bluuebunny/crossref_metadata_2025 huggingface dataset:
Dose response characteristics of new models of GAFCHROMIC films: Dependence on densitometer light source and radiation energy
Authors: Chiu‐Tsao Sou‐Tung , Duckworth Tamara , Zhang Chuanfang , Patel Neil S. , Hsiung Chih‐Yun , Wang Lin , Shih J. Allen , Harrison Louis B.
Published: September 2004

This paper presents a systematic study of the dose response characteristics of two new models and one commonly used model of GAFCHROMIC film: HS, XR‐T, and MD55‐2, respectively. We irradiated these film models with three different radiation sources: I‐125, Ir‐192, and 6 MV photon beam (6 MVX). We scanned the films with three different densitometers: a He–Ne laser with a wavelength of 633 nm, a spot densitometer with a wavelength of 671 nm, and a CCD camera densitometer with interchangeable LED boxes with wavelengths of 665 nm (red), 520 nm (green), and 465 nm (blue). We compared the film sensitivities in terms of net optical density (NOD) per unit dose in Gy. The sensitivity of each film model depends on radiation energy and the densitometer light source. Using He–Ne laser based densitometer as a reference standard, we found the sensitivities (NOD/Gy) for the red lights of wavelengths, 671 nm and 665 nm, are higher by factors of about 2.5 and 2, respectively. The sensitivities for green (520 nm) and blue (465 nm) lights are lower than that for He–Ne laser (633 nm) by factors of about 2 and 4, respectively. The energy dependence of the sensitivity varies with the film model, but is similar for all densitometer light sources. Comparing I‐125 to Ir‐192 and 6MVX, we note that (a) model XR‐T is about eight times more sensitive, and (b) models HS and MD55‐2 are about 40% less sensitive. Relative to MD55‐2, XR‐T is 12 times more sensitive for I‐125 but comparable for Ir‐192 and 6MVX, whereas HS is 2 to 3 times more sensitive in all cases. This set of results can serve as useful information for making decisions in selecting the film model and compatible densitometer to achieve the best accuracy of dosimetry in the appropriate dose range.
An EGSnrc Monte Carlo study of the microionization chamber for reference dosimetry of narrow irregular IMRT beamlets
Authors: Capote Roberto , Sánchez‐Doblado Francisco , Leal Antonio , Lagares Juan Ignacio , Arráns Rafael , Hartmann Günther H.
Published: September 2004

Intensity modulated radiation therapy (IMRT) has evolved toward the use of many small radiation fields, or “beamlets,” to increase the resolution of the intensity map. The size of smaller beamlets can be typically about 1–5 cm2. Therefore small ionization chambers (IC) with sensitive volumes ⩽0.1 cm3are generally used for dose verification of IMRT treatment. The dosimetry of these narrow photon beams pertains to the so‐called nonreference conditions for beam calibration. The use of ion chambers for such narrow beams remains questionable due to the lack of electron equilibrium in most of the field. The present contribution aims to estimate, by the Monte Carlo (MC) method, the total correction needed to convert the IBA‐Wellhöfer NAC007 micro IC measured charge in such radiation field to the absolute dose to water. Detailed geometrical simulation of the microionization chamber was performed. The ion chamber was always positioned at a 10 cm depth in water, parallel to the beam axis. The delivered doses to air and water cavity were calculated using the CAVRZ EGSnrc user code. The 6 MV phase‐spaces for Primus Clinac (Siemens) used as an input to the CAVRZnrc code were derived by BEAM/EGS4 modeling of the treatment head of the machine along with the multileaf collimator [Sánchez‐Doblado et al., Phys. Med. Biol.48, 2081–2099 (2003)] and contrasted with experimental measurements. Dose calculations were carried out for two irradiation geometries, namely, the reference 10×10 cm2field and an irregular (∼2×2 cm2) IMRT beamlet. The dose measured by the ion chamber is estimated by MC simulation as a dose averaged over the air cavity inside the ion‐chamberThe absorbed dose to water is derived as the dose deposited inside the same volume, in the same geometrical position, filled and surrounded by waterin the absence of the ionization chamber. Therefore, thedose ratio is a MC direct estimation of the total correction factor needed to convert the absorbed dose in air to absorbed dose to water. The dose ratio was calculated for several chamber positions, starting from the penumbra region around the beamlet along the two diagonals crossing the radiation field. For this quantity from 0 up to a 3% difference is observed between the dose ratio values obtained within the small irregular IMRT beamlet in comparison with the dose ratio derived for the reference 10×10 cm2field. Greater differences from the reference value up to 9% were obtained in the penumbra region of the small IMRT beamlet.
Radial gradient‐based segmentation of mammographic microcalcifications: Observer evaluation and effect on CAD performance
Authors: Paquerault Sophie , Yarusso Laura M. , Papaioannou John , Jiang Yulei , Nishikawa Robert M.
Published: September 2004

Precise segmentation of microcalcifications is essential in the development of accurate mammographic computer‐aided diagnosis (CAD) schemes. We have designed a radial gradient‐based segmentation method for microcalcifications, and compared it to both the region‐growing segmentation method currently used in our CAD scheme and to the watershed segmentation method. Two observer studies were conducted to subjectively evaluate the proposed segmentation method. The first study (A) required observers to rate the segmentation accuracy on a 100‐point scale. The second observer evaluation (B) was a preference study in which observers selected their preferred method from three displayed segmentation methods. In study A, the observers gave an average accuracy rating of 88 for the radial gradient‐based and 50 for the region‐growing segmentation method. In study B, the two observers selected the proposed method 56% and 62% of the time. We also investigated the effect of the proposed segmentation method on the performance of computerized classification scheme in differentiating malignant from benign clustered microcalcifications. The performances of the classification scheme using a linear discriminant analysis (LDA) or a Bayesian artificial neural network classifier both showed statistically significant improvements when using the proposed segmentation method. The areas under the receiver‐operating characteristic curves for case‐based performance when using the LDA classifier were 0.86 with the proposed segmentation method, 0.80 with the region‐growing method, and 0.83 with the watershed method.
The phantom portion of the American College of Radiology (ACR) Computed Tomography (CT) accreditation program: Practical tips, artifact examples, and pitfalls to avoid
Authors: McCollough Cynthia H. , Bruesewitz Michael R. , McNitt‐Gray Michael F. , Bush Krista , Ruckdeschel Thomas , Payne J. Thomas , Brink James A. , Zeman Robert K.
Published: September 2004

The ACR CT accreditation program, begun in 2002, requires the submission of approximately 20 images, several completed data sheets and printouts of three Excel worksheets. The procedure manual is very detailed, yet participants unfamiliar with the program or having minimal CT experience have needed to redo aspects of their submission, or in some cases do not receive accreditation, due to mistakes made by the physicist. This review of the phantom portion of the ACR CT accreditation program supplements the ACR provided instructions with additional photos of phantom setup, region‐of‐interest (ROI), and image placement on the film sheets, and examples of completed portions of actual (but anonymous) submissions. Common mistakes, as well as uncommon but interesting images, are shown and explanations are given as to what could have been done to avoid the problem. Additionally, a review of CT dose measurement techniques and calculations will enable the physicist to better assist sites where typical exam doses are above the ACR reference values.
Evaluation of the goodness of fit of solar simulated radiation to a reference solar spectrum for photobiological experiments
Authors: Schauberger Günther , Maier Harald , Cabaj Alexander , Schmalwieser Alois W.
Published: September 2004

There is a great need for photochemical and photobiological experiments to mimic the spectral distribution of solar ultraviolet radiation by artificial ultraviolet sources. The spectral distribution of various ultraviolet sources were compared with a reference solar spectrum which represents a realistic maximum solar spectrum under cloudless sky. Various methods to compare the artificial sources with the COLIPA solar reference spectrum were presented. Beside a graphical method which is based on the Lorenz curve also integral indices were used to characterize the deviations from the solar spectrum. Following parameters should be used to characterize an artificial source: (1) total output of the biologically effective irradiance, (2) maximum deviation of the cumulative relative spectral irradiance of the biologically effective solar simulated radiation, (3) spectral distribution of the difference of the cumulative relative spectral irradiance of the biologically effective solar simulated radiation, and (4) the index of the goodness of the spectral fit. Further on upper and lower limit values are suggested to improve the quality of the spectral fit of solar simulated radiation and the irradiance maximum should be limited to the range in which dose reciprocity is known to be conserved.
Technical note: A novel boundary condition using contact elements for finite element based deformable image registration
Authors: Zhang Tiezhi , Orton Nigel P. , Mackie T. Rockwell , Paliwal Bhudatt R.
Published: September 2004

Deformable image registration is an important tool for image‐guided radiotherapy. Physics‐model‐based deformable image registration using finite element analysis is one of the methods currently being investigated. The calculation accuracy of finite element analysis is dependent on given boundary conditions, which are usually based on the surface matching of the organ in two images. Such a surface matching, however, is hard to obtain from medical images. In this study, we developed a new boundary condition to circumvent the traditional difficulties. Finite element contact‐impact analysis was employed to simulate the interaction between the organ of interest and the surrounding body. The displacement loading is not necessarily specified. The algorithm automatically deforms the organ model into the minimum internal energy state. The analysis was performed on CT images of the lung at two different breathing phases (exhalation and full inhalation). The result gave the displacement vector map inside the lung. Validation of the result showed satisfactory agreement in most parts of the lung. This approach is simple, operator independent and may provide improved accuracy of the prediction of organ deformation.
Asymmetric fan beams (AFB) for improvement of the craniocaudal dose distribution in helical tomotherapy delivery
Authors: Gladwish Adam , Kron Tomas , McNiven Andrea , Bauman Glenn , Van Dyk Jake
Published: September 2004

Helical tomotherapy (HT) is a novel radiotherapy technique that utilizes intensity modulated fan beams that deliver highly conformal dose distributions in a helical beam trajectory. The most significant limitation in dose delivery with a constant fan beam thickness (FBT) is the penumbra width of the dose distribution in the craniocaudal direction, which is equivalent to the FBT. We propose to employ a half‐blocked fan beam at start and stop location to reduce the penumbra width by half. By opening the jaw slowly during the helical delivery until the desired FBT is achieved it is possible to create a sharper edge in the superior and inferior direction from the target. The technique was studied using a tomotherapy beam model implemented on a commercial treatment planning system (Theraplan Plus V3.0). It was demonstrated that the dose distribution delivered using a 25 mm fan beam can be improved significantly, to reduce the dose to normal structures located superiorly and inferiorly of the target. Dosimetry for this technique is straightforward down to a FBT of 15 mm and implementation should be simple as no changes in couch movement are required compared to a standard HT delivery. We conclude that the use of asymmetric collimated fan beams for the start and stop of the helical tomotherapeutic dose delivery has the potential of significantly improving the dose distribution in helical tomotherapy.
A new theory of phase‐contrast x‐ray imaging based on Wigner distributions
Authors: Wu Xizeng , Liu Hong
Published: September 2004

There is a pressing need for a comprehensive theory for phase‐contrast x‐ray imaging to guide its development and clinical applications. This work presents such a theory as the foundation for deriving these guidelines. The new theory is based on the Wigner‐distributions for the parabolic wave equations, and it is more general than the present theories based on the Fresnel–Kirchhoff diffraction theory. The new theory shows for the first time how the complex degree of coherence (CDC) of the incident x‐ray beam determines the phase‐contrast visibility in general, and how the reduced complex degree of coherence (RCDC) for an anode‐source is equal to the system's optical transfer function for geometric unsharpness in particular. The role of detector resolution in phase visibility has been clarified as well. Computer simulations based on the new theory were conducted and optimal design parameters were derived for phase‐contrast mammography systems.
A novel reconstruction algorithm to extend the CT scan field‐of‐view
Authors: Hsieh J. , Chao E. , Thibault J. , Grekowicz B. , Horst A. , McOlash S. , Myers T. J.
Published: September 2004

For various reasons, a projection dataset acquired on a computed tomography (CT) scanner can be truncated. That is, a portion of the scanned object is positioned outside the scan field‐of‐view (SFOV) and the line integrals corresponding to those regions are not measured. A projection truncation problem causes imaging artifacts that lead to suboptimal image quality. In this paper, we propose a reconstruction algorithm that enables an adequate estimation of the projection outside the SFOV. We make use of the fact that the total attenuation of each ideal projection in a parallel sampling geometry remains constant over views. We use the magnitudes and slopes of the projection samples at the location of truncation to estimate water cylinders that can best fit to the projection data outside the SFOV. To improve the robustness of the algorithm, continuity constraints are placed on the fitting parameters. Extensive phantom and patient experiments were conducted to test the robustness and accuracy of the proposed algorithm.
An ROC comparison of four methods of combining information from multiple images of the same patient
Authors: Liu Bei , Metz Charles E. , Jiang Yulei
Published: September 2004

Variance of diagnostic information contained in an image degrades diagnostic accuracy. Acquiring multiple images of the same patient (e.g., mediolateral oblique and craniocaudal view mammograms) can, in principle, help reduce this degradation. We demonstrate how this can be accomplished in the context of computer‐aided diagnosis (CAD). Assuming that computer outputs obtained from multiple images of the same patient can be transformed monotonically to the same pair of truth‐conditional normal distributions and, for simplicity, ignoring correlation among images, we investigate theoretically four methods of combining the computer outputs: taking the average, the median, the maximum, or the minimum. We found, as one would expect, that both the average and the median always produce an improved area under the receiver operating characteristic (ROC) curve (AUC) compared to the single‐view images, while the average always produces better performance than the median. However, the maximum and minimum also can produce improved AUCs in some situations, and under certain conditions can outperform the average. Surprisingly, we found that the maximum and minimum of normally‐distributed decision variables produce nearly binormal ROC curves. These results can be used as a guide in attempting to increase the efficacy of CAD when multiple images are available from the same patient.
Dosimetric properties of improved GafChromic films for seven different digitizers
Authors: Devic Slobodan , Seuntjens Jan , Hegyi Gyorgy , Podgorsak Ervin B. , Soares Christopher G. , Kirov Assen S. , Ali Imad , Williamson Jeffrey F. , Elizondo Angel
Published: September 2004

Two recently introduced GafChromic film models, HS and XR‐T, have been developed as more sensitive and uniform alternatives to GafChromic MD‐55‐2 film. The HS model has been specifically designed for measurement of absorbed dose in high‐energy photon beams (above 1 MeV), while the XR‐T model has been introduced for dose measurements of low energy (0.1 MeV) photons. The goal of this study is to compare the sensitometric curves and estimated dosimetric uncertainties associated with seven different GafChromic film dosimetry systems for the two new film models. The densitometers tested are: LKB Pharmacia UltroScan XL, Molecular Dynamics Personal Densitometer, Nuclear Associates Radiochromic Densitometer Model 37‐443, Photoelectron Corporation CMR‐604, Laser Pro 16, Vidar VXR‐16, and AGFA Arcus II document scanner. Pieces of film were exposed to different doses in a dose range from 0.5 to 50 Gy using 6 MV photon beam. Functional forms for dose vs net optical density have been determined for each of the GafChromic film‐dosimetry systems used in this comparison. Two sources of uncertainties in dose measurements, governed by the experimental measurement and calibration curve fit procedure, have been compared for the densitometers used. Among the densitometers tested, it is found that for the HS film type the uncertainty caused by the experimental measurement varies from 1% to 3% while the calibration fit uncertainty ranges from 2% to 4% for doses above 5 Gy. Corresponding uncertainties for XR‐T film model are somewhat higher and range from 1% to 5% for experimental and from 2% to 7% for the fit uncertainty estimates. Notwithstanding the significant variations in sensitivity, the studied densitometers exhibit very similar precision for GafChromic film based dose measurements above 5 Gy.
An implantable radiation dosimeter for use in external beam radiation therapy
Authors: Scarantino Charles W. , Ruslander David M. , Rini Christopher J. , Mann Gregory G. , Nagle H. Troy , Black Robert D.
Published: September 2004

An implantable radiation dosimeter for use with external beam therapy has been developed and tested both in vitro and in canines. The device uses a MOSFET dosimeter and is polled telemetrically every day during the course of therapy. The device is designed for permanent implantation and also acts as a radiographic fiducial marker. Ten dogs (companion animals) that presented with spontaneous, malignant tumors were enrolled in the study and received an implant in the tumor CTV. Three dogs received an additional implant in collateral normal tissue. Radiation therapy plans were created for the animals and they were treated with roughly 300 cGy daily fractions until completion of the prescribed cumulative dose. The primary endpoints of the study were to record any adverse events due to sensor placement and to monitor any movement away from the point of placement. No adverse events were recorded. Unacceptable device migration was experienced in two subjects and a retention mechanism was developed to prevent movement in the future. Daily dose readings were successfully acquired in all subjects. A rigorous in vitro calibration methodology has been developed to ensure that the implanted devices maintain an accuracy of ±3.5% relative to an ionization chamber standard. The authors believe that an implantable radiation dosimeter is a practical and powerful tool that fosters individualized patient QA on a daily basis.
Metallic copper as a fiducial marker for both CT and PET
Authors: Robinson Don , Gagne Isabelle , Riauka Terence , Duke John , Roa Wilson
Published: September 2004

There is great interest in augmenting computed tomography (CT) with information gained from other imaging modalities. Positron emission tomography (PET) provides valuable data related to patient physiology to aid in the delineation of tumor volumes. Combining the information provided by these imaging modalities requires accurate spatial registration of the two data sets. Fiducial based mapping provides straightforward registration based on corresponding landmark points or fiducials in the two image sets. When external fiducials are employed, consistent intermodality marker placement and centroid identification are essential to achieving an accurate and reliable registration. Similarity of marker design between modalities greatly aides in achieving this goal. Solid copper may serve as a fiducial marker in both CT and PET. Small spheres or wires of copper are readily visible in CT while neutron activation of these same markers produces positron emitting Copper‐64 for detection by PET. The use of identical shaped markers in both imaging modalities greatly simplifies the task of intermodality centroid matching. Copper has excellent machining properties and, prior to activation, is easy and safe to handle. The feasibility of Cu as a marker for both CT and PET is demonstrated using imaging phantoms.
Development of a 30‐week‐pregnant female tomographic model from computed tomography (CT) images for Monte Carlo organ dose calculations
Authors: Shi Chengyu , Xu X. George
Published: September 2004

Assessment of radiation dose and risk to a pregnant woman and her fetus is an important task in radiation protection. Although tomographic models for male and female patients of different ages have been developed using medical images, such models for pregnant women had not been developed to date. This paper reports the construction of a partial‐body model of a pregnant woman from a set of computed tomography (CT) images. The patient was 30 weeks into pregnancy, and the CT scan covered the portion of the body from above liver to below pubic symphysis in 70 slices. The thickness for each slice is 7 mm, and the image resolution is in a field; thus, the voxel size is The images were segmented to identify 34 major internal organs and tissues considered sensitive to radiation. Even though the masses are noticeably different from other models, the three‐dimensional visualization verified the segmentation and its suitability for Monte Carlo calculations. The model has been implemented into a Monte Carlo code, EGS4‐VLSI (very large segmented images), for the calculations of radiation dose to a pregnant woman. The specific absorbed fraction (SAF) results for internal photons were compared with those from a stylized model. Small and large differences were found, and the differences can be explained by mass differences and by the relative geometry differences between the source and the target organs. The research provides the radiation dosimetry community with the first voxelized tomographic model of a pregnant woman, opening the door to future dosimetry studies.
An automatic seed finder for brachytherapy CT postplans based on the Hough transform
Authors: Holupka E. J. , Meskell P. M. , Burdette E. C. , Kaplan I. D.
Published: September 2004

Purpose: The purpose of the work is to describe a new algorithm for the automatic detection of implanted radioactive seeds within the prostate. The algorithm is based on the traditional Hough transform. A method of quality assurance is described as well as a quantitative phantom study to determine the accuracy of the algorithm. Methods and Materials: An algorithm is described which is based on the Hough transform. The Hough transform is a well known transform traditionally used to automatically segment lines and other well defined geometric objects from images. The traditional Hough transform is extended to three‐dimensions and applied to CT images of seed implanted prostate glands. A method based on digitally reconstructed radiographs is described to quality assure the determined three‐dimensional positions of the detected seeds. Two phantom studies utilizing eight seeds and nine seeds are described. All eight seeds form a contiguous a square while the nine seed phantom describes seeds which are placed side‐by‐side in groups of two and three. The algorithm is applied to the CT scans of both phantoms and the seed positions determined. Results: The algorithm has been commercially developed and used to perform postsurgical dosimetric assessment on approximately 1000 patients. Using the described quality assurance tool it was determined that the algorithm accurately determined the seed positions in all 1000 patients. The algorithm was also applied to the eight seed phantom. The algorithm successfully found all eight seeds as well as their seed coordinates. The average radial error was determined to be 0.9 mm. For the nine seed phantom, the algorithm correctly identified all nine seeds, with an average radial error of 3 mm. Conclusions: The described algorithm is a robust, accurate, automatic, three‐dimensional application for CT based seed determination.
How should low‐contrast detail detectability be measured in fluoroscopy?
Authors: Tapiovaara Markku J. , Sandborg Michael
Published: September 2004

The relationship and precision of four methods for measuring the low‐contrast detail detectability in fluoroscopic imaging were studied. These included the physical measurement of the accumulation rate of the square of the signal‐to‐noise ratio two‐alternative forced‐choice (2‐AFC) experiments, sixteen‐alternative forced‐choice (16‐AFC) experiments and subjective determination of the threshold contrast. The precision and sensitivity of the threshold contrast measurement were seen to be modest in the constancy testing of fluoroscopic equipment: only large changes in system performance could be reliably detected by that method. The measurement of the is suggested instead. The relationship between the results of the various methods were studied, and it was found that human performance can be related to by introducing the concept of the effective image information integration time When measured for an unlimited observation time, it depicts the saturation of human performance in detecting a static low‐contrast detail in dynamic image noise. Here, was found to be about 0.6 s in 2‐AFC tests and 0.3 s in 16‐AFC tests.
Using FDG‐PET activity as a surrogate for tumor cell density and its effect on equivalent uniform dose calculation
Authors: Zhou Su‐Min , Wong Terence Z. , Marks Lawrence B.
Published: September 2004

The concept of equivalent uniform dose (EUD) has been suggested as a means to quantitatively consider heterogeneous dose distributions within targets. Tumor cell density/function is typically assumed to be uniform. We herein propose to use ‐labeled 2‐deoxyglucose (FDG) positron emission tomography (PET) tumor imaging activity as a surrogate marker for tumor cell density to allow the EUD concept to include intratumor heterogeneities and to study its effect on EUD calculation. Thirty‐one patients with lung cancer who had computerized tomography (CT)‐based 3D planning and PET imaging were studied. Treatment beams were designed based on the information from both the CT and PET scans. Doses were calculated in 3D based on CT images to reflect tissue heterogeneity. The EUD was calculated in two different ways: first, assuming a uniform tumor cell density within the tumor target; second, using FDG‐PET activity as a surrogate for tumor cell density at different parts of tumor to calculate the functional‐imaging‐weighted EUD (therefore will be labeled fEUD for convenience). The EUD calculation can be easily incorporated into the treatment planning process. For 28/31 patients, their fEUD and EUD differed by less than 6%. Twenty‐one of these twenty‐eight patients had tumor volumes In the three patients with larger tumor volume, the fEUD and EUD differed by 8%–14%. Incorporating information from PET imaging to represent tumor cell density in the EUD calculation is straightforward. This approach provides the opportunity to include heterogeneity in tumor function/metabolism into the EUD calculation. The difference between fEUD and EUD, i.e., whether including or not including the possible tumor cell density heterogeneity within tumor can be significant with large tumor volumes. Further research is needed to assess the usefulness of the fEUD concept in radiation treatment.
Automatic measurements of image geometry for online calibration of C‐arm fluoroscopy
Authors: Archip Neculai , Rohling Robert
Published: September 2004

Calibrated C‐arm fluoroscopy is used for a variety of surgical procedures where surgical tools and anatomical structures need to be located in space. Calibration can be performed online by combining measurements of the geometry of the image (center and radius) with measurements of a grid of fiducial markers. This article focuses on the first aspect—image geometry—and describes a method to perform the geometry measurements automatically. The accuracy, robustness, and speed of the method are validated on 100 images obtained from several hospitals with different C‐arm scanners. All 100 images were successfully measured with an average error of 0.8 mm for the center and 0.8 mm for the radius. The execution time is less than one second per image.
Radiation absorbed dose distribution in a patient treated with yttrium‐90 microspheres for hepatocellular carcinoma
Authors: Sarfaraz Mehrdad , Kennedy Andrew S. , Lodge Martin A. , Li X. Allen , Wu Xingen , Yu Cedric X.
Published: September 2004

We have implemented a three‐dimensional dose calculation technique accounting for dose inhomogeneity within the liver and tumor of a patient treated withmicrospheres. Single‐photon emission computed tomography (SPECT) images were used to derive the activity distribution within liver. A Monte Carlo calculation was performed to create a voxel dose kernel for thesource. The activity distribution was convolved with the voxel dose kernel to obtain the three‐dimensional (3D) radiation absorbed dose distribution. An automated technique was developed to accurately register the computed tomography (CT) and SPECT scans in order to display the 3D dose distribution on the CT scans. In addition, dose‐volume histograms were generated to fully analyze the tumor and liver doses. The calculated dose‐volume histogram indicated that although the patient was treated to the nominal whole liver dose of 110 Gy, only 16% of the liver and 83% of the tumor received a dose higher than 110 Gy. The mean tumor and liver doses were 163 and 58 Gy, respectively.
Ionization chamber‐based reference dosimetry of intensity modulated radiation beams
Authors: Bouchard Hugo , Seuntjens Jan
Published: September 2004

The present paper addresses reference dose measurements using thimble ionization chambers for quality assurance in IMRT fields. In these radiation fields, detector fluence perturbation effects invalidate the application of open‐field dosimetry protocol data for the derivation of absorbed dose to water from ionization chamber measurements. We define a correction factor to correct the absorbed dose to water calibration coefficient for fluence perturbation effects in individual segments of an IMRT delivery and developed a calculation method to evaluate the factor. The method consists of precalculating, using accurate Monte Carlo techniques, ionization chamber, type‐dependent cavity air dose, and in‐phantom dose to water at the reference point for zero‐width pencil beams as a function of position of the pencil beams impinging on the phantom surface. These precalculated kernels are convolved with the IMRT fluence distribution to arrive at the dose‐to‐water‐dose‐to‐cavity air ratio for IMRT fields and with a 10×10 cm2 open‐field fluence to arrive at the same ratio for the 10×10 cm2 reference field. The correction factor is then calculated as the ratio of and The calculation method was experimentally validated and the magnitude of chamber correction factors in reference dose measurements in single static and dynamic IMRT fields was studied. The results show that, for thimble‐type ionization chambers the correction factor in a single, realistic dynamic IMRT field can be of the order of 10% or more. We therefore propose that for accurate reference dosimetry of complete n‐beam IMRT deliveries, ionization chamber fluence perturbation correction factors must explicitly be taken into account.
Brachytherapy dosimetry parameters calculated for a new source
Authors: Rivard Mark J. , Melhus Christopher S. , Kirk Bernadette L.
Published: September 2004

A new brachytherapy source having adsorbed onto silver beads has been designed. The dose distributions of this source have been characterized using version 5 of the MCNP Monte Carlo radiation transport code available from Oak Ridge National Laboratory. These results are presented in terms of the updated AAPM Task Group No. 43 (TG‐43U1) formalism, dosimetry parameters, and recommended calculation methodology.
Radiation dose estimate in small animal SPECT and PET
Authors: Funk Tobias , Sun Mingshan , Hasegawa Bruce H.
Published: September 2004

Calculations of radiation dose are important in assessing the medical and biological implications of ionizing radiation in medical imaging techniques such as SPECT and PET. In contrast, radiation dose estimates of SPECT and PET imaging of small animals are not very well established. For that reason we have estimated the whole‐body radiation dose to mice and rats for isotopes such as and that are used commonly for small animal imaging. We have approximated mouse and rat bodies with uniform soft tissue equivalent ellipsoids. The mouse and rat sized ellipsoids had a mass of 30 g and 300 g, respectively, and a ratio of the principal axes of 1:1:4 and 0.7:1:4. The absorbed fractions for various photon energies have been calculated using the Monte Carlo software package MCNP. Using these values, we then calculated MIRD S‐values for two geometries that model the distribution of activity in the animal body: (a) a central point source and (b) a homogeneously distributed source, and compared these values against S‐value calculations for small ellipsoids tabulated in MIRD Pamphlet 8 to validate our results. Finally we calculated the radiation dose taking into account the biological half‐life of the radiopharmaceuticals and the amount of activity administered. Our calculations produced S‐values between and for SPECT agents, and for the PET agent assuming mouse sized ellipsoids with uniform source distribution. The S‐values for a central point source in an ellipsoid are about 10% higher than the values obtained for the uniform source distribution. Furthermore, the S‐values for mouse sized ellipsoids are approximately 10 times higher than for the rat sized ellipsoids reflecting the difference in mass. We reviewed published data to obtain administered radioactivity and residence times for small animal imaging. From these values and our computed S‐values we estimated that the whole body dose in small animals ranges between 6 cGy and 90 cGy for mice and between about 1 cGy and 27 cGy for rats. The whole body dose in small animal imaging can be very high in comparison to the lethal dose to mice For this reason the dose in small animal imaging should be monitored carefully and the administered activity should be kept to a minimum. These results also underscore the need of further development of instrumentation that improves detection efficiency and reduces radiation dose in small animal imaging.
Automatic detection and classification of hypodense hepatic lesions on contrast‐enhanced venous‐phase CT
Authors: Bilello Michel , Gokturk Salih Burak , Desser Terry , Napel Sandy , Jeffrey R. Brooke , Beaulieu Christopher F.
Published: September 2004

The objective of this work was to develop and validate algorithms for detection and classification of hypodense hepatic lesions, specifically cysts, hemangiomas, and metastases from CT scans in the portal venous phase of enhancement. Fifty‐six CT sections from 51 patients were used as representative of common hypodense liver lesions, including 22 simple cysts, 11 hemangiomas, 22 metastases, and 1 image containing both a cyst and a hemangioma. The detection algorithm uses intensity‐based histogram methods to find central lesions, followed by liver contour refinement to identify peripheral lesions. The classification algorithm operates on the focal lesions identified during detection, and includes shape‐based segmentation, edge pixel weighting, and lesion texture filtering. Support vector machines are then used to perform a pair‐wise lesion classification. For the detection algorithm, 80% lesion sensitivity was achieved at approximately 0.3 false positives (FP) per slice for central lesions, and 0.5 FP per slice for peripheral lesions, giving a total of 0.8 FP per section. For 90% sensitivity, the total number of FP rises to about 2.2 per section. The pair‐wise classification yielded good discrimination between cysts and metastases (at 95% sensitivity for detection of metastases, only about 5% of cysts are incorrectly classified as metastases), perfect discrimination between hemangiomas and cysts, and was least accurate in discriminating between hemangiomas and metastases (at 90% sensitivity for detection of hemangiomas, about 28% of metastases were incorrectly classified as hemangiomas). Initial implementations of our algorithms are promising for automating liver lesion detection and classification.
X‐ray imaging performance of structured cesium iodide scintillators
Authors: Zhao Wei , Ristic Goran , Rowlands J. A.
Published: September 2004

Columnar structured cesium iodide (CsI) scintillators doped with Thallium (Tl) have been used extensively for indirect x‐ray imaging detectors. The purpose of this paper is to develop a methodology for systematic investigation of the inherent imaging performance of CsI as a function of thickness and design type. The results will facilitate the optimization of CsI layer design for different x‐ray imaging applications, and allow validation of physical models developed for the light channeling process in columnar CsI layers. CsI samples of different types and thicknesses were obtained from the same manufacturer. They were optimized either for light output (HL) or image resolution (HR), and the thickness ranged between 150 and 600 microns. During experimental measurements, the CsI samples were placed in direct contact with a high resolution CMOS optical sensor with a pixel pitch of 48 microns. The modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) of the detector with different CsI configurations were measured experimentally. The aperture function of the CMOS sensor was determined separately in order to estimate the MTF of CsI alone. We also measured the pulse height distribution of the light output from both the HL and HR CsI at different x‐ray energies, from which the x‐ray quantum efficiency, Swank factor and x‐ray conversion gain were determined. Our results showed that the MTF at 5 cycles/mm for the HR type was 50% higher than for the HL. However, the HR layer produces ∼36% less light output. The Swank factor below K‐edge was 0.91 and 0.93 for the HR and HL types, respectively, thus their DQE(0) were essentially identical. The presampling MTF decreased as a function of thicknessL. The universal MTF, i.e., MTF plotted as a function of the product of spatial frequencyfand CsI thicknessL, increased as a function ofL. This indicates that the light channeling process in CsI improved the MTF of thicker layers more significantly than for the thinner ones.
Fundamental imaging characteristics of a slot‐scan digital chest radiographic system
Authors: Samei Ehsan , Saunders Robert S. , Lo Joseph Y. , Dobbins James T. , Jesneck Jonathan L. , Floyd Carey E. , Ravin Carl E.
Published: September 2004

Our purpose in this study was to evaluate the fundamental image quality characteristics of a new slot‐scan digital chest radiography system (ThoraScan, Delft Imaging Systems/Nucletron, Veenendaal, The Netherlands). The linearity of the system was measured over a wide exposure range at 90, 117, and 140 kVp with added Al filtration. System uniformity and reproducibility were established with an analysis of images from repeated exposures. The modulation transfer function (MTF) was evaluated using an established edge method. The noise power spectrum (NPS) and the detective quantum efficiency (DQE) of the system were evaluated at the three kilo‐voltages over a range of exposures. Scatter fraction (SF) measurements were made using a posterior beam stop method and a geometrical chest phantom. The system demonstrated excellent linearity, but some structured nonuniformities. The 0.1 MTF values occurred between The DQE(0.15) and DQE(2.5) were 0.21 and 0.07 at 90 kVp, 0.18 and 0.05 at 117 kVp, and 0.16 and 0.03 at 140 kVp, respectively. The system exhibited remarkably lower SFs compared to conventional full‐field systems with anti‐scatter grid, measuring 0.13 in the lungs and 0.43 in the mediastinum. The findings indicated that the slot‐scan design provides marked scatter reduction leading to high effective DQE of the system and reduced patient dose required to achieve high image quality.
Relationship between the generalized equivalent uniform dose formulation and the Poisson statistics‐based tumor control probability model
Authors: Zhou Su‐Min , Das Shiva , Wang Zhiheng , Marks Lawrence B.
Published: September 2004

The generalized equivalent uniform dose (GEUD) model uses a power‐law formalism, where the outcome is related to the dose via a power law. We herein investigate the mathematical compatibility between this GEUD model and the Poisson statistics based tumor control probability (TCP) model. The GEUD and TCP formulations are combined and subjected to a compatibility constraint equation. This compatibility constraint equates tumor control probability from the original heterogeneous target dose distribution to that from the homogeneous dose from the GEUD formalism. It is shown that this constraint equation possesses a unique, analytical closed‐form solution which relates radiation dose to the tumor cell survival fraction. It is further demonstrated that, when there is no positive threshold or finite critical dose in the tumor response to radiation, this relationship is not bounded within the realistic cell survival limits of 0%–100%. Thus, the GEUD and TCP formalisms are, in general, mathematically inconsistent. However, when a threshold dose or finite critical dose exists in the tumor response to radiation, there is a unique mathematical solution for the tumor cell survival fraction that allows the GEUD and TCP formalisms to coexist, provided that all portions of the tumor are confined within certain specific dose ranges.
Optimization of dose distributions for bifurcated coronary vessels treated with catheter‐based photon and beta emitters using the simulated annealing algorithm
Authors: Yue Ning , Roberts Kenneth B. , Son Haijun , Khosravi Sheida , Pfau Steven E. , Nath Ravinder
Published: September 2004

The dose distributions in the bifurcated vessels treated with a catheter‐based delivery system are complicated by the geometry of bifurcation consisting of a main and a branch vessel at different angles, and it is difficult to generate satisfactory dose distributions. We hypothesize that increasing the number of dwell positions can result in better dose distributions. An optimization method based on the simulated annealing was developed to demonstrate the validity of this idea. In this method, the source in the branch treatment was allowed to take up to five dwell positions. A cost function was constructed to deliver the prescription dose to the planning targets with penalties for both overdosing and underdosing. By using the optimization algorithm, it was found that for 90° to 60° bifurcated vessels an optimized single dwell position treatment scheme can be as effective as the schemes that include up to five dwell positions. As the bifurcation angle becomes smaller than 60°, the dose distributions generated with the single dwell position scheme become less satisfactory than the more complicated treatment schemes with multiple dwell positions. By using a three‐dwell‐position treatment scheme for the source, the overdosing can be kept under 166% even at a bifurcation angle of 20°.
Effects of the intensity levels and beam map resolutions on static IMRT plans
Authors: Sun Xuepeng , Xia Ping , Yu Naichang
Published: September 2004

In this study we focus on how the intensity level and multileaf collimator (MLC) resolution affect the quality of IMRT plans using the static MLC delivery technique. The planning process is based on a least‐square dose‐based quadratic function and uses a simulated annealing algorithm to sample the discrete variables. Three clinical cases are studied empirically: a medulloblastoma, a prostate, and an oropharyngeal carcinoma. The intensity levels used are 3, 5, 10, 20, and continuous; the map resolution varies from 0.15–1.5 cm, with the leaf width equal to the step size. The influence of these two parameters are studied by comparing the cost value and the cost of delivery time from a trade‐off point of view. An “efficient frontier” is drawn by connecting the plans with the lowest cost value at any given resolutions. For each case, a practical delivery region is defined by doubling the delivery time needed at a normal setting (five levels, 1.0 cm). Within this region, the “efficient frontier” demonstrates that the plans with five intensity levels are the most efficient comparing with plans with higher levels. This is a confirmation of the conclusion from Keller‐Reichenbecher et al. [Int. J. Radiat. Oncol., Biol., Phys. 45, 1315–1324 (1999)]. It indicates that to further improve the plan quality with the minimal cost of extra delivery time, the most economical way is to improve the resolution rather than using higher intensity levels.
New x‐ray tube performance in computed tomography by introducing the rotating envelope tube technology
Authors: Schardt Peter , Deuringer Josef , Freudenberger Jörg , Hell Erich , Knüpfer Wolfgang , Mattern Detlef , Schild Markus
Published: September 2004

The future demands of computed tomography imaging regarding the x‐ray source can be summarized with higher scan power, shorter rotation times, shorter cool down times and smaller focal spots. We report on a new tube technology satisfying all these demands by making use of a novel cooling principle on one hand and of a novel beam control system on the other hand. Nowadays tubes use a rotating anode disk mainly cooled via radiation. The Straton® x‐ray tube is the first tube available for clinical routine utilizing convective cooling exclusively. It is demonstrated that this cooling principle makes large heat storage capacities of the anode disk obsolete. The unprecedented cooling rate of 4.8 MHU/min eliminates the need for waiting times due to anode cooling in clinical workflow. Moreover, an electronic beam deflection system for focal spot position and size control opens the door to advanced applications. The physical backgrounds are discussed and the technical realization is presented. From this discussion the superior suitability of this tube to withstand g‐forces well above 20 g created by fast rotating gantries will become evident. Experience from a large clinical trial is reported and possible ways for future developments are discussed.