J.A.M. Santos

The role of dipolar interactions in magnetic nanoparticles : Ferromagnetic resonance in discontinuous magnetic multilayers

We have performed detailed ferromagnetic resonance (FMR) studies of [Co80Fe20(t)∕Al2O3(40 A)]10 (t=7−13 A) discontinuous multilayers, with measurements taken as a function of the angle of the applied static magnetic field with respect to the sample at room temperature. We outline an approach to FMR in granular systems by using the magnetic dipolar interaction between magnetic nanoparticles. This can be easily incorporated into the free energy of the system and provides a method for evaluating the average interaction strength in particulate media. Angular dependent measurements show how FMR can be used to evaluate interparticle interactions. Our measurements indicate that there is a change of coupling regime, going from superferromagnetic, for thicker samples (with larger average particle sizes and smaller separations), to superparamagnetic in thinner samples.

Peculiar magnetic and electrical properties near structural percolation in metal-insulator granular layers

We study CoFe∕Al2O3 multilayers, varying from granular to continuous structure with CoFe nominal thickness (10A⩽t⩽18A). Structural percolation takes place at t=tc≈18A, changing from activated to metallic conductance. A sharp anomaly in ac coercivity of magneto-optical Kerr effect and in magnetization M by superconducting quantum interferometer device is found at t=t*≈17A, interpreted as transition from superferromagnetism (by dipolar coupled CoFe clusters) to exchange ferromagnetism. Onset at t=t* of anisotropic magnetoresistance (AMR) is attributed to skew scattering of bandlike electrons within giant fractal clusters. These structures may cause the observed Barkhausen noises in M and AMR at t≈t*, disappearing at t>t*.

Characterization of Geometric Uncertainties of a Dose Calibrator During Measurement of 90Y Activity

The performance and measurement of 90Y with a dose calibrator has always been a key point of several international recommendations, specifically in the case of the radiopharmaceutical 90Y-ibritumomab tiuxetan, among other applications, used in the treatment of non-Hodgkin lymphoma. The objective of the present work was to examine in detail some important sources of geometric errors that, if neglected, can lead to an increase in uncertainty about the measured activity of this β− emitter radionuclide. Methods: A CRC-15R dose calibrator was used to measure and quantify some of these sources, although the same methodology can easily be applied to any other similar equipment. The depth response along the main axis of the dose calibrator was carefully characterized, as well as syringe volume effects and source angular dependence. Results: It was found that, if not taken properly into account, these issues can contribute to an increase in the activity uncertainty (e.g., 5.1% in the present example). This finding implies the possibility of easily reaching higher values than the internationally suggested 5% uncertainty in activity measurement for therapeutic purposes. Conclusion: These errors can be greatly reduced by previous characterization of the dose calibrator and careful implementation of the methodology for measurement.

Time-dependent transport effects in CoFe/Al2O3 discontinuous multilayers

Discontinuous multilayered metal-insulator Co80Fe20(t)/Al2O3(30 A) thin films with different values of nominal thickness t of metal layer were prepared by ion beam deposition. At t<14 A, a new phenomenon of slow electric relaxation was found, suggesting formation of highly nonequilibrium electronic states in the process of tunnel transport in such systems. An extension of the well-known Sheng–Abeles approach on the case of finite concentrations of charged granules is proposed. Within mean-field approximation, it gives a general picture of this phenomenon, though the corrections due to correlation between charges can be essential.

Experimental characterization of megavoltage beams for orthogonal ray imaging

OrthoCT (orthogonal computed tomography) is a potential new imaging technique that aims to acquire images of the volume to be irradiated immediately before or during a radiotherapy treatment. It potentially provides imaging with very low to eventually null dose, allowing to check if the morphology/anatomy of the patient and tumour are in agreement with the planned one. This technique relies on the detection of photons that are scattered in the patient and are emitted perpendicularly to the incident beam direction. To acquire the OrthoCT morphological images the scanning of the volume to be irradiated is done using pencil-like mega-voltage beams. The corresponding scan profile requires: (1) high homogeneity, so that variations can be associated only to dose/morphological alterations, and (2) high velocity, which favors multi-leaf collimator-based scans in respect to jaw-based ones. To compare the variability of a homogeneous beam with the variability of a scanned profile two scans with a cross-section of 5mm × 5mm (MLC-collimated) and 6mm × 6mm (jaw-collimated) were experimentally evaluated. The transverse profiles obtained with MLC-collimation in this work reveal a homogeneity with an intensity variability inferior to 1%, thus supporting OrthoCT imaging with morphology/anatomy changes superior to that value.

Magnetic phase behavior for Gd-rich Gd–Y

The elastic constants and electrical transport properties of single-crystal Gd and Gd–Y are measured for Y⩽26 at. %. There are clear features in the elastic constant C33(T) and in the temperature derivative of the a-axis resistivity, dρa/dT, at the accepted ferromagnetic Curie temperature (TC), and at the accepted spin-reorientation at TSR. In the Gd-rich Gd–Y, there is a third feature at T* below TSR, which is consistent with evidence for a further second-order transition. A modified magnetic phase diagram for Gd–Y is proposed.

Configuration of Volumetric Arc Radiotherapy Simulations Using PRIMO Software: A Feasibility Study

Volumetric Modulated Arc Therapy (VMAT) uses non-uniform intensity fields allowing volumetric complex dose distributions. The simultaneous MultiLeaf Collimator (MLC) motion and Gantry rotation pose difficulties in the dose distribution calculation by Treatment Planning Systems (TPS). Furthermore, a dedicated Quality Assurance (QA) program and patient-specific dose verifications are requested. Monte Carlo dose calculation in Radiotherapy (RT) is a gold standard due to its most detailed description of radiation-matter interaction. Recently, the PRIMO software was proposed, providing several built-in RT units models, including TrueBeam. Nevertheless, VMAT is not implemented yet. In this work, TrueBeam was simulated in PRIMO using 6 and 10 MV in Flatness Filter Free (FFF) mode and at 15 MV with Flatness Filter inserted. The results were validated by Gamma Function (2%, 2 mm) based on reference measurements in water tank. The VMAT complex dynamic delivery is divided into a customizable number of probabilistically sampled static configurations of jaws, leaves and gantry angles. In-house algorithms were developed to interpolate the LINAC geometrical information along the process once the planned information is retrieved from the TPS output DICOM file. A Graphical User Interface (GUI) was developed to assist the user to configure PRIMO to simulate complex deliveries. Static simulations in reference conditions showed always >97% of Gamma points <1 for PDD and profiles at various depths and fields sizes for the 6, 10 and 15 MV primary beam respectively. The GUI properly read, manipulated and wrote the configuration data in a “ppj” format, which was accepted by PRIMO. The dynamic jaws, MLC and gantry motion were positively assessed by visual inspection of the static beam configuration in PRIMO. Dynamic irradiations were simulated and the gamma function tests against reference dose distributions showed good agreement with typical QA criteria. A GUI to configure PRIMO for VMAT irradiations allowed to enable a flexible workflow for simulating a general dynamic treatment.

[P088] Optimization of yttrium-90 bremstrahlung imaging for evaluation of microspheres post-therapeutic liver radioembolization distribution

Purpose Post-therapy imaging of 90Y-Bremsstrahlung x-rays using SPECT, although 90Y is not the best isotope for imaging, has the potential of providing a reliable activity distribution of the 90Y-microspheres inside the liver. In literature, several image acquisition parameters and subsequent different reconstruction protocols can be found. With the aim of obtaining an optimized 90Y-Bremsstrahlung SPECT image, two reconstruction methods, Filtered-Backprojection (FBP) and Ordered-Subsets-Expectation-Maximization (OSEM), were applied both to phantoms and post-therapeutic images using several reconstruction parameters. Methods A 90Y-Bremsstrahlung image of a water filled Jaszczak phantom was acquired. A line source containing 222 MBq of 90Y and uniformly distributed activity was placed inside, along the longitudinal axis. The image was acquired using a gamma camera (e.cam, Siemens) equipped with a MEGP parallel-hole collimator and an energy window of 136–184 keV. By modifying the order and critical frequency (FBP) and number of iterations, subsets and Gaussian-filter FWHM (OSEM), SPECT resolution, image contrast, noise and counts per voxel ( C voxel ) were obtained from the phantom images. 24 different reconstruction protocols (10 FBP and 14 OSEM) were used and compared. Results The results of the reconstruction methodologies evaluated showed that in the FBP algorithm, improving the image resolution implies a loss of contrast and an increase of the noise. Regarding the OSEM algorithm, an increase of the noise level occurs with a higher number of iterations, although both contrast and image resolution improved. In addition, the C voxel also depends on the reconstruction algorithm parameters. It was showed that the C voxel is strongly connected with noise, and that increasing the noise also increases the C voxel . Conclusion Although in the past FBP algorithms were thoroughly used as standard methods for SPECT reconstruction, nowadays, iterative methods can be used in an acceptable reconstruction time due to an increase in computer performance and, often, present better quantitative and qualitative results. Still, optimization is required to provide the best trade-off between noise/contrast and image resolution. Nevertheless, the availability of FBP reconstruction algorithms is still desirable due to the higher number of parameters (e.g. different filters).

The use of needle holders in CTF guided biopsies as a dose reduction tool

Abstract Purpose The purpose of this study was to evaluate the efficacy of needle holders in reducing staff hand exposure during biopsies guided by computed tomography fluoroscopy (CTF), through the analysis of data acquired during a detailed monitoring study, undertaken in parallel with an ongoing optimization process to reduce hand irradiation. Methods Hand monitoring was performed with 11 extremity detectors, two per finger (base and tip) and one on the back of the wrist, for the left (dominant) hand, during two series of biopsies with comparable characteristics. The first series (47 biopsies) were performed with only quick‐check method (QC) and occasional side‐handle (SH) manipulation of the needle. The second series (63 biopsies) were performed after introducing needle holders (NH) in the course of an optimization process. Results Choice of technique (QC, QC + NH, QC + SH) by the interventional radiologist (IR) was related to biopsy difficulty. Measured hand exposure was low (< 1 mSv) for all QC‐only procedures, and for most of the QC + NH procedures. Occasional side‐handle manipulation still occurred during challenging biopsies, so that 8% of biopsies in the second series accounted for ~70% of total fingertip dose (~90 mSv). The methodology used allowed a detailed insight into the dose reduction achievable with needle holders during real procedures, without the limitations of phantom measurements. Conclusions Needle holders proved effective in reducing mean hand exposure during clinical procedures where real‐time manipulation was necessary. Occasional side‐handle manipulation was found to contribute disproportionately to hand exposure. This highlights the importance of individual hand monitoring during CTF guided procedures.

Prostate SBRT: The use of ERB and MOSFET in vivo dosimetry feasibility

Introduction Hypofractionated treatment regimens are indicated for some stages of prostate cancer. In order to provide the reproducibility of the relative position between rectum and prostate and to allow that only a small volume of the rectal wall remains close to the prostate, an endorectal balloon (ERB) may be used. Purpose This work aims to assess the dose on the rectal wall in the presence of the ERB filled with water or air and evaluate the deviation between the measured and calculated doses using two different algorithms (Eclipse AAA and iPlan PencilBeam) Materials and methods Two CT scans were obtained for a modified Rando phantom where an ERB (filled with water or air) with three MOSFET detectors was inserted. A simple 4 field in box plan and an IMRT (6MV beam) clinical plan were calculated in both CT sets, using two different algorithms. The treatment plans were delivered to the phantom using a Varian Novalis linac. Four sets of measurements were obtained and the results were compared with the calculated values. Results The difference between the calculated and measured doses around the ERB is lower when it is filled with water for both algorithms. The maximum relative differences when the ERB is filled with air are 1.8% for Eclipse and 5.5% for iPlan. Conclusion Both algorithms show a better a performance in the presence of water. Water filled ERB seems to be a suitable option. MOSFET dosimetry in association with ERBs for real-time in vivo during hypofractionated treatment of the prostate is a practical and reliable procedure. Disclosure The authors have no relevant financial or non-financial relationships to disclose.