1 research outputs found
Development of Methodologies for Diffusion-weighted Magnetic Resonance Imaging at High Field Strength
Diffusion-weighted imaging of small animals at high field strengths is a challenging prospect
due to its extreme sensitivity to motion. Periodically rotated overlapping parallel lines with
enhanced reconstruction (PROPELLER) was introduced at 9.4T as an imaging method that
is robust to motion and distortion. Proton density (PD)-weighted and T2-weighted
PROPELLER data were generally superior to that acquired with single-shot, Cartesian and
echo planar imaging-based methods in terms of signal-to-noise ratio (SNR), contrast-to-noise
ratio and resistance to artifacts.
Simulations and experiments revealed that PROPELLER image quality was dependent on
the field strength and echo times specified. In particular, PD-weighted imaging at high field
led to artifacts that reduced image contrast. In PROPELLER, data are acquired in
progressively rotated blades in k-space and combined on a Cartesian grid. PROPELLER
with echo truncation at low spatial frequencies (PETALS) was conceived as a postprocessing
method that improved contrast by reducing the overlap of k-space data from different blades
with different echo times.
Where the addition of diffusion weighting gradients typically leads to catastrophic motion
artifacts in multi-shot sequences, diffusion-weighted PROPELLER enabled the acquisition of
high quality, motion-robust data. Applications in the healthy mouse brain and abdomen at
9.4T and in stroke patients at 3T are presented.
PROPELLER increases the minimum scan time by approximately 50%. Consequently,
methods were explored to reduce the acquisition time. Two k-space undersampling regimes
were investigated by examining image fidelity as a function of degree of undersampling.
Undersampling by acquiring fewer k-space blades was shown to be more robust to motion
and artifacts than undersampling by expanding the distance between successive phase
encoding steps. To improve the consistency of undersampled data, the non-uniform fast
Fourier transform was employed. It was found that acceleration factors of up to two could be
used with minimal visual impact on image fidelity.
To reduce the number of scans required for isotropic diffusion weighting, the use of rotating
diffusion gradients was investigated, exploiting the rotational symmetry of the PROPELLER
acquisition. Fixing the diffusion weighting direction to the individual rotating blades yielded
geometry and anisotropy-dependent diffusion measurements. However, alternating the
orientations of diffusion weighting with successive blades led to more accurate
measurements of the apparent diffusion coefficient while halving the overall acquisition time.
Optimized strategies are proposed for the use of PROPELLER in rapid high resolution
imaging at high field strength