- 312 views
- 563 downloads
Relaxometry in the Human Brain Using High Field Magnetic Resonance Imaging
-
- Author / Creator
- McPhee, Kelly C
-
Quantitative relaxation mapping allows for direct non-invasive tissue quantification with removal of variations arising from RF fields and non-uniform coil sensitivity profiles. T2 may be quantified using multi-echo spin echo sequences; however, in practical imaging situations, imperfect refocussing due to RF interference and imperfect slice profiles results in contamination of the signal decay curve by contributions from indirect and stimulated echoes. Modelling of the multi-echo spin echo sequence by either Bloch equations or Extended Phase Graph (EPG) allows for indirect and stimulated echo compensation (ISEC). In this work, it is demonstrated that EPG-ISEC and Bloch-ISEC yield very similar but systematically different T2 results. EPG fitting provides reasonably accurate T2, but is limited by poor accuracy in resulting flip angles, and T2 errors increase when flip angles are provided. Bloch simultaneous fitting of T2 and flip angle provides excellent results, but can be limited by multiple solutions which can be overcome by including a flip angle map. Despite the availability of methods to compensate for indirect and stimulated echoes, exponential fitting remains persistent. It is common to discard the first or all odd echoes, to improve fitting. However, it is demonstrated in this work that this is insufficient to remove stimulated echo contamination, and errors in T2 fitting will remain significant, and will vary with T2, flip angle, and echo train length. In clinical practice, relaxation weighted images are routinely acquired, but quantification of relaxation times typically requires additional specialized pulse sequences and hence additional imaging time, which is not typically feasible in standard clinical practice. In this work, it is demonstrated that the Bloch-based ISEC approach permits T2 to be quantified using only a Proton Density-weighted and a T2-weighted fast spin echo image and a flip angle map. With the addition of a T1 weighted image to the protocol, simultaneous T1 and T2 quantification is demonstrated. This approach may enable the introduction of quantitative relaxation into radiology protocols without substantial time penalties.
-
- Graduation date
- Spring 2018
-
- Type of Item
- Thesis
-
- Degree
- Doctor of Philosophy
-
- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.