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The imaging dose for intra- and extra-cranial CyberKnife radiosurgery applications was calculated and the scattered radiation reaching the digital detectors was quantified and analyzed with regard to its origin.The image guidance subsystem of the CyberKnife was modeled based on vendor-provided information. The emitted X-ray energy spectrum for 120 kV was estimated using the SpekPy software tool. Monte Carlo (MC) image acquisition simulations were performed to calculate the total, primary and scattered photon fluences reaching each detector as a function of the imaged object dimensions. MC calculations of the imaging dose were performed for intra- and extra-cranial applications assuming 120 kV and 10 mAs acquisition settings.The amount of scattered radiation reaching each detector was found to depend on the dimensions of the imaged anatomical region, contributing more than 40 % to the total photon fluence for regions more than 20 cm thick. More than 20 % of this scattered radiation originates from the contralateral imaging field. A maximum organ dose of 1.5 mGy at the nasal bones and an average dose of 0.37 mGy to the eye lenses per image pair acquisition was calculated for head applications. An entrance imaging dose of 0.4 mGy was calculated for extracranial applications.Scattered radiation reaching each detector in the skull and spine tracking applications can be reduced by acquiring the pair of radiographs sequentially instead of simultaneously. A dose of 3.7 cGy to the eye lenses is estimated assuming 100 image pair exposures required for treatment completion.

Purpose Α customized approach to determine R2* relaxation rate for hepatic iron concentration (HIC) estimation is presented, and is evaluated in the context of concurrent liver fat infiltration. Methods The proposed method employs a customized acquisition protocol, featuring a 16-echo, gradient-echo sequence, and a bi-exponential least squares fitting that considers baseline noise and uses a cosine function to correct for fat-induced signal oscillation. 193 patients with wide-ranging HIC and liver fat fraction (FF) were imaged at 1.5 T. In severely iron-overload patients, a four-echo train technique was applied to enforce all 16 echoes in the 1.2–4.0 ms range. Acquired data were compared to corresponding results obtained with the IDEAL IQ method. Results Techniques employed to counter the rapid signal decay in iron-overloaded liver, such as the offset and the truncation methods, have to be combined with the appropriate calibration curve to provide reliable HIC estimation. When high grade steatosis and siderosis co-exist, fat-suppression may downgrade siderosis. A high correlation was observed between data obtained with the proposed technique and the IDEAL IQ method, except from the high R2* region. However, systematic differences were detected. In the concurrent presence of high FF and non-severe iron overload, it is postulated that the bi-exponential model may attribute patient siderosis grading more accurately than IDEAL IQ, while simultaneously providing reliable FF estimation. Conclusions The proposed approach is widely available and seems capable of providing reliable R2* measurements regardless of liver steatosis grading, whilst it succeeds in averting significant R2* underestimation in severely iron-overloaded liver.

Purpose MR images inherently suffer from spatial distortions. Spatial distortions play an important role particularly in quantitative MRI, where MRI parameters are employed as quantitative biomarkers, and in MRI-based treatment planning for advanced radiotherapy applications, which require high spatial accuracy in target localization and definition. Methods and results Spatial distortions stem from sources which either relate (sequence dependent distortions) or do not relate (non-sequence dependent distortions) to the measurement conditions (field strength, pulse sequence, scanning parameters, etc). Non-sequence dependent distortions arise from gradient field non-linearity and, thus, are related to the MRI system. Although MRI scanners utilize post-imaging correction algorithms which reduce gradient field non-linearity distortions, residual distortions may still be considerable at areas distant from the isocenter. These can be estimated and corrected with the aid of specially designed phantoms. Sequence dependent distortions arise from static magnetic field inhomogeneity (ΔBo), magnetic susceptibility (ΔBx) and chemical shift (ΔBcs) phenomena, collectively referred to as field inhomogeneities, which may be machine-related (ΔBo), or patient-induced ((ΔBx) and (ΔBcs)). Sequence dependent spatial distortions scale linearly with B0 and can be partly controlled through the optimization of the scanning parameters and conditions that affect them. However, patient-induced relevant phenomena cannot be predicted and may not be constant in time (e.g., susceptibility effects related to the presence of gadolinium-based contrast agents). Therefore, patient-specific distortion characterization and/or correction has drawn considerable attention. Relevant studies rely on either the field mapping technique or the read gradient polarity reversal method. Both methodologies not only account for susceptibility and chemical shift distortions but also for ΔBo related distortion. It is worth noting that the total distortion magnitude measured is mostly affected by the location dependent relative signs of distortion components stemming from different sources, such as B0 inhomogeneity and susceptibility effects. Spatial distortions also result in signal intensity distortions, since the signal intensity of a homogeneous voxel is either compressed or extended in a voxel of different size, shape and position. Conclusions Overall, it is essential to characterize, reduce and correct spatial distortions, so as not to adversely affect MRI quantitative results or introduce inaccuracies in MRI-based treatment planning applications.

Introduction Gliomatosis cerebri was not recognised as a separate entity till a few years ago and it was thought to be a rare entity. Advanced imaging methods and In Vivo Proton MR spectroscopy (MRS) have contributed to the safe diagnosis of the disease, which is not such a rare entity as once thought. Purpose To explore the capabilities of In Vivo Proton MR Spectroscopy not only for diagnosing the disease but also for exploring the metabolism of the tumor cells. Materials and methods In an extended period of 16 years, 147 patients were diagnosed with gliomatosis cerebri by In Vivo MR Spectroscopy (single voxel at selected areas and 3D Spectroscopic Imaging). Metabolic maps of choline and creatines showed tumor infiltration. For some patients serial MRS was also performed. Results In virtually all cases except one – a patient diagnosed with gliomatosis cerebri for whom the biopsy showed limbic encephalitis – biopsy confirmed a ”glioma” or in cases without biopsy the progress of the disease confirmed the MRS diagnosis. What was a surprise was that in most cases creatine concentration was higher than normal leading to the notion that part of the tumor cells maintain for some time normal cell metabolism (aerobic glycolysis). With advance of time, the metabolism switches to the primitive tumor cell metabolism. Conclusion In Vivo Proton MR Spectroscopy is a potent probe for the diagnosis of gliomatosis cerebri as well as for exploring tumor cell metabolism. Disclosure None of the authors has anything to declare.

Introduction Geometric disposition in MR images is partly attributed to patient/object induced distortions stemming from susceptibility differences and chemical shift artifacts. Purpose To develop and implement a procedure for patient-induced distortion characterization, as well as to evaluate and compare three distortion correction schemes in MR images used for target localization in stereotactic radiosurgery. Materials and methods A specially designed phantom was modified to accommodate two cylindrical-shape volumes lying in close proximity to control points (CPs) for distortion correction. These volumes, simulating brain metastases, were filled with Gd-solution, while the rest phantom was filled with standard copper sulfate solution. Following 1.5T MRI, Gd-induced distortion was estimated using CPs disposition analysis. Moreover, three distortion correction schemes were employed: the phase difference map (based on a dual-echo gradient echo sequence), as well as the average-image and the signal integration techniques (both based on frequency-encoding direction reversal). Residual distortion was assessed by comparing with corresponding CT scans. Results Gd-induced distortion results in a structure disposition of the of the order of 1 mm. The phase difference method proves excellent at high-signal regions but lack of phase information at low-signal areas results in unwrapping errors. The signal integration method relies on the accuracy of edge detection and noise removal at low-signal areas. The average-image technique is a simple and efficient method to improve geometric accuracy in such applications. Conclusion Patient-induced distortion in MR images should be considered and corrected for high fidelity target localization. One should be aware of the relative advantages/drawbacks of alternative correction schemes.

Introduction Mean Fractional Anisotropy (FA) along with Longitudinal and Transverse Apparent Diffusion Coefficients (ADC) are the established measures in assessing white matter (WM) integrity in TBI patients. A question is raised whether mean FA performs better than Shannon Entropy (SE) as a measure of discrimination between controls and patients with moderate to severe TBI. Purpose To assess WM integrity in moderate to severe TBI patients utilizing the established diffusion tensor measures, and compare FA with Shannon Entropy as measures of assessment of WM integrity. Materials and methods A group of 13 patients with moderate to severe TBI, 2–8 years post-ictus, has been compared with a group of 14 matched healthy controls. Diffusion Tensor Imaging (DTI) data were obtained at 3T with a pulse sequence employing 32 diffusion-encoding gradient directions. After data preprocessing, Tract Based Spatial Statistics (FSL-FMRIB) for FA, whole-brain histogram analysis for SE and Receiver Operating Curve (ROC) analysis for both FA and SE have been performed. Results Higher axial and transverse diffusivity and lower FA have been observed in TBI patients ( p P = 0.007), Global mean FA (0.8, P = 0.0066) and SE (0.8, p = 0.008). Conclusion Axial and transverse diffusivity are increased in TBI patients compared to controls. Diffuse Axonal Injury (DAI) is evident as global reduction of FA in TBI patients, which is driven by increased transverse diffusion changes. No significant difference is observed between FA and SE in TBI patient discrimination. Disclosure The authors declare no conflicts of interest including financial, consultant, institutional, and other relationships that might lead to bias or a any other conflict of interest. This work was supported by the Cyprus Research Promotion Foundation through a grant co-funded by the Cyprus Government and the European Regional Development Fund (F.C., PI; NEW INFRASTRUCTURE/STRATEGIC/0309/37). The authors would like to thank the study volunteers who participated in the project.

Introduction This study refers to effective dose (ED) estimation using the size-specific dose estimator (SSDE). Purpose This work aims at determining DLP SSDE -to-ED (k SSDE ) conversion factors where DLP SSDE is the length product multiplied by the appropriate f-factor (AAPM report 204) for chest-abdomen-pelvis (CAP) CT examinations in three weight-adjusted scan protocols and to compare the ED SSDE derived from k SSDE to that derived from k factors available in the literature. Materials and methods Three anthropomorphic phantoms of 55 kg, 73.5 kg and 96.2 kg body weight filled with 190 TLDs were scanned on a 64-slice multi-detector CT employing the respective weight-adjusted CAP-CT protocol used in clinical routine. Effective diameter was measured in the abdomen area of the phantoms and DLP SSDE was estimated using the respective f factor. For ED estimation, ICRP 103 tissue weighting factors for all organs inside and outside the irradiation field were used. Relevant k SSDE conversion factors were then calculated. These factors were then applied to 140 patients underwent CAP-CT examinations and the derived ED SSDE values were compared to corresponding ED k values. Results In the SSDE was 15.4% higher than ED k while in the medium category (60–90 kg) the corresponding percentage was 28.3%. For the over-weighted category (>90 kg) no significant difference in ED was observed. Conclusion Weight-adjusted DLP SSDE -to-ED conversion factors were determined. In low and medium weight categories, radiation dose may be underestimated when the patient size is not taken into consideration.

Introduction Thyroid remnant ablation with radioiodine is a well-established treatment for patients with Differentiated Thyroid Carcinoma (DTC) following thyroidectomy. After 3–4 days of hospitalization, these patients return home, presenting a possible radiation hazard to their family members. Purpose This work aims at estimating the radiation burden to the cohabitants of DTC patients treated with radioiodine, after hospital release. Materials and methods 724 patients (183/541 men/women) suffering from DTC were treated with radioiodine activities ranging from 1850 to 9250MBq. Thyroid Hormone Withdrawal (THW) was applied in 305 patients (42.1%), whereas 419 (57.9%) patients were prepared with recombinant human TSH (rhTSH). Radioiodine T eff was calculated from dose rate measurements at 1 and 2 m from the patients, performed at regular time intervals throughout their hospitalization. All patients were discharged 3–4 days post-treatment and were provided with written and oral instructions about radiation protection precautions. Patients were grouped according to their family status and radiation doses to their cohabitants were estimated based on appropriate exposure scenarios. Results Mean radioiodine T eff was 15.3 h (range 1.9–37.5 h). T eff in the rhTSH group was lower than the THW group (14.5 h and 17.1 h respectively, p〈0.05). Mean absorbed doses to all family members were estimated to be well below the corresponding dose constraints. Maximum dose to any child was slightly above 0.5 mSv, whereas maximum dose to any non-relative co-traveller was Conclusion Provided necessary precautions are followed, radiation burden to the family members of DTC patients treated with radioiodine following thyroidectomy can be kept lower than the corresponding dose constraints.

Introduction With a plethora of soft-tissue contrast mechanisms, lack of ionizing radiation and on-the-fly computer controller adjustment of imaging parameters, MRI has emerged as an alternative modality for guiding interventions. However, due to its inherent low signal sensitivity, conventional MRI cannot achieve the high speeds of X-ray fluoroscopy. Purpose To address this, we describe a novel approach for 3D MRI of tubular structures such as blood vessels or catheters, based on the collection of thick slab spatially matched projections. Materials and methods The implemented method includes the following: three elements. (1) Collection of three orthogonal projections of the same volume that contains the structure with a GRE (TR/TE = 26.07/3.71 ms, angle = 75°, matrix = 256 × 256, FOV = 200 × 200 mm 2 , slice = 200 mm). (2) Segmentation of the 2D structures on the three projections. (3) Reconstruction of the 3D structure by back projection. The method was tested on phantoms with vessel-mimicking structures made of tubing filled with 2% Gd-agent in a fatty matrix. The ground truth was an MRA (128 slices, TR/TE = 3.8/1.52 ms, angle = 40°, matrix = 384 × 264, FOV = 191 × 131 mm 2 , slice = 1.3 mm). Results The 3D centerline of the rendered structures was extracted and then found to be virtually the same (±pixel) to this extracted from a multislice MRA of the same structure. Conclusion The method can accurately image 3D tubular object in 20 s as compared to 186 s with the used MRA. Disclosure None of the authors has anything to declare.

Introduction The penumbra region is an intrinsic characteristic of any beam, weather photon or electron. Therefore, its correct determination constitutes a vital part in the dosimetric chain of a modern radiation therapy department. Purpose The purpose of this study is to compare and analyze results concerning the penumbra size determination obtained using six different measuring systems. Materials and methods Penumbra measurements were performed for two photon beams (6, 15 MV) for 4×4, 10×10 and 25×25 cm2 square field sizes and for five electron beams (6, 9, 12, 16, 20 MeV) using a 10×10 cm2 applicator at several depths. Six different measuring systems were used: (a) a Semiflex, (b) a Markus, (c) a Roos ionisation chamber, (d) a one dimensional Linnear Array (LA48, PTW), (e) a p-type diode (PTW 60008) and (f) an e-type diode (PTW 60017). All measurements were taken with standard water phantom geometry (MP3, PTW) using water as medium. Results Our results indicate that penumbra size determination is strongly dependant on the measuring system. More specifically, the diodes showed the narrowest penumbras followed by the LA48, while the largest penumbras were presented by the ionisation chambers. In terms of effectiveness, the LA48 method showed significant lower beam-on time. Conclusion Regarding penumbra size determination, the LA48 method is acceptable in terms of accuracy, and it is associated with considerable time-saving compared to other measuring systems. When the greatest possible accuracy is required, the use of silicon diode dosimeters is recommended.

Introduction QA of modern radiotherapy/radiosurgery techniques is quite challenging, requiring dosimetric systems with fine spatial resolution, water equivalence, lack of energy/dose rate dependence and, hopefully, 3D dose registering. Purpose A comparative dosimetric assessment of two 3D dosimeters, one custom and one commercially available, towards a dosimetric system with favorable characteristics for radiotherapy/radiosurgery QA. Materials and methods MRI scanned VIP Polymer Gel and Optical-CT scanned PRESAGE radiochromic dosimeters were calibrated side by side. Both were subjected to the same irradiation scheme, in an extensive dose range. The dosimetric characteristics compared included dose sensitivity, dose resolution, dose rate dependence, useful dose range, minimum detectable dose and, ultimately, the uncertainty budget of the measured dose distribution. Results & conclusion Both 3D dosimetric systems exhibited measured dose uncertainty appropriate for clinical dosimetry. The dose sensitivity of PRESAGE/OCT was higher resulting to improved dose resolution. This adds up to the increased ease of use of PRESAGE/OCT and availability of a dedicated optical scanner. However the increased noise in comparison to Polymer Gel/MRI and the occurrence of concentric ring and significant edge artifacts render this favorable characteristics debatable. Moreover indications of dose rate and energy dependence due to higher density have to be further investigated in order to end up with a dosimetric system to be used in the verification of complex radiotherapy/radiosurgery treatment plans. Disclosure Research conducted with financing by “IKY Postdoc Fellowships of Excellence in Greece – Siemens Programme” in the framework of the Hellenic Republic – Siemens Settlement Agreement.

Introduction In the past few years there have been several methods to determine fat infiltration of the liver, some of them being expensive commercial applications for MRI or expensive services provided by some companies to which the MRI data are sent for evaluation. β -Thalassemia patients with liver iron overload and fat infiltration require determination of both. Purpose To develop and certify a method for simultaneous determination of iron overload and fat infiltration of the liver with a 16-echo single breath-hold gradient echo sequence and compare the calculated results of fat infiltration with those determined by Proton MR spectroscopy (MRS) of the liver. Materials and methods 500 patients were examined at 1.5 Tesla (Signa HDxt, GE, Milwaukee, USA) with a 16-echo sequence at two different echo time ranges (a short and a long one for moderate and low iron overload, respectively). A least-squares-fit of the acquired data yielded T2* values and fat fraction. MR spectra from the liver were acquired from 40 patients. Results The agreement for liver fat infiltration determined by the proposed MRI technique and the MRS method was excellent. The gradient echo method is sensitive enough for fat fractions as low as 1%. Fat infiltration results are accurate for iron concentrations as high as 10 mg/g dry tissue. Conclusion The gradient-echo method is quick and accurate and is a reliable alternative to other expensive methods. Furthermore it provides iron overload and fat infiltration determination simultaneously, in a single breath-hold. Disclosure None of the authors has anything to declare.

There is a continuously evolving paradigm shift in MRI, that of quantitative MRI (qMRI). Therefore, a gradual transition exists from a simplified, phenomenological and descriptive procedure to a quantitative measurement used for early diagnosis, disease evaluation and monitoring, as well as for therapeutic guidance. There is a plethora of parameters that can be used as quantitative markers in MRI, ranging from relaxation times to indices related to complex mechanisms such as diffusion and perfusion. The advent of qMRI has introduced new concepts in clinical MRI practice such as sensitivity, accuracy and specificity. The measurement precision and accuracy can be determined through a carefully designed program of quality control employing both phantoms and healthy volunteers. The need for high specificity lies in choosing the optimal parameter to be measured and monitored. The main requirement for increasing the measurement’s sensitivity and accuracy and for achieving high sensitivity is to fully understand the underlying q-MRI mechanism. With regard to the various sources of error and inaccuracy involved, q-MRI can be divided into two sub-processes: data collection and data analysis. A quality assurance program has to address issues related to both sub-processes. The second one refers to post-processing and, thus, it is a repeatable process which can adopt suitable measures for mitigating pertinent inaccuracies. The former refers to scanning. Spatial distortions and signal inaccuracies stem from many factors related to the measurement conditions. It is essential, therefore, to characterize, reduce and correct these distortions and inaccuracies, so as not to adversely affect quantitative results.