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Dec 2011

Volume 38, Issue 12, pp. i-6789

Spotlight Figure

Med. Phys. 38, 6585 (2011); http://dx.doi.org/10.1118/1.3662911 (7 pages)

I. Kantemiris, P. Karaiskos, P. Papagiannis, and A. Angelopoulos
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EDITORIAL: Scientific journals and impact factors

William Hendee, Editor, Matt A. Bernstein, Editor-in-Chief, and Deborah Levine, Senior Deputy Editor

Med. Phys. 38, i (2011); http://dx.doi.org/10.1118/1.3660554 (2 pages)

Online Publication Date: 29 November 2011

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Abstract Unavailable
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01.30.-y Physics literature and publications
87.00.00 Biological and medical physics
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EDITORIAL: The tenuous state of clinical medical physics in diagnostic imaging

Ehsan Samei and J. Anthony Seibert

Med. Phys. 38, iii (2011); http://dx.doi.org/10.1118/1.3664002 (2 pages)

Online Publication Date: 29 November 2011

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Abstract Unavailable
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87.85.Pq Biomedical imaging
87.53.Jw Therapeutic applications, including brachytherapy
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POINT/COUNTERPOINT: Tumor hypoxia is an important mechanism of radioresistance in hypofractionated radiotherapy and must be considered in the treatment planning process

David J. Carlson, Ph.D., Kamil M. Yenice, Ph.D., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 38, 6347 (2011); http://dx.doi.org/10.1118/1.3639137 (4 pages) | Cited 1 time

Online Publication Date: 9 November 2011

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Abstract Unavailable
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87.53.Jw Therapeutic applications, including brachytherapy
87.17.-d Cell processes
87.53.Bn Dosimetry/exposure assessment

RADIATION IMAGING PHYSICS: Robust automatic segmentation of multiple implanted cylindrical gold fiducial markers in cone-beam CT projections

Walther Fledelius, Esben Worm, Ulrik V. Elstrøm, Jørgen B. Petersen, Cai Grau, Morten Høyer, and Per R. Poulsen

Med. Phys. 38, 6351 (2011); http://dx.doi.org/10.1118/1.3658566 (11 pages) | Cited 5 times

Online Publication Date: 9 November 2011

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Purpose: Implanted fiducial markers, which are used to correct for day-to-day variations, may potentially also be used to correct for intrafraction motion measurements. However, before any treatment can make use of, and react to, the position of the inserted markers they have to be segmented, either manually through expert user intervention or automatically from an imaging system. In the current study, we aimed to establish a robust and autonomous segmentation method for implanted cylindrical gold markers in a single set of projections from a cone-beam computed tomography (CBCT).
Methods: Multiple cylindrical gold markers were segmented in the projection images of CBCT scans by five sequential steps. Initially, marker candidates were identified in all projections with a blob detection routine, and then traced in subsequent projections. Traces inconsistent with a 3D marker position were rejected, and the best remaining traces were identified and used for the construction of a 3D marker constellation model, consisting of the size, position and orientation of the markers. Finally, projections of the model were used to generate templates for the final template-based marker segmentation. Hereby, challenging situations such as overlap of markers and low contrast regions were taken into account. The segmentation method was tested in 63 CBCT scans from 11 patients with 2–4 cylindrical gold markers implanted in the prostate and for 62 CBCT scans from six patients each with 2–3 cylindrical gold markers implanted in the liver and up to two cylindrical markers placed externally on the abdomen. After segmentation all projections of the 125 scans were manually inspected, and a successful segmentation was registered if the segmented position was within the projection of the marker.
Results: For prostate markers, the segmentation was successful in 99.8% of the projections. For the liver patients, liver markers and external markers were segmented successfully in 99.9 and 99.8% of the projections, respectively. All markers were identified in the 3D marker constellation model. The most common source of segmentation error was low contrast and motion of markers relative to each other, which resulted in a discrepancy between the template and actual projection appearance during marker overlap. Markers were overlapping in 20, 2.7, and 0.1% of the projections for prostate, liver, and external, respectively.
Conclusions: We have successfully implemented a new method that, without prior knowledge on marker size, position, orientation, and number, autonomously segments cylindrical gold markers from CBCT projections with a high success rate, despite overlap, motion, and low contrast.
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87.57.Q- Computed tomography
87.57.nm Segmentation
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RADIATION THERAPY PHYSICS: Evaluations of an adaptive planning technique incorporating dose feedback in image-guided radiotherapy of prostate cancer

Han Liu and Qiuwen Wu

Med. Phys. 38, 6362 (2011); http://dx.doi.org/10.1118/1.3658567 (9 pages)

Online Publication Date: 9 November 2011

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Purpose: Online image guidance (IG) has been used to effectively correct the setup error and inter-fraction rigid organ motion for prostate cancer. However, planning margins are still necessary to account for uncertainties such as deformation and intra-fraction motion. The purpose of this study is to investigate the effectiveness of an adaptive planning technique incorporating offline dose feedback to manage inter-fraction motion and residuals from online correction.
Methods: Repeated helical CT scans from 28 patients were included in the study. The contours of prostate and organs-at-risk (OARs) were delineated on each CT, and online IG was simulated by matching center-of-mass of prostate between treatment CTs and planning CT. A seven beam intensity modulated radiation therapy (IMRT) plan was designed for each patient on planning CT for a total of 15 fractions. Dose distribution at each fraction was evaluated based on actual contours of the target and OARs from that fraction. Cumulative dose up to each fraction was calculated by tracking each voxel based on a deformable registration algorithm. The cumulative dose was compared with the dose from initial plan. If the deviation exceeded the pre-defined threshold, such as 2% of the D99 to the prostate, an adaptive planning technique called dose compensation was invoked, in which the cumulative dose distribution was fed back to the treatment planning system and the dose deficit was made up through boost radiation in future treatment fractions. The dose compensation was achieved by IMRT inverse planning. Two weekly compensation delivery strategies were simulated: one intended to deliver the boost dose in all future fractions (schedule A) and the other in the following week only (schedule B). The D99 to prostate and generalized equivalent uniform dose (gEUD) to rectal wall and bladder were computed and compared with those without the dose compensation.
Results: If only 2% underdose is allowed at the end of the treatment course, then 11 patients fail. If the same criteria is assessed at the end of each week (every five fractions), then 14 patients fail, with three patients failing the 1st or 2nd week but passing at the end. The average dose deficit from these 14 patients was 4.4%. They improved to 2% after the weekly compensation. Out of these 14 patients who needed dose compensation, ten passed the dose criterion after weekly dose compensation, three patients failed marginally, and one patient still failed the criterion significantly (10% deficit), representing 3.6% of the patient population. A more aggressive compensation frequency (every three fractions) could successfully reduce the dose deficit to the acceptable level for this patient. The average number of required dose compensation re-planning per patient was 0.82 (0.79) per patient for schedule A (B) delivery strategy. The doses to OARs were not significantly different from the online IG only plans without dose compensation.
Conclusions: We have demonstrated the effectiveness of offline dose compensation technique in image-guided radiotherapy for prostate cancer. It can effectively account for residual uncertainties which cannot be corrected through online IG. Dose compensation allows further margin reduction and critical organs sparing.
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87.53.Bn Dosimetry/exposure assessment
87.53.Jw Therapeutic applications, including brachytherapy
87.59.-e X-ray imaging
87.19.xj Cancer
87.57.Q- Computed tomography

RADIATION IMAGING PHYSICS: Virtual monochromatic imaging in dual-source dual-energy CT: Radiation dose and image quality

Lifeng Yu, Jodie A. Christner, Shuai Leng, Jia Wang, Joel G. Fletcher, and Cynthia H. McCollough

Med. Phys. 38, 6371 (2011); http://dx.doi.org/10.1118/1.3658568 (9 pages) | Cited 6 times

Online Publication Date: 9 November 2011

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Purpose: To evaluate the image quality of virtual monochromatic images synthesized from dual-source dual-energy computed tomography (CT) in comparison with conventional polychromatic single-energy CT for the same radiation dose.
Methods: In dual-energy CT, besides the material-specific information, one may also synthesize monochromatic images at different energies, which can be used for routine diagnosis similar to conventional polychromatic single-energy images. In this work, the authors assessed whether virtual monochromatic images generated from dual-source CT scanners had an image quality similar to that of polychromatic single-energy images for the same radiation dose. First, the authors provided a theoretical analysis of the optimal monochromatic energy for either the minimum noise level or the highest iodine contrast to noise ratio (CNR) for a given patient size and dose partitioning between the low- and high-energy scans. Second, the authors performed an experimental study on a dual-source CT scanner to evaluate the noise and iodine CNR in monochromatic images. A thoracic phantom with three sizes of attenuating rings was used to represent four adult sizes. For each phantom size, three dose partitionings between the low-energy (80 kV) and the high-energy (140 kV) scans were used in the dual-energy scan. Monochromatic images at eight energies (40 to 110 keV) were generated for each scan. Phantoms were also scanned at each of the four polychromatic single energy (80, 100, 120, and 140 kV) with the same radiation dose.
Results: The optimal virtual monochromatic energy depends on several factors: phantom size, partitioning of the radiation dose between low- and high-energy scans, and the image quality metrics to be optimized. With the increase of phantom size, the optimal monochromatic energy increased. With the increased percentage of radiation dose on the low energy scan, the optimal monochromatic energy decreased. When maximizing the iodine CNR in monochromatic images, the optimal energy was lower than that when minimizing noise level. When the total radiation dose was equally distributed between low and high energy in dual-energy scans, for minimum noise, the optimal energies were 68, 71, 74, and 77 keV for small, medium, large, and extra-large (xlarge) phantoms, respectively; for maximum iodine CNR, the optimal energies were 66, 68, 70, 72 keV. With the optimal monochromatic energy, the noise level was similar to and the CNR was better than that in a single-energy scan at 120 kV for the same radiation dose. Compared to an 80 kV scan, however, the iodine CNR in monochromatic images was lower for the small, medium, and large phantoms.
Conclusions: In dual-source dual-energy CT, optimal virtual monochromatic energy depends on patient size, dose partitioning, and the image quality metric optimized. With the optimal monochromatic energy, the noise level was similar to and the iodine CNR was better than that in 120 kV images for the same radiation dose. Compared to single-energy 80 kV images, the iodine CNR in virtual monochromatic images was lower for small to large phantom sizes.
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87.57.Q- Computed tomography
87.59.-e X-ray imaging
87.57.cm Noise

MAGNETIC RESONANCE PHYSICS: An improved model for describing the contrast bolus in perfusion MRI

Vishal Patil and Glyn Johnson

Med. Phys. 38, 6380 (2011); http://dx.doi.org/10.1118/1.3658570 (4 pages) | Cited 2 times

Online Publication Date: 9 November 2011

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Purpose: Quantification of perfusion measurements using dynamic, susceptibility-weighted contrast-enhanced (DSC) MRI depends on estimating the size and shape of the tracer bolus. Typically, the bolus is described as a gamma variate function (GV) fitted to the bolus portion of tracer concentration time curve (CTC). However, the last point to fit is arbitrary which can lead to considerable variation in the fitted curve in the presence of noise. In this technical note, we present a model which takes into account recirculation explicitly and fits robustly to the entire CTC in the presence of noise.
Methods: Signal data measurements from ten DSC MRI patients were fitted with our new model and a GV function using four different methods of estimating the end of the bolus. Estimates of the area under the curves (AUC) and first moments (FMs) of the bolus were compared at different noise levels.
Results: The new model gave errors similar to or smaller than those of the most effective methods for fitting a GV.
Conclusions: The single compartment recirculation (SCR) model is the most robust fitting technique with respect to noise both for bias and variability.
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87.61.Bj Theory and principles
02.50.Ng Distribution theory and Monte Carlo studies

RADIATION THERAPY PHYSICS: Four-dimensional magnetic resonance imaging (4D-MRI) using image-based respiratory surrogate: A feasibility study

Jing Cai, Zheng Chang, Zhiheng Wang, William Paul Segars, and Fang-Fang Yin

Med. Phys. 38, 6384 (2011); http://dx.doi.org/10.1118/1.3658737 (11 pages) | Cited 4 times

Online Publication Date: 9 November 2011

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Purpose: Four-dimensional computed tomography (4D-CT) has been widely used in radiation therapy to assess patient-specific breathing motion for determining individual safety margins. However, it has two major drawbacks: low soft-tissue contrast and an excessive imaging dose to the patient. This research aimed to develop a clinically feasible four-dimensional magnetic resonance imaging (4D-MRI) technique to overcome these limitations.
Methods: The proposed 4D-MRI technique was achieved by continuously acquiring axial images throughout the breathing cycle using fast 2D cine-MR imaging, and then retrospectively sorting the images by respiratory phase. The key component of the technique was the use of body area (BA) of the axial MR images as an internal respiratory surrogate to extract the breathing signal. The validation of the BA surrogate was performed using 4D-CT images of 12 cancer patients by comparing the respiratory phases determined using the BA method to those determined clinically using the Real-time position management (RPM) system. The feasibility of the 4D-MRI technique was tested on a dynamic motion phantom, the 4D extended Cardiac Torso (XCAT) digital phantom, and two healthy human subjects.
Results: Respiratory phases determined from the BA matched closely to those determined from the RPM: mean (±SD) difference in phase: −3.9% (±6.4%); mean (±SD) absolute difference in phase: 10.40% (±3.3%); mean (±SD) correlation coefficient: 0.93 (±0.04). In the motion phantom study, 4D-MRI clearly showed the sinusoidal motion of the phantom; image artifacts observed were minimal to none. Motion trajectories measured from 4D-MRI and 2D cine-MRI (used as a reference) matched excellently: the mean (±SD) absolute difference in motion amplitude: −0.3 (±0.5) mm. In the 4D-XCAT phantom study, the simulated “4D-MRI” images showed good consistency with the original 4D-XCAT phantom images. The motion trajectory of the hypothesized “tumor” matched excellently between the two, with a mean (±SD) absolute difference in motion amplitude of 0.5 (±0.4) mm. 4D-MRI was able to reveal the respiratory motion of internal organs in both human subjects; superior–inferior (SI) maximum motion of the left kidney of Subject #1 and the diaphragm of Subject #2 measured from 4D-MRI was 0.88 and 1.32 cm, respectively.
Conclusions: Preliminary results of our study demonstrated the feasibility of a novel retrospective 4D-MRI technique that uses body area as a respiratory surrogate.
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87.61.-c Magnetic resonance imaging
87.19.Wx Pneumodyamics, respiration
87.19.xj Cancer
87.50.cm Dosimetry/exposure assessment

RADIATION THERAPY PHYSICS: Pitfalls of tungsten multileaf collimator in proton beam therapy

Vadim Moskvin, Chee-Wai Cheng, and Indra J. Das

Med. Phys. 38, 6395 (2011); http://dx.doi.org/10.1118/1.3658655 (12 pages) | Cited 3 times

Online Publication Date: 10 November 2011

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Purpose: Particle beam therapy is associated with significant startup and operational cost. Multileaf collimator (MLC) provides an attractive option to improve the efficiency and reduce the treatment cost. A direct transfer of the MLC technology from external beam radiation therapy is intuitively straightforward to proton therapy. However, activation, neutron production, and the associated secondary cancer risk in proton beam should be an important consideration which is evaluated.
Methods: Monte Carlo simulation with FLUKA particle transport code was applied in this study for a number of treatment models. The authors have performed a detailed study of the neutron generation, ambient dose equivalent [H*(10)], and activation of a typical tungsten MLC and compared with those obtained from a brass aperture used in a typical proton therapy system. Brass aperture and tungsten MLC were modeled by absorber blocks in this study, representing worst-case scenario of a fully closed collimator.
Results: With a tungsten MLC, the secondary neutron dose to the patient is at least 1.5 times higher than that from a brass aperture. The H*(10) from a tungsten MLC at 10 cm downstream is about 22.3 mSv/Gy delivered to water phantom by noncollimated 200 MeV beam of 20 cm diameter compared to 14 mSv/Gy for the brass aperture. For a 30-fraction treatment course, the activity per unit volume in brass aperture reaches 5.3 × 104 Bq cm−3 at the end of the last treatment. The activity in brass decreases by a factor of 380 after 24 h, additional 6.2 times after 40 days of cooling, and is reduced to background level after 1 yr. Initial activity in tungsten after 30 days of treating 30 patients per day is about 3.4 times higher than in brass that decreases only by a factor of 2 after 40 days and accumulates to 1.2 × 106 Bq cm−3 after a full year of operation. The daily utilization of the MLC leads to buildup of activity with time. The overall activity continues to increase due to 179Ta with a half-life of 1.82 yr and thus require prolonged storage for activity cooling. The H*(10) near the patient side of the tungsten block is about 100 μSv/h and is 27 times higher at the upstream side of the block. This would lead to an accumulated dose for therapists in a year that may exceed occupational maximum permissible dose (50 mSv/yr). The value of H*(10) at the upstream surface of the tungsten block is about 220 times higher than that of the brass.
Conclusions: MLC is an efficient way for beam shaping and overall cost reduction device in proton therapy. However, based on this study, tungsten seems to be not an optimal material for MLC in proton beam therapy. Usage of tungsten MLC in clinic may create unnecessary risks associated with the secondary neutrons and induced radioactivity for patients and staff depending on the patient load. A careful selection of material for manufacturing of an optimal MLC for proton therapy is thus desired.
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87.56.nk Collimators
87.53.Bn Dosimetry/exposure assessment
87.53.Jw Therapeutic applications, including brachytherapy
87.55.kh Applications
87.56.jk Field shaping

THERMOTHERAPY PHYSICS: Bypassing absorbing objects in focused ultrasound using computer generated holographic technique

Y. Hertzberg and G. Navon

Med. Phys. 38, 6407 (2011); http://dx.doi.org/10.1118/1.3651464 (9 pages) | Cited 2 times

Online Publication Date: 10 November 2011

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Purpose: Focused ultrasound (FUS) technology is based on heating a small volume of tissue, while keeping the temperature outside the focus region with minimal heating only. Several FUS applications, such as brain and liver, suffer from the existence of ultrasound absorbers in the acoustic path between the transducer and the focus. These absorbers are a potential risk for the FUS therapy since they might cause to unwanted heating outside the focus region. An acoustic simulation based solution for reducing absorbers’ heating is proposed, demonstrated, and compared to the standard geometrical solution. The proposed solution uses 3D continuous acoustic holograms, generated by the Gerchberg–Saxton (GS) algorithm, which are described and demonstrated for the first time using ultrasound planar phased-array transducer.
Methods: Holograms were generated using the iterative GS algorithm and fast Fourier transform (FFT) acoustic simulation. The performances of the holograms are demonstrated by temperature elevation images of the absorber, acquired by GE 1.5T MRI scanner equipped with InSightec FUS planar phased-array transducer built out of 986 transmitting elements.
Results: The acoustic holographic technology is demonstrated numerically and experimentally using the three letters patterns, “T,” “A,” and “U,” which were manually built into 1 × 1 cm masks to represent the requested target fields. 3D holograms of a focused ultrasound field with a hole in intensity at the absorber region were generated and compared to the standard geometrical solution. The proposed holographic solution results in 76% reduction of heating on absorber, while keeping similar heating at the focus.
Conclusions: In the present work we show for the first time the generation of efficient and uniform continuous ultrasound holograms in 3D. We use the holographic technology to generate a FUS beams that bypasses an absorber in the acoustic path to reduce unnecessary heating and potential clinical risk. The developed technique is superior in performance and flexibility compared to the intuitive geometrical technique that is being used in clinical practice.
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87.50.yt Therapeutic applications
42.40.Jv Computer-generated holograms
87.57.-s Medical imaging
02.60.-x Numerical approximation and analysis
87.17.-d Cell processes
87.85.Ox Biomedical instrumentation and transducers, including micro-electro-mechanical systems (MEMS)
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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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