We present a new imaging trajectory utilizing spiral-in and spiral-out interleaves efficiently. When compared with conventional spiral-in/out methods having the same resolution, the proposed interleaved spiral-in/out trajectory is half as long in readout duration so that susceptibility-induced signal dropout is reduced. As a result, activation within susceptibility-compromised regions is better detected using the proposed trajectory, and/or higher spatial or temporal resolution can be achieved.
When comparing the number of activated pixels within , keep in mind that conventional spiral-in/out has a readout duration of 120ms (60ms for spiral-in and spiral-out interleaves) while interleaved spiral-in/out has a total readout-duration of 60ms. Thus, the combined spiral-in and spiral-out images from conventional spiral-in/out trajectory (time series 5), in theory, have an SNR advantage due to longest readout duration (120ms). On the other hand, spiral-in and spiral-out images reconstructed with the UNFOLD technique from the interleaved spiral-in/out trajectory (time series 2 and 3) have SNR disadvantage in uniform brain since their readout durations are only 30ms.
Activation maps () alone cannot be used to decide on the best trajectory. When the imaging region is homogeneous, the readout duration can be relatively long to achieve high SNR. Then use of conventional spiral-out is a good choice. When the imaging region is heterogeneous, on the other hand, reducing readout duration can minimize signal loss. In that case, use of interleaved spiral-in/out is recommended. Interleaved spiral-in/out offers the advantage of using a two-shot trajectory (improved spatial resolution and reduced signal loss), but without the disadvantage of losing temporal resolution.
The interleaved spiral-in/out trajectory is effective in reducing susceptibility-induced signal dropout and detecting activation in the affected regions (as shown in and , ). With the compact readout-duration of the interleaved spiral-in/out trajectory, repetition time can be reduced such that more time frames are gathered in a fixed scan-time so as to increase SNR efficiency and temporal resolution. On the other hand, interleaved spiral-in/out can provide higher spatial resolution than conventional spiral-in/out if readout-duration is fixed.
What motivated our choice of spiral sampling, instead of Cartesian, is motion insensitivity, low distortion (12
), and the potential for high speed of acquisition when used in conjunction with acceleration techniques like parallel imaging. The proposed method is easy to incorporate into parallel imaging techniques such as spiral SENSE (25
), for example. Combining parallel imaging with interleaved spiral-in/out can further reduce readout duration (and signal dropout), certainly a direction for continuing research.