We evaluated 2 whole-body counters that had different types of detectors and geometry: the Fastscan (Model 2250; Canberra, Meriden, CT, USA) and the Accuscan (Model 2260; Canberra, Meriden, CT, USA).

The Fastscan is a stand up type whole-body counter, in which the subject stands encircled by a shield encompassing the detector, which rests parallel to the subject. The Fastscan contains 2 NaI(T1) detectors with a dimension of 7.6× 12.7×40.6 cm³ that are positioned serially at the top and bottom. The Fastscan was designed on the basis of anthropometric data; the detectors are positioned such that they are able to scan the entire body. The counter is able to monitor nuclides with energies between 300 keV and 1.8 MeV.1),2)

The Accuscan is a horizontal bed type whole-body counter in which the subject undergoes counting in a lying position rather than a standing position. Since the subject is in a relatively comfortable position, the length of the count can be extended and subjects with disabilities can be measured more easily. The Accuscan contains 2 high-purity Ge (HPGe) detectors. The HPGe detector has a relative efficiency of 25% to the 7.6×7.6 cm² NaI(T1) detector. The scanning range of the detectors measures 2 m and fully encompasses the areas of nuclide deposition in the body, and this configuration reduces bias and inaccuracies in measurement arising from variations in body size or in the location of nuclide deposits.3)

Both types of whole-body counters are encased on all sides except the anterior face in 10-cm-thick, low-background, lead shielding. Counts from background radiation were eliminated by using specially-manufactured ⁶⁰Co-free lead shielding.

The efficiency calibration of whole-body counters requires the use of a phantom that contains radioactive material with known certified values. Usually, the form and the mass attenuation coefficient of the calibration phantom are similar to the human body in order to reflect the counting efficiency that can be obtained using real subjects. In this study, we used an adult male bottle manikin absorption (BOMAB) phantom and a Radiation Management Corporation (RMC-II) phantom to calibrate the counting efficiency of the whole-body counters. Further, we compared the counting efficiency of each phantom by photon energy relative to the standard counting efficiency of the BOMAB phantom. The calibrated efficiency was expressed using a fourth-order polynomial function.

The BOMAB phantom is the standard calibration phantom for whole-body counting [11]. The energy source of this phantom is able to release over 200 keV of photon energy, and it has been designed as a phantom for measuring isotopes homogenously distributed over the body [12, 13]. The BOMAB phantom was manufactured on the basis of a report of the International Commission on Radiological Protection (ICRP) that provided detailed anthropometric data [14]. The BOMAB phantom is composed of 10 cylinders corresponding to different body parts. Depending on the body part to which they correspond, the cylinders are either elliptical or circular. Elliptical cylinders are used for the head, chest, and gut, while circular cylinders are used for the calves, thighs, arms, and neck. Each cylinder is made of high-density polyethylene and the hollow space is filled with 0.1 M HCl. Polyethylene is the preferred constituent material of phantoms because the mass attenuation coefficient of polyethylene remains within 10% of the mass attenuation coefficient of soft tissue across photon energies ranging from 200 keV to 3 MeV, thereby closely resembling the conditions of soft tissue [12]. The certified reference material (CRM) used in the BOMAB phantom is a liquid solution containing 9 types of nuclides that emit gamma rays (Eckert & Ziegler Analytics, Braunschweig, Germany). The range of energy emission of this CRM is 59.5–1,836.1 keV, and the relative expanded uncertainty of the activity is within 2.8% (κ=2). In order to simulate the homogeneous distribution of the isotope throughout the body, the CRM was homogenously distributed within the BOMAB phantom.

The RMC-II phantom was developed by Canberra (Meriden, CT, USA) to make phantoms more user-friendly and cost-effective. The counting efficiency of the American National Standards Institute (ANSI) N13.30 phantom was taken into account in its manufacture, and the RMC-II phantom was designed to be used for calibrating whole-body counters with a linear geometry [15]. The ANSI N13.30 phantoms, which are reflected in the RMC-II phantom, include the BOMAB phantom, the ANSI N44.3 thyroid phantom, and the Lawrence Livermore National Laboratory (LLNL) lung phantom [11]. The RMC-II phantom is manufactured so that 20 mL of a CRM can be inserted in any one of the 4 cavities, which are geometric imitations of the following anatomical structures (from top to bottom): the thyroid, the lungs, the whole body, and the gastrointestinal tract. The structure of the RMC-II phantom can be divided into the neck and body. The body is made from an acrylic board, the thickness of which is manufactured in a way that most accurately simulates how radiation sources are received by detectors. The neck is also made from an acrylic board and its cylindrical structure is manufactured according to the dimensions suggested in ANSI N44.3 [16]. When the RMC-II phantom is used to calibrate the counting efficiency, the height of the whole-body counter is leveled such that the floor of the phantom is at the level of the waist of the examiner (with a resulting height of 91 cm) [15]. Thus, the phantom was placed at the appropriate height in the Fastscan and in the Accuscan to perform the efficiency calibration. The CRM used in the RMC-II phantom was sealed in a 20-mL container; it contained eight types of nuclides in the form of a gel-type mixed gamma source, and was manufactured by the Korea Research Institute of Standards and Science (KRISS). The rang e of the photon energy of the CRM is 59.5–1,836.1 keV, which is similar to that of the CRM used in the BOMAB phantom, and the relative expanded uncertainty of activity is within 4.0% (κ=2). Of the ray portals of the RMC-II phantom, the portal most reflective of a homogeneously distributed isotope was used.

Performance test was conducted subsequent to the efficiency calibration. For the performance testing, we used a BOMAB phantom containing gamma ray-emitting nuclides distinct from CRM used for the efficiency calibration. The BOMAB phantom used for verification testing had the same geometry as the male adult BOMAB phantom used for the efficiency calibration and included the following nuclides: ²⁴¹Am, ¹³⁴Cs, ¹³⁷Cs, ⁶⁰Co, and ¹³³Ba. Of these, we used ¹³⁴Cs and ¹³³Ba to analyze the performance parameters, as these nuclides had energies different from those used during the calibration.

The whole-body verification counts for the Fastscan were collected for 240 seconds for the BOMAB phantom, whereas a duration of 1,800 seconds was used for the Accuscan. Each measurement was taken 5 times. These verification counting times were the same as those used for actual counting procedures. The measurements of both the Fastscan and Accuscan were analyzed using the counting efficiencies of the BOMAB phantom and the RMC-II phantom. Additionally, the measurements were evaluated in relation to the certified values of the verification BOMAB phantom in order to determine whether the counters were successfully calibrated. We assessed the measurements according to the International Organization for Standardization (ISO) 28218 criteria; specifically, we evaluated relative bias (Br) and repeatability (S_Br), which can be used to estimate the accuracy and the precision of the measurement. The following performance criteria were applied: -0.25<Br<0.50 and S_Br≦0.4. We calculated B_r and S_Br using the equations presented below [17].

In this equation, B_ri denotes the relative bias of the i^th measurement, Ai denotes the i^th measurement of radioactivity, and A_ai denotes the certified value of the radioactive sample. The relative bias (B_r) is the average of the relative biases of the measurements and can be evaluated using the following equation:

Here, n denotes the number of total measurements. Repeatability can be calculated using the following equation:

The intercomparison necessitates transportation of the phantoms between laboratories. As they are designed to simulate the whole body, it is unsurprising that they have very large volumes, and their transportation is hazardous due to potential radioactive leakage. IAEA sliced BOMAB phantoms, the radioactive material sealed in paper filters, was used in intercomparison in order to prevent incidents of radioactive leakage during delivery from the IAEA to the laboratory.

Previous intercomparison have shown that the agreement between the sliced BOMAB phantom and the conventional BOMAB phantom was ±8% in whole-body counters of various geometries [18]. These findings support the fact that the sliced BOMAB phantom closely resembles the homogenous distribution of radioactive material in the body. The body of the sliced BOMAB phantom is divided into several slices, with the thickness of each slice either 1.9 cm or 1.27 cm. The body is mainly made up of the thicker slices, and the thinner slices are integrated in between the larger slices to make the lengths of the phantom even [18]. As shown in Figure 1, each region of the sliced BOMAB phantom consists of 2 column, and the slices are connected to one another through these column. Between each slice, a surface source, which contains a homogeneous composite of the CRM, was inserted, and this conformation simulates how the radioactive material is homogeneously distributed across the body.

The counts were collected for 240 seconds for the Fastscan and for 1,800 seconds for the Accuscan; these durations are those used in actual procedures. Each measurement was repeated 5 times. Figure 2 shows how the the sliced BOMAB phantom was positioned in the Fastscan and in the Accuscan. The gamma spectrum of radionuclides that was measured with each whole-body counter was analyzed based on efficiency calibrations made using the BOMAB phantom and the RMC-II phantom. Additionally, we performed an intercomparison using the certified values of the sliced BOMAB phantom as the reference. The results of the intercomparison, comprising the assessment parameters and the ISO 28218 performance criteria, are illustrated in paragraph 2.3.

The counting efficiency of the Fastscan calibrated using the BOMAB phantom and the RMC-II phantom is presented in Figure 3. The disparity in counting efficiency between the phantoms at high photon energies (above 600 keV) was minimal (0%–2%). However, the disparity in counting efficiency between the phantoms at low photon energies (below 600 keV) was very large (-16% to 47%). The disparity seen in counting efficiency at low photon energies is thought to stem from the fact that low photon energies tend to be more affected by the geometry of the counters [19].

The counting efficiency of the Accuscan calibrated using the BOMAB phantom and the RMC-II phantom is presented in Figure 4. We found that the disparity in counting efficiency between the phantoms was large across all photon energies for the Accuscan. We compared 2 distinct levels of photon energy (above and below 600 keV). We found that the disparity in counting efficiency between the 2 phantoms at low photon energies was between -19% and -33%, defined relative to the BOMAB phantom counting efficiency. The disparity in the counting efficiency between phantoms at high photon energies ranged from -7% to -13%; thus, the disparity was less prominent than that seen at low photon energies but larger than that observed in the Fastscan at equivalent energies. We believe that this striking disparity can be ascribed to the geometry of the Accuscan, which scans the body from the legs to the head. The Accuscan is able to scan the BOMAB phantom from the lower end to the upper end, but is able to scan only the region of the RMC-II phantom equivalent to the upper body of the subject; as the scanning area of the Accuscan fails to encompass the lower end of the RMC-II phantom, which in fact lies beyond the shielding of the detector.

For the RMC-II phantom to be a valid alternative to the conventional BOMAB phantom, the counting efficiency of the RMC-II phantom must be comparable to that of the conventional BOMAB phantom, which is the standard phantom for calibration. However, we found that the counting efficiencies of the RMC-II phantom and the BOMAB phantom showed large discrepancies in the Fastscan at low photon energies (below 600 keV) and in the Accuscan counter across all photon energies. We suggest that further studies investigate this discrepancy.

A BOMAB phantom was used to calibrate the Fastscan and the Accuscan prior to analyzing them with the counting efficiency of either the RMC-II or the BOMAB phantom. The S_Br and the B_r of the measurements are summarized in Figure 5. The bars with diagonal lines denote the performance criteria (0.25<B_r<0.50 and S_Br ≦ 0.4) derived from bioassays as indicated in the ISO 28218 criteria. Measurements within these parameter ranges can be considered to indicate that the performance criteria were met.

When we analyzed the measurement results of the Fastscan using counting efficiency of the BOMAB phantom, we found that the B_r of ¹³³Ba (356 keV) was -0.09 and that the B_r of ¹³⁴Cs (605 keV) was -0.06. The S_Br values of ¹³³Ba and ¹³⁴Cs were 0.02. We likewise analyzed the results of the Fastscan using counting efficiency of the RMC-II phantom. The B_r for ¹³³Ba was -0.13, and the B_r of ¹³⁴Cs was -0.02. The S_Br of ¹³³Ba was 0.06, and that of ¹³⁴Cs was 0.02. All analyzed results of Fastscan using 2 phantoms were within the acceptable performance criteria.

We analyzed the measurement results of the Accuscan using counting efficiency of the BOMAB phantom and the RMC-II phantom. For the BOMAB phantom, the B_r of ¹³³Ba was 0.07 and the B_r for ¹³⁴Cs was -0.01. The S_Br of ¹³³Ba was 0.06 and the S_Br for ¹³⁴Cs was 0.02. For the RMC-II phantom, the B_r of ¹³³Ba was 0.31 and the B_r of ¹³⁴Cs was 0.12. The S_Br of ¹³³Ba was 0.06, and the S_Br of ¹³⁴Cs was 0.02. These findings show that the Accuscan satisfied the performance criteria regardless of the calibration phantom. However, using the counting efficiency of the RMC-II phantom led to a B_r value of ¹³³Ba that was more than 4 times greater than that observed when the counting efficiency of the BOMAB phantom was used, and a B_r value of ¹³⁴Cs that was more than 12 times greater. We therefore suggest that the assessment of radionuclides in the whole body with counting efficiency calibrated using the RMC-II phantom may impair the accuracy.

The measurements for the sliced BOMAB phantom are illustrated in Figure 6. The bars filled with diagonal lines denote values derived from the Fastscan, whereas those filled with horizontal lines denote values derived from the Accuscan. The data are also grouped to indicate results obtained using counting efficiency calibrated using the BOMAB phantom versus results obtained using the RMC-II phantom, and the reference activity of each nuclide is indicated with a horizontal line. The nuclides contained in the IAEA sliced BOMAB phantom are ⁶⁰Co, ¹³⁷Cs, and ²⁴¹Am. As described earlier, in our intercomparison of counting efficiency by phantom type, at photon energies exceeding 600 keV, such as for ⁶⁰Co (1,172 keV/1,332 keV) and ¹³⁷Cs (662 keV), we found that the measurements were similar between the phantoms, as indicated by minimal differences in their counting efficiencies. When we evaluated the difference in the measurements using counting efficiency of the BOMAB phantom and the RMC-II phantom in the 2 types of counters, the difference in measurements of ⁶⁰Co, which has the highest photon energy, was only 3% for the Fastscan and 7% for the Accuscan. The difference in measurements of ¹³⁷Cs was -5% for the Fastscan and 15% for the Accuscan; these differences were larger than were found for ⁶⁰Co but still about ±10%. However, for nuclides with photon energies lower than 600 keV, such as ²⁴¹Am (59 keV), we found that the difference in measurements was -38% for the Fastscan counter and 108% for the Accuscan counter, which is a much larger difference.

The radioactivity of the sliced BOMAB phantom, which was analyzed through either the counting efficiency of the BOMAB phantom or that of the RMC-II phantom, was assessed following the ISO 28218 criteria. The results of the intercomparative analysis are presented in Figure 7 for Fastscan and in Figure 8 for Accuscan.

The S_Br of the radioactivity measurements of the Fastscan ranged from 0.02 to 0.10, and all measurements satisfy the acceptable performance criteria irrespective of the type of calibration phantom used. For the Fastscan, the B_r of measurements derived from the counting efficiency of the BOMAB phantom ranged from -0.16 to 0.01, satisfying the performance criteria, but those of measurements derived from the counting efficiency of the RMC-II phantom were 0.01 for ⁶⁰Co, -0.05 for ¹³⁷Cs, and -0.49 for ²⁴¹Am. Therefore, the B_r values of ²⁴¹Am measurements obtained from the Fastscan calibrated with the RMC-II phantom were suboptimal and failed to meet the performance criteria.

Similarly to the values obtained from the Fastscan, the S_Br values of the Accuscan (0.02–0.34) satisfied the performance criteria, irrespective of the calibration phantom used. However, whether the B_r of the Accuscan was satisfactory depended on the calibration phantom. The B_r of the nuclides ranged from -0.01 to and 0.03 when the BOMAB phantom counting efficiency was used, satisfying the performance criteria. However, the B_r was 0.03 for ⁶⁰Co, 0.15 for ¹³⁷Cs, and 0.55 for ²⁴¹Am when the RMC-II phantom counting efficiency was used, which means that ⁶⁰Co and ¹³⁷Cs met the performance criteria but ²⁴¹Am did not.

All performance criteria were met when the efficiency calibration was performed using the BOMAB phantom, but not when the efficiency calibration was performed using the RMC-II phantom. The counting efficiency of the RMC-II phantom at low photon energies failed to satisfy the criteria for accurate measurements.