Experimentally determined and Monte Carlo-calculated energy dependence of NaCl pellets read by optically stimulated luminescence for photon beams in the energy range 30 keV to 1.25 MeV.

Ordinary salt, NaCl, has many properties suitable for dosimetry and has been suggested for both retrospective and prospective optically stimulated luminescence (OSL) dosimetry. Lately, the focus has been on NaCl that is compressed into solid pellets, as this improves both its handling and dosimetric properties. In this project, the energy dependence of NaCl pellets produced in-house was investigated for photon energies between 30 and 1.25 MeV. The NaCl pellets were first exposed to free-in-air conditions, and the estimated absorbed dose to the NaCl pellets was compared to the air kerma,Kair, at the point of exposure. Second, a backscatter medium of polymethyl methacrylate (PMMA) was added, and NaCl pellets were exposed when positioned on a ISO slab phantom to relate the response in the NaCl to the personal dose equivalent,Hp(10). The results show a significant energy dependence for exposure to low-energy photons with a peak over-response compared toKairandHp(10) of up to 18. Comparisons with Monte Carlo simulations show good agreement, even though the simulations cannot account for properties related to the intrinsic luminescence effects of the NaCl pellets or the readout and calibration process. The finite thickness of the NaCl pellet makes it an imperfect Bragg-Grey cavity, which complicates the behaviour of the energy dependence. The results presented here may serve as an important basis for further experimental and theoretical modelling of a build-up layer and filters in efforts to develop a passive personal dosemeter based on NaCl.

Pseudohelical scan for the dose profile measurements of 160-mm-wide cone-beam MDCT.

OBJECTIVE: Our objective was to determine what the reasonable total phantom length should be for the measurements and determination of length equilibrium, specifically for the Aquilion ONE cone-beam MDCT system. MATERIALS AND METHODS: Radiation dose measurements of a 160-mm-wide cone-beam-MDCT scanner and its radiation dose profile require a different approach than the traditional or conventional method using thermoluminescent dosimeters or small ionization chambers, which have been suggested by some investigators. In order to obtain the radiation dose profile of a cone-beam MDCT, two key elements must be addressed: proper instrumentation for the detection of radiation beam and inclusion of the tails of the radiation dose profile. In this study, a small (2 x 2 x 0.3 mm) solid-state detector was used to measure the dose profile, which required the introduction of a stepping motor to pull the detector through the phantom. Inclusion of the tails of the radiation dose profile meant more than one standard CT dose index phantom would be required to encompass the dose profile tails as much as practically possible. In fact, at minimum, a total of five standard CT dose index (CTDI) phantoms would be required to ensure the entire dose profile is included and detected. RESULTS: In the case of Toshiba Aquilion ONE MDCT with the maximum beam width of 160 mm, the phantom length that is required for the radiation dose profile measurement should be at least 750 mm, or 5 standard CTDI body phantoms. Current CTDI measurements utilizing 150 mm or 350 mm phantom lengths significantly underestimate the total dose of wide cone-beam MDCT. CONCLUSION: The measurement method outlined in this study amounts to an introduction of a new CT dose profile measurement using a pseudohelical scan.

Absorbed dose and dose rate using the Varian OBI 1.3 and 1.4 CBCT system.

According to published data, the absorbed dose used for a CBCT image acquisition with Varian OBI v1.3 can be as high as 100 mGy. In 2008 Varian released a new OBI version (v1.4), which promised to reduce the imaging dose. In this study, absorbed doses used for CBCT image acquisitions with the default irradiation techniques of Varian OBI v1.3 and v1.4 are measured. TLDs are used to derive dose distributions at three planes inside an anthropomorphic phantom. In addition, point doses and dose profiles inside a 'stack' of three CTDI body phantoms are measured using a new solid state detector, the CT Dose Profiler. With the CT Dose Profiler, the individual pulses from the X-ray tube are also studied. To verify the absorbed dose measured with the CT Dose Profiler, it is compared to TLD. The image quality is evaluated using a Catphan phantom. For OBI v1.3, doses measured in transverse planes of the Alderson phantom range between 64 mGy and 144 mGy. The average dose is around 100 mGy. For OBI v1.4, doses measured in transverse planes of the Alderson phantom range between 1 mGy and 51 mGy. Mean doses range between 3-35 mGy depending on CBCT mode. CT Dose Profiler data agree with TLD measurements in a CTDI phantom within the uncertainty of the TLD measurements (estimated SD +/- 10%). Instantaneous dose rate at the periphery of the phantom can be higher than 20 mGy/s, which is 10 times the dose rate at the center. The spatial resolution in v1.4 is not as high as in v1.3. In conclusion, measurements show that the imaging doses for default modes in Varian OBI v1.4 CBCT system are significantly lower than in v1.3. The CT Dose Profiler is proven fast and accurate for CBCT applications.

Construction and evaluation of a real-time personal dosemeter based on a Si sensor for eye-dose measurement.

The objective was to design a thin, flex card based personal dosemeter with low angular and energy dependence. It is based on silicon diodes that can measure the personal dose equivalent. Anisotropic conducting adhesive was used to connect the Si sensor to the flex card. Its intended use is for interventional radiology and after nuclear or radiological accidents, as it provides immediate detailed information about the dose rate to the wearer during shorter periods and integrates the dose rate during hours. It can be freely placed all over the body. By placing the dosemeter close to the eyes, it may be possible to estimate the personal dose equivalent as well as the dose at the lens of the eye.

SILICON DIODE AS AN ALPHA PARTICLE DETECTOR AND SPECTROMETER FOR DIRECT FIELD MEASUREMENTS.

A windowless silicon (Si) diode (4 mm(2)) was evaluated as alpha particle detector and spectrometer for field measurements. It was irradiated with alpha particles from a (241)Am (2.3 kBq) and a (210)Po (9 kBq) source at source-detector distances (SDD) of 0.5, 1.0 and 1.8 cm. The energy resolution in terms of full width at half maximum was 281, 148 and 113 keV for SDD of 0.5, 1.0 and 1.8 cm, respectively. The minimum detectable activity increased from 0.08 to 0.83 Bq when the SDD increased from 0.5 to 1.8 cm. The detector has the potential for several alpha spectrometric applications, such as monitoring for wound, skin and surface contamination at nuclear fuel facilities, nuclear power plants and facilities handling radioactive waste. Other areas are environmental surveys following releases of actinides at accidents in nuclear power plants and in connection with other radiological or nuclear scenarios.

CAN AN ENERGY-COMPENSATED SOLID-STATE X-RAY DETECTOR BE USED FOR RADIATION PROTECTION APPLICATIONS AT HIGHER PHOTON ENERGIES?

The objective of this study was to investigate the characteristics of a solid-state detector commonly available at hospitals for parallel use as a real-time personal radiation monitor following radiation emergency situations. A solid-state detector probe with an inherent filtration (R100, RTI Electronics AB, Mölndal, Sweden) was chosen for evaluation. The energy dependence and the linearity in signal response with kerma in air were examined, and the detector was exposed to both X-ray beams using a conventional X-ray unit with effective photon energies ranging between 28.5 and 48.9 keV and to gamma rays 1.17 and 1.33 MeV from (60)Co. The R100 exhibited ∼1.7 times over-response at the lowest X-ray energy relative to the (60)Co source. The detector demonstrated a linear response (R(2) = 1) when irradiated with (60)Co to air kerma values in the range of 20-200 mGy. The conclusion is that high-energy photons such as those from (60)Co can be detected by the R100 with an energy response within a factor of <2 over the energy range examined and that the detector can provide real-time dose measurements following nuclear or radiological events.

Average glandular dose in routine mammography screening using a Sectra MicroDose Mammography unit.

The Sectra MicroDose Mammography system is based on direct photon counting (with a solid-state detector), and a substantially lower dose to the breast than when using conventional systems can be expected. In this work absorbed dose measurements have been performed for the first unit used in routine mammography screening (at the Hospital of Helsingborg, Sweden). Two European protocols on dosimetry in mammography have been followed. Measurement of half value layer (HVL) cannot be performed as prescribed, but this study has demonstrated that non-invasive measurements of HVL can be performed accurately with a sensitive and well collimated solid-state detector with simultaneous correction for the energy dependence. The average glandular dose for a 50 mm standard breast with 50% glandularity, simulated by 45 mm polymethylmethacrylate, was found to be 0.21 and 0.28 mGy in March and December 2004, respectively. These values are much lower than for any other mammography system on the market today. It has to be stressed that the measurements were made using the current clinical settings and that no systematic optimisation of the relationship between absorbed dose and diagnostic image quality has been performed within the present study. In order to further increase the accuracy of absorbed dose measurements for this unit, the existing dose protocols should be revised to account also for the tungsten/aluminium anode/filter combination, the multi-slit pre-collimator device and the occurrence of a dose profile in the scanning direction.