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.

Introduction: Uveal melanoma is the most frequently occurring primary eye tumour. The aim of this research was to determine the difference in geometrical tumour measurements between ultrasound and magnetic resonance imaging (MRI), to compare MRI and ultrasound-based tumour models, and to evaluate the clinical implications of these differences for uveal melanoma patients.
Methods: Tumours of 42 uveal melanoma patients were segmented on T2-weighted MRI scans. Prominence, largest basal diameter (LBD), and second basal diameter (SBD) were calculated automatically and compared to clinical ultrasound measurements. Volumes of segmented MRI tumours were compared to the volumes of ellipsoid ultrasound-based models. Results: For the prominence, LBD, and SBD, the mean absolute differences were 0.8 ± 0.7 mm, 1.6 ± 1.3 mm and 1.6 ± 1.4 mm between the conventional ultrasound measurement and the MRI measurement. A significant difference was observed for the prominence between anteriorly and posteriorly located tumours (0.6 ± 0.5 mm versus 1.1 ± 0.8 mm, p = 0.02). Tumour volume on MRI was on average 70 mm³ smaller than the ultrasound tumour model volume, with a dice similarity coefficient of 0.81 ± 0.10.
Conclusion: The largest clinical impact of the use of MRI measurements may be observed in tumours with a prominence between 6 and 8 mm and a LBD between 14 and 18 mm. Furthermore, the use of MRI could add valuable shape information to proton beam therapy planning.