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Identifier 000451422
Title Quality assurance in modern techniques of functional Magnetic Resonance Imaging (fMRI) based on BOLD using special anthropomorphic phantoms. : Applications in the stimulation of the motor and visual cerebral cortex
Alternative Title Διασφάλιση ποιότητας στις σύγχρονες τεχνικές λειτουργικής Απεικόνισης Μαγνητικού Συντονισμού (λΑΜΣ) που βασίζονται στο φαινόμενο (BOLD) με χρήση ειδικών ανθρωπόμορφων ομοιωμάτων.
Author Μπουρσιάνης, Θεμιστοκλής
Thesis advisor Μαρής, Θωμάς
Reviewer Καραντάνας, Απόστολος
Παπαδάκη, Εφροσύνη
Abstract Magnetic Resonance Imaging is one of the most advanced techniques used worldwide for the human body imaging nowadays. Different physiological mechanisms are used to perform advanced examinations such as functional MRI (f MRI) which depends on BOLD(Blood Oxygen Level Dependant) contrast. The results of BOLD measurements rely on statistical analysis of the acquired signal and are sensitive to noise, SNR and even the examination conditions of the patientwith the use of stimulation devices. Quality Assurance protocols and Clinical studies are needed to compare and optimize the imaging parameters. QA protocols are implementing the use of phantoms to ensure imaging quality and rigid quantitative results from the systems. The existing quality assurance protocols for MRI do not include any method for measuring and evaluating the BOLD contrast, as it would suggest the use of a dynamic phantom to simulate the 2 states(baseline-activation) of the fMRI examination. Such phantoms have been presented in the literature with several limitations such as complexity, high cost of manufacturing and a lack of BOLD physical basis simulation. The first chapter includes a historical overview and a brief description of the MRI systems and the physical principles of the imaging. In the second chapter of the thesis a new method for BOLD contrast simulation is proposed. This method uses a dynamic, easy to construct and operate, low-cost physical phantom.A structure of thin pipelines passing through a gel volume was used to simulate blood vessels in human tissue. Quantitative T2*, R2* measurements were used to study the signal change of the phantom. BOLD fMRI experiments and analysis were performed to evaluate its potential use as an fMRI simulator. Experimental T2*, R2* measurements showed similar behavior with published references. BOLD contrast was successfully achieved with the proposed method. In addition, there were several proposed parameters, like the angle of the phantom relative to B0, which can easily adjust the signal change and the activation area. Coefficients of variation showed good reproducibility within a month period. Statistical t-maps were produced with in-house software for the BOLD measurements. T2*maps and BOLD images confirm the potential use of this phantom as an fMRI simulator and as a tool for studying sensitivity and specificity of BOLD sequences/algorithmsIn chapter 3, the new method for BOLD effect simulation is applied on an anthropomorphic head phantom trying to further simulate the fMRI examination. 3D printing technology and a calcium-based material is used to manufacture a head phantom to accommodate the previously proposed BOLD feature. The constructed dynamic head phantom provides a more realistic simulation of the fMRI experiment in terms of Β0 susceptibility effects and B1 uniformity. In order to demonstrate the phantom’s usefulness, the phantom was MRI scanned and the acquired images were compared to a human’s images in terms of susceptibility effects and homogeneity distortions. Additionally, the phantom was used to evaluate the SNR and BOLD strength dependence on the fMRI hardware commonly used during the examination to apply visual and auditory stimulations to the examined human subject.After visual inspection of the fieldmap images, it can be noticed that the head phantom is very similar to a real patient as its overall shape and size was the outcome of a real patient CT dataset.BOLD activation was successfully achieved with great timing during the states’ alternations as seen in the signal difference vs time graphs. The measured SNR and SSNR values show that neither the off-center positioning nor the presence of the goggles, headphones and cables affect drastically the T1 image quality. The 4 cm off center positioning does not seem to affect the SNR or SSNR. The hardware presence reduces the SNR by 9.6% and SSNR by 10.4% when it’s switched off. But when the devices were switched on, the same values were reduced to approximately 50% and even more when the cables were looped. The area of activation is strongly affected in a similar way from the different condition of examination. The MoCo area of activation appears reduced from the off-center positioning by 10% and reduced even more in the other scenarios. Almost half area appears activated when the stimulation devices are on and the cables looped. The mean and maximum t-values of the statistical difference between the states are not affected by any of the 5 conditions examined. The BOLD strength though is affected as it can be expressed by the t-value and area product. This dynamic head phantom is proved to be an excellent means for studying fMRI protocols and procedures in order to optimize them and evaluate the dependence of image quality and BOLD strength on the use of hardware devices inside the imaging area of an fMRI examination. Finally, in chapter 4 a clinical study is performed. This study, evaluates the dependence of fMRI results on echo time (TE) during concurrent activation of the visual and motor cortex at 1.5 T in a large sample of 21 healthy volunteers. The experiment was repeated using two different TE values (50 and 70 ms) in counterbalanced order. Furthermore, T2* measurements of the gray matter were performed. Results indicated that both peak beta value and number of voxels were significantly higher using TE=70ms compared to TE=50 ms in primary motor, somatosensory and supplementary motor cortices (p<.007). In addition, the amplitude of activation in visual cortices and the dorsal premotor area was also higher using TE=70 ms (p<.001). Gray matter T2* of the corresponding areas did not vary significantly. In conclusion, the optimal TE value (between the two studied) for visual and motor activity is 70 ms affecting both the amplitude and extent of regional hemodynamic activation.
Language English
Subject Phantom
Ομοίωμα
Ποιοτικός έλεγχος
Issue date 2022-12-07
Collection   School/Department--School of Medicine--Department of Medicine--Doctoral theses
  Type of Work--Doctoral theses
Permanent Link https://elocus.lib.uoc.gr//dlib/f/a/1/metadata-dlib-1668583521-929867-4401.tkl Bookmark and Share
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