Radioisopes

Wilhelm Conrad Roentgen

(1845-1923)

On November 8, 1895, at the University of Wurzburg….

Glowing fluorescent screen on a nearby table.

fluorescence was caused by invisible rays originating from the partially evacuated glass Hittorf-Crookes tube he was using to study cathode rays (i.e., electrons).

Surprisingly, these mysterious rays penetrated the opaque black paper wrapped around the tube. Roentgen had discovered X rays.

However, prior to his first formal correspondence to the University Physical-Medical Society, Roentgen spent two months thoroughly investigating the properties of X rays. Silvanus Thompson complained that Roentgen left "little for others to do beyond elaborating his work."

For his discovery, Roentgen received the first Nobel Prize in physics in 1901.

As a student in Holland, Roentgen was expelled from the Utrecht Technical School for a prank committed by another student.

Even after receiving a doctorate, his lack of a diploma initially prevented him from obtaining a position at the University of Wurzburg. He even was accused of having stolen the discovery of X rays by those who failed to observe them.

He rejected a title (i.e., von Roentgen) that would have provided entry into the German nobility.

Donated the money he received from the Nobel Prize to his University.

Roentgen did accept the honorary degree of Doctor of Medicine offered to him by the medical faculty of his own University of Wurzburg.

At the time of his death, Roentgen was nearly bankrupt from the inflation that followed World War I.

Antoine Henri Becquerel

(1852-1908)

Henri Becquerel was born into a family of scientists. His grandfather had made important contributions in the field of electrochemistry while his father had investigated the phenomena of fluorescence and phosphorescence.

Becquerel not only inherited their interest in science, he also inherited the minerals and compounds studied by his father.

And so, upon learning how Wilhelm Roentgen discovered X rays from the fluorescence they produced, Becquerel had a ready source of fluorescent materials with which to pursue his own investigations of these mysterious rays.

The material Becquerel chose to work with was potassium uranyl sulfate, K2UO2(SO4)2, which he exposed to sunlight and placed on photographic plates wrapped in black paper.

When developed, the plates revealed an image of the uranium crystals. Becquerel concluded "that the phosphorescent substance in question emits radiation which penetrates paper opaque to light." Initially he believed that the sun's energy was being absorbed by the uranium which then emitted X rays.

Further investigation, on the 26th and 27th of February, was delayed because the skies over Paris were overcast and the uranium-covered plates Becquerel intended to expose to the sun were returned to a drawer.

On the first of March, he developed the photographic plates expecting only faint images to appear. To his surprise, the images were clear and strong.

This meant that the uranium emitted radiation without an external source of energy such as the sun.

Becquerel had discovered radioactivity, the spontaneous emission of radiation by a material.

For his discovery of radioactivity,

… Becquerel was awarded the 1903

Nobel Prize for physics.

Pierre Curie (1859-1906)

Marie Curie (1867-1934)

In 1880, he and his brother Jacques had discovered piezoelectricity whereby physical pressure applied to a crystal resulted in the creation of an electric potential.

He also had made important investigations into the phenomenon of magnetism including the identification of a temperature, the curie point, above which a material's magnetic properties disappear.

After marriage with Curie…

Together, they began investigating the phenomenon of radioactivity recently discovered in uranium ore.

Although the phenomenon was discovered by Henri Becquerel, the term radioactivity was coined by Marie.

After chemical extraction of uranium from the ore, Marie noted the residual material to be more "active" than the pure uranium.

She concluded that the ore contained, in addition to uranium, new elements that were also radioactive. This led to their discoveries of the elements of polonium and radium.

For their work on radioactivity, the Curies were awarded the 1903 Nobel Prize in physics.

Tragically, Pierre was killed three years later in an accident while crossing a street in a rainstorm.

Pierre's teaching position at the Sorbonne was given to Marie. Never before had a woman taught there in its 650 year history!

Her first lecture began with the very sentence her husband had used to finish his last.

In his honor, the 1910 Radiology Congress chose the curie as the basic unit of radioactivity: the quantity of radon in equilibrium with one gram of radium (current definition: 1 Ci = 3.7x1010 dps).

A year later, Marie was awarded the Nobel Prize in chemistry for her discoveries of radium and polonium, thus becoming the first person to receive two Nobel Prizes.

For the remainder of her life she tirelessly investigated and promoted the use if radium as a treatment for cancer.

Marie Curie died July 4, 1934, overtaken by pernicious anemia no doubt caused by years of overwork and radiation exposure.

Ernest Rutherford

(1871-1937)

Father of nuclear physics.

Particles named and characterized by him include the alpha particle, beta particle and proton.

Even the neutron, discovered by James Chadwick, owes its name to Rutherford. The exponential equation used to calculate the decay of radioactive substances was first employed for that purpose by Rutherford and he was the first to elucidate the related concepts of the half-life and decay constant.

Rutherford won the 1908 Nobel Prize in chemistry.

In 1909, now at the University of Manchester, Rutherford was bombarding a thin gold foil with alpha particles when he noticed that although almost all of them went through the gold, one in eight thousand would "bounce" (i.e., scatter) back.

From this simple observation, Rutherford concluded that the atom's mass must be concentrated in a small positively-charged nucleus while the electrons inhabit the farthest reaches of the atom.

In 1919, Rutherford returned to Cambridge to become director of the Cavendish laboratory where he had previously done his graduate work under J.J. Thomson. It was here that he made his final major achievement, the artificial alteration of nuclear and atomic structure.

After his death in 1937, Rutherford's remains were buried in Westminster Abbey near those of Sir Isaac Newton.

By bombarding nitrogen with alpha particles, Rutherford demonstrated the production of a different element, oxygen.

"Playing with marbles" is what he called; the newspapers reported that Rutherford had "split the atom."

The radioisotopes have numerous applications in medicine, agriculture, industry and pure research.

Many applications employ a special technique known as “ tracer technique”.

A small quantity of a radioisotope is introduced into the substance to be studied and its path is traced by means of a Geiger-Muller (G. M.) counter.

What are radioisotopes?

Many of the chemical elements have a number of isotopes.

The isotopes of an element have the same number of protons in their atoms (atomic number) but different masses due to different numbers of neutrons.

In an atom in the neutral state, the number of external electrons also equals the atomic number.

These electrons determine the chemistry of the atom.

The atomic mass is the sum of the protons and neutrons. There are 82 stable elements and about 275 stable isotopes of these elements.

When a combination of neutrons and protons, which does not already exist in nature, is produced artificially, the atom will be unstable and is called a radioactive isotope or radioisotope.

There are also a number of unstable natural isotopes arising from the decay of primordial uranium and thorium. Overall there are some 1800 radioisotopes.

Radioisotopes in Medicine

NUCLEAR MEDICINE

This is a branch of medicine that uses radiation to provide information about the functioning of a person's specific organs or to treat disease. The thyroid, bones, heart, liver and many other organs can be easily imaged, and disorders in their function revealed. In some cases radiation can be used to treat diseased organs, or tumours.

Diagnostic techniques in nuclear medicine use radioactive tracers which emit gamma rays from within the body.

They can be given by injection, inhalation or orally.

The first type are where single photons are detected by a gamma camera which can view organs from many different angles.

The camera builds up an image from the points from which radiation is emitted; this image is enhanced by a computer and viewed by a physician on a monitor for indications of abnormal conditions.

In a similar way, the passage of a particular element in the body and the rate at which it accumulates in different organs can be studied.

A positron-emitting radionuclide is introduced, usually by injection, and accumulates in the target tissue. As it decays it emits a positron, which promptly combines with a nearby electron resulting in the simultaneous emission of two identifiable gamma rays in opposite directions.

These are detected by a PET camera and give very precise indication of their origin.

PET's most important clinical role is in oncology, with fluorine-18 as the tracer, since it has proven to be the most accurate non-invasive method of detecting and evaluating most cancers. It is also well used in cardiac and brain imaging.

Positron emission tomography (PET) is nuclear medicine imaging technique that produces a three-dimensional image or picture of functional processes in the body.

The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule.

Three-dimensional images of tracer concentration within the body are then constructed by computer analysis.

In modern scanners, three dimensional imaging is often accomplished with the aid of a CT X-ray scan performed on the patient during the same session, in the same machine.

Gamma imaging by either method described provides a view of the position and concentration of the radioisotope within the body. Organ malfunction can be indicated if the isotope is either partially taken up in the organ (cold spot), or taken up in excess (hot spot). If a series of images is taken over a period of time, an unusual pattern or rate of isotope movement could indicate malfunction in the organ.

A distinct advantage of nuclear imaging over x-ray techniques is that both bone and soft tissue can be imaged very successfully. This has led to its common use in developed countries where the probability of anyone having such a test is about one in two and rising.

RADIOTHERAPYRapidly dividing cells are particularly sensitive to damage by radiation. For this reason, some cancerous growths can be controlled or eliminated by irradiating the area containing the growth.

External irradiation can be carried out using a gamma beam from a radioactive cobalt-60 source, though in developed countries the much more versatile linear accelerators are now being utilised as a high-energy x-ray source (gamma and x-rays are much the same).

Internal radiotherapy is by administering or planting a small radiation source, usually a gamma or beta emitter, in the target area.

Iodine-131 is commonly used to treat thyroid cancer, probably the most successful kind of cancer treatment. It is also used to treat non-malignant thyroid disorders. Iridium-192 implants are used especially in the head and breast.

They are produced in wire form and are introduced through a catheter to the target area. After administering the correct dose, the implant wire is removed to shielded storage. This brachytherapy (short-range) procedure gives less overall radiation to the body, is more localised to the target tumour and is cost effective.

Targeted Alpha Therapy (TAT): especially for the control of dispersed cancers.

The short range of very energetic alpha emissions in tissue means that a large fraction of that radiative energy goes into the targeted cancer cells, once a carrier has taken the alpha-emitting radionuclide to exactly the right place. Laboratory studies are encouraging and clinical trials for leukaemia, cystic glioma and melanoma are under way.

An experimental development of this is Boron Neutron Capture Therapy using boron-10 which concentrates in malignant brain tumours.

The patient is then irradiated with thermal neutrons which are strongly absorbed by the boron, producing high-energy alpha particles which kill the cancer. This requires the patient to be brought to a nuclear reactor, rather than the radioisotopes being taken to the patient.

BIOCHEMICAL ANALYSIS

It is very easy to detect the presence or absence of some radioactive materials even when they exist in very low concentrations. Radioisotopes can therefore be used to label molecules of biological samples in vitro (out of the body). Pathologists have devised hundreds of tests to determine the constituents of blood, serum, urine, hormones, antigens and many drugs by means of associated radioisotopes. These procedures are known as radioimmuno assays and, although the biochemistry is complex, kits manufactured for laboratory use are very easy to use and give accurate results.

DIAGNOSTIC RADIOPHARMACEUTICALS:

Every organ in our bodies acts differently from a chemical point of view. Doctors and chemists have identified a number of chemicals which are absorbed by specific organs.

The thyroid, for example, takes up iodine, the brain consumes quantities of glucose, and so on.

With this knowledge, radiopharmacists are able to attach various radioisotopes to biologically active substances.

Once a radioactive form of one of these substances enters the body, it is incorporated into the normal biological processes and excreted in the usual ways.

Diagnostic radiopharmaceuticals can be used to examine blood flow to the brain, functioning of the liver, lungs, heart or kidneys, to assess bone growth, and to confirm other diagnostic procedures.

Another important use is to predict the effects of surgery and assess changes since treatment.

A radioisotope used for diagnosis must emit gamma rays of sufficient energy to escape from the body and it must have a half-life short enough for it to decay away soon after imaging is completed.

The radioisotope most widely used in medicine is technetium-99m, employed in some 80% of all nuclear medicine procedures.

Myocardial Perfusion Imaging (MPI) uses thallium-201 chloride or technetium-99m and is important for detection and prognosis of coronary artery disease.

For PET imaging, the main radiopharmaceutical is Fluoro-deoxy glucose (FDG) incorporating F-18 - with a half-life of just under two hours, as a tracer. The FDG is readily incorporated into the cell without being broken down, and is a good indicator of cell metabolism.

THERAPEUTIC RADIOPHARMACEUTICALS:

For some medical conditions, it is useful to destroy or weaken malfunctioning cells using radiation. The radioisotope that generates the radiation can be localised in the required organ in the same way it is used for diagnosis - through a radioactive element following its usual biological path, or through the element being attached to a suitable biological compound.

In most cases, it is beta radiation which causes the destruction of the damaged cells. This is radiotherapy.

Short-range radiotherapy is known as brachytherapy.