- Workshop presentation
- Open Access
Effects from RF spoiling disequilibrium in the background offsets of phase-contrast velocity imaging
© Gatehouse et al; licensee BioMed Central Ltd. 2012
- Published: 1 February 2012
- Lower Flip Angle
- Gradient Waveform
- Biomedical Research Unit
- Unit Funding
- Spoiler Gradient
To investigate how disequilibrium of spoiling affects background offset errors in phase-contrast velocity images.
Phase-contrast imaging normally uses gradient-echoes with RF spoiling and phase-encode rewinding with a fixed dephasing by gradient “spoiler” pulses in each TR (time between RF pulses) studied in detail (1) where effective artifact suppression in magnitude images required >≈ 8PI dephasing over the slice thickness, with >≈ 2PI dephasing over an FE pixel. However, the effectiveness of spoiling depends on establishing a steady-state. In phase-contrast imaging, background offsets are known to stabilise in continuous scanning such as retrospective gating, but sequence interruptions can be necessary eg in navigator-gating or slice-tracking. Unsteady background offsets are generally ascribed to disturbed equilibrium in eddy-current effects. This abstract is a first investigation of contributions from disequilibrium in spoiling.
To separate the two sources of unsteady background, the RF pulses in a non-segmented prospectively-triggered 50-frame cine phase-contrast 1m/s sequence were disabled in frames 21-30 while the gradient waveforms continued for those frames. Therefore data from frames 1-20 (“pre” RF interruption) include both sources while frames 31-50 (“post”) contain only spoiling disequilibrium effects. Background velocity offsets in muscle and fat were measured at 3T as functions of flip angle (20 - 30°), spoiler gradients (50,100,150% of the values in ref. 1 ) and TR (4.2 - 13.1ms).
Temporal stdev (cm/s) of tissue ROIs
Most of the results are consistent with incomplete spoiling in the early frames of a cine as the spoiling equilibrium is established, for example the longer T2 of fat explains its larger signals in higher-order pathways, which are reduced by lower flip angle. This effect stabilizes after a longer series of continuous gradient and RF activity, and only marginally exceeds random noise in muscle at lower flip-angles. However, for some phase-contrast sequences this effect may be significant in the background offset.
NIHR Cardiovascular Biomedical Research Unit funding.
- Leupold , Hennig , Scheffler : 2008Google Scholar
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