Person:

Varma, Gopal

Purpose—In balanced steady state free precession (bSSFP), the signal intensity has a wellknown dependence on the off-resonance frequency, or, equivalently, the phase advance between successive radiofrequency (RF) pulses. The signal profile can be used to resolve the contributions from the spectrally separated metabolites. This work describes a method based on use of a variable RF phase advance to acquire spatial and spectral data in a time-efficient manner for hyperpolarized 13C MRI.

Theory and Methods—The technique relies on the frequency response from a bSSFP acquisition to acquire relatively rapid, high-resolution images that may be reconstructed to separate contributions from different metabolites. The ability to produce images from spectrally separated metabolites was demonstrated in-vitro, as well as in-vivo following administration of hyperpolarized 1-13C pyruvate in mice with xenograft tumors.

Results—In-vivo images of pyruvate, alanine, pyruvate hydrate and lactate were reconstructed from 4 images acquired in 2 seconds with an in-plane resolution of 1.25 × 1.25mm2 and 5mm slice thickness.

Conclusions—The phase advance method allowed acquisition of spectroscopically selective images with high spatial and temporal resolution. This method provides an alternative approach to hyperpolarized 13C spectroscopic MRI that can be combined with other techniques such as multiecho or fluctuating equilibrium bSSFP.

Purpose To characterize a new approach to magnetization transfer (MT) imaging with improved specificity for myelinated tissues relative to conventional MT.

Methods Magnetization transfer preparation sequences were implemented with all radiofrequency power centered on a single frequency and also with power evenly divided between positive and negative frequencies. Dual frequency saturation was achieved both with short, alternating frequency pulses and with sinusoidal modulation of continuous irradiation. Images following preparation were acquired with a single shot fast spin echo sequence. Single and dual frequency preparation should achieve similar saturation of molecules except for those with inhomogenously broadened lines. Inhomogenous MT (IHMT) images were generated by subtraction of dual from single frequency prepared images. IHMT imaging was performed with different power and frequency in the brains of normal volunteers.

Results The (IHMT) method demonstrated a greater white/gray matter ratio than conventional MT and virtual elimination of signal in scalp and other unmyelinated tissues. IHMT exceeded 5% of the fully relaxed magnetization in white matter. A broad frequency spectrum and signs of axonal angular dependence at high frequency were observed that are consistent with dipolar broadening.

Conclusion IHMT shows promise for myelin specific imaging. Further study of physical mechanisms and diagnostic sensitivity are merited.