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Internship: B0 mapping for geometrical corrections

Duration: 3 months (bachelor project or short master internship)


Rationale:

Radiotherapy treatment planning in the UMCU relies more and more on MR imaging. The images are used by the clinicians to delineate the tumour. The radiotherapy treatment is planned on the delineated volume. Therefore, it is important that the MRI images are geometrically correct. This, however, is not necessarily the case. Diffusion weighted images, which give important information about the tumour extent, are for example easily geometrically disturbed. Those geometrical disturbances are all related the inhomogeneities in the main magnetic field.

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Figure 1:Diffusion weighted images of the oesophageal cancer. A) T2w-MRI scan, the tumour is delineated in red. B,C) Diffusion-weighted scan (b = 800 s/mm2) before geometrical correction. A clear mismatch is present between the tumour delineation and the area with diffusion restriction. D) Magnetic map, measured with MRI. The variation in the main magnetic field in the tumor leads to a pixel shift of 0.77 pixels on average. E,F) Diffusion-weighted scan (b = 800 s/mm2) after geometrical correction. The magnetic field map shown in figure D was used for the correction. After correction the tumor delineation is in line with the diffusion restriction area.

During the last years, we implemented a method to retrospectively correct the geometrically errors using a map of the main magnetic field. However, we have not investigated which field mapping method is best suited for retrospective geometrical corrections. Various field mapping methods which are available, such as: 
• 2-point DIXON: currently only gives the field map. 
• mDIXON: additionally gives information about water and fat content of tissue and T2*. 
• multi-echo techniques: can also be used to obtain information about T2*


Your task:
You will start with a short literature survey on field mapping methods and retrospective geometrical corrections. Hereafter, you will perform phantom and volunteer scans with various field mapping methods at various anatomical sites. For these scans you need to set up the MRI measurement (supervised). If required, the measurements are processed to obtain field maps. Then you will perform the geometrical correction and compare the results. Based on those results you will select the best suited method for B0 mapping of geometrical distortions. Finally, you will optimize the selected field mapping method to fit in clinically acceptable scan times.

Your profile:
• You have a basic understanding of MR imaging
• You like to do experiments and image processing
• You have a basic understanding of programming/scripting (e.g. matlab, bash scripting, C++)

Contact information:
Dr. Ir. Astrid van Lier,a.l.h.m.w.vanlier[at]umcutrecht.nl, 088-7550284
Dr. Ir. Marielle Philippens, m.philippens[at]umcutrecht.nl