Lucite Phantom Research Articles

Monte Carlo and experimental characterization of the first AMIRS 103Pd brachytherapy source

TG-43U1 dosimetric parameters of a new brachytherapy 103Pd source, including dose-rate constant, radial dose function, 2D anisotropy function, 1D anisotropy function and anisotropy constant, have been determined using MCNP4C code and have been verified by measurements in Perspex phantoms, using TLD-100 dosimeters calibrated in 60Co radiation field. The comparison of calculated and measured dosimetric parameters showed the validity of Monte Carlo calculations and experimental results. The anisotropy constant was calculated as 0.87 in water and 0.88 in Perspex; and measured as 0.92 in Perspex. Comparing dosimetric parameters of the new source with other source models showed acceptable agreement.

SU-GG-I-157: Quantification of Radiographic Image Contrast Enhancement Using Gold Nanoparticles

Purpose: To quantify radiologic imagecontrast enhancement using goldnanoparticles compared to iodinated contrast media (CM) over the entire diagnostic range of x‐ray energies. Method and Materials: A Perspex phantom with 4mm cylindrical wells was used to simulate small portions of vasculature. Each well was loaded with either goldnanoparticle solution or iodinated CM at equal concentration (0.5077 M radiopaque element). The phantom was imaged under full scatter conditions in computed radiography(CR) (40–80 kVp) and computed tomography(CT) (80–140 kVp). Images obtained at low energies (≈ 40 kVp) were validated using diagnostic type gafchromic film (Gafchromic® XRQA). CdTedetector with MCA was used to obtain transmission spectra after x‐ray beam at 130 kVp passed through solutions of goldnanoparticles or iodinated CM. Results:CT and CRimages were evaluated for contrast enhancement by contrast‐to‐noise ratio (CNR). Low energy results support previous findings, with gold exhibiting a 60% greater CNR than iodine. Goldnanoparticles also displayed excellent imagecontrast in CT, producing over two times greater signal than iodinated CM at 140 kVp. Over the x‐ray energy range of 70–100 kVp, however, both samples displayed similar contrast values. CdTe attenuation spectra are in accordance with image results where goldnanoparticles show a greater probability of attenuation than iodine for photons below approximately 35 keV and above 80 keV. Conclusion: Data indicates that a solution bearing goldnanoparticles would be an effective alternative to iodinated CM diagnostic radiology particularly at lower and higher ends of x‐ray energies used in radiology, such as mammography and CT.Conflict of Interest (only if applicable): Funding provided by NanoVic (Nanotechnology Victoria, Ltc.)

SU‐GG‐J‐37: Assessing Image Guided Radiation Therapy Targeting Accuracy: Coincidence Of Imaging System Isocenter With Treatment Machine Mechanical And Radiation Isocenters

Purpose: The purpose of this study is to introduce a method for quantifying the accuracy of linac and imaging system isocenters and demonstrate its use by measuring the targeting of two kilovoltage (kV) imaging systems, and one gantry mounted megavoltage Electronic Portal Imaging Device (EPID). One is the kV imaging system (OBI) integrated with the linear accelerator which supports radiographic and cone beam CT imaging. The other system is the BrainLab Exac Trac stand alone fixed to the treatment room ceiling and floor. Method and Materials: A cylindrical Lucite phantom was designed and fabricated to perform this study. The cylindrical phantom is 20 cm in diameter and 20 cm long. The phantom contains 13 radio‐opaque fiducial markers. A 5 mm spherical marker is positioned at the center of the cylinder. The phantom was imaged and the CT data sets were imported to two planning systems. Treatment beams were placed and both 3D data sets and isocenter information were transferred to the Linac delivery and imaging workstations. For each imaging system, phantom was initially positioned according to machine cross‐hair. Then images were acquired and measured shifts were applied to precisely position the phantom. Then, Winston‐Lutz test was performed for different gantry and collimator configuration. The radiation beam was collimated by a tray‐mounted 30 mm diameter circular collimator. Results: The radius of spherical volume in which all isocenter intersect or lie was measured. For EPID, OBI, CBCT, and Exac Trac, the radius is: 0.54 mm, 1.4 mm, 1.7 mm and 1.46 mm respectively. From the Winston‐Lutz analysis, the average isocenter deviation from all angles is 1.18 ± 0.28 mm. Conclusion: The error in mechanical, radiation and imaging isocenter is about one millimeter. Regardless what imaging system is used for patient setup, it is important to incorporate this systematic error in any planning margins.

MO‐E‐332‐05: The Effect of Copper Beam Filtration On the Transmission of Scattered X‐Rays Through a Typical Lead Barrier

Purpose: Standard use of copper beam filtration in modern cardiac catheterization and angiography systems can substantially change the x‐ray beam quality and ultimately have an impact on scattered x‐ray transmission through shielded barriers. A study was performed to investigate these effects. Method and Materials: Scatter was measured using broad‐beam geometry at 50 and 100 cm from the center of an 8″‐thick Lucite phantom with an 1800 cc ionization chamber. A Siemens Axiom Artis system was employed using 60, 81, 102 and 125 kVp with filtrations of 0, 0.1, 0.2, 0.3, 0.6 and 0.9mm of copper. The ionization chamber was wrapped in 1/16″‐thick lead to determine the transmission of scattered x‐rays through a typical shielded barrier. Results: Transmission at 125 kVp varied from 1.7×10−3 to 3.0×10−3 for beams with 0 to 0.9 mm of Cu, respectively. Similarly, transmission varied at 102 kVp from 1.2×10−3 to 2.2×10−3 and at 81 kVp from 7.1×10−4 to 9×10−4. Transmission at 60 kVp was about 7×10−4 for beams with 0 to 0.3 mm of Cu. Transmissions without Cu filtration at 125 and 102 kVp were measured to be about half the theoretical value reported by Simpkin and Dixon, which could result from variations in lead thickness. At 60 and 81 kVp transmissions without Cu filtration are more than an order of magnitude higher than that reported at 70 kVp by Simpkin and Dixon. Conclusion: Additional copper filtration can increase the barrier transmission up to a factor of two over unfiltered beams. However, it is unlikely that this amount of increase in transmission will significantly modify a shielding calculation. Further investigation is needed to determine the changes in typical workloads and scattered air kerma at different angles for these systems to understand the combined effect of these changes on barrier shielding calculations.

Monte Carlo calculations and experimental measurements of dosimetric parameters of the brachytherapy source

This article presents a brachytherapy source having 103Pd adsorbed onto a cylindrical silver rod that has been developed by the Agricultural, Medical, and Industrial Research School for permanent implant applications. Dosimetric characteristics (radial dose function, anisotropy function, and anisotropy factor) of this source were experimentally and theoretically determined in terms of the updated AAPM Task group 43 (TG-43U1) recommendations. Monte Carlo simulations were used to calculate the dose rate constant. Measurements were performed using TLD-GR200A circular chip dosimeters using standard methods employing thermoluminescent dosimeters in a Perspex phantom. Precision machined bores in the phantom located the dosimeters and the source in a reproducible fixed geometry, providing for transverse-axis and angular dose profiles over a range of distances from 0.5 to 5 cm. The Monte Carlo N-particle (MCNP) code, version 4C simulation techniques have been used to evaluate the dose-rate distributions around this model 103Pd source in water and Perspex phantoms. The Monte Carlo calculated dose rate constant of the IRA-103Pd source in water was found to be 0.678 cGy h(-1) U(-1) with an approximate uncertainty of +/-0.1%. The anisotropy function, F(r, theta), and the radial dose function, g(r), of the IRA- 103Pd source were also measured in a Perspex phantom and calculated in both Perspex and liquid water phantoms.

Experimental measurements and Monte Carlo calculations of dosimetric parameters of the IRA1- 103Pd brachytherapy source

This work presents a brachytherapy source having 103Pd adsorbed onto a cylindrical silver rod that has been developed by Agricultural, Medical and Industrial Research School for permanent implant applications. Dosimetric characteristics (dose-rate constant, radial dose function, anisotropy function and anisotropy factor) of this source were experimentally and theoretically determined in terms of the updated AAPM Task Group 43 (TG-43U1) recommendations. Measurements were performed using TLD-GR200A circular chip dosimeters using standard methods employing thermoluminescent dosimeters in a Perspex phantom. Precision machined bores in the phantom located dosimeters and source in a reproducible fixed geometry providing for transverse-axis and angular dose profiles over a range of distances from 0.5 to 5 cm. The Monte Carlo N-Particle (MCNP) code, version 4C was used to evaluate the dose-rate distributions around this model 103Pd source in water and Perspex phantoms. The Monte Carlo calculated dose-rate constant of the IRA1- 103Pd source in water was found equal to Λ=0.669 cGy/h/U with approximate uncertainties of ±0.1%. The anisotropy function, F( r, θ), and the radial dose function, g L ( r), of the IRA1- 103Pd source were also measured in Perspex phantom and calculated in both Perspex and liquid water phantom.

Surface dosimetry in a CT scanner using MOSFET detectors and Monte Carlo simulations

Diagnostic imaging with CT procedures is responsible for significant radiation doses to patients. To enable individual patient dose estimates, a combination of MOSFET detectors and Monte Carlo (MC) simulations was investigated for the determination of patient surface dose. The behaviour of MOSFETs in kV x-rays from a CT scanner was investigated with experiments and MC simulations with a CT scanner model. A dose reproducibility of 5% and a mean loss of sensitivity with accumulated dose of about 10% was noted for the MOSFETs. Beam energy increase from 80-140 kVp resulted in a response decrease of 10%. The MOSFET detectors were calibrated in terms of absolute surface dose with the aid of MC simulations. Good agreement was achieved between measured and calculated surface dose on a cylindrical Lucite phantom. Experiments with a stationary x-ray tube and contiguous axial scanning led to differences limited by 8%. Surface dose in helical scanning was investigated with measurements with radiological film and an array of five MOSFET detectors, leading to good agreement. It is concluded that an array of MOSFET detectors, calibrated in terms of surface dose, is a valuable tool to assess individual patient surface dose. In combination with MC simulations this may lead to estimations of effective dose.

Radiation Exposure to Premature Infants in a Neonatal Intensive Care Unit in Turkey

ObjectiveThe aim of this work was to determine the radiation dose received by infants from radiographic exposure and the contribution from scatter radiation due to radiographic exposure of other infants in the same room.Materials and MethodsWe retrospectively evaluated the entrance skin doses (ESDs) and effective doses of 23 infants with a gestational age as low as 28 weeks. ESDs were determined from tube output measurements (ESDTO) (n = 23) and from the use of thermoluminescent dosimetry (ESDTLD) (n = 16). Scattered radiation was evaluated using a 5 cm Perspex phantom. Effective doses were estimated from ESDTO by Monte Carlo computed software and radiation risks were estimated from the effective dose. ESDTO and ESDTLD were correlated using linear regression analysis.ResultsThe mean ESDTO for the chest and abdomen were 67 µGy and 65 µGy per procedure, respectively. The mean ESDTLD per radiograph was 70 µGy. The measured scattered radiation range at a 2 m distance from the neonatal intensive care unit (NICU) was (11-17 µGy) per radiograph. Mean effective doses were 16 and 27 µSv per procedure for the chest and abdomen, respectively. ESDTLD was well correlated with ESDTO obtained from the total chest and abdomen radiographs for each infant (R2 = 0.86). The radiation risks for childhood cancer estimated from the effective dose were 0.4 × 10-6 to 2 × 10-6 and 0.6 × 10-6 to 2.9 × 10-6 for chest and abdomen radiographs, respectively.ConclusionThe results of our study show that neonates received acceptable doses from common radiological examinations. Although the contribution of scatter radiation to the neonatal dose is low, considering the sensitivity of the neonates to radiation, further protective action was performed by increasing the distance of the infants from each other.

A method for estimation of accuracy of dose delivery with dynamic slit windows in medical linear accelerators

Intensity-modulated radiotherapy (IMRT) clinical dose delivery is based on computer-controlled multileaf movements at different velocities. To test the accuracy of modulation of the beam periodically, quality assurance (QA) methods are necessary. Using a cylindrical phantom, dose delivery was checked at a constant geometry for sweeping fields. Repeated measurements with an in-house designed methodology over a period of 1 year indicate that the method is very sensitive to check the proper functioning of such dose delivery in medical linacs. A cylindrical perspex phantom with facility to accurately position a 0.6-cc (FC 65) ion chamber at constant depth at isocenter, (SA 24 constancy check tool phantom for MU check, Scanditronix Wellhofer) was used. Dosimeter readings were integrated for 4-mm, 10-mm, 20-mm sweeping fields and for 3 angular positions of the gantry periodically. Consistency of standard sweeping field output (10-mm slit width) and the ratios of outputs against other slit widths over a long period were reported. A 10-mm sweeping field output was found reproducible within an accuracy of 0.03% (n = 25) over 1 year. Four-millimeter, 20-mm outputs expressed as ratio with respect to 10-mm sweep output remained within a mean deviation of 0.2% and 0.03% respectively. Outputs at 3 gantry angles remained within 0.5%, showing that the effect of dynamic movements of multileaf collimator (MLC) on the output is minimal for angular positions of gantry. This method of QA is very simple and is recommended in addition to individual patient QA measurements, which reflect the accuracy of dose planning system. In addition to standard output and energy checks of linacs, the above measurements can be complemented so as to check proper functioning of multileaf collimator for dynamic field dose delivery.

Solid State Microdosimetry With Heavy Ions for Space Applications

This work provides information pertaining to the performance of Silicon-On-Insulator (SOI) microdosimeters in heavy ion radiation fields. SOI microdosimeters have been previously tested in light ion radiation fields for both space and therapeutic applications, however their response has not been established in high energy, heavy ion radiation fields which are experienced in space. Irradiations were completed at the NASA Space Radiation Laboratory at BNL using 0.6 GeV/u Fe and 1.0 GeV/u Ti ions. Energy deposition and lineal energy spectra were obtained with this device at various depths within a Lucite phantom along the central axis of the beam. The response of which was compared with existing proportional counter data to assess the applicability of SOI microdosimeters to future deployments in space missions.

Field calibration and comparison of personal neutron dosemeter designs based on CR-39 for the use around high-energy accelerators

PADC (poly-allyl-diglycol-carbonate) material, also known as CR-39, is widely used for the detection of charged particles. Additionally, thermal or fast neutrons can be measured through detection of secondary charged particles produced in a dedicated radiator material. Around high-energy accelerators neutron dose is mainly dominated by neutrons with energies above ∼ 1 MeV . In respect to radiation protection in such fields, the detection of fast neutrons is imperative and detection of thermal neutrons can be set aside. Providing that a dosemeter holder contains sufficient hydrogen as replacement of a dedicated radiator, the CR-39 detectors can easily be used in different dosemeter designs without losing sensitivity for high-energy neutrons. These considerations were taken into account when introducing the CR-39 detectors for individual neutron dosimetry at the high-energy accelerator facilities CERN and DESY. To demonstrate the feasibility of such an approach, measurements at the High-Energy Reference Field Facility (CERF) at CERN were performed with three different types of dosemeters using CR-39 detectors from two different manufacturers. The dosemeters were placed on all faces of Perspex phantoms at reference locations, in order to study the influence of the mounting position on the phantoms. Moreover, a field calibration factor was determined.

A randomised controlled study comparing conventional and magnetic guidewires in a two‐dimensional branching tortuous phantom simulating angulated coronary vessels

To directly compare the magnetic navigation system (MNS) guidewires with conventional guidewires in branching tortuous phantoms with operators of varying MNS and percutaneous coronary intervention experience. Vessel tortuosity, angulation, and side branches remain limiting factors in coronary interventions. The MNS addresses these limitations by precisely directing the tip of a magnetised guidewire in vivo aided by two permanent adjustable external magnets. Crossing and fluoroscopy times of six operators were evaluated in five tortuous Perspex(R) phantom vessels in three consecutive attempts. Standard guidewire (SG) usage was unrestricted. Two 2nd generation magnetic guidewires (MG) were used. Failure was noted if the cross was unsuccessful within 5 min. The magnetic navigation was vastly superior to SG techniques with increasingly tortuous phantoms. It dramatically decreased both the crossing and fluoroscopy times with maximal reduction from 201.7 +/- 111 to 36.4 +/- 13 sec, P < 0.001 and 204.7 +/- 24 to 47.2 +/- 19 sec, P < 0.001, respectively. The MNS had a 98.8% procedural success rate compared to 68% with SG techniques. Moreover it considerably limited the amount of wire usage from 5.5 to 1.3. Operators with prior MG experience performed significantly better than those without, except in the simplest phantom where the difference was nonsignificant (33.8 +/- 13 sec vs. 41.7 +/- 17 sec, P = 0.2). MNS significantly reduces both the crossing and fluoroscopy times in tortuous coronary phantom models achieving excellent success rates with dramatic reductions in guidewire usage. Operators with prior MNS experience had an advantage over the inexperienced.

A gradient-based method for segmenting FDG-PET images: methodology and validation

A new gradient-based method for segmenting FDG-PET images is described and validated. The proposed method relies on the watershed transform and hierarchical cluster analysis. To allow a better estimation of the gradient intensity, iteratively reconstructed images were first denoised and deblurred with an edge-preserving filter and a constrained iterative deconvolution algorithm. Validation was first performed on computer-generated 3D phantoms containing spheres, then on a real cylindrical Lucite phantom containing spheres of different volumes ranging from 2.1 to 92.9 ml. Moreover, laryngeal tumours from seven patients were segmented on PET images acquired before laryngectomy by the gradient-based method and the thresholding method based on the source-to-background ratio developed by Daisne (Radiother Oncol 2003;69:247-50). For the spheres, the calculated volumes and radii were compared with the known values; for laryngeal tumours, the volumes were compared with the macroscopic specimens. Volume mismatches were also analysed. On computer-generated phantoms, the deconvolution algorithm decreased the mis-estimate of volumes and radii. For the Lucite phantom, the gradient-based method led to a slight underestimation of sphere volumes (by 10-20%), corresponding to negligible radius differences (0.5-1.1 mm); for laryngeal tumours, the segmented volumes by the gradient-based method agreed with those delineated on the macroscopic specimens, whereas the threshold-based method overestimated the true volume by 68% (p=0.014). Lastly, macroscopic laryngeal specimens were totally encompassed by neither the threshold-based nor the gradient-based volumes. The gradient-based segmentation method applied on denoised and deblurred images proved to be more accurate than the source-to-background ratio method.

Image-guided vascular neurosurgery based on three-dimensional rotational angiography

Three-dimensional rotational angiography is capable of exquisite visualization of cerebral blood vessels and their pathophysiology. Unfortunately, images obtained using this modality typically show a small region of interest without exterior landmarks to allow patient-to-image registration, precluding their use for neuronavigation purposes. The aim of this study was to find an alternative technique to enable 3D rotational angiography-guided vascular neurosurgery. Three-dimensional rotational angiograms were obtained in an angiographic suite with direct navigation capabilities. After image acquisition, a navigated pointer was used to touch fiducial positions on the patient's head. These positions were located outside the image volume but could nevertheless be transformed into image coordinates and stored in the navigation system. Prior to surgery, the data set was transferred to the navigation system in the operating room, and the same fiducial positions were touched again to complete the patient-to-image registration. This technique was tested on a Perspex phantom representing the cerebral vascular tree and on two patients with an intracranial aneurysm. In both the phantom and patients, the neuronavigation system provided 3D images representing the vascular tree in its correct orientation, that is, the orientation seen by the neurosurgeon through the microscope. In one patient, tissue shift was clearly observed without significant changes in the orientation of the structures. Results in this study demonstrate the feasibility of using 3D rotational angiography data sets for neuronavigation purposes. Determining the benefit of this type of navigation should be the subject of future studies.

Evaluation methods for detecting changes in beam output and energy in radiation beams from high-energy linear accelerators

There is need for simple methods for checking consistency of beam outputs and energy in linear accelerators used for radiotherapy. A method was designed by the department using perspex phantom with which the dosimetric data of two medical linear accelerators (Clinac 600 CD, Clinac 2300 CD) were evaluated over a period of 30 months. The efficacy of methods followed was checked. Routine beam consistency checks were designed for photon beams with 15 cm/ 5 cm depth ionizations in perspex phantom and variable depth combinations for electron beams. Calculated ionization ratios were compared with measured values to show their significance. The dose/MU for all radiation beams was maintained within 2% accuracy over the period of 30 months. Clinac 600 CD machine showed decreasing trend of cGy/MU, while Clinac 2300 CD showed increasing trend of cGy/MU over a period, which needed tuning of monitor chamber two times each. Tuning of output to achieve standard value was carried out once, for all electron energies when the output dose/MU exceeded 3%. During one week (June 2005), there were slight changes in electron energy detected using the ratio method, which did not recur anytime afterwards. The methods designed are adequate to find the consistency in the beam output and energies in the radiotherapy linacs.

A PHANTOM TEST OF PROTON-INDUCED DUAL-ENERGY X-RAY ANGIOGRAPHY USING IODINATED CONTRAST MEDIA

Characteristic-line radiation from heavy metal targets bombarded by MeV proton beams has been tested as an X-ray source for dual-energy K-edge subtraction imaging for human angiography (blood vessel imaging) based on iodinated contrast media. To utilize the strong absorption by iodine (Z = 53) at its K-absorption edge (33.2 keV), we used K α-line of La (lanthanum, Z = 57) at 33.4 keV. As a reference, also K α X emission of Sn (tin, Z = 50) at 25.2 keV was employed. Metallic plates of La and Sn were irradiated by 7-MeV protons to produce these characteristic X-rays. Energy-subtraction method was tested using Lucite phantoms which contain aqueous solutions of KI (potassium iodide) with different concentrations. Also Ca ( H 2 PO 4)2 powder was stuffed in these phantoms to simulate bones. The transmission images of the phantoms were recorded on imaging plates. During the exposure, the energy spectra of the X-rays were monitored by a CdTe detector. We found that the contrast of images of iodide solutions taken with La X-rays was higher than that with Sn X-rays. Also the energy subtraction procedure was successfully applied to reduce the graphical noise due to the bones and inhomogeneity of the soft tissue. However, to apply the present method to actual clinical use, the X-ray intensity must be increased by several orders of magnitude. Also the transmission of the “lower-energy” photons has to be a few orders higher for imaging of objects as thick as human chest.

Dosimetric characterization of a novel intracavitary mold applicator for high dose rate endorectal brachytherapy treatment

The dosimetric properties of a novel intracavitary mold applicator for 192Ir high dose rate (HDR) endorectal cancer treatment have been investigated using Monte Carlo (MC) simulations and experimental methods. The 28 cm long applicator has a flexible structure made of silicone rubber for easy passage into cavities with deep-seated tumors. It consists of eight source catheters arranged around a central cavity for shielding insertion, and is compatible for use with an endocavitary balloon. A phase space model of the HDR source has been validated for dose calculations using the GEANT4 MC code. GAFCHROMIC EBT model film was used to measure dose distributions in water around shielded and unshielded applicators with two loading configurations, and to quantify the shielding effect of a balloon injected with an iodine solution (300 mg I/mL). The film calibration procedure was performed in water using an 192Ir HDR source. Ionization chamber measurements in a Lucite phantom show that placing a tungsten rod in the applicator attenuates the dose in the shielded region by up to 85%. Inserting the shielded applicator into a water-filled balloon pushes the neighboring tissues away from the radiation source, and the resulting geometric displacement reduces the dose by up to 53%; another 8% dose reduction can be achieved when the balloon is injected with an iodine solution. All experimental results agree with the GEANT4 calculations within measurement uncertainties.

Feasibility of real time dual-energy imaging based on a flat panel detector for coronary artery calcium quantification

The feasibility of a real-time dual-energy imaging technique with dynamic filtration using a flat panel detector for quantifying coronary arterial calcium was evaluated. In this technique, the x-ray beam was switched at 15 Hz between 60 kVp and 120 kVp with the 120 kVp beam having an additional 0.8 mm silver filter. The performance of the dynamic filtration technique was compared with a static filtration technique (4 mm Al+0.2 mm Cu for both beams). The ability to quantify calcium mass was evaluated using calcified arterial vessel phantoms with 20-230 mg of hydroxylapatite. The vessel phantoms were imaged over a Lucite phantom and then an anthropomorphic chest phantom. The total thickness of Lucite phantom ranges from 13.5-26.5 cm to simulate patient thickness of 16-32 cm. The calcium mass was measured using a densitometric technique. The effective dose to patient was estimated from the measured entrance exposure. The effects of patient thickness on contrast-to-noise ratio (CNR), effective dose, and the precision of calcium mass quantification (i.e., the frame to frame variability) were studied. The effects of misregistration artifacts were also measured by shifting the vessel phantoms manually between low- and high-energy images. The results show that, with the same detector signal level, the dynamic filtration technique produced 70% higher calcium contrast-to-noise ratio with only 4% increase in patient dose as compared to the static filtration technique. At the same time, x-ray tube loading increased by 30% with dynamic filtration. The minimum detectability of calcium with anatomical background was measured to be 34 mg of hydroxyapatite. The precision in calcium mass measurement, determined from 16 repeated dual-energy images, ranges from 13 mg to 41 mg when the patient thickness increased from 16 to 32 cm. The CNR was found to decrease with the patient thickness linearly at a rate of (-7%/cm). The anatomic background produced measurement root-mean-square (RMS) errors of 13 mg and 18 mg when the vessel phantoms were imaged over a uniform (over the rib) and nonuniform (across the edge of rib) bone background, respectively. Misregistration artifacts due to motions of up to 1.0 mm between the low- and high-energy images introduce RMS error of less than 4.3 mg, which is much smaller than the frame to frame variability and the measurement error due to anatomic background. The effective dose ranged from 1.1 to 6.6 microSv for each dual-energy image, depending on patient thickness. The study shows that real-time dual-energy imaging can potentially be used as a low dose technique for quantifying coronary arterial calcium.

Energy responses of the LiF series TL pellets to high-energy photons in the energy range from 1.25 to 21 MV

The energy responses for the KLT-300(LiF:Mg,Cu,Na,Si, Korea), GR-200(LiF:Mg,Cu,P, China) and MCP-N(LiF:Mg,Cu,P, Poland) thermoluminescence(TL) pellets were studied for a photon radiation with energies from 1.25 MeV(60Co) to 21 MV (Microtron) to verify the usefulness of the calibration for the radiotherapy beams. The International Atomic Energy Agency (IAEA) and the World Health Organization (WHO) have performed thermoluminescence dosimetry (TLD) audits to verify the calibration of the beams by TL powder, but TL pellets were used in this study because the element correction factor (ECF), defined as the factor to correct the variations that all TL dosemeters cannot be manufactured to have exactly the same TL efficiency, for each TL pellet could be accurately derived and be handled conveniently when compared with the powder. Also several works for the energy response of the TLDs were done for the low-energy photon beams up to 60Co, but they will be extended in this experiment to the high photon energies (up to 20 MV), which are widely used in the therapy level of a radiation. The PTW 30006 ionisation chamber was calibrated by the Korea primary standards to establish the air-kerma rates and the TL pellets were irradiated in a specially designed waterproof pellet holder in a water phantom (30 x 30 x 30 cm3) just like the IAEA postal audits programme. This result was compared with that of another type of phantom [10 (W) x 10 (L) x 10 (H) cm3 PMMA Perspex phantom for the 60Co and 6 MV photon, and 10 x 10 x 20 (H) cm3 for the 10 and 21 MV photon] for its convenient use and easy handling and installation in a hospital. The results show that the differences of the responses for the water phantom and PMMA Perspex phantom were negligible, which is contrary to the general conception that a big difference would be expected. For an application of these results to verify the therapy beams, an appropriate energy correction factor should be applied to the energies and phantom types in use.

Development and characterization of a tissue equivalent plastic scintillator based dosimetry system

High precision techniques in radiation therapy, such as intensity modulated radiation therapy, offer the potential for improved target coverage and increased normal tissue sparing compared with conformal radiotherapy. The complex fluence maps used in many of these techniques, however, often lead to more challenging quality assurance with dose verification being labor-intensive and time consuming. A prototype dose verification system has been developed using a tissue equivalent plastic scintillator that provides easy-to-acquire, rapid, digital dose measurements in a plane perpendicular to the beam. The system consists of a water-filled Lucite phantom with a scintillator screen built into the top surface. The phantom contains a silver coated plastic mirror to reflect scintillation light towards a viewing window where it is captured using a charge coupled device camera and a personal computer. Optical photon spread is removed using a microlouvre optical collimator and by deconvolving a glare kernel from the raw images. A characterization of the system was performed that included measurements of linear output response, dose rate dependence, spatial linearity, effective pixel size, signal uniformity and both short- and long-term reproducibility. The average pixel intensity for static, regular shaped fields between 3 cm X 3 cm and 12 cm x 12 cm imaged with the system was found to be linear in the dose delivered with linear regression analysis yielding a correlation coefficient r2 > 0.99. Effective pixel size was determined to be 0.53 mm/pixel. The system was found to have a signal uniformity of 5.6% and a long-term reproducibility/stability of 1.7% over a 6 month period. The system's ability to verify a dynamic treatment field was evaluated using 60 degrees dynamic wedged fields and comparing the results to two-dimensional film dosimetry. Results indicate agreement with two-dimensional film dosimetry distributions within 8% inside the field edges. With further development, this system promises to provide a fast, directly digital, and tissue equivalent alternative to current dose verification systems.