OSLDs exhibited good batch homogeneity (<5%) and reproducibility (3.3%), as well as a linear response. In addition, they showed no statistically significant difference with TLDs in CT measurements (p > 0.1). However, high uncertainty (42%) in the dose estimate was found as a result of relatively high accumulated dose. Furthermore, nanoDots showed high angular dependence (up to 70%) in low kVp techniques. Energy dependence of about 60% was found, and correction factors were suggested for the range of energies investigated. Therefore, if angular and energy dependences are taken into consideration and the uncertainty associated with accumulated dose is avoided, OSLDs (nanoDots) can be suitable for use as point dosimeters in diagnostic settings.

Thyroid doses in 71% of study patients fell within the dose range historically correlated with an increased risk of thyroid cancer and whole body effective doses fell within the range of historical doses correlated with an increased risk of all solid cancers and leukemia. Selective scanning of body areas as compared with whole body scanning results in a statistically significant decrease in all doses.

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Purpose Metal-oxide-semiconductor field-effect transistors (MOSFETs) serve as a helpful tool for organ radiation dosimetry, and their use has grown in computed tomography (CT). While different approaches have been used for MOSFET calibration, those using the commonly-available 100 mm pencil ionization chamber have not incorporated measurements performed throughout its length, and moreover no previous work has rigorously evaluated the multiple sources of error involved in MOSFET calibration. In this paper we propose a new MOSFET calibration approach to translate MOSFET voltage measurements into absorbed dose from CT, based on serial measurements performed throughout the length of a 100 mm ionization chamber, and perform an analysis of the errors of MOSFET voltage measurements and four sources of error in calibration. Methods MOSFET calibration was performed at two sites, to determine single calibration factors for tube potentials of 80, 100, and 120 kVp, using a 100 mm long pencil ion chamber and a cylindrical computed tomography dose index (CTDI) phantom of 32 cm diameter. The dose profile along the 100 mm ion chamber axis was sampled in 5 mm intervals by nine MOSFETs in the nine holes of the CTDI phantom. Variance of the absorbed dose was modeled as a sum of the MOSFET voltage measurement variance and the calibration factor variance, the latter being comprised of three main subcomponents: ionization chamber reading variance, MOSFET-to-MOSFET variation and a contribution related to the fact that the average calibration factor of a few MOSFETs was used as an estimate for the average value of all MOSFETs. MOSFET voltage measurement error was estimated based on sets of repeated measurements. The calibration factor overall voltage measurement error was calculated from the above analysis. Results Calibration factors determined were close to those reported in the literature and by the manufacturer (∼ 3mV/mGy), ranging from 2.87 to 3.13 mV/mGy. The error σV of a MOSFET voltage measurement was shown to be proportional to the square root of the voltage V: σV=cV where c = 0.11 mV. A main contributor to the error in the calibration factor was the ionization chamber reading error with 5% error. The usage of a single calibration factor for all MOSFETs introduced an additional error of about 5-7%, depending on the number of MOSFETs that were used to determine the single calibration factor. The expected overall error in a high dose region (∼30 mGy) was estimated to be about 8%, compared to 6% when an individual MOSFET calibration was performed. For a low-dose region (∼3 mGy) these values were 13% and 12%. Conclusions A MOSFET calibration method was developed using a 100 mm pencil ion chamber and a CTDI phantom, accompanied by an absorbed dose error analysis reflecting multiple sources of measurement error. When using a single calibration factor, per tube potential, for different MOSFETs, only a small error was introduced into absorbed dose determinations, thus supporting the use of such a calibration factor for experim...

The purpose of this study was to investigate the possibility of estimating pediatric thyroid doses from CT using surface neck doses. Optically stimulated luminescence dosimeters were used to measure the neck surface dose of 25 children ranging in ages between one and three years old. The neck circumference for each child was measured. The relationship between obtained surface doses and thyroid dose was studied using acrylic phantoms of various sizes and with holes of different depths. The ratios of hole-to-surface doses were used to convert patients' surface dose to thyroid dose. ImPACT software was utilized to calculate thyroid dose after applying the appropriate age correction factors. A paired t-test was performed to compare thyroid doses from our approach and ImPACT. The ratio of thyroid to surface dose was found to be 1.1. Thyroid doses ranged from 20 to 80 mGy. Comparison showed no statistical significance (p = 0.18). In addition, the average of surface dose variation along the z-axis in helical scans was studied and found to range between 5% (in 10 cm diameter phantom/24 mm collimation/pitch 1.0) and 8% (in 16 cm diameter phantom/12 mm collimation/pitch 0.7). We conclude that surface dose is an acceptable predictor for pediatric thyroid dose from CT. The uncertainty due to surface dose variability may be reduced if narrower collimation is used with a pitch factor close to 1.0. Also, the results did not show any effect of thyroid depth on the measured dose.

Purpose The purpose of this paper was to present a method of determining the dose profile of the beam bow‐tie filter (BTF) without the need for fixing the x ray tube in a position or using special instruments or dosimeters other than the ordinary types of ion chambers used for CT dosimetry (e.g., Farmer chamber). Methods The idea behind this method is to try to invert the integral of exposures from axial measurements by decomposing it into fractions per degree of tube rotation. Measurements of the CT tube output were taken with a full tube rotation while the chamber was fixed in air. Starting with isocenter the output measurements were performed at 1‐cm interval above the isocenter. Measurements were repeated for three sizes of BTFs; small, medium, and large. Maximum fan angle per chamber position was computed and an effective fan angle was defined to account for the new angular range encountered per chamber position. Variation due to inverse‐square law was isolated from each measurement and contribution from the effective fan angle was computed. Resulted profiles from this method were then compared to profiles obtained with direct measurements, when the tube was in a fixed position. Effects of over and under 360° rotation per scan on results accuracy were also investigated. Results Using the direct measurements as the gold standard, results from this method were accurate to 4% for most of the BTFs angular ranges. The average relative error in the small BTF was 1.5%. In the medium BTF, the average relative error was <3% for up to 16° fan angle. With the large BTF, the mean error was about 4% for up to 22°. The relative error appeared to increase at larger fan angles especially with the large BTF; reaching an average of about 32% for fan angles between 22° and 27.5°. Conclusion The presented method is relatively easy to perform and provides BTFs profiles with reasonably good accuracy. Associated errors of >10% only appear in high angles of large and medium BTFs.

Purpose: Use of GAFCHROMIC® XR‐QA film to estimate entrance skin dose and skin dose measurements resulting from computed tomography‐guided procedures such as tumor ablation cyst aspiration and needle biopsies. Materials and Methods: Small pieces of GAFCHROMIC® XR‐QA each measuring approximately 1 × 6 cm2 were exposed to different measured radiation doses using a conventional x‐ray tube. The beam energy used was 117 kVp with a measured a HVL of 5.15 mm Al. An additional 1.85 mm Al were added to better approximate the beam characteristics of a computed tomography x‐ray beam of 120 kVp and a HVL of 7 mm Al. After 24 hours post‐exposure the films were scanned with a commercially available reflective‐type scanner. The mean pixel value measured for each film was used to calculate a net pixel value. The film calibration curve was generated by plotting the radiation dose as a function of net pixel value and then performing a curve fit. For each of seven computed‐tomography procedures selected a GAFCHROMIC® XR‐QA film strip measuring approximately 1 × 30 cm2 was positioned on the patient's skin parallel to the direction of table movement during the entirety of the procedure. The GAFCHROMIC® XR‐QA film strips were scanned and a dose profile generated. Results: The GAFCHROMIC® XR‐QA film skin dose measurements for all the patients so far investigated had a maximum dose ranging from 8–267 mGy with an average skin dose between 0.7–125 mGy. Conclusion: GAFCHROMIC® XR‐QA film has the potential for performing quick and reliable radiation skin dose measurements for patients undergoing computed tomography guided procedures. Skin dose profiles may be used to determine maximum or peak skin dose to provide feedback to physicians on their radiation management techniques and to optimize the CT scan parameters in order to minimize patient dose while maintaining diagnostic quality imaging.

Purpose: To compare the performance between two available metal artifact reduction software (MAR) packages from different CT vendors to determine applicability for surgical screws, even though such use is not supported. Methods: Two vertebral screws (Al and stainless steel) were placed in the center of a water phantom (20 cm × 15 cm), with long axis perpendicular to the scan direction. Scans were performed using a GE Discovery HD750 with Dual Energy (DE) and MARS software, and Philips iCT with iDose4 and OMAR software. Reconstruction was performed with and without MAR. Assessment was performed qualitatively and quantitatively by measuring the SD in areas surrounding the screws. For the GE scanner, mono‐energy reconstructions were performed for 70, 127, and 140 keV in addition to the MARS routine. Thickness of the screws was also measured and compared to the expected size. Results: DE reduced streak artifacts and MARS further reduced artifacts for both screws. However, a portion of the middle was distorted. SD of region next to the screw tips was reduced up to 97%; whereas SD in the region under the long axis increased in some MARS images by more than 50%. O‐MAR reduced streak artifacts but less effectively than MARS but with no loss of the middle section. With OMAR, in general most images showed reduced SD in both regions by 2% to 54%. Additionally, width was significantly reduced (20% to 50%) by MARS. This effect was not noted with O‐MAR. Conclusion: Performance of MARS and O‐MAR differed considerably. MARS effectively reduced streaks along the long axis but introduced distortions in regions near the center of the screw. With O‐MAR, streaks were reduced by a lesser degree than MARS along the screws axis, but by higher degree above and below the screws with no distortion at the center.