Abstract
We present a new method for magnetization transfer (MT) ratio imaging in the brain that requires no separate saturation pulse. Interslice MT effects that are inherent to multi-slice balanced steady-state free precession (bSSFP) imaging were controlled via an interslice delay time to generate MT-weighted (0 s delay) and reference images (5–8 s delay) for MT ratio (MTR) imaging of the brain. The effects of varying flip angle and phase encoding (PE) order were investigated experimentally in normal, healthy subjects. Values of up to ∼50% and ∼40% were observed for white and gray matter MTR. Centric PE showed larger MTR, higher SNR, and better contrast between white and gray matter than linear PE. Simulations of a two-pool model of MT agreed well with in vivo MTR values. Simulations were also used to investigate the effects of varying acquisition parameters, and the effects of varying flip angle, PE steps, and interslice delay are discussed. Lastly, we demonstrated reduced banding with a non-balanced SSFP-FID sequence and showed preliminary results of interslice MTR imaging of meningioma.
Highlights
Protons that are bound to macromolecules can exchange magnetization with free water protons leading to magnetization transfer (MT) phenomena [1]
The magnetization transfer ratio (MTR) images of the agar phantom showed relatively homogeneous MTR values with the exception of dark spots caused by air bubbles trapped in the phantom
The signals in white matter (WM) were higher than gray matter (GM) for all MTR images (Fig. 3a)
Summary
Protons that are bound to macromolecules can exchange magnetization with free water protons leading to magnetization transfer (MT) phenomena [1]. Macromolecular protons cannot be observed directly with magnetic resonance imaging (MRI) because of fast transverse relaxation (T2 * 10 μs); macromolecular protons can be preferentially saturated by offresonance (with respect to free water) radio frequency (RF) irradiation, since the absorption spectrum of macromolecular protons is much broader than that of free water. Exchange between the two pools of protons can transfer an observable decrease in magnetization to the free water pool. The percent signal decrease due to MT is called the magnetization transfer ratio (MTR). Changes in MTR can often give information about the macromolecules involved in PLOS ONE | DOI:10.1371/journal.pone.0117101. Changes in MTR can often give information about the macromolecules involved in PLOS ONE | DOI:10.1371/journal.pone.0117101 February 9, 2015
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