Whole brain inter-slice quantitative magnetization transfer imaging with database-driven fitting approach

Abstract

Macromolecules such as myelin could not be detected directly in magnetic resonance imaging due to their short relaxation times. Therefore, they were observed indirectly from the magnetization transfer (MT) effect. Conventional methods for quantifying MT was magnetization transfer ratio (MTR), however, it varies depending on scan conditions and does not complicate enough to represent complex information. Quantitative MT (qMT) can overcome these limitations by giving intrinsic MT parameters as a map but requires data acquisition at multiple frequency offsets and saturation powers, which increases scan time. Moreover, long fitting time to get a qMT map is another challenge in qMT. Therefore, qMT still needs improvements in data acquisition and processing. Recently balanced steady state free precession (bSSFP) based inter-slice imaging has been proposed as a fast MT imaging method, while its feasibility as qMT has not been investigated. Also, database driven approaches are tried in many research areas to improve processing time, but their applications in qMT have not be demonstrated yet. In this study, we investigated methods to decrease MT data acquisition time by modulation of the offset frequency and saturation power via changes in inter-slice gap and flip angle to advance the inter-slice MT imaging to qMT. And database driven fitting approach was used for decreasing map fitting time, in comparison with Bloch equation based approach, the results showed about 200 times reduction in processing time with the database approach. The inter-slice qMT and database based mapping efficiency were validated by comparison with conventional pre-saturation qMT method in single slice. After that, the approach was expanded to the whole brain in clinically acceptable scan time (< 10 min). The proposed approaches of inter-slice qMT imaging and database-driven processing will be useful for applying qMT imaging in diagnostic situations.