N. Lanconelli

Differences among Monte Carlo codes in the calculations of voxel S values for radionuclide targeted therapy and analysis of their impact on absorbed dose evaluations

Several updated Monte Carlo (MC) codes are available to perform calculations of voxel S values for radionuclide targeted therapy. The aim of this work is to analyze the differences in the calculations obtained by different MC codes and their impact on absorbed dose evaluations performed by voxel dosimetry. Voxel S values for monoenergetic sources (electrons and photons) and different radionuclides (90Y, 131I, and 188Re) were calculated. Simulations were performed in soft tissue. Three general-purpose MC codes were employed for simulating radiation transport: MCNP4C, EGSnrc, and GEANT4. The data published by the MIRD Committee in Pamphlet No. 17, obtained with the EGS4 MC code, were also included in the comparisons. The impact of the differences (in terms of voxel S values) among the MC codes was also studied by convolution calculations of the absorbed dose in a volume of interest. For uniform activity distribution of a given radionuclide, dose calculations were performed on spherical and elliptical volumes, varying the mass from 1 to 500 g. For simulations with monochromatic sources, differences for self-irradiation voxel S values were mostly confined within 10% for both photons and electrons, but with electron energy less than 500 keV, the voxel S values referred to the first neighbor voxels showed large differences (up to 130%, with respect to EGSnrc) among the updated MC codes. For radionuclide simulations, noticeable differences arose in voxel S values, especially in the bremsstrahlung tails, or when a high contribution from electrons with energy of less than 500 keV is involved. In particular, for 90Y the updated codes showed a remarkable divergence in the bremsstrahlung region (up to about 90% in terms of voxel S values) with respect to the EGS4 code. Further, variations were observed up to about 30%, for small source-target voxel distances, when low-energy electrons cover an important part of the emission spectrum of the radionuclide (in our case, for 131I). For 90Y and 188Re, the differences among the various codes have a negligible impact (within few percents) on convolution calculations of the absorbed dose; thus either one of the MC programs is suitable to produce voxel S values for radionuclide targeted therapy dosimetry. However, if a low-energy beta-emitting radionuclide is considered, these differences can affect also dose depositions at small source-target voxel distances, leading to more conspicuous variations (about 9% for 1311) when calculating the absorbed dose in the volume of interest.

Physical and psychophysical characterization of a novel clinical system for digital mammography

PURPOSEnIn recent years, many approaches have been investigated on the development of full-field digital mammography detectors and implemented in practical clinical systems. Some of the most promising techniques are based on flat panel detectors, which, depending on the mechanism involved in the x-ray detection, can be grouped into direct and indirect flat panels. Direct detectors display a better spatial resolution due to the direct conversion of x rays into electron-hole pairs, which do not need an intermediate production of visible light. In these detectors the readout is usually achieved through arrays of thin film transistors (TFTs). However, TFT readout tends to display noise characteristics worse than those from indirect detectors. To address this problem, a novel clinical system for digital mammography has been recently marketed based on direct-conversion detector and optical readout. This unit, named AMULET and manufactured by FUJIFILM, is based on a dual layer of amorphous selenium that acts both as a converter of x rays (first layer) and as an optical switch for the readout of signals (second layer) powered by a line light source. The optical readout is expected to improve the noise characteristics of the detector. The aim is to obtain images with high resolution and low noise, thanks to the combination of optical switching technology and direct conversion with amorphous selenium. In this article, the authors present a characterization of an AMULET system.nnnMETHODSnThe characterization was achieved in terms of physical figures as modulation transfer function (MTF), noise power spectra (NPS), detective quantum efficiency (DQE), and contrast-detail analysis. The clinical unit was tested by exposing it to two different beams: 28 kV Mo/Mo (namely, RQA-M2) and 28 kV W/Rh (namely, W/Rh).nnnRESULTSnMTF values of the system are slightly worse than those recorded from other direct-conversion flat panels but still within the range of those from indirect flat panels: The MTF values of the AMULET system are about 45% and 15% at 5 and 8 lp/mm, respectively. On the other hand, however, AMULET NNPS results are consistently better than those from direct-conversion flat panels (up to two to three times lower) and flat panels based on scintillation phosphors. DQE results lie around 70% when RQA-M2 beams are used and approaches 80% in the case of W/Rh beams. Contrast-detail analysis, when performed by human observers on the AMULET system, results in values better than those published for other full-field digital mammography systems.nnnCONCLUSIONSnThe novel clinical unit based on direct-conversion detector and optical reading presents great results in terms of both physical and psychophysical characterizations. The good spatial resolution, combined with excellent noise properties, allows the achievement of very good DQE, better than those published for clinical FFDM systems. The psychophysical analysis confirms the excellent behavior of the AMULET unit.

Experimental validation of the filtering approach for dose monitoring in proton therapy at low energy

The higher physical selectivity of proton therapy demands higher accuracy in monitoring of the delivered dose, especially when the target volume is located next to critical organs and a fractionated therapy is applied. A method to verify a treatment plan and to ensure the high quality of the hadrontherapy is to use Positron Emission Tomography (PET), which takes advantage of the nuclear reactions between protons and nuclei in the tissue during irradiation producing beta(+)-emitting isotopes. Unfortunately, the PET image is not directly proportional to the delivered radiation dose distribution; this is the reason why, at the present time, the verification of depth dose profiles with PET techniques is limited to a comparison between the measured activity and the one predicted for the planned treatment by a Monte Carlo model. In this paper we test the feasibility of a different scheme, which permits to reconstruct the expected PET signal from the planned radiation dose distribution along beam direction in a simpler and more direct way. The considered filter model, based on the description of the PET image as a convolution of the dose distribution with a filter function, has already demonstrated its potential applicability to beam energies above 70 MeV. Our experimental investigation provides support to the possibility of extending the same approach to the lower energy range ([40, 70] MeV), in the perspective of its clinical application in eye proton therapy.

Development of a Multi-Energy CT for Small Animals: Characterization of the Quasi-Monochromatic X-Ray Source

A new multi-energy CT for small animals is operative (now only in scanning mode) at the Physics Department of the University of Bologna. The system makes use of a set of quasi-monochromatic X-ray beams produced by means of an Highly-Oriented Pyrolytic Graphite Bragg monochromator. This source is able to provide beams with energy tunable in a range from 20 to 70 keV. Here we present a complete characterization of the source. A theoretical model of the source has been analyzed, according to the known Zachariasens theory for diffracting crystals. The beams have also been characterized in resolution and intensity, over the accessible range, and we present here some measured spectra. The monochromator system demonstrated an energy spread of about 3 keV (Full Width at Half Maximum-FWHM) at a beam energy of 26 keV. At the same energy, the intensity of the output beam is about 6% of the primary beam.

An Innovative CCD-Based High-Resolution CT System for Analysis of Trabecular Bone Tissue

Synchrotron-based digital radiography and microtomography devices are powerful, nondestructive, high-resolution research tools. In this paper, we present a linear system with a pixel size of 22.5 mum and a field-of-view (FOV) 13 cm long and about 1 mm high. The system is composed of a linear converter GOS screen coupled to an intensified electron-bombarded CCD (EBCCD) camera, by means of a rectangular-to-linear fiber optic adapter. This optical guide is composed of seven bundles, each one transporting light in a coherent way to preserve spatial information. In this way, a high spatial resolution over an extended FOV is obtained. The detector works as an X-ray scanner by means of a high-precision translation mechanical device with 18 cm travel range. The total FOV obtained this way is 13 cm long and 18 cm high. The aim of this paper is to demonstrate the feasibility of this system to investigate a large area of a bone and to calculate the appropriate histomorphometric parameters. Here we present an investigation gained at ELETTRA synchrotron facility at Trieste, Italy. A monochromatic 34-keV beam has been used for imaging a human proximal femur, about 9 cm in width, with our system. The reconstructed images (13 cmtimes13 cm) were cross sections containing femoral head, femoral neck, and greater trochanter. The local variations in trabecular and cortical structure of the examined bone were clearly visible at a level not obtainable with medical CT scanners. The used spatial resolution allowed the visualization of thin trabeculae, which typically lie in a range of 100 mum or lower. The quality of the reconstructed cross-section images confirmed that the system presented is a novel tool for high resolution three-dimensional (3-D) imaging of bone structure, with a pixel size over a volume of interest not achievable with conventional microCT scanners

Development of a multi-energy CT for small animals: Characterization of the quasi-monochromatic X-ray source

A new multi-energy CT for small animals is operative (now only in scanner mode) at the Physics Department of the University of Bologna. The system makes use of a set of quasi-monochromatic X-ray beams produced by means of an highly-oriented pyrolytic graphite Bragg monochromator. This source is able to provide beams with energy tunable in a range from 20 to 70 keV. Here we present a complete characterization of the source. A theoretical model of the source has been analyzed, according to a very-well known theory for diffracting crystals. The beams have also been characterized in resolution and intensity, all over the accessible range, and we present here some measured spectra.

CsI(Tl) Micro-Pixel Scintillation Array for Ultra-high Resolution Gamma-ray Imaging

The aim of this paper is to investigate the intrinsic spatial resolution limit by coupling a CsI(Tl) micro-pixel scintillation array to position sensitive photomultipliers (PSPMTs) for ultra-high resolution gamma-ray imaging. On this purpose, 1 mm thick array with 0.2 mm pixel side, 0.4 mm pitch has been realized by Spectra Physics (Hilger). The present scintillation arrays technology is suitable to produce larger crystal areas. In this paper we present spatial resolution and positioning results obtained by coupling the micro-pixel scintillation array to Hamamatsu square PSPMTs: 1rdquo R8520-C12, 1rdquo R5900-L16 and 2rdquo H8500 Flat panel PMT. Preliminary measurements demonstrate better performance in term of uniformity response when micro-pixel array is coupled to a H8500 PSPMT model. This setup carries out an intrinsic spatial resolution lower limit of about 0.6 mm FWHM at 50% FWHM energy resolution, defining it as the minimum scintillation array pitch detectable at 122 keV. The results obtained by R5900-L16 with a better sampling of the scintillation light has shown an improvement of the position linearity in spite of a worse spatial resolution due to the poor light output of scintillation array.

Physical and clinical comparison between a screen-film system and a dual-side reading mammography-dedicated computed radiography system

Background: Digital mammography systems, thanks to a physical performance better than conventional screen-film units, have the potential of reducing the dose to patients, without decreasing the diagnostic accuracy. Purpose: To achieve a physical and clinical comparison between two systems: a screen-film plate and a dual-side computed radiography system (CRM; FUJIFILM FCR 5000 MA). Material and Methods: A unique feature of the FCR 5000 MA system is that it has a clear support medium, allowing light emitted during the scanning process to be detected on the back of the storage phosphor plate, considerably improving the systems efficiency. The systems physical performance was tested by means of a quantitative analysis, with calculation of the modulation transfer function, detective quantum efficiency, and contrast-detail analysis; subsequently, the results were compared with those achieved using a screen-film system (SFM; Eastmann Kodak MinR-MinR 2000). A receiver operating characteristic (ROC) analysis was then performed on 120 paired clinical images obtained in a craniocaudal projection with the conventional SFM system under standard exposure conditions and also with the CRM system working with a dose reduced by 35% (average breast thickness: 4.3 cm; mean glandular dose: 1.45 mGy). CRM clinical images were interpreted both in hard copy and in soft copy. Results: The ROC analysis revealed that the performances of the two systems (SFM and CRM with reduced dose) were similar (P>0.05): the diagnostic accuracy of the two systems, when valued in terms of the area underneath the ROC curve, was found to be 0.74 for the SFM, 0.78 for the CRM (hard copy), and 0.79 for the CRM (soft copy). Conclusion: The outcome obtained from our experiments shows that the use of the dual-side CRM system is a very good alternative to the screen-film system.

SPEMT imaging with a dedicated VAoR dual-head camera: preliminary results

We have developed a SPEMT (Single Photon Emission MammoTomography) scanner that is made up of two cameras rotating around the pendulous breast of the prone patient, in Vertical Axis of Rotation (VAoR) geometry. Monte Carlo simulations indicate that the device should be able to detect tumours of 8 mm diameter with a tumour/background uptake ratio of 5:1. The scanner field of view is 41.6 mm height and 147 mm in diameter. Each head is composed of one pixilated NaI(Tl) crystal matrix coupled to three Hamamatsu H8500 64-anodes PMTs read out via resistive networks. A dedicated software has been developed to combine data from different PMTs, thus recovering the dead areas between adjacent tubes. A single head has been fully characterized in stationary configuration both in active and dead areas using a point-like source in order to verify the effectiveness of the readout method in recovering the dead regions. The scanner has been installed at the Nuclear Medicine Division of the University of Pisa for its validation using breast phantoms. The very first tomographic images of a breast phantom show a good agreement with Monte Carlo simulation results.

Monte Carlo optimization of an industrial tomography system

Computed tomography (CT) is becoming a very useful non-destructive testing technique, in the industrial field, since it permits the detection of small inner defects in a reliable and accurate way. In order to get very good performance, in terms of image contrast and spatial resolution, the configuration of the tomography system has to be optimized carefully. Monte Carlo simulations can be a very helpful method, for choosing different conditions and selecting the best configuration of a CT system. In this paper we present a preliminary optimization of an industrial CT apparatus, obtained by means of Monte Carlo simulations. The system is composed of an X-ray tube, filtering and collimation devices, and a detector made of a scintillator coupled to a CCD camera. We focus our attention on large aluminum objects and investigate the contribution of the scattered radiation. Some options have been simulated, for reducing the scattering photons, thus improving the overall image quality