Αρκετές έρευνες, παλαιές και πιο πρόσφατες, έχουν ασχοληθεί με την αξιολόγηση της ποιότητας της εικόνας των απεικονιστικών συστημάτων πυλαίας απεικόνισης (EPID). Έχουν επίσης, δημοσιεύσει αποτελέσματα ποιοτικών μετρήσεων όπως MTF, DQE, CNR και SNR. Ωστόσο, σε αυτές τις μελέτες η επίδραση των διαφόρων ενεργειών δεν αξιολογήθηκε συστηματικά[G.Jarry and F.Verhaegen, 2005]. Επιπλέον, ελάχιστες εργασίες έχουν χρησιμοποιήσει το ομοίωμα QC-3 για τις αντίστοιχες μελέτες ποιοτικής αξιολόγησης[Filipe Martins Garcia de Moura, 2008, Poonam Yadav et al, 2010]. Το προαναφερθέν ομοίωμα χρησιμοποιείται επί το πλείστον, κλινικά, για τον προσδιορισμό θέσης του ασθενούς[U. Ramm et al, 2013]. Η παρούσα διπλωματική εργασία αφορά την πειραματική αξιολόγηση της ποιότητας της εικόνας των συστημάτων πυλαίας απεικόνισης (Portal Imaging). Η αξιολόγηση πραγματοποιήθηκε με τη χρήση ποιοτικών δεικτών όπως είναι, η Συνάρτηση μεταφοράς διαμόρφωσης (MTF), Φάσμα ισχύος θορύβου (NPS), Κανονικοποιημένο φάσμα ισχύος του θορύβου (NNPS), Σχετική Ανιχνευτική κβαντική αποδοτικότητα (R-DQE), Λόγος αντίθεσης προς θόρυβο (CNR), Λόγος σήματος προς θόρυβο (SNR), Δείκτης Ποιότητας Εικόνας (FIQ). To MTF υπολογίστηκε με χρήση του Συνάρτηση τετραγωνικής απόκρισης (SWRF), το NPS υπολογίστηκε μέσω του μετασχηματισμού Fourrier στην περιοχή ενδιαφέροντος της ακτινοβολούμενης εικόνας, το R-DQE υπολογίστηκε χρησιμοποιώντας το MTF και το κανονικοποιημένο NPS, το SNR υπολογίστηκε από το κλάσμα, με αριθμητή τη τιμή του κάθε εικονοστοιχείου του σημείου ενδιαφέροντος της εικόνας και παρανομαστή την τυπική απόκλιση της αντίστοιχης περιοχής ενδιαφέροντος, το CNR υπολογίστηκε από την διαίρεση της αντίθεσης της περιοχής ενδιαφέροντος με τον στατιστικό θόρυβο, το FIQ υπολογίστηκε από το MTF της κάθε χωρικής συχνότητας διαιρούμενη με τον συντελεστή μεταβλητότητας (Coefficient of Variation (CV)). Οι προαναφερθέντες ποιοτικοί δείκτες χρησιμοποιήθηκαν σε ψηφιακές εικόνες DICOM χρησιμοποιώντας το εξειδικευμένο QC-3 test phantom. Πραγματοποιήθηκαν 48 απεικονίσεις, σε συνθήκες Ακτινοθεραπείας (20x20cm2, SSD=100cm), του εν λόγω φάντομ για έναν αριθμό από Monitor Units (MUs), για διάφορους Ρυθμούς δόσης (DR) και χρησιμοποιώντας 4 διαφορετικές ενέργειες. Συμπερασματικά διαπιστώθηκε ότι η ποιότητα της εικόνας βελτιώνεται όσο αυξάνουν τα MUs και τα DR, αλλά παρατηρήθηκε μία μικρή υποβάθμιση της εικόνας στις χαμηλές χωρικές συχνότητες. Για το ίδιο απεικονιστικό σύστημα, στα 6MV, με χαμηλές τιμές DR το CNR και το SNR είναι αυξημένο. Ενώ, συγκρίνοντας την ενέργεια δέσμης 6MV με 18MV, παρατηρήθηκε ότι το SNR και το CNR είναι υψηλότερα στα 6MV. Για τέσσερις διαφορετικές ενέργειες δέσμης φωτονίων παρατηρήθηκε παρόμοια συμπεριφορά του R-DQE στις τρεις ενέργειες(10MV, 15MV, 18MV), στα 6MV το R-DQE είχε φθίνουσα πορεία. Στα γραφήματα R-DQE για ίδιο DR υπήρξε μία μικρή διαφοροποίηση, αυξήθηκαν σε μικρό ποσοστό οι τιμές του R-DQE για τα 6MV και 18MV. / Several studies, older and more recent, have dealt with the evaluation of image quality of portal imaging systems (EPID). They also publish qualitative measurements such as MTF, DQE, CNR and SNR. However, in these studies the effect of different actions was not evaluated systematically [G.Jarry and F.Verhaegen, 2005]. Moreover, few works have used the QC-3 phantom for the respective quality evaluation studies [Filipe Martins Garcia de Moura, 2008, Poonam Yadav et al, 2010]. The phantom is used mostly clinically, for positioning the patient [U. Ramm et al, 2013]. This thesis concerns the experimental evaluation of the image quality of portal imaging systems (Portal Imaging). The evaluation was conducted using qualitative indicators such as the modulation transfer function (MTF), noise power spectrum (NPS), Normalized power spectrum of noise (NNPS), Relative detective quantum efficiency (R-DQE), contrast to noise ratio (CNR), Signal to Noise Ratio (SNR), Image Quality Index (FIQ). The MTF was calculated using the square wave response function (SWRF), the NPS was calculated via Fourrier transform the region of interest of the image radiated, the R-DQE was calculated using the MTF and the normalized NPS, the SNR calculated from the fraction with the numerator value of each pixel point of interest of the image and the denominator as the standard deviation of the corresponding region of interest, the CNR was calculated by dividing the contrast of the region of interest with the statistical noise, FIQ was calculated from the MTF of each spatial frequency divided by the coefficient of variation (Coefficient of Variation (CV)). The quality indicators were used in digital DICOM images using the specialized QC-3 test phantom. There were 48 displays, under Radiotherapy conditions (20x20cm2, SSD = 100cm), of that phantom for a variety of Monitor Units (MUs), for various dose rate (DR) and using 4 different energies. In conclusion, it was found that the image quality is improved by increasing the MUs and DR, but a small deterioration in the image was observed in low spatial frequencies. At the same imaging system at 6MV, low DR values the CNR and SNR were increased. While comparing the energy beam 6MV to 18MV, it was observed that the SNR and CNR were higher at 6MV. For four different photon energy beams were observed a similar behavior of R-DQE at three energy beams (10MV, 15MV, 18MV), at 6MV the R-DQE was declining. In the R-DQE graphs at the same DR there was a slight differentiation. The R-DQE was increased at small percentage rates for 6MV and 18MV.

<p>Currently film is being replaced by electronic detectors for portal imaging in radiation therapy. This development offers obvious advantages such as on-line quality assurance and digital images that can easily be accessed, processed and communicated. In spite of the improvements, the image quality has not been significantly enhanced, partly since the quantum efficiency compared to film is essentially the same, and the new electronic devices also suffer from sensitivity to the harsh radiation environment. In this thesis we propose a third generation electronic portal imaging device with increased quantum efficiency and potentially higher image quality.</p><p>Due to the parallel readout capability it is much faster than current devices, providing at least 200 frames per second (fps), and would even allow for a quality assurance and adaptive actions after each accelerator pulse. The new detector is also sensitive over a broader range of energies (10 keV - 50 MeV) and can be used to obtain diagnostic images immediately prior to the treatment without repositioning the patient. The imaging could be in the form of portal imaging or computed tomography. The new detector is based on a sandwich design containing several layers of Gas Electron Multipliers (GEMs) in combination with, or integrated with, perforated converter plates. The charge created by the ionizing radiation is drifted to the bottom of the assembly where a tailored readout system collects and digitizes the charge. The new readout system is further designed in such a way that no sensitive electronics is placed in the radiation beam and the detector is expected to be radiation resistant since it consists mainly of kapton, copper and gas.</p><p>A single GEM detector was responding linearly when tested with a 50 MV photon beam at a fluence rate of ~10¹⁰ photons mm^-2 s^-1 during 3-5 μs long pulses, but also with x-ray energies of 10-50 keV at a fluence rate of up to ~10⁸ photons mm^-2 s^-1. The electron transmission of a 100 μm thick Cu plate with an optical transparency of ~46% was found to be ~15.4%, i.e. the effective hole transmission for the electrons was about one third of the hole area. A low effective GEM gain is enough to compensate for the losses in converters of this dimension. A prototype for the dedicated electronic readout system was designed with 50 x 100 pixels at a pitch of 1.27 mm x 1.27 mm. X-ray images were achieved with a single GEM layer and also in a double GEM setup with a converter plate interleaved. To verify the readout speed a Newton pendulum was imaged at a frame rate of 70 fps and alpha particles were imaged in 188 fps. The experimental studies indicates that the existing prototype can be developed as a competitive alternative for imaging in radiation therapy.</p>

Currently film is being replaced by electronic detectors for portal imaging in radiation therapy. This development offers obvious advantages such as on-line quality assurance and digital images that can easily be accessed, processed and communicated. In spite of the improvements, the image quality has not been significantly enhanced, partly since the quantum efficiency compared to film is essentially the same, and the new electronic devices also suffer from sensitivity to the harsh radiation environment. In this thesis we propose a third generation electronic portal imaging device with increased quantum efficiency and potentially higher image quality. Due to the parallel readout capability it is much faster than current devices, providing at least 200 frames per second (fps), and would even allow for a quality assurance and adaptive actions after each accelerator pulse. The new detector is also sensitive over a broader range of energies (10 keV - 50 MeV) and can be used to obtain diagnostic images immediately prior to the treatment without repositioning the patient. The imaging could be in the form of portal imaging or computed tomography. The new detector is based on a sandwich design containing several layers of Gas Electron Multipliers (GEMs) in combination with, or integrated with, perforated converter plates. The charge created by the ionizing radiation is drifted to the bottom of the assembly where a tailored readout system collects and digitizes the charge. The new readout system is further designed in such a way that no sensitive electronics is placed in the radiation beam and the detector is expected to be radiation resistant since it consists mainly of kapton, copper and gas. A single GEM detector was responding linearly when tested with a 50 MV photon beam at a fluence rate of ~1010 photons mm-2 s-1 during 3-5 μs long pulses, but also with x-ray energies of 10-50 keV at a fluence rate of up to ~108 photons mm-2 s-1. The electron transmission of a 100 μm thick Cu plate with an optical transparency of ~46% was found to be ~15.4%, i.e. the effective hole transmission for the electrons was about one third of the hole area. A low effective GEM gain is enough to compensate for the losses in converters of this dimension. A prototype for the dedicated electronic readout system was designed with 50 x 100 pixels at a pitch of 1.27 mm x 1.27 mm. X-ray images were achieved with a single GEM layer and also in a double GEM setup with a converter plate interleaved. To verify the readout speed a Newton pendulum was imaged at a frame rate of 70 fps and alpha particles were imaged in 188 fps. The experimental studies indicates that the existing prototype can be developed as a competitive alternative for imaging in radiation therapy.

Intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) have been introduced in radiation therapy to achieve highly conformal dose distributions around the tumour while minimizing dose to surrounding normal tissues. These techniques have increased the need for comprehensive quality assurance tests, to verify that customized patient treatment plans are accurately delivered during treatment. In vivo dose verification, performed during treatment delivery, confirms that the actual dose delivered is the same as the prescribed dose, helping to reduce treatment delivery errors. In vivo measurements may be accomplished using entrance or exit detectors. The objective of this project is to investigate a novel entrance detector designed for in vivo dose verification. This thesis is separated into three main investigations, focusing on a prototype entrance transmission detector (TRD) developed by IBA Dosimetry, Germany. First contaminant electrons generated by the TRD in a 6 MV photon beam were investigated using Monte Carlo (MC) simulation. This study demonstrates that modification of the contaminant electron model in the treatment planning system is required for accurate patient dose calculation in buildup regions when using the device. Second, the ability of the TRD to accurately measure dose from IMRT and VMAT was investigated by characterising the spatial resolution of the device. This was accomplished by measuring the point spread function with further validation provided by MC simulation. Comparisons of measured and calculated doses show that the spatial resolution of the TRD allows for measurement of clinical IMRT fields within acceptable tolerance. Finally, a new general research tool was developed to perform MC simulations for VMAT and IMRT treatments, simultaneously tracking dose deposition in both the patient CT geometry and an arbitrary planar detector system, generalized to handle either entrance or exit orientations. It was demonstrated that the tool accurately simulates dose to the patient CT and planar detector geometries. The tool has been made freely available to the medical physics research community to help advance the development of in vivo planar detectors. In conclusion, this thesis presents several investigations that improve the understanding of a novel entrance detector designed for patient in vivo dosimetry.

Intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) have been introduced in radiation therapy to achieve highly conformal dose distributions around the tumour while minimizing dose to surrounding normal tissues. These techniques have increased the need for comprehensive quality assurance tests, to verify that customized patient treatment plans are accurately delivered during treatment. In vivo dose verification, performed during treatment delivery, confirms that the actual dose delivered is the same as the prescribed dose, helping to reduce treatment delivery errors. In vivo measurements may be accomplished using entrance or exit detectors. The objective of this project is to investigate a novel entrance detector designed for in vivo dose verification. This thesis is separated into three main investigations, focusing on a prototype entrance transmission detector (TRD) developed by IBA Dosimetry, Germany. First contaminant electrons generated by the TRD in a 6 MV photon beam were investigated using Monte Carlo (MC) simulation. This study demonstrates that modification of the contaminant electron model in the treatment planning system is required for accurate patient dose calculation in buildup regions when using the device. Second, the ability of the TRD to accurately measure dose from IMRT and VMAT was investigated by characterising the spatial resolution of the device. This was accomplished by measuring the point spread function with further validation provided by MC simulation. Comparisons of measured and calculated doses show that the spatial resolution of the TRD allows for measurement of clinical IMRT fields within acceptable tolerance. Finally, a new general research tool was developed to perform MC simulations for VMAT and IMRT treatments, simultaneously tracking dose deposition in both the patient CT geometry and an arbitrary planar detector system, generalized to handle either entrance or exit orientations. It was demonstrated that the tool accurately simulates dose to the patient CT and planar detector geometries. The tool has been made freely available to the medical physics research community to help advance the development of in vivo planar detectors. In conclusion, this thesis presents several investigations that improve the understanding of a novel entrance detector designed for patient in vivo dosimetry.

A technique for the in vivo dose verification of intensity modulated radiation therapy (IMRT) has been developed. An electronic portal image, calibrated in terms of absolute dose, is acquired for each radiation field following transmission through the patient at the time of treatment. For an IMRT field, the portal image signal is back-projected through a model of the patient in order to calculate the dose at the isocentric plane perpendicular to the beam central axis. The IMRT in vivo dose verification technique was adapted for volumetric modu- lated arc therapy (VMAT) treatments when a single dosimetric image is acquired over an arc. The patient dose along axis of gantry rotation can be directly related to the signal along the vertical axis of EPIs in integrated mode. In this novel VMAT in vivo dosimetry technique, the portal image signal is back-projected through a rotationally averaged model of the patient to calculate a 1D in vivo dose along the axis of gantry rotation. A research ethics board clinical study was approved and transmission portal images were acquired at regular intervals from human subjects. Portal image-derived isocenter point doses were in good agreement with treatment planning system (TPS) calculations for IMRT (mean difference δ=0.0%, standard deviation of the differences σ=4.3%) and VMAT (δ=1.1%, σ=1.7%). The one-dimensional (VMAT) and two-dimensional (IMRT) reconstructed doses were further analyzed by calculating mean dose differences and γ−evaluation pass-rates, which were also shown to be in good agreement with TPS calculations. The portal image-based in vivo dosimetry techniques were shown to be clinically feasible, with reconstruction times on the order of minutes for the first fraction and less than one minute for each fraction thereafter. / Graduate / 0760 / 0574 / 0760

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Nowadays more and more radiotherapy departments use intensity modulated beams for treatment of patients. Intensity modulated radiotherapy (ImRT) is able to modificate intensity of radiation across the iradiated field. In this way it is posible to achieve better dose conformity than in conventional radiotherapy. Implementation of ImRT allows us to escalate dose to target volume with same side effects of organs at risk as in conventional radiotherapy or to reduce normal tissue complication - decrease dose to organ at risk with the same tumour dose. This fact reguires extension of our guality system to all network of delivery dose to patients, inclusive linear accelerator with multileaf collimator, treatment planning system, electronic portal imaging device and so on. Quality assurance is guaranteed both periodical user tests and independent verification of The State Office for Nuclear Safety. The aim of this work is finding the optimal and effective way for the verification treatment plans, determining criteria for evaluation measured results, proposing summary all aspects of radiation protection patients which are treate ionisation beams with intensity modulated radiotherapy. The optimization one of the principles of radiation protection will be provided by routin verification treatment plans.

Recent work has demonstrated improvement of image quality with low atomic number (Z) linear accelerator (linac) targets and energies as low as 3.5 MV compared to a standard 6 MV therapeutic beam. In this work, the incident electron beam energy has been lowered to energies between 1.90 and 2.35 MeV. The improvement of megavoltage planar image quality with the use of carbon and aluminum linac targets has been assessed compared to a standard 6 MV therapeutic beam. Common electronic portal imaging devices contain a 1.0 mm copper conversion plate to increase detection efficiency of a therapeutic megavoltage spectrum. When used in imaging with a photon beam generated with a low-Z target, the conversion plate attenuates a substantial proportion of photons in the diagnostic range, thereby reducing the achievable image quality. Image quality as a function of copper plate thickness has been assessed for planar imaging and cone beam computed tomography.