Benefits and pitfalls of keyhole imaging, especially in first-pass perfusion studies.

A comparison of dynamic results of a multi-echo contrast-enhanced perfusion study obtained from a keyhole imaging experiment and the results from low-resolution updates is presented. If, for each dynamic state, a separate reference image exists, high spatial resolution in the dynamic results can be preserved through keyhole imaging. If only one reference image can be used, the dynamic key-hole results still offer high spatial frequency content due to spatial phase discontinuities in the images. These often exist at the outline of organs and result from the fat in connective tissues. If the basic assumption of keyhole imaging, namely, that the relevant information is centered in k-space, is violated, as in T2*-weighted gradient-echo images, keyhole imaging can lead to erroneous results even though the update images themselves seem to be free of any artifacts.