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5 Focus on Health & Medicine: Medical Uses of Radioisotopes

5 Focus on Health & Medicine: Medical Uses of Radioisotopes

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312







NUCLEAR CHEMISTRY



FIGURE 10.4



HIDA Scan Using Technetium-99m



a.



b.



liver



bile duct



liver

gall bladder

bile ducts

stomach



gall bladder



a. Schematic showing the location of the liver, gall bladder, and bile ducts

b. A scan using technetium-99m showing bright areas for the liver, gall bladder, and bile ducts, indicating normal function



technetium-99m travels through the bloodstream and into the liver, gall bladder, and bile ducts,

where, in a healthy individual, the organs are all clearly visible on a scan. When the gall bladder

is inflamed or the bile ducts are obstructed by gallstones, uptake of the radioisotope does not

occur and these organs are not visualized because they do not contain the radioisotope.

Red blood cells tagged with technetium-99m are used to identify the site of internal bleeding in

an individual. Bone scans performed with technetium-99m can show the location of metastatic

cancer, so that specific sites can be targeted for radiation therapy (Figure 10.5).

Thallium-201 is used in stress tests to diagnose coronary artery disease. Thallium injected into

a vein crosses cell membranes into normal heart muscle. Little radioactive thallium is found in

areas of the heart that have a poor blood supply. This technique is used to identify individuals

who may need bypass surgery or other interventions because of blocked coronary arteries.



PROBLEM 10.19



The half-life of thallium-201 is three days. What fraction of thallium-201 is still present in an

individual after nine days?



10.5B



RADIOISOTOPES USED IN TREATMENT



The high-energy radiation emitted by radioisotopes can be used to kill rapidly dividing tumor cells.

Two techniques are used. Sometimes the radiation source is external to the body. For example, a

beam of radiation produced by decaying cobalt-60 can be focused at a tumor. Such a radiation source

must have a much longer half-life—5.3 years in this case—than radioisotopes that are ingested for

diagnostic purposes. With this method some destruction of healthy tissue often occurs, and a patient

may experience some signs of radiation sickness, including vomiting, fatigue, and hair loss.

A more selective approach to cancer treatment involves using a radioactive isotope internally at

the site of the tumor within the body. Using iodine-131 to treat hyperthyroidism has already been

discussed (Section 10.1). Other examples include using radioactive “seeds” or wire that can be

implanted close to a tumor. Iodine-125 seeds are used to treat prostate cancer and iridium-192

wire is used to treat some cancers of the breast.

Figure 10.6 illustrates radioisotopes that are used for diagnosis or treatment.



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FOCUS ON HEALTH & MEDICINE: MEDICAL USES OF RADIOISOTOPES







313



FIGURE 10.5 Bone Scan Using Technetium-99m

b.



a.



kidneys



bladder



The bone scan of a patient whose lung cancer has spread to other organs. The anterior view

[from the front in (a)] shows the spread of disease to the ribs, while the posterior view [from the

back in (b)] shows spread of disease to the ribs and spine. The bright areas in the mid-torso

and lower pelvis are due to a collection of radioisotope in the kidneys and bladder, before it is

eliminated in the urine.







FIGURE 10.6 Common Radioisotopes Used in Medicine



Xenon-133

lung function



Technetium-99m

bone scan



Technetium-99m

gall bladder function



Technetium-99m

visualizing gastrointestinal

bleeding



Iodine-131

hyperthyroidism

and thyroid tumors



Phosphorus-32

treating leukemia

and lymphomas



Iridium-192

cancers of

the breast



Thallium-201

heart function



10.5C POSITRON EMISSION TOMOGRAPHY—PET SCANS

Positron emission tomography (PET) scans use radioisotopes that emit positrons when the

nucleus decays. Once formed, a positron combines with an electron to form two γ rays, which

create a scan of an organ.



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314



NUCLEAR CHEMISTRY







a.



FIGURE 10.7 PET Scans

b.



c.



kidneys

bladder



a. The PET scan shows cancer of the lymph nodes in the neck and abdomen, as well as

scattered areas of tumor in the bone marrow of the arms and spine before treatment.

b. The schematic of selected organs in the torso and pelvis.

c. The PET scan shows significant clearing of disease after chemotherapy by the decrease in

intensity of the radioisotope. The dark regions in the kidneys (in the torso) and bladder (in the

lower pelvis) are due to the concentration of the radioisotope before elimination in the urine.



0

+1e



+



0

–1e



positron electron





gamma rays



Carbon-11, oxygen-15, nitrogen-13, and fluorine-18 are common radioactive isotopes used in

PET scans. For example, a carbon-11 or fluorine-18 isotope can be incorporated in a glucose

molecule. When this radioactive molecule is taken internally, its concentration becomes highest

in areas in the body that continually use glucose. A healthy brain shows a high level of radioactivity from labeled glucose. When an individual suffers a stroke or has Alzheimer’s disease, brain

activity is significantly decreased and radioactivity levels are decreased.

PET scans are also used to detect tumors and coronary artery disease, and determine whether

cancer has spread to other organs of the body. A PET scan is also a noninvasive method of monitoring whether cancer treatment has been successful (Figure 10.7).



PROBLEM 10.20



Write a nuclear equation for the emission of a positron from nitrogen-13.



10.6 NUCLEAR FISSION AND NUCLEAR FUSION

The nuclear reactions used in nuclear power plants occur by a process called nuclear fission,

whereas the nuclear reactions that take place in the sun occur by a process called nuclear

fusion.

• Nuclear fission is the splitting apart of a heavy nucleus into lighter nuclei and neutrons.

• Nuclear fusion is the joining together of two light nuclei to form a larger nucleus.



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NUCLEAR FISSION AND NUCLEAR FUSION



315



10.6A



NUCLEAR FISSION



When uranium-235 is bombarded by a neutron, it undergoes nuclear fission and splits apart into

two lighter nuclei. Several different fission products have been identified. One common nuclear

reaction is the fission of uranium-235 into krypton-91 and barium-142.

+



235

92U



n



1

0n



+



91

36Kr



+



142

56Ba



1



3 0n



p

Each neutron can react

with more uranium-235.

+



+



+



More fission products and

more neutrons are formed.



Three high-energy neutrons are also produced in the reaction as well as a great deal of energy.

Whereas burning 1 g of methane in natural gas releases 13 kcal of energy, fission of 1 g of

uranium-235 releases 3.4 × 108 kcal. Each neutron produced during fission can go on to bombard

three other uranium-235 nuclei to produce more nuclei and more neutrons. Such a process is

called a chain reaction.

In order to sustain a chain reaction there must be a sufficient amount of uranium-235. When that

amount—the critical mass—is present, the chain reaction occurs over and over again and an

atomic explosion occurs. When less than the critical mass of uranium-235 is present, there is a

more controlled production of energy, as is the case in a nuclear power plant.

A nuclear power plant utilizes the tremendous amount of energy produced by fission of the

uranium-235 nucleus to heat water to steam, which powers a generator to produce electricity

(Figure 10.8). While nuclear energy accounts for a small but significant fraction of the electricity

needs in the United States, most of the electricity generated in some European countries comes

from nuclear power.





FIGURE 10.8



a.



A Nuclear Power Plant



b.



electricity

steam generator



containment

building



turbine



generator



cooling

tower



reactor

core



coolant

pump



cooling loop

condenser



primary loop



secondary loop



water supply pump



a. Nuclear power plant with steam rising from a cooling tower

b. Fission occurs in a nuclear reactor core that is housed in a containment facility. Water surrounding the reactor is heated by the energy

released during fission, and this energy drives a turbine, which produces electricity. Once the steam has been used to drive the turbine,

it is cooled and re-circulated around the core of the reactor. To prevent the loss of any radioactive material to the environment, the water

that surrounds the reactor core never leaves the containment building.



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316



NUCLEAR CHEMISTRY



Two problems that surround nuclear power generation are the possibility of radiation leaks and

the disposal of nuclear waste. Plants are designed and monitored to contain the radioactive materials within the nuclear reactor. The reactor core itself is located in a containment facility with

thick walls, so that should a leak occur, the radiation should in principle be kept within the building. The nuclear reactor in Chernobyl, Russia, was built without a containment facility and in

1986 it exploded, releasing high levels of radioactivity to the immediate environment and sending

a cloud of reactivity over much of Europe.

The products of nuclear fission are radioactive nuclei with long half-lives, often hundreds or even

thousands of years. As a result, nuclear fission generates radioactive waste that must be stored in

a secure facility so that it does not pose a hazard to the immediate surroundings. Burying waste

far underground is currently considered the best option, but this issue is still unresolved.



PROBLEM 10.21



Write a nuclear equation for each process.

a. Fission of uranium-235 by neutron bombardment forms strontium-90, an isotope of xenon,

and three neutrons.

b. Fission of uranium-235 by neutron bombardment forms antimony-133, three neutrons, and

one other isotope.



10.6B



NUCLEAR FUSION



Nuclear fusion occurs when two light nuclei join together to form a larger nucleus. For example,

fusion of a deuterium nucleus with a tritium nucleus forms helium and a neutron. Recall from

Section 2.3 that deuterium is an isotope of hydrogen that contains one proton and one neutron in

its nucleus, while tritium is an isotope of hydrogen that contains one proton and two neutrons in its

nucleus.

+



2

1H



3

1H



4

2He



+



1

0n



n

p

+



deuterium



+



tritium



helium



Like fission, fusion also releases a great deal of energy—namely, 5.3 × 108 kcal/mol of helium

produced. The light and heat of the sun and other stars result from nuclear fusion.

One limitation of using fusion to provide energy for mankind is the extreme experimental conditions needed to produce it. Because it takes a considerable amount of energy to overcome the

repulsive forces of the like charges of two nuclei, fusion can only be accomplished at high temperatures (greater than 100,000,000 °C) and pressures (greater than 100,000 atm). Since these

conditions are not easily achieved, using controlled nuclear fusion as an energy source has yet to

become a reality.

Controlled nuclear fusion has the potential of providing cheap and clean power. It is not plagued

by the nuclear waste issues of fission reactors, and the needed reactants are readily available.



PROBLEM 10.22



Nuclear fusion in the stars occurs by a series of reactions. Identify X, Y, and Z in the following

nuclear reactions that ultimately convert hydrogen into helium.

a. 11H + X

b. 11H + 21H

c. 11H + 32He



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2

1H



+



0

+1e



Y

4

2He



+ Z



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FOCUS ON HEALTH & MEDICINE: MEDICAL IMAGING WITHOUT RADIOACTIVITY



317



10.7 FOCUS ON HEALTH & MEDICINE

MEDICAL IMAGING WITHOUT RADIOACTIVITY

X-rays, CT scans, and MRIs are also techniques that provide an image of an organ or extremity

that is used for diagnosis of a medical condition. Unlike PET scans and other procedures discussed thus far, however, these procedures are not based on nuclear reactions and they do not

utilize radioactivity. In each technique, an energy source is directed towards a specific region in

the body, and a scan is produced that is analyzed by a trained medical professional.

X-rays are a high-energy type of radiation called electromagnetic radiation. Tissues of different density interact differently with an X-ray beam, and so a map of bone and internal organs is

created on an X-ray film. Dense bone is clearly visible in an X-ray, making it a good diagnostic

technique for finding fractures (Figure 10.9a). Although X-rays are a form of high-energy radiation, they are lower in energy than the γ rays produced in nuclear reactions. Nonetheless, X-rays

still cause adverse biological effects on the cells with which they come in contact, and the exposure of both the patient and X-ray technician must be limited.

CT (computed tomography) scans, which also use X-rays, provide high resolution images of “slices”

of the body. Historically, CT images have shown a slice of tissue perpendicular to the long axis of

the body. Modern CT scanners can now provide a three-dimensional view of the body’s organs. CT

scans of the head are used to diagnose bleeding and tumors in the brain (Figure 10.9b).

MRI (magnetic resonance imaging) uses low-energy radio waves to visualize internal organs. Unlike

methods that use high-energy radiation, MRIs do not damage cells. An MRI is a good diagnostic

method for visualizing soft tissue (Figure 10.9c), and thus it complements X-ray techniques.





a.



FIGURE 10.9



Imaging the Human Body

b.



c.



herniated

disc



a. X-ray of a broken humerus in a patient’s arm

b. A color-enhanced CT scan of the head showing the site of a stroke

c. MRI of the spinal cord showing spinal compression from a herniated disc



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318



NUCLEAR CHEMISTRY



CHAPTER HIGHLIGHTS

KEY TERMS

Alpha (α) particle (10.1)

Becquerel (10.4)

Beta (β) particle (10.1)

Chain reaction (10.6)

Critical mass (10.6)

Curie (10.4)

Gamma (γ) ray (10.1)

Geiger counter (10.4)



Gray (10.4)

Half-life (10.3)

LD50 (10.4)

Nuclear fission (10.6)

Nuclear fusion (10.6)

Nuclear reaction (10.1)

Positron (10.1)

Rad (10.4)



Radioactive decay (10.2)

Radioactive isotope (10.1)

Radioactivity (10.1)

Radiocarbon dating (10.3)

Rem (10.4)

Sievert (10.4)

X-ray (10.7)



KEY CONCEPTS

❶ Describe the different types of radiation emitted by a

radioactive nucleus. (10.1)

• A radioactive nucleus can emit α particles, β particles,

positrons, or γ rays.

• An α particle is a high-energy nucleus that contains two

protons and two neutrons.

• A β particle is a high-energy electron.

• A positron is an antiparticle of a β particle. A positron has a

+1 charge and negligible mass.

• A γ ray is high-energy radiation with no mass or charge.

❷ How are equations for nuclear reactions written? (10.2)

• In an equation for a nuclear reaction, the sum of the mass

numbers (A

(A) must be equal on both sides of the equation.

The sum of the atomic numbers (Z

(Z) must be equal on both

sides of the equation as well.

❸ What is the half-life of a radioactive isotope? (10.3)

• The half-life (t

(t1/2) is the time it takes for one-half of a

radioactive sample to decay. Knowing the half-life and the

amount of a radioactive substance, one can calculate how

much sample remains after a period of time.

• The half-life of radioactive C-14 can be used to date

archaeological artifacts.

❹ What units are used to measure radioactivity? (10.4)

• Radiation in a sample is measured by the number of

disintegrations per second, most often using the curie (Ci);

1 Ci = 3.7 × 1010 disintegrations/s. The becquerel (Bq) is

also used; 1 Bq = 1 disintegration/s; 1 Ci = 3.7 ì 1010 Bq.

The exposure of a substance to radioactivity is measured

with the rad (radiation absorbed dose) or the rem (radiation

equivalent for man).



smi26573_ch10.indd 318



❺ Give examples of common radioisotopes used in medicine.

(10.5)

• Iodine-131 is used to diagnose and treat thyroid disease.

• Technetium-99m is used to evaluate the functioning of the

gall bladder and bile ducts, and in bone scans to evaluate

the spread of cancer.

• Red blood cells tagged with technetium-99m are used to

find the site of a gastrointestinal bleed.

• Thallium-201 is used to diagnose coronary artery disease.

• Cobalt-60 is used as an external source of radiation for

cancer treatment.

• Iodine-125 and iridium-192 are used in internal radiation

treatment of prostate cancer and breast cancer, respectively.

• Carbon-11, oxygen-15, nitrogen-13, and fluorine-18 are

used in positron emission tomography.

❻ What are nuclear fission and nuclear fusion? (10.6)

• Nuclear fission is the splitting apart of a heavy nucleus into

lighter nuclei and neutrons.

• Nuclear fusion is the joining together of two light nuclei to

form a larger nucleus.

• Both nuclear fission and nuclear fusion release a great deal

of energy. Nuclear fission is used in nuclear power plants to

generate electricity. Nuclear fusion occurs in stars.

❼ What medical imaging techniques do not use

radioactivity? (10.7)

• X-rays and CT scans both use X-rays, a high-energy form

of electromagnetic radiation.

• MRIs use low-energy radio waves to image soft tissue.



12/4/08 10:57:50 AM



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