Br-80 is produced in the 79Br(n, Y)80mBr!80Br nuclear reaction and in the decay of the metastable 80mBr. The half-life of the metastable 80mBr is 4.4 h, and this disin­tegrates to the 80Br isotope, the half-life of which is 18 min (isomeric transition). By irradiation of organic bromine compounds, the bond of bromine-80m can break as a result of the Szilard—Chalmers effect, producing inorganic bromine. When the inorganic bromine is separated immediately after the irradiation, it contains the ions of both radioactive isotopes, namely 80mBr and 80Br. After a few hours, only the product of the isomeric transition (80Br) can break out of the organic molecules, so it can be separated with high purity. The Br-80 isotope disintegrates with elec­tron capture and negative and positive beta decays.

Br-82 is produced in the 81Br(n, Y)82Br nuclear reaction. Its half-life is 35.9 h, and it emits (3_ and gamma radiation. For more information, see Section 8.7.1.1.

In a homogeneous system containing isotope molecules of the different substances, an isotope exchange may occur. The rate of the isotope exchange depends on the binding energies. Through kinetic studies of the isotope exchange, the binding energy of the isotope in a given substance can be determined. The reaction is directed by the increase of the entropy and reaches equilibrium when the distribu­tion of the isotopes becomes homogeneous (maximum mixing entropy). This means that in equilibrium, the specific activities will be the same for each substance.

The McKay equation can be used for the kinetic description of the isotope exchange reaction. As an example, the isotope exchange reaction of ethyl iodide and sodium iodide labeled with 131I is mentioned. AX and BX mean the ethyl

iodide and the sodium iodide, respectively. The labeled forms of these compounds are indicated by 0. The exchange reaction is defined as:

AX 1BX0 з AX01BX (9.44)

C2H511 Na131IoC2H513111 NaI (9.45)

At a given time (t), the specific activity of the two compounds can be defined by the following:

aA 5 JA and aB 5 (9:46)

[A] [B]

where [A0] is the activity of the labeled AX0 and [A] is the quantity of AX. Similarly, [B0] is the activity of the labeled BX0 and [B] is the quantity of BX. For the isotope exchange reaction of ethyl iodide and sodium iodide, [A0] and [B0] are the activities of 131I in the ethyl iodide and sodium iodide, respectively. As postu­lated by the definition of the specific activity, the mass of the iodine has to be taken into account. (NB: Similar equations can be written when the molar activities of the two compounds are expressed where the moles of AX and BX are substituted into Eq. (9.46).)

The total activity of the system is constant at any time (a closed system):

[A0] 1 [B0] 5 [AN] 1 [BN] (9.47)

where [AN] and [BN] are the equilibrium activities of AX0 and BX0, respectively. In equilibrium, the mixing entropy has maximum value, and the distribution of 131I is directed by the ratio of the quantity of AX and BX:

IAN 5 [A]

[BN] [B]

that is, both the ratio of the activities and the ratio of quantities are the same.

To be able to solve the kinetic equation of the exchange reaction (see Eq. (9.51)), only one parameter should be variable. This means that the change of the activity of only one of the reactants is to be taken into account. To eliminate the activity of the other reactant, some equivalent mathematical transformations are necessary. At first, [BN ] is expressed from Eq. (9.48):

(9.49)

Let us substitute [BN] into Eq. (9.47) and then express [B0]:

0 1 [AN][B0]

nJ [A0

Since the system is closed, the change of the activity of AX0 ([A0]) as a function of time is as follows:

where R is the rate constant of the isotope exchange. Equation (9.51) expresses both the transmission of the labeling atom from BX0 to AX and the reversed pro­cess. After an equivalent mathematical transformation, we obtain:

By substituting the specific activities (Eq. (9.46)), after some transformation:

By substituting [B0] from Eq. (9.50), we obtain:

= ([A] [AN ] 1 [AN ] [B] — [A] [A0] — [B] [A0]) = ^ ([A’m ] — [A ])([A] 1 [B])

(9.54)

By the separation of the variates, the equation is given as follows:

d A 0

[AN] — [A0] [A] [B]

The solution of Eq. (9.55) is:

The integration constant (C) can be determined by assuming that at t = 0, [A0] = [A00]. Thus,

C = ln( [AN] — [A0 ]) The solution is:

when t = 0, [A00] = 0, there are no labeling atoms in AX (only sodium iodide is labeled with the 131I isotope), the solution is as follows:

02 ra)=(12 F)=2 eRt

The ratio of the activities at any time and in equilibrium is frequently denoted as:

This means that the ratio of exchanged activity to equilibrium activity is to be measured. This is a widely used technique in radiotracer experiments because the relative activity measurements are often much easier.

Equation (9.59) is similar to the kinetic equation of first-order chemical reac­tions; however, R’ depends on the quantities of the two compounds, AX and BX. The half-time of the reaction depends on the quantities of AX and BX. Thus, the reaction is only formally a first-order reaction; instead, it is a pseudo first-order reaction.

When [A] and [B] are constant, the activation-free energy (i. e., the binding energy of the labeling atom) can be calculated from the Rs determined at different temperatures using the Arrhenius equation.

An isotope exchange reaction can have three different kinetic profiles:

Nuclear reactions with charged particles are induced by irradiation, e. g., by protons with high energy (in the range of MeV; see Sections 6.2.3 and 6.2.3.1). The (p, Y) reactions are relatively simple because the emitted particle is neutral. The prompt

gamma photons are detected. Since the Coulomb repulsion is smaller for light ele­ments, the proton-induced gamma emission is especially suitable for the analysis of light elements (e. g., Li, Be, B, F, Na, and Al). The analysis of light elements is the main advantage of PIGE. A PIGE spectrum of glass is shown in Figure 10.28.

Further Reading

De Soete, D., Gijbels, R. and Hoste, J. (1972). Neutron Activation Analysis. Wiley, New York, NY.

European Spallation Centre, 2011. EES Fields of Research. <http://ess-scandinavia. eu/ess- fields-of-research> (accessed 28.03.12.)

Glascock, M. D., 1996—2011. Overview of Neutron Activation Analysis. <http://archaeometry.

missouri. edu/naa_overview. html> (assessed 26.12.11.)

Kertesz, Zs., Szikszai, Z., Szoboszlai, Z., Simon, A., Huszrink, R. and Uzonyi, I. (2009). Study of individual atmospheric aerosol particles at the Debrecen ion microprobe. Nucl. Instrum. Methods B 267:2236—2240.

Molnar, G. L. (ed.) (2004). Handbook of Prompt Gamma Activation Analysis with Neutron Beams. Kluwer Academic Publishers, Dordrecht/Boston, MA/London.

Molnar, Zs., 2003—2009. Neutron Activation Analysis. <http://www. reak. bme. hu/Wigner_ Course/WignerManuals/Budapest/NEUTRON_ACTIVATION_ANALYSIS. htm> (accessed 15.11.11.)

Nagy, N. M. and Konya, J. (2009). Interfacial Chemistry of Rocks and Soils. Taylor & Francis, Boca Raton, FL.

Nagy, N. M., Konya, J., Beszeda, M., Beszeda, I., Kalman, E., Keresztes, Zs., et al. (2003). Physical and chemical formations of lead contaminants in clay and sediment. J. Colloid Interface Sci. 263:13—22.

Revay, Zs. (2009). Determining elemental composition using prompt gamma activation anal­ysis. Anal. Chem. 81:6851.

Simon, A., Matiskainen, H., Uzonyi, I., Csedreki, L., Szikszai, Z., Kertesz, Zs., et al. (2011). PIXE analysis of Middle European 18th and 19th century glass seals. X-Ray Spectrom. 40:224-228.

Szentmiklosi, L., Kis, Z., Belgya, T., Kasztovszky, Zs., Kudejova, P., Materna, T., et al., and the Ancient Charm Collaboration. (2008). A new PGAI-NT setup and elemental imaging experiments at the Budapest Research Reactor, NRC7—Seventh International Conference on Nuclear and Radiochemistry, Budapest, Hungary, 24-29 August 2008.

Tolgyesi, J. (1962). Jadrove ziarenie v chemickej analyse (Nuclear Radiation in Chemical Analysis). SVTL, Bratislava.

Vass, Sz., Gilanyi, T. and Borbely, S. (2000). SANS study of the structure of sodium alkyl sulfate micellar solutions in terms of the one-component macrofluid model. J. Phys. Chem. B 104:2073-2081.

Vass, Sz., Grimm, H., Banyai, I., Meier, G. and Gikrnyi, T. (2005). Slow water diffusion in micellar solutions. J. Phys. Chem. Lett. 109:11870-11874.

From the radionuclides used for medical applications (shown in Table 12.3), the (metastable) technetium-99m (Tc-99m) is the most commonly used with a share of

about 80% since it optimally meets the previously mentioned requirements, having a gamma energy of 141 keV and a half-life of 6 h. Last but not least, it is a genera­tor product.

Some substances emit scintillations under the effect of radiation. These substances can be used as scintillation detectors. Rutherford was the first to use a scintillation detector to measure alpha radiation. However, scintillation detectors became really important when Zoltan Bay constructed photomultipliers which transform scintilla­tions into electric impulses. As a result of the improvement of scintillator and photomultipliers, scintillation detectors became one of the most important and widely applied tools in radiation measurement.

The advantages of using scintillation methods are as follows:

• The scintillator material is solid or liquid; its density is higher by about three orders of magnitude than the density of the gas in gas-filled detectors. Thus, a significant portion of radiation is converted into light photons. The free path of the light photons can be in the range of 1 m, while the free path of the ions in the gas-filled detectors is only a few centimeters.

• The time of the formation, collection of the light photons, electron conversion and multi­plication, and signal processing is much shorter in scintillation detectors than in gas-filled detectors, so the dead time of scintillation detectors can be as low as about 10_8 s.

• Some scintillator material can be produced as a single crystal. In this case, the intensity of the scintillation is proportional to the energy of the radiation within a wide energy range, so the scintillation detectors are suitable to measure the energy of the radiation.

• The shape of the signals in the scintillation detectors depends on the type of radiation. Thus, it is possible to differentiate between the different types of radiation.

• The scintillation detectors can detect and measure alpha, beta, and gamma radiations. The detectors and the measuring techniques, however, are different for each types of radiation. This will be discussed later.

The scintillation system consists of the following parts:

• The scintillator material converts the radiation into light photons.

• The photomultiplier transforms the light photons into electric impulses.

• The electric processing units amplify, discriminate, process, count, record, the electric impulses.

The isotopes are used as chemical indicators, so they must fulfill the same require­ments as the usual chemical indicators. This means that they have to indicate the presence and concentration of a substance in a given place at a given time, but they should not have any effect on the studied process. The radioactive isotopes usually satisfy these criteria, since they are chemically the same as the inactive isotopes of the same elements. For example, when a 12C atom of an organic substance is

replaced by a 14C isotope, the type and strength of the chemical bond and the physical and chemical properties of the compound do not change significantly. Of course, the differences between the masses of the isotopes can result in some differences—in other words, isotope effects (see Chapter 3). These isotope effects, however, can usually be observed only for light elements. For heavier elements, the effects are negligible because the experimental errors are generally higher than the influences of the isotope effects. Therefore, the isotope effects are usually negligible in radioactive indicator methods.

The requirements can be classified into two groups: (1) the usual requirements for all chemical indicators and (2) special requirements only for radioactive indica­tors. The usual requirements are as follows: the indicator should be sensitive and selective, and can be measured precisely; and the distribution of the indicator should be homogeneous in the system.

The sensitivity of a radioactive indicator depends on its decay rate, and as a con­sequence, so does the activity of the isotope. The sensitivity can be shown in the next example. Let’s take a radioactive indicator with 1000 counts/minute (cpm) radioactive intensity; this intensity can usually be measured easily. The standard deviation of the measurement of 1000 cpm is ±3.3% (see Section 14.7.1). When the measuring efficiency is 10%, this intensity can be obtained if the activity is 10,000 disintegrations/minute (dpm). Assume that the half-life of the radioactive isotope is 60 min (such as for 212Bi). These data can be substituted into Eq. (4.12) as follows:

dN л ln 2

A = -■ = Xn= N

dt t1=2

From Eq. (8.1), N = 9 X 105 nuclides are obtained. Expressing this number in mols (dividing by the Avogadro number), you get 9 X 105/6 X 1023 = 1.5 X 10_18 mol. Therefore, this very small quantity can be measured easily on the basis of its radio­activity. When the decay constant is higher, the sensitivity decreases.

The homogeneous distribution of the radioactive indicator is very important. The radioactive indicator is homogeneously mixed in a system, that is the mixing entropy is maximal (see Section 9.1) if the chemical species of the radioactive indi­cator and the studied substance is ab initio the same or a fast isotope exchange can be possible between the chemical species of the radioactive indicator and the stud­ied substance. The rate of the isotope exchange depends on the strength of the chemical bonds; therefore, it changes in a very wide timescale. For example, the binding energy of iron(II) ion in hemoglobin is high, so the rate of the isotope exchange between iron(II) ions in hemoglobin and in 55Fe ions dissolved in water is practically zero. In such cases, the indicator gives no information on the sub­stance in the other chemical species.

In radioactive tracer experiments, the concentration of the indicator is frequently very small. (As seen in the previous example, even 10_18mol can be measured.) For instance, carrier-free radioactive isotopes could be mentioned. The term
denotes a radioisotope of an element in pure form; i. e., essentially undiluted with a stable isotope. In fact, the term “no-carrier-added” is more correct than “carrier — free.” This means that no carrier is added. However, one can still be present in the system during the experiments, for example, as the result of the interaction with the environment or as the pollutants in the chemicals used.

In other applications, the inactive isotopes (carriers) of the radioactive isotope are added to the system containing the isotope at a low concentration. However, the chemical identity does not need to be the same; chemically similar substances may also behave as carriers. It is known, for example, that potassium or calcium ions are carriers of Cs-137 and Sr-90, respectively, in geological as well as biologi­cal systems. It should be mentioned, though, that in such cases, the ratios of the isotopes and the carrier may be different in each phase of the heterogeneous system.

The two cases (carrier-added or no-carrier-added) must be treated differently. In the presence of a carrier, the usual (macroscopic) chemical concentrations are pres­ent for the chemical species in question; thus, the usual chemical relations and laws apply.

The chemical properties at these very low concentration ranges (no-carrier — added), however, can be different from the usual properties seen in macroscopic concentrations. In the case of such low concentrations, no chemical system can be considered homogeneous because the number of molecules on the surfaces (the wall of the laboratory vessels, any contaminants in the solution, air bubbles, small particles, great molecules, etc.) is more than, or at least similar to, the number of radioactive nuclides in the solution. The radioactive isotopes can accumulate in bulk on these surfaces and can take part in the subsequent formation of heteroge­neous phases (adsorption, colloid formation, precipitation, etc.). Similar accumula­tion also takes place in the range of macroscopic concentration; however, its effects (the accumulation of 10_7—10_8 mol of the substances) can be neglected. In carrier-free systems, however, the total quantity of the radioactive isotope can be accumulated on the surfaces, resulting in the significant decrease in its concentra­tion and activity of the bulk. The adsorption of thorium isotopes on the walls of laboratory vessels is shown in Figure 8.2.

Besides adsorption on the macroscopic interfaces, the ions in very low concen­tration can precipitate even when the concentrations do not reach the solubility product (i. e., normally, they would stay in solution) and/or pseudocolloids (or radiocolloids) may form. Even in a solution containing an alkali metal ion, Cs-137 ion in carrier-free concentration, colloid particles are formed under conditions where it would not usually be expected based on thermodynamic parameters.

For the understanding of the formation of heterogeneous phases at extremely low concentrations, two rules called Hahn’s rules (Hahn, 1926a, b, Hahn and Imre, 1929, Wahl and Bonner, 1951) may serve as starting points:

1. The precipitation rule says that an element (isotope) in extremely low concentration coprecipitates with another element in high concentration if it fits into the crystal lattice of this other element.

2. The adsorption rule says that an ion in extremely low concentration adsorbs well on the surface of a solid crystal when the surface charge of solid and ion are opposite and the adsorbed compound is very insoluble. Thus, the ions in very low concentration can be precipitated even when the concentrations do not reach the solubility product (i. e., nor­mally, they would stay in solution), and/or radiocolloids may form.

The formation of radiocolloids can be interpreted by impurities present in the solution (air bubble, dust, cellulose fiber, hydrolytic products, giant molecules, etc.) that act as active sites for heterogeneous nucleation where the radioactive iso­topes can adsorb. In the case of the formation of radiocolloids, the radioactive iso­tope is not evenly distributed in the solution (Figure 8.3), so the formation of radiocolloids must be avoided, for example, by the application of very pure sub­stances, including water, and keeping the isotopes in acidic solutions.

At very low concentrations of the carrier-free radioactive isotopes, the interfaces are not covered totally, and the coverage is smaller than monomolecular. For this reason, the usual thermodynamic requirement, namely that the activity of the sur­face is one, is not satisfied. As a result, the thermodynamic relations, such as the Nernst equation, have to be applied in their complete form:

RT flox

є = £0 — ln (8.2)

zF flred

where є is the redox potential, є0 is the standard redox potential, R is the gas constant, T is the temperature, z is the change of charges, F is the Faraday constant, and aox

and ored are the activities of the oxidized and reduced species, respectively. In addi­tion, the activity of the solid phase (reduced species of metals) is not known, so the value of the redox potential has to be determined experimentally in each case.

As mentioned in the Hahn’s rules, an element (radioactive isotope) in extremely low concentration (microcomponent) coprecipitates with another element in high concentration (macrocomponent) if it fits into the crystal lattice of this other ele­ment. The distribution of the micro — and macrocomponents in the coprecipitate can be expressed by different formulas, namely by the Doerner—Hoskins equation:

ln—— = A ln -— (8.3)

a — x b — y

or the Henderson—Kracek equation:

У = Db-—y (8.4)

x a— x

In Eqs. (8.3) and (8.4), D and A mean the fractionation coefficients, a and b are the quantities of the macro — and microcomponents in the whole system, and x and y are the quantities of the macro- and microcomponents in the crystal phase. When the fractionation coefficient is 1, as in Hevesy’s experiments (see Section 8.1), the macro — and microcomponents are chemically the same; they are the isotopes of the same elements. When the fractionation factor is not equal to 1, the micro — and macrocomponents are different (e. g., RaCl2 and BaCl2) even if they have similar properties (e. g., isomorphous crystal lattice).

Radioactive indicators have specific properties due to the fact that they are radioactive: (1) the properties originating from the radioactive decay and (2) the properties coming from the different mass numbers.

The radioactive indicator disintegrates, that is, the activity decreases in time. This decrease is determined by the decay constant or half-life. In the tracer studies, it is desirable if the half-life of the radioactive isotopes is comparable to the time needed for the studies. If the half-life is too short, the radioisotope can disintegrate before finishing the investigations. If the half-life is too long, the measurable activ­ity demands a greater quantity of the radioactive nuclides. In addition, as the result of the decay, radioactive wastes form, the treatment of which necessitates particular procedures, and the radiation impact of the environment is also higher. This is especially important in in vivo biological and medical applications.

Radiation, especially at high activities, can also initiate chemical and biological processes (radiolytic effects as discussed in Sections 6.4 and 13.4.2). Any operation with radioactive material demands special laboratory conditions, radiation protec­tion, and waste treatment.

As mentioned previously, the sensitivity of the radioactive indication is deter­mined by the properties of the isotope. The same is true for the selectivity. As a result of the radioactive decay, daughter nuclides that are chemically different from the parent nuclides are formed which can contaminate the studied system. A special problem arises when the daughter nuclides are also radioactive. In this case, the measurements of radioactivity can become more complex. The radiations with dis­crete energy (e. g., alpha and gamma) can be separated by spectrometry. The mea­surement and separation of the continuous radiation (e. g., beta) may be difficult. As an example, the pure beta emitter parent/daughter nuclide pair, 90Sr—90Y, is mentioned. Precise activity measurements can be done only in secular equilibrium (see Section 4.1.6). Since the half-life of the daughter nuclide, 90Y, is 64 h, estab­lishing this equilibrium demands a rather long time. However, when the beta ener­gies of the parent and daughter nuclides are different enough, the spectra can be separated by mathematical methods, or the parent—daughter elements can be sepa­rated chemically.

Another special property of the radioactive indicators is due to the difference between the mass numbers of the isotopes. This is the isotope effect (discussed in Section 3.1), which is important mainly for light elements. When an inactive carrier is added to the radiotracer, the specific activity decreases because of the isotope dilution. As will be discussed in Section 9.4.1, isotope dilution is the basis of numerous analytical methods. The inactive isotopes of the carrier and the radioac­tive isotopes participate in an isotope exchange reaction, which was mentioned pre­viously relating to homogeneity.

Isotope preparations belonging to this group are generated with the (n, Y) nuclear reaction, followed by dissolution and chemical processing of the target. Radiochemical separation is needed only in those cases when other atoms in the compound besides the desired one are highly activated. The products typically con­tain carriers; for this reason, their specific activity is relatively low.

Among the production methods described in the following section, the first two technologies are very simple because, due to the homogeneous isotope composition and ideal chemical composition of the target, only the target nuclide is generated during irradiation without contaminating radionuclides, and so radiochemical sepa­ration is not needed.

The 90Y radionuclide (Table 8.5) with its energy belongs to the group of high — range beta-emitting radionuclides: its penetration in tissues of the living organ is 1—2 cm, so it is suitable for the therapeutic treatment of bone metastases of similar size. The organ-specific behavior of this radionuclide—e. g., penetration of 90Y radionuclide into bone metastases—is ensured by adding ethylene diamine methy­lene phosphonate (EDTMP) to the radionuclide at the treatment site and the formed

complex is intravenously injected to the patient. Typical activity of the injection is 17.5—37 MBq. The total activity of the production batches is approximately 370 MBq.

The 153Sm radionuclide (shown in the Table 8.6) with its energy belongs to the group of low-range beta-emitting radionuclides. Its penetration in tissues of the living organ is 1 —2 mm, so it is suitable for the therapeutic treatment of bone metastases of similar size. The organ-specific behavior of this radionuclide—e. g., penetration of 153Sm radionuclide into bone metastases—is ensured by adding EDTMP to the radionuclide on the treatment site, and the formed complex is intra­venously injected into the patient. The typical activity of the injection is 150—260 MBq. The total activity of the production batches is approximately 2600 MBq.

Other radionuclides belonging to the group that does not require isotope separa­tion are 186Re, 166Ho 169Yb, 165Dy, 177Lu, 89Sr, 59Fe, and 198Au (for medical appli­

The following two production procedures (51Cr and 82Br) are examples of hav­ing multiple chemical elements present in the target. Consequently, the activation of the target, in addition to the target isotope, results in contaminating radionuclide of significant activity, thus requiring subsequent radiochemical separation.

In medical applications, the typical injected activity of 51Cr administered to patient is 10—18.5 MBq, so typical batch activity of the production is around 370 MBq.

In industry, the 51Cr radioisotope (shown in Table 8.7) is widely used for tracer investigation of metallurgical processes and for studying corrosion processes due to the fact that chromium is an important component of the iron — and steel-based structures.

The 82Br radioisotope (see Table 8.8), as a halogen element, can be used as a tracer isotope for halogenation of various industrial components. It has high-energy gamma radiation; therefore, it can be detected outside industrial equipment or pipe­lines. Due to its short half-life, it decays rapidly after the investigation.

The most important industrial application of the 82Br radioisotope is the leakage test of oil pipelines. This 82Br-labeled methyl bromide with an activity of

As mentioned in Section 10.1.2, traditional analytical methods can be coupled with radioactive tracer techniques, or qualitative and quantitative analysis of the labeled compounds can be performed by any traditional analytical technique. One such method is radiochromatography, which is frequently used for purification and qual­ity control of radiochemicals and radiopharmaceuticals (radiochemical purity con­trol, as discussed in Section 8.3). Any types of chromatography, namely, HPLC, thin-layer chromatography, and ion exchange chromatography, can be used for this purpose. The detection is done by measuring the radioactivity of the fractions. Radiochromatogram scanners are commercially available.

11.3.1 Principle of the Measurements

Investigations executed with sealed radioactive sources are based on absorption or scattering of the radiation. Gamma radiation of a sealed radiation source installed

on one side of a piece of equipment penetrates into the equipment/contained mate­rial and generates signals in a detector placed on the opposite side of the equip­ment. This signal indicates the presence or lack of the material, while its intensity is proportional with the rate of absorption which depends on the physical features of the material in the equipment. In this way, some physical features (density, thickness) of the material inside the equipment can be determined from the inten­sity measured.

Now we shall present some examples when gamma imaging has particular impor­tance (see Table 12.5).

12.5.1 Thyroid Scintigraphy

A significant part (normally 15—25%) of iodine will be trapped in thyroid cells from the circulation, and stay there for a long time (in healthy persons it will drop to half in about four months). It will be built into tyrosine, which is converted to the thyroid hormones thyroxin (T4) and triiodo-L-thyronine (T3). Pertechnetate (obtained directly from a technetium generator) gets trapped in the thyroid the

Figure 12.8 A “cold” thyroid nodule at the upper pole of the right lobe.

Figure 12.9 A “hot” thyroid nodule in the right lobe (anterior view).

same way, but—as it cannot be built into organic molecules—it will be washed out faster. Both can be used for thyroid scintigraphy, most frequently in order to clarify the functional status of nodules that were palpated or visualized by ultrasound. While some benign and malignant tumors accumulate less radiopharmaceutical than the normal thyroid tissue (“cold” nodules; see Figure 12.8), in the so-called autonomous adenomas there will be high accumulation, escaping the regular feed­back mechanism (Figure 12.9).