Ionizing Radiation

They are those able to produce ionization in matter, in other words, to pull

an electron in the atom and, in particular, in biological structures.

Ionizing radiation are the and particles, neutrons and radiation with

short wavelength (high energy) such as , X e UV.

In the particular case of and X radiation, ionization is due to electrons

that are released after the primary interactions of photons with the atoms of

the medium, followed by the secondary ionization of the electrons with the

other electrons of the medium until they lose their energy.

Radioactive decay

If the core of a certain nuclide is in a situation of instability due to an

excess of protons or neutrons, or excess of both, it tends to become another

nuclide more stable. nuclide.

This process of nuclear transformation, which change the proportion

between protons and neutrons, is radioactive decay.

Decline by emission

The gamma emission results from an excess energy released by the

nucleus of an atom in the form of electromagnetic radiation.

Gamma decay may be associated with other decays, as the or , if the

nuclides descendants remain in an excited state.

photons are as light and X-rays, due to electromagnetic nature, but they

have a higher energy that comes from the electronic layers of atoms.

Mapping of radon in Portugal

The release of radon to the atmosphere is

conditioned by the permeability and

porosity of soils and rocks.

Meteorological parameters such as

atmospheric pressure, humidity and

temperature also influence the release of

radon.

Photoelectric effect

It is an interaction process in which a photon with energy E interacts with

an atom, with the emission of an orbital electron.

The photon is absorbed, losing all their energy in the ionization process.

The kinetic energy (Ec) of the electron released (photoelectron) is given by

the difference between the photon energy (h) and the extraction work of the

electron (we).

Ec = h - we

Moving photoelectrons will interact with matter.

The probability of the photoelectric effect occur is greater the bigger is the

electron bond to the atom, in other words, it is more likely to occur

photoelectric effect in an electronic layer nearest the core.

This effect is predominant for electromagnetic radiation of low energy and

it is more likely in materials with higher atomic number.

The photoelectron becomes a secondary ionizing particle and it will also

be an ionization agent.

Compton effect

It is a process of interaction involving an elastic collision between a photon

and a free or little connected electron to atom.

The initial photon gives rise to a new photon of lower energy. Energy

remainder is transferred to the Compton electron (or recoil electron).

The transferred energy of the incident photon to the recoil electron is

maximum if the collision is frontal and it will be minimum in case of a

tangential collision.

The probability of Compton effect occur decreases when the photon

energy decreases and increase with the atomic number of the materials,

being the released electron a secondary ionizing particle.

Pair production

It is an electromagnetic interaction process of the photon with the

electric field of the nucleus of the atom. From this interaction results that

the photon ceases to exist, forming a pair of particles electron/positron.

The kinetic energy of the pair electron/positron will be greater the

greater is the excess of the energy of the photon in relation to 1,02 MeV.

This process only occurs in the presence of

matter, since it is necessary a change in the

amount of movement with a heavy core to

conserve the energy and the amount of

movement.

Nuclear Medicine

The basic principle of nuclear medicine imaging is to get an image from the

radiation that comes from the organ to examine.

To obtain this image, it is administered in the patient a radiopharmaceutical

(which contain a radioactive element in its structure) which is important in the

specific organic function that we want to assess.

In therapeutic terms, Nuclear Medicine is generally used to thyroid

treatments.

Iodine-131 is absorbed by the thyroid and thyroid cancer cells and it does

not accumulate significantly in healthy cells outsider the thyroid.

Image formation

The scintigraphic images obtained in Nuclear Medicine are based on the

ability to detect gamma radiation emitted by a radioactive material

administered in the human body.

The detectors are able to detect the distribution of the radioactive material,

obtaining functional images of certain organs of human body.

The gamma camera is basically composed by a crystal of large

dimensions of NaI doped with thallium (usually with 40cm x 50cm and 1cm

thick) coupled, via a light guide, to several photomultipliers and a collimator

placed in front of the crystal.

The output signals from the photomultipliers are processed, providing

information about the amount of energy absorbed by the crystal and the

spatial coordinates of the incidence of radiation on the crystal.

The collimator is an important component of the gamma camera and its

selection depends on the type of study that is intended to do, which may be

parallel (only allows the normal radiation passing through the detector), of

type pinhole (allows to do amplifications), convergent (for a good resolution

and sensitivity) and divergent (when the object has higher dimensions than

the camera).

The shield that covers the entire chamber isolates it from all sources of

radiation that are not in its field of vision.

The radioisotope most widely used in nuclear medicine is technetium-99m

(Tc-99m), having a half-life of 6 hours, emitting gamma radiation with an

energy of 140 KeV.

The power of penetration of this radiation is sufficiently high that a fraction

of about 40% pass through the human body and reach an external detector.

During this way, radiation can interact with some tissues of human body or

with its own gamma camera, by photoelectric or Compton interactions.

For image analysis only interests radiation which has not undergone

Compton effect, so, only interests that radiation that reaches the detector

with an energy of 140 KeV, being that radiation that provides the useful

information about the process that it is being studied in the patient.

The fact of filtering the remaining information due to interactions between

the radiation and human tissues is due to the fact that such radiation change

their direction and energy and, giving indications that are considered noise.

Detectors

There are several types of detectors for Nuclear Medicine, being the most

used the scintillators, using a crystal of sodium iodide containing small

amounts of thallium.

These crystals are connected to a photomultiplier tube. The crystal is

scintillator because when a gamma ray interacts with it is produced visible

light.

The photomultiplier tube coupled to crystal transforms the light gathering

into electrical pulses. A scintillation detector is not only an imaging device

since it only detects the presence of high energy photons.

To obtain an image it is necessary to obtain the direction in which the

photons are moving and the position in which the interaction of photons with

the detector occurs.

The spatial resolution obtained with these systems is 5 to 8 mm.

The collimator has great influence on the spatial resolution

because the more narrow and long the holes are (see figure)

the best be achieved that only photons from disintegration of

the radioisotope inserted into the patients body with a course

perpendicular to the collimator are able to reach the detectors.

So that the collimator was perfect it would only accept photons that had a

trajectory perpendicular to this but in reality this is not possible, accepting photons

with different angles of incidence.

This fact causes little sharpness in the image obtained which will have better

resolution according to the characteristics of the collimator (longer and narrower

holes) or the shortest distance from the radiation source to the camera.

Regarding the noise in the image, (statistical variation between a pixel and

the next) it will be necessary to increase the number of counts to decrease.

ex.: if the diameter of the collimator holes are increased noise will decrease

but otherwise the resolution will decrease. The solution is to search the ideal

relationship between noise and resolution to obtain the best image possible.

Other difficulties are related to the fact that many of the photons that reach

the detectors do not follow a direct path from the source to the camera,

having interacted with the tissues (Scattering) and changed the direction.

Single Photon Emission Tomography (SPECT)

This technique is used to obtain three-dimensional images (acquired in

different planes) by measuring the isotopes activity previously administered

to the patients body, which decay emitting gamma radiation.

Advantage:

Disadvantage:

Obtaining a 3D image of the distribution of tracer into

the patient.

Time required for data collection and image

formation.

High dose of radioisotope that is necessary

administer to obtain a signal with good quality.

This technique has been refined in order to reduce the mentioned

disadvantages, particularly in the treatment of noise during the process of

image reconstruction.

Most modern cameras now contain 3 sensing heads to increase the

sensitivity to emitted radiation.

Positron Emission Tomography (PET)

In this technique the radionuclide administered to the patient disintegrates

emitting positrons with kinetic energy of 1 MeV order.

If these positrons are in proximity to soft tissues, they can go a few

millimeters before interacting with electrons of matter annihilating

themselves.

The annihilation yields 2 photons of energies of 511 KeV (equivalent to the

rest mass of an electron), emitted in opposite directions.

The patient is surrounded by detectors, that should answer to each event

from its body.

These detectors are scintillators and electronically linked in order to detect

coincidences, i.e., if the radiation recorded by each detector was emitted

simultaneously or with a slight time difference.

Short half-life of the positron emitters which implies that

need to be used almost immediately after its production.

Some of the most commonly used radionuclides in PET

Disadvantage:

Of radionuclides from the previous table only 82Rb can be produced in a

generator (from 82Sr), all the others are produced in cyclotrons.

Since the detection is made by coincidences and not exclusively by

collimation and there are more events, this technique becomes with better

resolution than SPECT.

After the analysis of coincidences, this technique allows , such as SPECT,

allows the formation of a 3D image.

Accelerators Cyclotron

The Cyclotron Accelerator was invented by Ernest Lawrence in 1929.

It was first constructed in 1932, at the University of California, Berkeley,

and accelerated charged particles (electrons, protons, heavy ions as

deuterium nuclei, helium, etc.) so that they can be used in various

applications (for example, Oncology therapies).

The Cyclotrons are used in a hospital for the production of radioisotopes

for PET and SPECT techniques or as s radiation source for radiotherapy.

Scintillation detectors

A scintillation detector usually comprises a crystal coupled to a

photomultiplier.

These detectors have the property that certain crystals have. Being

traversed by ionizing radiation, they excite part of their electrons to a higher

energy level, emitting, after the decay, low energy photons (scintillation

photons).

The greater the amount of absorbed radiation, the grater the emitted light.

These emitted scintillation photons will produce, by photoelectric effect in

the photocathode of the photomultiplier, photoelectrons.

The electronic signal produced will allow, after the processing of data, to

provide information about the amount of energy absorbed by the crystal.

The most commonly used scintillation detectors are sodium iodide doped with

thallium (NaI:Tl), cesium iodide doped with thallium (CsI:Tl), bismuth and

germanium oxide (BGO), yttrium and aluminum perovskite doped with cerium

(YAP:Ce), lutetium and yttrium orthosilicate doped with cerium (LYSO:Ce).

Of these, the most commonly used in commercial Anger cameras are NaI (Tl).

The photomultipliers which are coupled to the detector expand and

convert the emitted photoelectrons, yielding an electronic signal with greater

amplitude, that can be processed , as shown in next figure.

Basic operation of a photomultiplier

Germanium detectors

The germanium detectors (Z=32) are semiconductor detectors which are

used for the detection of lower energy radiation because they are transparent

to gamma radiation due to their atomic number be less than that of crystals

used in scintillation detectors.

However this type of detectors has a better energy resolution when

operated at low temperatures, usually being cooled by liquid nitrogen.

The energy required to create electron/gap pairs is about 10 to 100 times

lower than in the scintillation detectors and if they were not cooed they could,

electrons at room temperature, be excited spontaneously, resulting in a

unfavorable signal/noise ratio.

Silicon detectors

The silicon detectors (Z=14), such as germanium, are semiconductor

detectors and permeable to high-energy radiation being used for the

detection of charged particles or the formation of images by X-rays or

gamma radiation of low energy (around 100 KeV), having high resolution (for

example, mammography).