With autocalibrated PC-MRI, using pairwise mapping and split averaging, mSENSE and GRAPPA reconstructions were able to perform accurate flow and peak velocity measurements. Regardless of the presence of aliasing artifact, mSENSE and GRAPPA had flow measurements with small mean differences relative to the flow measurements from conventional PC-MRI. When there is no aliasing artifact, then comparable mean differences were obtained between SENSE and autocalibrated PC-MRI. When there is aliasing artifact, measurements from mSENSE and GRAPPA reconstructions had smaller mean differences than SENSE, which indicates there is correlation between the presence of the aliasing artifact and its impact on flow measurement accuracy in SENSE. For the peak velocity the differences between SENSE and autocalibrated PC-MRI are not appreciable. The aliasing may not be as obstructive when dealing with peak velocity measurements because the peak velocity may not lie where the aliasing occurs.
An alternative to counter the aliasing would be to prescribe the field of view to avoid this wraparound effect. In the in vivo examples of the aorta this was certainly possible for some of the cases where the slice can be shifted to avoid wraparound. An autocalibrated approach would, however, reduce the chance of operator error in prescribing these slices. Also, most of the cases for the pulmonary artery were prescribed in a fashion such that aliasing could not be avoided. The obliquity of the slice may prevent anatomy, such as parts of the torso, to be entirely excluded from extending greater than the field of view in the phase encoding direction and thus causing wraparound. In addition, another solution would be to prescribe a larger field of view. The image matrix can be preserved although at the cost of decreased spatial resolution. Additionally, the image matrix can be increased at the cost of increased scan time.
The effect of the aliasing artifact may cause underestimation of velocity. The effect is more pronounced in the systolic phases of the heart, as seen in . If one were to treat the aliased tissue as if it were static, then the effect would be analogous to the effect resulting from partial volume. Assuming that the magnitudes for the two flow echoes are approximately equal and that bipolar gradients are used for the two flow-encoding steps, the static tissue will cause the phase difference to be smaller than expected and the velocity will be underestimated.
The RMS values with GRAPPA are smaller than the values with mSENSE, although the flow measurements are comparable. The simulations indicate that GRAPPA is more robust when using four central lines for ORF = 2 and 8 central lines for ORF = 4. In the in vivo experiments the mean differences are comparable in mSENSE and GRAPPA for both flow and peak velocity. Despite some residual aliasing artifact from inaccuracies in the mSENSE coil sensitivity maps, there is still good agreement between mSENSE and the conventional PC-MRI images. In addition, integrating the central phase-encodes into a reconstruction such as vdSENSE or GEM may be problematic with PC-MRI. These techniques are similar to SENSE and require coil sensitivity maps to reconstruct images. They would also have to address the issue raised in flow-encode pairing.
An advantage of autocalibrated PC-MRI is that patient motion is minimized between the calibration scans and data acquisition scans. The patient may shift positions between the calibration scan and SENSE PC-MRI scan. The position of the chest wall may slightly differ with different breathholds, possibly leading to inaccurate coil sensitivity maps and inaccurate measurements. When this type of artifact becomes noticeable the calibration scan must be repeated. Autocalibrated PC-MRI would be insensitive to these types of movement.
Other GRAPPA-based techniques that improve image reconstruction (22
) may additionally increase the accuracy of phase-contrast MRI and merits further investigation.
In conclusion, a practical autocalibrated PC-MRI sequence has been implemented and tested in normal volunteers. Flow velocity data derived from autocalibrated PC-MRI reconstructed with mSENSE and GRAPPA are found to be comparable with conventional PC-MRI in the imaging of the great arteries. GRAPPA and mSENSE PC-MRI are shown to have more robust measurements than SENSE when there is aliasing artifact caused by insufficient coil sensitivity maps. The peak velocity measurements show no significant differences. Thus, autocalibrated PC-MRI may be a valuable and powerful tool for enhancing the performance of PC-MRI.