Littérature scientifique sur le sujet « Ionoacoustic »

This paper analyzes how to improve the precision of ionoacoustic proton range verification by optimizing the analog signal processing stages with particular emphasis on analog filters. The ionoacoustic technique allows one to spatially detect the proton beam penetration depth/range in a water absorber, with interesting possible applications in real-time beam monitoring during hadron therapy treatments. The state of the art uses nonoptimized detectors that have low signal quality and thus require a higher total dose, which is not compatible with clinical applications. For these reasons, a comprehensive analysis of acoustic signal bandwidth, signal-to-noise-ratio and noise power/bandwidth will be presented. The correlation between these signal-quality parameters with maximum achievable proton range measurement precision will be discussed. In particular, the use of an optimized analog filter allows one to decrease the dose required to achieve a given precision by as much as 98.4% compared to a nonoptimized filter approach.

Oncological hadron therapy utilizes a beam of charged particles to destroy the tumor cells, exploiting the particular deposition curve that allow minimum damage to the surrounding healty tissues compared to traditional radiotherapy. Sulak and Hayakawa’s works have shown the applicability of this technique in clinical scenarios, but the lack of dedicated electronics for this type of experiments affects the spatial resolution that can be obtained with this technique [1]. This work presents an integrated analog front-end dedicated to ionoacoustic experiments that allows to estimate the position of the Bragg Peak with an average deviation of 1% with respect to the real position.

Abstract Accurate knowledge of the exact stopping location of ions inside the patient would allow full exploitation of their ballistic properties for patient treatment. The localized energy deposition of a pulsed particle beam induces a rapid temperature increase of the irradiated volume and leads to the emission of ionoacoustic (IA) waves. Detecting the time-of-flight (ToF) of the IA wave allows inferring information on the Bragg peak location and can henceforth be used for in-vivo range verification. A challenge for IA is the poor signal-to-noise ratio at clinically relevant doses and viable machines. We present a frequency-based measurement technique, labeled as ionoacoustic tandem phase detection (iTPD) utilizing lock-in amplifiers. The phase shift of the IA signal to a reference signal is measured to derive the ToF. Experimental IA measurements with a 3.5 MHz lead zirconate titanate (PZT) transducer and lock-in amplifiers were performed in water using 22 MeV proton bursts. A digital iTPD was performed in-silico at clinical dose levels on experimental data obtained from a clinical facility and secondly, on simulations emulating a heterogeneous geometry. For the experimental setup using 22 MeV protons, a localization accuracy and precision obtained through iTPD deviates from a time-based reference analysis by less than 15 μm. Several methodological aspects were investigated experimentally in systematic manner. Lastly, iTPD was evaluated in-silico for clinical beam energies indicating that iTPD is in reach of sub-mm accuracy for fractionated doses < 5 Gy. iTPD can be used to accurately measure the ToF of IA signals online via its phase shift in frequency domain. An application of iTPD to the clinical scenario using a single pulsed beam is feasible but requires further development to reach <1 Gy detection capabilities.

Proton range verification by ionoacoustic wave sensing is a technique under development for applications in adron therapy as an alternative to nuclear imaging. It provides an acoustic imaging of the proton energy deposition vs. depth using the acoustic wave Time of Flight (ToF). State-of-the-art (based on simulations and experimental results) points out that this detection technique achieves better spatial resolution (< 1 mm) of the proton range comparing with Positron-Emission-Tomography (PET) and prompt gamma ray techniques. This work presents a complete Geant4/k-Wave model that allows to understand several physical phenomena and to evaluate the key parameters that affect the acoustic field generated by the incident proton radiation.

La tecnica acustica di verifica sperimentale del range di protoni (ionoacustica) si basa sul rilevamento del debole segnale termoacustico emesso dalla rapida deposizione di energia che avviene alla fine range del fascio, in corrispondenza del picco di Bragg. In questo contesto, questa tesi presenta le principali caratteristiche della strumentazione microelettronica utilizzata per i Proton Sound Detector introducendo specifiche tecniche di progettazione fortemente orientate sia alla massimizzazione del Rapporto Segnale Rumore SNR (a livello di sensore acustico) che minimizzazione della figura di rumore (a livello di amplificatore analogico). La prima parte di questa tesi tratta delle sfide strumentali relative agli esperimenti ionoacustici fornendo dettagli tecnici specifici riguardanti sia la progettazione del sensore acustico (ovvero come costruire il sensore massimizzando l'SNR) sia il design dell'amplificatore a basso rumore (LNA). Verranno presentati i risultati sperimentali di un primo esperimento effettuato presso il Laboratorio Maier-Leibniz di Garching, Monaco, con un fascio di protoni a 20 MeV (scenario preclinico) e verrà mostrato come una progettazione elettronica dedicata a segnali misti permetta di migliorare significativamente il rapporto segnale-rumore e l'accuratezza della localizzazione del picco di Bragg di 6 dB. In questo contesto, questo primo sviluppo del rivelatore raggiunge due importanti obiettivi: il miglioramento dell'SNR a parità di dose e una forte semplificazione della strumentazione del rivelatore rispetto allo stato dell'arte, consentendo una maggiore precisione della misurazione dell'impulso acustico, e allo stesso tempo incrementando la portabilità e la compattezza del dispositivo. Nelle applicazioni cliniche di adroterapia, l'energia del fascio (da 65 MeV fino a 200 MeV) e la dose vengono scelte in funzione dello specifico scenario clinico. Ciò comporta segnali acustici di ampiezza e larghezza di banda diverse, costringendo l’adozione di soluzioni tecnologiche avanzate in grado di gestire un ampio spettro di segnali in termini di larghezza di banda, ampiezza e rumore. Per questo motivo, la seconda parte di questa tesi propone un modello Matlab efficiente e innovativo del fenomeno fisico ionoacustico, che condensa in un unico sistema lineare tempo invariante tutti i processi di conversione dell'energia coinvolti. Il modello ionoacustico proposto sostituisce i complessi strumenti di simulazione classici (usati per caratterizzare il segnale acustico indotto dal fascio di protoni) e facilita lo sviluppo di rivelatori dedicati fornendo una descrizione precisa del segnale acustico nei diversi scenari. Infine, verrà presentato il progetto di una seconda versione del Proton Sound Detector che introduce il concetto di media nel dominio dello spazio (invece della media nel dominio del tempo, basata sull’elaborazione di più shot del fascio che comporta una significativa extra-dose). Questo rilevatore utilizza un sensore multicanale per eseguire una media spaziale dei segnali acquisiti e aumentare l'SNR di 18 dB a parità di dose rispetto al classico approccio monocanale. Questo approccio tuttavia richiede lo sviluppo di elettronica altamente miniaturizzata che non può essere implementata con componenti standard su circuiti stampati. Viene quindi presentato il progetto e la caratterizzazione di un front-end analogico multicanale implementato su un Application-Specified-Integrated-Circuit (ASIC) in tecnologia CMOS 28 nm che permette di elaborare in parallelo tutti i 64 canali del sensore acustico. Questo High-Resolution Proton Sound Detector (HR-ProSD) è completato da un circuito digitale dedicato implementato su FPGA (Field Programmable Gate Array) che consente di mappare in tempo reale e 2D la deposizione di dose nello spazio.
Acoustic proton range experimental verification technique (iono-acoustics) is based on sensing the weak thermoacoustic signal emitted by the fast energy deposition (and/or the heating process) at the end of the beam range (Bragg Peak). In this context, this thesis presents the main characteristics of the micro-electronics instrumentation used for proton sound detectors introducing specific design techniques strongly oriented to both maximization of the acoustic Signal-to-Noise-Ratio (at the Acoustic Sensor level) and Noise-Figure minimization (at analog amplifier level). The first part of this thesis addresses all the instrumentation challenges related to iono-acoustic experiments providing specific technical details regarding both acoustic sensor design (i.e. how to build the sensor while maximizing the SNR) and the LNA design. The experimental results of a first experiment carried out at Maier-Leibniz Laboratory in Garching, Munich, with a proton beam at 20 MeV (sub-clinical energy) will be presented and it will be shown how a dedicated mixed-signal electronics design allows to significantly improve the signal-to-noise ratio and the accuracy of the BP localization by 6 dB. In this context, this first detector development achieves two important objectives: the improvement of the acoustic SNR and a strong simplification of the detector instrumentation w.r.t. state-of-the-art, enabling increasing accuracy of the acoustic pulse measurement, and at the same time the portability and compactness of the device. In clinical hadron-therapy applications, variable beam energy (from 65 MeV up to 200 MeV) and variable doses are used as a function of the selected medical treatment. This induces different acoustic pulses amplitude and bandwidth, forcing advanced technological solutions capable of handling a wide spectrum of signals in terms of bandwidth, amplitude, and noise. For this reason, the second part of this thesis proposes an efficient and innovative Matlab Model of the ionoacoustic physical phenomenon, based on englobing in a single mathematical Linear-Time-Invariant-System all energy conversion processes involved in iono-acoustics. The proposed ionoacoustics model replaces classical and complex simulation tools (used to characterize the proton induced acoustic signal) and facilitates the development of dedicated detectors. Finally, the design of a second version of the Proton Sound Detector will be presented that introduces the concept of space-domain averaging (instead of time-domain averaging based on multiple beam shot processing for noise attenuation and thus extra-doses). This detector uses a multi-channel sensor to perform a spatial average of the acquired signals and increase the SNR by 18 dB at the same dose compared to the classic single channel approach. This approach however requires the development of highly miniaturized electronics that cannot be implemented with off-the-shelf components on Printed Circuit Boards. The design and characterization of a multichannel analog front-end implemented on a CMOS 28 nm Application-Specified-Integrated-Circuit (ASIC) which allows to process the 64 channels of the acoustic sensor in parallel is then presented. This High-Resolution Proton Sound Detector (HR-ProSD) is completed by digital circuits implemented on Field Programmable Gate Array (FPGA) that allow to locate in real time the deposition of energy in space.