AbstractID: 4154 Title: Fast 4D Imaging: Breaking the Speed Limits in MR and CT
Advances in MRI and CT have made it possible to produce time-resolved series of 3D
volume images. In CT the increase in detector rows to the present value of 64 - or even
256 - has brought a significant increase in image quality and reduced acquisition times.
The emergence of volumetric cone beam systems employing flat panel detectors also
offers the possibility of rapid 4D acquisition. In MRI, 4D sequences have been facilitated
by the introduction of multi-coil parallel imaging methods and several acquisition
schemes using novel k-space sampling schemes.
The vast majority of MRI applications use Cartesian acquisitions employing phase
encoding. This line-by-line scan through k-pace produces spatial resolution that is
linearly related to acquisition time. This is basically a k-space speed limit imposed by the
Nyquist theorem. For dynamic applications this tradeoff between temporal and spatial
resolution is unacceptable. When highly undersampled radial acquisitions are used,
spatial resolution is determined by readout resolution rather than the number of k-space
lines acquired. The price paid for violating the Nyquist theorem is streak artifacts.
However, when the data acquisition is done using a time-resolved 3D radial acquisition
called VIPR(Vastly undersampled Isotropic PRojection imaging), the streaks are often
tolerable and have permitted acceleration factors up to 60 relative to Cartesian acquisition
for applications such as phase contrast imaging where background signals are cancelled
and streaks can only be generated by the vessels of interest. Undersampled cone beam
acquisition using an array of x-ray sources combined with large area detectors has been
proposed as a means of increasing the temporal resolution of CT and appears to have
similar properties to 3D undersampled MRI acquisition.