Abstract
EPI with Keyhole (EPIK) is a hybrid imaging technique that overcomes many of the performance disadvantages associated with EPI. Previously, EPIK was shown to provide a higher temporal resolution and fewer image distortions than EPI whilst maintaining comparable performance for the detection of BOLD-based signals. This work carefully examines the putative enhanced sensitivity of EPIK in a typical fMRI setting by using a robust fMRI paradigm – visually guided finger tapping – to demonstrate the advantages of EPIK for fMRI at 3 T. The data acquired were directly compared to the community standard fMRI protocol using single-shot EPI to ascertain a clear comparison. Each sequence was optimised to offer its highest possible spatial resolution for a given set of imaging conditions, i.e., EPIK and EPI achieved an in-planar resolution of 2.08 × 2.08 mm2 with 32 slices and 3.13 × 3.13 mm2 with 36 slices, respectively. EPIK demonstrated a number of clear improvements, such as superior spatial resolution with favourable robustness against susceptibility artefacts. Both imaging sequences revealed robust activation within primary motor, premotor and visual regions, although significantly higher BOLD amplitudes were detected using EPIK within the primary and supplementary motor areas. Dynamic causal modelling, in combination with Bayesian model selection, identified identical winning models for EPIK and EPI data. Coupling parameters reflecting task-related modulations and the connectivity of fixed connections were comparably robust for both sequences. However, fixed connections from the left motor cortex to the right visual cortex were estimated as being significantly more robust for EPIK data.
Highlights
In order to effectively shorten the acquisition window in EPI, an alternative acquisition strategy, EPI with Keyhole (EPIK), has been presented[2,3] and validated at 1.5T4,5 and 3T6,7
Visual inspection of the figure suggests that the EPIK image were reconstructed without any significant loss of signal or any severe degradation of image quality and, the slices exhibit a noticeably enhanced spatial resolution, when compared with the EPI images
Due to the reduced number of lines per shot in EPIK and the use of an effectively shorter readout per phase encoding line in EPIK (0.445 ms) than in EPI (0.510 ms), a substantial reduction of geometric distortions was observed in the EPIK images
Summary
In order to effectively shorten the acquisition window in EPI, an alternative acquisition strategy, EPI with Keyhole (EPIK), has been presented[2,3] and validated at 1.5T4,5 and 3T6,7. A sliding window technique is used to ensure that the periphery of k-space is continually updated, albeit at a slower rate than the keyhole In this way, a higher temporal resolution and fewer image distortions occur in EPIK, when compared to EPI. In one of our previous studies[6], it was shown that EPIK achieved comparable temporal stability and a similar BOLD detection performance to EPI (community standard technique) under the ‘same’ imaging parameters and the ‘same’ image resolution. In addition to the analyses of BOLD amplitudes, performance differences of EPIK and EPI were evaluated with regard to effective connectivity analyses between brain regions using dynamic causal modelling (DCM)[8]. We hypothesise that the coupling parameters estimated on the basis of EPIK data may be more robust across different scanning sessions
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