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Chapter: Medical Electronics - Radiological Equipments

Ionising Radiation

In general, ionizing radiation is harmful and potentially lethal to living beings but can have health benefits in radiation therapy for the treatment of cancer and thyrotoxicosis.

IONISING RADIATION

 

In general, ionizing radiation is harmful and potentially lethal to living beings but can have health benefits in radiation therapy for the treatment of cancer and thyrotoxicosis.

 

Most adverse health effects of radiation exposure may be grouped in two general categories:

 

·        Deterministic effects (harmful tissue reactions) due in large part to the killing/ malfunction of cells following high doses; and

 

·        Stochastic effects, i.e., cancer and heritable effects involving either cancer development in exposed individuals owing to mutation of somatic cells or heritable disease in their offspring owing to mutation of reproductive (germ) cells.

 

Its most common impact is the stochastic induction of cancer with a latent period of years or decades after exposure. The mechanism by which this occurs is well understood, but quantitative models predicting the level of risk remain controversial. The most widely accepted model posits that the incidence of cancers due to ionizing radiation increases linearly with effective radiation dose at a rate of 5.5% per sievert. If this linear model is correct, then natural background radiation is the most hazardous source of radiation to general public health, followed by medical imaging as a close second. Other stochastic effects of ionizing radiation are teratogenesis, cognitive decline, and heart disease.

 

High radiation dose gives rise to Deterministic effects which reliably occur above a threshold, and their severity increases with dose. Deterministic effects are not necessarily more or less serious than stochastic effects; either can ultimately lead to a temporary nuisance or a fatality. Examples are: radiation burns, and/or rapid fatality through acute radiation syndrome, chronic radiation syndrome, and radiation-induced thyroiditis.

 

Beneficially, controlled doses are used for medical imaging and radiotherapy, and some scientists suspect that low doses may have a mild hormetic effect that can improve health, but the US National Academy of Sciences Biological Effects of Ionizing Radiation Committee "has concluded that there is no compelling evidence to indicate a dose threshold below which the risk of tumor induction is zero

 

When alpha particle emitting isotopes are ingested, they are far more dangerous than their half-life or decay rate would suggest. This is due to the high relative biological effectiveness of alpha radiation to cause biological damage after alpha-emitting radioisotopes enter living cells. Ingested alpha emitter radioisotopes such as transuranics or actinides are an average of about 20 times more dangerous, and in some experiments up to 1000 times more dangerous than an equivalent activity of beta emitting or gamma emitting radioisotopes.

 

·        Medical use of rad iation based on the fact that that it can dest roy cells (instrument sterilisation, treatm ent of cancer)

 

·        Medical use of radiation based on the fact that radiation is easy to detect

 

Types of ionizing radiation


 

Alpha (α) radiation cons ists of a fast-moving helium-4 (4He) nucleus and is stopped by a sheet of paper. Beta (β) radiation, consisting of electrons, is halted by an aluminium plate. Gamma (γ) radiation, consistingg of energetic photons, is eventually absorbed as it penetrates a dense material. Neutron (n) radiation consists of free neutrons that are blocked using light elements, like hydrogen, which slow and/or capture them. Not shown: gala ctic cosmic rays that consist of energetic charged nu clei like protons, helium nuclei, and high-charged nuclei called HZE ions.

 

Ionizing radiation is cate gorized by the nature of the particles or ele ctromagnetic waves creating the ionising effect. The se have different ionization mechanisms, an d may be grouped as directly or indirectly ionizing.

 

Directly ionizing

 

Any charged massive particle can ionize atoms directly by funddamental interaction through the Coulomb force if it carries sufficient kinetic energy. This includes atomic nuclei, electrons, muons, charged pions, protons, and energetic charged nuclei stripped of their electrons, all of which must be moving at relativistic speeds to reach the required kinetic energy. The first two to be recognized were given special names, which are used today: Helium nuclei at relativistic speeds are called alpha particles, and electrons at relativistic speeds are called beta particles. Natural cosmic rays are made up primarily of relativistic protons but also include heavier atomic nuclei like helium ions and HZE ions and muons. Charged pions are very short-lived and seen only in large amounts in particle accelerators.

 

Alpha particles

 

Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. They are generally produced in the process of alpha decay, but may

 

also be produced in other ways. Alpha particles are named after the first letter in the Greek alphabet, α. The symbol for the alpha particle is α or α2+. Because they are identical to helium

 

nuclei, theyarealsosometimeswrittenasHe2+or 42He2+indicating a Helium ion with a +2 charge (missing its two electrons). If the ion gains electrons from its environment, the alpha particle can be written as a normal (electrically neutral) Helium atom 42He.

 

They are a highly ionizing form of particle radiation, and when resulting from radioactive alpha decay have low penetration depth. They can be stopped by a few centimetres of air, or by the skin. However, so-called long range alpha particles from ternary fission are three times as energetic, and penetrate three times as far. The helium nuclei that form 10-12% of cosmic rays are also usually of much higher energy than those produced by nuclear decay processes, and are thus capable of being highly penetrating and able to traverse the human body and dense shielding, depending on their energy.

 

Beta particles

 

Beta particles are high-energy, high-speed electrons or positrons emitted by certain types

 

of radioactive nuclei, such as potassium-40. The production of beta particles is termed beta decay. They are designated by the Greek letter beta (β). There are two forms of beta decay, β

and β+, which respectively give rise to the electron and the positron.

 

High-energy beta particles may produce X-rays known as bremsstrahlung ("braking radiation") or secondary electrons (delta ray)as they pass through matter. Both of these can subsequently ionize as an indirect ionization effect.

 

Bremsstrahlung is of concern when shielding beta emitters, because interaction of beta particles with the shielding material produces Bremsstrahlung radiation. This effect is greater with material of high atomic numbers, so material with low atomic numbers is used for beta source shielding.

 

Positrons

 

The positron or ant electron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron. When a low-energy positron collides with a low-energy electron, annihilation occurs, resulting in the production of two or more gamma ray photons (see electron–positron annihilation).

 

Positrons may be generated by positron emission radioactive decay (through weak interactions), or by pair production from a sufficiently energetic photon. Positrons are common artificial sources of ionizing radiation in medical PET scans.As positrons are positiv ely charged particles they can also directly ionize an atom through Coulomb interactions.

 

Indirectly ionizing


 

 

Radiation interaction - gamma rays are represented by wavy lines, charged particles and neutrons by straight lines. The small circles show where ionization occ urs.Indirect ionizing radiation is electrically neutral a nd therefore does not interact strongly with matter. The bulk of the ionization effects are due to secondary ionizations.

 

3 main uses of ionising radiatio n in medicine:

 

         Treatment

 

         Diagnosis

 

         Sterilisation

 

Cancer

 

Cancers are growths of cells (cancerous tumours) which are out of co ntrol. As a result of this, they do not perform their intended function.

 

Treatment of Cancer

 

Cancerous tumours can be treated  using the following main methods:

 

         Chemotherapy (drugs).

 

             Radiation therapy (radi otherapy and brachytherapy).

 

Surgery.

 

Factors which affect the choice of treatment for cancer. The choice of treatment depends on a number of factors including:

 

         The size of the tumour.

 

         The position of the tumo ur.

 

The Aims of Radiation Therapy

 

The aim of radiation therapy is to cause damage to the cancerous cells whilst minimising the risk to surrounding healthy tissue

 

The damage inflicted by radiation therapy causes the cancerous cells to stop reproducing and thus the tumour shrinks. Unfortunately, healthy cells can also be damaged by the radiation. The amount of radiation given to the patient has to be accurately calculated so that the damage is limited to the cancerous cells only.

 

Radiation Therapy

 

Radiation therapy uses ionising radiation to treat cancer i.e. to destroy cancerous cells. There are two techniques in radiation therapy that are used to treat cancer using ionising radiation:

 

Diagnosis

 

Static 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

 

Dynamic Imaging

 

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

 

         Dynamic studies are performed on the kidneys and heart.

 

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

         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).

 

Sterilisation

 

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

 

         Nowadays, medical instruments (e.g. syringes) are prepacked and then irradiation using an intense gamma ray source.

 

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

 

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