This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to the use of k-space spiral trajectories in acquiring MRI data.
The invention will be described with reference to publications listed in the attached appendix.
MRI signals for reconstructing an image of an object are obtained by placing the object in a magnetic field, applying magnetic gradients for slice selection, applying a magnetic excitation pulse to tilt nuclei spins in the desired slice, and then detecting MRI signals emitted from the tilted nuclei spins. The detected signals can be envisioned as traversing lines in a Fourier transformed space (k-space) with the lines aligned in spaced parallel Cartesian trajectories and in spiral trajectories emanating from an origin in k-space.
Spiral trajectories (1) not only have been extensively used in fast MRI imaging, but also were increasingly applied to quantitative applications, such as functional MRI (2,3), spectroscopy (4), apparent diffusion coefficient measurement (5) and flow quantitation using the phase-contrast (PC) method (6,7).
The quality of reconstructed spiral images depends on whether the actual k-space sampling points are at their nominal positions (8). To obtain accurate sampling positions, high-fidelity gradient waveforms must be generated, and data acquisition must be precisely synchronized with the gradient systems. Although newer gradient systems can provide more accurate gradient waveforms, timing mis-registration between data acquisition and gradient systems can significantly degrade the accuracy of sampling positions. The anti-aliasing filter introduces an additional delay to the data acquisition system. As a result, delays have been a very common source of k-space trajectory distortion (9). Although these delays can be partly compensated in pulse sequence programming, it is difficult to totally correct for these delays because they are usually different in the three physical axes. Furthermore, the hardware imposes restrictions on the timing positions of gradient waveforms and data acquisition. For example, on our scanner, the temporal resolutions of gradient waveforms and data acquisition are 4-.mu.s and 1-.mu.s, respectively. Those uncompensated delays shift the echo time and merely generate a linear phase in a conventional 2DFT image, but they cause more significant artifacts with other k-space trajectories. One well-known example is echo-planar imaging (EPI) (10), where ghosts appear when the readout gradient and data acquisition are not aligned. For spiral imaging, such delays may significantly degrade image quality if they are not carefully tuned. Even on a scanner whose delays have been adjusted, we found that the minor residual delays may still cause shading artifacts which are problematic for quantitative applications.
Because an ideal k-space trajectory is difficult to achieve, several methods to measure the actual trajectory have been proposed in the literature (8,9,11-15). Such a measurement usually needs to be performed whenever the image orientation is changed. To avoid repetitive measurements, Kerr et al. (16) assumes that each gradient system is a linear system and can be modeled with a transfer function. The advantage of this method is that the characterization procedure is done only once. Although using the measured k-space trajectories in reconstruction improves image quality, the accuracy of the measured trajectories is limited by noise and assumptions made in the modeling of those methods.