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Feb 2012

Volume 39, Issue 2, pp. 569-1153

Spotlight Figure

Med. Phys. 39, 788 (2012); http://dx.doi.org/10.1118/1.3676692 (10 pages)

D. M. Santos, J. St. Aubin, B. G. Fallone, and S. Steciw
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POINT/COUNTERPOINT: Vendor provided machine data should never be used as a substitute for fully commissioning a linear accelerator

Indra J. Das, Ph.D., Christopher F. Njeh, Ph.D., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 39, 569 (2012); http://dx.doi.org/10.1118/1.3658740 (4 pages)

Online Publication Date: 11 January 2012

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Abstract Unavailable
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87.56.bd Accelerators
29.20.Ej Linear accelerators

RADIATION THERAPY PHYSICS: A method to estimate the effect of deformable image registration uncertainties on daily dose mapping

Martin J. Murphy, Francisco J. Salguero, Jeffrey V. Siebers, David Staub, and Constantin Vaman

Med. Phys. 39, 573 (2012); http://dx.doi.org/10.1118/1.3673772 (8 pages)

Online Publication Date: 11 January 2012

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Purpose: To develop a statistical sampling procedure for spatially-correlated uncertainties in deformable image registration and then use it to demonstrate their effect on daily dose mapping.
Methods: Sequential daily CT studies are acquired to map anatomical variations prior to fractionated external beam radiotherapy. The CTs are deformably registered to the planning CT to obtain displacement vector fields (DVFs). The DVFs are used to accumulate the dose delivered each day onto the planning CT. Each DVF has spatially-correlated uncertainties associated with it. Principal components analysis (PCA) is applied to measured DVF error maps to produce decorrelated principal component modes of the errors. The modes are sampled independently and reconstructed to produce synthetic registration error maps. The synthetic error maps are convolved with dose mapped via deformable registration to model the resulting uncertainty in the dose mapping. The results are compared to the dose mapping uncertainty that would result from uncorrelated DVF errors that vary randomly from voxel to voxel.
Results: The error sampling method is shown to produce synthetic DVF error maps that are statistically indistinguishable from the observed error maps. Spatially-correlated DVF uncertainties modeled by our procedure produce patterns of dose mapping error that are different from that due to randomly distributed uncertainties.
Conclusions: Deformable image registration uncertainties have complex spatial distributions. The authors have developed and tested a method to decorrelate the spatial uncertainties and make statistical samples of highly correlated error maps. The sample error maps can be used to investigate the effect of DVF uncertainties on daily dose mapping via deformable image registration. An initial demonstration of this methodology shows that dose mapping uncertainties can be sensitive to spatial patterns in the DVF uncertainties.
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87.55.dk Dose-volume analysis
87.57.Q- Computed tomography
87.57.nj Registration
87.59.-e X-ray imaging
87.53.Jw Therapeutic applications, including brachytherapy
87.53.Bn Dosimetry/exposure assessment

NUCLEAR MEDICINE PHYSICS: Development of a pixelated GSO gamma camera system with tungsten parallel hole collimator for single photon imaging

S. Yamamoto, H. Watabe, Y. Kanai, E. Shimosegawa, and J. Hatazawa

Med. Phys. 39, 581 (2012); http://dx.doi.org/10.1118/1.3673774 (8 pages)

Online Publication Date: 11 January 2012

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Purpose: In small animal imaging using a single photon emitting radionuclide, a high resolution gamma camera is required. Recently, position sensitive photomultiplier tubes (PSPMTs) with high quantum efficiency have been developed. By combining these with nonhygroscopic scintillators with a relatively low light output, a high resolution gamma camera can become useful for low energy gamma photons. Therefore, the authors developed a gamma camera by combining a pixelated Ce-doped Gd2SiO5 (GSO) block with a high quantum efficiency PSPMT.
Methods: GSO was selected for the scintillator, because it is not hygroscopic and does not contain any natural radioactivity. An array of 1.9 mm × 1.9 mm × 7 mm individual GSO crystal elements was constructed. These GSOs were combined with a 0.1-mm thick reflector to form a 22 × 22 matrix and optically coupled to a high quantum efficiency PSPMT (H8500C-100 MOD8). The GSO gamma camera was encased in a tungsten gamma-ray shield with tungsten pixelated parallel hole collimator, and the basic performance was measured for Co-57 gamma photons (122 keV).
Results: In a two-dimensional position histogram, all pixels were clearly resolved. The energy resolution was ∼15% FWHM. With the 20-mm thick tungsten pixelated collimator, the spatial resolution was 4.4-mm FWHM 40 mm from the collimator surface, and the sensitivity was ∼0.05%. Phantom and small animal images were successfully obtained with our developed gamma camera.
Conclusions: These results confirmed that the developed pixelated GSO gamma camera has potential as an effective instrument for low energy gamma photon imaging.
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87.57.U- Nuclear medicine imaging
07.85.-m X- and γ-ray instruments
85.60.Ha Photomultipliers; phototubes and photocathodes

NUCLEAR MEDICINE PHYSICS: Gap compensation during PET image reconstruction by constrained, total variation minimization

Seonmin Ahn, Soo Mee Kim, Jungah Son, Dong Soo Lee, and Jae Sung Lee

Med. Phys. 39, 589 (2012); http://dx.doi.org/10.1118/1.3673775 (14 pages)

Online Publication Date: 11 January 2012

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Purpose: Positron emission tomography (PET) is a noninvasive molecular imaging tool with various clinical and preclinical applications. The polygonal structure of small-diameter PET scanners that are designed for specific purposes can lead to gaps between the detector modules and result in loss of PET data during measurement. In the current study, the authors applied the compressed sensing (CS)-based total variation (TV) minimization method to PET image reconstructions to reduce the artifacts caused by gaps in small-diameter PET systems.
Methods: The first step in each iteration estimates whether an image is consistent with the measured PET data using the existing common reconstruction algorithms (ART, OSEM, and RAMLA). The second step recovers sparsity in the gradient domain of the image by minimizing the TV of an estimated image. The authors evaluated the gap-compensable reconstruction algorithms with uniform disk and Shepp-Logan phantoms by simulating sinograms which contained Poisson random noise and a data loss due to detector gaps. In addition, these methods were applied to real high resolution research tomography (HRRT)-like sinograms of human brain and uniform phantom. A comparison with other methods for gap compensation prior to or during image reconstruction was also made. Quantitative evaluations were performed by computing the uniformity, root mean squared error, and difference between the reconstructed images of nongapped and gapped sinograms.
Results: The simulation results showed that the gap-compensable methods incorporating TV minimization could control gap artifacts, as well as Poisson random noise. In particular, OSEM-TV and RAMLA-TV showed distinct potential via the properties of convergence and robustness to different noise levels and gap angle.
Conclusions: A TV minimization strategy incorporated into commonly used PET reconstruction algorithms was useful for reducing the occurrence of artifacts due to gaps between detector modules in small-diameter PET scanners.
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87.57.uk Positron emission tomography (PET)
87.57.nf Reconstruction

NUCLEAR MEDICINE PHYSICS: A filtered backprojection algorithm with characteristics of the iterative landweber algorithm

Gengsheng L. Zeng

Med. Phys. 39, 603 (2012); http://dx.doi.org/10.1118/1.3673956 (5 pages) | Cited 1 time

Online Publication Date: 11 January 2012

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Purpose: In order to eventually develop an analytical algorithm with noise characteristics of an iterative algorithm, this technical note develops a window function for the filtered backprojection (FBP) algorithm in tomography that behaves as an iterative Landweber algorithm.
Methods: Based on the formulation of the iterative Landweber algorithm, a frequency domain window function is derived for each iteration of the Landweber algorithm. The resultant window function has an index k, emulating the characteristics of the Landweber algorithm at the kth iteration. The window function is used to modify the ramp filter in the FBP algorithm.
Results: Computer simulations show that the windowed FBP algorithm with window function index k and the iterative Landweber algorithm iteration number k give similar reconstructions in terms of resolution and noise.
Conclusions: Analytical FBP algorithms are able to provide similar results to the iterative Landweber algorithm if the ramp filter in the FBP algorithm is modified by a set of specially designed window functions.
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87.57.Q- Computed tomography
02.60.-x Numerical approximation and analysis

RADIATION IMAGING PHYSICS: Measured and calculated K-fluorescence effects on the MTF of an amorphous-selenium based CCD x-ray detector

David M. Hunter, George Belev, Safa Kasap, and Martin J. Yaffe

Med. Phys. 39, 608 (2012); http://dx.doi.org/10.1118/1.3673957 (15 pages)

Online Publication Date: 11 January 2012

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Purpose: Theoretical reasoning suggests that direct conversion digital x-ray detectors based upon photoconductive amorphous-selenium (a-Se) could attain very high values of the MTF (modulation transfer function) at spatial frequencies well beyond 20 cycles mm−1. One of the fundamental factors affecting resolution loss, particularly at x-ray energies just above the K-edge of selenium (12.66 keV), is the K-fluorescence reabsorption mechanism, wherein energy can be deposited in the detector at locations laterally displaced from the initial x-ray interaction site. This paper compares measured MTF changes above and below the Se K-edge of a CCD based a-Se x-ray detector with theoretical expectations.
Methods: A prototype 25 μm sampling pitch (Nyquist frequency = 20 cycles mm−1, 200 μm thick a-Se layer based x-ray detector, utilizing a specialized CCD readout device (200 × 400 area array), was used to make edge images with monochromatic x-rays above and below the K-edge of Se. A vacuum double crystal monochromator, exposed to polychromatic x-rays from a synchrotron, formed the monochromatic x-ray source. The monochromaticity of the x-rays was 99% or better. The presampling MTF was determined using the slanted edge method. The theory modeling the MTF performance of the detector includes the basic x-ray interaction physics in the a-Se layer as well as effects related to the operation of the CCD and charge trapping at a blocking layer present at the CCD/a-Se interface.
Results: The MTF performance of the prototype a-Se CCD was reduced from the theoretical value prescribed by the basic Se x-ray interaction physics, principally by the presence of a blocking layer. Nevertheless, the K-fluorescence reduction in the MTF was observed, approximately as predicted by theory. For the CCD prototype detector, at five cycles mm−1, there was a 14% reduction of the MTF, from a value of 0.7 below the K-edge of Se, to 0.6 just above the K-edge.
Conclusions: The MTF of an a-Se x-ray detector has been measured using monochromatic x-rays above and below the K-edge of selenium. The MTF is poorer above the K-edge by an amount consistent with theoretical expectations.
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87.59.bf Digital radiography
07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors

RADIATION THERAPY PHYSICS: Detection and correction for EPID and gantry sag during arc delivery using cine EPID imaging

Pejman Rowshanfarzad, Mahsheed Sabet, Daryl J. O’Connor, Peter M. McCowan, Boyd M. C. McCurdy, and Peter B. Greer

Med. Phys. 39, 623 (2012); http://dx.doi.org/10.1118/1.3673958 (13 pages)

Online Publication Date: 11 January 2012

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Purpose: Electronic portal imaging devices (EPIDs) have been studied and used for pretreatment and in-vivo dosimetry applications for many years. The application of EPIDs for dosimetry in arc treatments requires accurate characterization of the mechanical sag of the EPID and gantry during rotation. Several studies have investigated the effects of gravity on the sag of these systems but each have limitations. In this study, an easy experiment setup and accurate algorithm have been introduced to characterize and correct for the effect of EPID and gantry sag during arc delivery.
Methods: Three metallic ball bearings were used as markers in the beam: two of them fixed to the gantry head and the third positioned at the isocenter. EPID images were acquired during a 360° gantry rotation in cine imaging mode. The markers were tracked in EPID images and a robust in-house developed MATLAB code was used to analyse the images and find the EPID sag in three directions as well as the EPID + gantry sag by comparison to the reference gantry zero image. The algorithm results were then tested against independent methods. The method was applied to compare the effect in clockwise and counter clockwise gantry rotations and different source-to-detector distances (SDDs). The results were monitored for one linear accelerator over a course of 15 months and six other linear-accelerators from two treatment centers were also investigated using this method. The generalized shift patterns were derived from the data and used in an image registration algorithm to correct for the effect of the mechanical sag in the system. The Gamma evaluation (3%, 3 mm) technique was used to investigate the improvement in alignment of cine EPID images of a fixed field, by comparing both individual images and the sum of images in a series with the reference gantry zero image.
Results: The mechanical sag during gantry rotation was dependent on the gantry angle and was larger in the in-plane direction, although the patterns were not identical for various linear-accelerators. The reproducibility of measurements was within 0.2 mm over a period of 15 months. The direction of gantry rotation and SDD did not affect the results by more than 0.3 mm. Results of independent tests agreed with the algorithm within the accuracy of the measurement tools. When comparing summed images, the percentage of points with Gamma index <1 increased from 85.4% to 94.1% after correcting for the EPID sag, and to 99.3% after correction for gantry + EPID sag.
Conclusions: The measurement method and algorithms introduced in this study use cine-images, are highly accurate, simple, fast, and reproducible. It tests all gantry angles and provides a suitable automatic analysis and correction tool to improve EPID dosimetry and perform comprehensive linac QA for arc treatments.
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87.53.Bn Dosimetry/exposure assessment
87.57.nj Registration
87.53.Jw Therapeutic applications, including brachytherapy

OPTICAL PHYSICS: Design and evaluation of an optically-tracked single-CCD laser range scanner

Thomas S. Pheiffer, Amber L. Simpson, Brian Lennon, Reid C. Thompson, and Michael I. Miga

Med. Phys. 39, 636 (2012); http://dx.doi.org/10.1118/1.3675397 (7 pages)

Online Publication Date: 11 January 2012

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Purpose: Acquisition of laser range scans of an organ surface has the potential to efficiently provide measurements of geometric changes to soft tissue during a surgical procedure. A laser range scanner design is reported here which has been developed to drive intraoperative updates to conventional image-guided neurosurgery systems.
Methods: The scanner is optically-tracked in the operating room with a multiface passive target. The novel design incorporates both the capture of surface geometry (via laser illumination) and color information (via visible light collection) through a single-lens onto the same charge-coupled device (CCD). The accuracy of the geometric data was evaluated by scanning a high-precision phantom and comparing relative distances between landmarks in the scans with the corresponding ground truth (known) distances. The range-of-motion of the scanner with respect to the optical camera was determined by placing the scanner in common operating room configurations while sampling the visibility of the reflective spheres. The tracking accuracy was then analyzed by fixing the scanner and phantom in place, perturbing the optical camera around the scene, and observing variability in scan locations with respect to a tracked pen probe ground truth as the camera tracked the same scene from different positions.
Results: The geometric accuracy test produced a mean error and standard deviation of 0.25 ± 0.40 mm with an RMS error of 0.47 mm. The tracking tests showed that the scanner could be tracked at virtually all desired orientations required in the OR set up, with an overall tracking error and standard deviation of 2.2 ± 1.0 mm with an RMS error of 2.4 mm. There was no discernible difference between any of the three faces on the lasers range scanner (LRS) with regard to tracking accuracy.
Conclusions: A single-lens laser range scanner design was successfully developed and implemented with sufficient scanning and tracking accuracy for image-guided surgery.
Show PACS
87.63.lt Laser imaging
85.60.Gz Photodetectors (including infrared and CCD detectors)
42.62.Be Biological and medical applications

RADIATION IMAGING PHYSICS: Small animal PET scanner based on monolithic LYSO crystals: Performance evaluation

F. Sánchez, L. Moliner, C. Correcher, A. González, A. Orero, M. Carles, A. Soriano, M. J. Rodriguez-Alvarez, L. A. Medina, F. Mora, and J. M. Benlloch

Med. Phys. 39, 643 (2012); http://dx.doi.org/10.1118/1.3673771 (11 pages)

Online Publication Date: 12 January 2012

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Purpose: The authors have developed a small animal Positron emission tomography (PET) scanner based on monolithic LYSO crystals coupled to multi-anode photomultiplier tubes (MA-PMTs). In this study, the authors report on the design, calibration procedure, and performance evaluation of a PET system that the authors have developed using this innovative nonpixelated detector design.
Methods: The scanner is made up of eight compact modules forming an octagon with an axial field of view (FOV) of 40 mm and a transaxial FOV of 80 mm diameter. In order to fully determine its performance, a recently issued National Electrical Manufacturers Association (NEMA) NU-4 protocol, specifically developed for small animal PET scanners, has been followed. By measuring the width of light distribution collected in the MA-PMT the authors are able to determine depth of interaction (DOI), thus making the proper identification of lines of response (LORs) with large incidence angles possible. PET performances are compared with those obtained with currently commercially available small animal PET scanners.
Results: At axial center when the point-like source is located at 5 mm from the radial center, the spatial resolution measured was 1.65, 1.80, and 1.86 mm full width at half maximum (FWHM) for radial, tangential, and axial image profiles, respectively. A system scatter fraction of 7.5% (mouse-like phantom) and 13% (rat-like phantom) was obtained, while the maximum noise equivalent count rate (NECR) was 16.9 kcps at 12.7 MBq (0.37 MBq/ml) for mouse-like phantom and 12.8 kcps at 12.4 MBq (0.042 MBq/ml) for rat-like phantom The peak absolute sensitivity in the center of the FOV is 2% for a 30% peak energy window. Several animal images are also presented.
Conclusions: The overall performance of our small animal PET is comparable to that obtained with much more complex crystal pixelated PET systems. Moreover, the new proposed PET produces high-quality images suitable for studies with small animals.
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87.57.uk Positron emission tomography (PET)

RADIATION IMAGING PHYSICS: XCOM intrinsic dimensionality for low-Z elements at diagnostic energies

Hans Bornefalk

Med. Phys. 39, 654 (2012); http://dx.doi.org/10.1118/1.3675399 (4 pages)

Online Publication Date: 12 January 2012

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Purpose: To determine the intrinsic dimensionality of linear attenuation coefficients (LACs) from XCOM for elements with low atomic number (Z = 1–20) at diagnostic x-ray energies (25–120 keV). H0q, the hypothesis that the space of LACs is spanned by q bases, is tested for various q-values.
Methods: Principal component analysis is first applied and the LACs are projected onto the first q principal component bases. The residuals of the model values vs XCOM data are determined for all energies and atomic numbers. Heteroscedasticity invalidates the prerequisite of i.i.d. errors necessary for bootstrapping residuals. Instead wild bootstrap is applied, which, by not mixing residuals, allows the effect of the non-i.i.d residuals to be reflected in the result. Credible regions for the eigenvalues of the correlation matrix for the bootstrapped LAC data are determined. If subsequent credible regions for the eigenvalues overlap, the corresponding principal component is not considered to represent true data structure but noise. If this happens for eigenvalues l and l + 1, for any l ≤ q, H0q is rejected.
Results: The largest value of q for which H0q is nonrejectable at the 5%-level is q = 4. This indicates that the statistically significant intrinsic dimensionality of low-Z XCOM data at diagnostic energies is four.
Conclusions: The method presented allows determination of the statistically significant dimensionality of any noisy linear subspace. Knowledge of such significant dimensionality is of interest for any method making assumptions on intrinsic dimensionality and evaluating results on noisy reference data. For LACs, knowledge of the low-Z dimensionality might be relevant when parametrization schemes are tuned to XCOM data. For x-ray imaging techniques based on the basis decomposition method (Alvarez and Macovski, Phys. Med. Biol. 21, 733–744, 1976), an underlying dimensionality of two is commonly assigned to the LAC of human tissue at diagnostic energies. The finding of a higher statistically significant dimensionality thus raises the question whether a higher assumed model dimensionality (now feasible with the advent of multibin x-ray systems) might also be practically relevant, i.e., if better tissue characterization results can be obtained.
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87.59.B- Radiography
02.50.-r Probability theory, stochastic processes, and statistics
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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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