[Dosimetric Properties of Brass Mesh Bolus for High-energy Photon Beam].

PURPOSE: To investigate fundamental dosimetric properties of surface dose, exit dose, and beam profile of the brass mesh bolus for 4, 6, and 10 MV high-energy photon beams in radiation therapy. METHODS: Surface dose and exit dose in the water-equivalent phantom were measured, and percent depth doses (PDDs) were calculated with no bolus, one layer of brass mesh, two layers of brass mesh bolus, three layers of brass mesh bolus, and 0.5 cm tissue-equivalent (TE) bolus. Exit dose was measured at a phantom thickness of 10 cm. Beam profiles were measured at phantom depths of 0 cm and 10 cm. All dosimetry was performed for 4, 6, and 10 MV photon beams using a linear accelerator. RESULTS: The surface dose at a phantom depth of 0 cm increased to 37.3%, 36.3%, and 31.0% for 4, 6, and 10 MV, respectively, with the brass mesh bolus compared to the case of no bolus. The surface dose decreased with one layer of brass mesh bolus compared to that with the 0.5 cm TE bolus. On the other hand, the exit dose increased to 22.0%, 23.1%, and 22.8% for 4, 6, and 10 MV, respectively, with the brass mesh bolus compared to the case of no bolus. The beam profile at the depth of 0 cm showed oscillations, and the difference between the maximum and minimum doses was up to 13.1% with one layer of brass mesh bolus. CONCLUSION: It was suggested that the brass mesh bolus not only increases the surface dose but also has different properties from the conventional TE bolus.

[Reduction of Operator Exposure Radiation Dose and Evaluation of Image Quality Using Half Scan in CT Fluoroscopy].

In recent years, the number of examinations and treatments using computed tomography fluoroscopy (CTF) has been increasing, and there is concern about an increase in the exposure radiation dose of the operator. Use of half scan CTF can be expected to reduce the exposure radiation dose, but there is no report. The purpose of this study was to evaluate the exposure radiation dose at the operator's position and image quality when using a half scan CTF. The left side facing the gantry was the operator's position, and the ambient dose equivalent at 160 cm, 130 cm, and 100 cm from the floor was measured using an ionization chamber survey meter. The absorbed dose at the forceps holding position of the operator was measured using a fluorescent glass dosimeter with the forceps holding position 15 cm caudal from the scan center. The imaging conditions used a tube voltage of 120 kV and a tube current of 50 mA. Half scan CTF was performed by changing the center angle of the half scan on the console every 45°. As a result, the set angles were 135°and 90°at the operator's position, and 135°at the operator's forceps holding position. In addition, we evaluated the effect of half scan CTF on image quality. CTF images were collected with a cryogenic needle used for cryotherapy punctured in a water-equivalent self-made phantom. The profile curves of the obtained images were drawn and compared using analysis software to evaluate the effects of artifacts. Then, the SD of the CT value of the region of interest with and without the artifact was measured, and the relative artifact index was calculated and evaluated. Using the same image, CT value and SD were tested to evaluate noise. Half scan CTF had no effect on the image quality due to artifacts and noise.

[Evaluation of the Number of Sampling Angles in Three Dimensional Ordered Subset Expectation Maximization: Comparison of Step and Shoot Acquisition and Continuous Acquisition].

Three dimensional ordered subset expectation maximization (3D-OSEM) improves spatial resolution and contrast. Continuous acquisition, and step and shoot acquisition are used in single photon emission computed tomography (SPECT). The purpose of this study was to evaluate the effect of 3D-OSEM when acquisition method was different. We evaluated spatial resolution using a line source phantom and uniformity using a pool phantom. The phantoms were acquired by step and shoot acquisition and continuous acquisition at changing step angles. These projection data were reconstructed using filtered back projection (FBP) and 3D-OSEM. We evaluated reconstruction images using the full width half maximum (FWHM) of line source and root mean square uncertainty (%RMSU) of pool phantom. 3D-OSEM improved spatial resolution and uniformity compared with FBP. Change of FWHM in radial direction and %RMSU by using 3D-OSEM was approximately equal to continuous acquisition in step and shoot acquisition. However, even if using 3D-OSEM, distance between center of rotation and the location of line source is long, a large sampling step angle produced an increase FWHM in tangential direction using continuous acquisition. Step angles need to be set based on the sampling theorem using continuous acquisition.

Evaluation of the distribution of activation inside a compact medical cyclotron.

The distribution of activation inside a compact medical cyclotron was evaluated by measuring 1cm dose equivalent rates and γ-ray spectra. Analysis of the distribution of activation showed high activation at the deflector and the magnetic channel. Radionuclides 60Co, 57Co, 65Zn, and 54Mn were detected. Different radionuclides were generated from different components of the cyclotron, and low-activity radionuclides could be detected under low-background-radiation conditions.

Decontamination of the Activation Product Based on a Legal Revision of the Cyclotron Vault Room on the Non-self-shield Compact Medical Cyclotron.

The non-self-shield compact medical cyclotron and the cyclotron vault room were in operation for 27 years. They have now been decommissioned. We efficiently implemented a technique to identify an activation product in the cyclotron vault room. Firstly, the distribution of radioactive concentrations in the concrete of the cyclotron vault room was estimated by calculation from the record of the cyclotron operation. Secondly, the comparison of calculated results with an actual measurement was performed using a NaI scintillation survey meter and a high-purity germanium detector. The calculated values were overestimated as compared to the values measured using the NaI scintillation survey meter and the high-purity germanium detector. However, it could limit the decontamination area. By simulating the activation range, we were able to minimize the concrete core sampling. Finally, the appropriate range of radioactivated area in the cyclotron vault room was decontaminated based on the results of the calculation. After decontamination, the radioactive concentration was below the detection limit value in all areas inside the cyclotron vault room. By these procedures, the decommissioning process of the cyclotron vault room was more efficiently performed.

Monte Carlo simulation of PET and SPECT imaging of 90Y.

PURPOSE: Yittrium-90 ((90)Y) is traditionally thought of as a pure beta emitter, and is used in targeted radionuclide therapy, with imaging performed using bremsstrahlung single-photon emission computed tomography (SPECT). However, because (90)Y also emits positrons through internal pair production with a very small branching ratio, positron emission tomography (PET) imaging is also available. Because of the insufficient image quality of (90)Y bremsstrahlung SPECT, PET imaging has been suggested as an alternative. In this paper, the authors present the Monte Carlo-based simulation-reconstruction framework for (90)Y to comprehensively analyze the PET and SPECT imaging techniques and to quantitatively consider the disadvantages associated with them. METHODS: Our PET and SPECT simulation modules were developed using Monte Carlo simulation of Electrons and Photons (MCEP), developed by Dr. S. Uehara. PET code (MCEP-PET) generates a sinogram, and reconstructs the tomography image using a time-of-flight ordered subset expectation maximization (TOF-OSEM) algorithm with attenuation compensation. To evaluate MCEP-PET, simulated results of (18)F PET imaging were compared with the experimental results. The results confirmed that MCEP-PET can simulate the experimental results very well. The SPECT code (MCEP-SPECT) models the collimator and NaI detector system, and generates the projection images and projection data. To save the computational time, the authors adopt the prerecorded (90)Y bremsstrahlung photon data calculated by MCEP. The projection data are also reconstructed using the OSEM algorithm. The authors simulated PET and SPECT images of a water phantom containing six hot spheres filled with different concentrations of (90)Y without background activity. The amount of activity was 163 MBq, with an acquisition time of 40 min. RESULTS: The simulated (90)Y-PET image accurately simulated the experimental results. PET image is visually superior to SPECT image because of the low background noise. The simulation reveals that the detected photon number in SPECT is comparable to that of PET, but the large fraction (approximately 75%) of scattered and penetration photons contaminates SPECT image. The lower limit of (90)Y detection in SPECT image was approximately 200 kBq/ml, while that in PET image was approximately 100 kBq/ml. CONCLUSIONS: By comparing the background noise level and the image concentration profile of both the techniques, PET image quality was determined to be superior to that of bremsstrahlung SPECT. The developed simulation codes will be very useful in the future investigations of PET and bremsstrahlung SPECT imaging of (90)Y.

Distribution of residual long-lived radioactivity in the inner concrete walls of a compact medical cyclotron vault room.

OBJECTIVE: Compact medical cyclotrons have been set up to generate the nuclides necessary for positron emission tomography. In accelerator facilities, neutrons activate the concrete used to construct the vault room; this activation increases with the use of an accelerator. The activation causes a substantial radioactive waste management problem when facilities are decommissioned. In the present study, several concrete cores from the walls, ceiling and floor of a compact medical cyclotron vault room were samples 2 years after the termination of operations, and the radioactivity concentrations of radionuclides were estimated. METHODS: Cylindrical concrete cores 5 cm in diameter and 10 cm in length were bored from the concrete wall, ceiling and floor. Core boring was performed at 18 points. The gamma-ray spectrum of each sample was measured using a high-purity germanium detector. The degree of activation of the concrete in the cyclotron vault room was analyzed, and the range and tendency toward activation in the vault room were examined. RESULTS: (60)Co and (152)Eu were identified by gamma-ray spectrometry of the concrete samples. (152)Eu and (60)Co are produced principally from the stable isotopes of europium and cobalt by neutron capture reactions. The radioactivity concentration did not vary much between the surface of the concrete and at a depth of 10 cm. Although the radioactivity concentration near the target was higher than the clearance level for radioactive waste indicated in IAEA RS-G-1.7, the mean radioactivity concentration in the walls and floor was lower than the clearance level. CONCLUSION: The radioactivity concentration of the inner concrete wall of the medical cyclotron vault room was not uniform. The areas exceeding the clearance level were in the vicinity of the target, but most of the building did not exceed the clearance levels.

An anthropomorphic phantom study of brain dopamine transporter SPECT images obtained using different SPECT/CT devices and collimators.

UNLABELLED: The aim of this study was to evaluate differences in dopamine transporter SPECT images among different SPECT/CT devices and to determine the most appropriate region of interest (ROI) for semiquantitative evaluation. METHODS: An anthropomorphic striatal phantom was filled with (123)I solutions of different striatum-to-background radioactivity ratios. Data were acquired using 2 SPECT/CT devices equipped with low- to medium-energy general-purpose and low-energy high-resolution (LEHR) collimators. The SPECT images were reconstructed by filtered backprojection with both attenuation and scatter correction and then were analyzed using specific binding ratio (SBR). The most appropriate of 7 ROI types was determined, and we then compared the linearity and recovery of SBR among the different SPECT/CT devices and collimators. RESULTS: The linearity of SBR was excellent for all types of ROIs. The ROI contouring the striatum based on the CT images showed the best recovery of SBR using mean activity in the striatal ROI (SBRmean) (47.8%). For this ROI, the recovery of SBRmean for SPECT/CT with a LEHR collimator with thick septa and a long hole length was 61.6%-significantly higher than that of other devices. CONCLUSION: The ROI contouring the striatum based on CT images was considered appropriate for evaluating dopamine transporter SPECT/CT. Among the different SPECT/CT devices, an LEHR collimator designed for (123I)I imaging is recommended.

Accuracy of amplitude-based respiratory gating for PET/CT in irregular respirations.

OBJECTIVE: We evaluated the accuracy of amplitude gating PET (AG-PET) compared with phase gating PET (PG-PET) in relation to respiratory motion patterns based on a phantom analysis. METHOD: We used a NEMA IEC body phantom filled with an (18)F solution with a 4:1 sphere-to-background radioactivity ratio (12.6 and 2.97 kBq/mL). PET/CT scans were acquired in a motionless and moving state on a Biograph mCT. The respiratory movements were simulated by four different waveform patterns consisting of ideal breathing, breathing with a pause period, breathing with a variable amplitude and breathing with a changing baseline. AG-PET selects the narrow bandwidth containing 20 % of the respiratory cycle. PG-PET was reconstructed with five gates. The image quality was physically assessed using the percent contrast (Q H,10mm), background variability (N 10mm) recovery coefficient (RC), and sphere volumes. RESULT: In regular motion patterns with ideal breathing and breathing with a pause period, the Q H,10mm, RC and sphere volumes were not different between AG-PET and PG-PET. In the variable amplitude pattern, the Q H,10mm of AG-PET was higher than that of PG-PET (35.8 vs 28.2 %), the RC of AG-PET was higher than that of PG-PET and sphere volume of AG-PET was smaller than that of PG-PET (6.4 vs 8.6 mL). In the changing baseline pattern, the Q H,10mm of AG-PET was higher than that of PG-PET (42.4 vs 16.7 %), the RC of AG-PET was higher than that of PG-PET and sphere volume of AG-PET was smaller than that of PG-PET (6.2 vs 9.8 mL). The N 10mm did not differ between AG-PET and PG-PET, irrespective of the motion pattern. CONCLUSION: Amplitude gating PET is considered to be more accurate than phase gating PET for examining unstable respiratory motion patterns, such as those involving a variable amplitude or changing baseline.

[Patient dose measurement with fluorescent glass dosimeter: characteristics evaluation and patient skin dose measurement in abdominal interventional radiology].

In this study, we investigated the usefulness of the fluorescent glass dosimeter for measuring patient dose. The fluorescent glass dosimeter is constructed of a glass element and its holder. One type has a tin (Sn) filter and the other does not. The characteristics of these two types of fluorescent glass dosimeters were studied in the range of diagnostic X-ray energy. The result was excellent for each characteristic. Directional dependency, however, was recognized in the fluorescent glass dosimeter with tin (Sn) filter. Based on these evaluations, patient skin dose was measured for abdominal interventional radiology and diagnostic digital subtraction angiography using the holder without filter, which is less direction-dependent and eliminates obstructive shadows in radiography and fluoroscopy. The average skin dose of 30 patients for abdominal IVR was 1.17+/-0.44 Gy (0.51-1.94 Gy), while those for diagnostic DSA examination was 0.54+/-0.21 Gy (0.15-1.02 Gy). The fluorescent glass dosimeter provides high capability for skin dose measurement. The fluorescent glass dosimeter is also useful for controlling patient dose during IVR procedures.