**3.1.1 Methods and installations for primary (absolute) standardization**

The distinction between the radionuclide metrology and other metrology branches consists of the necessity to elaborate specific standardization methods almost for each radionuclide, due to the variability of decay schemes, and in the impossibility to construct an immuable standard, due to the radioactive dizintegration.

In radionuclide metrology, an absolute standardization is done by the following procedure: one detects the radiations emitted by a radioactive source and the method for establishing an adequate relation between the counting rate and the activity of the source is elaborated. A general relation is expressed as:

$$\mathbf{N}\_{\rm rad} = \boldsymbol{\varepsilon} \text{ s} \, \mathbf{A} = \boldsymbol{\varepsilon} \, \text{s} \, \mathbf{N}\_{\rm o} \tag{1}$$

*Nrad* is the counting rate, s-1 (impulses per second) for the detected radiation; *ε* is the detection efficiency of the system; *s*, denoted also as *(I, p)* is the intensity of detected

Role of the Radionuclide Metrology in Nuclear Medicine 149

*N NN NN N PC PC* =

*NN N N PC c* γ

Relation (4) shows that the efficiencies of the two detectors are eliminated and consequently the activity can be determined directly from the three counting rates. Relations (3) and (4) were written in very simplifying assumptions, which in reality can not be reached: the radiouclides have a simple decay scheme - an alpha or beta decay, followed by a gamma transition to the daughter ground level; the detectors are fully specialized; no perturbations due to the installation, such as background counting rate, dead times, resolution time of the coincidence circuit modifies the counting rates. The coincidence relations are more complex, and were written by (Gandy.1961) and (Baerg.1973). For decay scheme corrections the coincidence method is applied associated with an efficiency extrapolation variant, and for the instrumental corrections various formulae were deduced by (Grigorescu.1973) and (Smith.1978). Many specific methods were elaborated in the primary activity standard laboratories, among them being the Radionuclide Metrology Laboratory from the Horia Hulubei National Institute for Research & Development in Physics and Nuclear Engineering (IFIN-HH, RML) Bucharest, the Romanian owner of the primary activity standard. A coincidence installation was set up in the sixties and was continuously updated in this laboratory. The variant of the efficiency extrapolation, allowing to accomplish the decay scheme corrections, was developed and applied for many types of radionuclides. The classical extrapolation coincidence method was applied for the beta-gamma medical radionuclide 131I, for which the whole traceability chain was established (Sahagia et al.2008a). The therapy radionuclides 153Sm, 177Lu and 186,188Re, strong beta - weak gamma-ray emitters, have a "triangular" decay scheme and were standardized by a special extrapolation procedure (Sahagia et al.2005a,2005b,2002). For the radionuclide 99mTc, decaying by isomer transition in competition with internal conversion, a new type of coincidence scheme was applied, based on the coincidence between 119.5-138 keV conversion electrons and Tc *x*-rays (Sahagia.2006). Radionuclide 125I is an electron capture gamma emitter; it was standardized by the *x - x, γ* coincidence method (Sahagia et al.2008b). The positron emitters, used for PET, PET/CT systems, like 68(Ge + Ga) generator, or other radionuclides decaying by electron capture in competition with positron emission, were standardized by the positronannihilation-ray coincidence, or combinations of radiations (Grigorescu et al.2004; Grigorescu et al.1996). In the present time, in many laboratories, the classical, analog, coincidence set up is replaced by the Digital Coincidence Counting (DCC) (Buckman et al.1998). At the same time, instead of the PC, a liquid scintillation counter (LSC), associated with a NaI(Tl) detector

ε

γ γ

=

ε

is used for coincidence measurement (Chylinski & Radoszewski.1996)

ε

ε

**Efficiency tracer method** is an extension of the coincidence method applied for the standardization of pure beta decaying radionuclides. The solution to be standardized is mixed with a standard solution from a beta-gamma emitting radionuclide, preferable with a simple decay scheme, with a beta spectrum close to that of the radionuclide to standardize and chemically compatible with it. For the two radionuclide's mixture, relation (3) becomes:

=

Index *1* refers to the tracer and *2* to the pure beta emitter. By an extrapolation procedure, a relation is established between the two *εPC1* and *εPC2* efficiencies, allowind the calculation of

+ ==

1 01 2 02 <sup>01</sup> 01 ; ; γ γ

εε

*N N NN NN N PC PC PC c PC* (5)

γ

 ε

Relations (3) are equivalent with:

00 0 ; ;

γ

/ = 0 (4)

*c PC* = (3)

 εε

radiations and *A,* or *(N0)* is the activity of the source, Becquerel (*Bq*). Every standardization operation aims to determine as precisely as possible the quantity *ε*, or to find a method to eliminate it from relation (1)

Two basic methods are used, NCRP58 (1978). Methods based on the detection in a welldefined geometry (defined solid angle), the most used being the 4πsr geometry, applicable for radionuclides emitting a single type of radiations, and coincidence methods used for those emitting coincident radiations.

#### **3.1.1.1 Methods based on determined geometry**

**Solid angle method** is based on the precise calculation of the solid angle, of the absorption in air and in detector window, and of the radiations scattering correction. The method is used for the standarization of the alpha sources, in vacuum chambers, provided with Si barrier surface detectors; in other cases, for example beta radiations, it is not satisfactory precise.

**Methods based on 4πsr geometry** use a detection system assuring the quasi total detection of the radiations emitted by the source, which are used for measurement, what means that the following condition is fulfilled:

$$
\mathfrak{a} \approx 1 \tag{2}
$$

The method of the *4π* proportional counter, known as the method *4πPC*, is the most known*.*  It is applicable to solid sources, or to radioactive solutions gravimetrically dropped on very thin solid supports, from pure beta emitters with high maximum energy, generally > 150 keV. The remaining corrections to be applied are due to: beta rays absorption in source mass (selfabsorption) and in its support. The method is frequently used for the standardization of the radioactive gases and is known as "gas fillig counter" (Stanga et al.2002). The corrections are due to the wall nondetection, mainly the extremity of counters. Another applied method is the sum peak counting, based on the use of a large NaI(Tl) well type crystal, with a thin window (Nedjadi et al.2007). It is used for radionuclides emitting gamma radiations with high energies and intensities, as for example: 124Sb, 222Rn decay chain, etc. The corrections are done by using Monte Carlo programs.

**Liquid scintillator methods**. The method is applicable to the radioactive solutions or gases, which are dissolved in a liquid scintillator. The detection geometry is 4πsr. The selfabsorption and source support absorption are eliminated but other processes occur: the quenching and non-detection of low energy radiations. It is applicable to pure alpha emitters, for which *ε* = 1, and high energy pure beta emitters.

#### **3.1.1.2 Coincidence methods**

The methods are generally applicable for the radionuclides decaying with the emission of mixed radiations, such as: *α – γ; β - γ*; *x, eA – γ.* It is extended to the pure beta or electron capture radionuclides.

**Coincidence method and installation** *4πPC-γ.* A classical coincidence installation contains: a proportional counter (PC), one or two NaI(Tl) detectors and electronic modules, allowing the individual recording of the impulses provided by the two detection channels, and the coincidences recorded by a coincidence selector. The principle of the method is the following: the relations between the counting rates on the three channels: proportional counter – *NPC*, gamma ray counting - *Nγ*, coincidence – *Nc* , the activity *N0* and the corresponding detection efficiencies *εPC* and *ε <sup>γ</sup>* , are:

$$N\_{\rm PC} = \varepsilon\_{\rm PC} N\_0; \quad N\_{\gamma} = \varepsilon\_{\gamma} N\_0; \quad N\_c = \varepsilon\_{\rm PC} \varepsilon\_{\gamma} N\_0 \tag{3}$$

Relations (3) are equivalent with:

148 12 Chapters on Nuclear Medicine

radiations and *A,* or *(N0)* is the activity of the source, Becquerel (*Bq*). Every standardization operation aims to determine as precisely as possible the quantity *ε*, or to find a method to

Two basic methods are used, NCRP58 (1978). Methods based on the detection in a welldefined geometry (defined solid angle), the most used being the 4πsr geometry, applicable for radionuclides emitting a single type of radiations, and coincidence methods used for

**Solid angle method** is based on the precise calculation of the solid angle, of the absorption in air and in detector window, and of the radiations scattering correction. The method is used for the standarization of the alpha sources, in vacuum chambers, provided with Si barrier surface detectors; in other cases, for example beta radiations, it is not satisfactory

**Methods based on 4πsr geometry** use a detection system assuring the quasi total detection of the radiations emitted by the source, which are used for measurement, what means that

ε

The method of the *4π* proportional counter, known as the method *4πPC*, is the most known*.*  It is applicable to solid sources, or to radioactive solutions gravimetrically dropped on very thin solid supports, from pure beta emitters with high maximum energy, generally > 150 keV. The remaining corrections to be applied are due to: beta rays absorption in source mass (selfabsorption) and in its support. The method is frequently used for the standardization of the radioactive gases and is known as "gas fillig counter" (Stanga et al.2002). The corrections are due to the wall nondetection, mainly the extremity of counters. Another applied method is the sum peak counting, based on the use of a large NaI(Tl) well type crystal, with a thin window (Nedjadi et al.2007). It is used for radionuclides emitting gamma radiations with high energies and intensities, as for example: 124Sb, 222Rn decay chain, etc. The corrections

**Liquid scintillator methods**. The method is applicable to the radioactive solutions or gases, which are dissolved in a liquid scintillator. The detection geometry is 4πsr. The selfabsorption and source support absorption are eliminated but other processes occur: the quenching and non-detection of low energy radiations. It is applicable to pure alpha

The methods are generally applicable for the radionuclides decaying with the emission of mixed radiations, such as: *α – γ; β - γ*; *x, eA – γ.* It is extended to the pure beta or electron

**Coincidence method and installation** *4πPC-γ.* A classical coincidence installation contains: a proportional counter (PC), one or two NaI(Tl) detectors and electronic modules, allowing the individual recording of the impulses provided by the two detection channels, and the coincidences recorded by a coincidence selector. The principle of the method is the following: the relations between the counting rates on the three channels: proportional counter – *NPC*, gamma ray counting - *Nγ*, coincidence – *Nc* , the activity *N0* and the

≈ 1 (2)

eliminate it from relation (1)

precise.

those emitting coincident radiations.

the following condition is fulfilled:

are done by using Monte Carlo programs.

**3.1.1.2 Coincidence methods** 

capture radionuclides.

emitters, for which *ε* = 1, and high energy pure beta emitters.

corresponding detection efficiencies *εPC* and *ε <sup>γ</sup>* , are:

**3.1.1.1 Methods based on determined geometry**

$$\mathbf{N}\_{\rm PC} \mathbf{N}\_{\gamma} \ne \mathbf{N}\_{c} = \mathbf{N}\_{0} \tag{4}$$

Relation (4) shows that the efficiencies of the two detectors are eliminated and consequently the activity can be determined directly from the three counting rates. Relations (3) and (4) were written in very simplifying assumptions, which in reality can not be reached: the radiouclides have a simple decay scheme - an alpha or beta decay, followed by a gamma transition to the daughter ground level; the detectors are fully specialized; no perturbations due to the installation, such as background counting rate, dead times, resolution time of the coincidence circuit modifies the counting rates. The coincidence relations are more complex, and were written by (Gandy.1961) and (Baerg.1973). For decay scheme corrections the coincidence method is applied associated with an efficiency extrapolation variant, and for the instrumental corrections various formulae were deduced by (Grigorescu.1973) and (Smith.1978). Many specific methods were elaborated in the primary activity standard laboratories, among them being the Radionuclide Metrology Laboratory from the Horia Hulubei National Institute for Research & Development in Physics and Nuclear Engineering (IFIN-HH, RML) Bucharest, the Romanian owner of the primary activity standard. A coincidence installation was set up in the sixties and was continuously updated in this laboratory. The variant of the efficiency extrapolation, allowing to accomplish the decay scheme corrections, was developed and applied for many types of radionuclides. The classical extrapolation coincidence method was applied for the beta-gamma medical radionuclide 131I, for which the whole traceability chain was established (Sahagia et al.2008a). The therapy radionuclides 153Sm, 177Lu and 186,188Re, strong beta - weak gamma-ray emitters, have a "triangular" decay scheme and were standardized by a special extrapolation procedure (Sahagia et al.2005a,2005b,2002). For the radionuclide 99mTc, decaying by isomer transition in competition with internal conversion, a new type of coincidence scheme was applied, based on the coincidence between 119.5-138 keV conversion electrons and Tc *x*-rays (Sahagia.2006). Radionuclide 125I is an electron capture gamma emitter; it was standardized by the *x - x, γ* coincidence method (Sahagia et al.2008b). The positron emitters, used for PET, PET/CT systems, like 68(Ge + Ga) generator, or other radionuclides decaying by electron capture in competition with positron emission, were standardized by the positronannihilation-ray coincidence, or combinations of radiations (Grigorescu et al.2004; Grigorescu et al.1996). In the present time, in many laboratories, the classical, analog, coincidence set up is replaced by the Digital Coincidence Counting (DCC) (Buckman et al.1998). At the same time, instead of the PC, a liquid scintillation counter (LSC), associated with a NaI(Tl) detector is used for coincidence measurement (Chylinski & Radoszewski.1996)

**Efficiency tracer method** is an extension of the coincidence method applied for the standardization of pure beta decaying radionuclides. The solution to be standardized is mixed with a standard solution from a beta-gamma emitting radionuclide, preferable with a simple decay scheme, with a beta spectrum close to that of the radionuclide to standardize and chemically compatible with it. For the two radionuclide's mixture, relation (3) becomes:

$$N\_{\rm PC} = \varepsilon\_{\rm PC1} N\_{01} + \varepsilon\_{\rm PC2} N\_{02}; \quad N\_{\gamma} = \varepsilon\_{\gamma} N\_{01}; \quad N\_c = \varepsilon\_{\rm PC} \varepsilon\_{\gamma} N\_{01} \tag{5}$$

Index *1* refers to the tracer and *2* to the pure beta emitter. By an extrapolation procedure, a relation is established between the two *εPC1* and *εPC2* efficiencies, allowind the calculation of

Role of the Radionuclide Metrology in Nuclear Medicine 151

These relations are calculated theoretically, from the radionuclide spectrum, by itteration for a big number of values of the free parameter λ, by using computer programs, like SPEBETA; TDRCB-1, 2, 7; DETECSZ, etc., elaborated at LNHB-France and RC Poland. On the other hand, an experimental ratio between the counting rates of the two types of coincidences, equal to the efficiencies' ratio, is determined. They depend on the activity *N*0 and

> 0 0 *RN RN D dT t* = = ε;

The theoretical ratio of efficiencies, corresponding to the adjusted free parameter is the

*d D t T R R*

In this manner, the efficiencies are determined, and the activity *N*0 is calculated from

A TDCR system was set up in IFIN-HH,RML, with the assistance of Dr. Philippe Cassette from LNHB. It was used for the standardization of radionuclides as 3H, 89Sr, 63Ni and was validated by international comparisons (Razdolescu et al.2004; 2006). Recently, a new counter was set up in RML, replacing the three PMTs with 6 channel photomultipliers (CPM) and designing a new optical chamber (Ivan et al.2010). It has the advantage of being

**3.2 Validation of the primary installations and methods; international equivalence** 

The International Committee for Weights and Measures (CIPM) coordinates the metrology branches through the Consultative Committees, as the Comité Consultatif des Rayonnements Ionisants – CCRI. CCRI is divided in three sections. The Section II, CCRI(II) - Measurement of Radionuclides, coordinates the Radionuclide Metrology. The Bureau International des Poids et Mesures-BIPM, Sèvres, http://www.bipm.org France, has the custody of the international standards. The equivalence of the primary standards is assured at this level by absolute methods of standardization. At the regional level, the Regional Metrology Organizations (MRO) are operational. In Europe there exist: EURAMET - European Association of National Metrology Institutes and COOMET - Euro-Asian Cooperation of National Metrology Institutes. The connection between BIPM and MROs is assured by the Joint Committee of the Regional Metrology Organizations and the BIPM - JCRB. At this level one assures the traceability also by using secondary (relative) standardization methods. The CIPM-MRA, Mutual Recognition Arrangement (1999), defines the recognition of the calibration and measurement certificates issued by the National Metrology Institutes. For example, Romania is part of CIPM-MRA. The document contains four annexes: Annex A - List of signatories; the Romanian signatory is the Romanian Bureau of Legal Metrology - National Institute of Metrology (BRML-INM); Annex B - Key Comparison Data Base (KCDB) per Institute; Annex C- Calibration and Measurement Capabilities (CMC); Annex D - Key Comparison Data Base. The condition for the applicability of CIPM-MRA by the signatory countries is the demonstration of the equivalence of the primary standards, or of the traceability for the others, such as presented

ε

ε

ε

(8)

<sup>=</sup> (9)

efficiencies, as:

optimum, when:

relations (8)

compact and portable.

**3.2.1 The International System** 

activity *N02* of the pure beta radionuclide.The method is applied for therapeutic pure beta radionuclides, like 89Sr, or other important radionuclides as 137Cs (Razdolescu et al.2002a, Sahagia.1981) 32P, 90Y.

#### **3.1.1.3 Advanced methods based on the liquid scintillation counting**

The method consists of mixing the radioactive solution with a liquid scintillator (LS). The energy of the radiations is transferred to the LS; it emits light photons, which produce photoelectrons, multiplied in a photomultiplier (PMT). Electron impulses are collected at the PMT anode. The detection efficiency is superior to that of proportional counters, but it depends strongly on the radiations energy. A calculation model, known as the "Free parameter principle" was developed, leaving from the idea that the sum of fenomena: luminous photons emission in LS, their arrival at PMT photocathode and emission of a photoelectron as result of interaction, is described by a Poisson distribution law. Another influence of LS is the quenching; it can be due to its chemistry, but the most important is the ionization quenching, *Q*(*E*), which is described by an empirical relation, written by (Birks.1964)*.* Taking into account all these fenomena, a relation connecting the detection efficiency, *ε,* equal to the probability of producing a photoelectron, *P(λ,E)*, with the counter parameters and ionizing radiation energy, *E* was deduced*.* (Broda et al.2007; Pochwalski&Radoszewski.1979):

$$\mathfrak{a} = P(\mathcal{A}, E) = \mathbf{1} - e^{-\frac{EQ(E)}{\lambda R}} \tag{6}$$

*λ* is called the free parameter and *R* is the number of photomultipliers in the system. In relation (6) it is important to adjust the free parameter such as to reflect the measurement conditions. On this purpose, two main models were developed.

**CIEMAT/NIST method** consists of the use of a LS counter provided with two PMTs in coincidence, in order to diminish the influence of background; commercial counters can be used. The adjustment of the free parameter is achieved by using an efficiency tracer, consisting of a tritium standard solution. The model contains several steps: a relation is established between the tritium detection efficiency and a quantity equivalent of the free parameter, known as quenching indicator parameter (QIP) (Grau Malonda.1999), measured by using the Compon radiation of an external source; a theoretical relation is calculated between the efficiency of tritium and that of the nuclide to standardize, for different QIP values, based on the beta spectra characteristics. Using a determined QIP value, one calculates the nuclide efficiency from the theoretical efficiency relation, corresponding to that QIP.

**Triple to double coincidence ratio (TDCR) method** makes use of a special counter, provided with three PMTs, connected in double and triple coincidences. Leaving from the general equation (6), the efficiencies, equal to the probabilities of registration for the logical sum of double, *ε*d , and respectively triple, *ε*t , coincidences were written by (Broda et al.2007; Pochwalski&Radoszewski.1979):

$$\begin{aligned} \boldsymbol{\varepsilon}\_d &= d(\boldsymbol{E}) = 3 \left( \mathbf{1} - e^{-\frac{\boldsymbol{E}Q(\boldsymbol{E})}{3\lambda}} \right)^2 - 2 \left( \mathbf{1} - e^{-\frac{\boldsymbol{E}Q(\boldsymbol{E})}{3\lambda}} \right)^3 \\ \boldsymbol{\varepsilon}\_t &= \boldsymbol{t}(\boldsymbol{E}) = \left( \mathbf{1} - e^{-\frac{\boldsymbol{E}Q(\boldsymbol{E})}{3\lambda}} \right)^3 \end{aligned} \tag{7}$$

activity *N02* of the pure beta radionuclide.The method is applied for therapeutic pure beta radionuclides, like 89Sr, or other important radionuclides as 137Cs (Razdolescu et al.2002a,

The method consists of mixing the radioactive solution with a liquid scintillator (LS). The energy of the radiations is transferred to the LS; it emits light photons, which produce photoelectrons, multiplied in a photomultiplier (PMT). Electron impulses are collected at the PMT anode. The detection efficiency is superior to that of proportional counters, but it depends strongly on the radiations energy. A calculation model, known as the "Free parameter principle" was developed, leaving from the idea that the sum of fenomena: luminous photons emission in LS, their arrival at PMT photocathode and emission of a photoelectron as result of interaction, is described by a Poisson distribution law. Another influence of LS is the quenching; it can be due to its chemistry, but the most important is the ionization quenching, *Q*(*E*), which is described by an empirical relation, written by (Birks.1964)*.* Taking into account all these fenomena, a relation connecting the detection efficiency, *ε,* equal to the probability of producing a photoelectron, *P(λ,E)*, with the counter parameters and ionizing radiation energy, *E* was deduced*.* (Broda et al.2007;

( )

2 3 ( ) ( ) 3 3

 λ

(7)

*EQ E EQ E*

− −

⎛ ⎞⎛ ⎞

⎝ ⎠⎝ ⎠

<sup>−</sup> = = − (6)

λ*R*

(,) 1 *EQ E PE e*

*λ* is called the free parameter and *R* is the number of photomultipliers in the system. In relation (6) it is important to adjust the free parameter such as to reflect the measurement

**CIEMAT/NIST method** consists of the use of a LS counter provided with two PMTs in coincidence, in order to diminish the influence of background; commercial counters can be used. The adjustment of the free parameter is achieved by using an efficiency tracer, consisting of a tritium standard solution. The model contains several steps: a relation is established between the tritium detection efficiency and a quantity equivalent of the free parameter, known as quenching indicator parameter (QIP) (Grau Malonda.1999), measured by using the Compon radiation of an external source; a theoretical relation is calculated between the efficiency of tritium and that of the nuclide to standardize, for different QIP values, based on the beta spectra characteristics. Using a determined QIP value, one calculates the nuclide

**Triple to double coincidence ratio (TDCR) method** makes use of a special counter, provided with three PMTs, connected in double and triple coincidences. Leaving from the general equation (6), the efficiencies, equal to the probabilities of registration for the logical sum of double, *ε*d , and respectively triple, *ε*t , coincidences were written by (Broda et al.2007;

> <sup>3</sup> ( ) 3

λ

= =− −− ⎜ ⎟⎜ ⎟

() 31 2 1

*dE e e*

*EQ E*

−

⎛ ⎞ = = − ⎜ ⎟ ⎜ ⎟ ⎝ ⎠

λ

**3.1.1.3 Advanced methods based on the liquid scintillation counting** 

ε

efficiency from the theoretical efficiency relation, corresponding to that QIP.

() 1

*tE e*

*d*

ε

*t*

ε

conditions. On this purpose, two main models were developed.

 λ

Sahagia.1981) 32P, 90Y.

Pochwalski&Radoszewski.1979):

Pochwalski&Radoszewski.1979):

These relations are calculated theoretically, from the radionuclide spectrum, by itteration for a big number of values of the free parameter λ, by using computer programs, like SPEBETA; TDRCB-1, 2, 7; DETECSZ, etc., elaborated at LNHB-France and RC Poland. On the other hand, an experimental ratio between the counting rates of the two types of coincidences, equal to the efficiencies' ratio, is determined. They depend on the activity *N*0 and efficiencies, as:

$$R\_D = \mathcal{N}\_0 \mathcal{E}\_d \; ; \; R\_T = \mathcal{N}\_0 \mathcal{E}\_t \tag{8}$$

The theoretical ratio of efficiencies, corresponding to the adjusted free parameter is the optimum, when:

$$\frac{\mathcal{E}\_d}{\mathcal{E}\_t} = \frac{\mathcal{R}\_D}{\mathcal{R}\_T} \tag{9}$$

In this manner, the efficiencies are determined, and the activity *N*0 is calculated from relations (8)

A TDCR system was set up in IFIN-HH,RML, with the assistance of Dr. Philippe Cassette from LNHB. It was used for the standardization of radionuclides as 3H, 89Sr, 63Ni and was validated by international comparisons (Razdolescu et al.2004; 2006). Recently, a new counter was set up in RML, replacing the three PMTs with 6 channel photomultipliers (CPM) and designing a new optical chamber (Ivan et al.2010). It has the advantage of being compact and portable.

#### **3.2 Validation of the primary installations and methods; international equivalence 3.2.1 The International System**

The International Committee for Weights and Measures (CIPM) coordinates the metrology branches through the Consultative Committees, as the Comité Consultatif des Rayonnements Ionisants – CCRI. CCRI is divided in three sections. The Section II, CCRI(II) - Measurement of Radionuclides, coordinates the Radionuclide Metrology. The Bureau International des Poids et Mesures-BIPM, Sèvres, http://www.bipm.org France, has the custody of the international standards. The equivalence of the primary standards is assured at this level by absolute methods of standardization. At the regional level, the Regional Metrology Organizations (MRO) are operational. In Europe there exist: EURAMET - European Association of National Metrology Institutes and COOMET - Euro-Asian Cooperation of National Metrology Institutes. The connection between BIPM and MROs is assured by the Joint Committee of the Regional Metrology Organizations and the BIPM - JCRB. At this level one assures the traceability also by using secondary (relative) standardization methods. The CIPM-MRA, Mutual Recognition Arrangement (1999), defines the recognition of the calibration and measurement certificates issued by the National Metrology Institutes. For example, Romania is part of CIPM-MRA. The document contains four annexes: Annex A - List of signatories; the Romanian signatory is the Romanian Bureau of Legal Metrology - National Institute of Metrology (BRML-INM); Annex B - Key Comparison Data Base (KCDB) per Institute; Annex C- Calibration and Measurement Capabilities (CMC); Annex D - Key Comparison Data Base. The condition for the applicability of CIPM-MRA by the signatory countries is the demonstration of the equivalence of the primary standards, or of the traceability for the others, such as presented

Role of the Radionuclide Metrology in Nuclear Medicine 153

(CRPs) and are approved by the CCRI(II); they refer to special types of samples and relative methods are accepted. IFIN-HH, RML is registered as *"CCRI(II)-S6.Radionuclide*", for the

*The Activity Unit is established individually for each radionuclide, after the evaluation of the key* 

Four methods are used to calculate the KCRV (Ratel.2007): arithmetic and weighted mean, median and weighted median. The most adequate variant is selected; in most cases, the arithmetic mean is preferred. The outliers are established after applying the exclusion criteria. The KCRV and its uncertainty are approved by the CCRI(II) and may be modified in time, after accumulating new results for the respective radionuclide. The "outliers" are not eliminated from KCDB, but their difference from KCRV must be taken into account

When both the *SIR-BIPM.RI(II)-K1.Radionuclide* and the *CCRI(II)-K2. Radionuclide*  comparison results are registered for the gamma-ray emitters, a link is established between the two types of comparisons, through the measurement in the BIPM ionization chamber of

*Equivalence.* An equivalence matrix is calculated. Two main quantities are reported and compared: Difference between the individual result and KCRV, (*D*i) and its uncertainty (*U*i), for a coverage interval *k* =2. The ratio of these quantities is the measure of degree of equivalence. The CCRI(II) approved "Draft B" is published in the KCDB and in *Metrologia, Technical Supplement issues*. IFIN-HH, RML, is registered in KCDB with 28 radionuclides. *Equivalence validity.* A validity interval was adopted by consensus. Consequently, in order to maintain the equivalence for some radionuclides, IFIN-HH repeated the comparison. It participated in 1983 and in 2007 at the *SIR-BIPM.RI(II)-K1*.Co-60 comparisons. As for 134Cs, the participations were: *CCRI(II)-K2* in 1978 and *SIR-BIPM.RI(II)-K1* in 2006 and for 137Cs,

Example: *Analysis of the IFIN-HH, RML result at the medical 131I key comparison SIR-BIPM.RI(II)-K.1.I-131, 2007* (Ratel et al.2008). *KCRV* = (40400 ±40) *MBq*; *IFIN-HH* value = (40371 ± 139) *MBq*; *D*i =0.07%; *U*i=0.65% . Consequently, the IFIN-HH, RML's result was

A matrix of radionuclides, codified by colors, according to the standardization difficulty by method, was established by the CIPM-CCRI(II) Key Comparison Working Group (KCWG), within the document Grouping Criteria Radionuclides for Supporting CMCs (Karam.2007). It can be used as follows. A red coded radionuclide measured by LSC, such as 3H, supports

**3.3 Implementation and recognition of the Quality Management System – Approval of** 

An international agreement for the practical use of the results obtained by the National Metrology Institutes (NMI) in assurance of the traceability chain and recognition of the measurements and certificates was concluded through the document: *"Joint statement by the CIPM and ILAC on the roles and responsibilities of national metrology institutes and national recognized accreditation bodies*". At the level of EURAMET, the implementation of the Quality

the participations were: *CCRI(II)-K2* in 1982 and *SIR-BIPM.RI(II)-K1* in 2009.

radionuclides 57Co and 131I, of interest in nuclear medicine.

when reporting the calibration uncertainties.

the ampoules sent for the *K2* comparison.

taken into account in the calculation of the *KCRV*. **SI Activity Unit (Becquerel) for other radionuclides.** 

89Sr, 90Y, LSC measured, and green color coded.

**CMC files** 

**Establishment of the SI Activity Unit (Becquerel) by key comparisons.** 

*comparison result, which is expressed as the Key Comparison Reference Value – KCRV*.

in figure 1. The equivalence is demonstrated only by the participation at international comparisons. IFIN-HH, RML, has participated at international comparisons since 1962. *The Key Comparison Data Base (KCDB) Annex B of the CIPM – MRA*, can be found at the address: http://kcdb.bipm.org/AppendixB/KCDB\_ApB\_search.asp. The participants in CIPM-MRA are the National Institutes of Metrology (NMIs), but for specialized metrology branches as the ionizing radiations, the participants can be the Designated Institutes. In Romania, according to the protocol concluded between BRML-INM and IFIN-HH it is established that IFIN-HH is designed as the owner of the primary activity (Becquerel) and derived quantities standard, with national and international responsibilities in the field:


Due to its scientific authority, IFIN-HH became also: Member of the International Committee for Radionuclide Metrology (ICRM) in 1980 and Member of the Consultative Committee Section II, CCRI(II), in 2004
