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

Gantry angle dependence in IMRT pre-treatment patient-specific quality controls - Corrected Proof

Angelo Filippo Monti, Chiara Berlusconi, Stefania Gelosa — 2012-01-27

Abstract: Intensity Modulated Radiation Therapy (IMRT) is a complex treatment modality that requires pre-treatment patient-specific quality control (QC) in order to assess a correct treatment delivery. The aim of this work is to investigate pre-treatment patient-specific per-field QCs performed with an on-board EPID at the gantry angle of 0° and at the treatment ones, and to asses if measurements executed at 0° are able to guarantee a correct treatment.Ten patients with prostate cancer were evaluated. Two “verification” plans were created for each patient in order to calculate the dose at the EPID surface: one with all fields positioned at 0° and one with all fields at the actual treatment angles.EPID’s mechanical shifts due to gravity effects were always taken into account and corrected.0 and no-0 plans were compared using a gamma-index method (3%, 3 mm). The gamma index was found dependent on gantry angles but the difference between 0 and no-0 samples was small (−0.3% mean value) and the criteria of acceptability of the gamma method was always satisfied for every field delivered at angles different from 0.Therefore patient-specific pre-treatment QCs should be done at treatments angles, but, if periodical quality assurance is performed on dynamic MLC for different gantry angles, this requirement was shown not strictly mandatory and pre-treatment IMRTQC can be reasonably executed at 0° angles too.

Calculation of beam quality correction factors for various thimble ionization chambers using the Monte Carlo code PENELOPE - Corrected Proof

Fabián Erazo, Antonio M. Lallena — 2012-01-25

Abstract: The beam quality correction factor and the perturbation factor pQ, commonly considered in dosimetry with ionization chambers, were calculated for the NE2571 and the Standard Imaging A19 and A12S chambers, using the Monte Carlo simulation code PENELOPE. For the NE2571 chamber, the values of obtained are in very good agreement with those found in previous works by Wulff et al. and Muir and Rogers with the code EGSnrc and also with the experimental results summarized in the NCS code of practice. For pQ, a difference of ∼0.4% has been found between our results and those obtained with EGSnrc for 60Co and this difference increases slightly with values. These factors have been calculated also for the A19 and A12S chambers of Standard Imaging. The values of show reasonable agreement with those recently calculated by Muir and Rogers and the measurements of McEwen.

Evaluation of clinical benefits achievable by using different optimization algorithms during real-time prostate brachytherapy - Corrected Proof

Marta Adamczyk, Grzegorz Zwierzchowski, Julian Malicki, Janusz Skowronek — 2012-01-06

Abstract: Background and purpose: High dose rate (HDR) brachytherapy is a clinically used procedure in prostate cancer treatment. The purpose of this study was to present the influence of using different optimization algorithms in 3D-CBRT planning on the treatment plan quality.Materials and methods: Treatment plans were calculated for 15 patients – three plans for each patient using: geometrical optimization (GO), inverse optimization (IO) and blind inverse optimization (BIO). For each patient, PTV and OAR volumes, number of needles and geometry of the implant were set equal. Differences between dose distributions were tracked using: D90, V100, V200, Dmax (for prostate); D10, Dmax (for urethra); D10, V100, Dmax (for rectum).Results: The analysis of mean values of D90 and V100 in the prostate showed that inverse algorithms gave the best results (mean D90 was 12.1% for BIO and 9.3% for IO better than for GO, mean V100 was 8.2% for BIO and 6.3% for IO better than for GO). From a clinical point of view, GO diminished the doses in the PTV and urethra in all analyzed parameters. The lowest mean doses in the rectum were achieved for plans optimized with IO and BIO (mean D10: 61.2% for GO, 58.1% for IO, 58.0% for BIO; mean Dmax: 92.8% for GO, 85.1% for IO, 83.6% for BIO).Conclusions: Application of the blind inverse optimization (BIO) algorithm led to clinically best dose parameters for PTV and the rectum. Use of geometrical optimization (GO) led to smaller doses in the urethra, which was however associated with a certain dose decrease also in PTV.

Iterative reconstruction methods in two different MDCT scanners: Physical metrics and 4-alternative forced-choice detectability experiments – A phantom approach - Corrected Proof

2012-01-04

Abstract: This paper characterizes and evaluates the potential of three commercial CT iterative reconstruction methods (ASIR™, VEO™ and iDose4 (™)) for dose reduction and image quality improvement. We measured CT number accuracy, standard deviation (SD), noise power spectrum (NPS) and modulation transfer function (MTF) metrics on Catphan phantom images while five human observers performed four-alternative forced-choice (4AFC) experiments to assess the detectability of low- and high-contrast objects embedded in two pediatric phantoms. Results show that 40% and 100% ASIR as well as iDose4 levels 3 and 6 do not affect CT number and strongly decrease image noise with relative SD constant in a large range of dose. However, while ASIR produces a shift of the NPS curve apex, less change is observed with iDose4 with respect to FBP methods. With second-generation iterative reconstruction VEO, physical metrics are even further improved: SD decreased to 70.4% at 0.5 mGy and spatial resolution improved to 61% (MTF50%). 4AFC experiments show that few improvements in detection task performance are obtained with ASIR and iDose4, whereas VEO makes excellent detections possible even at an ultra-low-dose (0.3 mGy), leading to a potential dose reduction of a factor 3 to 7 (67%–86%). In spite of its longer reconstruction time and the fact that clinical studies are still required to complete these results, VEO clearly confirms the tremendous potential of iterative reconstructions for dose reduction in CT and appears to be an important tool for patient follow-up, especially for pediatric patients where cumulative lifetime dose still remains high.

Real-time tumor tracking with an artificial neural networks-based method: A feasibility study - Corrected Proof

2012-01-03

Abstract: The purpose of this study was to develop and assess the performance of a tumor tracking method designed for application in radiation therapy. This motion compensation strategy is currently applied clinically only in conventional photon radiotherapy but not in particle therapy, as greater accuracy in dose delivery is required.We proposed a tracking method that exploits artificial neural networks to estimate the internal tumor trajectory as a function of external surrogate signals. The developed algorithm was tested by means of a retrospective clinical data analysis in 20 patients, who were treated with state of the art infra-red motion tracking for photon radiotherapy, which is used as a benchmark. Integration into a hardware platform for motion tracking in particle therapy was performed and then tested on a moving phantom, specifically developed for this purpose.Clinical data show that a median tracking error reduction up to 0.7 mm can be achieved with respect to state of the art technologies. The phantom study demonstrates that a real-time tumor position estimation is feasible when the external signals are acquired at 60 Hz.The results of this work show that neural networks can be considered a valuable tool for the implementation of high accuracy real-time tumor tracking methodologies.

Development of cylindrical stepwedge phantom for routine quality controls of a helical tomotherapy machine - Corrected Proof

Krzysztof Mikołajczyk, Tomasz Piotrowski — 2012-01-03

Abstract: The aim of this study was to design a cylindrical stepwedge phantom and an appropriate treatment procedure, based on which parameters of tomotherapy machine and generated beam of radiation will be defined. The accuracy of parameter determination, which can be defined with the aid of the measurement system, was also evaluated.The cylindrical phantom that we developed and manufactured (stepwedge phantom) consists of four cylinders with different diameters made of polycaprolactam-PA-6, i.e. material with high mechanical strength, low water absorption (making measurements repeatable) and a density comparable to that of human soft tissues. The appropriate treatment procedure is carried out in a dynamic mode, which is focused on specific properties of the tomotherapy machine. It means that a phantom situated on the couch moves to the inside of the rotating linear accelerator.A total of 18 procedures were implemented in order to calculate the following parameters: couch velocity, dose rate value at a depth, Dose Ratio coefficients, dose variation (so-called Dose Flatness) coefficients, and the time of gantry rotation. Reference intervals for these parameters were determined to be as follows: for the couch velocity: ±1.2%, the average dose rate measured at depth: ±1.8%, the calculated values of the coefficients Dose Ratio: ±0.5% and Dose Flatness: (0.53–0.65)%, the time of gantry rotation: ±3%.The final results showed that during a single irradiation procedure, which lasts 5 min, the cylindrical stepwedge phantom allows to precisely determine the values of the above-mentioned parameters. Its use in the daily dosimetric measurements can ensure better control of the work of the tomotherapy machine.

Automation of clip localization in Digital Tomosynthesis for setup of breast cancer patients - Corrected Proof

2011-12-30

Abstract: The objective of this study is to develop an automatic clip localization procedure for breast cancer patient setup based on Digital Tomosynthesis (DTS) and to characterize its performance with respect to the overall registration accuracy and robustness. The study was performed under an IRB-approved protocol for 12 breast cancer patients with surgical clips implanted around the tumor cavity. The registration of DTS images to planning CTs was performed using an automatic algorithm developed to overcome specific challenges of localization and registration of clips in the breast setup images. The automatic method consisted of auto-segmentation (intensity-based thresholding with a priori knowledge about clip size and location to distinguish clips from bony features) and auto-registration of the segmented clip clusters. To determine the inherent accuracy and robustness of the registration algorithm, additional simulated DTS data was analyzed. The developed algorithm is efficient in removing false positives and negatives and provides an accuracy of better than 2.3mm for 60° and 3.3mm for 40° DTS. When incorporated in clinical software, this algorithm helps to facilitate fast and accurate setup evaluation with minimal dose delivered to patients.

On-line integration of computer controlled diagnostic devices and medical information systems in undergraduate medical physics education for physicians - Corrected Proof

Josef Hanus, Tomas Nosek, Jiri Zahora, Ales Bezrouk, Vladimir Masin — 2011-12-26

Abstract: We designed and evaluated an innovative computer-aided-learning environment based on the on-line integration of computer controlled medical diagnostic devices and a medical information system for use in the preclinical medical physics education of medical students. Our learning system simulates the actual clinical environment in a hospital or primary care unit. It uses a commercial medical information system for on-line storage and processing of clinical type data acquired during physics laboratory classes. Every student adopts two roles, the role of ‘patient’ and the role of ‘physician’. As a ‘physician’ the student operates the medical devices to clinically assess ‘patient’ colleagues and records all results in an electronic ‘patient’ record. We also introduced an innovative approach to the use of supportive education materials, based on the methods of adaptive e-learning. A survey of student feedback is included and statistically evaluated.The results from the student feedback confirm the positive response of the latter to this novel implementation of medical physics and informatics in preclinical education. This approach not only significantly improves learning of medical physics and informatics skills but has the added advantage that it facilitates students’ transition from preclinical to clinical subjects.

Automatic exposure control at MDCT based on the contrast-to-noise ratio: Theoretical background and phantom study - Corrected Proof

2011-12-19

Abstract: Purpose: To develop a new automatic exposure control (AEC) technique based on the contrast-to-noise ratio (CNR) and provide constant lesion detectability.Methods: Lesion detectability is affected by factors such as image noise, lesion contrast, and lesion size. We performed ROC analysis to assess the relationship between the optimum CNR and the lesion diameter at various levels of lesion contrast. We then developed a CNR-based AEC algorithm based on lesion detectability. Using CNR- based AEC algorithm, we performed visual evaluation of low-contrast detectability by 5 radiologists on a low-contrast module of the Catphan phantom, a contrast-difference level of 1.0% (difference in the CT number = 10 HU), and objects 3.0–9.0 mm in diameter.Results: On step-and-shoot scans the mean detection fraction with CNR-based AEC remained almost constant from 88 to 99 % regardless of the lesion size. We observed the same trend on helical scans, the mean detection fraction with CNR-based AEC exhibited a high score from 91 to 100%. Although CNR-based AEC maintains higher CNR for smaller size or lower contrast lesion, radiation dose on 3 mm lesion resulted in about 13 times larger than that of 9 mm lesion size. CTDIvol for the CNR-based AEC technique changed dramatically with the SDZ from 7.5 to 100.0 mGy for step-and-shoot scans and from 9.1 to 121.5 mGy for helical scans.Conclusions: From the viewpoint of ROC analysis-based CNR for lesion detection, CNR-based AEC potentially provide image quality advantages for clinical implementation.

Radio-protective effects of melatonin against irradiation-induced oxidative damage in rat peripheral blood - Corrected Proof

2011-12-16

Abstract: During radiotherapy, ionizing irradiation interacts with biological systems to produce free radicals, which attacks various cellular components. The hematopoietic system is well-known to be radiosensitive and its damage may be life-threatening. Melatonin synergistically acts as an immunostimulator and antioxidant. In this study we used a total of 120 rats with 20 rats in each group. Group 1 did not receive melatonin or irradiation (Control group), Group 2 received only 10 mg/kg melatonin (Mel group), Group 3 exposed to dose of 2 Gy irradiation (2 Gy Rad group), Group 4 exposed to 8 Gy irradiation (8 Gy Rad group), Group 5 received 2 Gy irradiation plus 10 mg/kg melatonin (Mel +2 Gy Rad group) and Group 6 received 8 Gy irradiation plus 10 mg/kg melatonin (Mel+8 Gy Rad group). Following exposure to radiation, five rats from each group were sacrificed at 4, 24, 48 and 72 h. Exposure to different doses of irradiation resulted in a dose-dependent decline in the antioxidant enzymes activity and lymphocyte count (LC) and an increase in the nitric oxide (NO) levels of the serum. Pre-treatment with melatonin (10 mg/kg) ameliorates harmful effects of 2 and 8 Gy irradiation by increasing lymphocyte count(LC) as well as antioxidant enzymes activity and decreasing NO levels at all time-points. In conclusion 10 mg/kg melatonin is likely to be a threshold concentration for significant protection against lower dose of 2 Gy gamma irradiation compared to higher dose of 8 Gy. Therefore, it seems that radio-protective effects of melatonin are dose-dependent.

Radiation leakage study for the Valencia applicators - Corrected Proof

D. Granero, J. Perez-Calatayud, F. Ballester, Z. Ouhib — 2011-12-12

Abstract: Introduction and purpose: The Valencia applicators which are accessories of the microSelectron-HDR afterloader (Nucletron, Veenendaal, The Netherlands) are designed to treat skin lesions. These cup-shaped applicators are an alternative to superficial/orthovoltage x-ray treatment units. They limit the irradiation to the required area using tungsten-alloy shielding, and are equipped with a tungsten-alloy flattering filter allowing the treatment of skin tumors, the oral cavity, vaginal cuff, etc. The tungsten-alloy thickness to shield radiation is not the same in all parts of the applicators. This fact led us to question whether the leakage radiation differs depending on where it is measured, and whether this may be relevant in some clinical cases. The purpose of this work is to study from the radiation protection point of view the radiation leakage of the Valencia applicators, and provide a solution for current users and for the manufacturer.Methods and materials: Simulations based on the Monte Carlo (MC) method using the Geant4 code have been realized studying the dose rate distribution in air around the cup of the Valencia applicators. An experimental study with radiochromic film has also been done to measure the dose distribution in the back side of the applicators and to compare it with MC results.Results and conclusions: Radiation leakage of up to 170% of the prescribed dose has been found at the back surface of these applicators. Although this side is not usually directed to the patient, in some applications such as the treatment of a lesion on the nose, special care must be taken to avoid unexpected and unnecessary irradiation of the eyes. A possible solution could be to add additional shielding to the applicator in order to reduce this leakage or to put some shielding to protect the eyes. Additionally, a new concept design of the Valencia applicators using more shielding material in the applicator backside is proposed.

Mechanical aspects of CO2 angiography - Corrected Proof

2011-12-05

Abstract: The aim of this paper is to clarify some physical–mechanical aspects involved in the carbon dioxide angiography procedure (CO2 angiography), with a particular attention to a possible damage of the vascular wall.CO2 angiography is widely used on patients with iodine intolerance. The injection of a gaseous element, in most cases manually performed, requires a long training period. Automatic systems allow better control of the injection and the study of the mechanical behaviour of the gas.CO2 injections have been studied by using manual and automatic systems. Pressures, flows and jet shapes have been monitored by using a cardiovascular mock. Photographic images of liquid and gaseous jet have been recorded in different conditions, and the vascular pressure rises during injection have been monitored.The shape of the liquid jet during the catheter washing phase is straight in the catheter direction and there is no jet during gas injection. Gas bubbles are suddenly formed at the catheter’s hole and move upwards: buoyancy is the only governing phenomenon and no bubbles fragmentation is detected. The pressure rise in the vessel depends on the injection pressure and volume and in some cases of manual injection it may double the basal vascular pressure values.CO2 angiography is a powerful and safe procedure which diffusion will certainly increase, although some aspects related to gas injection and chamber filling are not jet well known. The use of an automatic system permits better results, shorter training period and limitation of vascular wall damage risk.

Xenon for NMR biosensing – Inert but alert - Corrected Proof

Leif Schröder — 2011-11-28

Abstract: NMR studies with hyperpolarized xenon as functionalized sensor or contrast agent recently made notable progress in developing a new approach for detecting molecular markers and parameters of biomedical interest. Combining spin polarization enhancement with novel indirect detection schemes easily enables a 107-fold signal gain, thus having promising potential to solve the NMR sensitivity problem in many applications. Though an inert element, 129Xe has exquisite NMR properties to sense molecular environments. This review summarizes recent developments in the production of hyperpolarized xenon and the design and detection schemes of xenon biosensors.

Complication probability model for subcutaneous fibrosis based on published data of partial and whole breast irradiation - Corrected Proof

2011-11-28

Abstract: Purpose: To extend the application of current radiation therapy (RT) based normal tissue complication probability (NTCP) models of radiation-induced fibrosis (RIF) of the breast to include the effects of fractionation, inhomogeneous dose, incomplete recovery, and time after the end of radiotherapy in partial breast irradiation (PBI).Materials and methods: An NTCP Lyman model with biologically effective uniform dose (BEUD) with and without a correction for the effect of incomplete repair was used. The time to occurrence of RIF was also taken into account. The radiobiological parameters were determined by fitting incidences of moderate/severe RIF in published randomized studies on RT of the breast. The NTCP model was used to calculate the risk of toxicity in 35 patients treated with intensity modulated, non-accelerated PBI and the result was compared with observed incidence of RIF.Results: With α/β fixed at 3Gy the parameters of the model without correction for incomplete repair extracted from fitting were: 50% complication probability biologically effective dose BEUD50 = 107.2 Gy (95%CI = 95.9–118.6 Gy), volume parameter n = 0.06 (95%CI = 0–0.23), and slope of dose response m = 0.22, (95%CI = 0.20–0.23). After including the correction for incomplete repair with repair halftime for subcutaneous tissue of τ = 4.4 h we obtained BEUD50 = 105.8 Gy (95%CI = 96.9–114.6Gy), n = 0.15 (95%CI = 0–0.33), m = 0.22 (95%CI = 0.20–0.23). Average NTCP predicted by these models, 4.3% and 2.0% respectively, offered a good agreement with RIF incidence in our patients, 5.7%, after an average follow-up of 12 months.Conclusion: The NTCP models of RIF, incorporating the effects of fractionation, volume effect, and latency of toxicity look promising to model PBI. Clinical validation from a prospective PBI treatment study is under development and will help test this preliminary result.

Adaptive volumetric modulated arc treatment planning for esophageal cancers using cone beam computed tomography - Corrected Proof

2011-11-14

Abstract: Purpose: To assess the potential of cone beam CT (CBCT) derived adaptive RapidArc treatment for esophageal cancers in reducing the dose to organs at risk (OAR).Methods and materials: Ten patients with esophageal cancer were CT scanned in free breathing pattern. The PTV is generated by adding a 3D margin of 1 cm to the CTV as per ICRU 62 recommendations. The double arc RapidArc plan (Clin_RA) was generated for the PTV. Patients were setup using kV orthogonal images and kV-CBCT scan was acquired daily during first week of therapy, then weekly. These images were exported to the Eclipse TPS. The adaptive CTV which includes tumor and involved nodes was delineated in each CBCT image set for the length of the PTV. The composite CTV from first week CBCT was generated using Boolean union operator and 5 mm margin was added circumferentially to generate adaptive PTV (PTV1). Adaptive RapidArc plan (Adap_RA) was generated. NTCP and DVH of the OARs of the two plans were compared. Similarly, PTV2 was generated from weekly CBCT. PTV2 was evaluated for the coverage of 95% isodose of Adap_RA plan.Results: The PTV1 and PTV2 volumes covered by 95% isodose in adaptive plans were 93.51 ± 1.17% and 94.59 ± 1.43% respectively. The lung V10Gy, V20Gy and mean dose in Adap_RA plan was reduced by 17.43% (p = 0.0012), 34.64% (p = 0.0019) and 16.50% (p = 0.0002) respectively compared to Clin_RA. The Adap_RA plan reduces the heart D35% and mean dose by 17.35% (p = 0.0011) and 17.16% (p = 0.0012). No significant reduction in spinal cord and liver doses were observed. NTCP for the lung (0.42% vs. 0.08%) and heart (1.39% vs. 0.090%) was reduced significantly in adaptive plans.Conclusion: The adaptive re-planning strategy based on the first week CBCT dataset significantly reduces the doses and NTCP to OARs.

Boosting runtime-performance of photon pencil beam algorithms for radiotherapy treatment planning - Corrected Proof

M. Siggel, P. Ziegenhein, S. Nill, U. Oelfke — 2011-11-10

Abstract: Pencil beam algorithms are still considered as standard photon dose calculation methods in Radiotherapy treatment planning for many clinical applications. Despite their established role in radiotherapy planning their performance and clinical applicability has to be continuously adapted to evolving complex treatment techniques such as adaptive radiation therapy (ART). We herewith report on a new highly efficient version of a well-established pencil beam convolution algorithm which relies purely on measured input data. A method was developed that improves raytracing efficiency by exploiting the capability of modern CPU architecture for a runtime reduction. Since most of the current desktop computers provide more than one calculation unit we used symmetric multiprocessing extensively to parallelize the workload and thus decreasing the algorithmic runtime. To maximize the advantage of code parallelization, we present two implementation strategies – one for the dose calculation in inverse planning software, and one for traditional forward planning. As a result, we could achieve on a 16-core personal computer with AMD processors a superlinear speedup factor of approx. 18 for calculating the dose distribution of typical forward IMRT treatment plans.

A new isocentric technique for exact geometric matching in the radiotherapy of the breast and ipsilateral supraclavicular fossa using dual asymmetric jaws - Corrected Proof

Nando Romeo — 2011-11-07

Abstract: Background: In some clinical situations breast or chest wall radiotherapy for cancer is given in association with supraclavicular fossa irradiation. Often the treatment is delivered by two tangential fields to the breast or chest wall and an anterior field that irradiates the supraclavicular region. The tissue between the breast or chest wall and the supraclavicular region may be under or overdosed, because of the junction between the two tangential fields and the anterior field.Purpose: To present a new isocentric technique for exact geometric matching between the two tangential fields and the anterior field.Methods: Patients are positioned with both arms raised. Using three-dimensional trigonometry, two half-fields, with isocenter between the breast and the supraclavicular region, are easily matched. The tangential fields have a collimator rotation to protect the lung without additional shielding. The correct gantry, collimator and couch positions are defined for the anterior field to match the tangential fields.Conclusions: A general formula for exact geometric matching in radiotherapy of the breast and supraclavicular fossa is presented. The method does not require additional shielding to eliminate divergence other than the four independent jaws. The result is simple to implement in modern delivery facilities.

Performance of longitudinal and volumetric tube current modulation in a 64-slice CT with different choices of acquisition and reconstruction parameters - Corrected Proof

Aart J. Van der Molen, Raoul M.S. Joemai, Jacob Geleijns — 2011-11-07

Abstract: Aim of the study was to evaluate the performance of a tube current modulation (TCM) system (SUREExposure 3D).On a 64 detector-row CT scanner (Aquilion 64, Toshiba), performance of fixed tube current, longitudinal TCM, and volumetric TCM acquisitions were assessed. A homogeneous cone-shaped phantom and an anthropomorphic phantom were used. Tube current and noise profiles were quantitatively analysed by box and whisker plots when phantom size, acquisition, and reconstruction parameters were varied.At similar median noise, fixed tube current scanning showed a noise range of 16.8–38.3 HU, while longitudinal TCM showed a range of 19.4–31.4 HU and volumetric TCM showed an even lower range of 20.7–28.7 HU. When acquisitions resulting in similar image quality (noise) were compared, the use of volumetric compared to longitudinal TCM resulted in a variable radiation dose reduction up to 6.6%.In conclusion, SUREExposure 3D resulted in more uniform image quality at a lower dose. Volumetric TCM shows improved results over longitudinal TCM.

Angular smoothing and radial regularization of ODF fields: Application on deterministic crossing fiber tractography - Corrected Proof

K.M. Otto, H.-H. Ehricke, V. Kumar, U. Klose — 2011-11-03

Abstract: The advent of high angular resolution diffusion imaging (HARDI) has opened up new perspectives for the delineation of crossing and branching fiber pathways. However, image acquisition under clinical conditions with limited measurement time faces the problem of poor spatial and angular resolution and the technique’s high susceptibility to noise. In this paper we present a straightforward spatial filter for ODF fields that uses the data-inherent structural information around a voxel as part of a directionally selective method for angular smoothing and radial regularization (ASRR). Especially in regions where fibers cross (multimodal voxels), the method allows us to reduce noise, improve the accuracy of ODF diffusion peaks, and strengthen signals of non-dominant fibers. Moreover, we propose a dynamic scheme in which regularization is applied only to ODFs classified as multimodal. The approach is quantitatively evaluated on synthetic datasets of various configurations. With an in vivo dataset of a human subject, measured under clinical imaging conditions, we demonstrate the method’s ability to improve tractography of non-dominant transcallosal fiber pathways and the long fibers of the superior longitudinal fasciculus.

Biophysical characterization of gold nanoparticles-loaded liposomes - Corrected Proof

Mohsen Mahmoud Mady, Mohamed Mahmoud Fathy, Tareq Youssef, Wafaa Mohamed Khalil — 2011-10-24

Abstract: Gold nanoparticles were prepared and loaded into the bilayer of dipalmitoylphosphatidylcholine (DPPC) liposomes, named as gold-loaded liposomes. Biophysical characterization of gold-loaded liposomes was studied by transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy as well as turbidity and rheological measurements. FTIR measurements showed that gold nanoparticles made significant changes in the frequency of the CH2 stretching bands, revealing that gold nanoparticles increased the number of gauche conformers and create a conformational change within the acyl chains of phospholipids. The transmission electron micrographs (TEM) revealed that gold nanoparticles were loaded in the liposomal bilayer. The zeta potential of DPPC liposomes had a more negative value after incorporating of Au NPs into liposomal membranes. Turbidity studies revealed that the loading of gold nanoparticles into DPPC liposomes results in shifting the temperature of the main phase transition to a lower value. The membrane fluidity of DPPC bilayer was increased by loading the gold nanoparticles as shown from rheological measurements. Knowledge gained in this study may open the door to pursuing liposomes as a viable strategy for Au NPs delivery in many diagnostic and therapeutic applications.

Design and realisation of tissue-equivalent dielectric simulators for dosimetric studies on microwave antennas for interstitial ablation - Corrected Proof

V. Lopresto, R. Pinto, R. Lodato, G.A. Lovisolo, M. Cavagnaro — 2011-10-17

Abstract: Thermal ablation therapies, based on electromagnetic field sources (interstitial or intracavitary antennas) at radio and microwave frequencies, are increasingly used in medicine due to their proven efficacy in the treatment of many diseases (tumours, stenosis, etc). Such techniques need standardized procedures, still not completely consolidated, as to analyze the behaviour of antennas for treatment optimisation. Several tissue-equivalent dielectric simulators (also named phantoms) have been developed to represent human head tissues, and extensively used in the analysis of human exposure to the electromagnetic emissions from hand-held devices; yet, very few studies have considered other tissues, as those met in ablation therapies. The objective of this study was to develop phantoms of liver and kidney tissue to experimentally characterise interstitial microwave antennas in reference conditions. Phantom properties depend on the simulated target tissue (liver or kidney) and the considered frequency (2.45 GHz in this work), addressing the need for a transparent liquid to easily control the positioning of the probe with respect to the antenna under test. An experimental set-up was also developed and used to characterise microwave ablation antenna performances. Finally, a comparison between measurements and numerical simulations was performed for the cross-validation of the experimental set-up and the numerical model. The obtained results highlight the fundamental role played by dielectric simulators in the development of microwave ablation devices, representing the first step towards the definition of a procedure for the ablation treatment planning.

The persistent microbicidal effect in water exposed to the corona discharge - Corrected Proof

Jaroslav Julák, Vladimír Scholtz, Soňa Kotúčová, Olga Janoušková — 2011-09-19

Abstract: This article describes and particularly explains a new phenomenon of persistent microbicidal effect of water previously exposed to the low-temperature plasma, which cannot be attributed to the acidification only. The direct microbicidal action of plasma is well documented, being mediated by number of reactive particles with a short lifetime. However, we observed the microbicidal effect also in exposed water stored for a month, where it must be mediated by stable particles. In water and in phosphate-buffered saline, the formation of NOx and corresponding acids, H2O2 and O3 was confirmed after exposition to the low-temperature plasma generated in air by DC negative glow corona and positive streamer discharge. The time course of acidification, H2O2 and O3 formation were deremined. Except uncertain traces of HCN, SIFT-MS analysis of exposed liquids reveals no additional reactive compounds. The microbicidal effect persists almost unchanged during 4 weeks of storage, although O3 completely and H2O2 almost disappears. Staphylococcus epidermidis and Escherichia coli were inactivated within 10 min of incubation in exposed liquids, Candida albicans needs at least 1 h. The solutions prepared by artificial mixing of reactive compounds mimic the action of exposed water, but in lesser extent. The acid milieu is the main cause of the microbicidal effect, but the possibility of still unidentified additional compound remains open.

Dosimetric impact of fiducial markers in patients undergoing photon beam radiation therapy - Corrected Proof

2011-08-29

Abstract: Fiducial markers are widely used in image-guided radiation therapy to correct for setup error and organ motion. These markers, however, can cause dose perturbations in the target volume for patients undergoing external-beam radiation therapy. The goal of this study was to determine the dosimetric impact of various types of fiducial markers commonly used in patients receiving photon radiation therapy. Monte Carlo simulations based on a newly developed EGSnrcMP user code were used to investigate three types of gold fiducial markers and a carbon marker. A single photon field with each fiducial in various orientations and two parallel-opposed beams were simulated at 6-MV and 18-MV energies. The results indicated that dose perturbations depended on marker size, material, and orientation, as well as on incident beam energy. Maximum dose perturbations were found for a single 6-MV beam. The increase in dose reached a factor of 1.58 near the upstream surface of the gold marker because of electron backscatter. At the downstream surface, the dose was reduced to a factor of 0.53 at the same point without the marker. For the 18-MV beam, the maximum dose factor was 1.48 and the minimum dose factor was 0.66. For the two parallel-opposed beams, the maximum dose reduction was within 5% at 6 MV and 2% at 18 MV. Dose enhancement, however, remained significant, reaching factors of 1.20 and 1.33 for the two energies near the fiducial surface. Carbon fiducials caused dose perturbations of only ∼1%.

Modelling the influence of thermal effects induced by radio frequency electric field on the dynamics of the ATPase nano-biomolecular motors - Corrected Proof

A. Lohrasebi, S. Mohamadi, S. Fadaie, H. Rafii-Tabar — 2011-08-08

Abstract: We model the dynamics of the F0 component of the F0F1-ATPase mitochondrion-based nano-motor operating in a stochastically-fluctuating medium that represents the intracellular environment. The stochastic dynamics are modeled via Langevin equation of motion wherein fluctuations are treated as white noise. We have investigated the influence of an applied alternating electric field on the rotary motion of the F0 rotor in such an environment. The exposure to the field induces a temperature rise in the mitochondrion’s membrane, within which the F0 is embedded. The external field also induces an electric potential that promotes a change in the mitochondrion’s transmembrane potential (TMP). Both the induced temperature and the change in TMP contribute to a change in the dynamics of the F0. We have found that for external fields in the radio frequency (RF) range, normally present in the environment and encountered by biological systems, the contribution of the induced thermal effects, relative to that of the induced TMP, to the dynamics of the F0 is more significant. The changes in the dynamics of the F0 part affect the frequency of the rotary motion of the F0F1-ATPase protein motor which, in turn, affects the production rate of the ATP molecules.

Validation of a Monte Carlo simulation for dose assessment in dental cone beam CT examinations - Corrected Proof

2011-08-02

Abstract: A Monte Carlo (MC) simulation for calculating absorbed dose has been developed and applied for dental applications with an i-CAT cone beam CT (CBCT) system. To validate the method a comparison was made between calculated and measured dose values for two different clinical protocols. Measurements with a pencil CT chamber were performed free-in-air and in a CT dose head phantom; measurements were also performed with a transmission ionization chamber. In addition for each protocol a total number of 58 thermoluminescence dosemeters (TLD) were packed in groups and placed at 16 representative anatomical locations of an anthropomorphic phantom (Remab system) to assess absorbed doses. To simulate X-ray exposure, a software application based on the EGS4 package was applied. Dose quantities were calculated for different voxelized models representing the CT ionization and transmission chambers, the TLDs, and the phantoms as well. The dose quantities evaluated in the comparison were the accumulated dose averaged along the rotation axis (), the volume average dose, for the dosimetric phantom, the dose area product (DAP) and the absorbed dose for the TLDs. Absolute differences between measured and simulated outcomes were ≤2.1% for free-in-air doses; ≤6.2% in the 5 cavities of the CT dose head phantom; ≤13% for TLDs inside the primary beam. Such differences were considered acceptable in all cases and confirmed the validity of the MC program for different geometries.In conclusion, the devised MC simulation program can be a robust tool to optimize protocols and estimate patient doses for CBCT units in dental, oral and maxillofacial radiology.

Comparative clinical dosimetry with X-knife and gamma knife - Corrected Proof

M.K. Semwal, Sukhvir Singh, A. Sarin, S. Bhatnagar, H.C. Pathak — 2011-08-01

Abstract: X-knife and gamma knife techniques are well-established for cranial stereotactic radiosurgery (SRS). Due to differences in their radiation delivery methods, some of the dosimetric parameters of these two techniques differ which may have clinical significance. There are many dosimetric studies comparing linear accelerator based techniques such as X-knife with gamma knife but generally from different institutions. We carried out a retrospective comparative study of the dosimetric parameters of the SRS treatments performed at our centre with X-knife (circular cones) and gamma knife. Our results indicate that the dose conformity and dose fall-off in the vicinity of the target volumes were better for patients treated with gamma knife as compared to X-knife. However, the dose fall-off pattern shows a reversal at a larger distance from the target. It was better for the X-knife as compared to gamma knife in the low dose region.

The Education and training of clinical medical physicists in 25 European, 2 North American and 2 Australasian countries: Similarities and differences - Corrected Proof

2011-07-27

Abstract: Purpose: The clinical medical physicist is part of a team responsible for safe and competent provision of radiation-based diagnostic examinations and therapeutic practices. To ensure that the physicist can provide an adequate service, sufficient education and training is indispensable. The aim of this study is to provide a structured description of the present status of the clinical medical physicist education and training framework in 25 European, 2 North American and 2 Australasian countries.Methods: For this study, data collection was based on a questionnaire prepared by the European Federation of Organizations in Medical Physics (EFOMP) and filled-in either by the corresponding scientific societies-organizations or by the authors.Results: In the majority of cases, a qualified medical physicist should have an MSc in medical physics and 1–3 years of clinical experience. Education and training takes place in both universities and hospitals and the total duration of the programs ranges from 2.5 to 9 years. In 56% of all European countries, it is mandatory to hold a diploma or license to work as a medical physicist, the situation being similar in Australasian and 4 states of USA. Generally, there are national registers of medical physicists with inclusion on the register being voluntary. There are renewal mechanisms in the registers usually based on a Continuing Professional Development (CPD) system.Conclusions: In conclusion, a common policy is followed in general, on topics concerning education and training as well as the practice of the medical physicist profession, notwithstanding the presence of a few differences.

QA of dynamic MLC based on EPID portal dosimetry - Corrected Proof

2011-07-25

Abstract: Purpose: Dynamic delivery of intensity modulated beams (dIMRT) requires not only accurate verification of leaf positioning but also a control on the speed of motion. The latter is a parameter that has a major impact on the dose delivered to the patient. Time consumed in quality assurance (QA) procedures is an issue of relevance in any radiotherapy department. Electronic portal imaging dosimetry (EPID) can be very efficient for routine tests. The purpose of this work is to investigate the ability of our EPID for detecting small errors in leaf positioning, and to present our daily QA procedures for dIMRT based on EPID.Methods and materials: A Varian 2100 CD Clinac equipped with an 80 leaf Millennium MLC and with amorphous silicon based EPID (aS500, Varian) is used. The daily QA program consists in performing: Stability check of the EPID signal, Garden fence test, Sweeping slit test, and Leaf speed test.Results and discussion: The EPID system exhibits good long term reproducibility. The mean portal dose at the centre of a 10 × 10 cm2 static field was 1.002 ± 0.004 (range 1.013–0.995) for the period evaluated of 47 weeks. Garden fence test shows that leaf position errors of up to 0.2 mm can be detected. With the Sweeping slit test we are able to detect small deviations on the gap width and errors of individual leaves of 0.5 and 0.2 mm. With the Leaf speed test problems due to motor fatigue or friction between leaves can be detected.Conclusions: This set of tests takes no longer than 5 min in the linac treatment room. With EPID dosimetry, a consistent daily QA program can be applied, giving complete information about positioning/speed MLC.

Five-year experience of quality control for a 3D LSO-based whole-body PET scanner: Results and considerations - Corrected Proof

R. Matheoud, A.L. Goertzen, L. Vigna, J. Ducharme, G. Sacchetti, M. Brambilla — 2011-07-25

Abstract: PET scanners require routine monitoring and quality control (QC) to ensure proper scanner performance. QC helps to ensure that PET equipment performs as specified by the manufacturer and that there have not been significant changes in the system response since acceptance. In this work we describe the maintenance history and we report on the results obtained from the PET system QC testing program over 5 years at two centers, both utilizing a Siemens Biograph 16 HiRez PET/CT system. QC testing programs were based on international standards and included the manufacturer’s daily QC, monthly uniformity and sensitivity, quarterly cross-calibration and annual resolution and image quality.For the Winnipeg and Novara sites, two and one PET detector blocks have been replaced, respectively. Neither system has had other significant PET system related hardware replacements. The manufacturer’s suggested daily QC was sensitive to detecting problems in the function of PET detector elements. The same test was not sensitive for detecting long term drifts in the systems: the Novara system observed a significant deterioration over five years of testing in the sensitivity which exhibited a decrease of 16% as compared to its initial value measured at system installation. The measure of the energy spectrum, showed that the 511keV photopeak had shifted to a position of 468keV. This shift was corrected by having service personnel perform a complete system calibration and detector block setup.We recommend including tests of system energy response and of sensitivity as part of a QC program since they can provide useful information on the actual performance of the scanner. A modification of the daily QC test by the manufacturer is suggested to monitor the long term stability of the system. Image quality and spatial resolution tests have proven to be of limited value for monitoring the system over time.

Impact of miscentering on patient dose and image noise in x-ray CT imaging: Phantom and clinical studies - Corrected Proof

M.A. Habibzadeh, M.R. Ay, A.R. Kamali Asl, H. Ghadiri, H. Zaidi — 2011-07-11

Abstract: The operation of the bowtie filter in x-ray CT is correct if the object being scanned is properly centered in the scanner’s field-of-view. Otherwise, the dose delivered to the patient and image noise will deviate from optimal setting. We investigate the effect of miscentering on image noise and surface dose on three commercial CT scanners. Six cylindrical phantoms with different size and material were scanned on each scanner. The phantoms were positioned at 0, 2, 4 and 6 cm below the isocenter of the scanner’s field-of-view. Regression models of surface dose and noise were produced as a function of miscentering magnitude and phantom’s size. 480 patients were assessed using the calculated regression models to estimate the influence of patient miscentering on image noise and patient surface dose in seven imaging centers. For the 64-slice CT scanner, the maximum increase of surface dose using the CTDI-32 phantom was 13.5%, 33.3% and 51.1% for miscenterings of 2, 4 and 6 cm, respectively. The analysis of patients’ scout scans showed miscentering of 2.2 cm in average below the isocenter. An average increase of 23% and 7% was observed for patient dose and image noise, respectively. The maximum variation in patient miscentering derived from the comparison of imaging centers using the same scanner was 1.6 cm. Patient miscentering may substantially increase surface dose and image noise. Therefore, technologists are strongly encouraged to pay greater attention to patient centering.

A strategic development model for the role of the biomedical physicist in the education of healthcare professionals in Europe - Corrected Proof

2011-07-04

Abstract: This is the third of a series of articles targeted at biomedical physicists providing educational services to other healthcare professions, whether in a university faculty of medicine/health sciences or otherwise (e.g., faculty of science, hospital-based medical physics department). The first paper identified the past and present role of the biomedical physicist in the education of the healthcare professions and highlighted issues of concern. The second paper reported the results of a comprehensive SWOT (strengths, weaknesses, opportunities, threats) audit of that role. In this paper we present a strategy for the development of the role based on the outcomes of the SWOT audit. The research methods adopted focus on the importance of strategic planning at all levels in the provision of educational services. The analytical process used in the study was a pragmatic blend of the various theoretical frameworks described in the literature on strategic planning research as adapted for use in academic role development. Important results included identification of the core competences of the biomedical physicist in this context; specification of benchmarking schemes based on experiences of other biomedical disciplines; formulation of detailed mission and vision statements; gap analysis for the role. The paper concludes with a set of strategies and specific actions for gap reduction.

Quality control for system count rate performance with scatter in gamma cameras - Corrected Proof

D. Guirado, J.C. Ramírez, J.M. de la Vega, M. Vilches, A.M. Lallena — 2011-06-06

Abstract: We aimed to analyze the optimal conditions to carry out the periodical statistical control tests of the gamma camera count rate performance. First we focused in reproducing the actual R20 value of the gamma camera response. Second we studied the variability of this parameter in the statistical control test. We performed a reference measurement, which consisted of the determination of the complete curve relating observed and incident count rates, the counting model describing it and the reference R20. This reference determined the conditions for the statistical control tests and the way to analyze the results obtained. Results from three different gamma cameras were studied. Each gamma camera showed a different behavior and required specific data analysis. The optimal conditions to perform the statistical control test were determined in each case. Our procedure provides the information necessary to correlate the average value of R20 obtained in the quality control test with the reference one. The critical requirement to perform any statistical control test, that is to have a reduced variability of the control variable, can be fulfilled in this case only for relatively high activities.

Monte Carlo derivation of AAPM TG-43 dosimetric parameters for GZP6 Co-60 HDR sources - Corrected Proof

Sanaz Hariri Tabrizi, Alireza Kamali Asl, Zohreh Azma — 2011-05-30

Abstract: Cobalt 60 source is generally available on high dose rate (HDR) afterloading equipment especially for treatment of gynecological lesions. The GZP6 remote afterloader (Nuclear Power Institute of China) utilizes 60Co sources for treatment of intracavitary and intraluminal malignancies. In this study, the AAPM TG-43 dosimetric parameters of three sources in GZP6 system have been studied using MCNP4C Monte Carlo (MC) code; and the results are compared with other available 60Co HDR sources. The presented parameters consist of air kerma strength, dose rate constant, radial dose function and anisotropy function. They show less than 1% uncertainty. The TG-43 based dosimetry data can be used not only to validate the dedicated treatment planning software (TPS), but also to introduce new complementary software to enhance the system performance in gynecological treatments.

From analytic inversion to contemporary IMRT optimization: Radiation therapy planning revisited from a mathematical perspective - Corrected Proof

Yair Censor, Jan Unkelbach — 2011-05-27

Abstract: In this paper we look at the development of radiation therapy treatment planning from a mathematical point of view. Historically, planning for Intensity-Modulated Radiation Therapy (IMRT) has been considered as an inverse problem. We discuss first the two fundamental approaches that have been investigated to solve this inverse problem: Continuous analytic inversion techniques on one hand, and fully-discretized algebraic methods on the other hand. In the second part of the paper, we review another fundamental question which has been subject to debate from the beginning of IMRT until the present day: The rotation therapy approach versus fixed angle IMRT. This builds a bridge from historic work on IMRT planning to contemporary research in the context of Intensity-Modulated Arc Therapy (IMAT).

Clinical investigations of a CVD diamond detector for radiotherapy dosimetry - Corrected Proof

G.T. Betzel, S.P. Lansley, F. Baluti, L. Reinisch, J. Meyer — 2011-05-18

Abstract: X-ray detectors based on single crystal diamond film made via chemical vapor deposition were investigated to evaluate their performance under clinically relevant conditions for radiotherapy dosimetry. Studies focused on repeatability, dose rate dependence, tissue phantom ratios, output factors and beam profiling. Repeatability experiments revealed a temporary loss in sensitivity due to charge detrapping effects following irradiation, which was modeled to make corrections that improved short-term precision. Dose rate dependence was observed (Fowler fitting parameter Δ = 0.96 ± 0.2) using dose rates up to ∼2 Gy min^−1. The detector statistically distinguished (n = 5, P < 0.05) between dose values separated by 7.7 × 10^−3 Gy (1 MU). Depth dose measurements from 1 to 15 cm and output factors using 3 × 3 to 10 × 10 cm^2 field sizes compared well with a Farmer ion chamber (<1.3% difference). Output factor measurements indicate encouraging results for fields sizes <4 × 4 cm^2. Off-axis measurements showed that perturbation of the beam could be reduced when the detector is used in the edge-on orientation due to its thin-film sandwich configuration and ∼200 nm thick Ag contacts. This relatively inexpensive detector has potential to be used for routine dosimetry using conventional radiotherapy instrumentation.

Kilovoltage beam model for flat panel imaging system with bow-tie filter for scatter prediction and correction - Corrected Proof

M. Blessing, M.S. Bhagwat, Y. Lyatskaya, J.R. Bellon, J. Hesser, P. Zygmanski — 2011-05-16

Abstract: Purpose: Kilovoltage flat-panel imaging systems are used for cone-beam Computed Tomography (CBCT) and digital Tomosynthesis (DTS). Hereby, the presence of scatter and relatively large dose from imaging are challenging factors. In this study a phenomenological beam model was developed to characterize imager response to imaging beams with a bow-tie filter (Varian OBI system).Materials and method: The kilovoltage beam model was based on dose ratio formalism and thus was using standard concepts of megavoltage dose calculation such as scatter factors, tissue maximum ratio and off-axis ratio. Primary and scatter (head and phantom scatter) were modeled with three Gaussian kernels. Parameters were based on measured transmission images for slabs of solid water of different total thickness and various jaw settings.Results: The beam model was used to evaluate contributions from primary, secondary and tertiary contributions for different geometrical objects such as cylinders and step-like phantoms. Theoretical predictions of radiographs using the model for known objects are consistent with the measurements.Conclusion: Secondary and tertiary contributions were interpreted as scatter and can be subtracted from CBCT projections based on the analytical model. Therefore our model can provide a basis for improvement of image quality (less artifacts due to scatter, better contrast and resolution) in CBCT reconstruction.

First application of total skin electron beam irradiation in Greece: Setup, measurements and dosimetry - Corrected Proof

2011-04-25

Abstract: Total Skin Electron Beam (TSEB) irradiation is considered as the treatment of choice for cutaneous T-cell lymphoma internationally, for either curative purposes or palliative care. An attempt for the first application of this external radiation therapy technique in Greece took place at the Radiation Therapy Unit of 2nd Department of Radiology of University of Athens at University General Hospital “Attikon”. TSEB modality was developed on a linear accelerator VARIAN Clinac 2100C. To create a uniform and sufficiently large field () at , two symmetrical 6 MeV electron beams are combined with 17.5° tilts concerning the horizontal direction. An immobilization system was constructed to support patient during treatment and to modulate the composite electron field. Irradiation procedure demands a standing patient that takes, in total, six treatment positions. For the confirmation of treatment suitability and the determination of physical features of the clinical electron field, specific measurements were carried out using a parallel-plate ionization chamber and TLDs at water equivalent plastic and anthropomorphic phantoms. Measurements at the referred conditions showed a homogeneous total field with intensity variation of ±2% in the longitudinal axis and ±4% at horizontal axis. The mean energy of the composite field () is 3.4 MeV, the most probable energy () is 4.4 MeV and the half-value depth in water () is 1.5 g/cm2. The maximum X-ray background of the TSEB field is 2.1% at head and feet. The above results lead us to conclude that TSEB treatment using “Six-dual-field” technique can be applied in our department safely.

A dosimetry method in the transverse plane of HDR Ir-192 brachytherapy source using gafchromic EBT2 film - Corrected Proof

S.C. Uniyal, S.D. Sharma, U.C. Naithani — 2011-04-21

Abstract: Radiochromic film dosimetry is increasingly used in brachytherapy applications for its higher resolution ability as compared to other experimental methods. The present study was aimed to assess the accuracy and suitability of use of the improved radiochromic film model, Gafchromic EBT2, to evaluate the dose distribution in the transverse plane of microselectron HDR 192Ir source.A specially designed and locally fabricated Polymethyl methacrylate (PMMA) phantom was used in this work for the experimental measurement of dose distribution around the source in its transverse plane. The AAPM TG-43U1 recommended radial dose function, g (r), and dose rate constant, Λ, for the source were measured using Gafchromic EBT2 film and thermoluminescent dosimeters (TLD). The EBT2 film measured dosimetric quantities were validated against their values obtained from the TLD measurements and previously published values for the same source available in literature.The dose rate constant and radial dose function for microselectron HDR 192Ir source obtained from Gafchromic EBT2 film measurements are in agreement with their TLD measured results within 3.9% and 2.8% respectively. They also agree within the accepted range of uncertainty with their experimental and Monte Carlo calculated results reported in literature.This work demonstrates the suitability of using Gafchromic EBT2 film dosimetry in characterization of dose distribution in the transverse plane of HDR Ir-192 source. This is a more efficient method than TLD dosimetry at discrete and distant positions. Relative to TLD dosimetry, it is found to be better reproducible, easy to use and a less expensive method of dosimetry.

CT iterative reconstruction in image space: A phantom study - Corrected Proof

C. Ghetti, O. Ortenzia, G. Serreli — 2011-04-18

Abstract: Although iterative reconstruction is widely applied in SPECT/PET, its introduction in clinical CT is quite recent, in the past the demand for extensive computer power and long image reconstruction times have stopped the diffusion of this technique. Recently Iterative Reconstruction in Image Space (IRIS) has been introduced on Siemens top CT scanners. This recon method works on image data area, reducing the time-consuming loops on raw data and noise removal is obtained in subsequent iterative steps with a smoothing process. We evaluated image noise, low contrast resolution, CT number linearity and accuracy, transverse and z-axis spatial resolution using some dedicated phantoms in single, dual source and cardiac mode. We reconstructed images with a traditional filtered back-projection algorithm and with IRIS. The iterative procedure preserves spatial resolution, CT number accuracy and linearity moreover decreases image noise. These preliminary results support the idea that dose reduction with preserved image quality is possible with IRIS, even if studies on patients are necessary to confirm these data.

Analysis of image sharpness reproducibility on a novel engineered micro-CT scanner with variable geometry and embedded recalibration software - Corrected Proof

D. Panetta, N. Belcari, A. Del Guerra, A. Bartolomei, P.A. Salvadori — 2011-04-18

Abstract: This study investigates the reproducibility of the reconstructed image sharpness, after modifications of the geometry setup, for a variable magnification micro-CT (μCT) scanner. All the measurements were performed on a novel engineered μCT scanner for in vivo imaging of small animals (Xalt), which has been recently built at the Institute of Clinical Physiology of the National Research Council (IFC-CNR, Pisa, Italy), in partnership with the University of Pisa. The Xalt scanner is equipped with an integrated software for on-line geometric recalibration, which will be used throughout the experiments. In order to evaluate the losses of image quality due to modifications of the geometry setup, we have made 22 consecutive acquisitions by changing alternatively the system geometry between two different setups (Large FoV – LF, and High Resolution – HR). For each acquisition, the tomographic images have been reconstructed before and after the on-line geometric recalibration. For each reconstruction, the image sharpness was evaluated using two different figures of merit: (i) the percentage contrast on a small bar pattern of fixed frequency (f = 5.5 lp/mm for the LF setup and f = 10 lp/mm for the HR setup) and (ii) the image entropy. We have found that, due to the small-scale mechanical uncertainty (in the order of the voxel size), a recalibration is necessary for each geometric setup after repositioning of the system’s components; the resolution losses due to the lack of recalibration are worse for the HR setup (voxel size = 18.4 μm). The integrated on-line recalibration algorithm of the Xalt scanner allowed to perform the recalibration quickly, by restoring the spatial resolution of the system to the reference resolution obtained after the initial (off-line) calibration.

On impulse response functions computed from dynamic contrast-enhanced image data by algebraic deconvolution and compartmental modeling - Corrected Proof

Gunnar Brix, Mona Salehi Ravesh, Stefan Zwick, Jürgen Griebel, Stefan Delorme — 2011-04-15

Abstract: Concentration-time courses measured by dynamic contrast-enhanced (DCE) imaging can be described by a convolution of the arterial input with an impulse response function, QT(t), characterizing tissue microcirculation. Data analysis is based on two different approaches: computation of QT(t) by algebraic deconvolution (AD) and subsequent evaluation according to the indicator dilution theory (IDT) or parameterization of QT(t) by analytical expressions derived by compartmental modeling. Pitfalls of both strategies will be addressed in this study.Tissue data acquired by DCE-CT in patients with head-and-neck cancer and simulated by a reference model (MMID4) were analyzed by a two-compartment model (TCM), a permeability-limited two-compartment model (PL-TCM) and AD. Additionally, MMID4 was used to compute the ‘true’ response function that corresponds to the simulated tumor data.TCM and AD yielded accurate fits, whereas PL-TCM performed worse. Nevertheless, the corresponding response functions diverge markedly. The response curves obtained by TCM decrease exponentially in the early perfusion phase and overestimate the tissue perfusion, QT(0). AD also resulted in response curves starting with a negative slope and not – as the ‘true’ response function in accordance with the IDT – with a horizontal plateau. They are thus not valid responses in the sense of the IDT that can be used unconditionally for parameter estimation.Response functions differing considerably in shape can result in virtually identical tissue curves. This non-uniqueness makes a strong argument not to use algebraic but rather analytical deconvolution to reduce the class of solutions to representatives that are in accordance with a-priori knowledge. To avoid misinterpretations and systematic errors, users must be aware of the pitfalls inherent to the different concepts.