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Dec 2006

Volume 33, Issue 12, pp. 4455-4773

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POINT/COUNTERPOINT: It is important that medical physicists be involved in the development and implementation of integrated hospital information systems

George C. Nikiforidis, Ph.D., George C. Kagadis, Ph.D., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 33, 4455 (2006); http://dx.doi.org/10.1118/1.2242051 (4 pages) | Cited 5 times

Online Publication Date: 6 November 2006

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Abstract Unavailable
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87.80.-y Biophysical techniques (research methods)
87.57.-s Medical imaging

MAGNETIC RESONANCE PHYSICS: Characterization of the susceptibility artifact around a prostate brachytherapy seed in MRI

K. Wachowicz, S. D. Thomas, and B. G. Fallone

Med. Phys. 33, 4459 (2006); http://dx.doi.org/10.1118/1.2364052 (9 pages) | Cited 7 times

Online Publication Date: 6 November 2006

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Magnetic distortions surrounding a typical brachytherapy seed (IMC6711, OncoSeed™) within a clinical magnetic resonance imager were modeled for a number of different seed orientations with respect to the main magnetic field. From these distortion maps, simulated images were produced. The simulated images were then compared to images experimentally acquired using a spin echo technique on a Philips 1.5 T magnetic resonance imaging scanner. The modeled images were found to conform very well to those acquired experimentally, thus allowing one to establish where the seed is positioned within the complex image distortion patterns. The artifact patterns were dependent on the orientation of the seed with the main magnetic field, as well as the direction of the read encode gradient. While all imaging schemes which employ a unidirectional linear read encode trajectory should produce the artifacts modeled in this article, sequences other than spin echo may produce additional artifacts. Gradient echo and steady-state free precession imaging techniques were also performed on the seed for comparison.
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87.61.Tg Clinical applications
87.61.Ff Instrumentation
87.53.Jw Therapeutic applications, including brachytherapy
87.63.-d Non-ionizing radiation equipment and techniques
87.19.X- Diseases

RADIATION THERAPY PHYSICS: Using fluence separation to account for energy spectra dependence in computing dosimetric a-Si EPID images for IMRT fields

Weidong Li, Jeffrey V. Siebers, and Joseph A. Moore

Med. Phys. 33, 4468 (2006); http://dx.doi.org/10.1118/1.2369468 (13 pages) | Cited 20 times

Online Publication Date: 6 November 2006

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This study develops a method to improve the dosimetric accuracy of computed images for an amorphous silicon flat-panel imager. Radially dependent kernels derived from Monte Carlo simulations are convolved with the treatment-planning system’s energy fluence. Multileaf collimator (MLC) beam hardening is accounted for by having separate kernels for open and blocked portions of MLC fields. Field-size-dependent output factors are used to account for the field-size dependence of scatter within the imager. Gamma analysis was used to evaluate open and sliding window test fields and intensity modulated patient fields. For each tested field, at least 99.6% of the points had γ<1 with a 3%, 3-mm criteria. With a 2%, 2-mm criteria, between 81% and 100% of points had γ<1. Patient intensity modulated test fields had 94%–100% of the points with γ<1 with a 2%, 2-mm criteria for all six fields tested. This study demonstrates that including the dependencies of kernel and fluence on radius and beam hardening in the convolution improves its accuracy compared with the use of radial and beam-hardening independent kernels; it also demonstrates that the resultant accuracy of the convolution method is sufficient for pretreatment, intensity modulated patient field verification.
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87.55.km Verification
87.56.ng Wedges and compensators
87.55.-x Treatment strategy
87.56.J- Collimation
87.55.K- Monte Carlo methods

RADIATION BIOLOGY: On relating the generalized equivalent uniform dose formalism to the linear-quadratic model

David Djajaputra and Qiuwen Wu

Med. Phys. 33, 4481 (2006); http://dx.doi.org/10.1118/1.2369469 (9 pages) | Cited 3 times

Online Publication Date: 6 November 2006

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Two main approaches are commonly used in the literature for computing the equivalent uniform dose (EUD) in radiotherapy. The first approach is based on the cell-survival curve as defined in the linear-quadratic model. The second approach assumes that EUD can be computed as the generalized mean of the dose distribution with an appropriate fitting parameter. We have analyzed the connection between these two formalisms by deriving explicit formulas for the EUD which are applicable to normal distributions. From these formulas we have established an explicit connection between the two formalisms. We found that the EUD parameter has strong dependence on the parameters that characterize the distribution, namely the mean dose and the standard deviation around the mean. By computing the corresponding parameters for clinical dose distributions, which in general do not follow the normal distribution, we have shown that our results are also applicable to actual dose distributions. Our analysis suggests that caution should be used in using generalized EUD approach for reporting and analyzing dose distributions.
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87.55.-x Treatment strategy
87.17.-d Cell processes
02.50.Ng Distribution theory and Monte Carlo studies

RADIATION THERAPY PHYSICS: Direct aperture deformation: An interfraction image guidance strategy

Yuanming Feng, Carlos Castro-Pareja, Raj Shekhar, and Cedric Yu

Med. Phys. 33, 4490 (2006); http://dx.doi.org/10.1118/1.2374675 (9 pages) | Cited 25 times

Online Publication Date: 7 November 2006

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A new scheme, called direct aperture deformation (DAD), for online correction of interfraction geometric uncertainties under volumetric imaging guidance is presented. Using deformable image registration, the three-dimensional geometric transformation matrix can be derived that associates the planning image set and the images acquired on the day of treatment. Rather than replanning or moving the patient, we use the deformation matrix to morph the treatment apertures as a potential online correction method. A proof-of-principle study using an intensity-modulated radiation therapy plan for a prostate cancer patient was conducted. The method, procedure, and algorithm of DAD are described. The dose-volume histograms from the original plan, reoptimized plan, and rigid-body translation plan are compared with the ones from the DAD plan. The study showed the feasibility of the DAD as a general method for both target dislocation and deformation. As compared with using couch translation to move the patient, DAD is capable of correcting both target dislocation and deformations. As compared with reoptimization, online correction using the DAD scheme could be completed within a few minutes rather than tens of minutes and the speed gain would be at a very small cost of plan quality.
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87.55.-x Treatment strategy
87.56.ng Wedges and compensators
87.57.N- Image analysis
87.59.bd Computed radiography

MAGNETIC RESONANCE PHYSICS: Low magnetic moment PIN diodes for high field MRI surface coils

Pavel Voskoboynik, Ronald D. Joos, W. E. Doherty, Jr., and Ron B. Goldfarb

Med. Phys. 33, 4499 (2006); http://dx.doi.org/10.1118/1.2372216 (3 pages)

Online Publication Date: 8 November 2006

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Positive-intrinsic-negative (PIN) silicon diodes are commonly used in magnetic resonance imaging (MRI) coils to perform active or passive blocking and detuning, or to disable circuit functions. However, diode packages with large magnetic moments are known to cause image artifacts in high field MRI systems. In this study, diode packages with low magnetic moment were designed by compensating components of ferromagnetic nickel and paramagnetic tungsten with diamagnetic silver. The new diodes have an initial positive susceptibility up to fields of 1 T and a negative susceptibility from 1 to 7 T. Their magnetic moments are one to two orders of magnitude smaller than those of standard diodes; moments as small as 20 nJ∕T at 7 T were achieved.
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87.61.Ff Instrumentation
87.63.-d Non-ionizing radiation equipment and techniques
85.30.Kk Junction diodes

RADIATION IMAGING PHYSICS: Modeling granular phosphor screens by Monte Carlo methods

Panagiotis F. Liaparinos, Ioannis S. Kandarakis, Dionisis A. Cavouras, Harry B. Delis, and George S. Panayiotakis

Med. Phys. 33, 4502 (2006); http://dx.doi.org/10.1118/1.2372217 (13 pages) | Cited 18 times

Online Publication Date: 8 November 2006

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The intrinsic phosphor properties are of significant importance for the performance of phosphor screens used in medical imaging systems. In previous analytical-theoretical and Monte Carlo studies on granular phosphor materials, values of optical properties, and light interaction cross sections were found by fitting to experimental data. These values were then employed for the assessment of phosphor screen imaging performance. However, it was found that, depending on the experimental technique and fitting methodology, the optical parameters of a specific phosphor material varied within a wide range of values, i.e., variations of light scattering with respect to light absorption coefficients were often observed for the same phosphor material. In this study, x-ray and light transport within granular phosphor materials was studied by developing a computational model using Monte Carlo methods. The model was based on the intrinsic physical characteristics of the phosphor. Input values required to feed the model can be easily obtained from tabulated data. The complex refractive index was introduced and microscopic probabilities for light interactions were produced, using Mie scattering theory. Model validation was carried out by comparing model results on x-ray and light parameters (x-ray absorption, statistical fluctuations in the x-ray to light conversion process, number of emitted light photons, output light spatial distribution) with previous published experimental data on Gd2O2S:Tb phosphor material (Kodak Min-R screen). Results showed the dependence of the modulation transfer function (MTF) on phosphor grain size and material packing density. It was predicted that granular Gd2O2S:Tb screens of high packing density and small grain size may exhibit considerably better resolution and light emission properties than the conventional Gd2O2S:Tb screens, under similar conditions (x-ray incident energy, screen thickness).
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87.59.B- Radiography
87.63.-d Non-ionizing radiation equipment and techniques
87.57.C- Image quality
02.50.Ng Distribution theory and Monte Carlo studies

RADIATION MEASUREMENT PHYSICS: Dosimetric characterization of a novel intracavitary mold applicator for 192Ir high dose rate endorectal brachytherapy treatment

Emily Poon, Brigitte Reniers, Slobodan Devic, Té Vuong, and Frank Verhaegen

Med. Phys. 33, 4515 (2006); http://dx.doi.org/10.1118/1.2364054 (12 pages) | Cited 10 times

Online Publication Date: 14 November 2006

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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.
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87.53.Jw Therapeutic applications, including brachytherapy
87.55.K- Monte Carlo methods
87.56.Da Ancillary equipment
87.19.X- Diseases

RADIATION THERAPY PHYSICS: Monte Carlo modelling of a-Si EPID response: The effect of spectral variations with field size and position

Laure Parent, Joao Seco, Phil M. Evans, Andrew Fielding, and David R. Dance

Med. Phys. 33, 4527 (2006); http://dx.doi.org/10.1118/1.2369465 (14 pages) | Cited 24 times

Online Publication Date: 15 November 2006

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This study focused on predicting the electronic portal imaging device (EPID) image of intensity modulated radiation treatment (IMRT) fields in the absence of attenuation material in the beam with Monte Carlo methods. As IMRT treatments consist of a series of segments of various sizes that are not always delivered on the central axis, large spectral variations may be observed between the segments. The effect of these spectral variations on the EPID response was studied with fields of various sizes and off-axis positions. A detailed description of the EPID was implemented in a Monte Carlo model. The EPID model was validated by comparing the EPID output factors for field sizes between 1×1 and 26×26 cm2 at the isocenter. The Monte Carlo simulations agreed with the measurements to within 1.5%. The Monte Carlo model succeeded in predicting the EPID response at the center of the fields of various sizes and offsets to within 1% of the measurements. Large variations (up to 29%) of the EPID response were observed between the various offsets. The EPID response increased with field size and with field offset for most cases. The Monte Carlo model was then used to predict the image of a simple test IMRT field delivered on the beam axis and with an offset. A variation of EPID response up to 28% was found between the on- and off-axis delivery. Finally, two clinical IMRT fields were simulated and compared to the measurements. For all IMRT fields, simulations and measurements agreed within 3%—0.2 cm for 98% of the pixels. The spectral variations were quantified by extracting from the spectra at the center of the fields the total photon yield (Ytotal), the photon yield below 1 MeV (Ylow), and the percentage of photons below 1 MeV (Plow). For the studied cases, a correlation was shown between the EPID response variation and Ytotal, Ylow, and Plow.
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87.53.Bn Dosimetry/exposure assessment
87.56.ng Wedges and compensators
87.55.km Verification
87.55.-x Treatment strategy
87.55.K- Monte Carlo methods
87.56.Da Ancillary equipment

RADIATION IMAGING PHYSICS: Dose and image quality for a cone-beam C-arm CT system

Rebecca Fahrig, Robert Dixon, Thomas Payne, Richard L. Morin, Arundhuti Ganguly, and Norbert Strobel

Med. Phys. 33, 4541 (2006); http://dx.doi.org/10.1118/1.2370508 (10 pages) | Cited 49 times

Online Publication Date: 16 November 2006

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We assess dose and image quality of a state-of-the-art angiographic C-arm system (Axiom Artis dTA, Siemens Medical Solutions, Forchheim, Germany) for three-dimensional neuro-imaging at various dose levels and tube voltages and an associated measurement method. Unlike conventional CT, the beam length covers the entire phantom, hence, the concept of computed tomography dose index (CTDI) is not the metric of choice, and one can revert to conventional dosimetry methods by directly measuring the dose at various points using a small ion chamber. This method allows us to define and compute a new dose metric that is appropriate for a direct comparison with the familiar CTDIW of conventional CT. A perception study involving the CATPHAN 600 indicates that one can expect to see at least the 9 mm inset with 0.5% nominal contrast at the recommended head-scan dose (60 mGy) when using tube voltages ranging from 70 kVp to 125 kVp. When analyzing the impact of tube voltage on image quality at a fixed dose, we found that lower tube voltages gave improved low contrast detectability for small-diameter objects. The relationships between kVp, image noise, dose, and contrast perception are discussed.
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87.59.bd Computed radiography
87.56.Da Ancillary equipment
87.57.C- Image quality
87.19.L- Neuroscience
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Medical Physics is an official science journal of the AAPM and of the COMP/CCPM/IOMP
William R. Hendee, Editor
Departments of Radiology, Radiation Oncology, Biophysics, Community and Public Health, Medical College of Wisconsin
One Physics Ellipse
College Park, Maryland 20740
301-209-3352

© 2012, American Association of Physicists in Medicine. Individual readers of this journal, and nonprofit libraries acting for them, are freely permited to make fair use of the material in it, such as to copy an article for use in teaching or research. (For other kinds of copying see "Copying Fees.") Permission is granted to quote from this journal in scientific works with the customary acknowledgment of the source. To reprint a figure, table, or other excerpt requires, in addition, AAPM may require that permission also be obtained from one of the authors. Address inquiries and notices to Penny Slattery, Journal Manager, Medical Physics Journal, AAPM, One Physics Ellipse, College Park, MD 20740-3846; email: journal@aapm.org.

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