Low Field Magnetic Resonance Imaging of the Eye

Inexpensive MRI for Ocular Conditions

Master thesis (2022)

Authors

C. Haasjes Mechanical Engineering

Contributors

J-W.M. Beenakker Leiden University Medical Center (supervisor 1)
R.F. Remis Signal Processing Systems - EEMCS (supervisor 1)
F.M. Vos ImPhys/Computational Imaging - AS (supervisor 2)
J.L. Herder Precision and Microsystems Engineering (supervisor 2)
Faculty
Mechanical Engineering
Copyright: © 2022 Corné Haasjes
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Published Date

01-12-2022

Language

English

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Faculty

Mechanical Engineering

Abstract

Ultrasound imaging is an important modality in ocular oncology, allowing for fast examination of the eye by the ophthalmologist themselves. It is clinically used to measure tumour sizes for treatment planning. However, ocular ultrasound is limited to two-dimensional imaging, and suffers from poor contrast between tumour and sclera, which negatively impacts the accuracy of tumour measurements. In this work, low field MRI is investigated as a possible alternative for ultrasound imaging.
Design requirements are a scan time of less than 4 minutes; resolution of 1.0 mm isotropic; Field of View (FOV) large enough to contain the eye and the orbit; contrast sufficient to distinguish the sclera, vitreous, tumour, lens and lipid. The experimental setup consists of a 46 mT Halbach-array based scanner, a volume coil as transmit coil and a custom-built surface coil as receive coil. Images are made of a water phantom to characterise the FOV, and a porcine eye to characterise the contrast.
The FOV is found to meet the requirements, and the contrast is sufficient to distinguish the sclera, vitreous, lens and lipid in porcine eyes. The resolution is too low and the scans take too long (about 5 minutes at a resolution of 1.0 × 1.0 × 7.5 mm). Increasing the resolution and decreasing the scan time will result in a low Contrast to Noise Ratio (CNR), causing the contrast requirement to be violated. Fast, high-resolution three-dimensional imaging is therefore not feasible on the current system.
The CNR can be improved by using a higher field strength, which requires the development of new hardware. Furthermore, in order to develop a clinically useable system, it is necessary to determine tumour contrast, design optimised pulse sequences, and test the method on human subjects.