Abstract
Daily quality control testing of a γ-camera is of the utmost importance in assessing whether the camera is suitable for clinical use. The aim of our study was to assess the suitability of a fillable 141Ce-based flood field phantom developed in-house for daily quality control testing of γ-cameras. Methods: Daily uniformity testing was performed for 113 d using the fillable 141Ce phantom and a commercially available sheet-type 57Co phantom, and the results were compared. Results: The average integral uniformity obtained by the 141Ce and 57Co phantoms was 3.24% and 2.72%, respectively, for detector 1 and 3.31% and 2.78%, respectively, for detector 2. Conclusion: The 141Ce phantom we developed is a suitable alternative to the commercially available 57Co phantom.
- 141Ce flood field uniformity
- 57Co flood field uniformity
- flood field phantom
- gamma camera
- surrogate radionuclide
The γ-camera, one of the oldest and most widely used imaging devices in nuclear medicine, is a sensitive piece of equipment that can be affected by such factors as power fluctuations and changes in temperature or humidity. Routine quality control testing is critical for any type of nuclear medicine imaging device to ensure the quality of the clinical images it provides. In γ-cameras, such testing evaluates performance regarding uniformity, spatial resolution, spatial linearity, energy resolution, and peaking. Uniformity, or flood, quality control tests check the response of the detectors to uniform irradiation. The detectors pass the test if the obtained uniformity is within the defined reference limits (1–3).
Many possible problems with γ-cameras can lead to degradation of uniformity. Hence, uniformity testing is performed daily before the cameras can be used on patients. The method of testing can be either intrinsic or extrinsic. The intrinsic method, which requires that a 99mTc point source be prepared and the collimator removed every morning, is a tedious, time-consuming, and technically demanding process. In addition, this method does not evaluate the quality of a collimator. Hence, for daily testing in a nuclear medicine department, the extrinsic method is usually chosen. This method requires one of two types of flood field phantom: the fillable type or the sheet type. Fillable phantoms generally need to be refilled with 99mTc each day that the testing is performed. Sheet phantoms generally are 57Co phantoms, which do not require refilling and can be used for up to 2 y (4–6). However, because the cost of 57Co sheet phantoms is quite high, their use in small nuclear medicine departments of developing countries is limited, and many such departments are compelled to use 99mTc fillable phantoms. As an alternative to the 57Co sheet phantom and the 99mTc fillable phantom, a new type of phantom—fillable with 141Ce—has been developed in-house by the Isotope Production and Applications Division of the Bhabha Atomic Research Centre. This phantom requires refilling only every 50–60 d because of the long half-life of 141Ce. 141Ce emits monochromatic γ-photons with an energy of 145.4 keV, which is close to the 140-keV γ-energy of 99mTc, the radioisotope most commonly used for γ-camera imaging (7,8).
MATERIALS AND METHODS
This study took place over a 113-d span during which uniformity tests were performed every morning using both a commercially available 57Co sheet phantom and the 141Ce fillable phantom. The data were stored and analyzed.
Characteristics of Uniformity Phantoms
A uniformity phantom is a rectangular box or sheet that extrinsically irradiates the detectors of a γ-camera with a uniform flux of γ-rays across the field of view. The characteristics of the two radioisotopes (57Co and 141Ce) used in the uniformity phantoms of this study are listed in Table 1.
Parameters for the 57Co and 141Ce Phantoms
Production of 141Ce
141Ce was produced by neutron irradiation of natural Ce(SO4)2⋅4H2O targets in the Dhruva nuclear reactor of Bhabha Atomic Research Centre at various neutron fluxes (ranging from 0.5 × 1013 to 0.5 × 1014 n/cm2/s) for 1–4 mo. A 141Ce solution was extracted from the irradiated targets by radiochemical processing and allowed to decay for a duration sufficient to eliminate the presence of relatively shorter-lived radionuclide impurities such as 137Ce, 139Ce, and 143Ce (9,10).
Preparation of 141Ce Phantom
The phantom, a fillable box made of 6-mm-thick acrylic glass, was designed and produced in-house (7,8). Its inner dimensions were 13.2 mm in depth, 655 mm in length, and 460 mm in width to cover the field of view of most commercially available γ-cameras (Fig. 1).
(A) Commercially available 57Co sheet phantom (RadLite; RadQual, LLC). (B) 141Ce fillable phantom developed in-house by Bhabha Atomic Research Centre.
A trained radiation professional with more than 10 y of experience in preparing phantoms and handling open radioactive materials was tasked with filling the phantom. A double-layer plastic sheet with thick layers of absorbent sheeting was kept beneath the phantom. The filling aperture was surrounded by thick paper to absorb any radioactive solution that might spill.
First, the phantom was filled with distilled water. Then, 740 MBq of 141Ce was added, the phantom was agitated to uniformly dissolve the isotope, and any air bubbles were removed. After preparation of the phantom was complete, the personnel, workplace, and all materials (e.g., gloves, absorbent paper, and plastic sheeting) were surveyed using a Ram Gene-1 radiation meter (Rotem Industries). Any material found to be contaminated was stored as radioactive waste for a duration sufficient for decay. The prepared phantom was not agitated again before being used for testing each morning, as it was assumed that the solution would remain uniform.
Uniformity Testing
The most commonly evaluated performance characteristic in the daily quality control testing of γ-cameras is uniformity. In intrinsic testing, the collimator is removed and a point source of 99mTc is centered over the detector at a distance 5 times the largest dimension of the crystal, providing a near-uniform photon flux impinging on the detector. In extrinsic testing, a fillable phantom source of 99mTc or 141Ce, or a sheet source of 57Co, is placed directly on the collimated detector. Four million counts are acquired, and the integral and differential uniformities of the flood image are quantitated to obtain the deviation from acceptable uniformity. Integral uniformity is calculated using the following equation (5):
where Cmax is the maximum count—and Cmin the minimum count—in a pixel in the useful field of view.
Differential uniformity can be calculated for every 5-pixel segment in every row and column of the flood image using the following equation (5):
where Cmax is the maximum count in a pixel within 5 consecutive pixels in a row or column, and Cmin is the minimum count in a pixel within the same 5 consecutive pixels in a row or column.
Uniformity testing of an Infinia Hawkeye 2 SPECT/CT scanner (GE Healthcare) was performed every morning before it was used on patients. Each day, testing was performed first using the 141Ce fillable phantom and then using the 57Co sheet phantom. The scanning parameters are summarized in Table 1. Uniformity and linearity corrections were applied for the 57Co and 141Ce acquisitions using 57Co and 99mTc uniformity and linearity maps, respectively. Data from the 57Co phantom were processed automatically by the software in the scanner, and data from the 141Ce phantom were processed using uniformity software in the workstation (Xeleris 1.123; GE Healthcare). Table 1 summarizes the parameters applied to calculate integral uniformity for both phantoms, with the integral uniformity of the 57Co phantom being considered the gold standard against which the 141Ce phantom was to be compared. We divided our 113-day study period into halves (i.e., the first 41 tests and the last 48 tests; the remaining 24 d fell on weekends or holidays). We compared the average integral uniformity of the 141Ce phantom with that of the 57Co phantom for the entire study, the first half of the study, and the second half of the study.
Radiation Safety Testing
For both phantoms, a Ram Gene-1 radiation meter was used to determine the radiation exposure of any staff performing the testing. The initial radiation field was measured on all sides of the phantom, both at the surface and at a distance of 1 m, and the maximum and minimum radiation exposure was recorded. The radiation exposure of the professional who filled the phantom was measured using a pocket dosimeter (Aloka Medical, Ltd.).
Statistical Analysis
Two-tailed Mann–Whitney U testing was performed to determine the significance of differences in integral uniformity between the two phantoms at significance levels of 0.1 and 0.5 for the entire study, the first half of the study, and the second half of the study.
RESULTS
The phantoms underwent a background quality control test every morning before the uniformity test and always passed. For both phantoms, integral uniformity was within permissible limits (5%, as prescribed by the manufacturer of the 57Co phantom) throughout the study. The average integral uniformity for the 141Ce and 57Co phantoms was 3.24% and 2.73%, respectively, for detector 1 and 3.32% and 2.78%, respectively, for detector 2 (Table 2), and the average scanning time for the entire study was 836.2 s and 208.6 s, respectively (Table 2). For both detector 1 and detector 2, the average integral uniformity for the 141Ce phantom was comparable to that for the 57Co phantom during the first half of the study (Table 3). The average integral uniformity for the 141Ce and 57Co phantoms during the first half of the study was 2.91% and 2.77%, respectively, for detector 1 and 2.88% and 2.81%, respectively, for detector 2. During the second half, the respective percentages were 3.61% and 2.65% for detector 1 and 3.79% and 2.75% for detector 2. The correlation of integral uniformity between the two phantoms was greater in the first half of the study than in the second half (Fig. 2). Table 4 shows the results of Mann–Whitney U tests; integral uniformity significantly differed between the 57Co and 141Ce phantoms during the entire study and during the second half of the study (P < 0.00001 [both detectors]) but not during the first half of the study (P = 0.18024 [detector 1] and 0.6818 [detector 2]).
Average Integral Uniformity and Scanning Time for 57Co and 141Ce Phantoms
Average Integral Uniformity and Scanning Time for 57Co and 141Ce Phantoms by Study Interval
Graphs of uniformity obtained for 57Co and 141Ce phantoms with detectors 1 (A) and 2 (B).
Mann–Whitney U Test Results
Radiation exposure was also comparable between the two phantoms. With shielding, the average maximum radiation exposure at the surface and 1 m away was 0.2 and 0 μSv/h, respectively, for the 141Ce phantom (containing 669.6 MBq at the beginning) and 2.0 and 0.2 μSv/h, respectively, for the 57Co phantom (containing 285.4 MBq at the beginning). Without shielding, the respective values were 131 and 5 μSv/h for the 141Ce phantom and 150 and 10.7 μSv/h for the 57Co phantom. The radiation exposure of the professional who filled the phantom averaged 8.6 μSv.
DISCUSSION
Various authors (1–6) have suggested different protocols for testing the integral uniformity of a γ-camera, but these can be performed for only a single detector at a time. The test needs to be repeated after the second detector has been rotated in front of the flood. Hence, the testing is cumbersome and time-consuming and requires technical expertise. Extrinsic uniformity testing is performed without removing the collimator and ensures accurate performance of not only the system but also the collimator. Therefore, most nuclear medicine centers use this method for daily uniformity testing of modern γ-cameras. Our in-house–developed 141Ce fillable phantom is an alternative to the 99mTc fillable phantom obviating daily filling with the radioisotope. The 141Ce fillable phantom also eliminates dependency on imported 57Co sheet phantoms. 99mTc is the workhorse isotope for diagnostic procedures on γ-cameras, and a 99mTc fillable phantom is the most suitable for quality control testing of γ-cameras. However, refilling such phantoms is cumbersome and tedious and can risk exposing the staff to an extra radiation burden. Our 141Ce phantom requires refilling only once every 50 d, and in our study the professional who refilled it was exposed to an average of only 8.6 μSv. 141Ce is a soft β-emitter, and the β-emission is attenuated by the acrylic glass of the phantom. 57Co is widely used in phantoms because it has a photopeak (122 keV) close to that of 99mTc (140 keV), has a long half-life (271 d), and is available in the form of ready-to-use sheets. The photopeak of 141Ce (145 keV) is even closer to that of 99mTc; hence, 141Ce may be a suitable replacement for 57Co (6,11).
In our study, a 57Co sheet source with no more than 1% nonuniformity was considered the standard against which the 141Ce filled phantom was to be compared. The comparative uniformity results between the two phantoms were consistent for the first half of the study (i.e., integral uniformity of 2.77% and 2.81% for detectors 1 and 2, respectively, for the 57Co phantom and 2.91% and 2.88% for detectors 1 and 2, respectively, for the 141Ce phantom). In the second half of the study, there were relatively more variations and significant differences in average integral uniformity for both phantoms. Mann–Whitney U tests also showed a significant difference in integral uniformity for both phantoms for the entire study and the second half of the study, whereas for the first half of the study there was no significant difference in integral uniformity. The difference in integral uniformity in the second half of the study can be attributed to the poor counting rate and gradual progression of nonuniformity in the distribution of the 141Ce source. The average scanning time required for the 141Ce phantom to complete the test was around 7 min in the first half of the study, or twice the average time required for the 57Co phantom (3.5 min). In addition, the average scanning time required for the 141Ce phantom to complete the test in the second half of the study was much higher (∼21 min) than that required for the 57Co phantom (3.5 min). The average radiation exposure at the surface of the 141Ce phantom and 1 m away was also comparable to that of the 57Co phantom.
Despite the encouraging results of our study, the suppliers of the 141Ce isotope need to ensure its production and regular delivery to the nuclear medicine department. Production of 141Ce in a reactor is a cumbersome, technically demanding process requiring much effort, expertise, and professional commitment. However, the scientists at the Bhabha Atomic Research Centre have mastered the technique for easily producing this isotope on a regular basis, thus ensuring a regular supply to the clinical nuclear medicine department.
Although the supply of the 141Ce isotope is not yet commercialized, the cost to produce it in our country (India) is not high (U.S. $30–$40/GBq), nor is the cost to supply it to the nuclear medicine department (U.S. $50–$60/GBq). Because the phantom will require refilling every 50 d, the total cost of a 2-y supply of the isotope will be approximately U.S. $750, as compared with U.S. $5,500 for one commercially available 57Co phantom, which will also need replacement every 2 y. Extra radiation safety precautions need to be taken to avoid any major spillage of the 141Ce isotope during phantom preparation. We follow and recommend stringent precautions and use the utmost care during handling of this isotope. The low cost and ease of handling of the 141Ce phantom will facilitate its use for daily uniformity testing in the nuclear medicine department.
CONCLUSION
Our in-house–developed 141Ce fillable phantom provided a reasonable result that was consistent with that of a standard 57Co sheet phantom. The 141Ce phantom may need to be refilled only once every 50 d, greatly reducing physical effort, time, and radiation exposure. Considering the easy availability and low cost of this 141Ce fillable phantom, it may be considered a suitable alternative to 57Co sheet phantoms and 99mTc fillable phantoms.
DISCLOSURE
No potential conflict of interest relevant to this article was reported.
Footnotes
Published online Apr. 13, 2017.
REFERENCES
- Received for publication January 6, 2017.
- Accepted for publication March 29, 2017.
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