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Dec 2004

Volume 31, Issue 12, pp. 3165-3533

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POINT/COUNTERPOINT: Noncompete clauses in employment contracts violate a physicist’s freedom to practice his/her profession

Shirish K. Jani, Prakash Shrivastava, and William R. Hendee, Moderator

Med. Phys. 31, 3165 (2004); http://dx.doi.org/10.1118/1.1818144 (3 pages)

Online Publication Date: 5 November 2004

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Abstract Unavailable
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87.90.+y Other topics in biological and medical physics (restricted to new topics in section 87)

RADIATION THERAPY PHYSICS: Predicting energy response of radiographic film in a 6 MV x-ray beam using Monte Carlo calculated fluence spectra and absorbed dose

Åsa Palm, Assen S. Kirov, and Thomas LoSasso

Med. Phys. 31, 3168 (2004); http://dx.doi.org/10.1118/1.1812911 (11 pages) | Cited 22 times

Online Publication Date: 5 November 2004

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The advantage of radiographic film is that it allows two-dimensional, high-resolution dose measurement. While there is concern over its photon energy dependence, these problems are considered acceptable within small fields, where the scatter component is small. The application of film dosimetry to intensity modulated radiotherapy (IMRT) raises additional concern since the primary fluence may vary significantly within the field. The varying primary fluence in combination with a large scatter fraction, present for large fields and large depths, causes the spectrum at various points within the IMRT field to differ from the spectrum in the uniform fields typically used for calibrating the film. As a result, significant artifacts are introduced in the measured dose distribution. The purpose of this work is to quantify and develop a method to correct for these artifacts. Two approaches based on Monte Carlo (MC) simulations are examined. In the first method, the film artifact, as quantified by film and ion chamber output measurements in uniform square fields, is derived from the MC calculated ratio of absorbed doses to film and to water. In the second method, the measured film artifact is correlated with MC calculated photon spectra, revealing a strong correlation between the measured artifact and the “scatter”-to-“primary” ratio, defined by the ratio of the number of photons below to the number of photons above 0.1 MeV, independent of field size and depth. These methods are evaluated in high- and low-dose regions of a large intensity-modulated field created with a central block. The spectral approach is also tested with a clinical IMRT field. The absorbed dose method accurately corrects the measured film dose in the open part of the field and in points under the block and outside the field. The dose error is reduced from as much as 16% of the open field dose to less than 1%, as verified with an ion chamber. The spectral method accurately corrects the measured film dose in the open region of the centrally blocked field, but does not fully correct for the film artifact for points under the block and outside the field, where the spectrum is substantially different. Applied to the clinical field, the corrected film measurement shows good agreement with data obtained with a two-dimensional diode array.
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87.56.Da Ancillary equipment
87.53.Bn Dosimetry/exposure assessment
87.55.K- Monte Carlo methods
87.59.B- Radiography
29.40.Cs Gas-filled counters: ionization chambers, proportional, and avalanche counters

NUCLEAR MEDICINE PHYSICS: Four-dimensional (4D) PET/CT imaging of the thorax

S. A. Nehmeh, Y. E. Erdi, T. Pan, A. Pevsner, K. E. Rosenzweig, E. Yorke, G. S. Mageras, H. Schoder, Phil Vernon, O. Squire, H. Mostafavi, S. M. Larson, and J. L. Humm

Med. Phys. 31, 3179 (2004); http://dx.doi.org/10.1118/1.1809778 (8 pages) | Cited 110 times

Online Publication Date: 8 November 2004

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We have reported in our previous studies on the methodology, and feasibility of 4D-PET (Gated PET) acquisition, to reduce respiratory motion artifact in PET imaging of the thorax. In this study, we expand our investigation to address the problem of respiration motion in PET/CT imaging. The respiratory motion of four lung cancer patients were monitored by tracking external markers placed on the thorax. A 4D-CT acquisition was performed using a “step-and-shoot” technique, in which computed tomography (CT) projection data were acquired over a complete respiratory cycle at each couch position. The period of each CT acquisition segment was time stamped with an “x-ray ON” signal, which was recorded by the tracking system. 4D-CT data were then sorted into 10 groups, according to their corresponding phase of the breathing cycle. 4D-PET data were acquired in the gated mode, where each breathing cycle was divided into ten 0.5 s bins. For both CT and PET acquisitions, patients received audio prompting to regularize breathing. The 4D-CT and 4D-PET data were then correlated according to respiratory phase. The effect of 4D acquisition on improving the co-registration of PET and CT images, reducing motion smearing, and consequently increase the quantitation of the SUV, were investigated. Also, quantitation of the tumor motions in PET, and CT, were studied and compared. 4D-PET with matching phase 4D-CTAC showed an improved accuracy in PET-CT image co-registration of up to 41%, compared to measurements from 4D-PET with clinical-CTAC. Gating PET data in correlation with respiratory motion reduced motion-induced smearing, thereby decreasing the observed tumor volume, by as much as 43%. 4D-PET lesions volumes showed a maximum deviation of 19% between clinical CT and phase- matched 4D-CT attenuation corrected PET images. In CT, 4D acquisition resulted in increasing the tumor volume in two patients by up to 79%, and decreasing it in the other two by up to 35%. Consequently, these corrections have yielded an increase in the measured SUV by up to 16% over the clinical measured SUV, and 36% over SUV’s measured in 4D-PET with clinical-CT Attenuation Correction (CTAC) SUV’s. Quantitation of the maximum tumor motion amplitude, using 4D-PET and 4D-CT, showed up to 30% discrepancy between the two modalities. We have shown that 4D PET/CT is clinically a feasible method, to correct for respiratory motion artifacts in PET/CT imaging of the thorax. 4D PET/CT acquisition can reduce smearing, improve the accuracy in PET-CT co-registration, and increase the measured SUV. This should result in an improved tumor assessment for patients with lung malignancies.
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87.59.bd Computed radiography
87.19.X- Diseases
87.57.N- Image analysis
87.19.U- Hemodynamics
87.19.Wx Pneumodyamics, respiration

RADIATION THERAPY PHYSICS: Effect of MLC leaf width on the planning and delivery of SMLC IMRT using the CORVUS inverse treatment planning system

Jay Burmeister, Patrick N. McDermott, Todd Bossenberger, Edgar Ben-Josef, Kenneth Levin, and Jeffrey D. Forman

Med. Phys. 31, 3187 (2004); http://dx.doi.org/10.1118/1.1812607 (7 pages) | Cited 8 times

Online Publication Date: 10 November 2004

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This study investigates the influence of multileaf collimator (MLC) leaf width on intensity modulated radiation therapy (IMRT) plans delivered via the segmented multileaf collimator (SMLC) technique. IMRT plans were calculated using the Corvus treatment planning system for three brain, three prostate, and three pancreas cases using leaf widths of 0.5 and 1 cm. Resulting differences in plan quality and complexity are presented here. Plans calculated using a 1 cm leaf width were chosen over the 0.5 cm leaf width plans in seven out of nine cases based on clinical judgment. Conversely, optimization results revealed a superior objective function result for the 0.5 cm leaf width plans in seven out of the nine comparisons. The 1 cm leaf width objective function result was superior only for very large target volumes, indicating that expanding the solution space for plan optimization by using narrower leaves may result in a decreased probability of finding the global minimum. In the remaining cases, we can conclude that we are often not utilizing the objective function as proficiently as possible to meet our clinical goals. There was often no apparent clinically significant difference between the two plans, and in such cases the issue becomes one of plan complexity. A comparison of plan complexity revealed that the average 1 cm leaf width plan required roughly 60% fewer segments and over 40% fewer monitor units than required by 0.5 cm leaf width plans. This allows a significant decrease in whole body dose and total treatment time. For very complex IMRT plans, the treatment delivery time may affect the biologically effective dose. A clinically significant improvement in plan quality from using narrower leaves was evident only in cases with very small target volumes or those with concavities that are small with respect to the MLC leaf width. For the remaining cases investigated in this study, there was no clinical advantage to reducing the MLC leaf width from 1 to 0.5 cm. In such cases, there is no justification for the increased treatment time and whole body dose associated with the narrower MLC leaf width.
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87.55.-x Treatment strategy
87.56.J- Collimation
87.56.Da Ancillary equipment
87.53.Kn Conformal radiation treatment

RADIATION PROTECTION PHYSICS: Potential third-party radiation exposure from outpatients treated with 131I for hyperthyroidism

Roberta Matheoud, Eugenio Reschini, Cristina Canzi, Franco Voltini, and Paolo Gerundini

Med. Phys. 31, 3194 (2004); http://dx.doi.org/10.1118/1.1809781 (7 pages) | Cited 5 times

Online Publication Date: 10 November 2004

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Thirty-three hyperthyroid patients treated with radioiodine (mean administered activity 414 MBq, range 163–555) were studied to determine if pretreatment dosimetry could be used to give radiation protection advice that could assure compliance with the effective dose constraints suggested by the European Commission. Effective doses to travelers, co-workers, and sleeping partners were estimated by integrating the effective dose rate-versus-time curve obtained by fitting the dose rates measured several times after radioiodine administration to a biexponential function. The mean estimated effective doses to travelers, co-workers, and sleeping partners were 0.11 mSv (0.05–0.24), 0.24 mSv (0.07–0.52), and 1.8 mSv (0.6–4.1), respectively. The best correlation was found between effective dose (D) in mSv and maximum activity (AUmax) in MBq taken up in the thyroid: Dtraveler=0.0005*(AUmax)+0.04 (r=0.88,p<0.01); Dco-worker=0.0013*(AUmax)+0.03(r=0.89,p<0.01); Dsleeping partners=0.0105*(AUmax)+0.16 (r=0.93,p<0.01). Private/public transports are always allowed. For the co-workers the effective dose constraint of 0.3 mSv is met without restrictions and with 3 days off work if AUmax is lower or higher than 185 MBq, respectively. For the sleeping partners the effective dose constraint of 3 mSv is met without restriction and with 4 nights separate sleeping arrangements if AUmax is lower or higher than 185 MBq, respectively. The potential for contamination by the patients was determined from perspiration samples taken from the patient’s hands, forehead, and neck and in saliva at 4, 24, and 48 h after radioiodine treatment. The mean highest 131I activity levels for hands, forehead, neck, and saliva were 4.1 Bq∕cm2, 1.9 Bq∕cm2, 0.9 Bq∕cm2, and 796 kBq∕g, respectively. The results indicate that there is minimal risk of contamination from these patients.
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87.55.N- Radiation monitoring, control, and safety
87.53.-j Effects of ionizing radiation on biological systems

RADIATION MEASUREMENT PHYSICS: Influence of field size on a PTW type 23342 plane-parallel ionization chamber’s response

C. Austerlitz, H. P. Villar, and M. A. P. Santos

Med. Phys. 31, 3201 (2004); http://dx.doi.org/10.1118/1.1819554 (5 pages) | Cited 1 time

Online Publication Date: 10 November 2004

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The response of a PTW type 23342 plane-parallel ionization chamber, both in air and in phantom, was evaluated for x-ray tube potentials between 30 and 100 kV and radiation field diameters ranging from 30 to 70 mm. The experiments were performed with a calibrated Pantak x-ray machine and made use of the same set of x-ray qualities adopted by the PTB primary laboratory for the calibration of such chambers. A Plexiglas® phantom (1.18 g cm−3) 110 mm long, 110 mm wide, and 80 mm deep was used for phantom measurements. X-ray qualities were characterized by using 99.99% pure aluminum filters. On the basis of the IAEA’s TRS 398, the article discusses the dependence of the plane-parallel ionization chamber readings with field size in air and in phantom, its implication with regard to clinical dosimetry, cross-calibration, and dissemination of calibration factors.
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87.53.Bn Dosimetry/exposure assessment
87.59.-e X-ray imaging
87.63.-d Non-ionizing radiation equipment and techniques

RADIATION IMAGING PHYSICS: An error-reduction-based algorithm for cone-beam computed tomography

Kai Zeng, Zhiqiang Chen, Li Zhang, and Ge Wang

Med. Phys. 31, 3206 (2004); http://dx.doi.org/10.1118/1.1809792 (7 pages) | Cited 12 times

Online Publication Date: 11 November 2004

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Image reconstruction from cone-beam projections collected along a single circular source trajectory is commonly done using the Feldkamp algorithm, which performs well only with a small cone angle. In this report, we propose an error-reduction-based algorithm to increase the cone angle by several folds to achieve satisfactory image quality at the same radiation dose. In our scheme, we first reconstruct the object using the Feldkamp algorithm. Then, we synthesize cone-beam projection data from the reconstructed volume in the same geometry, and reconstruct the volume again from the synthesized projections. Finally, these two reconstruction results are combined to reduce the reconstruction error and produce a superior image volume. The merit of this algorithm is demonstrated in numerical simulation.
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87.59.bd Computed radiography
02.60.Gf Algorithms for functional approximation
87.57.N- Image analysis

RADIATION THERAPY PHYSICS: Transmission and dose perturbations with high-Z materials in clinical electron beams

Indra J. Das, Chee-Wai Cheng, Raj K. Mitra, Alireza Kassaee, Zelig Tochner, and Lawrence J. Solin

Med. Phys. 31, 3213 (2004); http://dx.doi.org/10.1118/1.1819551 (9 pages) | Cited 6 times

Online Publication Date: 11 November 2004

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High density and atomic number (Z) materials used in various prostheses, eye shielding, and beam modifiers produce dose enhancements on the backscatter side in electron beams and is well documented. However, on the transmission side the dose perturbation is given very little clinical importance, which is investigated in this study. A simple and accurate method for dose perturbation at metallic interfaces with soft tissues and transmission through these materials is required for all clinical electron beams. Measurements were taken with thin-window parallel plate ion chambers for various high-Z materials (Al, Ti, Cu, and Pb) on a Varian and a Siemens accelerator in the energy range of 5–20 MeV. The dose enhancement on both sides of the metallic sheet is due to increased electron fluence that is dependent on the beam energy and Z. On the transmission side, the magnitude of dose enhancement depends on the thickness of the high-Z material. With increasing thickness, dose perturbation reduces to the electron transmission. The thickness of material to reduce 100% (range of dose perturbation), 50% and 10% transmission is linear with the beam energy. The slope (mm∕MeV) of the transmission curve varies exponentially with Z. A nonlinear regression expression {t=E[α+β exp(−0.1Z)]} is derived to calculate the thickness at a given transmission, namely 100%, 50%, and 10% for electron energy, E, which is simple, accurate and well suited for a quick estimation in clinical use. Caution should be given to clinicians for the selection of thickness of high-Z materials when used to shield critical structures as small thickness increases dose significantly at interfaces.
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87.53.Bn Dosimetry/exposure assessment
87.85.J- Biomaterials
87.53.-j Effects of ionizing radiation on biological systems

RADIATION IMAGING PHYSICS: Adaptive algebraic reconstruction technique

Wenkai Lu and Fang-Fang Yin

Med. Phys. 31, 3222 (2004); http://dx.doi.org/10.1118/1.1812606 (9 pages) | Cited 7 times

Online Publication Date: 12 November 2004

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Algebraic reconstruction techniques (ART) are iterative procedures for reconstructing objects from their projections. It is proven that ART can be computationally efficient by carefully arranging the order in which the collected data are accessed during the reconstruction procedure and adaptively adjusting the relaxation parameters. In this paper, an adaptive algebraic reconstruction technique (AART), which adopts the same projection access scheme in multilevel scheme algebraic reconstruction technique (MLS-ART), is proposed. By introducing adaptive adjustment of the relaxation parameters during the reconstruction procedure, one-iteration AART can produce reconstructions with better quality, in comparison with one-iteration MLS-ART. Furthermore, AART outperforms MLS-ART with improved computational efficiency.
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87.59.bd Computed radiography
87.57.C- Image quality
87.57.N- Image analysis
02.30.Uu Integral transforms

INFRARED AND MICROWAVE IMAGING: Model-based microwave image reconstruction: simulations and experiments

Razvan Ciocan and Huabei Jiang

Med. Phys. 31, 3231 (2004); http://dx.doi.org/10.1118/1.1812871 (11 pages) | Cited 7 times

Online Publication Date: 12 November 2004

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We describe an integrated microwave imaging system that can provide spatial maps of dielectric properties of heterogeneous media with tomographically collected data. The hardware system (800–1200 MHz) was built based on a lock-in amplifier with 16 fixed antennas. The reconstruction algorithm was implemented using a Newton iterative method with combined Marquardt-Tikhonov regularizations. System performance was evaluated using heterogeneous media mimicking human breast tissue. Finite element method coupled with the Bayliss and Turkel radiation boundary conditions were applied to compute the electric field distribution in the heterogeneous media of interest. The results show that inclusions embedded in a 76-diameter background medium can be quantitatively reconstructed from both simulated and experimental data. Quantitative analysis of the microwave images obtained suggests that an inclusion of 14 mm in diameter is the smallest object that can be fully characterized presently using experimental data, while objects as small as 10 mm in diameter can be quantitatively resolved with simulated data.
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87.59.E- Mammography
87.57.N- Image analysis
87.19.X- Diseases
02.70.Dh Finite-element and Galerkin methods
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Medical Physics is an official science journal of the AAPM and of the COMP/CCPM/IOMP
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Departments of Radiology, Radiation Oncology, Biophysics, Community and Public Health, Medical College of Wisconsin
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