Experimental method to assess the absorbed dose in mammography

Lo scopo di questo lavoro di tesi è quello di sviluppare un metodo di calcolo della dose media assorbita dalla mammella durante le procedure mammografiche. La mammografia, tecnica di imaging di riferimento per esplorare in modo completo la mammella, viene effettuata nei programmi regionali di screening per la diagnosi precoce del carcinoma mammario che coinvolgono la popolazione femminile in età 50-69 anni e si tratta di una procedura non invasiva e con un’elevata sensibilità. Tuttavia la mammella è ritenuta un organo superficiale altamente radiosensibile e la cadenza raccomandata è biennale, perciò è necessario adottare misure di radioprotezione adeguate per ridurre il rischio che si presenti un tumore radio-indotto in seguito all’esposizione a radiazioni ionizzanti. Le norme fondamentali di sicurezza relative all’esposizione alle radiazioni ionizzanti sono state recentemente aggiornate con la pubblicazione della nuova Direttiva Europea 59/2013/EURATOM che dovrà essere recepita entro Febbraio 2018. La nuova Direttiva pone l’attenzione sul tema del controllo delle dosi erogate durante le procedure radiologiche sotto la diretta responsabilità del fisico che deve occuparsi della dosimetria per la valutazione della dose al paziente. La Direttiva dispone anche che all’interno del referto dell’esame venga inserito un dato dosimetrico riferito all’esposizione stessa. Al fine di adempiere a quest’ultima richiesta è fondamentale disporre di un indicatore dosimetrico direttamente correlato al rischio da radiazioni che possa fornire le informazioni opportune al paziente, al medico radiologo, al medico prescrivente, e che possa essere monitorato mediante software dedicati che permettano di avere a disposizione un gran numero di dati provenienti dalle diverse apparecchiature radiologiche. L’attuale indice dosimetrico di riferimento per stimare la dose in mammografia è la dose ghiandolare media (AGD, Average Glandular Dose), rappresentativa della dose assorbita in media dal tessuto ghiandolare (più radiosensibile rispetto a cute e tessuto adiposo), e che dipende dalla qualità del fascio X, dallo spessore e dalla composizione della mammella. Essa si calcola a partire dal kerma in aria incidente Ka,i e, utilizzando fattori di conversione ottenuti da simulazioni Monte Carlo, si ottiene la formula: D= Ka,i ·g·c·s. Tale metodo è stato sviluppato negli anni ’90 da Dance e collaboratori e i fattori g, c e s sono fattori empirici tabulati. La dose media assorbita (Mean Absorbed Dose, MAD) può essere definita come funzione dei parametri espositivi impostati (kVp, mAs, accoppiamento anodo/filtro) e dello spessore della mammella (d). Risolvendo l’integrale che tiene conto dei parametri sopra detti, le quantità necessarie che devono essere individuate per la corretta valutazione della MAD sono il kerma in aria incidente sulla superficie di ingresso del volume target e il coefficiente di assorbimento di energia µ relativo al materiale considerato. Per ottenere questi parametri si è proceduto a fare delle misure sperimentali utilizzando i mammografi situati presso la SDO Senologia Radiologica dell’Azienda Ospedaliero Universitaria Pisana. Per il calcolo del kerma in aria è stato utilizzato un rivelatore a stato solido (Barracuda) e con misure ripetute di esposizione al variare dei kVp, dei mAs e dell’accoppiamento anodo/filtro è stato possibile caratterizzare i fasci X in modo esaustivo. Successivamente è stato misurato il coefficiente di assorbimento di energia µ per ogni accoppiamento anodo/filtro e per diversi valori di kVp utilizzando dei fantocci acqua equivalenti e dei rivelatori a termoluminescenza (TLD). A conclusione del lavoro è possibile quindi calcolare la MAD per ciascuna proiezione mammografica una volta noti i parametri di esposizione e lo spessore della mammella, senza dover ricorrere a coefficienti di conversione tabulati. A termine del lavoro è stato eseguito un confronto tra il valore di MAD ottenuto con il metodo sperimentale e il metodo standard di riferimento che prevede il calcolo della MGD secondo il metodo di Dance. I valori ottenuti risultano in perfetto accordo entro l’errore sperimentale

Methods for muscle Magnetic Resonance Imaging at Ultra High Field

The work described in this thesis has been carried out in the framework of a research topic devoted to the study of NeuroMuscular Diseases (NMD) by Magnetic Resonance Imaging (MRI) as reliable and non invasive instrument for diagnosis and follow up. In particular, this preliminary study has been realized to assess the opportunities offered by musculoskeletal imaging with MRI (MSK-MRI) at Ultra High Field (UHF). The first and unique MR scanner at 7T in Italy for human studies has been recently installed by the IMAGO 7 Foundation (Calambrone, Pisa). In the diagnostic study of NMD, MRI has become an important imaging method able to detect muscle involvement and to evaluate the severity of the disease. It already exists a pediatric protocol routinely used in clinical MRI at low magnetic field (1.5T) in which lower extremities are examined in the same session and the total acquisition time is about 30 minutes. The evaluation of these images is now only visual and software tools for quantitative analysis do not exist. Standardization procedures and quantitative methods could be very useful instruments to optimize the diagnostic performances. Moreover, in the past few years static magnetic fields of increasing strength have been employed to increase Signal to Noise Ratio (SNR), to improve image contrast and spatial resolution, and to decrease the acquisition time. Increasing the magnetic field the behavior of spins in the sample changes, and in particular change the relaxation times of tissues which are related to the magnetic field strength. These modifications should be studied in order to optimize and implement acquisition parameters for muscle MRI at 7T. In fact, the sequences parameters used at low magnetic field to extract different information from the sample can not be employed at Ultra High Field.In this context, the thesis work has been focused on these two main topics: the development of a software for the quantitative analysis of the clinical muscle MRI images currently available and the realization of an acquisition protocol to broadly characterize the tissues of interest in NMD (fat and muscle) in ex-vivo samples at 7T and acquire the first in-vivo images of human volunteers.After a description of the physical principles and technological equipment of nuclear magnetic resonance and magnetic resonance imaging, a method to develop an automatic algorithm of segmentation has been exposed, able also to quantifying fat infiltration percentage in muscles of the leg. Such algorithm has been preliminary optimized and validated by means of a total of 26 MRI scans (10 healthy subjects and 16 NMD patients) already available at the MR laboratory of IRCCS Stella Maris and acquired with a clinical 1.5T MR scanner Signa GE Medical Systems. The scans of healthy subjects have been studied in order to extract and quantify from the MR images a reference standard, absent in literature, of anatomical parameters as geometry and signal intensity of muscle and fat tissues. The results have then been used as reference parameters to evaluate the grade of disease of NMD patients. The resulting automatic system has been developed and implemented as an original hierarchical module network in the MeVisLab medical image analysis framework. The separation between fat and muscle done at single voxel (volume elements) level allowed a quantitative automatic evaluation of fat infiltration percentage. This result has then been related to the grade of muscle impairment in the NMD patients. The scans of 8 of the 16 NMD patients have been selected by the child neuropsychiatrist involved in this study to form the gold standard for 4 grades of effective muscle involvement. By applying this method to the scans of the 8 NMD patients of the validation set, a great agreement with respect to the diagnosis of the child neuropsychiatrist has been obtained.About the topic related to image acquisition at 7T, a complete characterization and optimization of acquisition sequences to realize muscle and fat images has been conducted on the GE 7T Human research MRI system of IMAGO7 foundation. Relaxation times T1, T2 and T2* at 7T for ex-vivo muscle and fat tissues have been measured in pork leg samples respectively acquiring images with Inversion Recovery (varying Inversion Time), Spin Echo (varying Echo Time) and Gradient Echo (varying Echo Time) and then calculating the desired parameters. Moreover, the Dixon technique has been studied for a possible use at 7T, which is based on chemical shift between water and fat. The signals acquired at different Echo Time (TE) are used to realize two images with different information, with water and fat spins in phase and out of phase, respectively. By means of a numerical processing of these images is possible to obtain other two images, one with the fat signal suppressed ("water image") and one with the water signal suppressed ("fat image"). The implementation as additional feature in Spoiled Gradient Recalled Echo (SPGR) sequence has been tested and optimized by evaluating the Ernst angle, which is the flip angle that maximizes the echo signal. Taking into account the results of this preliminary work, the differences between ex-vivo and in-vivo samples and the data available in the literature, a first attempt of protocol for human calf has been implemented and used to acquire images of a first healthy volunteer at 7T. In conclusion, the software for automatic and quantitative analysis of muscle MR images resulted a promising instrument both in the characterization of muscle and fat patterns in healthy subjects and in the classification of NMD patients for the clinical images currently available, acquired at 1.5T. The data acquisition for this work is still in progress in order to obtain a statistically significant dataset. Moreover, a preliminary protocol suitable for human in-vivo muscle acquisitions at 7T has been realized and the first images of human calf have been acquired

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

The uncertain climate footprint of wetlands under human pressure

Significant climate risks are associated with a positive carbon–temperature feedback in northern latitude carbon-rich ecosystems,making an accurate analysis of human impacts on the net greenhouse gas balance of wetlands a priority. Here, we provide a coherent assessment of the climate footprint of a network of wetland sites based on simultaneous and quasi-continuous ecosystem observations of CO2 and CH4 fluxes. Experimental areas are located both in natural and in managed wetlands and cover a wide range of climatic regions, ecosystem types, and management practices. Based on direct observations we predict that sustained CH4 emissions in natural ecosystems are in the long term (i.e., several centuries) typically offset by CO2 uptake, although with large spatiotemporal variability. Using a space-for-time analogy across ecological and climatic gradients, we represent the chronosequence from natural to managed conditions to quantify the “cost” of CH4 emissions for the benefit of net carbon sequestration. With a sustained pulse– response radiative forcing model, we found a significant increase in atmospheric forcing due to land management, in particular for wetland converted to cropland. Our results quantify the role of human activities on the climate footprint of northern wetlands and call for development of active mitigation strategies for managed wetlands and new guidelines of the Intergovernmental Panel on Climate Change (IPCC) accounting for both sustained CH4 emissions and cumulative CO2 exchange

Quantitative assessment of noise and spatial resolution properties of ASIR-reconstructed CT images at different percentages of reconstruction

Introduction Computed tomography (CT) is the largest source of population exposure to ionizing radiation in industrialized countries. Recent advances in CT imaging include the development of iterative reconstruction algorithms in order to obtain a dose reduction without compromising the diagnostic quality of images. The purpose of this study was to quantitatively assess noise reduction and spatial resolution in CT imaging with the ASIR (Adaptive Statistical Iterative Reconstruction, GE Healthcare) reconstruction algorithm at different percentages of reconstruction as well as different kVp, mAs and contrasts. Acquisitions of the Catphan-504 phantom were performed on a PET/CT scanner (Discovery-710, GE Healthcare). CT images were reconstructed using both filtered back projection (FBP) and ASIR with different percentages of reconstruction (20%, 40%, 60%, 80%, 100%). The image noise was estimated for different values of scanning parameters (i.e. mAs, kVp, pitch, slice thickness). Then, 3D/2D/1D noise power spectrum was estimated. Also, spatial resolution was assessed by obtaining the modulation transfer function (MTF) for a wide range of scanning parameters values and different contrast objects by the circular Edge Spread Function method and the Point Spread Function method. Image noise decreased (up to 50% as compared to FBP) when increasing the percentage of ASIR reconstruction (behaviour more relevant for higher spatial frequencies). Only for low tube load (<56 mAs) and lower contrast objects (with respect to phantom background - i.e. PMP, Delrin, LDPE and Polystyrene) acquisitions, MTF analysis showed that, increasing the ASIR percentage, the reconstructed images were characterized by an appreciable reduction in spatial resolution, when compared to FBP-reconstructed images. When compared to FBP, ASIR allows a relevant noise reduction without appreciably affecting image quality, except for very low dose and contrast acquisitions

A non-invasive method for a quantitative evaluation of muscle involvement in MRI of Neuromuscular Diseases

This work reports a study of Neuromuscular Diseases (NMD) by Magnetic Resonance Imaging as reliable and non invasive instrument for NMD diagnosis and follow up. The evaluation of the images is now only visual, while standardization procedures and quantitative methods could be very useful instruments to optimize the diagnostic performances. We propose a new method to evaluate the fat infiltration in tissues developed and retrospectively applied to images of the human leg. Through a muscle segmentation algorithm on structural T1-weighted magnetic resonance images (MRIs), the estimated non-muscle percentage (eNMP) in the segmented muscle area has been evaluated in healthy subjects as a reference value. A semi-automated procedure allows extending the algorithm to MRIs of NMD patients. A strong correlation has been demonstrated between this index and the disease severity. The final aim is to obtain a quantitative evaluation of fat infiltration percentage and to relate it to the grade of muscle impairment in subjects affected by Neuromuscular Diseases

Average absorbed breast dose in mammography: a new possible dose index matching the requirements of the European Directive 2013/59/EURATOM

Background The new European Directive 2013/59/EURATOM requires that patients are informed about the risk associated with ionising radiation and that detailed information on patient exposure is included in the radiological report. This implies a revision of the routinely used dose indexes to obtain quantities related to individual exposure evaluable from acquisition parameters. Here we propose a new mammography dose index consistent with the average glandular dose (AGD). Methods An equation has been developed for calculating the average absorbed breast dose (2ABD). It depends on incident air kerma k a,i and on energy absorption coefficient μen; k a,i can be calculated for each anode-filter combination, based on kVp, mAs, the yield of the tube used Y tb , and the breast thickness d; μ en depends on kVp and has been evaluated for each anode-filter combination. 2ABD has been compared to AGD evaluated by Dance or Wu methods, which represent the reference standards, for 20 patients of our university hospital. Results The incident air kerma k a,i , calculated as a function of kVp, mAs, Y tb and d, was in good agreement with the same quantity directly measured: the relative uncertainty is < 0.10. The results of the comparison between 2ABD and AGD evaluated by both Dance and Wu methods appear to be consistent within the uncertainties. Conclusions 2ABD is easily evaluable for each mammogram from the acquisition parameters. It can be proposed as a new suitable dose index, consistent with AGD, matching the requirements of the 2013 European Directive

Performance Studies of the Associative Memory System of the ATLAS Fast Tracker

The Associative Memory (AM) system of the Fast Tracker (FTK) processor has been designed to perform pattern matching using the hit information of the ATLAS experiment silicon tracker. The system is one of the two main processing elements of FTK and is mainly based on the use of Application Specific Integrated Circuits, the AM chips, specifically designed to execute pattern matching with a high degree of parallelism. It finds track candidates at low resolution that are seeds for a full resolution track fitting. The AM system implementationis based on a collection of “AM boards”, the “Serial Link Processors” (AMBSLP). The AMBSLP is based on a network of high speed serial links to sustain very high data traffic. It has a high consumption (250 W) because of its high performance requirements and, therefore, the AM system needs custom power and cooling. This paper reports on the performance studies of the system made with the first production of 64 AMBSLPs integrated in FTK and results from the first ATLAS data during 2018

The uncertain climate footprint of wetlands under human pressure

Significant climate risks are associated with a positive carbon–temperature feedback in northern latitude carbon-rich ecosystems, making an accurate analysis of human impacts on the net greenhouse gas balance of wetlands a priority. Here, we provide a coherent assessment of the climate footprint of a network of wetland sites based on simultaneous and quasi-continuous ecosystem observations of CO2 and CH4 fluxes. Experimental areas are located both in natural and in managed wetlands and cover a wide range of climatic regions, ecosystem types, and management practices. Based on direct observations we predict that sustained CH4 emissions in natural ecosystems are in the long term (i.e., several centuries) typically offset by CO2 uptake, although with large spatiotemporal variability. Using a space-for-time analogy across ecological and climatic gradients, we represent the chronosequence from natural to managed conditions to quantify the “cost” of CH4 emissions for the benefit of net carbon sequestration. With a sustained pulse–response radiative forcing model, we found a significant increase in atmospheric forcing due to land management, in particular for wetland converted to cropland. Our results quantify the role of human activities on the climate footprint of northern wetlands and call for development of active mitigation strategies for managed wetlands and new guidelines of the Intergovernmental Panel on Climate Change (IPCC) accounting for both sustained CH4 emissions and cumulative CO2 exchange

Radiation dose from medical imaging in end stage renal disease patients: a Nationwide Italian Survey

Background and objectives: End stage renal disease (ESRD) patients are exposed to the risk of ionizing radiation during repeated imaging studies. The variability in diagnostic imaging policies and the accompanying radiation doses across various renal units is still unknown. We studied this variability at the centre level and quantified the associated radiation doses at the patient level. Methods: Fourteen Italian nephrology departments enrolled 739 patients on haemodialysis and 486 kidney transplant patients. The details of the radiological procedures performed over one year were recorded. The effective doses and organ doses of radiation were estimated for each patient using standardized methods to convert exposure parameters into effective and organ doses RESULTS: Computed tomography (CT) was the major contributor (> 77%) to ionizing radiation exposure. Among the haemodialysis and kidney transplant patients, 15% and 6% were in the high ( 65 20 mSv per year) radiation dose groups, respectively. In haemodialysis patients, the most exposed organs were the liver (16 mSv), the kidney (15 mSv) and the stomach (14 mSv), while the uterus (6.2 mSv), the lung (5.7 mSv) and the liver (5.5 mSv) were the most exposed in kidney transplant patients. The average cumulative effective dose (CED) of ionizing radiation among centres in this study was highly variable both in haemodialysis (from 6.4 to 18.8 mSv per patient-year; p = 0.018) and even more so in kidney transplant (from 0.6 to 13.7 mSv per patient-year; p = 0.002) patients. Conclusions: Radiation exposure attributable to medical imaging is high in distinct subgroups of haemodialysis and transplant patients. Furthermore, there is high inter-centre variability in radiation exposure, suggesting that nephrology units have substantially different clinical policies for the application of diagnostic imaging studies