With centric K-Space we observed that artifact slacks off to 14% (p<0.001). With sequential K-Space high and medium frequencies are acquired with a low signal discontinuity, so K-Space is undersampled along phase encoding direction. We have a frequency filtering of a region that shows an high leap of signal when Gd-EOB-DTPA arrives. Furthermore, a significant part of low but mostly medium frequencies is lost in case of early acquisition, needed for a correct representation of contrast resolution. Instead, with centric K-Space, medium frequencies are all acquired after the low ones without abrupt changes occurring in the concentration of the bolus of contrast between high and low frequencies. The probability that the matrix is championed with an uniform bolus, and that this is represented with enough low and medium frequencies in order that contrast resolution is properly described, increases. The artifact is more frequent with matrix 320 rather than 256; the acquisition time with an inferior matrix decreases and the bolus has less abrupt variations in concentration during K-Space filling.
In conclusion, we observed that the artifact is a type of truncation artifact for contrast agent variation during the K-Space filling and corresponds to Maki artifact of angiographic field [4]. Also Maki artifact occurs as a Gibbs diffraction but is mentioned in literature limited to the angiographic context. In a “parenchymal field”, like a dynamic study of liver with Gd-EOB-DTPA, that has considerable troubles in the synchronization of acquisition with bolus transition, a similar artifact occurs that we should name “Ripples on the water” artifact. As a matter of fact, ripples similar to those formed when we throw a rock into a pond are created around the tissue impregnated of gadoxetate disodium (as with high signal: aorta, renal cortex, hepatic parenchymal, spleen). The lines are mostly oriented perpendicularly to the phase encoding direction. This is due to an undersampling along phase encoding for the signal variation during K-Space filling, which is why we acquire the center (low and progressively medium frequencies) of K-Space with an insufficient or abruptly variable contrast medium concentration. Therefore, the artifact is a low-medium frequencies loss Gibbs one. (Figure 6).
In order to preserve spatial resolution, and further reduce number of exams with artifact, we can associate centric K-Space to:
1- real time visualization of medium contrast, to synchronize with bolus transit and reveal the correct arterial phase according to patient age and cardiovascular disease [3].
2- 3D advanced parallel imaging techniques, that subsample data concurrently in both phase encoding directions, working on sensitivity variation of the coils, preserving image quality but reducing acquisition time in a consistent way [5][6].
3- GRASP sequences, that combine Compressed Sensing, Parallel Imaging and a continuous radial sampling of K-Space, using between spokes an increment of 111,25°. GRASP technique acquires continuously and allows to subsequently rebuild high resolution images with time windows chosen by the operator [7][8].