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Jun 2010

Volume 37, Issue 6, pp. 2401-3495

Spotlight Figure

Med. Phys. 37, 2414 (2010); doi:10.1118/1.3395554 (11 pages)

Martina Marinelli, Axel Martinez-Möller, Brian Jensen, Vincenzo Positano, Susanne Weismüller, Nassir Navab, Luigi Landini, Markus Schwaiger, and Stephan G. Nekolla
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POINT/COUNTERPOINT: Radiation therapists should not have to wear personnel dosimetry badges

Scott Dube, M.S., R. Paul King, M.S., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 37, 2401 (2010); doi:10.1118/1.3371682 (3 pages) | Cited 1 time

Online Publication Date: 5 May 2010

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Abstract Unavailable
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87.55.dk Dose-volume analysis
87.55.N- Radiation monitoring, control, and safety

RADIATION THERAPY PHYSICS: Investigation of three radiation detectors for accurate measurement of absorbed dose in nonstandard fields

Eunah Chung, Hugo Bouchard, and Jan Seuntjens

Med. Phys. 37, 2404 (2010); doi:10.1118/1.3392247 (10 pages) | Cited 6 times

Online Publication Date: 5 May 2010

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Purpose: To establish accurate experimental dosimetry techniques for reference dose measurements in nonstandard composite fields.
Methods: A cylindrical PMMA phantom filled with water was constructed, at the center of which reference absorbed dose to water for a head and neck IMRT delivery was measured. Based on the proposed new formalism for reference dosimetry of nonstandard fields [ Alfonso et al., Med. Phys. 35, 5179–5186 (2008) ], a candidate plan-class specific reference (pcsr) field for a typical head and neck IMRT delivery was created on the CT images of the phantom. The absorbed dose to water in the pcsr field normalized to that in a reference 10×10 cm2 field was measured using three radiation detectors: Gafchromic® EBT films, a diamond detector, and a guarded liquid-filled ionization chamber developed in-house (GLIC-03). Pcsr correction factors kQpcsr,Qfpcsr,fref were determined for five different types of air-filled ionization chambers (Exradin A12, NE2571, Exradin A1SL, Exradin A14, and PinPoint® 31006) in a fully rotated delivery and in a delivery with the same MLC settings and weights but from a single gantry angle (a collapsed delivery).
Results: The combined standard uncertainty in measuring the correction factor kQpcsr,Qfpcsr,fref using the three dosimetry techniques was 0.3%. For all the air-filled ionization chambers and the pcsr field tested, the correction factor was not different from unity by more than ±0.8%. For the fully rotated delivery, the correction factors were in a narrow range of 0.9955–0.9986, while in the collapsed delivery, they were in a slightly broader range of 0.9922–1.0048. In the collapsed delivery, the Farmer-type chambers (Exradin A12 and NE2571) had very similar correction factors (0.9922 and 0.9931, respectively), whereas the correction factors for the smaller chambers showed more distinct chamber-type dependence.
Conclusions: The authors have established three experimental dosimetry techniques that allow reference measurements of nonstandard field correction factors kQpcsr,Qfpcsr,fref for air-filled ionization chambers at the 0.3% 1σ uncertainty level. These techniques can be used to determine criteria for the selection of plan-class specific reference fields and ultimately improve clinical reference dosimetry of nonstandard fields.
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87.50.wj Dosimetry/exposure assessment
29.40.Cs Gas-filled counters: ionization chambers, proportional, and avalanche counters
87.50.wp Therapeutic applications
87.55.dk Dose-volume analysis
87.57.Q- Computed tomography

NUCLEAR MEDICINE PHYSICS: Registration of myocardial PET and SPECT for viability assessment using mutual information

Martina Marinelli, Axel Martinez-Möller, Brian Jensen, Vincenzo Positano, Susanne Weismüller, Nassir Navab, Luigi Landini, Markus Schwaiger, and Stephan G. Nekolla

Med. Phys. 37, 2414 (2010); doi:10.1118/1.3395554 (11 pages) | Cited 1 time

Online Publication Date: 5 May 2010

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Purpose: The combination of sequentially acquired cardiac PET and SPECT data integrating metabolic and perfusion information allows the assessment of myocardial viability, a relevant clinical parameter for the management of patients who have suffered myocardial infarction and are now candidates for complex and cost intensive therapies such as bypass surgery. However, registration of cardiac functional datasets acquired on different imaging systems is limited by the difficulty to define anatomical landmarks and by the relatively poor inherent spatial resolution. In this article, the authors sought to evaluate whether it is possible to automatically register FDG-PET and sestamibi-SPECT cardiac data.
Methods: Automatic rigid registration was implemented with the ITK framework using Mattes mutual information as the similarity measure and a quaternion to represent the rotational component. The goodness of the alignment was evaluated by computing the mean target registration error (mTRE) at the myocardial wall. The registration parameters were optimized for robustness and speed using the data from 11 cardiac patients undergoing both PET and SPECT examinations (training datasets). The optimized algorithm was applied on the PET and SPECT data from 11 further patients (evaluation datasets). Quantitative (mTRE calculation) and visual (scoring method) comparisons were performed between automatic and manual registrations. Moreover, the automatic registration was also compared to the registration implicitly defined in the standard clinical analysis.
Results: The registration parameters were successfully optimized and resulted in a mean mTRE of 1.13 mm and 1.2 s average runtime on standard computer hardware for the training datasets. Automatic registration in the 11 validation datasets resulted in an average mTRE of 2.3 mm, with 7.5 mm mTRE in the worst case and an average runtime of 1.6 s. Automatic registration outperformed manual registrations both for the mTRE and for the visual assessment. Automatic registration also resulted in higher accuracy and better visual assessment as compared to the registration implicitly performed in the standard clinical analysis.
Conclusions: The results demonstrate the possibility to successfully perform mutual information based registration of PET and SPECT cardiac data, allowing an improved workflow for the sequentially acquired cardiac datasets, in general, and specifically for the assessment of myocardial viability.
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87.57.uk Positron emission tomography (PET)
87.57.uh Single photon emission computed tomography (SPECT)
87.57.nj Registration
02.60.Pn Numerical optimization

RADIATION MEASUREMENT PHYSICS: A field size specific backscatter correction algorithm for accurate EPID dosimetry

Sean L. Berry, Cynthia S. Polvorosa, and Cheng-Shie Wuu

Med. Phys. 37, 2425 (2010); doi:10.1118/1.3400043 (10 pages) | Cited 3 times

Online Publication Date: 5 May 2010

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Purpose: Portal dose images acquired with an amorphous silicon electronic portal imaging device (EPID) suffer from artifacts related to backscattered radiation. The backscatter signal varies as a function of field size (FS) and location on the EPID. Most current portal dosimetry algorithms fail to account for the FS dependence. The ramifications of this omission are investigated and solutions for correcting the measured dose images for FS specific backscatter are proposed.
Methods: A series of open field dose images were obtained for field sizes ranging from 2×2 to 30×40 cm2. Each image was analyzed to determine the amount of backscatter present. Two methods to account for the relationship between FS and backscatter are offered. These include the use of discrete FS specific correction matrices and the use of a single generalized equation. The efficacy of each approach was tested on the clinical dosimetric images for ten patients, 49 treatment fields. The fields were evaluated to determine whether there was an improvement in the dosimetric result over the commercial vendor’s current algorithm.
Results: It was found that backscatter manifests itself as an asymmetry in the measured signal primarily in the inplane direction. The maximum error is approximately 3.6% for 10×10 and 12.5×12.5 cm2 field sizes. The asymmetry decreased with increasing FS to approximately 0.6% for fields larger than 30×30 cm2. The dosimetric comparison between the measured and predicted dose images was significantly improved (p⪡.001) when a FS specific backscatter correction was applied. The average percentage of points passing a 2%, 2 mm gamma criteria increased from 90.6% to between 96.7% and 97.2% after the proposed methods were employed.
Conclusions: The error observed in a measured portal dose image depends on how much its FS differs from the 30×40 cm2 calibration conditions. The proposed methods for correcting for FS specific backscatter effectively improved the ability of the EPID to perform dosimetric measurements. Correcting for FS specific backscatter is important for accurate EPID dosimetry and can be carried out using the methods presented within this investigation.
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87.55.dk Dose-volume analysis

RADIATION THERAPY PHYSICS: Verification of MLC based real-time tumor tracking using an electronic portal imaging device

Sarah Han-Oh, Byong Yong Yi, Fritz Lerma, Barry L. Berman, Minzhi Gui, and Cedric Yu

Med. Phys. 37, 2435 (2010); doi:10.1118/1.3425789 (6 pages)

Online Publication Date: 6 May 2010

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Purpose: The authors have developed a novel technique using an electronic portal imaging device (EPID) to verify the geometrical accuracy of delivery of dose-rate-regulated tracking (DRRT). This technique, called verification of real-time tracking with EPID (VORTE), can potentially be used for both on-line and off-line quality assurance (QA) of MLC-based dynamic tumor tracking.
Methods: The shape and position of target as a function of time, which is assumed to be known, is projected onto the EPID plane. This projected sequence of apertures as a function of time (target motion) is then used as the reference. The accuracy of dynamic MLC tracking can then be assessed by how well the delivered beam follows this projected target motion without the use of a physical moving phantom. The beam apertures controlled by DRRT (aperture motion) is detected by the EPID as a function of time. The aperture motion is compared to the target motion to evaluate tracking error introduced by DRRT. The accuracy of VORTE was measured using film measurements of ten static fields. The VORTE for dynamic tumor tracking was tested with several target motions, including (1) rigid-body two-dimensional (2-D) cyclic motion in the superior-inferior direction with various period and amplitude; (2) the above 2-D cyclic motion plus cyclic deformation; and (3) 2-D cyclic motion with both deformation and rotation. For each target motion, the controlled aperture motion resulting from DRRT was acquired at ∼ 8 Hz using EPID in the continuous-acquisition mode. Leaf positions in all captured frames were measured from the EPID and compared to their expected positions. The passing rate of 2 mm criteria for all leaves from all frames was calculated for each of the four patterns of tumor motion. Additionally, the root-mean-square (RMS) deviations of the centroid of the apertures between the designed and delivered beams were calculated for all three cases.
Results: The accuracy of MLC-leaf position determination by VORTE is 0.5 mm (1 standard deviation) by comparison to film measurements. With DRRT, the passing rates using the 2 mm criteria for all acquired frames are 100% for the 2-D displacement, 99% for the 2-D displacement with deformation, and 88% for the 2-D displacement combined with both deformation and rotation. The RMS deviations are 0.6 mm for the 2-D displacement, 1.0 mm for the 2-D displacement with deformation, and 1.1 mm for the 2-D displacement combined with both deformation and rotation.
Conclusions: The VORTE can measure the accuracy of MLC-based tumor tracking without the necessity of employing a moving phantom. Moreover, it can be used for complex target motion (i.e., 2-D displacement combined with deformation and rotation) that is difficult to create with physical moving phantoms. Therefore, the VORTE and the novel QA process illustrated by this study have a great potential for verifying real-time tumor tracking.
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87.59.-e X-ray imaging
87.57.U- Nuclear medicine imaging
87.55.dk Dose-volume analysis
87.56.J- Collimation

RADIATION THERAPY PHYSICS: Dose correction strategy for the optimization of volumetric modulated arc therapy

Pengpeng Zhang, Jie Yang, Margie Hunt, and Gig Mageras

Med. Phys. 37, 2441 (2010); doi:10.1118/1.3426001 (4 pages) | Cited 1 time

Online Publication Date: 6 May 2010

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Purpose: Dose calculation during optimization of volumetric modulated arc therapy (VMAT) is necessarily simplified to keep computation time manageably low; however the approximations used in the scatter dose calculation lead to discrepancy with more accurate dose calculation following optimization. The purpose of this study is to develop a dose correction strategy in optimization that can minimize the disagreement.
Methods: VMAT delivery is modeled using a number of static equispaced beams. Dose correction factors (Cij) are associated with each beam i and point j inside the region of interest. Cij is calculated as the ratio of dose obtained from the full scatter dose calculation over that from the partial scatter dose calculation in optimization. VMAT optimization algorithm is a multiple resolution approach. The dose correction factors are calculated at the beginning of each resolution and applied as multiplicative corrections to the partial scatter dose during optimization. Clinical cases for brain, prostate, paraspinal, and esophagus are utilized to evaluate the method. Treatment plans created with and without the correction scheme are normalized such that the complication rates of organs at risk (OARs) are comparable. The resulting planning target volume (PTV) mean doses are used to compare plan quality.
Results: The difference between the dose calculated at the end of optimization and at the end of the final forward dose calculation is reduced from 7% and 5% for the PTV and OAR mean doses without correction to approximately 1% with correction. Applying dose correction during optimization saves planners 2–4 h in average in treatment planning, and has a positive impact on plan quality, evidenced by a noticeably higher PTV mean dose: 2.1%, 2.4%, 0.5%, and 9.3% of the corresponding prescription dose in the brain, esophagus, prostate, and paraspinal cases, respectively.
Conclusions: When dose correction is applied during optimization, dose discrepancies between optimization and full dose calculation are reduced. Integrating dose correction in VMAT optimization allows planners to adjust the optimization constraints more easily and confidently during optimization and has the potential to improve plan quality.
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87.55.dk Dose-volume analysis
87.53.Jw Therapeutic applications, including brachytherapy

RADIATION THERAPY PHYSICS: Treatment plans optimization for contrast-enhanced synchrotron stereotactic radiotherapy

M. Edouard, D. Broggio, Y. Prezado, F. Estève, H. Elleaume, and J. F. Adam

Med. Phys. 37, 2445 (2010); doi:10.1118/1.3327455 (12 pages) | Cited 6 times

Online Publication Date: 6 May 2010

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Purpose: Synchrotron stereotactic radiotherapy (SSRT) is a treatment that involves the targeting of high-Z elements into tumors followed by stereotactic irradiation with monochromatic x-rays from a synchrotron source, tuned at an optimal energy. The irradiation geometry, as well as the secondary particles generated at a higher yield by the medium energy x-rays on the high-Z atoms (characteristic x-rays, photoelectrons, and Auger electrons), produces a localized dose enhancement in the tumor. Iodine-enhanced SSRT with systemic injections of iodinated contrast agents has been successfully developed in the past six years in the team, and is currently being transferred to clinical trials. The purpose of this work is to study the impact on the SSRT treatment of the contrast agent type, the beam quality, the irradiation geometry, and the beam weighting for defining an optimized SSRT treatment plan.
Methods: Theoretical dosimetry was performed using the MCNPX particle transport code. The simulated geometry was an idealized phantom representing a human head. A virtual target was positioned in the central part of the phantom or off-centered by 4 cm. The authors investigated the dosimetric characteristics of SSRT for various contrast agents: Iodine, gadolinium, and gold; and for different beam qualities: Monochromatic x-ray beams from a synchrotron source (30–120 keV), polychromatic x-ray beams from an x-ray tube (80, 120, and 180 kVp), and a 6 MV x-ray beam from a linear accelerator. Three irradiation geometries were studied: One arc or three noncoplanar arcs dynamic arc therapy, and an irradiation with a finite number of beams. The resulting dose enhancements, beam profiles, and histograms dose volumes were compared for iodine-enhanced SSRT. An attempt to optimize the irradiation scheme by weighing the finite x-ray beams was performed. Finally, the optimization was studied on patient specific 3D CT data after contrast agent infusion.
Results: It was demonstrated in this study that an 80 keV beam energy was a good compromise for treating human brain tumors with iodine-enhanced SSRT, resulting in a still high dose enhancement factor (about 2) and a superior bone sparing in comparison with lower energy x-rays. This beam could easily be produced at the European Synchrotron Radiation Facility medical beamline. Moreover, there was a significant diminution of dose delivered to the bone when using monochromatic x-rays rather than polychromatic x-rays from a conventional tube. The data showed that iodine SSRT exhibits a superior sparing of brain healthy tissue in comparison to high energy treatment. The beam weighting optimization significantly improved the treatment plans for off-centered tumors, when compared to nonweighted irradiations.
Conclusions: This study demonstrated the feasibility of realistic clinical plans for low energy monochromatic x-rays contrast-enhanced radiotherapy, suitable for the first clinical trials on brain metastasis with a homogeneous iodine uptake.
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87.53.Jw Therapeutic applications, including brachytherapy
87.55.dk Dose-volume analysis
87.53.Bn Dosimetry/exposure assessment
87.56.-v Radiation therapy equipment
87.59.-e X-ray imaging

RADIATION BIOLOGY: Significant dose can be lost by extended delivery times in IMRT with x rays but not high-LET radiations

Michael C. Joiner, Nagaraju Mogili, Brian Marples, and Jay Burmeister

Med. Phys. 37, 2457 (2010); doi:10.1118/1.3425792 (9 pages) | Cited 3 times

Online Publication Date: 6 May 2010

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Purpose: To experimentally simulate IMRT delivery using two human cell models in vitro and test the hypothesis that a loss in effective dose resulting from the prolongation of megavoltage x-ray treatment delivery time would be greatly reduced in corresponding IMRT simulations using higher-LET radiation.
Methods: The effect of prolonging the delivery time of a treatment fraction was investigated in vitro using human PC-3 prostate and HGL21 glioblastoma tumor cell lines. Cells were irradiated with x rays from a conventional linear accelerator or with neutrons from a clinical d(48.5)+Be radiotherapy beam and maintained at 37 °C throughout. The delivery time for six closely spaced doses, simulating six multiple-port segments, was varied from acute to 60 min for x-ray irradiation, and acute to 120 min for neutron irradiation. Cell survival was measured following summed doses for the six segments of 0.5–6 Gy for x rays and 0.16–2 Gy for neutrons, covering the most likely range of dose per fraction used in clinical radiotherapy.
Results: Prolonging x-ray delivery time (from initiation of segment 1 to initiation of segment 6) from 5 to 45 min resulted in a loss in effective total dose (in equivalent 2 Gy multifraction treatments) of 5.6% in the PC-3 cell line and 11.7% in the HGL21 cell line. More clinically common prolongations of 5–30 and 5–15 min resulted in effective dose reductions of 3.8% and 1.7% for PC-3, and 7.3% and 2.9% for HGL21. A loss of less than 0.5% in effective dose was observed for prolongations up to 45 min of similarly effective neutron irradiation of PC-3 and HGL21 cells.
Conclusions: Prolonged delivery times of photon fractions could have a significant impact on treatment outcome especially for tumors with a low α/β ratio and short repair halftime. These effects are significant at delivery times commonly associated with IMRT and are variable with cell type. X-ray IMRT should therefore always be planned to minimize dose-fraction delivery time. However, if IMRT treatments are delivered with high-LET radiation, this considerably reduces the dependence of the biological effect on fraction delivery time even out to 2 h.
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87.53.Bn Dosimetry/exposure assessment
87.59.-e X-ray imaging
87.53.Jw Therapeutic applications, including brachytherapy

RADIATION IMAGING PHYSICS: A method to correct the influence of carbon fiber couchtop and patient positioning device on image quality of cone beam CT

Kuo Men, Jianrong Dai, Minghui Li, and Yin Zhang

Med. Phys. 37, 2466 (2010); doi:10.1118/1.3425999 (7 pages) | Cited 2 times

Online Publication Date: 10 May 2010

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Purpose: To evaluate the influence of carbon fiber couchtop and patient positioning devices on cone beam CT (CBCT) image quality and develop an effective method to correct the influence.
Methods: A standard CT phantom (Catphan 500) was used to evaluate the influence of iBeam evo carbon fiber couchtop on the quality of CBCT image obtained from an Elekta synergy machine. The evaluation indices were contrast-to-noise ratio (CNR), spatial resolution, image uniformity, and image noise. With using the Beer–Lambert law and the energy-response function of the flat-panel imager, a method was applied to deduct the image signal of the couchtop (and the positioning devices) from each projection image of a phantom/patient, and then used all corrected projection images to reconstruct a CBCT image. The performance of the correction method was evaluated using the CBCT images of a Catphan 500 phantom, a head-and-neck cancer patient, and a prostate cancer patient.
Results: In two phantom studies (the phantom to simulate a human head and neck and the one to simulate a human body), the CNR of the CBCT images obtained with the couchtop reduced 18.1% and 29.8%, respectively with respect to those obtained without the couchtop; meanwhile, the image uniformity reduced 16.4% and 24.1% due to the use of the carbon fiber couchtop. The couchtop also induced extra image noise (16.5% for the h&n phantom and 4.2% for the body phantom). However, CBCT imaging with the couchtop did not affect the spatial resolution. After applying the projection image correction, there was a significant improvement in CNR (by 19.5% and 25.8%), image uniformity (by 9.2% and 13.1%), and image noise (by 10.2% and 3.9%), with respect to CBCT images obtained with the couchtop.
Conclusions: The presence of the carbon fiber couchtop and the patient positioning devices can significantly impair CBCT image quality in terms of the CNR, the image uniformity, and the image noise. By removing the influence of the couchtop and the patient-positioning devices from CB projection images, the correction method improves CBCT image quality and thus image guidance in radiotherapy.
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87.59.-e X-ray imaging
87.57.cf Spatial resolution
87.57.cm Noise
87.57.Q- Computed tomography
87.19.xj Cancer

MEDICAL PHYSICS LETTERS: Radiation dose efficiency comparison between differential phase contrast CT and conventional absorption CT

Joseph Zambelli, Nicholas Bevins, Zhihua Qi, and Guang-Hong Chen

Med. Phys. 37, 2473 (2010); doi:10.1118/1.3425785 (7 pages) | Cited 7 times

Online Publication Date: 10 May 2010

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Purpose: The superior radiation dose efficiency of a newly implemented differential phase contrast CT imaging method compared to the conventional absorption CT method is demonstrated.
Methods: A differential phase contrast CT imaging method has recently been implemented using conventional x-ray sources with a grating interferometer consisting of three gratings. This approach offers the possibility of simultaneous reconstruction of both attenuation contrast and phase contrast images from a single acquisition. This enables a direct comparison of radiation dose efficiency of both types of reconstructed images under identical conditions. Radiation dose efficiency was studied by measuring the change in contrast-to-noise ratio (CNR) with different exposure levels. A physical phantom of 28.5 mm diameter was constructed and used for measurement of CNR in both the absorption and phase contrast CT images, which were reconstructed from the same data set.
Results: For three of the four materials studied, at any given exposure level, the CNR of the differential phase contrast CT images was superior to that of the corresponding absorption contrast CT images. The most dramatic improvement was noted in the contrast between PMMA and water, where the CNR was improved by a factor of approximately 5.5 in the differential phase contrast CT images. Additionally, the CNR of phase contrast CT is empirically shown to have the same square root dependence on exposure, as is the case for absorption contrast CT.
Conclusions: The differential phase contrast CT method provided higher CNR than conventional absorption CT at equivalent dose levels for most of the materials studied, and so may enable achievement of the same object visibility as conventional absorption CT methods at a lower exposure level.
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87.57.Q- Computed tomography
87.57.nf Reconstruction
87.59.-e X-ray imaging
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