  Three-dimensional (3D) projection acquisition (PA) imaging has recently
gained attention because of its advantages, such as achievability of very short
echo time, less sensitivity to motion, and undersampled acquisition of
projections without sacrificing spatial resolution. However, larger subjects
require a stronger Nyquist criterion and are more likely to be affected by
outer-volume signals outside the field of view (FOV), which significantly
degrades the image quality. Here, we proposed a variable slab-selective
projection acquisition (VSS-PA) method to mitigate the Nyquist criterion and
effectively suppress aliasing streak artifacts in 3D PA imaging. The proposed
method involves maintaining the vertical orientation of the slab-selective
gradient for frequency-selective spin excitation and the readout gradient for
data acquisition. As VSS-PA can selectively excite spins only in the width of
the desired FOV in the projection direction during data acquisition, the
effective size of the scanned object that determines the Nyquist criterion can
be reduced. Additionally, unwanted signals originating from outside the FOV
(e.g., aliasing streak artifacts) can be effectively avoided. The mitigation of
the Nyquist criterion owing to VSS-PA was theoretically described and confirmed
through numerical simulations and phantom and human lung experiments. These
experiments further showed that the aliasing streak artifacts were nearly
suppressed.
