-
A. Adare,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
A. Angerami,
K. Aoki,
N. Apadula,
Y. Aramaki, [......],
R. Yang,
A. Yanovich,
J. Ying,
S. Yokkaichi,
Z. You,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
S. Zhou
[show abstract]
[hide abstract]
ABSTRACT: The PHENIX collaboration at the Relativistic Heavy Ion Collider (RHIC)
reports measurements of azimuthal dihadron correlations near midrapidity in
d+Au collisions at sqrt(s_NN)=200 GeV. These measurements complement recent
analyses by experiments at the Large Hadron Collider (LHC) involving central
p+Pb collisions at sqrt(s_NN)=5.02 TeV, which have indicated strong anisotropic
long-range correlations in angular distributions of hadron pairs. The origin of
these anisotropies is currently unknown. Various competing explanations include
parton saturation and hydrodynamic flow. We observe qualitatively similar
anisotropies at RHIC to those seen at the LHC, and when both are divided by an
estimate of the initial-state eccentricity, the anisotropies follow a common
multiplicity scaling. This scaling is also found to extend to heavy ion data at
RHIC and the LHC, where the anisotropies are widely thought to be due to
hydrodynamic flow. The results presented here, at much lower collision energy
and with a deuteron projectile (instead of a proton), provide important new
information for understanding the origin of these new long-range correlations.
03/2013;
-
A Adare,
C Aidala,
N N Ajitanand,
Y Akiba,
H Al-Bataineh,
J Alexander,
A Angerami,
K Aoki,
N Apadula,
Y Aramaki, [......],
R Yang,
A Yanovich,
J Ying,
S Yokkaichi,
Z You,
G R Young,
I Younus,
I E Yushmanov,
W A Zajc,
S Zhou
[show abstract]
[hide abstract]
ABSTRACT: The PHENIX experiment has measured electrons and positrons at midrapidity from the decays of hadrons containing charm and bottom quarks produced in d+Au and p+p collisions at sqrt[s_{NN}]=200 GeV in the transverse-momentum range 0.85≤p_{T}^{e}≤8.5 GeV/c. In central d+Au collisions, the nuclear modification factor R_{dA} at 1.5<p_{T}<5 GeV/c displays evidence of enhancement of these electrons, relative to those produced in p+p collisions, and shows that the mass-dependent Cronin enhancement observed at the Relativistic Heavy Ion Collider extends to the heavy D meson family. A comparison with the neutral-pion data suggests that the difference in cold-nuclear-matter effects on light- and heavy-flavor mesons could contribute to the observed differences between the π^{0} and heavy-flavor-electron nuclear modification factors R_{AA}.
Physical Review Letters 12/2012; 109(24):242301. · 7.37 Impact Factor
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
R. Akimoto,
H. Al-Bataineh,
H. Al-Ta'ani,
J. Alexander,
A. Angerami, [......],
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
A. Zelenski,
C. Zhang,
S. Zhou,
J. Zimányi,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: The jet fragmentation function is measured with direct photon-hadron
correlations in p+p and Au+Au collisions at sqrt(s_NN)=200 GeV. The p_T of the
photon is an excellent approximation to the initial p_T of the jet and the
ratio z_T=p_T^h/p_T^\gamma is used as a proxy for the jet fragmentation
function. A statistical subtraction is used to extract the direct photon-hadron
yields in Au+Au collisions while a photon isolation cut is applied in p+p. I_
AA, the ratio of jet fragment yield in Au+Au to that in p+p, indicates
modification of the jet fragmentation function. Suppression, most likely due to
energy loss in the medium, is seen at high z_T. The fragment yield at low z_T
is enhanced at large angles. Such a trend is expected from redistribution of
the lost energy into increased production of low-momentum particles.
12/2012;
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
R. Akimoto,
H. Al-Bataineh,
H. Al-Ta'ani,
J. Alexander,
K. R. Andrews, [......],
Z. You,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
A. Zelenski,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: The three Upsilon states, Upsilon(1S+2S+3S), are measured in d+Au and p+p
collisions at sqrt(s_NN)=200 GeV and rapidities 1.2<|y|<2.2 by the PHENIX
experiment at the Relativistic Heavy-Ion Collider. Cross sections for the
inclusive Upsilon(1S+2S+3S) production are obtained. The inclusive yields per
binary collision for d+Au collisions relative to those in p+p collisions
(R_dAu) are found to be 0.62 +/- 0.26 (stat) +/- 0.13 (syst) in the gold-going
direction and 0.91 +/- 0.33 (stat) +/- 0.16 (syst) in the deuteron-going
direction. The measured results are compared to a nuclear-shadowing model,
EPS09 [JHEP 04, 065 (2009)], combined with a final-state breakup cross section,
sigma_br, and compared to lower energy p+A results. We also compare the results
to the PHENIX J/psi results [Phys. Rev. Lett. 107, 142301 (2011)]. The rapidity
dependence of the observed Upsilon suppression is consistent with lower energy
p+A measurements.
11/2012;
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
H. Al-Ta'ani,
A. Angerami,
K. Aoki, [......],
S. Yokkaichi,
G. R. Young,
I. Younus,
Z. You,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: We previously reported [Phys. Rev. D 82, 112008 (2010)] measurements of
transverse single-spin asymmetries, A_N, in J/psi production from transversely
polarized p+p collisions at sqrt(s)=200 GeV with data taken by the PHENIX
experiment at the Relativistic Heavy Ion Collider in 2006 and 2008.
Subsequently, we have found errors in the analysis procedures for the 2008
data, which resulted in an erroneous value for the extracted A_N. The errors
affected the sorting of events into the correct left/right and forward/backward
bins. This produced an incorrect value for the 2008 result, but the 2006 result
is unaffected. We have conducted two independent reanalyses with these errors
corrected, and we present here the corrected values for the 2008 data and the
combined results for 2006 and 2008. The new combined spin asymmetry in the
forward region is A_N = -0.026+/-0.026(stat)+/-0.003(sys). Since this asymmetry
is consistent with zero, we no longer claim that our results suggest a possible
non-zero trigluon correlation function in transversely polarized protons.
10/2012;
-
S Afanasiev,
C Aidala,
N N Ajitanand,
Y Akiba,
A Al-Jamel,
J Alexander,
K Aoki,
L Aphecetche,
R Armendariz,
S H Aronson, [......],
A Yanovich,
S Yokkaichi,
G R Young,
I Younus,
I E Yushmanov,
W A Zajc,
O Zaudtke,
C Zhang,
J Zimányi,
L Zolin
[show abstract]
[hide abstract]
ABSTRACT: We report the measurement of direct photons at midrapidity in Au+Au collisions at sqrt[s_{NN}]=200 GeV. The direct photon signal was extracted for the transverse momentum range of 4 GeV/c<p_{T}<22 GeV/c, using a statistical method to subtract decay photons from the inclusive photon sample. The direct photon nuclear modification factor R_{AA} was calculated as a function of p_{T} for different Au+Au collision centralities using the measured p+p direct photon spectrum and compared to theoretical predictions. R_{AA} was found to be consistent with unity for all centralities over the entire measured p_{T} range. Theoretical models that account for modifications of initial direct photon production due to modified parton distribution functions in Au and the different isospin composition of the nuclei predict a modest change of R_{AA} from unity. They are consistent with the data. Models with compensating effects of the quark-gluon plasma on high-energy photons, such as suppression of jet-fragmentation photons and induced-photon bremsstrahlung from partons traversing the medium, are also consistent with this measurement.
Physical Review Letters 10/2012; 109(15):152302. · 7.37 Impact Factor
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
K. Aoki,
L. Aphecetche,
J. Asai, [......],
J. Ying,
S. Yokkaichi,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: The energy dependence of the single-transverse-spin asymmetry, A_N, and the
cross section for neutron production at very forward angles were measured in
the PHENIX experiment at RHIC for polarized p+p collisions at sqrt(s)=200 GeV.
The neutrons were observed in forward detectors covering an angular range of up
to 2.2 mrad. We report results for neutrons with momentum fraction of x_F=0.45
to 1.0. The energy dependence of the measured cross sections were consistent
with x_F scaling, compared to measurements by an ISR experiment which measured
neutron production in unpolarized p+p collisions at sqrt(s)=30.6--62.7 GeV. The
cross sections for large x_F neutron production for p+p collisions, as well as
those in e+p collisions measured at HERA, are described by a pion exchange
mechanism. The observed forward neutron asymmetries were large, reaching
A_N=-0.08+/-0.02 for x_F=0.8; the measured backward asymmetries, for negative
x_F, were consistent with zero. The observed asymmetry for forward neutron
production is discussed within the pion exchange framework, with interference
between the spin-flip amplitude due to the pion exchange and nonflip amplitudes
from all Reggeon exchanges. Within the pion exchange description, the measured
neutron asymmetry is sensitive to the contribution of other Reggeon exchanges
even for small amplitudes.
09/2012;
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
K. Aoki,
L. Aphecetche,
R. Armendariz, [......],
S. Yokkaichi,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
C. Zhang,
S. Zhou,
J. Zimanyi,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: Heavy-flavor production in p + p collisions is a good test of perturbative-quantumchromodynamics
(pQCD) calculations. Modification of heavy-flavor production in heavy-ion collisions relative
to binary-collision scaling from p + p results, quantified with the nuclear-modification factor (RAA), provides
information on both cold- and hot-nuclear-matter effects. Midrapidity heavy-flavor RAA measurements at the Relativistic Heavy Ion Collider have challenged parton-energy-loss models and resulted in upper limits on the viscosity-entropy ratio that are near the quantum lower bound. Such measurements have not been made in the forward-rapidity region. The charm-production cross section in p + p collisions at √s = 200 GeV, integrated over pT and in the rapidity range 1.4 < y < 1.9, is found to be dσc ¯ c/dy = 0.139 ± 0.029 (stat) +0.051 −0.058 (syst) mb. This result is consistent with a perturbative fixed-order-plus-next-to-leading-log calculation within scale uncertainties and is also consistent with expectations based on the corresponding midrapidity charm-production cross section measured by PHENIX. The RAA for heavy-flavor muons in Cu + Cu collisions is measured in three centrality bins for 1 < pT < 4 GeV/c. Suppression relative to binary-collision scaling (RAA < 1) increases with centrality. Within experimental and theoretical uncertainties, the measured charm yield in p + p collisions is consistent with state-of-the-art pQCD calculations. Suppression in central Cu + Cu collisions suggests the presence of significant cold-nuclear-matter effects and final-state energy loss.
Physical Review C 08/2012; 86:024909. · 3.31 Impact Factor
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
K. Aoki,
Y. Aramaki,
E. T. Atomssa, [......],
J. Ying,
S. Yokkaichi,
Z. You,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: The PHENIX experiment has measured the production of neutral pions in Au+Au
collisions at sqrt(s_NN)=200 GeV. The new data offer a fourfold increase in
recorded luminosity, providing higher precision and a larger reach in
transverse momentum, p_T, to 20 GeV/c. The production ratio of eta/pi^0 is
0.46+/-0.01(stat)+/-0.05(syst), constant with p_T and collision centrality. The
observed ratio is consistent with earlier measurements, as well as with the p+p
and d+Au values. The production of pi^0 is suppressed by a factor of 5, as in
earlier findings. However, with the improved statistical precision a small but
significant rise of the nuclear modification factor, R_AA, vs p_T, with a slope
of 0.0106+/-^(0.0034)_(0.0029)[GeV/c]^-1, is discernible in central collisions.
A phenomenological extraction of the average fractional parton energy loss
shows a decrease with increasing p_T. To study the path length dependence of
suppression, the pi^0 yield was measured at different angles with respect to
the event plane; a strong azimuthal dependence of the pi^0 R_AA is observed.
The data are compared to theoretical models of parton energy loss as a function
of the path length, L, in the medium. Models based on pQCD are insufficient to
describe the data, while a hybrid model utilizing pQCD for the hard
interactions and AdS/CFT for the soft interactions is consistent with the data.
08/2012;
-
A. Adare,
S. S. Adler,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
A. Al-Jamel,
J. Alexander,
A. Angerami, [......],
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
C. Zhang,
S. Zhou,
J. Zimányi,
L. Zolin,
X. Zong
[show abstract]
[hide abstract]
ABSTRACT: Direct photons have been measured in sqrt(s_NN)=200 GeV d+Au collisions at
midrapidity. A wide p_T range is covered by measurements of nearly-real virtual
photons (1<p_T<6 GeV/c) and real photons (5<p_T<16 GeV/c). The invariant yield
of the direct photons in d+Au collisions over the scaled p+p cross section is
consistent with unity. Theoretical calculations assuming standard cold nuclear
matter effects describe the data well for the entire p_T range. This indicates
that the large enhancement of direct photons observed in Au+Au collisions for
1.0<p_T<2.5 GeV/c is due to a source other than the initial-state nuclear
effects.
08/2012;
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
A. Angerami,
K. Aoki,
N. Apadula, [......],
A. Yanovich,
J. Ying,
S. Yokkaichi,
Z. You,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: The PHENIX experiment has measured electrons and positrons at midrapidity
from the decays of hadrons containing charm and bottom quarks produced in d+Au
and p+p collisions at sqrt(s_NN)=200 GeV at the Relativistic Heavy Ion
Collider, in the transverse-momentum range 0.85 < pT < 8.5 GeV/c. In central
d+Au collisions, the nuclear modification factor R_dA at 1.5 < pT < 5 GeV/c
displays evidence of enhancement of these electrons, relative to those produced
in p+p collisions, and shows that the mass-dependent Cronin enhancement
observed at RHIC extends to the heavy-D-meson family. A comparison with the
neutral-pion data suggests that the difference in cold-nuclear-matter effects
on light- and heavy-flavor mesons could contribute to the observed differences
between the pi0 and heavy-flavor-electron nuclear modification factor R_AA.
08/2012;
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
K. Aoki,
Y. Aramaki,
E. T. Atomssa, [......],
J. Ying,
S. Yokkaichi,
Z. You,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: Measurements of the anisotropy parameter v2 of identified hadrons (pions, kaons, and protons) as a function of centrality, transverse momentum pT, and transverse kinetic energy KET at midrapidity (|η|<0.35) in Au + Au collisions at √sNN=200 GeV are presented. Pions and protons are identified up to pT= 6 GeV/c, and kaons up to pT= 4 GeV/c, by combining information from time-of-flight and aerogel Čerenkov detectors in the PHENIX Experiment. The scaling of v2 with the number of valence quarks (nq) has been studied in different centrality bins as a function of transverse momentum and transverse kinetic energy. A deviation from previously observed quark-number scaling is observed at large values of KET/nq in noncentral Au + Au collisions (20–60%), but this scaling remains valid in central collisions (0–10%).
Physical Review C 06/2012; 85:064914. · 3.31 Impact Factor
-
A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
K. Aoki,
Y. Aramaki,
E. T. Atomssa, [......],
J. Ying,
S. Yokkaichi,
Z. You,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: Measurements of the anisotropy parameter v2 of identified hadrons (pions, kaons, and protons) as a function of centrality, transverse momentum pT, and transverse kinetic energy KET at midrapidity (|η|<0.35) in Au + Au collisions at √sNN=200 GeV are presented. Pions and protons are identified up to pT= 6 GeV/c, and kaons up to pT= 4 GeV/c, by combining information from time-of-flight and aerogel Čerenkov detectors in the PHENIX Experiment. The scaling of v2 with the number of valence quarks (nq) has been studied in different centrality bins as a function of transverse momentum and transverse kinetic energy. A deviation from previously observed quark-number scaling is observed at large values of KET/nq in noncentral Au + Au collisions (20–60%), but this scaling remains valid in central collisions (0–10%).
Phys. Rev. C. 06/2012; 85(6).
-
A. Adare,
S. Afanasiev,
C. Aidala,
N.N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
K. Aoki,
L. Aphecetche,
J. Asai, [......],
J. Ying,
S. Yokkaichi,
G.R. Young,
I. Younus,
I.E. Yushmanov,
W.A. Zajc,
O. Zaudtke,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: The differential cross section for the production of direct photons in p+p collisions at √s = 200
GeV at midrapidity was measured in the PHENIX detector at the Relativistic Heavy Ion Collider.
Inclusive-direct photons were measured in the transverse momentum range from 5.5–25 GeV/c,
extending the range beyond previous measurements. Event structure was studied with an isolation
criterion. Next-to-leading-order perturbative-quantum-chromodynamics calculations give a good
description of the spectrum. When the cross section is expressed versus xT , the PHENIX data
are seen to be in agreement with measurements from other experiments at different center-of-mass
energies.
Physical Review C 06/2012; · 3.31 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Purpose: With no stable landmarks available for localization, a 'virtual isocenter' "'or surrogate landmark near the target'" can be used for image guidance. However, using a virtual isocenter in ExacTrac has not been thoroughly validated. This study evaluates its target localization accuracy and investigates the impact of two different couch correction sequences.Methods: A CT scan was acquired on an anthropomorphic thoracic phantom with a 2mm-diameter ball bearing (BB) marker implanted in thelung region. A treatment plan was created with isocenter placed at the BB center, and exported to ExacTrac. In ExacTrac, a virtual isocenter wasplaced on a spine vertebral body where three translational shifts (8.8cm laterally, 1.5cm longitudinally and 6cm vertically) were present. A series ofcouch rotations (+/-3 degrees, 1 degree increment) was intentionally applied to simulate angular setup variations. For each rotation, two stereoscopic x-rayimages were acquired and fused using the ExacTrac 6D registrationalgorithm. Calculated shifts were applied using two sequences: (1)automatic 5D corrections (three translations/two robotic couch rotations) followed by manual couch rotation; (2) manual couch rotation then automatic 5D corrections. After each ExacTrac localization, orthogonal (anterior-posterior and right-lateral) portal images were acquired to quantify BB center deviations from the radiation isocenter as an indicator of residual error. Results: Minimal difference between investigated table correction sequences was observed. Average translational deviations between the BB and radiation isocenter (mean+/-1SD) were 0.3+/-0.3mm and 1.0+/-0.2mm for lateral and vertical axis respectively. Longitudinally, the deviations were 0.8+/-0.4mm from the anterior-posterior image and 0.1+/-0.3mm from the right-lateral image. The systematic difference (0.7+/-0.1 mm) between thetwo may have been attributed to gantry sagging during rotation. Conclusions: ExacTrac system successfully corrected angular shifts using the virtual isocenter method in a rigid phantom setup. The sequence ofcouch correction did not influence the localization accuracy. Further patient study is warranted.
Medical Physics 06/2012; 39(6):3662. · 2.83 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Purpose: Imaging lag and scatter are 2 major problems in CBCT. Previously we demonstrated that our proposed synchronized-moving-grid (SMOG) system mitigates the scatter problem. The SMOG-system is placed between the radiation source and patient. Multiple projections with alternating image and 'shadow' bands are taken at different grid positions at each gantry angle, and are merged together to form a full projection. Scatter can be physically reduced by the grid, and further correction is facilitated by scatter measurements in the shadow bands. We hypothesize that the imaging lag may also be detected in the shadow bands and thus be corrected. Methods: Three imaging lag models, which provide a good fit for the first 10 frames of measured lag, but have differences for larger frame numbers, were used for the simulation. The lag effect was considered as the lag sum of the 100 previous frames. With the SMOG-system, a pixel in a projection is alternately in the shadow or image band for different exposures. The intensities of the pixel for different exposures were obtained from 680 CBCT projections of a rando phantom without or with a static grid. The lag effect was calculated for arbitrary pixels without the SMOG-system, and for paired pixels in the neighboring image and shadow bands with the SMOG- system for 2 and 3 exposures. The residual lag of the SMOG-system was calculated as the lag difference of the paired pixels. Results: The maximum lag was 17%, 23% and 38%, respectively, for the 3 models without SMOG. The maximum residual lag was 4.1%, 4.1% and 3.9% with the SMOG- system for 2-exposures, and was 0.7%, 0.8% and 0.8% for 3-exposures for the 3 models. Conclusions: Using a model to correct for the lag effect is not reliable. However, the SMOG-system is able to correct for the lag effect effectively.
Medical Physics 06/2012; 39(6):3893. · 2.83 Impact Factor
-
N Wen,
J Kim,
S Kim,
C Glide-Hurst, J Jin,
J Gordon,
T Nurushev,
I Chetty,
K Levin,
B Movsas,
S Ryu
[show abstract]
[hide abstract]
ABSTRACT: Purpose: To compare localization accuracies between an ExacTrac and cone beam computed tomography (CBCT) systems for single fraction spine adiosurgery. The work also aimed to evaluate the inherent systematic deviation of both ExacTrac and CBCT systems to achieve highly accurate localization in the spine radiosurgery. Methods: ExacTrac and CBCT imaging systems were evaluated using the linac isocenter as the mutual reference point. First, a BB was placed in an anthropomorphic pelvic phantom. The phantom was localized with both imaging systems and the procedure was repeated 12 times. These results were used to devise a localization protocol using both imaging systems in spine radiosurgery, and employed for 51 patients (81 isocenters) prescribed for single fraction treatment. The displacement discrepancy between the isocenter and two systems were quantified in four dimensions (three translations, one rotation). A Student's two-tailed t-test was used to test for significant differences between the two imaging systems. Results: The phantom study showed 1.4±0.5, 0.6±0.5, and 0.1±0.5 mm differences between the two imaging systems in the anterior/posterior (A/P), superior/inferior (S/I) and left/right (L/R) directions, respectively. The angular difference was minimal along all three axes. The patient study revealed similar isocenter discrepancies between ExacTrac and CBCT of 1.1 ± 0.7 mm, 1.0±0.9 mm, and 0.2±0.9 mm in the A/P, S/I, and L/R directions, respectively, with the A/P and S/I directions showing statistical significance ((t(80) = 13.5 and 7.6 respectively, p = 0.000). The couch yaw discrepancy was 0 ± 0.3°. Overall, 1 mm systematic differences were observed in the A/P and S/I directions between ExacTrac and CBCT localization systems, both in phantom and patient. A procedure was developed to mitigate this systematic discrepancy. Conclusions: These findings have justified our patient localization tolerance levels of 2 mm translation and 1 degree rotation for spine SRS treatment.
Medical Physics 06/2012; 39(6):3666. · 2.83 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Purpose: To develop a synchronized moving grid (SMOG) system to enhance the contrast-to-noise ratio (CNR) and correct image lag in cone-beam CT (CBCT). Methods: The SMOG system uses a rapidly oscillating, synchronized moving grid attached to the kV source. Multiple exposures are taken at each gantry angle with the grid offset by a distance equal to the grid interspace width after each exposure, before the gantry moves to the next position during a scan. In each projection image, patient image data are acquired within the grid interspace area and scatter is measured under the area blocked by the grid for post-scan scatter correction. The grid provides direct scatter reduction by blocking part of the beam, which enhances CNR in the reconstructed CBCT images. Image lag was also estimated from the blocked area and used for lag correction in the image area since both areas have similar radiation history during SMOG acquisition. Experimental studies were performed to evaluate the CNR enhancement for different numbers of exposures (1, 2, and 4) taken at each gantry angle using an enlarged CATphan. Simulation studies were performed to evaluate the lag correction using a lag model. Results: Experimental results showed that SMOG enhanced the CNR by 16% and 13% when increasing exposure number from 1 to 2 and from 2 to 4, respectively. This enhancement was more dramatic for large phantoms. The gain in CNR by increasing the exposure number gradually diminished as the exposure number became large. The CNR values from 4-exposure SMOG were similar to those from the quasi-fan beam scan. Simulation results showed that image lag was reduced from 19% to 0.6% using 4-exposure SMOG. Conclusions: SMOG has the advantages of substantially enhancing the CNR and correcting image lag while removing the scatter artifacts in CBCT with similar scanning time and imaging dose as conventional CBCT.
Medical Physics 06/2012; 39(6):3890. · 2.83 Impact Factor
-
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
A. Al-Jamel,
J. Alexander,
K. Aoki,
L. Aphecetche,
R. Armendariz,
S. H. Aronson, [......],
A. Yanovich,
S. Yokkaichi,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
C. Zhang,
J. Zimányi,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: We report the measurement of direct photons at midrapidity in Au+Au
collisions at sqrt{s_NN} = 200 GeV. The direct photon signal was extracted for
the transverse-momentum range of 4 GeV/c < p_T < 22 GeV/c, using a statistical
method to subtract decay photons from the inclusive-photon sample. The
direct-photon nuclear-modification factor R_AA was calculated as a function of
p_T for different Au+Au collision centralities using the measured p+p
direct-photon spectrum and compared to theoretical predictions. R_AA was found
to be consistent with unity for all centralities over the entire measured p_T
range. Theoretical models that account for modifications of
initial-direct-photon production due to modified-parton-distribution functions
in Au and the different isospin composition of the nuclei, predict a modest
change of R_AA from unity and are consistent with the data. Models with
compensating effects of the quark-gluon plasma on high-energy photons, such as
suppression of jet-fragmentation photons and induced-photon bremsstrahlung from
partons traversing the medium, are also consistent with this measurement.
05/2012;
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A. Adare,
S. Afanasiev,
C. Aidala,
N. N. Ajitanand,
Y. Akiba,
H. Al-Bataineh,
J. Alexander,
K. Aoki,
L. Aphecetche,
J. Asai, [......],
J. Ying,
S. Yokkaichi,
G. R. Young,
I. Younus,
I. E. Yushmanov,
W. A. Zajc,
O. Zaudtke,
C. Zhang,
S. Zhou,
L. Zolin
[show abstract]
[hide abstract]
ABSTRACT: The differential cross section for the production of direct photons in p+p
collisions at sqrt(s)=200 GeV at midrapidity was measured in the PHENIX
detector at the Relativistic Heavy Ion Collider. Inclusive-direct photons were
measured in the transverse-momentum range from 5.5--25 GeV/c, extending the
range beyond previous measurements. Event structure was studied with an
isolation criterion. Next-to-leading-order perturbative-quantum-chromodynamics
calculations give a good description of the spectrum. When the cross section is
expressed versus x_T, the PHENIX data are seen to be in agreement with
measurements from other experiments at different center-of-mass energies.
05/2012;
