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Dec 2012

Volume 39, Issue 12, pp. 7181-7731

Spotlight Figure

Med. Phys. 39, 7571 (2012); http://dx.doi.org/10.1118/1.4761952 (9 pages)

Giampaolo Tomasi, Tony Shepherd, Federico Turkheimer, Dimitris Visvikis, and Eric Aboagye
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POINT/COUNTERPOINT: Physicists who are responsible for high-tech radiotherapy procedures should have to be specially credentialed

Brian D. Kavanagh, MD, Geoffrey S. Ibbott, Ph.D., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 39, 7181 (2012); http://dx.doi.org/10.1118/1.4748333 (4 pages)

Online Publication Date: 26 November 2012

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Abstract Unavailable
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87.53.Ly Stereotactic radiosurgery

MAGNETIC RESONANCE PHYSICS: Characterization of tissue magnetic susceptibility-induced distortions for MRIgRT

T. Stanescu, K. Wachowicz, and D. A. Jaffray

Med. Phys. 39, 7185 (2012); http://dx.doi.org/10.1118/1.4764481 (9 pages)

Online Publication Date: 26 November 2012

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Purpose: MR image geometric integrity is one of the building blocks of MRI-guided radiotherapy. In particular, tissue magnetic susceptibility-induced effects are patient-dependent and their behavior is difficult to assess and predict. In this study, the authors investigated in detail the characteristics of susceptibility (χ) distortions in the context of MRIgRT, including the case of two common MR-linac system configurations.
Methods: The magnetic field distortions were numerically simulated for several imaging parameters and anatomical sites, i.e., brain, lung, pelvis (with air pockets), and prostate. The simulation process consisted of (a) segmentation of patient CT data into susceptibility relevant anatomical volumes (i.e., soft-tissue, bone and air/lung), (b) conversion of CT data into susceptibility masks by assigning bulk χ values to the structures defined at (a), (c) numerical computations of the local magnetic fields by using a finite difference algorithm, and (d) generation of the geometric distortion maps from the magnetic field distributions. For each patient anatomy, the distortions were quantified at the interfaces of anatomical structures with significantly different χ values. The analysis was performed for two specific orientations of the external main magnetic field (B0) characteristic to the MR-linac systems, specifically along the z-axis for a bore MR scanner and in the (x,y)-plane for a biplanner magnet. The magnetic field local perturbations were reported in ppm. The metrics used to quantify the geometric distortions were the maximum, mean, and range of distortions. The numerical simulation algorithm was validated using phantom data measurements.
Results: Susceptibility-induced distortions were determined for both quadratic and patient specific geometries. The numerical simulations showed a good agreement with the experimental data. The measurements were acquired at 1.5 and 3 T and with an encoding gradient varying between 3 and 20 mT/m by using an annular phantom mimicking the water-air and water-oil χ interfaces. For quadratic geometries, the magnitude of field distortion increased rapidly with the size of the inhomogeneity up to about 10 mm and then tended to plateau. This trend became more evident for materials with a larger Δχ relative to water. The simulations showed only a slight increase in the maximum distortion values when the B0 orientation was varied with regard to the shape of the χ inhomogeneity. In the case of patient anatomy, the largest distortion values arose at the air-soft-tissue interface. Considering the two MR-linac system configurations and comparing the field distortion values corresponding to all organ structures, the distortions tended to be larger for the biplanar magnet. The authors provide a reference table with ppm values which can be used to easily evaluate the geometric distortions for patient data as a function of B0 and the strength of the encoding gradient.
Conclusions: The susceptibility distortions were quantified as a function of multiple parameters such as the χ inhomogeneity size and shape, the magnitude of B0 and the readout gradient, and the orientation of B0 with respect to the sample geometry. The analysis was performed for several anatomical sites and corresponding to two B0 orientations as featured by MR-linac systems.
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87.61.Jc Anatomic imaging
87.61.Ff Instrumentation
87.53.Jw Therapeutic applications, including brachytherapy
87.56.bd Accelerators
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RADIATION THERAPY PHYSICS: A real time dose monitoring and dose reconstruction tool for patient specific VMAT QA and delivery

Neelam Tyagi, Kai Yang, David Gersten, and Di Yan

Med. Phys. 39, 7194 (2012); http://dx.doi.org/10.1118/1.4764482 (11 pages)

Online Publication Date: 26 November 2012

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Purpose: To develop a real time dose monitoring and dose reconstruction tool to identify and quantify sources of errors during patient specific volumetric modulated arc therapy (VMAT) delivery and quality assurance.
Methods: The authors develop a VMAT delivery monitor tool called linac data monitor that connects to the linac in clinical mode and records, displays, and compares real time machine parameters with the planned parameters. A new measure, called integral error, keeps a running total of leaf overshoot and undershoot errors in each leaf pair, multiplied by leaf width, and the amount of time during which the error exists in monitor unit delivery. Another tool reconstructs Pinnacle3™ format delivered plan based on the saved machine logfile and recalculates actual delivered dose in patient anatomy. Delivery characteristics of various standard fractionation and stereotactic body radiation therapy (SBRT) VMAT plans delivered on Elekta Axesse and Synergy linacs were quantified.
Results: The MLC and gantry errors for all the treatment sites were 0.00 ± 0.59 mm and 0.05 ± 0.31°, indicating a good MLC gain calibration. Standard fractionation plans had a larger gantry error than SBRT plans due to frequent dose rate changes. On average, the MLC errors were negligible but larger errors of up to 6 mm and 2.5° were seen when dose rate varied frequently. Large gantry errors occurred during the acceleration and deceleration process, and correlated well with MLC errors (r = 0.858, p = 0.0004). PTV mean, minimum, and maximum dose discrepancies were 0.87 ± 0.21%, 0.99 ± 0.59%, and 1.18 ± 0.52%, respectively. The organs at risk (OAR) doses were within 2.5%, except some OARs that showed up to 5.6% discrepancy in maximum dose. Real time displayed normalized total positive integral error (normalized to the total monitor units) correlated linearly with MLC (r = 0.9279, p < 0.001) and gantry errors (r = 0.742, p = 0.005). There is a strong correlation between total integral error and PTV mean (r = 0.683, p = 0.015), minimum (r = 0.6147, p = 0.033), and maximum dose (r = 0.6038, p = 0.0376).
Conclusions: Errors may exist during complex VMAT planning and delivery. Linac data monitor is capable of detecting and quantifying mechanical and dosimetric errors at various stages of planning and delivery.
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87.53.Bn Dosimetry/exposure assessment
87.55.dk Dose-volume analysis

RADIATION THERAPY PHYSICS: Localization of deformable tumors from short-arc projections using Bayesian estimation

W. Hoegele, P. Zygmanski, B. Dobler, M. Kroiss, O. Koelbl, and R. Loeschel

Med. Phys. 39, 7205 (2012); http://dx.doi.org/10.1118/1.4764483 (10 pages) | Cited 1 time

Online Publication Date: 26 November 2012

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Purpose: The authors present a stochastic framework for radiotherapy patient positioning directly utilizing radiographic projections. This framework is developed to be robust against anatomical nonrigid deformations and to cope with challenging imaging scenarios, involving only a few cone beam CT projections from short arcs.
Methods: Specifically, a Bayesian estimator (BE) is explicitly derived for the given scanning geometry. This estimator is compared to reference methods such as chamfer matching (CM) and the minimization of the median absolute error adapted as tools of robust image processing and statistics. In order to show the performance of the stochastic short-arc patient positioning method, a CIRS IMRT thorax phantom study is presented with movable markers and the utilization of an Elekta Synergy® XVI system. Furthermore, a clinical prostate CBCT scan of a Varian® On-Board Imager® system is utilized to investigate the robustness of the method for large variations of image quality (anterior-posterior vs lateral views).
Results: The results show that the BE shifts reduce the initial setup error of up to 3 cm down to 3 mm at maximum for an imaging arc as short as 10° while CM achieves residual errors of 7 mm at maximum only for arcs longer than 40°. Furthermore, the BE can compensate robustly for low image qualities using several low quality projections simultaneously.
Conclusions: In conclusion, an estimation method for marker-based patient positioning for short imaging arcs is presented and shown to be robust and accurate for deformable anatomies.
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87.59.bd Computed radiography
87.57.Q- Computed tomography
87.53.Jw Therapeutic applications, including brachytherapy
06.30.Bp Spatial dimensions (e.g., position, lengths, volume, angles, and displacements)
02.50.Tt Inference methods
02.50.Ey Stochastic processes

RADIATION IMAGING PHYSICS: A Monte Carlo tool to study the mortality reduction due to breast screening programs

Luis I. Zamora, Cristina Forastero, Damián Guirado, Rafael J. Martínez-Luna, and Antonio M. Lallena

Med. Phys. 39, 7215 (2012); http://dx.doi.org/10.1118/1.4764484 (9 pages)

Online Publication Date: 26 November 2012

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Purpose: To develop a Monte Carlo tool that permits to study the reduction in breast cancer mortality rate due to breast screening programs.
Methods: Simulations implement woman histories undergoing a screening program, include a model of survival after local treatment of invasive cancers and use distributions of time gained due to screening detection against symptomatic detection and overall sensitivity of the screening obtained previously. Mortalities for the whole woman population and for those women with ages within the range considered in the program have been calculated.
Results: For the whole woman population, a reduction in breast cancer mortality up to 29% has been found for a configuration that includes women aged between 50 and 70 years, with a screening interval of two years and 100% acceptance rate. If an acceptance of 70% is considered, this percentage reduces to 20%. If, in the same conditions, the program starts at 40 years, the reduction of the mortality reaches 24% while if the screening interval is one year, this percentage raises to 28%. If mortalities are calculated for those women with ages within the range included in the program these reductions are greater and no significant differences are found between the programs with age ranges [50–70] and [40–70]. In the model, radio-induced cancers have no effect in survival.
Conclusions: The results agree reasonably well with those of different trials. Mortality reductions of 12%–20% (between two and four deaths per year and 105 women) are obtained only for acceptances above 50%. This could be considered as a threshold for the acceptance, which appears to be a critical parameter.
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87.10.Rt Monte Carlo simulations
87.19.xj Cancer

INFRARED AND MICROWAVE IMAGING: Infrared thermal imaging as a novel evaluation method for deep vein thrombosis in lower limbs

Fangge Deng, Qing Tang, Yujiang Zheng, Guangqiao Zeng, and Nanshan Zhong

Med. Phys. 39, 7224 (2012); http://dx.doi.org/10.1118/1.4764485 (8 pages)

Online Publication Date: 26 November 2012

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Purpose: Early detection of deep vein thrombosis (DVT) is critical to prevent clinical pulmonary thromboembolism. However, most conventional methods for diagnosing DVT are functionally limited and complicated. The aim of this study was to evaluate the value of infrared-thermal-imaging (IRTI), a novel imaging detection or screening technique, in diagnosis of DVT in animal models.
Methods: DVT model of femoral veins was established in nine New Zealand rabbits. The right hind femoral vein was embolized and the contralateral one served as a nonembolized control. Measurements of IRTI, compression ultrasonography (CPUS), and angiography under ultrasonic observation (AGUO) were performed at three time points: T1 (baseline, 10 min prior to surgery), T2 (2 h after thrombin injection), and T3 (48 h postoperatively). Qualitative pseudocolor analysis and quantitative temperature analysis were performed based on mean area temperature (Tav) and mean curvilinear temperature (Tca) of the region of interest as shown in IRTI. Temperature differences (TD) in Tav (TDTav) and Tca (TDTca) between the DVT and control sides were computed. Comparative statistical analysis was carried out by paired t-test and repeated measure, while multiple comparisons were performed by using Greenhouse-Geisser and Bonferroni approach. Values of P < 0.05 and P < 0.01 were considered statistically significant and highly significant.
Results: Modeling of DVT was successful in all rabbits, as confirmed by CPUS and AGUO and immediately detected by IRTI. IRTI qualitative analysis of pseudocolor revealed that the bilateral temperatures were apparently asymmetrical and that there were abnormally high temperature zones on the DVT side where thrombosis formed. The results of paired t-test of Tav and Tca between DVT side and control sides did not reveal statistical difference at T1 (Tav: P = 0.817; Tca: P = 0.983) yet showed statistical differences at both T2 (Tav: P = 0.023; Tca: P = 0.021) and T3 (Tav: P = 0.016; Tca: P = 0.028). Results of repeated measure and multiple comparisons of TDTav and TDTca were highly different and significant differences across the T2 (TDTav: P = 0.009; TDTav: P = 0.03) and T3 (TDTav: P = 0.015; TDTav: P = 0.021).
Conclusions: IRTI temperature quantitative analysis may help further detection of DVT. Additionally, IRTI could serve as a novel detection and screening tool for DVT due to its convenience, rapid response, and high sensitivity.
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87.63.D- Ultrasonography
02.50.-r Probability theory, stochastic processes, and statistics

RADIATION THERAPY PHYSICS: Temperature and temporal dependence of the optical response for a radiochromic dosimeter

Peter S. Skyt, Isak Wahlstedt, Ludvig P. Muren, Jørgen B. B. Petersen, and Peter Balling

Med. Phys. 39, 7232 (2012); http://dx.doi.org/10.1118/1.4764486 (5 pages)

Online Publication Date: 26 November 2012

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Purpose: Both temporal and thermal dependencies of the dose response have been observed in radiochromic dosimeters. As these dependencies may be influenced by the dose level, the present study investigates the temperature dependence during irradiation and the temporal change of the optical response following irradiation of radiochromic dosimeters at a range of doses.
Methods: Cuvette samples of the PRESAGE™ radiochromic dosimeter were irradiated within a dose range of 0–10 Gy at irradiation temperatures within 5–35 °C and postirradiation storage within 6–30 °C. The optical response due to irradiation was measured using a standard spectrophotometer and the data were analyzed in terms of thermal and temporal change.
Results: The initial dose response was linear over the applied dose range independent of irradiation temperature. However, the optical response to a specific dose increased exponentially with irradiation temperature corresponding to an activation energy of 0.114 ± 0.007 eV. The temporal change in dose response after irradiation consisted of an offset, an auto-oxidation rate with activation energy 0.84 ± 0.03 eV, and an initial exponential increase in optical response (1.6 ± 0.2 eV) followed by an exponential decrease in optical response (0.98 ± 0.08 eV). These contributions depended on both storage temperature and the dose given, leading to a nonlinear dose response with time at low storage temperatures and a high auto-oxidation rate at high storage temperatures.
Conclusions: Thermal equilibration is important to the radiochromic dosimeter investigated due to an exponential change in dose response with irradiation temperature and a considerable postirradiation temporal change in response. For the dosimeter version investigated in this study, a compromise in storage temperature has to be made between increasing the nonlinearity of the dose response with time and inducing a high auto-oxidation rate.
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87.53.Bn Dosimetry/exposure assessment
78.40.-q Absorption and reflection spectra: visible and ultraviolet

RADIATION IMAGING PHYSICS: The second-order differential phase contrast and its retrieval for imaging with x-ray Talbot interferometry

Yi Yang and Xiangyang Tang

Med. Phys. 39, 7237 (2012); http://dx.doi.org/10.1118/1.4764901 (17 pages)

Online Publication Date: 26 November 2012

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Purpose: The x-ray differential phase contrast imaging implemented with the Talbot interferometry has recently been reported to be capable of providing tomographic images corresponding to attenuation-contrast, phase-contrast, and dark-field contrast, simultaneously, from a single set of projection data. The authors believe that, along with small-angle x-ray scattering, the second-order phase derivative Φ″s(x) plays a role in the generation of dark-field contrast. In this paper, the authors derive the analytic formulae to characterize the contribution made by the second-order phase derivative to the dark-field contrast (namely, second-order differential phase contrast) and validate them via computer simulation study. By proposing a practical retrieval method, the authors investigate the potential of second-order differential phase contrast imaging for extensive applications.
Methods: The theoretical derivation starts at assuming that the refractive index decrement of an object can be decomposed into δ = δs + δf, where δf corresponds to the object's fine structures and manifests itself in the dark-field contrast via small-angle scattering. Based on the paraxial Fresnel-Kirchhoff theory, the analytic formulae to characterize the contribution made by δs, which corresponds to the object's smooth structures, to the dark-field contrast are derived. Through computer simulation with specially designed numerical phantoms, an x-ray differential phase contrast imaging system implemented with the Talbot interferometry is utilized to evaluate and validate the derived formulae. The same imaging system is also utilized to evaluate and verify the capability of the proposed method to retrieve the second-order differential phase contrast for imaging, as well as its robustness over the dimension of detector cell and the number of steps in grating shifting.
Results: Both analytic formulae and computer simulations show that, in addition to small-angle scattering, the contrast generated by the second-order derivative is magnified substantially by the ratio of detector cell dimension over grating period, which plays a significant role in dark-field imaging implemented with the Talbot interferometry.
Conclusions: The analytic formulae derived in this work to characterize the second-order differential phase contrast in the dark-field imaging implemented with the Talbot interferometry are of significance, which may initiate more activities in the research and development of x-ray differential phase contrast imaging for extensive preclinical and eventually clinical applications.
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87.57.Q- Computed tomography
87.59.B- Radiography
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RADIATION IMAGING PHYSICS: Comprehensive assessment of the slice sensitivity profiles in breast tomosynthesis and breast CT

Anita Nosratieh, Kai Yang, Shadi Aminololama-Shakeri, and John M. Boone

Med. Phys. 39, 7254 (2012); http://dx.doi.org/10.1118/1.4764908 (8 pages)

Online Publication Date: 26 November 2012

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Purpose: This study experimentally evaluated the slice sensitivity profile (SSP) and its relationship between acquisition angle, object size, and cone angle. The sensitivity profile metric was used to characterize a breast tomosynthesis system's resolution in the z-axis. The SSP was also measured on a prototype breast computed tomography (bCT) system.
Methods: The SSP was measured using brass disks placed within adipose tissue-equivalent breast phantoms. The digital tomosynthesis system (Selenia Dimensions, Hologic Corporation, Bedford, MA) acquires projection images over a 15° angular range and the bCT scanner acquires projection images over a 360° angular range. Angular ranges between 15° and 360° were studied by using a subset of the projection images acquired on the bCT scanner. The SSP was determined by measuring a background-corrected mean gray scale value as a function of the z-position (axis normal to the plane of the detector).
Results: The results show that SSP improves when the angular acquisition range is increased and the SSP approaches a delta function for angles greater than 180°. Smaller objects have a narrower SSP and the SSP is not significantly dependent on the cone angle. For a 2.5, 5, 10 mm disk, the full width at half maximum of the SSP was 35, 61, 115 mm, respectively, on the tomosynthesis system (at 15°) and was 0.5 mm for all disk diameters on the bCT scanner (at 360°).
Conclusions: The SSP is dependent on object size and angular acquisition range. These dependencies are overcome once the angular acquisition range is increased beyond 180°.
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87.57.Q- Computed tomography

RADIATION IMAGING PHYSICS: Speed-of-sound compensated photoacoustic tomography for accurate imaging

Jithin Jose, Rene G. H. Willemink, Wiendelt Steenbergen, C. H. Slump, Ton G. van Leeuwen, and Srirang Manohar

Med. Phys. 39, 7262 (2012); http://dx.doi.org/10.1118/1.4764911 (10 pages)

Online Publication Date: 26 November 2012

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Purpose: In most photoacoustic (PA) tomographic reconstructions, variations in speed-of-sound (SOS) of the subject are neglected under the assumption of acoustic homogeneity. Biological tissue with spatially heterogeneous SOS cannot be accurately reconstructed under this assumption. The authors present experimental and image reconstruction methods with which 2D SOS distributions can be accurately acquired and reconstructed, and with which the SOS map can be used subsequently to reconstruct highly accurate PA tomograms.
Methods: The authors begin with a 2D iterative reconstruction approach in an ultrasound transmission tomography setting, which uses ray refracted paths instead of straight ray paths to recover accurate SOS images of the subject. Subsequently, they use the SOS distribution in a new 2D iterative PA reconstruction approach, where refraction of rays originating from PA sources is accounted for in accurately retrieving the distribution of these sources. Both the SOS reconstruction and SOS-compensated PA reconstruction methods utilize the Eikonal equation to model acoustic wavefront propagation. The equation is solved using a high accuracy fast marching method.
Results: The authors validated the new reconstruction algorithms using numerical phantoms. For experiments they utilized the recently introduced PER-PACT method which can be used to simultaneously acquire SOS and PA data from subjects.
Conclusions: It is first confirmed that it is important to take SOS inhomogeneities into account in high resolution PA tomography. The iterative reconstruction algorithms, that model acoustic refractive effects, in reconstructing SOS distributions, and subsequently using these distributions to correct PA tomograms, yield artifact-free highly accurate images. The approach of using the hybrid measurement method and the new reconstruction algorithms is successful in substantially improving the quality of PA images with a minimization of blurring and artifacts.
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87.63.dh Ultrasonographic imaging
87.57.cf Spatial resolution
87.57.cp Artifacts and distortion
87.57.nf Reconstruction
87.57.nj Registration
87.57.Q- Computed tomography
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