MRI USING PIECEWISE-LINEAR SPIRAL TRAJECTORY

A new generation of high power gradient systems which allow much faste r MR imaging as well as shorter echo times has recently become availab le. Some of these high-speed gradient systems impose limits an the per centage of time during which the gradient can change in amplitude (sle wing duty cycle), While this limitation may be immaterial to many 2DFT and echo planar imaging methods, a traditional circular spiral trajec tory is difficult to use on these systems because its gradient wavefor ms change during the entire course of the trajectory so that the slewi ng duty cycle during the readout period is 100%. We describe a piecewi se-linear spiral trajectory which is composed of linear segments and r ounded corners. This trajectory reduces the slewing duty cycle while m aintaining the desirable imaging properties of circular spirals includ ing interleaving by simple gradient rotation. For one representative e xample, the slewing duty cycle is reduced to 46%. A conventional gridd ing method was used for image reconstruction, but a new numerical algo rithm to calculate the density compensation factor was required, Use o f piecewise-linear spiral trajectories reduces the impact imposed by l imited gradient slewing duty cycle.