Magnetization transfer imaging using steady-state free precession MR sequences

Magnetic resonance imaging (MRI) benefits from an exceptional soft tissue
contrast and is therefore an important tool for medical diagnosis. While
contrast in conventional MRI is generated by protons with free mobility,
magnetization transfer (MT) imaging generates contrast from protons bound to
macromolecules. This contrast is based on tissue microstructure and tissue
integrity. Additional information about tissue changes is desirable for better
understanding, for early diagnosis, and for monitoring treatment response of
many pathologies. However, most MT imaging techniques are still not suitable for
application in the daily clinical routine due to long acquisition times. In
contrast, steady-state free precession (SSFP) sequences offer short acquisition
times and high signal-to-noise ratios (SNR) in combination with their inherent
MT-sensitivity. In this thesis, new MT imaging methods are developed using SSFP
sequences.
In Chapter 2, a quantitative MT imaging technique is implemented based on
balanced SSFP (bSSFP). The derived analytical solution is used to determine
quantitative MT parameters, such as the bound proton fraction and the forward
exchange rate as well as the relaxation times in human brain. In Chapter 3, a
protocol is introduced that benefits from an even shorter acquisition time and
from the possibility to incorporate phase-cycled acquisitions to reduce banding
artifacts. In the second part of this chapter, the effect of finite RF pulses is
accounted for by a modification to the two-pool bSSFP signal equation.
While bSSFP techniques are well applicable in targets with low susceptibility
variations such as the human brain, targets of the musculoskeletal system, such
as cartilage and muscle cause signal loss from off-resonance effects. As a
result, in Chapter 4, the proposed quantitative MT imaging principle is adapted
to nonbalanced SSFP. Quantitative MT parameters are derived from human femoral
muscle and human patellar cartilage. In addition to quantitative MT imaging,
bSSFP-based magnetization transfer ratio (MTR) measurements are performed in
significantly shorter times and with an improved SNR compared to conventional
methods. In Chapter 5, the reproducibility of bSSFP-MTR is analyzed in brain
tissue of healthy subjects. In order to establish bSSFP-based quantitative MT
imaging in a clinical setting, a reference data set of normal appearing brain
structures is analyzed in Chapter 6. First experiences of the new MT technique
in imaging of tumor and ischemia patients are presented.