Departmental Oral Examination (Thesis Title: "Validation and Optimization of Myelin Water Imaging in Preclinical Model of Spinal Cord Injury")

Henry S. Chen
Event Date and Time: 
Thu, 2016-08-18 09:00 - 11:00
Henn 309
Local Contact: 
Physics and Astronomy, UBC
Intended Audience: 

Myelin  content is an important marker for neuropathology; however, direct imaging of myelin is difficult. Consequently, quantitative T2 based myelin water imaging measures myelin content indirectly by probing the property of the surrounding water. Typically, a lengthy multi-echo spin-echo sequence is used to obtain decay curves that are fitted to produce T2 distributions. In white matter, two peaks are observed, one with short and one with long T2 associated with water trapped between the myelin lipid bilayers and intra/extracellular water. The ratio of myelin water peak to the entire distribution is called the myelin water fraction (MWF) and correlates well the myelin content.

This thesis has two parts. The first half deals with the use of compressed sensing (CS) to accelerate the lengthy sequence used in myelin water imaging. CS CPMG sequence was implemented in 2D utilizing group-sparse reconstruction in order to take advantage of the correlation between echoes. Simulated undersampling and real undersampling experiments were performed. It was found that acceleration up to 2× was possible without significant impacting MWF map quality wherever adequate SNR was available. This is followed by a brief investigation into 3D CS CPMG, where similar results were achieved.

The second part of the thesis focuses on myelin water imaging in the presence of myelin debris. Because MWF is associated with the water trapped in between the myelin lipid bilayers, the reading depends heavily on myelin morphology. We compared MWF to transmission electron microscopy (TEM) derived myelin content using a rat injury model at normal (normal myelin), 3 weeks post-injury (a large amount of myelin debris), and 8 weeks post-injury (myelin debris largely cleared). We found that myelin water fraction correlates strongly with the amount of myelin lipid bilayers in both intact myelin and myelin debris. From the TEM images, it appears that myelin debris consists of areas of either normally spaced myelin or large watery spaces. No significant difference was found in myelin spacing among the post-injury time points.

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