Physica Medica: European Journal of Medical Physics - Articles in Press

Fluid dynamic simulation of rat brain vessels, geometrically reconstructed from MR-angiography and validated using phase contrast angiography - Corrected Proof

Monika Carola Lehmpfuhl, Andreas Hess, M. André Gaudnek, Michael Sibila — 2010-08-09

Abstract: The exact knowledge of the blood vessel geometry plays an important role, not only in clinical applications (stroke diagnosis, detection of stenosis), but also for deeper analysis of hemodynamic functional data, such as fMRI. Such vessel geometries can be obtained by different MR angiographic measurements. It is shown that simulations using computational fluid dynamics (CFD) can be used to validate the vessel geometry, automatically reconstructed from time of flight (TOF) angiograms or phase contrast angiography (PC-MRA) data. CFD simulations are based on PC-MRA data, since these data contain additionally rheological information (phases) besides merely amplitudes as is the case for TOF measurements. Parts of the rat brain vessel system are carefully modeled consisting of a main tube and second order branches. By analyzing velocity changes up and downstream of bifurcations, it is shown that CFD can be used to help detecting missing vessels in the TOF based reconstruction. It is demonstrated by artificially deleting a branch from the reconstruction and compared the flow in both resulting CFD simulations. Finally the simulations help to understand the effects of secondary branches on the flow in the main tube.The aim of this study is to compare the measured (PCA) flow data with the CFD simulation results, based on the vessel geometry gained from the PCA image using an in house reconstruction algorithm. If a more accurate simulation method is found and if in principal the simulation matches the PCA data, it might be possible to deduct that in cases where the measured data varies from the CFD simulation, the reconstruction is not complete, i.e. branches are missing or wrong branches were reconstructed.

Evaluation of dose area product vs. patient dose in diagnostic X-ray units - Corrected Proof

K. Kisielewicz, A. Truszkiewicz, S. Wach, M. Wasilewska–Radwańska — 2010-08-02

Abstract: Dose Area Product (DAP) meters which measure in units of [Gy*m2], are widely used in radiology, fluoroscopy and interventional cardiology X-ray units. However, assessment of the radiological hazard to the patient undergoing a given diagnostic procedure cannot be readily obtained from the measured value of DAP. We also developed simple relationships between Entrance Surface Dose (ESD) and the DAP value measured in air, for different collimator field sizes and patient thickness. To establish these relationships, measurements were performed in a water phantom using high-sensitive thermoluminescent detectors (LiF:Mg,Cu,P) calibrated in terms of Kerma in water. Using these relationships developed for a given X-ray unit, calculations of ESD (in mGy) could be performed on the basis of DAP by the X-Ray unit software itself, if the X-ray unit could also evaluate the Focus-to-Skin Distance (FID), e.g. by ultra-sound techniques.

Assessment of computerized treatment planning system accuracy in calculating wedge factors of physical wedged fields for 6 MV photon beams - Corrected Proof

2010-07-26

Abstract: Wedge filters are commonly used in external beam radiotherapy to achieve a uniform dose distribution within the target volume. The main objective of this study was to investigate the accuracy of the beam modifier algorithm of Theraplan plus (TPP version 3.8) treatment planning system and to confirm that either the beam hardening, beam softening and attenuation coefficients along with wedge geometry and measured wedge factor at single depth and multiple fields sizes can be the replacement of wedged profile and wedged cross-sectional data or not. In this regard the effect of beam hardening and beam softening was studied with physical wedges for 6 MV photons. The Normalized Wedge Factors (NWFs) were measured experimentally as well as calculated with the Theraplan plus, as a function of depth and field size in a water phantom for 15°, 30°, 45°, and 60° wedge filters. The beam hardening and softening was determined experimentally by deriving the required coefficients for all wedge angles. The TPP version 3.8 requires wedge transmission factor at single depth and multiple field sizes. Without incorporating the hardening and softening coefficients the percent difference between measured and calculated NFWs was as high as 7%. After the introduction of these parameters into the algorithm, the agreement between measured and TPP (V 3.8) calculated NWFs were improved to within 2 percent for various depths. Similar improvement was observed in TPP version 3.8 while calculating NWFs for various field sizes when the required coefficients were adjusted. In conclusion, the dose calculation algorithm of TPP version 3.8 showed good accuracy for a 6 MV photon beam provided beam hardening and softening parameters are taken into account. From the results, it is also concluded that, the beam hardening, beam softening and attenuation coefficients along with wedge geometry and measured wedge factor at single depth and multiple fields sizes can be the replacement of wedged profile and wedged cross-sectional data in the TPS. The study also indicated that by ignoring the beam softening and beam hardening will result in an inaccurate dose to the target volume of the patient.

In vivo dose verification from back projection of a transit dose measurement on the central axis of photon beams - Corrected Proof

Pascal Francois, Philippe Boissard, Lucie Berger, Alejandro Mazal — 2010-07-08

Abstract: Purpose: In vivo dose verification is used to prevent major deviations between the prescribed dose and the dose really delivered to the patient. This work presents a quick and simple alternative method for verification of dose delivered to the patient using photon beams. During the treatment session, a transit dose is measured with the EPID and the dose in the patient is estimated from back projection of the portal dose.Methods and Materials: The formalism for dose calculation is described. It is independent of the detector and has been validated for different beam energies using an ionization chamber (IC). Central axis doses estimated by this formalism were compared with measured dose. Subsequently, the IC was replaced by the EPID appropriately calibrated. The feasibility of the method and its applicability in clinical use has been evaluated on 3 8 patients treated with conformal therapy for various localizations.Results: Ratios between stated and measured doses are reported. They are within the accepted tolerance of classical in vivo dosimetry (SD of 3.5%).Conclusions: The proposed method for in vivo dose verification is very simple to implement and to use in clinics. Measurements can be repeated during several sessions giving the opportunity to built new strategies for the validation by statistical evaluation of the data. The trending of in vivo dose along the treatment becomes also possible. The number of checkable beams is also increased by this method.

A comparative contrast perception phantom image of brain CT study between high-grade and low-grade liquid crystal displays (LCDs) in electronic medical charts - Corrected Proof

2010-06-30

Abstract: The purpose of this study was to clarify whether non-medical-grade liquid crystal displays (LCDs) are acceptable for the soft-copy reading of brain CTs. Four kinds of color LCDs with different image quality levels were used: medical-grade LCD, low-grade general LCD calibrated with the grayscale display function (GSDF), low-grade general LCD calibrated with gamma 2.2 and a notebook personal computer display panel. In Osirix’s standard window setting for brain CTs, the average CT values of brain parenchyma in 100 cases were correlated with a grayscale level ranging from 71 to 91 in a 256-step grayscale. At these gray levels, the image contrast on the two low-grade LCDs calibrated with gamma 2.2 was higher than that on the medical-grade LCD. Eleven healthy volunteers participated in the contrast perception study, which used electronically generated target phantom images that simulated subtle abnormalities with a low or high attenuation difference in brain parenchyma. The three low-grade LCDs showed correct response rates and reaction times that were superior to those of the medical-grade display. The grayscale calibrations, GSDF or gamma 2.2, are likely to be more critical than the display grade, suggesting that the use of a low-grade LCD may be acceptable in the image contrast of brain CT.

Dose calculation algorithm of fast fine-heterogeneity correction for heavy charged particle radiotherapy - Corrected Proof

Nobuyuki Kanematsu — 2010-06-28

Abstract: This work addresses computing techniques for dose calculations in treatment planning with proton and ion beams, based on an efficient kernel-convolution method referred to as grid-dose spreading (GDS) and accurate heterogeneity-correction method referred to as Gaussian beam splitting. The original GDS algorithm suffered from distortion of dose distribution for beams tilted with respect to the dose-grid axes. Use of intermediate grids normal to the beam field has solved the beam-tilting distortion. Interplay of arrangement between beams and grids was found as another intrinsic source of artifact. Inclusion of rectangular-kernel convolution in beam transport, to share the beam contribution among the nearest grids in a regulatory manner, has solved the interplay problem. This algorithmic framework was applied to a tilted proton pencil beam and a broad carbon-ion beam. In these cases, while the elementary pencil beams individually split into several tens, the calculation time increased only by several times with the GDS algorithm. The GDS and beam-splitting methods will complementarily enable accurate and efficient dose calculations for radiotherapy with protons and ions.

Practical use of Gafchromic® EBT films in electron beams for in-phantom dose distribution measurements and monitor units verification - Corrected Proof

2010-06-04

Abstract: Purpose: The possibility of using the Gafchromic® EBT films parallel to incident electron beams was assessed in order to facilitate quality assurance tests for electron dose calculation algorithms.Materials and methods: Calibration curves were made for electron energies of 6, 9 and 12MeV. A set-up was suggested for EBT film irradiation parallel to the beam, and the dose measurements were compared to Ionization Chamber (IC) measurements in standard and small electrons beams. A more complex Quality Assurance (QA) set-up was performed with the cylindrical CARPET® phantom in order to test our Treatment Planning System (TPS) (Eclipse, Varian Medical Systems, Palo Alto, California) for the clinical situation of a chest wall electron beam therapy. Two dimensional dose distribution and gamma index results were compared to the calculated distribution given by the TPS.Results: The reproducibility was found to be better than 1.5%. We found that applying strong pressure and aligning carefully the film edge with the phantom surface, as recommended for radiographic films, did not completely eliminate the air gap effect. Adding an ultrasound transmission gel and 2 complementary EBT films on the surface gave satisfactory results. The absolute dose for the reference 10×10cm2 field was always within 1% of IC measurements and for smaller elongated fields (5×10, 4×10 and 3×10cm2) the mean difference was −1.4% for the three energies. The mean difference with the IC measurements in R100, R90 and R50 was 0.9mm for all fields and for the three energies. The mean difference in the width of the 90% and the 50% isodoses at R100 was 0.6mm. With the CARPET® phantom set-up very good agreement was found in the 2D dose distribution; 99% of the points satisfied the γ<1 criteria (3%–3mm).Conclusion: EBT films parallel to the beam axis could be used for absolute measurements of 2D dose distribution if ultrasound gel and overlying perpendicular films are added on the phantom surface.

A liquid-filled ionization chamber for high precision relative dosimetry - Corrected Proof

2010-05-31

Abstract: Radiosurgery and intensity modulated radiation therapy (IMRT) treatments are based on the delivery of narrow and/or irregularly shaped megavoltage photon beams. This kind of beams present both lack of charged particle equilibrium and steep dose gradients. Quality assurance (QA) measurements involved in these techniques must therefore be carried out with a dosimeter featuring high small volume.In order to obtain a good signal to noise ratio, a relatively dense material is needed as active medium. Non-polar organic liquids were proposed as active mediums with both good tissue equivalence and showing high signal to noise ratio.In this work, a liquid-filled ionization chamber is presented. Some results acquired with this detector in relative dosimetry are studied and compared with results obtained with unshielded diode. Medium-term stability measurements were also carried out and its results are shown. The liquid-filled ionization chamber presented here shows its ability to perform profile measurements and penumbrae determination with excellent accuracy. The chamber features a proper signal stability over the period studied.

A simple analytical expression to calculate the backscatter factor for low energy X-ray beams - Corrected Proof

U. Chica, G. Flórez, M. Anguiano, A.M. Lallena — 2010-04-30

Abstract: A simple analytical expression aiming to calculate the backscatter factor used in dosimetry protocols to determine the absorbed dose in water for low energy X-rays beams is presented. This expression is based on the linear dependence of the backscatter factor with the generating potential, for fixed values of the half-value layer. The results of a recent work in which 74 X-ray beams with different spectroscopic characteristics, generated with the code xcomp5r and transported with the Monte Carlo code penelope have been used. The expression derived permits to calculate the backscatter factor within 5% accuracy. The predictive power of this expression has been tested for 20 X-ray beams generated with potentials from 50 to 250kV, for which half-value layers and backscatter factors have been experimentally determined.

The impact of overscan on patient dose with first generation multislice CT scanners - Corrected Proof

Ioannis A. Tsalafoutas — 2010-04-26

Abstract: Helical scanning requires the irradiation of larger lengths than those planned. This is referred to as overscan and results to an increase of patient dose. Its impact on patient dose was investigated for three first generation multislice CT scanners; a six-, a quad- and a dual-slice. The amount of overscan was determined using the scanners’ dose-length product (DLP) indications and films positioned on the CT table. With the preset protocol for the chest examination selected in all CT scanners, the overscan length calculated from the DLP indications was 6.3, 3.5 and 2cm respectively, whereas the corresponding figures derived from the films were 6.6, 4.8 and 2.5 to 3.2cm. For a 30cm scan length, the respective contributions of overscan to the DLP values were 17, 10 and 6%, whereas for a scan length of 20cm the respective values increased to 24, 15 and 9%. For the smallest scan lengths allowed in helical mode, the respective contributions reached 53, 88 and 67% because for the six-slice scanner the smallest scan length was limited to twice the collimation, whereas in the quad and dual scanners no limitation existed. For small scan lengths the presence of overscan cancels out any dose reduction offered by helical scanning with pitch factor values larger than one and therefore the axial mode should be preferred, when this is not prohibited by the diagnostic task in question.

Influence of different contributions of scatter and attenuation on the threshold values in contrast-based algorithms for volume segmentation - Corrected Proof

2010-04-20

Abstract: The aim of this work is to evaluate the role of different amount of attenuation and scatter on FDG-PET image volume segmentation using a contrast-oriented method based on the target-to-background (TB) ratio and target dimensions. A phantom study was designed employing 3 phantom sets, which provided a clinical range of attenuation and scatter conditions, equipped with 6 spheres of different volumes (0.5–26.5 ml). The phantoms were: (1) the Hoffman 3-dimensional brain phantom, (2) a modified International Electro technical Commission (IEC) phantom with an annular ring of water bags of 3 cm thickness fit over the IEC phantom, and (3) a modified IEC phantom with an annular ring of water bags of 9 cm. The phantoms cavities were filled with a solution of FDG at 5.4 kBq/ml activity concentration, and the spheres with activity concentration ratios of about 16, 8, and 4 times the background activity concentration. Images were acquired with a Biograph 16 HI-REZ PET/CT scanner. Thresholds (TS) were determined as a percentage of the maximum intensity in the cross section area of the spheres. To reduce statistical fluctuations a nominal maximum value is calculated as the mean from all voxel >95%. To find the TS value that yielded an area A best matching the true value, the cross section were auto-contoured in the attenuation corrected slices varying TS in step of 1%, until the area so determined differed by less than 10 mm2 versus its known physical value. Multiple regression methods were used to derive an adaptive thresholding algorithm and to test its dependence on different conditions of attenuation and scatter.The errors of scatter and attenuation correction increased with increasing amount of attenuation and scatter in the phantoms. Despite these increasing inaccuracies, PET threshold segmentation algorithms resulted not influenced by the different condition of attenuation and scatter. The test of the hypothesis of coincident regression lines for the three phantoms used provided no statistical basis for believing that the three lines are not coincident.Calibration curves needed to implement contouring algorithms based on adaptive TS segmentation of PET volumes can be devised in different conditions of attenuation and scatter. This opens the possibility of defining a unified contrast-based method for target delineation in different anatomical districts.

Have panoramic indices the power to identify women with low BMD at the axial skeleton? - Corrected Proof

John Damilakis, Konstantinos Vlasiadis — 2010-04-02

Abstract: The purpose of this study was to investigate whether panoramic mandibular indices i.e. mandibular cortical width (MCW), alveolar bone resorption degree (M/M ratio) and panoramic mandibular index (PMI) can be used as pre-selection tests to identify women with low bone mineral density (BMD) at the axial skeleton. MCW, PMI and M/M ratio were measured on dental panoramic radiographs in a group of 151 postmenopausal women aged 38–80 years. BMD at the lumbar spine and hip was measured by dual energy X-ray absorptiometry. BMD values were categorized as normal, osteopenic or osteoporotic according to the WHO classification. ROC analysis was used to determine cut-off values for MCW. MCW performed better than PMI and M/M ratio in its power to differentiate women with osteopenia or osteoporosis from healthy subjects. Cut-off values for the MCW estimated to detect women with T-score < −1 and those with T-score ≤ −2.5 identified a total of 45% and 34% of subjects respectively. Sensitivity and specificity was found to be low ranging between 55 and 70 for both cut-off values. Panoramic indices appear to have limited power in their ability to identify women with low BMD at the axial skeleton.

Evaluation of an uncollimated printed paper transmission source used under scatter limiting conditions - Corrected Proof

2010-03-08

Abstract: Transmission sources used for image attenuation correction, allowing image quantification, are collimated to reduce scatter. We propose the same effect can be achieved for an uncollimated source by increasing source to patient distance. The aim was to compare planar image performance characteristics and absorbed doses of uncollimated and collimated radioactive printed paper transmission sources.The scatter contribution to the uncollimated 99mTc source data was evaluated for different combinations of detector phantom distance, detector source distance and phantom source distance. Measurements were performed by increasing the Lucite phantom thickness in 1cm steps to 20 cm. Spatial resolution, detection efficiency and entrance absorbed dose rate were measured for the uncollimated and collimated transmission source images.Results derived from the energy spectra, obtained with the uncollimated transmission source indicate that scatter contribution increases with decreasing detector source distance. The scatter component in the uncollimated transmission images (detector source distances≥60cm; phantom source distances≥40cm) was comparable to that obtained with collimated transmission images. Attenuation coefficients obtained compared well (0.168cm−1 vs. 0.171cm−1). The full widths at half maxima differed by less than 0.9mm. The detection efficiency of the uncollimated source was 2.5 times higher than obtained with the collimated source. The entrance absorbed dose obtained from an uncollimated source was 3.75 times larger than that obtained from the collimated source.An uncollimated transmission source (detector source distance≥60cm) results in acceptable image characteristics and presents a low cost, low dose, high efficiency option for transmission imaging.

Physical characterization and comparison of two commercially available micro-MLCs - Corrected Proof

Tarun K. Podder, Greg Bednarz, Yan Yu, James M. Galvin — 2010-03-04

Abstract: In this study, the physical characteristics (penumbra width variation with the source size and shape, interleaf leakage, transmission through the leaves, and the tongue-and-groove effect) of two linear accelerators (BrainLAB's Novalis and Elekta's Synergy-S Beam Modulator) have been investigated. For similar square fields (about 4.5cm×4.5cm) with source-to-surface/skin-distance (SSD) ranging from 90cm to 115cm and measurements taken at the depth of Dmax=1.5cm for 6MV photon beam. The Novalis MLC has penumbra width of 2.4±0.11mm–2.8±0.11mm at the leaf-end and 2.2±0.1mm–2.7±0.1mm on the leaf-side; and those for the Synergy-S MLC are 4.4±0.17mm–5.2±0.2mm and 3.0±0.12mm–3.5±0.12mm. Upon rotating the Synergy-S collimator by 90° (i.e., shifting the leaf movement to the gun–target direction), significant reduction of the leaf-end penumbra width (17%) and increase of leaf-side penumbra width (28%) suggest an elliptical shape of the radiation source spot. Similar rotation of the collimator yielded reduction of the penumbras on both leaf-end (34%) and leaf-side (28%) for Novalis, indicating that the Novalis has a more symmetric source size. For all the field sizes and settings, BrainLAB's Novalis μMLC produce a smaller penumbra for simple square fields compared to the Elekta's Synergy-S. However, this difference became less pronounced for leaf-side penumbra and also for circular fields. The tongue-and-groove effect of the Novalis (23±0.9%) is slightly smaller than that of the Synergy-S (25±1%); while the interleaf leakage and leakage directly through leaves for Synergy-S (1.6±0.07% & 0.9±0.04%) are lower than that of Novalis (2±0.08% & 1.3±0.05%).

Generalized EPID calibration for in vivo transit dosimetry - Corrected Proof

2010-03-03

Abstract: Many researchers are studying new in vivo dosimetry methods based on the use of Elelctronic portal imaging devices (EPIDs) that are simple and efficient in their daily use. However the need of time consuming implementation measurements with solid water phantoms for the in vivo dosimetry implementation can discourage someone in their use.In this paper a procedure has been proposed to calibrate aSi EPIDs for in vivo transit dosimetry. The dosimetric equivalence of three aSi Varian EPIDs has been investigated in terms of signal reproducibility and long term stability, signal linearity with MU and dose per pulse and signal dependence on the field dimensions. The signal reproducibility was within ±0.5% (2SD), while the long term signal stability has been maintained well within ±2%. The signal linearity with the monitor units (MU) was within ±2% and within ±0.5% for the EPIDs controlled by the IAS 2, and IAS 3 respectively. In particular it was verified that the correction factor for the signal linearity with the monitor units, klin, is independent of the beam quality, and the dose per pulse absorbed by the EPID.For 6, 10 and 15MV photon beams, a generalized set of correlation functions F(TPR,w,L) and empirical factors f(TPR,d,L) as a function of the Tissue Phantom Ratio (TPR), the phantom thickness, w, the square field side, L, and the distance, d, between the phantom mid-plane and the isocentre were determined to reconstruct the isocenter dose.The tolerance levels of the present in vivo dosimetry method ranged between ±5% and ±6% depending on the tumor body location.In conclusion, the procedure proposed, that use generalized correlation functions, reduces the effort for the in vivo dosimetry method implementation for those photon beams with TPR within ±0.3% as respect those here used.

Experimental analysis of Hb oxy–deoxy transition in single optically stretched red blood cells - Corrected Proof

G. Rusciano — 2010-02-25

Abstract: Raman confocal microscopy, combined with an optical stretcher, is used to study the spatial distribution and the oxidation state of hemoglobin in erythrocytes under stretching condition. In particular, a near infrared laser (λ = 1064 nm) is used to generate multiple time-sharing Optical Tweezers to trap and stretch a single erythrocyte, while a second laser (λ = 532 nm) acts as Raman probe. Our study demonstrates that stretching induces hemoglobin transition to the deoxygenated state. Moreover, by using Principal Component Analysis we prove the reversibility of the hemoglobin transition after application of the optically induced mechanical stress.

Comparison of monitor units calculated by radiotherapy treatment planning system and an independent monitor unit verification software - Corrected Proof

P. Sellakumar, C. Arun, S.S. Sanjay, S.B. Ramesh — 2010-02-22

Abstract: In radiation therapy, the monitor units (MU) needed to deliver a treatment plan are calculated by treatment planning systems (TPS). The essential part of quality assurance is to verify the MU with independent monitor unit calculation to correct any potential errors prior to the start of treatment. In this study, we have compared the MU calculated by TPS and by independent MU verification software. The MU verification software was commissioned and tested for the data integrity to ensure that the correct beam data was considered for MU calculations. The accuracy of the calculations was tested by creating a series of test plans and comparing them with ion chamber measurements. The results show that there is good agreement between the two. The MU difference (MUdiff) between the monitor unit calculations of TPS and independent MU verification system was calculated for 623 fields from 245 patients and was analyzed by treatment site for head & neck, thorax, breast, abdomen and pelvis. The mean MUdiff of −0.838% with a standard deviation of 3.04% was observed for all 623 fields. The site specific standard deviation of MUdiff was as follows: abdomen and pelvis (<1.75%), head & neck (2.5%), thorax (2.32%) and breast (6.01%). The disparities were analyzed and different correction methods were used to reduce the disparity.

Scanning orientation and polarization effects for XRQA radiochromic film - Corrected Proof

Hani Alnawaf, Martin J. Butson, Tsang Cheung, Peter K.N. Yu — 2010-02-12

Abstract: Gafchromic XRQA radiochromic film, is an effective tool for quality assurance and dose assessment in kilovoltage radiotherapy and diagnostic applications. Like other Gafchromic film products, XRQA film exhibits a variation in dose to reflected optical density response with angle of rotation when analysed with a light source that is partially or fully polarised such as a desktop scanner. Although warnings are not given on manufacturers specifications, this can affect dosimetry accuracy and we recommend that it is essential to scan all XRQA films in the same orientation. The effect is not as pronounced as EBT Gafchromic film. The magnitude of this variation has been measured and shown to be up to 16±2% (1SD) using a fully linear polarised light source was seen with a 90° angle rotation. This would be the maximum variation seen on a desktop scanner with a fully polarised light source. For our standard desktop scanner (Epson v700) a mean variation of 2±1% from 0cGy to 20cGy applied dose was measured as compared to 8±2% for EBT Gafchromic. We recommend that to decrease uncertainty in dose measurement, accurate alignment of the calibration films to experimental films be performed on a regular basis. This is especially important if your desktop scanner has a high degree of polarization of its light source.

Reduction of patient dose in medical radiography by utilizing scattered X-rays: Relation between permissible limit of scatter fraction, viewer brightness, and perceptibility of vision - Corrected Proof

Yoshiyuki Asai, Masanobu Uemura, Masao Matsumoto, Hitoshi Kanamori — 2010-02-04

Abstract: This paper proposes a new technique for reducing the patient dose when employing medical radiographs prepared by using screen-film systems. In this technique the patient dose can be reduced by employing scattered X-rays in order to obtain the same film density as that realized without the use of scattered X-rays. The minimum perceptible thickness difference ΔXmin, which can be recognized by liminal vision, was psychophysically calculated by considering the energy spectrum of incident X-ray, sensitivity spectrum of the screen layer, and the perception capability of human vision. From the calculated ΔXmins in various conditions, the permissible upper limit of scatter fraction for obtaining the same ΔXmin for three kinds of luminances, and the fraction of reduction in the primary X-rays were determined.As an example of the results, when the object size required for perception is 1.3 mm, a scatter fraction up to 42% can be permitted at a density D of 1.0 for a luminance of 2548 cd m–2. When we increase the luminance of the viewer from 478 cd m–2 to 2548 cd m–2, the upper limit of the permitted scatter fraction varies from 30% to 42% at a D of 1.0, i.e., the patient dose can be reduced by 17% under the same perceptibility of ΔXmin by utilizing scattered X-rays. This reduction can be successfully achieved by changing the lead content of the grid from 0.45 to 0.38 g cm–2.

Coronary angiography and percutaneous transluminal coronary angioplasty procedures: Evaluation of patients' maximum skin dose using Gafchromic films and a comparison of local levels with reference levels proposed in the literature - Corrected Proof

C. Giordano, L. D'Ercole, R. Gobbi, M. Bocchiola, F. Passerini — 2010-02-01

Abstract: The aim of this study was to evaluate the maximum skin dose (MSD) in patients undergoing interventional cardiology procedures, obtaining local reference levels and comparing these with the reference levels proposed in the literature.The patients' MSD was measured using Gafchromic XR type R films. In order to evaluate reference levels, the number of images acquired, the fluoroscopy times and the KAPTOTAL were recorded for each procedure.For the evaluation of the MSD, 8 coronary angiography (CA) and 16 percutaneous transluminal coronary angioplasty (PTCA) procedures, carried out in the period from May to June 2008, were analyzed.For the CA procedures the MSD values were below 0.5 Gy.For the PTCA procedures, we found a fairly good correlation between fluoroscopy time and MSD (r = 0.80, p = 0.0002) and between MSD and WFP (r = 0.863, p < 0.0001); there was a strong correlation between KAPTOTAL and MSD (r = 0.904, p < 0.0001). Since the correlation between KAPTOTAL and MSD is more striking than that with fluoroscopic time and the WFP, KAP measurements are suitable for online skin dosimetry and may, therefore, be used to avoid radiation-induced skin injuries. A MSD greater than 3 Gy occurred in only one procedure.For calculus of the local reference levels, we extended the data-gathering to 30 procedure CA and to 40 PTCA: we compared local practice with that in other centers using the guidance levels proposed by Balter et al. Our median KAP values were below these proposed guidance levels; our mean KAP values were above these proposed action levels. From a first application of the proposed reference levels, it appears that, according to the recommendations of Balter et al. an investigation into local practice is not necessary.

Influence of Geant4 parameters on dose distribution and computation time for carbon ion therapy simulation - Corrected Proof

Nabil Zahra, Thibault Frisson, Loic Grevillot, Philippe Lautesse, David Sarrut — 2010-01-27

Abstract: The aim of this work was to study the influence of Geant4 parameters on dose distribution and computational time for simulations of carbon ion therapy. The study was done using Geant4 version 9.0. The dose distribution in water for incident monoenergetic carbon ion beams of 300 MeV/u were compared for different values of secondary particle production threshold and different step limits. Variations of depth dose of about 2 mm were observed in some cases, which induced a 30% variation of dose deposit in the Bragg peak region. Other tests were done using Geant4 version 9.2 to verify the results from this study. The two versions provided converging results and led to the same conclusions.

Accuracy of real-time MR temperature mapping in the brain: A comparison of fast sequences - Corrected Proof

A. Kickhefel, Jörg Roland, Clifford Weiss, Fritz Schick — 2010-01-25

Abstract: Purpose: To compare magnetic resonance (MR) thermometry based on the proton resonance frequency (PRF) method using a single shot echoplanar imaging (ss EPI) sequence to both of the standard sequences, gradient echo (GRE) and segmented echoplanar imaging (seg EPI) in the in vivo human brain, at 1.5T and 3T.Material and methods: Repetitive MR thermometry was performed on the brain of six volunteers using GRE, seg EPI, and ss EPI sequences on whole-body 1.5T and 3T clinical systems using comparable acquisition parameters. Phase stability and temperature data precision in the human head were determined over 12 min for the three sequences at both field strengths. An ex-vivo swine skeletal muscle model was used to evaluate temperature accuracy of the ss EPI sequence during heating by high intensity focused ultrasound (HIFU).Results: In-vivo examinations of brain revealed an average temperature precision of 0.37 °C/0.39 °C/0.16 °C at 3T for the GRE/seg EPI/ss EPI sequences. At 1.5T, a precision of 0.58 °C/0.63 °C/0.21 °C was achieved. In the ex-vivo swine model, a strong correlation of temperature data derived using ss EPI and GRE sequences was found with a temperature deviation <1 °C.Conclusion: The ss EPI sequence was the fastest and the most precise sequence for MR thermometry, with significantly higher accuracy compared to GRE.

Stochastic modelling of the role of cisplatin in altered fractionation schedules for head and neck cancer - Corrected Proof

Loredana Marcu, Eva Bezak — 2009-12-25

Abstract: Advanced head and neck cancers are one of the most challenging cancers facing the oncologists due to their aggressiveness attributable to the high hypoxic content and the tumour's ability to repopulate during radiotherapy. Alterations of radiotherapy fractionation schedules are possible ways to improve tumour control. Clinical trials have shown that both hyperfractionated radiotherapy (multiple fractions a day, over the same treatment time), and accelerated radiotherapy (higher doses per fraction, six days a week, over 5 weeks or less) are more effective than conventional radiotherapy in the management of head and neck cancer. However, the treatment choice between hyperfractionated and accelerated radiotherapy is still debated, due to very similar results obtained regarding tumour control. Furthermore, while radiotherapy alone has an impact on the short-term prognosis of advanced head and neck cancer, the long-term benefits have been moderate. Cisplatin is a chemotherapeutic agent which combined with conventional radiotherapy has shown to improve patient survival. The present paper employs a Monte Carlo modelling approach in assessing the effect of combined cisplatin-altered fractionation schedule on tumour response. The growth of a head and neck carcinoma has been modelled using probabilistic functions sampled by computer generated random number sequences, maintaining the biological constitution of a tumour. The tumour growth model has been built to simulate the in vivo processes taking place before and after radiotherapy/chemotherapy. The model has shown that adding cisplatin to radiotherapy improves tumour control in both hyperfractionated and accelerated radiotherapy.

Viewing boxes: A survey in diagnostic radiology departments of Moroccan hospitals - Corrected Proof

F. Bentayeb, K. Nfaoiu, O. Basraoui, A.C.P. Azevedo — 2009-12-25

Abstract: This work consists on a survey of the performance of viewing boxes installed in diagnostic radiology departments and included several aspects: checking the illuminance of the diagnostic rooms where the viewing boxes are located as well as the viewing boxes luminance levels and homogeneity. Seven hospitals took part in the survey, being three in the city of Casablanca and four in Rabat. A total of 136 viewing boxes and 18 diagnostic rooms have been checked. It was found that one diagnostic room works under normal conditions and that 80% of the viewing boxes present luminance levels below the international recommendations and the homogeneity is inadequate in 85%. Some simple and cheap initiatives can be performed in these departments to increase luminance and improve homogeneity levels such as cleaning and bulbs replacement.

Calculating and controlling the error of discrete representations of Pareto surfaces in convex multi-criteria optimization - Corrected Proof

David Craft — 2009-12-21

Abstract: A discrete set of points and their convex combinations can serve as a sparse representation of the Pareto surface in multiple objective convex optimization. We develop a method to evaluate the quality of such a representation, and show by example that in multiple objective radiotherapy planning, the number of Pareto optimal solutions needed to represent Pareto surfaces of up to five dimensions grows at most linearly with the number of objectives. The method described is also applicable to the representation of convex sets.