Report No 1951/AP - Instytut Fizyki JÄ drowej PAN

NON-CARTESIAN SAMPLING IN MRI:
RADIAL AND SPIRAL SEQUENCES
Katarzyna Cieślar, Katarzyna Suchanek, Mateusz Suchanek, Tadeusz Pałasz,
Tomasz Dohnalik, and Zbigniew Olejniczak*
Institute of Physics, Jagellonian University, Kraków, Poland,
*Institute of Nuclear Physics, Kraków, Poland
Data acquisition in the k-space can be accomplished in many different ways. The most
commonly used strategy is the 2DFT (spin-wrap) method (Fig 1 a), where the k-space is
sampled on a regular cartesian grid. The reconstruction of the image is then performed by the
2D FFT algorithm. Among alternative methods of sampling the k-space there are radial and
spiral acquisition schemes. We present two versions of these sequences based on the FID
sampling (Fig 1 b and c). The k-space trajectory is given either by a set of radial lines starting
from the origin of coordinate system, or by a set of interleaved spirals.
The main disadvantage of non-cartesian imaging is a multistep reconstruction process.
The data sets collected along either radial or spiral trajectories need to be regridded onto a
cartesian coordinate system. This process has to account for an uneven distribution of the
sampled data points by applying a proper density compensation weighting. Following this, the
standard 2D FFT is performed to obtain the image.
Due to their inherently high signal-to-noise ratio and short total acquisition time, the
non-cartesian sequences offer great advantages for MRI of living objects, especially in the
case when the motion artifacts have to be eliminated. The first images obtained by these
methods will be presented. Consequently, they will be used in static and dynamic imaging of
rat lungs using polarised helium-3 on our 0.08 T MRI system.
References:
[1]. G.H. Glover, J. M. Pauly, Magn. Reson. Med., 28: 275-289 (1992)
[2]. C. H. Meyer, B. Hu, D. G. Nishimura, A. Mackovski, Magn. Reson. Med., 28: 202-213
(1992)
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