21.5: Los usos de los radioisótopos

Habilidades para desarrollar

• Liste las aplicaciones comunes de los isótopos radiactivos

Los isótopos radiactivos tienen las mismas propiedades químicas que los isótopos estables del mismo elemento, pero emiten radiación, que se puede detectar. Si reemplazamos uno (o más) átomo(s) con radioisótopos en un compuesto, podemos rastrearlos monitoreando sus emisiones radioactivas. Este tipo de compuesto se llama el marcador radiactivo. Los radioisótopos se usan para seguir los caminos de las reacciones bioquímicas o para determinar cómo se distribuye una sustancia dentro de un organismo. Los trazadores radiactivos también se usan en muchas aplicaciones médicas, incluidos el diagnóstico y el tratamiento. Se usan para medir el desgaste del motor, analizar la formación geológica alrededor de los pozos de petróleo y mucho más.

Los radioisótopos han revolucionado la práctica médica, donde se usan extensivamente. En los Estados Unidos se realizan anualmente más de 10 millones de procedimientos de medicina nuclear y más de 100 millones de pruebas de medicina nuclear. Cuatro ejemplos típicos de trazadores radiactivos utilizados en la medicina son tecnecio-99 \(\ce{(^{99}_{43}Tc)}\), talio-201 \(\ce{(^{201}_{81}Tl)}\), yodo-131 \(\ce{(^{131}_{53}I)}\) y sodio-24 \(\ce{(^{24}_{11}Na)}\). Los tejidos dañados en el corazón, el hígado y los pulmones absorben preferentemente ciertos compuestos de tecnecio-99. Cuando se inyecta, se puede determinar la ubicación del compuesto de tecnecio y, por tanto, el tejido dañado, detectando los rayos γ emitidos por el isótopo Tc-99. El talio-201 (Figura \(\PageIndex{1}\)) se concentra en el tejido cardíaco sano, por eso los dos isótopos, Tc-99 y Tl-201, se usan juntos para estudiar el tejido cardíaco. El yodo-131 se concentra en la glándula tiroides, el hígado y algunas partes del cerebro. Por eso, se puede usar para controlar el bocio y tratar afecciones de la tiroides, como la enfermedad de Grave, así como los tumores hepáticos y cerebrales. Las soluciones salinas que contienen compuestos de sodio-24 se inyectan en el torrente sanguíneo para ayudar a localizar las obstrucciones en el flujo sanguíneo.

Figura \(\PageIndex{1}\): Administering thallium-201 to a patient and subsequently performing a stress test offer medical professionals an opportunity to visually analyze heart function and blood flow. (credit: modification of work by “Blue0ctane”/Wikimedia Commons)

Radioisotopes used in medicine typically have short half-lives—for example, the ubiquitous Tc-99m has a half-life of 6.01 hours. This makes Tc-99m essentially impossible to store and prohibitively expensive to transport, so it is made on-site instead. Hospitals and other medical facilities use Mo-99 (which is primarily extracted from U-235 fission products) to generate Tc-99. Mo-99 undergoes β decay with a half-life of 66 hours, and the Tc-99 is then chemically extracted (Figure \(\PageIndex{2}\)). The parent nuclide Mo-99 is part of a molybdate ion, \(\ce{MoO4^2-}\); when it decays, it forms the pertechnetate ion, \(\ce{TcO4-}\). These two water-soluble ions are separated by column chromatography, with the higher charge molybdate ion adsorbing onto the alumina in the column, and the lower charge pertechnetate ion passing through the column in the solution. A few micrograms of Mo-99 can produce enough Tc-99 to perform as many as 10,000 tests.

Figura \(\PageIndex{2}\): (a) The first Tc-99m generator (circa 1958) is used to separate Tc-99 from Mo-99. The \(\ce{MoO4^2-}\) is retained by the matrix in the column, whereas the \(\ce{TcO4-}\) passes through and is collected. (b) Tc-99 was used in this scan of the neck of a patient with Grave’s disease. The scan shows the location of high concentrations of Tc-99. (credit a: modification of work by the Department of Energy; credit b: modification of work by “MBq”/Wikimedia Commons)

Radioisotopes can also be used, typically in higher doses than as a tracer, as treatment. Radiation therapy is the use of high-energy radiation to damage the DNA of cancer cells, which kills them or keeps them from dividing (Figure \(\PageIndex{3}\)). A cancer patient may receive external beam radiation therapy delivered by a machine outside the body, or internal radiation therapy (brachytherapy) from a radioactive substance that has been introduced into the body. Note that chemotherapy is similar to internal radiation therapy in that the cancer treatment is injected into the body, but differs in that chemotherapy uses chemical rather than radioactive substances to kill the cancer cells.

Figura \(\PageIndex{3}\): The cartoon in (a) shows a cobalt-60 machine used in the treatment of cancer. The diagram in (b) shows how the gantry of the Co-60 machine swings through an arc, focusing radiation on the targeted region (tumor) and minimizing the amount of radiation that passes through nearby regions.

Cobalt-60 is a synthetic radioisotope produced by the neutron activation of Co-59, which then undergoes β decay to form Ni-60, along with the emission of γ radiation. The overall process is:

\[\ce{^{59}_{27}Co + ^1_0n⟶ ^{60}_{27}Co⟶ ^{60}_{28}Ni + ^0_{−1}β + 2^0_0γ}\]

The overall decay scheme for this is shown graphically in Figure \(\PageIndex{4}\).

Figura \(\PageIndex{4}\): Co-60 undergoes a series of radioactive decays. The γ emissions are used for radiation therapy.

Radioisotopes are used in diverse ways to study the mechanisms of chemical reactions in plants and animals. These include labeling fertilizers in studies of nutrient uptake by plants and crop growth, investigations of digestive and milk-producing processes in cows, and studies on the growth and metabolism of animals and plants.

For example, the radioisotope C-14 was used to elucidate the details of how photosynthesis occurs. The overall reaction is:

\[\ce{6CO2}(g)+\ce{6H2O}(l)⟶\ce{C6H12O6}(s)+\ce{6O2}(g),\]

but the process is much more complex, proceeding through a series of steps in which various organic compounds are produced. In studies of the pathway of this reaction, plants were exposed to CO₂ containing a high concentration of \(\ce{^{14}_6C}\). At regular intervals, the plants were analyzed to determine which organic compounds contained carbon-14 and how much of each compound was present. From the time sequence in which the compounds appeared and the amount of each present at given time intervals, scientists learned more about the pathway of the reaction.

Commercial applications of radioactive materials are equally diverse (Figure \(\PageIndex{5}\)). They include determining the thickness of films and thin metal sheets by exploiting the penetration power of various types of radiation. Flaws in metals used for structural purposes can be detected using high-energy gamma rays from cobalt-60 in a fashion similar to the way X-rays are used to examine the human body. In one form of pest control, flies are controlled by sterilizing male flies with γ radiation so that females breeding with them do not produce offspring. Many foods are preserved by radiation that kills microorganisms that cause the foods to spoil.

Figura \(\PageIndex{5}\): Common commercial uses of radiation include (a) X-ray examination of luggage at an airport and (b) preservation of food. (credit a: modification of work by the Department of the Navy; credit b: modification of work by the US Department of Agriculture)

Americium-241, an α emitter with a half-life of 458 years, is used in tiny amounts in ionization-type smoke detectors (Figure \(\PageIndex{6}\)). The α emissions from Am-241 ionize the air between two electrode plates in the ionizing chamber. A battery supplies a potential that causes movement of the ions, thus creating a small electric current. When smoke enters the chamber, the movement of the ions is impeded, reducing the conductivity of the air. This causes a marked drop in the current, triggering an alarm.

Figura \(\PageIndex{6}\): Inside a smoke detector, Am-241 emits α particles that ionize the air, creating a small electric current. During a fire, smoke particles impede the flow of ions, reducing the current and triggering an alarm. (credit a: modification of work by “Muffet”/Wikimedia Commons)

Resumen

Compounds known as radioactive tracers can be used to follow reactions, track the distribution of a substance, diagnose and treat medical conditions, and much more. Other radioactive substances are helpful for controlling pests, visualizing structures, providing fire warnings, and for many other applications. Hundreds of millions of nuclear medicine tests and procedures, using a wide variety of radioisotopes with relatively short half-lives, are performed every year in the US. Most of these radioisotopes have relatively short half-lives; some are short enough that the radioisotope must be made on-site at medical facilities. Radiation therapy uses high-energy radiation to kill cancer cells by damaging their DNA. The radiation used for this treatment may be delivered externally or internally.

Glosario

chemotherapy

similar to internal radiation therapy, but chemical rather than radioactive substances are introduced into the body to kill cancer cells

external beam radiation therapy

radiation delivered by a machine outside the body

internal radiation therapy

(also, brachytherapy) radiation from a radioactive substance introduced into the body to kill cancer cells

radiation therapy

use of high-energy radiation to damage the DNA of cancer cells, which kills them or keeps them from dividing

radioactive tracer

(also, radioactive label) radioisotope used to track or follow a substance by monitoring its radioactive emissions

Contribuyentes y atribuciones

• Paul Flowers (Universidad de Carolina del Norte - Pembroke), Klaus Theopold (Universidad de Delaware) y Richard Langley (Stephen F. Austin Universidad del Estado) con autores contribuyentes. Contenido del libro de texto producido por la Universidad de OpenStax tiene licencia de Atribución de Creative Commons Licencia 4.0 licencia. Descarge gratis en http://cnx.org/contents/85abf193-2bd...a7ac8df6@9.110)."

• Ana Martinez ([e-mail]) contribuyó a la traducción de este texto.