Spatial uncertainties in Magnetic Resonance Imaging, and their correction: Comparison of 1.5 T and 3T systems

Postgraduate Thesis uoadl:2916577 314 Read counter

Unit:
Κατεύθυνση Ιατρική Φυσική-Ακτινοφυσική
Library of the School of Health Sciences
Deposit date:
2020-06-12
Year:
2020
Author:
Theocharis Stefanos
Supervisors info:
Παντελής Καραΐσκος, Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ, Επιβλέπων
Ιωάννης Σεϊμένης, Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ
Κωνσταντίνος Λουκάς, Επίκουρος Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ
Original Title:
Χωρικές ανακρίβειες στην Απεικόνιση Μαγνητικού Συντονισμού, και διόρθωση τους: Σύγκριση συστημάτων 1.5 Τ και 3Τ
Languages:
Greek
Translated title:
Spatial uncertainties in Magnetic Resonance Imaging, and their correction: Comparison of 1.5 T and 3T systems
Summary:
This paper aims to evaluate geometric distortion and apply methods of correction for clinically used images of Magnetic Resonance Imaging (MRI). Despite their inherent distortion, MRI images are increasingly used in applications of Radiotherapy with a high demand of geometric accuracy, for example in the treatment planning of Stereotactic Radiosurgery (SRS), where it is essential that there is high conformity of the dose field and high dosage gradient. It is therefore essential, to evaluate the different types of distortions present in MRI images, and the development of a robust methodology to eliminate or reduce them. In addition, the increase in the number of higher intensity MRI machines, it is expected that the geometric distortions will increase.
Following the deposition of the relevant theoretical background, the second part of the current thesis has performed an evaluation of the geometric distortions associated with MRI images. The reverse gradient technique is employed, in order to evaluate the sequence dependent distortions. Next is calculated independently, the net geometric uncertainty (total distortion) in MRI images, and the sum of the frequency independent distortion and the distortion of spatial registration of the images. The images were acquired using static fields of intensity 1.5 T and 3 T, with an average vector of total distortion of 1.17 mm and 1.74mm and a maximum value of 1.75 mm and 3.20 mm respectively. The calculations were performed on MRI images of VIP polymer, chemical dosimeter which was irradiated in a Gamma Knife unit.
Finally, the ”Mean Image” methodology is applied for correction of the images, in which gradient reversed images are used to produce the corrected image. Reverse gradient images are used, in which the signal of each voxel has been replaced with the average signal of the equivalent tumor element of each of the initial images. The correction of the images using the aforementioned method, results in the measurable improvement of the total geometric uncertainty, not only regarding the average values - 0.29 mm (from 1.17 mm) in 1.5T and 1.11 mm (from 1.74 mm) in 3T - but also in the maximum values - 0.75 mm (from 1.75 mm) in 1.5T and 1.85 mm (from 3.09 mm) in 3T, whilst remaining quick and easy in its application.
Main subject category:
Health Sciences
Keywords:
MRI
Index:
No
Number of index pages:
0
Contains images:
Yes
Number of references:
42
Number of pages:
91
File:
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Διπλωματική εργασία-Θεοχάρης Στέφανος.pdf
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