Cube Phantom Research Articles

SU-E-J-74: Evaluation of the Commercial Prototype Open Face Mask Used with Surface Rendering Imaging System for Proton Therapy Patients with Cancers of the Brain and Head: Successful Preclinical Study.

Evaluation of the commercial prototype open face mask (Fibreplast, Q-FixTM) used with surface rendering imaging system (AlignRTTM, London, UK) for setup and monitoring of proton therapy patients with cancers of the brain and head. Fibreplastic open face masks were used with Base-of-Skull frame/moldcare pillow. Two sizes of the pre-made cutouts in the masks were investigated: leaveing open skin surface in the oval-shaped area between (I) upper lip and forehead boss, II) between mandible and forehead boss in superior-inferior direction; and between zygomatic (cheek-) bones/sphenoid bones laterally. Calibration of AlignRT system was verified with SRS cube phantom (VisionRT)and 2D X-ray patient positioning system (DIPS, IBA). The accuracy of translational shifts and rotations was a) verified using a head&neck phantom and b) tested using volunteers. Within AlignRT software, only the open area was used as ROI for the registration; the option 'Intracranial SRS' used for surface capture. For the isocenter determination accuracy within 0.3mm/0.2 degrees, the head&neck phantom registration was performed with the 0.5mm/0.5 degree accuracy; the volunteer tests showed the registration accuracy within 0.5mm ±0.2mm (1 s)/0.7±0.2 (1 s) degree. The simulation of the noise in the surface data such as eye blinking and attempts to swallow/chew was performed, with negligible effect on the registration. No significant differences between the study results between the masks of type (I) and (II) were noticed. The AlignRT surface rendering system has sufficient accuracy when used with the open face head&neck mask for localization/registration of the proton therapy patients with tumors of the head and brain.

Using matrix summation method for three dimensional dose calculation in brachytherapy

AimThe purpose of this study is to calculate radiation dose around a brachytherapy source in a water phantom for different seed locations or rotation the sources by the matrix summation method. BackgroundMonte Carlo based codes like MCNP are widely used for performing radiation transport calculations and dose evaluation in brachytherapy. But for complicated situations, like using more than one source, moving or rotating the source, the routine Monte Carlo method for dose calculation needs a long time running. Materials and methodsThe MCNPX code has been used to calculate radiation dose around a 192Ir brachytherapy source and saved in a 3D matrix. Then, we used this matrix to evaluate the absorbed dose in any point due to some sources or a source which shifted or rotated in some places by the matrix summation method. ResultsThree dimensional (3D) dose results and isodose curves were presented for 192Ir source in a water cube phantom shifted for 10 steps and rotated for 45 and 90° based on the matrix summation method. Also, we applied this method for some arrays of sources. ConclusionThe matrix summation method can be used for 3D dose calculations for any brachytherapy source which has moved or rotated. This simple method is very fast compared to routine Monte Carlo based methods. In addition, it can be applied for dose optimization study.

SU-E-T-717: Research on the Inhomogeneous Correction Algorithm Based on Monte Carlo Simulation

Purpose: Batho is one of inhomogeneous correction algorithms which has high correction accuracy in homogeneous tissue, but has lower correction accuracy in cavum and the interface between different density tissues. In this paper, the database of dose distribution with every kinds of different density tissues combination by Monte Carlo simulation is built up for the purpose of development for fast and accurate dose calculation method. Methods: The MC simulation models are built as follow. Model 1: in 30cm×30cm×30cm cube phantom full of one homogeneity tissue. In module 2:1cm×1cm×1cm cube inhomogeneous tissue is put in the central axis (0,0,0.5),(0,0,1.5,)…(0,0,29.5) (each down 1cm) of model 1 respectively. These phantoms are segmented into 0.2cm × 0.2cm × 0.2cm volumes. For simplicity, the inhomogeneous tissue of human body is classified to typical tissues (water, bone etc.) by electron densities. The dose distribution of homogeneous tissue and inhomogeneous tissue is simulated by MC in model 1 and model 2.All dose value is normalized to the point (0,0,1.5). Then database of dose distribution relative error between dose distribution of mode 2 and mode l could be established. At any point p, the destiny change along the primary ray path judged by program firstly. Batho algorithm is used in uniform region. In the interface between different tissues and inside cavity, the dose value will be corrected by the database of relative error. Results: Comparing with the correction result of pure Batho algorithm, this program can gain more accurate correction results close to MC in short time in the interface between different tissues and cavity. Conclusions: In this paper, the higher correction accuracy could be achieved and the method is simple, easy to implement. The result of this study can provide fast and accurate dose calculation module radiation planning system. The work is supported by National 973 Planned Project (2006CB708307), the National Natural Science Foundation of China (60872112 and 10805012), the Natural Science Foundation of Zhejiang Province (Z207588), the College Science Research Projects of Anhui Province (KJ2008B268).

Positioning accuracy of a new image-guided radiotherapy system

To evaluate the accuracy of the patient-positioning function of a newly developed image-guided radiotherapy system, the MHI-TM2000 (Mitsubishi Heavy Industries, Ltd., Japan). The isocenter positions prescribed by the lasers, MV treatment beam, and image guidance systems (kV X-ray image and kV-CBCT) were calculated using a cube phantom with a 10-mm-diameter steel ball fixed to the center of the phantom. Then, their location discrepancies were estimated. In addition, to verify the scale and orientation of the coordinate axes of the kV X-ray imaging system, positional measurements were repeated with the phantom placed at 50 mm off-isocenter along the vertical, longitudinal, and lateral directions, respectively. Further, image fusions of an anthropomorphic phantom image and the corresponding image translated by a pre-determined amount were performed. The isocenter alignment among the coordinate systems was coincident within 0.5 mm in translation for the vertical, longitudinal, and lateral axes, respectively. The geometrical errors at 50 mm off-isocenter for kV X-ray images and CBCT were within 0.2 mm and 1.0 mm, respectively. The image fusion errors were within 1.0 mm in translation and 1.0 degrees in rotation, respectively. No significant difference in the image fusion accuracy was observed between the chest and pelvis phantoms. The isocenter alignment among the coordinate systems was performed with high accuracy. Furthermore, the automatic image fusion function achieved sufficient patient positioning accuracy and precision for image-guided radiotherapy.

SU-GG-T-250: 3D Dose Reconstruction for Delivery Quality Assurance (DQA) from Multiple 2D Planes Using the OCTAVIOUS Phantom

Purpose: To perform a 3D dose reconstruction for delivery quality assurance (DQA) from multiple 2D planes using the OCTAVIOUS phantom. Method and Materials: Ten RapidArc patient treatment plans of different sites were delivered on two phantoms. Two different DQA plans were delivered for each of the 10 patients: 1) OCTAVIOUS phantom and 2) 30×30×30 cube solid water phantom in which the detector array (Seven29) was placed. The corresponding 3D dose of each of the DQA plans was exported. All DQA plans were delivered by means of a NovalisTX with the HD120 MLC. For the solid water phantom, the same plan was delivered six times with the array varying in the coronal plane in increments of 0.5cm. For the OCTAVIOUS, the same plan was delivered four times by rotating the phantom in 45° increments along its longitudinal axis. An in‐house MATLAB code was used to read the planar dose information. A linear reconstruction was performed for the cube phantom while a circular reconstruction was used for the OCTAVIOUS. Dose statistics per plane were obtained to validate the reconstruction method. Results: Both interpolation methods showed good agreement to the planned dose distribution in the high dose region (<1.0%) but showed discrepancies in the low dose region. A DAH comparison shows good agreement for the sagittal and coronal planes but demonstrates some discrepancies in the transversal plane. Conclusion: A simple linear interpolation method is able to predict good matching in the high dose region between the reconstructed dose and the planned dose. This technique is a good starting point to establish a benchmark in the level of manipulation necessary to obtain good 3D dose delivery quality assurance using current technology. Conflict of Interest: Research Sponsored by PTW Company

A trust region method in adaptive finite element framework for bioluminescence tomography

Bioluminescence tomography (BLT) is an effective molecular imaging (MI) modality. Because of the ill-posedness, the inverse problem of BLT is still open. We present a trust region method (TRM) for BLT source reconstruction. The TRM is applied in the source reconstruction procedure of BLT for the first time. The results of both numerical simulations and the experiments of cube phantom and nude mouse draw us to the conclusion that based on the adaptive finite element (AFE) framework, the TRM works in the source reconstruction procedure of BLT. To make our conclusion more reliable, we also compare the performance of the TRM and that of the famous Tikhonov regularization method after only one step of mesh refinement of the AFE framework. The conclusion is that the TRM can get faster and better results after only one mesh refinement step of AFE framework than the Tikhonov regularization method when handling large scale data. In the TRM, all the parameters are fixed, while in the Tikhonov method the regularization parameter needs to be well selected.

A positioning QA procedure for 2D/2D (kV/MV) and 3D/3D (CT/CBCT) image matching for radiotherapy patient setup

A positioning QA procedure for Varian's 2D/2D (kV/MV) and 3D/3D (planCT/CBCT) matching was developed. The procedure was to check: (1) the coincidence of on‐board imager (OBI), portal imager (PI), and cone beam CT (CBCT)'s isocenters (digital graticules) to a linac's isocenter (to a pre‐specified accuracy); (2) that the positioning difference detected by 2D/2D (kV/MV) and 3D/3D(planCT/CBCT) matching can be reliably transferred to couch motion. A cube phantom with a 2 mm metal ball (bb) at the center was used. The bb was used to define the isocenter. Two additional bbs were placed on two phantom surfaces in order to define a spatial location of 1.5 cm anterior, 1.5 cm inferior, and 1.5 cm right from the isocenter. An axial scan of the phantom was acquired from a multislice CT simulator. The phantom was set at the linac's isocenter (lasers); either AP MV/R Lat kV images or CBCT images were taken for 2D/2D or 3D/3D matching, respectively. For 2D/2D, the accuracy of each device's isocenter was obtained by checking the distance between the central bb and the digital graticule. Then the central bb in orthogonal DRRs was manually moved to overlay to the off‐axis bbs in kV/MV images. For 3D/3D, CBCT was first matched to planCT to check the isocenter difference between the two CTs. Manual shifts were then made by moving CBCT such that the point defined by the two off‐axis bbs overlay to the central bb in planCT. (PlanCT can not be moved in the current version of OBI1.4.) The manual shifts were then applied to remotely move the couch. The room laser was used to check the accuracy of the couch movement. For Trilogy (or Ix‐21) linacs, the coincidence of imager and linac's isocenter was better than 1 mm (or 1.5 mm). The couch shift accuracy was better than 2 mm.PACS number: 87.55.Qr, 87.57.Q‐

Variations of lung density and geometry on inhomogeneity correction algorithms: A Monte Carlo dosimetric evaluation

This work contributed the following new information to the study of inhomogeneity correction algorithm: (1) Evaluation of lung dose calculation methods as a function of lung relative electron density (rhoe,lung) and treatment geometry and (2) comparison of doses calculated using the collapsed cone convolution (CCC) and adaptive convolution (AC) in lung using the Monte Carlo (MC) simulation with the EGSnrc-based code. The variations of rhoe,lung and geometry such as the position and dimension of the lung were studied with different photon beam energies and field sizes. Three groups of inhomogeneous lung phantoms, namely, "slab," "column," and "cube," with different positions, volumes, and shapes of lung in water as well as clinical computed tomography lung images were used. The rhoe,lung in each group of phantoms vary from 0.05 to 0.7. 6 and 18 MV photon beams with small (4 x 4 cm2) and medium (10 x 10 cm2) field sizes produced by a Varian 21 EX linear accelerator were used. This study reveals that doses in the inhomogeneous lung calculated by the CCC match well with those by AC within +/- 1%, indicating that the AC, with an advantage of shorter computing times (three to four times shorter than CCC), is a good substitute for CCC. Comparing the CCC and AC to MC in general, significant dose deviations are found when the rhoe,lung is < or =0.3. The degree of deviation depends on the photon beam energy and field size and is relatively large when high-energy photon beams with small fields are used. For penumbra widths (20%-80%), the CCC and AC agree well with MC for the slab and cube phantoms with the lung volumes at the central beam axis (CAX). However, deviations (>2 mm) occur in the column phantoms, with two lung volumes separated by a unit density column along the CAX in the middle using the 18 MV beam with 4 x 4 cm2 field for rhoe,lung < or =0.1. This study provides new dosimetric data to evaluate the impact of the variations of rhoe,lung and geometry on dose calculations in inhomogeneous media using CCC and AC.

SU-FF-T-559: Effect of Cardiac Motion On the Cyberknife Synchrony Tracking System for Radiosurgical Cardiac Ablation

Purpose: To characterize the effect of cardiac motion on the CyberKnife Synchrony respiratory tracking system for radiosurgical cardiac ablation. Method and Materials: A high precision programmable haptic device (Force Dimensions Omega.6) was used to simulate cardiac and respiratory motion of a ball cube phantom. The CyberKnife Synchrony system was used to track the position of the 25 mm diameter target via spatial correlation of a fiducial within the phantom with the external LEDs placed on a second haptic device simulating chest wall motion. The targeting and tracking error were analyzed by comparing the actual target positions with the linac offset positions predicted from the Synchrony model. Film dosimetry was used to evaluate the effect of cardiac motion on target coverage. Results: Examination of log files revealed that the Synchrony modeling averaged out cardiac motion not present in the surrogate with some error. Superior‐inferior motion from both respiratory and cardiac motion contributed to the largest Synchrony modeling error. This modeling error led to misdirection of the beams from the time averaged target position. Additionally, untracked motion resulted in the target moving into and out of the radiation beams which manifested as softening of the dose gradient changes. Analysis of the films indicated that this was most severe in the superior inferior direction consistent with the extent of motion in that direction. Conclusion: A small additional margin would enable treatment of cardiac targets with the current CyberKnife if Synchrony tracking were used. For the motions and dose distributions tested, a 2 mm margin was sufficient to provide good target coverage in the presence of untracked cardiac motion. S. Dieterich consults for Cyberheart Inc.

SU‐GG‐J‐45: CBCT Based Quality Assurance for Optically Guided Patient Positioning and Monitoring

Purpose: To provide image based quality assurance procedure to evaluate and eliminate systematic discrepancies between an optical positioning system and Cone Beam CT (CBCT). Materials/Method: An optically‐guided positioning system (Varian Medical Systems, Palo Alto, CA) is routinely used to setup and monitor patients undergoing fractionated and hypo‐fractionated head and neck treatments at Stanford University. We have typically assessed the accuracy of this positioning with matching of (CBCT) images acquired in treatment position to the patients planning CT scans. When the two positioning systems did not agree, the optical one was recalibrated via CBCT using a custom made imaging device consisting of the reference fiducial frame mounted to a cube phantom that is used for routine OBI QA. A CT scan of this system was acquired and exported to the Son Array workstation where the registration of the CT was performed. Phantom positioning was achieved using optical guidance. Setup was verified by volumetric matching of CBCT images to the CT. Results: To date, three patients have been treated in our clinic using the combined frameless array/CBCT positioning systems. In two cases, the solutions of the frameless and CBCT systems coincided within &lt;1 mm and &lt;1° rotations. In one case, differences of 3 mm were observed. The application of the above QA procedure ruled out systematic discrepancies related to system performance. It was concluded that for this patient the bite block positioning varied significantly between planning and setup. In this case positioning of the patient was based on volumetric anatomical matching via CBCT. Conclusion: The use of CBCT for both positioning and verification provides an ideal quality assurance procedure for frameless patient setup as it permits calibration of the frameless array and direct visualization of the proposed re‐positioning shifts.

Comparison of calibration methods for spatial tracking of a 3-D ultrasound probe

Real-time 3-D ultrasound (US) is a new-generation US system that uses a dedicated probe to create volume data sets instead of standard 2-D cross-sectional images. For applications in image-guided surgery and radiation therapy, a position tracker is added to the probe so that the volumes can be located in space. Calibration plays a critical role in determining the overall accuracy of an US volume-tracking system. In this paper, three calibration methods are developed specifically for 3-D probes. The three methods are based on a IXI-shaped wire phantom, a cube phantom and a stylus. The performance of each method was evaluated in terms of calibration reproducibility, point accuracy and reconstruction accuracy by distance measurement. The mean errors in the reproducibility tests were 1.50 mm (IXI-wire), 1.16 mm (cube) and 5.13 mm (stylus). The root mean square errors of the point accuracy measure were 2.15 mm (IXI-wire), 4.91 mm (cube) and 2.36 mm (stylus). The root mean square errors of the reconstruction accuracy by distance measure were 1.52 mm (IXI-wire), 1.59 mm (cube) and 1.85 mm (stylus). Overall, the IXI-wire phantom achieved the best results. (E-mail: rohling@ece.ubc.ca)

SU-FF-T-77: IMRT of Superficial Tumors

Purpose: Due to inadequate beam modeling in the buildup region, large differences exist between planned and delivered IMRT doses for superficial targets. Our measurements show that plans underestimate doses delivered to superficial target regions by upto 40%. This results in unexpected skin reactions in patients and overdosing of critical structures. In this work, surface dose for such cases was investigated. Method and Materials: Using Rando phantom, IMRT plans were developed for PTVs located at: (1) skin surface (PTV0); (2) 10mm from skin (PTV10), and (3) phantom center (PTVctr). Dose distributions in Rando phantom were measured with EDR films and verified by MOSFET detectors. QA for all IMRT plans was done in a cube phantom using EDR films and ion chamber.Results: For PTV0, measurements in Rando phantom showed surface receiving full dose. In contrast, planned surface dose was 40% lower. In fact, superficial regions (up to 7mm) of PTV were underdosed. In distal target regions, agreement between planned and measured doses was good. For PTV10, agreement between planned and delivered surface doses was significantly better (∼10%). MOSFET measurements indicated 42.5% and 8.5% surface overdose(average) for PTV0 and PTV10 respectively. For centrally located tumors, measured and planned doses were in excellent agreement. In QA phantom, dose uniformity of transferred targets depended on their location. For PTV0, the side of the tumor that was facing surface received 30% higher dose than prescription. For PTV10, dose increase was 10%. Conclusion: In IMRT plans, adequate target dose is delivered without bolus. Extreme care must be exercised in delineating superficial PTVs. Whenever possible, a minimum distance of 1cm between skin surface and PTV must be preserved. With smaller distances, the skin may appear to be erroneously spared. This phenomenon is not detected by commonly used IMRT QA procedures. Therefore, in‐vivo measurements are recommended.

Evaluation of an innovative plastic cube phantom designed to improve the efficiency of accelerator QA

A new cubic phantom was designed to improve the efficiency on the QA measurement of accelerator. It has a variety of applications, such as dose constancy check, depth dose verifications, and symmetry and flatness evaluations. In particular, this new design makes it much easier to perform the check on output constancy vs. gantry angles as a cylindrical ion chamber positioned at the center of the phantom. The reproducibility of the setup using this phantom has been investigated. The charge effect of the phantom is found to be insignificant. It also reduces the monthly dosimetric QA time spent on a typical multimodality accelerator by approximately 40%. PACS number(s): 87.66.–a

Measurement of absorbed dose by 7-GeV bremsstrahlung in a PMMA phantom

High-energy electron storage rings generate energetic bremsstrahlung photons through radiative interaction of the particle beam with the residual gas molecules and other components inside the storage ring. At synchrotron radiation facilities, where beamlines are channeled out of the storage ring, a continuous bremsstrahlung spectrum, with a maximum energy of the stored particle beam, will be present. At the advanced photon source (APS), where the stored beam energy is 7 GeV, bremsstrahlung generated in the straight sections of the insertion device beamlines, which are a total of 15.38 m in length, can be significant. The contribution from each bremsstrahlung interaction adds up to produce a narrow mono-directional bremsstrahlung beam that comes down through the insertion device beamlines. The resulting absorbed dose distributions by this radiation in a 300 mm×300 mm×300 mm tissue substitute cube phantom were measured with LiF:Mg,Ti (TLD-700) thermoluminescent dosemeters. The normalized absorbed dose, in a cross-sectional area of 100 mm 2 at a depth of 150 mm of the PMMA phantom, was measured as 3.3×10 6 mGy h −1W −1 for 7-GeV bremsstrahlung spectrum.

Phantoms with 10BF3 detectors for boron neutron capture therapy applications.

Two acrylic cube phantoms have been constructed for BNCT applications that allow the depth distribution of neutrons to be measured with miniature 10BF3 detectors in 0.5-cm steps beginning at 1-cm depth. Sizes and weights of the cubes are 14 cm, 3.230 kg, and 11 cm, 1.567 kg. Tests were made with the epithermal neutron beam from the patient treatment port of the Brookhaven Medical Research Reactor. Thermal neutron depth profiles were measured with a bare 10BF3 detector at a reactor power of 50 W, and Cd-covered detector profiles were measured at a reactor power of 1 kW. The resulting plots of counting rate versus depth illustrate the dependence of neutron moderation on the size of the phantom. But more importantly the data can serve as benchmarks for testing the thermal and epithermal neutron profiles obtained with accelerator-based BNCT facilities. Such tests could be made with these phantoms at power levels about five orders of magnitude lower than that required for the treatment of patients with brain tumors.

Analysis and convergence of the iterative convolution/superposition dose reconstruction technique for multiple treatment beams and tomotherapy.

An iterative convolution/superposition (C/S) algorithm has been created to reconstruct dose distributions in patients from exit dose measurements during a radiotherapy treatment. The method is based on an extended phantom which includes the patient CT representation and an electronic portal imaging device (EPID). The patient CT is assumed to be a true and rigid representation of the patient at the time of treatment. The C/S method computes the dose throughout the extended phantom which allows the exit dose to be predicted in the EPID. The process is then reversed to take the exit dose measurement and infer what the dose distribution must have been to produce the measured exit dose. The dose distribution is modeled without knowledge of the incident intensity distribution, and includes the effects of scatter in the computation. The iterative method begins by assuming that the exit primary energy fluence (PEF) is equal to the exit dose, the PEF is then backprojected through the extended phantom and superposed with the dose deposition kernel to determine a new prediction of the exit dose. The ratio of the computed PEF to exit dose is then multiplied by the measured exit dose image to produce a better representation of the exit PEF. Successive iterations then converge to the exit PEF image that would produce the measured exit dose image. Once convergence is established, the dose distribution is determined by backprojecting the exit PEF followed by superposition with the dose deposition kernel. The method is used to reconstruct the dose from a stimulated dynamic wedge and verified with film. Convergence and termination of the algorithm is then investigated with no noise and in the presence of noise. The method is then expanded to handle multiple treatment beams by separating the representation of the EPID from the patient or phantom representation in the computation process. Investigation of the effects of noise during the process of iterative dose reconstruction is necessary to understand the capabilities of the algorithm using exit dose images that may contain significant amounts of noise. The capability of the algorithm is evaluated for multiple field treatments to a cube phantom and a prostate patient CT representation in the presence of noise. The method is then used to simulate the dose reconstruction process for tomotherapy using 72 intensity-modulated fan beams. Dose reconstruction is shown to be capable of verifying the dose distributions in patients including multiple beams and dynamic collimation, provided the patient CT is known at the time of treatment.

Improved determination of biplane imaging geometry from two projection images and its application to three‐dimensional reconstruction of coronary arterial trees

A technique has been developed for accurate estimation of three-dimensional (3D) biplane imaging geometry and reconstruction of 3D objects based on two perspective projections acquired at arbitrary orientations, without the need of calibration. The required prior information (i.e., the intrinsic parameters of each single-plane imaging system) for determination of biplane imaging geometry includes (a) the distance between each focal spot and its image plane, SID (the focal-spot to imaging-plane distance); (b) the pixel size, psize (e.g., 0.3 mm/pixel); (c) the distance between the two focal spots ff' or the known 3D distance between two points in the projection images; and (d) for each view, an approximation of the magnification factor, MF (e.g., 1.2), which is the ratio of the SID and the approximate distance of the object to the focal spot. Item (d) is optional but may provide a more accurate estimation if it is available. Given five or more corresponding object points in both views, a constrained nonlinear optimization algorithm is applied to obtain an optimal estimate of the biplane imaging geometry in the form of a rotation matrix R and a translation vector t that characterize the position and orientation of one imaging system relative to the other. With the calculated biplane imaging geometry, 3D spatial information concerning the object can then be reconstructed. The accuracy of this method was evaluated by using a computer-simulated coronary arterial tree and a cube phantom object. Our simulation study showed that a computer-simulated coronary tree can be reconstructed from two views with less than 2 and 8.4 mm root-mean-square (rms) configuration (or relative-position) error and absolute-position error, respectively, even if the input errors in the corresponding 2D points are fairly large (more than two pixels = 0.6 mm). In contrast, input image error of more than one pixel (= 0.3 mm) can yield 3D position errors of 10 cm or more when other existing methods based on linear approaches are employed. For the cube phantom images acquired from a routine biplane system, rms errors in the 3D configuration of the cube and the 3D absolute position were 0.6-0.9 mm and 3.9-5.0 mm, respectively.

Determination of three-dimensional positions of known sparse objects from a single projection.

A new technique is developed for accurate determination of the three-dimensional position and orientation of known sparse objects, e.g., a configuration of points, from a single-perspective projection. In this technique, a computer model of the known object is translated and rotated so as to align it optimally in a least-squares sense with the projection lines connecting the image points with the focal spot by using a modification of the projection-Procrustes technique. The translational and rotational adjustments are repeated iteratively until the angular change between iterations is less than 0.25 degree. Simulations indicate that, for rms input image errors of 0.03 cm, the three-dimensional positions and orientations can be determined to within approximately 0.2 cm and 0.3 degree for a wide range of initially guessed positions and orientations, and positions can be determined with an accuracy of approximately 0.3 cm for objects having as few as four points. In phantom experiments, three-dimensional positions and orientations of a cube phantom were reproducibly determined to within 0.23 cm and 0.13 degree. The entire calculation requires only 10 s on a VAX 3500 to converge to the solution. The accuracy, precision, and speed of the technique indicate that it will be a useful tool for determination of three-dimensional positions and orientations of known sparse objects.

Coefficients for the Conversion of Air Collision Kerma to Dose Equivalent for the Calibration of Individual Dosemeters in X Ray Fields

For the purpose of calibrating individual dosemeters in X ray fields when using slab or cube phantoms, the coefficients used to convert air collision kerma to dose equivalent at various depths of a water cube (side length 30 cm), a PMMA slab (30 cm x 30 cm front face area, 15 cm thick) and a tissue-equivalent slab of the same dimensions have been numerically determined for most of the radiation qualities of X rays recommended by the ISO. The new results are based on Monte Carlo data. the X ray beam was assumed to be parallel and extended. Besides the conversion coefficients to be used in the case of irradiation with an X ray beam of normal incidence on the phantom's front face, the angular dependence factor for photon beams of oblique incidence was also studied.

Phantom for Calibrating and Type-Testing Individual Dosemeters

The suitability of two polyhedron phantoms for type-testing and calibrating individual dosemeters in connection with the introduction of the operational quantities H p (d) and H s (d) in individual monitoring is discussed. It is concluded that an icosahedron phantom with an edge length of 18.64 cm is preferable. It combines the advantages of a sphere phantom with those of a cube or slab phantom and allows a number of angles of radiation incidence to be used for a fixed radiation field. The expense of producing an icosahedron phantom is comparable with that of producing a sphere phantom