Considering the characteristics of nuclear data, the time needed for the investiga­tions (several days) should match the half-life of the radionuclides. For measure­ments outside the equipment wall, gamma-emitting radionuclides with relatively high energy (>300 keV) are suitable. Radionuclides emitting a high number of gamma quantums per decay are advantageous; because of their higher count rate, lower activity is necessary for the investigations (see the role of the a factor in Eq. (11.3)). Certainly an important consideration when selecting the radionuclide is simplicity of its preparation, which should take place in the research reactor through the (n, Y) nuclear reaction, favorably with a high activity yield (as discussed in Section 8.5.2).

In addition to nuclear data, physical and chemical features will determine which radionuclides can be selected for a given tracer study. Radionuclides most fre­quently used for industrial tracer studies are summarized in Table 11.1.

In the preparatory phase of the tracer study, at the preparation of the radionu­clide, and during the investigation, rules governing the handling of radioisotopes must be complied with. The fate of the radioactive isotopes used for industrial tracer studies is important to define in advance and solve in an authorized manner.

The simplest way is to store the material labeled with relatively short-lived radionuclides in a well-separated place until its radioactivity decays below the

exempted activity level. Storing time can be considerably reduced if the labeled material is diluted during the technological processes or is artificially diluted after the study. In such cases, the exempted radioactive concentration will be the precon­dition of the release. For instance, the dilution rate of a radioactive-labeled compo­nent that is introduced into a huge storing container with a great volume of nonradioactive material can even grant an exemption from separated storage.

The main field of nuclear medicine today is imaging with gamma cameras. What can justify the in vivo use of radioactive preparations delivering radiation dose to humans? Other generally applied modalities of medical imaging, including ultrasound, X-ray, and X-ray computed tomography (CT), and most routine proce­dures of magnetic resonance imaging (MRI) are structural imaging methods. A pathological process is visible in these images only when it has already caused structural changes. For instance:

• The borders between tissues of different acoustic impedance (determined by the elasticity and density) can be seen in ultrasound images. An abnormal process will be visible only when it has already altered the structure of the tissue.

• X-ray (including CT) images will distinguish tissues with different radiation attenuation,

i. e., density. The easiest is to differentiate solid bones from soft tissues.

On the contrary, by applying radioactive tracers, we can follow the accumula­tion, secretion, metabolism, and excretion process of various molecules, so that a pathological process can be identified even in an early stage, when the structure of the investigated organ is not yet significantly different from its normal state. That is why the imaging methods of nuclear medicine are considered functional rather than anatomical.

Moreover, much smaller molar concentrations of radiotracers can be detected than the usual concentrations of contrast materials used for structural imaging

(see Table 12.4); thus, the application of a radiotracer does not interfere with or change the function of the organ investigated.

PET studies contribute about 6% to nuclear medical imaging, and the distribu­tion of gamma camera studies depends on both health-care protocols and the reimbursement policy of a country (see Table 12.5 and Figure 12.3).

As discussed previously in this chapter, the most important detectors (gas-filled tubes, the photomultipliers of the scintillation, and the semiconductor detectors) produce electric impulses. The electric impulses formed in the gas-filled tubes can be measured, e. g., by a simple electric circuit, as illustrated in Figure 14.8.

In Figure 14.8, the detector is considered to be a resistance that tends to be infinity (the resistance of the detector is much higher than the resistance of R,

Figure 14.8 A simple electric circuit for the measurement of electric impulses in an ionization detector (D—detector, R—resistance, I—current meter, U—voltage meter).

which is about GO) when there are no ions produced in the gas-filled tube (i. e., no radiation). In this case, the current tends to be zero; thus, the voltage measured by the U voltage meter also tends to be zero, as expected by Ohm’s law (U = R X I). Under the ionizing effect of radiation, electrons and ions are induced, decreasing the resistance (the resistance of the detector becomes less than the resistance of R). As a result, the current, as well as the voltage on the U voltage meter, increases, and an electric impulse is formed. The voltage is proportional to the amplitude of the electric impulse.

When using scintillation detectors, an anodic resistance and a condenser (RC circle) are used. In the case of semiconductor detectors, a charge-sensitive pream­plifier is required.

The obtained electric impulses can also be amplified or attenuated linearly depending on the voltage required by the signal-processing units. Impulses with different amplitudes can be separated by discriminators in one-channel analyzers or multichannel amplitude analyzers.

In one-channel analyzers, the electric impulses with different amplitudes are separated by differential discriminators. These are filters that permit the signals in the range of VD ± Д VD to pass. The level VD and the width ДVD can be varied. By varying VD, the total gamma spectrum can be scanned step by step. The total gamma spectrum is obtained if the VD is incremented by Д VD in each step.

In multichannel analyzers, the total spectrum is recorded in one measurement. It is very comfortable and saves time, which is especially important when the radio­nuclide has a short half-life or low activity.

The signals produced in the amplifier of the detector transfer to an analog/digital converter (ADC). The condenser in the ADC is charged up, as determined by the amplitude of the electric impulse. Then the condenser partially discharges, and in the meantime, an oscillator emits impulses at a constant speed. The number of the impulses emitted during the discharge of the condenser is proportional to the ampli­tude of the signal input into the ADC, i. e., to the energy of the radiation. The num­ber of the impulses coming out of the oscillator determines the position of the
input signals in the memory. Each position of the memory corresponds to an input signal with a particular energy, and its value always increases by 1 when a signal with the same energy is input into the analog/digital converter.

Previously, the data stored in the memory was recorded by two digital—analog converters (DACs) and an oscilloscope. One of the DACs treated the positions in the memory that define the position of the electron beam along the x-axis of the oscilloscope. The other DAC shows the deviation of the electron beam from the x-axis. As a result, the spectrum is continuously presented on the monitor of the oscilloscope. Nowadays, computers equipped with analyzer cards are used for sig­nal processing. Their operation is not discussed here.

Since the cross section of cadmium for neutrons is about 104 barns, cadmium tar­gets have to be placed in the nuclear reactors very carefully. Cd-115m is produced by the 114Cd(n, Y)115mCd nuclear reaction. Its half-life is 44.6 days, and it emits в_ and gamma radiation.

8.6.16 Indium Isotopes

In-111 is produced in cyclotrons. Its half-life is 2.8 days, and it disintegrates with electron capture and gamma radiation.

In-114 is produced by the 113In(n, Y)114 in nuclear reaction. Its half-life is 72 s, and it disintegrates with electron capture and в_ and gamma radiation. This nuclear reaction is used for the measurement of neutron flux in neutron generators.

In-114m is produced by the 113In(n, Y)114mIn nuclear reaction. Its half-life is 50 days, and it disintegrates with electron capture and в_ and gamma radiation.

The Paneth surface determination postulates that the heterogeneous isotope exchange is much faster on the surface of the bulk solid phase than inside it. Thus, the specific surface area of a solid substance can be determined from the ratio of the radioactiv­ities of the solid and its saturated solution. For example, the specific surface area of lead sulfate can be determined using radioactive lead ions (e. g., 212Pb21). The isotope exchange takes place between the lead ions in the solution and on the surface of the solid lead sulfate:

PbSO4 1 *Pb21 з* PbSO4 1 Pb21 (9.115b)

Since there are no chemical reactions, the process is directed by the change of the mixing entropy. When the exchange takes place only on the surface, the ratio of the activities of the solution and solid is determined by the ratio of the atoms/ ions in the solution and on the surface of the solid, as follows:

Activity of solution cV

= 9.116)

Activity of solid X

where X is the number of the atoms/ions on the surface, c is the concentration of saturated solution (solubility), and V is the volume of the solution. After multiply­ing X by the cross section of the surface atoms/ions, the specific surface area is expressed in area units.

The radioactive atoms/ions can be buried into the bulk by isotherm transcrystal­lization. This effect is corrected by the kinetics of the isotope exchange; the radio­activity is measured as a function of time (Figure 9.15). The kinetic curve has two sections: a sharp and a slight increase; and the fast surface exchange is followed by a slow exchange (transcrystallization) with the bulk phase. Extrapolating the slight

Figure 9.15 The determination of the surface exchange from the radioactivity versus time function by a heterogeneous isotope exchange.

increase to the initial time (t = 0), it cut the vertical axis at the radioactivity belong­ing to the pure surface exchange.

For crystalline substances, only a portion of the surface sites, the so-called active surface, exchanges with the radioactive atoms/ions in the solution. The active sur­face depends on the temperature. According to Imre, the relation of the total surface (X) and the active surface (X*) is as follows:

X* = X exp (-R0 (9.117)

where E is the activation energy of the isotope exchange and T is the temperature. The total surface area can be determined from the active surface areas measured at different temperatures.

The best-known application of measuring methods based on neutron scattering and attenuation is moisture content determination, which relies on the special character of hydrogen as a neutron-scattering medium. When high-energy neutrons collide with hydrogen atoms, the former lose their energy and slow down. The rate of slowing down is great since the masses of the neutron and the hydrogen nucleus are similar.

If an Am-241/Be radiation source with activity of some GBq emitting fast neu­trons and a detector sensitive only to slow neutrons are placed into the medium to be tested, the intensity of low-energy neutrons detected by the sensor will be pro­portional to the hydrogen content of the material tested (Figure 11.24).

The fact that hydrogen, but not water content, is determined must be taken into consideration because this method also measures, for example, chemically bound hydrogen in organic compounds or crystal water. The measurement is also dis­turbed by the presence of other neutron-absorbing elements (B, Cd) and modifica­tion of the composition with elements that have low atomic numbers (Cl, O, S).

For reliable measurement, a permanent consistency must be maintained. If the consistency of the material is changing, the density must be measured separately. Today, moisture-measuring instruments with two separate radiation sources (one is gamma, and the other is a neutron emitter) and two detectors, already performing density compensation, are available (Figure 11.25).

Nuclear moisture-measuring instruments operated with a neutron source are mostly used in foundries and in producing materials in industry for the sampling — free, quick, and accurate measurement of the moisture content of various mixtures and the consistency of additives.

Examples of industrial applications include the following:

• Water addition is determined in concrete panel factories by the continuous measurement of the moisture content of river sand (this is the component that has the highest moisture content, as compared to dry components).

Intensity on the opposite side

• For determining the moisture content of soils, portable soil moisture-measuring instru­ments have been developed.

• Moisture content measurement is very important in road, railway, and basement construction.

12.6.1 The PET Camera

In a PET camera, there are rings of detectors around the patient. The two photons resulting from annihilation fly in opposite directions and will be detected by two detectors at almost the same time. The line of event can be determined by con­necting the two detectors (Figure 12.12). So, in contrast to gamma cameras, we do not need a collimator for PET imaging, and consequently both the sensitivity and spatial resolution of PET are better than those of a regular gamma camera for human imaging. Moreover, since the pairs of photons hitting a particular pair of detectors always travel the same path length inside the patient’s body (independent of the position where the annihilation occurred along the line), the correction for attenuation is more straightforward.

The rings can simultaneously detect radiation emitted in all directions, so PET is capable of acquiring dynamic tomographic studies as well. Utilizing this, in the beginning, PET was mostly used for research purposes, primarily for pharmaco — and receptor kinetic brain studies. Today, most of the PET studies are clinical: they are used to search for tumors and metastases.

PET scanners are rather expensive, and we need positron-emitting radionuclides produced in cyclotrons, both limiting the number of studies. The price of a PET study is typically higher by an order of magnitude than that of a gamma camera procedure.

Among the members of the decay series of U-238 (see Figure 4.4) and Th-234 (see Figure 4.6), Ra-226, Rn-222, Pb-210, Bi-210, Po-210, and Ra-228, Th-228, Rn-220, Pb-212 isotopes, respectively, have been used in radiotracer studies. Their applications are discussed briefly here.

8.5.1.1 Th-234

As already discussed, the Th-232 isotope is the parent nuclide in a radioactive decay series. Its half-life and decay constant are 14 billion years and 5 X 1011/ years, respectively, so its radioactivity is also very low and can be measured only with difficulty. The half-life of the other thorium isotope, Th-234, is much shorter (24.1 days), so the same number of Th-234 nuclides gives 1011 times higher radio­activity than Th-232. Obviously, therefore, Th-234 is used for the labeling of Th-232.

The characteristic oxidation state of thorium is +4, so it has been used as a radioindicator of other elements with +4 oxidation state (e. g., Ce(IV)). These studies have been especially significant in colloid chemistry, where the effect of the tetravalent “ions” on the colloid processes (e. g., adsorption and coagulation) has been investigated.

The Th-234 isotope was separated from U-238 series by extraction with ether by Fajans and Goring.

Charged particles (e. g., protons and deuterons) generated in cyclotrons may partici­pate in nuclear reactions, resulting in several new radionuclides used mainly for medical purposes. According to proton energy, the following cyclotron types are known:

•
Medical or “baby” cyclotrons installed on the site of the application of radionuclide with maximum proton energy of 10—12 MeV, suitable for producing very short-lived (T1/2 < 2 h) radionuclides (18F, 13N, 11C, 15O; see Table 8.4). These radionuclides are tracers for PET, and all except 18F are found in living organs as chemical elements. Thus, they have the advantage that no foreign atom is used for labeling the organ-specific molecule and atoms naturally present in living organs can be labeled with their PET radionuclides.

• In industrial cyclotrons with higher proton energy (30—40 MeV), radionuclides of longer half-lives used both as industrial and medical tracers (67Ga, 201Tl, 111In, 123I, and 81Rb) can be produced. The latter is the parent of the isotope generator 81Rb/81Kr, the daughter of which, as noble gas, is used for lung diagnostics.

• The main application of the very-high-energy cyclotrons (70—200 MeV) is tumor ther­apy. In addition, cyclotrons with a high current density are used for producing radionu­clides of a low-proton-absorption cross section (e. g., 103Pd).

For cyclotron irradiation, the yield Y (Bq/pA • h) of the nuclear reaction can be calculated with the following formula:

where N is the number of target atoms in a given volume, Ф is the flux of the bom­barding particle, a is the cross section of the target element, E is the energy of the bombarding particle, and X is the thickness of the target. (This equation is another form of Eq. (6.9).)

Among cyclotron isotopes, the 18F radionuclide and its labeled compound, 18F-fluorodeoxyglucose (18FDG), has the most important and the highest utilization.

Today, the fluorination reaction following the target irradiation is a fully automated, computer-controlled process using “synthesis panels,” which carry out computed steps of the reaction without human intervention (see Table 8.16).

Among radionuclides with longer half-lives produced in industrial cyclotrons, 67Ga, 201Tl, and 123I have practical importance in medical applications. These

detecting glucose consumption that cells use for energy supply. Tumor cells, for instance, consume glucose at an increased rate, so diagnosis of such cells is possible with FDG. In addition to this, it is also suitable for detecting certain myocardial disorders and inflammations.

products—due to their longer half-lives—can be transported over longer distances. Nuclear reactions that serve to generate these radionuclides are:

66Zn (d, n)67Ga (8.26)

203Tl (p, 3n)201Pb, followed by 201Pb! 201Tl (в 2 decay) (8.27)

122Te (p, n)123I (8.28)

Some radionuclides produced in high-energy cyclotrons are important radionu­clide generators.

Irradiation: 69Ga(p, 2n)68Ge!68Ge/68Ga generator

Irradiation: 85Rb(p, 4n)82Sr ! 82Sr/82Rb generator

As daughter nuclides emit positrons, these generators are used for PET images.

Table 8.17 The Most Frequently Used Quality Control Methods for Open-Vessel
Radioactive Preparations

Tested Parameter Test Method

Activity

Specific activity

Radioactive concentration Radionuclide purity Radiochemical purity pH

Separation yield of the parent and daughter radionuclides (at generators)

Parent nuclide concentration (as contamination) in the separated daughter nuclide—called a parent breakthrough Sterility

Endotoxin content (pyrogenity)

The reverse isotope dilution method is used for the quantitative analysis of radioac­tive substances, especially in mixtures of radioactive substances. The quantity of the radioactive substance (m0 + mx) is determined by adding inactive substance (m) (standard). The steps of the procedure are the same as in the simple isotope dilution
method. It is usually true that mx«m0 and (mx + m0)^m0. From Eq. (10.1) or (10.2), the quantity is:

m

m0 5 50—~ (1a3)

a

or is expressed by intensities:
m

m0 = i

^ -1 i

Similar to the simple isotope dilution method, the specific activity or intensity has to be measured before and after the isotope dilution. Since the quantity m is known, m0 can be calculated. The sensitivity is determined by the minimum quantity that is needed to measure a0.

Reverse isotope dilution is used in microanalyses (e. g., for the analysis of purity), and the yield of nuclear reactions is used in the activation analysis (see Section 10.2.2.1).