Polymethyl Methacrylate Phantom Research Articles

TH‐A‐213CD‐03: Polychromatic Cone‐Beam X‐Ray Fluorescence Computed Tomography of Gold Nanoparticle‐Loaded Objects

Purpose: To determine the spatial distribution and amount of gold nanoparticles (GNPs) within small‐animal‐sized objects using polychromatic cone‐beam x‐ray fluorescence computed tomography (XFCT) under realistic constraints on x‐ray dose, scan time, and image resolution. Methods: 6‐mm‐diameter cylindrical tubes containing saline solution and 0.5–2.0 wt. % of GNPs were inserted into a cylindrical polymethyl methacrylate (PMMA) phantom, 3 cm in diameter and 5 cm in height. The phantom was irradiated by a cone‐beam of polychromatic 105 kVp x‐rays filtered by 0.9 mm of tin. Energy‐sensitive cadmium telluride detectors behind a 2.5 mm diameter lead pinhole collimator collected the spectrum of emitted gold K‐shell fluorescence and Compton scattered photons at an angle of 90° relative to the beam central axis as the phantom was rotated to a series of 60 projection angles in 6° steps. Sinograms of gold fluorescence photon signals were constructed by extracting the gold fluorescence peak height from the Compton background, and the image of GNP location and concentration was reconstructed using a maximum likelihood iterative reconstruction algorithm. Results: Using the measured sinograms, XFCT images of GNP‐loaded objects were successfully reconstructed, accurately determining both the GNP location and concentration. The x‐ray dose delivered during the XFCT scanning was measured using the AAPM TG‐61 protocol, and it was determined that, using an array detector, it would be possible to acquire these images in one hour with a tissue dose of approximately 20 cGy. Conclusions: With a few easily achievable modifications, the current benchtop setup would be capable of producing useful XFCT images of small animals injected with GNPs with the total scanning time and a tissue dose comparable to other modalities currently available for routine pre‐clinical in‐vivo imaging such as micro‐CT. Supported in part by NIH/NCI grant 1R01CA155446. Supported in part by NIH/NCI grant 1R01CA155446

SU-E-T-155: Dose Response Curve of EBT2 and EBT3 Radiochromic Films to a Synchrotron-Produced Monochromatic X-Ray Beam.

This work investigates the dose-response curves of Gafchromic EBT2 and EBT3 radiochromic films using synchrotron-produced monochromatic x-ray beams. These dosimeters are being utilized for dose verification in photoactivated Auger electron therapy at the LSU Center for Advanced Microstructures and Devices (CAMD) synchrotron facility. Monochromatic beams of 25, 30 and 35 keV were generated on the tomography beamline at CAMD. Ion chamber depth-dose measurements were used to calculate the dose delivered to films irradiated simultaneously at depths from 0.7 - 8.5 cm in a 10×10×10-cms polymethylmethacrylate phantom. AAPM TG-61 protocol was applied to convert measured ionization into dose. Calibrations of films at 4 MV were obtained for comparison using a Clinac 21 EX radiotherapy accelerator at Mary Bird Perkins Cancer Center. Films were digitized using an Epson 1680 Professional flatbed scanner and analyzed using the optical density (OD) derived from the red channel. For EBT2 film the average sensitivity (OD/dose) at 50, 100, and 200 cGy relative to that for 4-MV x- rays was 1.07, 1.20, and 1.23 for 25, 30, and 35 keV, respectively. For EBT3 film the average sensitivity was within 3 % of unity for all three monochromatic beams. EBT2 film sensitivity shows strong energy dependence over an energy range of 25 keV - 4 MV. EBT3 film shows weak energy dependence, indicating that it would be the better dosimeter for Auger electron therapy. This research was supported by contract W81XWH-10-1-0005 awarded by The U.S. Army Research Acquisition Activity, 820 Chandler Street, Fort Detrick, MD 21702-5014. This report does not necessarily reflect the position or policy of the Government, and no official endorsement should be inferred.

Comparison of spatial and contrast resolution for cone-beam computed tomography scanners

The purpose was to evaluate the perceived spatial and contrast resolution for a wide range of cone-beam computed tomography (CBCT) devices. A customized polymethyl methacrylate (PMMA) phantom was developed. Inserts containing a line-pair and rod pattern were used. The phantom was scanned with 13 CBCT devices and 1 multislice CT (MSCT) device using a variety of scanning protocols. The images were presented to 4 observers for scoring. The observer scores showed excellent agreement. A wide range was seen in image quality between CBCT exposure protocols. Compared with the average CBCT scores, the MSCT protocols scored lower for the line-pair insert but higher for the rod insert. CBCT devices are generally suitable for the visualization of high-contrast structures. Certain exposure protocols can be used for depicting low-contrast structures or fine details. The user should be able to select appropriate exposure protocols according to varying diagnostic requirements.

Fast on‐site Monte Carlo tool for dose calculations in CT applications

Monte Carlo (MC) simulation is an established technique for dose calculation in diagnostic radiology. The major drawback is its high computational demand, which limits the possibility of usage in real-time applications. The aim of this study was to develop fast on-site computed tomography (CT) specific MC dose calculations by using a graphics processing unit (GPU) cluster. GPUs are powerful systems which are especially suited to problems that can be expressed as data-parallel computations. In MC simulations, each photon track is independent of the others; each launched photon can be mapped to one thread on the GPU, thousands of threads are executed in parallel in order to achieve high performance. For further acceleration, the authors considered multiple GPUs. The total computation was divided into different parts which can be calculated in parallel on multiple devices. The GPU cluster is an MC calculation server which is connected to the CT scanner and computes 3D dose distributions on-site immediately after image reconstruction. To estimate the performance gain, the authors benchmarked dose calculation times on a 2.6 GHz Intel Xeon 5430 Quad core workstation equipped with two NVIDIA GeForce GTX 285 cards. The on-site calculation concept was demonstrated for clinical and preclinical datasets on CT scanners (multislice CT, flat-detector CT, and micro-CT) with varying geometry, spectra, and filtration. To validate the GPU-based MC algorithm, the authors measured dose values on a 64-slice CT system using calibrated ionization chambers and thermoluminesence dosimeters (TLDs) which were placed inside standard cylindrical polymethyl methacrylate (PMMA) phantoms. The dose values and profiles obtained by GPU-based MC simulations were in the expected good agreement with computed tomography dose index (CTDI) measurements and reference TLD profiles with differences being less than 5%. For 10(9) photon histories simulated in a 256 × 256 × 12 voxel thorax dataset with voxel size of 1.36 × 1.36 × 3.00 mm(3), calculation times of about 70 and 24 min were necessary with single-core and multiple-core central processing unit (CPU) solutions, respectively. Using GPUs, the same MC calculations were performed in 1.27 min (single card) and 0.65 min (two cards) without a loss in quality. Simulations were thus speeded up by factors up to 55 and 36 compared to single-core and multiple-core CPU, respectively. The performance scaled nearly linearly with the number of GPUs. Tests confirmed that the proposed GPU-based MC tool can be easily adapted to different types of CT scanners and used as service providers for fast on-site dose calculations. The Monte Carlo software package provides fast on-site calculation of 3D dose distributions in the CT suite which makes it a practical tool for any type of CT-specific application.

Image‐based spectral distortion correction for photon‐counting x‐ray detectors

To investigate the feasibility of using an image-based method to correct for distortions induced by various artifacts in the x-ray spectrum recorded with photon-counting detectors for their application in breast computed tomography (CT). The polyenergetic incident spectrum was simulated with the tungsten anode spectral model using the interpolating polynomials (TASMIP) code and carefully calibrated to match the x-ray tube in this study. Experiments were performed on a Cadmium-Zinc-Telluride (CZT) photon-counting detector with five energy thresholds. Energy bins were adjusted to evenly distribute the recorded counts above the noise floor. BR12 phantoms of various thicknesses were used for calibration. A nonlinear function was selected to fit the count correlation between the simulated and the measured spectra in the calibration process. To evaluate the proposed spectral distortion correction method, an empirical fitting derived from the calibration process was applied on the raw images recorded for polymethyl methacrylate (PMMA) phantoms of 8.7, 48.8, and 100.0 mm. Both the corrected counts and the effective attenuation coefficient were compared to the simulated values for each of the five energy bins. The feasibility of applying the proposed method to quantitative material decomposition was tested using a dual-energy imaging technique with a three-material phantom that consisted of water, lipid, and protein. The performance of the spectral distortion correction method was quantified using the relative root-mean-square (RMS) error with respect to the expected values from simulations or areal analysis of the decomposition phantom. The implementation of the proposed method reduced the relative RMS error of the output counts in the five energy bins with respect to the simulated incident counts from 23.0%, 33.0%, and 54.0% to 1.2%, 1.8%, and 7.7% for 8.7, 48.8, and 100.0 mm PMMA phantoms, respectively. The accuracy of the effective attenuation coefficient of PMMA estimate was also improved with the proposed spectral distortion correction. Finally, the relative RMS error of water, lipid, and protein decompositions in dual-energy imaging was significantly reduced from 53.4% to 6.8% after correction was applied. The study demonstrated that dramatic distortions in the recorded raw image yielded from a photon-counting detector could be expected, which presents great challenges for applying the quantitative material decomposition method in spectral CT. The proposed semi-empirical correction method can effectively reduce these errors caused by various artifacts, including pulse pileup and charge sharing effects. Furthermore, rather than detector-specific simulation packages, the method requires a relatively simple calibration process and knowledge about the incident spectrum. Therefore, it may be used as a generalized procedure for the spectral distortion correction of different photon-counting detectors in clinical breast CT systems.

Determination of absorbed dose to water around a clinical HDR192Ir source using LiF:Mg,Ti TLDs demonstrates an LET dependence of detector response

Experimental radiation dosimetry with thermoluminescent dosimeters (TLDs), calibrated in a (60)Co or megavoltage (MV) photon beam, is recommended by AAPM TG-43U1for verification of Monte Carlo calculated absorbed doses around brachytherapy sources. However, it has been shown by Carlsson Tedgren et al. [Med. Phys. 38, 5539-5550 (2011)] that for TLDs of LiF:Mg,Ti, detector response was 4% higher in a (137)Cs beam than in a (60)Co one. The aim of this work was to investigate if similar over-response exists when measuring absorbed dose to water around (192)Ir sources, using LiF:Mg,Ti dosimeters calibrated in a 6 MV photon beam. LiF dosimeters were calibrated to measure absorbed dose to water in a 6 MV photon beam and used to measure absorbed dose to water at distances of 3, 5, and 7 cm from a clinical high dose rate (HDR) (192)Ir source in a polymethylmethacrylate (PMMA) phantom. Measured values were compared to values of absorbed dose to water calculated using a treatment planning system (TPS) including corrections for the difference in energy absorption properties between calibration quality and the quality in the users' (192)Ir beam and for the use of a PMMA phantom instead of the water phantom underlying dose calculations in the TPS. Measured absorbed doses to water around the (192)Ir source were overestimated by 5% compared to those calculated by the TPS. Corresponding absorbed doses to water measured in a previous work with lithium formate electron paramagnetic resonance (EPR) dosimeters by Antonovic et al. [Med. Phys. 36, 2236-2247 (2009)], using the same irradiation setup and calibration procedure as in this work, were 2% lower than those calculated by the TPS. The results obtained in the measurements in this work and those obtained using the EPR lithium formate dosimeters were, within the expanded (k = 2) uncertainty, in agreement with the values derived by the TPS. The discrepancy between the results using LiF:Mg,Ti TLDs and the EPR lithium formate dosimeters was, however, statistically significant and in agreement with the difference in relative detector responses found for the two detector systems by Carlsson Tedgren et al. [Med. Phys. 38, 5539-5550 (2011)] and by Adolfsson et al. [Med. Phys. 37, 4946-4959 (2010)]. When calibrated in (60)Co or MV photon beams, correction for the linear energy transfer (LET) dependence of LiF:Mg,Ti detector response will be needed as to measure absorbed doses to water in a (192)Ir beam with highest accuracy. Such corrections will depend on the manufacturing process (MTS-N Poland or Harshaw TLD-100) and details of the annealing and read-out schemes used.

Dosimetry concepts for scanner quality assurance and tissue dose assessment in micro-CT

At present, no established methods exist for dosimetry in micro computed tomography (micro-CT). The purpose of this study was therefore to investigate practical concepts for both dosimetric scanner quality assurance and tissue dose assessment for micro-CT. The computed tomography dose index (CTDI) was adapted to micro-CT and measurements of the CTDI both free in air and in the center of cylindrical polymethyl methacrylate (PMMA) phantoms of 20 and 32 mm diameter were performed in a 6 month interval with a 100 mm pencil ionization chamber calibrated for low tube voltages. For tissue dose assessment, z-profile measurements using thermoluminescence dosimeters (TLDs) were performed and both profile and CTDI measurements were compared to Monte Carlo (MC) dose calculations to validate an existing MC tool for use in micro-CT. The consistency of MC calculations and TLD measurements was further investigated in two mice cadavers. CTDI was found to be a reproducible quantity for constancy tests on the micro-CT system under study, showing a linear dependence on tube voltage and being by definition proportional to mAs setting and z-collimation. The CTDI measured free in air showed larger systematic deviations after the 6 month interval compared to the CTDI measured in PMMA phantoms. MC calculations were found to match CTDI measurements within 3% when using x-ray spectra measured at our micro-CT installation and better than 10% when using x-ray spectra calculated from semi-empirical models. Visual inspection revealed good agreement for all z-profiles. The consistency of MC calculations and TLD measurements in mice was found to be better than 10% with a mean deviation of 4.5%. Our results show the CTDI implemented for micro-CT to be a promising candidate for dosimetric quality assurance measurements as it linearly reflects changes in tube voltage, mAs setting, and collimation used during the scan, encouraging further studies on a variety of systems. For tissue dose assessment, MC calculations offer an accurate and fast alternative to TLD measurements allowing for dose calculations specific to any geometry and scan protocol.

Mammography dosimetry using an in-house developed polymethyl methacrylate phantom

Phantom-based measurements in mammography are well-established for quality assurance (QA) and quality control (QC) procedures involving equipment performance and comparisons of X-ray machines. Polymethyl methacrylate (PMMA) is among the best suitable materials for simulation of the breast. For carrying out QA/QC exercises in India, a mammographic PMMA phantom with engraved slots for keeping thermoluminescence dosemeters (TLD) has been developed. The radiation transmission property of the developed phantom was compared with the commercially available phantoms for verifying its suitability for mammography dosimetry. The breast entrance exposure (BEE), mean glandular dose (MGD), percentage depth dose (PDD), percentage surface dose distribution (PSDD), calibration testing of automatic exposure control (AEC) and density control function of a mammography machine were measured using this phantom. MGD was derived from the measured BEE following two different methodologies and the results were compared. The PDD and PSDD measurements were carried out using LiF: Mg, Cu, P chips. The in-house phantom was found comparable with the commercially available phantoms. The difference in the MGD values derived using two different methods were found in the range of 17.5-32.6 %. Measured depth ranges in the phantom lie between 0.32 and 0.40 cm for 75 % depth dose, 0.73 and 0.92 cm for 50 % depth dose, and 1.54 and 1.78 cm for 25 % depth dose. Higher PSDD value was observed towards chest wall edge side of the phantom, which is due to the orientation of cathode-anode axis along the chest wall to the nipple direction. Results obtained for AEC configuration testing shows that the observed mean optical density (O.D) of the phantom image was 1.59 and O.D difference for every successive increase in thickness of the phantom was within±0.15 O.D. Under density control function testing, at -2 and -1 density settings, the variation in film image O.D was within±0.15 O.D of the normal density setting '0' and at +2 and +1 density setting, it was observed to be within±0.30 O.D. This study indicates that the locally made PMMA TLD slot phantom can be used to measure various mammography QC parameters which are essentially required for better outcomes in mammography.

Influence of image metrics when assessing image quality from a test object in cardiac X-ray systems: Part II.

The images generated in modern IC laboratories are created with high-quality standard (1,024×1,024 pixels and 10-12 bits/pixel) enabling cardiologists to perform interventions in the best conditions. But these images are in most of the cases archived in a basic quality standard (512×512 pixels and 8 bits/pixel). The purpose of this work is to complete the research developed in a previous paper and analyze the influence of the matrix size and the bit depth reduction on the image quality acquired on a polymethylmethacrylate (PMMA) phantom with a test object. The variation in contrast-to-noise ratio (CNR) and high contrast spatial resolution (HCSR) were investigated when the matrix size and the bit depth were independently modified for different phantom thicknesses. These two image quality parameters did not suffer noticeable alterations under bits depth reduction from 10 to 8 bits. Such a result seems to imply that bits depth reduction could be used to reduce file sizes with a suitable algorithm and without losing perceptible image quality information. But when the matrix size was reduced from 1,024×1,024 to 512×512 pixels, a reduction from 17% to 25% in HCSR was noticed when changing phantom thickness, and an increase of 27% in CNR was observed. These findings should be taken into account and it would be wise to conduct further investigations in the field of clinical images.

A new mammography dosimetric phantom

Breast phantoms produced with tissue-equivalent materials are used in an attempt to simulate glandular and adipose tissues, in terms of X-ray attenuation and density. In this work, a set of breast tissue-equivalent phantoms (BTE phantoms) with semicircular shapes of different thicknesses and compositions were produced. Such phantoms may be used in the measurement of the incident air kerma (K(i)) and the mean glandular dose (D(G)) delivered to patients undergoing mammography. To characterise the materials used to produce the phantoms, a series of 17-keV X-ray attenuation coefficient measurements were performed. The carbon-nitrogen-hydrogen elemental composition and the densities of the tissue-equivalent materials were also determined and compared with values available in the literature. Linear attenuation coefficients of 0.724 and 0.923 cm(-1) were determined, respectively, for adipose and glandular tissues. Such values agree with data available in the literature. On the basis of the results obtained in this work, it is suggested that BTE phantoms are used instead of polymethyl methacrylate phantoms to select exposure parameters (kV, mAs and target/filter combination) specific for breast glandularities from 0 to 50 % in the optimisation of doses in mammography.

Quantification of metal artifacts on cone beam computed tomography images

To quantify metal artifacts obtained from a wide range of cone beam computed tomography (CBCT) devices and exposure protocols, to compare their tolerance to metals of different densities, and to provide insights regarding the possible implementation of metal artifact analysis into a QC protocol for CBCT. A customized polymethyl methacrylate (PMMA) phantom, containing titanium and lead rods, was fabricated. It was scanned on 13 CBCT devices and one multi-slice computed tomography (MSCT) device, including high-dose and low-dose exposure protocols. Artifacts from the rods were assessed by two observers by measuring the standard deviation of voxel values in the vicinity of the rods, and normalizing this value to the percentage of the theoretical maximum standard deviation. For CBCT datasets, artifact values ranged between 6.1% and 27.4% for titanium, and between 10.% and 43.7% for lead. Most CBCT devices performed worse than MSCT for titanium artifacts, but all of them performed better for lead artifacts. In general, no clear improvement of metal artifacts was seen for high-dose protocols, although certain devices showed some artifact reduction for large FOV or high exposure protocols. Regions in the vicinity of the metal rods were moderately or gravely affected, particularly in the area between the rods. In practice, the CBCT user has very limited possibilities to reduce artifacts. Researchers and manufacturers need to combine their efforts in optimizing exposure factors and implementing metal artifact reduction algorithms.

Automated Framework for Digital Radiation Dose Index Reporting From CT Dose Reports

Radiation exposure from CT studies has increased over the past 30 years in the United States and now constitutes approximately 50% of the radiation dose index administered in the health care setting. Tracking CT dose index (CTDI) is cumbersome because it relies on a manufacturer-generated screen capture, which contains the estimated dose index exposure for the patient. The radiation dose index information is not digital but, rather, is "burned" into the image (i.e., not in numeric form, not as part of the image header or elsewhere associated with the study), making it difficult to automatically share these data with other information systems. The purpose of the dose index reporting application (DIRA) we developed for CT is to extract the radiation dose index information from the CTDI reports to eventually perform automated quality control, promote radiation safety awareness, and provide a longitudinal record of patient-specific health care-related radiation exposure. A random selection of 518 CTDI reports were processed by the DIRA and the dose index information was extracted. CTDI reports using a standard DICOM C-STORE to the DIRA allow an automated process to compile radiation dose index and patient information in a Web-based framework using a structured query language (SQL) database. Our initial tests showed that the DIRA accurately extracted dose index information from 518 of 518 CTDI reports (100%). Because the extracted CTDI descriptor-dose-length product-is based on standard CTDI measurements obtained using fixed-size cylindric polymethylmethacrylate phantoms, preliminary studies have been performed to correct for patient size by applying correction factors derived from CTDI measurements using a range of phantom sizes from 6 to 32 cm in diameter. Our system provides a way to automatically track CTDI on existing CT scanners and does not rely on the DICOM SR Dose Index Report standard, which is available on only the newest CT scanners. A modular and vendor-independent DIRA system can be integrated with any existing CT scanner. This system greatly facilitates digital dose index reporting and makes it possible to provide a longitudinal record of the health care radiation exposure estimate in an individual patient's health record.

WE‐G‐211‐04: Experimental Demonstration of Cone‐Beam Polychromatic X‐Ray Fluorescence Computed Tomography (XFCT) Imaging of Gold Nanoparticle‐Loaded Regions within Small Animal‐Sized Phantoms

Purpose: To demonstrate the feasibility of determining the location and gold concentration of gold nanoparticle (GNP)‐loaded regions within polymethyl methacrylate (PMMA) phantoms using cone‐beam polychromatic x‐ray fluorescence computed tomography (XFCT) technique. Methods: Tubes containing water and GNPs at 2%, 1%, and 0.5% by weight were inserted into two 5 cm‐long cylindrical PMMA phantoms with 3 cm and 5 cm in diameter, respectively. The phantoms were irradiated by a cone‐beam of polychromatic 110 kVp x‐rays filtered by 1.0 mm of lead. Energy‐sensitive cadmium telluride detectors behind a 2.5 mm diameter lead pinhole collimator collected the spectrum of emitted gold K‐shell fluorescence and Compton scattered photons at an angle of 90° relative to the beam central axis as the phantom was rotated to a series of 60 projection angles in 6° steps. Sinograms of gold fluorescence photon signals were constructed by extracting the gold fluorescence peak height from the Compton background, and the image of GNP location and concentration was reconstructed using a maximum likelihood iterative reconstruction algorithm. Results: Using the measured sinograms, images of GNP location were successfully reconstructed for both the 3 cm and 5 cm phantoms. Additionally, the signal intensity was linearly related to gold concentration. The x‐ray dose delivered during the XFCT scanning was measured using the TG‐61 protocol, and it was determined that, using an array detector, it would be possible to acquire these images with roughly 30 cGy of x‐ray dose delivered. Conclusions: This study demonstrates, for the first time to our knowledge, the feasibility of imaging GNP location and concentration within small animal‐sized objects using an ordinary cone‐beam polychromatic x‐ray source under realistic constraints on dose and scan time. This investigation strongly suggests a bench‐top XFCT system will ultimately become a viable option for in‐vivo molecular imaging with conjugated or unconjugated GNPs.

TU‐G‐110‐07: Quantitative Measurement of an Iodinated Contrast Agent with Dual Energy and Spectral Breast Computed Tomography

Purpose: To investigate the feasibility of quantifying an iodinated contrast agent with dual energy and spectral breast computed tomography (CT).Methods: A cylindrical Polymethyl methacrylate (PMMA) phantom with a diameter of 4.5 cm was constructed using various iodine concentrations that ranged from 0.4 to 4 mg/ml. Calibrations and measurements were carried out with (1) a flat panel detector using dual kVp technique and (2) a Cadmium‐Zinc‐Telluride (CZT) photon counting detector using a single kVp approach in a spectral CT system. Based on the simulation results, energies of 50 and 120 kVp were chosen for the dual kVp imaging. Energy splitting at 34 keV, which is just above the k‐edge of iodine, was used to bin photons into two images within a single energy acquisition at 50 kVp. Results: Iodinated solutions with different concentrations can be clearly identified in the dedicated iodine image obtained from both dual kVp flat panel imaging and CZT‐based spectral CT. The linear fitting of the experimental data as a function of the known values yielded slopes of 0.998 and 1.021 for the dual kVp and spectral CT techniques, respectively. This suggested that the measured iodine concentrations agreed very well with the known values. Conclusion: The experimental results suggest that both dual kVp and spectral CT can be used for quantitative decomposition of contrast‐enhanced breast imaging. Meanwhile, the CZT‐based spectral CT system may offer a better image quality than the comparable flat panel based dual energy CT system.

SU‐E‐I‐76: Investigation of Patient Dose in Mammography in Japan by 2010

Purpose: In Japan, mammography units are shifting from screen‐film (S/F) mammography to digital (DR) mammography. However, no studies have investigated the actual radiation dose received by patients undergoing mammography using DR units. In order to assess the present situation, we conducted a survey of the actual radiation dose at 92 participating mammography facilities in 2010. Methods: The units used were F/S (5%), computed radiography (CR) (47%), and flat panel display (FPD) (48%).The incident air kerma was obtained from the irradiation conditions of each facility, and the average glandular dose (AGD) was calculated. Specifically, the AGD was calculated using a quality control phantom (model 156 phantom) and a polymethyl methacrylate (PMMA) phantom (10, 20, 30, 40, 50 and 60 mm thicknesses) equipped with automatic exposure control (AEC) systems. An ionization chamber dosimeter （Radcal Model 9015） with a parallel plate ionization chamber calibrated based on national standards was used. Aluminum plates (99.997 % purity) were used for half value layer assessment. Results: The AGD was not greater than 3 mGy in all cases and was taken to be the patient dose. Mean dose was 1.8 mGy, and coefficient of variation (CV) was not greater than 0.3. Approximately 75% of facilities used a dose of not greater than 2 mGy. The range of doses was approximately three‐fold. Mean AGD was the lowest for F/S and highest for CR. Conclusions: There are about 10,000 radiologic technologists for mammography in Japan, and radiographic imaging at most facilities is performed by these technologists. As a result, there is little variation among facilities, and a minimum radiation dose is used.

SU-E-T-483: Investigations of Dose Delivery Accuracy of High Dose Rate Flattening Filter Free (FFF) Beams in RapidArc™ and IMRT Deliveries on TrueBeam™

Purpose: Medical linacs with dose rates as high as 2400 cGy/min at 10MV and 1400 cGy/min at 6MV have been clinically installed in many institutions. In reality, little is known about the accuracy of delivery at these high dose rates, particularly for complex treatment plans with simultaneous motion of gantry and multileaf‐collimator (MLC) aperture. We present measurements that examine the dosimetric accuracy of dynamically‐shaped RapidArc™ and IMRT beams at the highest available dose rates. Methods: An Eclipse™ ver. 8.9 planning system commissioned with the analytic anisotropy algorithm (AAA™) for the flattening‐filter‐free X‐ray beams of the Varian TrueBeam™ linear accelerator with a high‐definition 120‐leaf MLC (HDMLC) accessory was used to create RapidArc™ and IMRT lung‐ and abdominal‐target patient treatment plans deliverable in the FFF mode of TrueBeam™. Verification plans applied to a 22‐cm diameter cylindrical polymethylmethacrylate phantom configured as water‐ equivalent material with an electron density of 1.147 were delivered at the highest available dose rates. Dose was measured with TG‐51 cross‐ calibrated ScandiDos® Delta4™dual orthogonal‐plane 1069‐diode system for conventional 2 Gy fractions and for 15Gy–16.5 Gy fractions characteristic of stereotactic body radiotherapy(SBRT). Delivery was repeated with conventional dose rate (400 cGy/min). Measured results for the two dose rates were compared with each other, and with Eclipse treatment plans. Results: Excellent agreement was found between 2400 and 400 MU/min delivery. A sample 2‐arc RapidArc™ plan gave absolute relative dose difference ratio of 0.42%±0.21% for the 439 points measuring above 10% of prescription dose, with maximum dose difference over all 1069 measurement points being 0.76% of prescription. For a small‐field 1‐ arc plan, quantities were 0.96%±0.14% for 103 points, and maximum difference over all 1069 points was 1.61%. Gated versus ungated delivery for 200 cGy/fx showed negligible differences at both 400 and 2400 dose rates. Eclipse calculations demonstrated reasonable agreement with absolute dose measurement.

The feasibility of polychromatic cone-beam x-ray fluorescence computed tomography (XFCT) imaging of gold nanoparticle-loaded objects: a Monte Carlo study

A recent study investigated the feasibility to develop a bench-top x-ray fluorescence computed tomography (XFCT) system capable of determining the spatial distribution and concentration of gold nanoparticles (GNPs) in vivo using a diagnostic energy range polychromatic (i.e. 110 kVp) pencil-beam source. In this follow-up study, we examined the feasibility of a polychromatic cone-beam implementation of XFCT by Monte Carlo (MC) simulations using the MCNP5 code. In the current MC model, cylindrical columns with various sizes (5–10 mm in diameter) containing water loaded with GNPs (0.1–2% gold by weight) were inserted into a 5 cm diameter cylindrical polymethyl methacrylate phantom. The phantom was then irradiated by a lead-filtered 110 kVp x-ray source, and the resulting gold fluorescence and Compton-scattered photons were collected by a series of energy-sensitive tallies after passing through lead parallel-hole collimators. A maximum-likelihood iterative reconstruction algorithm was implemented to reconstruct the image of GNP-loaded objects within the phantom. The effects of attenuation of both the primary beam through the phantom and the gold fluorescence photons en route to the detector were corrected during the image reconstruction. Accurate images of the GNP-containing phantom were successfully reconstructed for three different phantom configurations, with both spatial distribution and relative concentration of GNPs well identified. The pixel intensity of regions containing GNPs was linearly proportional to the gold concentration. The current MC study strongly suggests the possibility of developing a bench-top, polychromatic, cone-beam XFCT system for in vivo imaging.

Quantification of Various Factors Influencing the Precision of Thermoluminescent Detector Calibrations for New and Used Chip Sets

Factors affecting the random and systematic error in calibrating three sets of 100 LiF:Mg,Ti thermoluminescent detector chips were investigated. The chips were held in a polymethyl methacrylate plate with 0.3 cm deep wells covered with a thin top plate, affixed to a polymethyl methacrylate phantom 150 cm from a 3.2 × 10(10) GBq 137Cs source, used to irradiate the chips to 4.52 mGy. Three sets of chips were used: one new, one heavily used, and one having relatively high degrees of visible physical damage. Variations in the exposure rate across the plate were measured with an ion chamber. Experimental drift was judged by performing successive calibrations on subsequent days, while always reading the chips in the same order. The chips were subject to manual examination to determine variations in mass and physical quality. This study indicates that more accurate calibrations can be obtained by accounting for the error caused by nonuniformity in the delivered dose, which was in this study as high as 4.4% from the center to the edges of the target. Making use of more than three calibrations only reduces the standard deviation as a percentage of the mean of a set by less than 1%. Desirable dosimeters in commercially rejected sets were identified by comparing each dosimeter's standard deviation of response across all calibrations to a commercially acceptable control set. Up to 50% variations in mass and visual quality, including opacity, fracture, and surface scratches to chips, showed little to no correlation with their response.

A dosimetry method in the transverse plane of HDR Ir-192 brachytherapy source using gafchromic EBT2 film

Radiochromic film dosimetry is increasingly used in brachytherapy applications for its higher resolution ability as compared to other experimental methods. The present study was aimed to assess the accuracy and suitability of use of the improved radiochromic film model, Gafchromic EBT2, to evaluate the dose distribution in the transverse plane of microselectron HDR 192Ir source.A specially designed and locally fabricated Polymethyl methacrylate (PMMA) phantom was used in this work for the experimental measurement of dose distribution around the source in its transverse plane. The AAPM TG-43U1 recommended radial dose function, g (r), and dose rate constant, Λ, for the source were measured using Gafchromic EBT2 film and thermoluminescent dosimeters (TLD). The EBT2 film measured dosimetric quantities were validated against their values obtained from the TLD measurements and previously published values for the same source available in literature.The dose rate constant and radial dose function for microselectron HDR 192Ir source obtained from Gafchromic EBT2 film measurements are in agreement with their TLD measured results within 3.9% and 2.8% respectively. They also agree within the accepted range of uncertainty with their experimental and Monte Carlo calculated results reported in literature.This work demonstrates the suitability of using Gafchromic EBT2 film dosimetry in characterization of dose distribution in the transverse plane of HDR Ir-192 source. This is a more efficient method than TLD dosimetry at discrete and distant positions. Relative to TLD dosimetry, it is found to be better reproducible, easy to use and a less expensive method of dosimetry.

Dosimetric and image quality comparison of two digital mammography units with different target/filter combinations: Mo/Mo, Mo/Rh, W/Rh, W/Ag

Our aim was to allocate a digital mammography unit to the screening programme on the basis of the ALARA (as low as reasonably achievable) radiation protection principle. Two Hologic Selenia mammography units were studied: one with a molybdenum anode and the other with a tungsten anode. After optimisation of the image production chain, we evaluated doses in a phantom under standard conditions. In this phase, we exposed a polymethyl-methacrylate (PMMA) phantom to the two mammography units and recorded the exposure parameters used by them. The phantom was subsequently replaced by a dedicated Radcal ionisation chamber on which preliminary dose assessments were conducted. Image quality of the two systems was compared by exposing a phantom containing geometrical inserts and setting the exposure parameters used for the dose assessments on each mammography unit. Dosimetric assessments of exposure data were recorded from the mammographic examinations of approximately 400 women (1,600 exposures). The unit with the tungsten anode achieved a lower patient dose. As a result, the Selenia-W device was allocated to the breast screening programme.