Chapter 2Radioactivity

The Atomic structure

Objectives:IntroductionFour types of radioactive decayPenetration power of radiationNatural Decay LawRadiation unitsBiological effects of radiationPocket dosimeterMedical applications

References:1- Medical Physics textbook by Cameron

What is Radioactivity? -It is the property of an atom to change into atoms of another element.

-Spontaneous change is called decay.

-Radioactivity is a property of the material and can be either natural or artificially induced.

-Radiation in simple terms is the “Transfer of energy through space and matter .”

Unstable elements are called radionuclides or radioisotopes which are characterised by:

1 -Half-life (T½) [Time needed for half of the radioactive nuclei to decay]

2 -Mode(s) of decay [α, β, γ and spontaneous fission] 3 -Type of radiation emitted

4 -Products of the decay which may also be radioactive.Radioactivity is a natural and spontaneous (decay) process by which the unstable atoms of an element emit or radiate excess energy in the form of particles and waves. These emissions are collectively called “ionizing radiations”. Depending on how the nucleus loses this energy either a lower energy atom of the same form will result, or a completely different nucleus and atom can be formed.

Radiology is concerned with studying the effect of radiation to the human body for diagnostically and therapeutically application. In order to evaluate the advantages and disadvantages of the various medical applications of radiation and its limitations, this requires an understanding of the following concepts:

1 -The basic nature of radiation.

2- Interaction between radiation and matter.

3 -Radiation detection.

4 -Biological effects of radiation.Nuclear energy is released by the splitting (fission) or merging (combining) together (fusion) of the nuclei of the atom (s).Nuclear energy is released via three exothermic processes:

1- Radioactive decay :A neutron or proton in the radioactive nucleus decays spontaneously by emitting either particles, electromagnetic radiation (gamma rays), neutrinos or all of them .

2 -Fusion: Two atomic nuclei fuse together to form a heavier nucleus .3 -Fission: The breaking of a heavy nucleus into two (or more rarely three)

lighter nuclei .

Radioactivity is associated with the transmutation of the nucleus from one

element to another. Thus, for example, when radium (Ra) emits an alpha

(α) particle, the nucleus is transformed into radon (Rn). These emissions

are collectively called ionizing radiations. Depending on how the nucleus

loses this energy either a lower energy atom of the same form will result, or

a completely different nucleus and atom can be formed .

What is Ionizing Radiation?

-It is a radiation that has the ability to strip electrons from atoms, this

process is called ionization.

-Ionization can initiate biological damage so slight as to be totally

unnoticeable or so severe as to cause death.

ينزع

Main ionizing radiation are: , , , x-rays and neutrons

]neutron p+ + - (proton & beta particle {negatron}), neutrons don’t ionize

directly, their energy range from 10-2 to 107 eV[ .

Note: eV is the unit used to measure the energy of a charged particle, and it

is defined as “The energy an electron can gain when it is accelerated

through a potential difference of 1 (V).”

Does not have enough energy to remove (strip) electrons from atoms, although

it can interact with matter through other processes, some of which may be

detrimental Sunburn is a good example of the effect of non-ionizing

radiation. Sun is composed of 74% by mass hydrogen (11H0), an element

whose nucleus is a single proton; energy is released when 4 protons combine

into a helium (42He2) nucleus, a process in which two of them are also

converted to neutrons (10n1).

What is Non-ionizing Radiation?

.( ضار(

- For nucleus to be stable a certain number of neutrons is required if this number is increased or decreased than required, then the nucleus will become unstable and start to disintegrate (decay) to reach a stable state.

-Atoms with completely filled orbits [atoms of the so-called noble gases: helium, neon, argon, krypton, and xenon (133Xe)] cannot share electrons with other elements, therefore they are chemically most inert.

- -Some man-made radionuclide emit types of radiation not emitted by natural radioactive substance such as technetium (Tc), which means that this element is half-stable .

-A metastable radionuclide decay by emitting γ rays only, and its daughter nucleus differs from its parent in having less energy.

Types of Atoms

StableUnstable

4 He2

235 U 92

HeliumProtons = 2Neutrons = 2Mass number = 4

UraniumProtons = 92Neutrons = 143Mass number =235

1432

Metastable

9943Tc56

TechnetiumProtons = 43Neutrons= 56Mass number = 99

A=Mass numberZ= Atomic number

The nucleus of mass is characterized by atomic number Z protons of mass mp and N

neutrons of mass mn. The mass number A = [Z+N]. Protons and neutrons are closely

related and are sometimes collectively known as nucleons. A simple description of

nuclear binding energy is the energy required to break apart, split, or break down, the

nucleus of the atom into its component parts (nucleons).

Proton

Neutron

NucleusNucleus

Nuclear binding energy (binding energy of nucleons into a

nuclide) can be easily computed from the easily measurable

difference in mass of a nucleus, and the sum of the masses

of the number of free neutrons and protons that make up

the nucleus. Once this mass difference, called the mass

defect or mass deficiency, is known, Einstein's mass-

energy equivalence formula can be used to compute the

nuclear binding energy of any nucleus:

E =Δmc²Δm = mass defect, and c = speed of light in a vacuum. From this formula, adding energy increases mass, removing energy, decreases it.

Δm =(sum of masses of protons and neutrons)-(measured mass of nucleus)

The difference between the actual mass of the nucleus (mnuclide) measured in

amu and the number of nucleons (A=Z+N) is called mass excess .

Mass excess = mnuclide– A

The mass of the atom's nucleus (mnuclide) is always less than the sum of the

individual masses of the constituent protons and neutrons (A). This notable

difference is a measure of the nuclear binding energy, which is a result of

forces that hold the nucleus together. A general and simple description of

nuclear binding energy is the energy required to break apart, split, or break

down, the nucleus of the atom into its component parts (nucleons). If the

binding energy for the products is higher when light nuclei fuse, or when

heavy nuclei split (break down), either of these processes will result in a

release of the "extra" binding energy, and this energy is referred to as

nuclear energy. It is also loosely called nuclear power.

The main and highest peak in the curve is at iron (most tightly bound element) then the curve will slowly decreases, also a narrow isolated peak at helium (4He), is very stable .

The heaviest nuclei in nature is uranium 238U , which is unstable, having a lifetime of 4.5 billion years, close to the age of the Earth. Uranium is still relatively abundant; it may

be formed in an explosion, preceding the formation of the solar system. In each of these, radioactive decay produces "daughter isotopes" which are also unstable, starting a chain of decays which ends in some stable isotope. Nickel (62Ni) isotope is the most tightly

bound of nuclides .

Ni62

Yield from nuclear fusion

Elements heavier than Fe can yield energy by nuclear fission

8.8

4

Yield from nuclear

fission

Iron is the third most tightly bound elements, its binding energy/nucleons ≈ 8.8 MeV

58F26

28

235118

26

Mass is often expressed in term of amu (atomic mass units) which is defined as

1/12 of the mass of carbon atom [126C6] by the following equation:

1/12 x12/No=1/(6.022 x 1023)=1.655 x 10-24 (g) ≈1.66 x 10-27(Kg)=931.25 (MeV)

As higher the binding energy (BE) is as more stable the nucleus is. To

calculate the "binding energy" we use this formula:

BE = P(mp + me) + N.mn - mnuclide

where P is the number of protons of the radionuclides and N its number of

neutrons. We take mp = 938.2723 MeV, me = 0.5110 MeV, mn = 939.5656

MeV and mnuclide is the actual mass of the nucleus in MeV.

]No = Avogadro’s number = 6.022 x1023 (g-1) = 6.022x1026 (Kg-1)[

The isotopes of a given element are chemically identical [i.e., they participate in

the same chemical reactions but they could be distinguished from each other

because their mass number (A) are different.

Nuclei with equal number of neutrons and different A, Z are called isotones

177N10, 18

8O10, and 199F10

Nuclei with equal mass number but different Z, N are called isobars

187N11, 18

8O10 and 189F9All these nuclei have different binding energies because they differ in the

number of protons and neutrons from each other.

Four types of radioactive decay:

1 (Alpha () decay [its charge is +2] - 24He (Helium) nucleus (2p + 2n)

ejected. The binding energy of helium is appreciable, and seems to be

the energy source of the Sun and of most stars.

2 (Beta () decay [charge -1]- change of nucleus charge, conserves mass

[their are three types of beta decay: -, + and electron capture (EC)].

3 (Gamma () decay [charge is zero] - photon emission, no change in A or

Z

4 (Spontaneous fission which generates two smaller nuclei .Unstable nuclei undergo transformation by the emission of energetic radiation

such as: (1) alpha (α) particles, which are high-speed helium nuclei consisting

of two protons and two neutrons; (2) beta (β) particles, which are very high-

speed electrons; and (3) gamma (γ) rays, which are highly energetic photons .

The nucleus can be in higher excitation if it rotates (rotational energy), if it vibrates

(vibrational energy) or if the single particles are in higher quantum mechanically allowed

states (single particle excitation).

-emission occurs by de-excitation of a high excitation level of the nucleus to the ground

state. The energy difference between the two excited states corresponds to the energy

of the -radiation.)High excitation level(

Cobalt

Nickel

=2.823-0.318

=2.505-1.173

=1.332-1.332

Nickel is the most tightly bound nucleus (per nucleon), followed by (5826Fe32) and

(5626Fe30).

decay is of three types

-Neutron rich atom is required. -Converts one neutron into a proton and emit

electron, negative beta particle (e- = β-). This is

because neutrons are more massive than protons . -No change of A, but different element is formed.

-Release of electron antineutrino.

1 (- decay

2 (+ decay

3 31 2 eH He e

11 116 5 eC B e

-Proton rich atom is required.

-Converts one proton into a neutron and emit

positron, positive beta particle (e+ = β+)[for energetic reason and not to free particles] .

-The conversion of protons to neutrons is the result of a weak (nuclear) force.

-No change of A, but different element is formed. -Release of electron neutrino (ve).

A is mass number = (Z + N)Z is Atomic number= protons number

]Blue ball indicate neutron and red ball indicate protons[

A

Z

Hydrogen Helium

Carbon Boron

P+

n

n

-

Positron neutrino

5 6

2 1 Negatron

electron antineutrino

nP+P+

Parent Daughter

74Be + e- EC

7

3Li + ve

If the mass difference between parent and daughter is less than 1.022 MeV

(the mass of 2 electrons) a proton-rich nucleus may still convert protons to

neutrons by the process of electron capture, in which a proton simply captures

one of the atom's K orbital electrons (K electron), emits a neutrino, and

becomes a neutron (i.e., number of protons decrease while, number of

neutrons increase).

Beryllium Lithium

3 (Electron capture (EC)

p

pp

pn

nn n

nn

np p

p

K-electron

Proton rich nucleus 3 4

)neutrino(

Neutron rich atom

)β-(

In -decay a neutron is converted into a proton by electron emission (-

decay) and

electron antineutrino (ve ), with no change in mass number (A) as shown in the example.

140 139

ActiniumRadiumAZ

)β-( )negatron(

Proton rich atom

Thorium (Daughter)

Most positrons emitters are produced by cyclotron, which accelerate positive particles to bombard the target material. To emit positron a proton is converted into a neutron by positron (antiparticle of the electron) emission (+decay) with no change in mass number. This is permitted if enough energy is available between parent and daughter nuclides to do this (the required energy difference is equal to 1.022 MeV, which is the

mass of 2 electrons) .

)β+(

139 140

Protactinium (Parent)

+

)Antiparticle of the electron(

Cyclotron =محطمالذره

)β+(

β - particles have a continuous spectrum of energies because they share their energy with the neutrino.

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

In the -decay processes the nucleus reduces its mass by emitting a 42He2

(helium) nucleus (-particle) to reach a less massive state.

-decay occurs in heavy massive nuclei. The kinetic energy of the emitted α-particles is determined by the mass of the parent (Ra) and daughter (Rn) system which is a member of the uranium series.

)Radium(

Radon

Radium Radon

-

Radon is an important terrestrial source of ionizing radiation because, although on average it is very rare. Radon is a decay product of uranium, which is relatively common in the earth’s crust.

α α

Parent

Daughter

138

136

γ

138

2

146 144

Nucleus reduces its mass by emitting 42He2(α particle)

UraniumPlutonium

)α(

p npn

Spontaneous of Plutonium nucleus

Penetration power of radiation

Note: that neutrons is the most penetrating as their energy range from 10-2 to 107 eV

Range of radiation particles (R) is defined as “the maximum distance traveled in the absorber material by the most energetic radiation particles”.

Rang of Alpha and Beta radiation in the medium

GM counter

Medium

Interaction of radiation with matter

Alpha (α) Beta (β) Gamma (γ)

Since the alpha particle is basically a helium nucleus (2

4He) it is the largest and most massive type of radiation.

The major energy loss mechanism for alpha particles is electronic excitation and ionization.

The specific ionization of an alpha particle is very high, in the order of thousands of ion pairs per centimeter of air.

Beta particles are charged particles with relatively light mass. There are 2 main mechanisms of interaction: 1- Excitation, Ionization 2- Bremsstrahlung

1- Photoelectric effect2- Compton scattering3- Pair production

Natural Decay Law

The rate of the decay process is determined by the activity A (number of decay processes per second) of the radioactive sample. [A= Disintegration/second= dps]

The activity [A(t)] is proportional to the number of radioactive nuclei (radionuclide), t is the time.

is the decay constant = ln2/t = 0.693/t

A(t) = Final activity Initial activity= A(t0)

Each radionuclide decays at a fixed rate commonly indicated by the half-life (t1/2). Each radioisotope has a unique and unchangeable half-life.

Activity of sample versus time plotted on a linear scale

Activity of sample has an exponential relation as: A = λN= Aoe-λt

A is the activity (disintegration/second), Ao is the initial activity, λ (hour-1) is the

decay constant and t is the time in hour since the activity was Ao. The unit of activity

(disintegration per second) was named Becquerel (Bq), another unit used is Curie (Ci) .

T½ 2T½ 3T½ 4T½

Time

1/8 A0

1/4 A0

1/2 A0

Ac

tiv

ity

of

Sam

ple

(B

q)

A=A0e-t

A0

1/16 A0

λ = ln2/t = 0.693/t

N = No e-λt

N, the number of nuclei present at time t. No the initial number of nuclei.

Initial activity

Num

ber

of n

ucle

i

Radioactivity versus time for the common radionuclide on a semi-log scale

100

50

25

12.5

6

241812601

The straight line on the semi-log graph indicate that the decay is exponential as shown in the equation. If the graph shows a straight line then only one radionuclide is present. While, a curved line indicate the presence of more than one radionuclide.

Time (hr)

Rad

ioac

tivity

% A = Aoe-λt

N(to), A(to) are the initial number of radionuclide and initial activity, respectively.

Half life (t1/2) of a radionuclide: Is the time required for the radioactive material to

decrease by one - half of their initial value. t1/2 = ln2/λ = 0.693/λ

There is an inverse correlation between half life time (t1/2) and decay constant (λ) for each radionuclide.

The life time () of a nucleus is defined by:

]Since λ = ln2/t1/2 then 1/λ = t1/2/ln2 [

Number of nuclei present at half life time = N(t½)

=t1/20.693

ln [e-λt1/2] = ln1/2= -λt1/2

Dividing both sides by N(t0)

]Take ln of both sides[

Radiation units

Activity Units:

The unit for measuring the activity of radiation source [number of decays (disintegration) per time] is Becquerel (Bq) or Curie (Ci):

1 Bq = 1 decay/ second = 2.7 x 10-11 (Ci)

One curie is defined as: The activity of 1 gram of pure Radium 1 Ci = 3.7 x 1010 (decay/s) = 3.7 x 1010 (Bq)

Dosimetry Units:

the measurement or calculation of energy deposition to material by

radiation”. Due to the interaction between radiation and material ionization

occurs in the radiated material (Energy transfer from the high energetic

radiation photons or particles to atomic electrons in the material).

Unit for exposure (E) is the Roentgen [R] which is defined by the “ionization

between EM-radiation and air .”

Roentgen is defined as” the amount of EM-radiation which produces in one

gram of air 2.58 x10-7 (Coulomb) at normal temperature (22°C) and pressure

(760 mmHg) conditions” R = 2.58 x 10-7 (C/g) = 2.58 x 10-4 (C/Kg)

Exposure rate (ER) Is the radiation intensity measured in [Roentgen/hour] it can be related to the activity A of a source (in units mCi) via this equation: ER = (Γ) x A (R/h)

i.e., ER is directly proportional to activity [A in (mCi)] Г is the exposure constant for a particular radionuclide in [(Rcm2)/(hmCi)],

and d is the distance between source and material in units (cm). The exposure

constant is characteristic for the different radiation sources.

Roentgen is used to measure the amount of energy in a photon beam just

before entering the skin of a patient. It is defined only for measurement in air.

And it applies only to x and gamma rays up to energies of about 3 MeV.

d2

Absorbed dose (D) of radiation in any kind of material depends on the typical

ionization energy of the particular material. The absorbed dose is defined as “the

energy deposited in any medium (tissue) due to radiation absorption”. It is

measured in ergs/gram or joules/kg = Gray = Gy = 100 rad

Absorbed dose therefore, clearly depends on the energy loss behavior of various

kinds of radiation: D = E x W1P

Where, E is energy deposited in any medium, W1P is the average ionization energy for air, W1P 34 (Joul/Coulomb).

Example: Calculate the absorbed dose in rad for 1(R) exposure of (EM) radiation,

knowing that 1 R = 2.58 x 10-4 (Coulomb/kg).

D = E x W1P = 1 R x 34 (J/C) = 2.58 x 10-4 (C/kg) x 34 (J/C) = 87.72 x 10-4 (J/kg)

= 87.72 x 10-4 (Gy) = 87.72 x 10-4 x 102 rad = 0.8772 rad

Absorbed dose rate (DR)

If radiation of energy Er and an activity A is completely absorbed in material of

mass (M) Kg. The absorbed dose rate DR is :

DR = A (Er / M) (Bq.MeV/Kg)

Example: A patient receives an injection of 1.1x 108 (Bq) of 131I, which

accumulate in the thyroid gland (Mthyroid = 20 g). The mean energy of the emitted

radiation is Er = 300 KeV. What is the dose rate to the thyroid?

Note: MeV =106 (eV) =1000 KeV hence, (KeV) = 1/1000 (MeV)

Answer: DR = A (Er / M)

M thyroid = 20 (g) = 0.02 (Kg), Er = 300 KeV = (300/1000)= 0.3 MeV

)Absorbed dose rate = (DR = 1.1 x 108 x (0.3/0.02)= 16.5 x 108 (Bq.MeV/Kg)

Equivalent dose (H): Is the absorbed dose in tissue or organ of T weight for the type and quality of radiation R .

H is calculated by multiplying absorbed dose (DT,R) averaged over organ or tissue due to radiation by the appropriate radiation weighting factor (WR).

H = DT, R x WR (Sievert =100 rem )

If DT,R is in rad, the dose equivalent (H) is in rem and If DT,R is in Gy then H will be in Sv.

Every radiation type has its own weighting factor WR

Effective dose (Hε): Accounts for the effectiveness or quality of the radiation. It also includes a

factor representing the sensitivity of the tissue to the radiation, so it also accounts for how the

tissue might react. H is defined as “The sum of the products of the equivalent doses (H) to the

organ and tissues exposed ,

each multiplied by the weighting factor WR for each irradiated organ.”

H= Σ H.WR (Sievert = 100 rem)

rem = rad x WR

Exposure rate and absorbed dose are independent of the nature of radiation.

Biological damage depends mainly on the energy loss of the radiation to the

body material. These energy losses differ considerably for the various kinds

of radiation. For x-rays equivalent dose is equal to absorbed dose.

WR depends strongly on the ionization power of the various kinds of radiation

per path length. The unit of dose equivalent (H) is the Sievert (Sv) or rem,

where 1 Sv= 100 rem [ Hence, 1 rem = 0.01 Sv ]

rem stands for “Roentgen equivalent man” and it is the traditional unit of

equivalent dose

Biological effects of radiation

Acute effects: are due to a single accidental exposure to a high dose of radiation during a short period of time. Symptoms are:

Delayed (chronic) effects: are due to long term low-level of exposure that is called continuous exposure. The most common delayed effects are various forms of cancer(leukemia, bone cancer, thyroid cancer, lung cancer) and genetic defects (malformations in children born to parents exposed to radiation).

- Nausea, vomiting and fatigue

- Increased temperature

- Blood changes

- Bone marrow damage

- Damage to cells lining the small intestine

- Damage to blood vessels in the brain

Somatic effects the health of the exposed individual.

Hereditary effects occur in the descendants of exposed individuals.

Deterministic effects are those which become evident after a large dose. There

is a threshold below which the effects do not occur, e.g. reddening of the

skin (3 Sv).

Stochastic effects are effects for which the probability of an effect occurring

rather than its severity is regarded as a function of dose with no threshold,

e.g. range of solid tumours as well as leukaemia.

Types of exposure are: Occupational, Medical and Public

Biological effects of ionizing radiation

How does ionizing radiation damage DNA?

Direct action: Photon directly hits the nucleus of the cell and destroys the DNA

of the cell, thereby killing it.

Indirect action: Photon hits the cytoplasm in the cell, surrounding the

nucleus. This causes the water in the cytoplasm to change chemically from

H2O to H2O2 (Hydrogen Peroxide) which produces the cell death.

]P+ is the protons found in the nucleus and e- is the electron spinning all the time in the space around the nucleus of an atom [

People who work with radioactive sources often wear a radiation badge. This monitors

the level of radiation a worker is being exposed to. There is a light-proof packet of

photographic film sealed in thin plastic inside the badge. The more radiation this

absorbs, the darker it becomes when it is developed. To get an accurate measure of

the dose received, the badge contains different materials that the radiation must

penetrate to reach the film. These may include aluminum, copper, lead-tin alloy and

plastic. There is also an open area at the centre of the badge.

Film badge Closed radiation badge

Opened radiation badge

Light - proof

TLD is a phosphor, such as lithium fluoride (LiF) or calcium fluoride (CaF), in a solid crystal structure. When a TLD is exposed to ionizing radiation at ambient temperatures, the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material. Some of the atoms in the material that absorb that energy become ionized, producing free electrons and areas lacking one or more electrons, called holes. Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place.Heating the crystal causes the crystal lattice to vibrate, releasing the trapped electrons in the process. Released electrons return to the original ground state, releasing the captured energy from ionization as light, hence the name thermoluminescent. Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor.

Thermoluminescent detector (TLD)

Pocket DosimeterThe pocket dosimeter or pen dosimeter is a common small sized ion chamber which measures the originated charge by direct collection on a quartz fiber electroscope.

The U-shaped quartz fiber is close to a U- shaped wire. If the fiber is charged it will be deflected away from the wire. The position of deflection is a measure of the accumulated radiation dose.

The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure. It also has the advantage of being reusable. The few main disadvantages of a pocket dosimeter is their limited range, inability to provide a permanent record, and the potential for discharging and reading loss due to dropping or bumping.

Medical application

Radiation Therapy

Radiation therapy uses ionizing radiation to treat cancer i.e. to destroy cancerous cells.

There are two techniques in radiation therapy that are used to treat cancer using ionizing radiation:

RadiotherapyBrachytherapy

Radiotherapy Treatment Planning

• Planning

• Simulation

• Treatment

Every treatment using radiotherapy has to be rigorously planned. The planning process consists of three phases:

Radiotherapy Treatment Planning

The cancerous tumour has to be located so that its size and position can be analysed. This information can be obtained from:

•X-rays•CT scans•MRI scans•Ultrasound images

Planning

Radiotherapy Treatment Planning

Simulation

Once the amount of radiation to be given has been accurately calculated, the patient then goes to the simulator to determine what settings are to be selected for the actual treatment using a linear accelerator.

The settings are determined by taking a series of x-rays to make sure that the tumour is in the correct position ready to receive the ionising radiation.

Radiotherapy Treatment Planning

Treatment

• Irradiation using high energy gamma rays.

• Irradiation using high energy x-rays.

Cancerous tumours can be treated using radiotherapy as follows:

Radiotherapy Treatment Planning

Treatment

• Irradiation using high energy gamma rays.

• Irradiation using high energy x-rays.

Cancerous tumours can be treated using radiotherapy as follows:

Radiotherapy TreatmentIrradiation Using High Energy Gamma Rays

• Gamma rays are emitted from a cobalt-60 source – a radioactive form of cobalt.

• The cobalt source is kept within a thick, heavy metal container.

• This container has a slit in it to allow a narrow beam of gamma rays to emerge.

Radiotherapy TreatmentIrradiation Using High Energy X-rays

• The x-rays are generated by a linear accelerator (linac).

• The linac fires high energy electrons at a metal target and when the electrons strike the target, x-rays are produced.

• The x-rays produced are shaped into a narrow beam by movable metal shutters.

Treatment of Cancer

Radiotherapy

• The apparatus is arranged so that it can rotate around the couch on which the patient lies.

• This allows the patient to receive radiation from different directions.

• The diseased tissue receives radiation all of the time but the healthy tissue receives the minimum amount of radiation possible.

• Treatments are given as a series of small doses because cancerous cells are killed more easily when they are dividing, and not all cells divide at the same time – this reduces some of the side effects which come with radiotherapy.

Radiation Therapy

Brachytherapy• This involves placing

implants in the form of seeds, wires or pellets directly into the tumour.

• Such implants may be temporary or permenant depending on the implant and the tumour itself.

• The benefit of such a method is that the tumour receives nearly all of the dose whilst healthy tissue hardly receives any.

Radiation Therapy

Brachytherapy

• Uterus• Cervix• Prostate• Intraocular• Skin• Thyroid• Bone

Brachytherapy is used to treat the following cancers:

TracersThere are many uses of ionising radiation based on the fact that it is easy to detect. In such applications, the radioactive material is used in the form of a tracer.

In nuclear medicine, a tracer is a radioactive substance which is taken into the body either, as an injection, or as a drink. Such a substance is normally a gamma emitter which is detected and monitored.

This gives an indication of any problems that may be present in body organs or tissues by how much, or how little, of the substance has been absorbed.

Nuclear MedicineTracers

• A drug which is chosen for the particular organ that is being studied.

• A radioactive substance which is a gamma emitter.

It is important to be able to study internal organs, or tissues, without the need for surgery. In such cases, radioactive tracers can be injected into the body so such studies can take place. The path of these tracers can be detected using a gamma camera because of their radioactivity.

Such tracers consist of two parts:

Tracers Used in Nuclear Medicine

Factors Which Affect the Choice of Tracer

• They will concentrate in the organ, or tissue, which is to be examined.

• They will lose their radioactivity (short t).

• They emit gamma rays which will be detected outside the body.

• Gamma rays are chosen since alpha and beta particles would be absorbed by tissues and not be detected outside the body.

• Technitium-99m is most widely used because it has a half-life of 6 hours.

Such tracers are chosen so that:

Factors Which Affect the Choice of Tracer

• Gamma rays are chosen since alpha and beta particles would be absorbed by tissues and not be detected outside the body.

• Technitium-99m is most widely used because it has a half-life of 6 hours.

Why is a half-life of 6 hours important?

• A shorter half-life would not allow sufficient measurements or images to be obtained.

• A longer half-life would increase the amount of radiation the body organs or tissues receive.

A half-life of 6 hours is important because:

The Gamma Camera

The tracer is injected into the patient. The radiation emitted from the patient is detected using a gamma camera.

A typical gamma camera is 40 cm in diameter – large enough to examine body tissues or specific organs. The gamma rays are given off in all directions but only the ones which travel towards the gamma camera will be detected.

The Gamma Camera

• A collimator.• A detector.• Electronic systems.

A gamma camera consists of three main parts:

electronic systems

detector

collimator

The Gamma Camera

• The collimator is usually made of lead and it contains thousands of tiny holes.

• Only gamma rays which travel through the holes in the collimator will be detected.

The Collimator

The Gamma Camera

• The detector is a scintillation crystal and is usually made of Sodium Iodide with traces of Thallium added.

• The detector is a scintillation crystal and it converts the gamma rays that reach it into light energy.

The Detector

The Gamma Camera

• The electronic systems detect the light energy received from the detector and converts it into electrical signals.

The Electronic Systems

DiagnosisStatic Imaging

• There is a time delay between injecting the tracer and the build-up of radiation in the organ.

• Static studies are performed on the brain, bone or lungs scans.

DiagnosisDynamic Imaging

• The amount of radioactive build-up is measured over time.

• Dynamic studies are performed on the kidneys and heart.

Dynamic ImagingThe Renogram

• To assess individual kidney and/or bladder function.

• To detect urinary tract infections.• To detect and assess obstructed kidney(s).• To detect and assess vesico-ureteric reflux.• To assess kidney transplant(s).

Renograms are dynamic images of the kidneys and they are performed for the following reasons:

Performing the Renogram

• The tracer is injected into the patient.• The radioactive material is removed from the

bloodstream by the kidneys.• Within a few minutes of the injection, the

radiation is concentrated in the kidneys.• After 10 – 15 minutes, almost all of the

radiation should be in the bladder.• The gamma camera takes readings every

few seconds for 20 minutes.

DiagnosisThe Renogram

•The computer adds up the radioactivity in each kidney and

the bladder.•This can be shown

as a graph of activity versus time – a

time-activity curve.

An Abnormal RenogramNormal Renogram

Renogram: Is X-ray of renal shadow following injection of opaque medium

Sterilization

-Radiation not only kills cells, it can also kill germs or

bacteria.

-Nowadays, medical instruments (e.g. syringes) are pre-packed and then irradiated using an intense gamma ray source.

-This kills any germs or bacteria but does not damage the syringe, nor make it radioactive.