the radioactive sample. (number of decay processes per second) of the radioactive sample.

The activity is proportional to the number of radioactive nuclei (radionuclide)The activity is proportional to the number of radioactive nuclei (radionuclide)

is the decay constant!is the decay constant!

Differential equation for Differential equation for N(t)N(t) can be solved can be solved

N(tN(t00), ), AA(t(t00) are the initial number of radionuclides and ) are the initial number of radionuclides and

initial activity, respectively.initial activity, respectively.

The half life The half life tt1/21/2 of a radionuclide is the time by which the of a radionuclide is the time by which the

number of radionuclides has reduced to 50%.number of radionuclides has reduced to 50%.

This shows a direct correlation This shows a direct correlation between half life and decay constant for between half life and decay constant for each radionuclide.each radionuclide.

The lifetime The lifetime r r of a nucleus is defined by:of a nucleus is defined by:

Quite often the expression Quite often the expression “lifetime” can be found for radionuclides.“lifetime” can be found for radionuclides.

This means that after a period corresponding to the “lifetime”This means that after a period corresponding to the “lifetime” of a radioactive nucleus the initial abundance has decreased to 36.8% of of a radioactive nucleus the initial abundance has decreased to 36.8% of its initial value, of a nucleus can be found!its initial value, of a nucleus can be found!

Unit for Unit for exposure exposure EE is theis the Roentgen [R]Roentgen [R] which is defined by thewhich is defined by the ionization between EM-radiation and air. 1 ionization between EM-radiation and air. 1 Roentgen is the amount of EM-radiation which produces in 1 gram of air Roentgen is the amount of EM-radiation which produces in 1 gram of air 2.58 2.58 1010-7-7 C at normal temperature (22°C) and pressure (760 Torr) C at normal temperature (22°C) and pressure (760 Torr) conditions.conditions.

Dosimetry UnitsDosimetry Units

Due to the interaction between radiation and material Due to the interaction between radiation and material ionization occurs in the radiated material! (Energy transfer from the ionization occurs in the radiated material! (Energy transfer from the high energetic radiation photons or particles to atomic electrons.) The high energetic radiation photons or particles to atomic electrons.) The ionization can be used as measure for the amount of exposure which ionization can be used as measure for the amount of exposure which the material had to radiation.the material had to radiation.

1 R = 2.58 1 R = 2.58 1010-4-4 C/kg C/kg

The The exposure rate exposure rate ERER (= ionization/time) can be related (= ionization/time) can be related to the to the activity activity AA of a source (in units mCi) viaof a source (in units mCi) via::

F is the exposure constant in units F is the exposure constant in units [ (R[ (Rcmcm22) / (h) / (hmCi) ]mCi) ], , and d is the distance between source and material in units [cm]. The and d is the distance between source and material in units [cm]. The exposure constant is characteristical for the radiation source:exposure constant is characteristical for the radiation source:

TheThe absorbed doseabsorbed dose DD of radiation in any kind of of radiation in any kind of material depends on the typical ionization energy of the particular material depends on the typical ionization energy of the particular material. The absorbed dose is defined in terms of the absorbed material. The absorbed dose is defined in terms of the absorbed radiation energy per mass radiation energy per mass WW1P 1P ..

It therefore clearly depends on the energy loss behavior It therefore clearly depends on the energy loss behavior of the various kinds of radiation.of the various kinds of radiation.

The unit for the absorbed dose isThe unit for the absorbed dose is::

1 Gray 1 Gray = = 1Gy = 1 J/kg = 101Gy = 1 J/kg = 1044 erg/kg = 100 rad erg/kg = 100 rad

The average ionization energy for air is The average ionization energy for air is WW1P1P 34 eV/ion. With 34 eV/ion. With

1 eV = 1.6022 1 eV = 1.6022 1010-19-19J and the charge per ion is 1.6J and the charge per ion is 1.61010-19-19, this yields for , this yields for the absorbed dose in air D for 1 R exposure of EM radiation:the absorbed dose in air D for 1 R exposure of EM radiation:

D D = = 11 R • 34 J/C R • 34 J/C = = 2.58 2.58 1010-4-4 C/kg C/kg 34 J/C = 8.8 34 J/C = 8.8 1010-3-3 J/kg = J/kg =8.8 8.8 1010-3-3 Gy = 0.88 rad Gy = 0.88 rad

The average ionization energy depends critically on the material.The average ionization energy depends critically on the material.

There is an empirical relation between the amount of There is an empirical relation between the amount of ionization in air and the absorbed dose for a given photon ionization in air and the absorbed dose for a given photon energy and absorber (body tissue).energy and absorber (body tissue).

The absorbed dose in rads per roentgen of exposure is The absorbed dose in rads per roentgen of exposure is known as the known as the roentgen-to-rad conversion factor Croentgen-to-rad conversion factor C

C is approximately equal to one for soft body tissue in the C is approximately equal to one for soft body tissue in the energy range of diagnostic radiology.energy range of diagnostic radiology.

The increase for bone material is due to higher The increase for bone material is due to higher photoelectric absorption cross section for low energy photons.photoelectric absorption cross section for low energy photons.

Dose (rad) = Exposure (R) x R to Rad Conversion factorDose (rad) = Exposure (R) x R to Rad Conversion factor

Exposure, exposure rate and absorbed dose are independent of Exposure, exposure rate and absorbed dose are independent of the nature of radiation. Biological damage depends mainly on the energy the nature of radiation. Biological damage depends mainly on the energy loss of the radiation to the body material. These energy losses differ loss of the radiation to the body material. These energy losses differ considerably for the various kinds of radiation. To assess the biological considerably for the various kinds of radiation. To assess the biological effects of the different kind of radiations better, as new empirical unit the effects of the different kind of radiations better, as new empirical unit the dose equivalent Hdose equivalent H is introduced:is introduced:

DOSE EQUIVALENTDOSE EQUIVALENT

with the quality factor Q which depends strongly on the ionization power of the with the quality factor Q which depends strongly on the ionization power of the various kinds of radiation per path length. In first approximation Qvarious kinds of radiation per path length. In first approximation QZ of radiation Z of radiation particles, Q(particles, Q(, X, , X, ) ) 1.1.

As higher Q as higher the damage the radiation does!As higher Q as higher the damage the radiation does!

EFFECTIVE DOSEEFFECTIVE DOSE

The various body organs have The various body organs have different response to radiation. To different response to radiation. To determine the specific sensitivity to determine the specific sensitivity to radiation exposure a tissue specific radiation exposure a tissue specific organ weighting factor organ weighting factor wwTT has been has been

established to assign a particular organ established to assign a particular organ or tissue or tissue TT a certain exposure risk.a certain exposure risk.

The given weighting factors in the table imply for example that an The given weighting factors in the table imply for example that an equivalent dose of 1 mSv to the lung entails the same probability of damaging equivalent dose of 1 mSv to the lung entails the same probability of damaging effects as an equivalent dose to the liver of (0.12/0.05) effects as an equivalent dose to the liver of (0.12/0.05) 1 mSv = 2.4 mSv1 mSv = 2.4 mSv

The sum of the products of the equivalent dose to the organ The sum of the products of the equivalent dose to the organ HHTT and the and the

weighting factor weighting factor wwTT for each organ irradiated is called the effective dose for each organ irradiated is called the effective dose HH::

H HT TT

Like HLike HTT, H, H is expressed in units Sv or rem!. is expressed in units Sv or rem!.