Brain MR perfusion-weighted imaging with alternate ascending/descending directional navigation.

Blood perfusion has been noninvasively imaged with arterial spin labeling (ASL). All ASL methods use a separate spin preparation for labeling of arterial blood in advance to data readout for pair-wise subtraction. Improvement of ASL techniques has been mostly about improvement of spin preparation methods, such as pulsed ASL (PASL) (1–5), continuous ASL (6–11), and pseudo-continuous (or pulsed-continuous) ASL (pCASL) (12–14). Despite these improvements and the noninvasive nature, ASL has not been routinely used in clinical applications because of its intrinsic limitations. One is relatively low percent signal change (PSC) between label and control images that require many averages to achieve reasonable signal-to-noise ratio (SNR) even with relatively low resolution. Another is the narrow time window permitted for the perfusion contrast that limits baseline image qualities. The third is sensitivity to tissues with heterogeneous arterial transit times. Perfusion in tissues with longer arterial transit time is likely underestimated in ASL due to enhanced T1 recovery of labeled arterial spins. The fourth is that perfusion directionality is not easily assessable with multiple slices, as labeling direction cannot be easily changed. The aforementioned limitations may be overcome, if we can put the spin labeling plane right next to the imaging planes while maintaining the multislice capability. This may be achieved by acquiring multiple slices in the sequential order but in two different directions of ascending and descending, with no separate spin preparation. For the axial imaging case, slices sequentially acquired in the direction away from the heart (“ascending” in this article) work as labeling and those in the direction toward the heart (“descending” in this article) work as a control. In this new perfusion-weighted imaging technique, alternate ascending/descending directional navigation (ALADDIN), spin preparation and imaging parts are not separated: imaging one plane works simultaneously as spin preparation for the next imaging planes. Balanced steady state free precession (bSSFP) (15) may be a good choice for ALADDIN because of its high flip angle, high temporal resolution, and good sensitivity to the initial magnetization differences (16,17). In this sense, bSSFP has also been used for perfusion-weighted imaging with the conventional labeling methods (16–18). The anticipated problems with the ALADDIN approach are transient magnetization transfer (MT) effects and incomplete T1 recovery between successive ascending/descending acquisitions in edge slices, MT asymmetry, field inhomogeneity, and gradient imperfection (9). The problems in the edge slices may be handled by discarding them. MT asymmetry, field inhomogeneity, and gradient imperfection may be resolved by averaging over alternating slice gradient polarities (9). As imaging a slice works simultaneously as labeling the following slices, different scan direction (e.g., axial, sagittal, and coronal) would provide differential perfusion information because of changes in the labeling direction. In this article, feasibility of this new technique, ALADDIN, was tested for brain perfusion imaging with various scan conditions using bSSFP, and the results were compared with a conventional PASL method.