Abstract
There is growing evidence as to the benefits of collecting BOLD fMRI data with increased sampling rates. However, many of the newly developed acquisition techniques developed to collect BOLD data with ultra-short TRs require hardware, software, and non-standard analytic pipelines that may not be accessible to all researchers. We propose to incorporate the method of shifted echo into a standard multi-slice, gradient echo EPI sequence to achieve a higher sampling rate with a TR of <1 s with acceptable spatial resolution. We further propose to incorporate temporal averaging of consecutively acquired EPI volumes to both ameliorate the reduced temporal signal-to-noise inherent in ultra-fast EPI sequences and reduce the data burden. BOLD data were collected from 11 healthy subjects performing a simple, event-related visual-motor task with four different EPI sequences: (1) reference EPI sequence with TR = 1440 ms, (2) shifted echo EPI sequence with TR = 700 ms, (3) shifted echo EPI sequence with every two consecutively acquired EPI volumes averaged and effective TR = 1400 ms, and (4) shifted echo EPI sequence with every four consecutively acquired EPI volumes averaged and effective TR = 2800 ms. Both the temporally averaged sequences exhibited increased temporal signal-to-noise over the shifted echo EPI sequence. The shifted echo sequence with every two EPI volumes averaged also had significantly increased BOLD signal change compared with the other three sequences, while the shifted echo sequence with every four EPI volumes averaged had significantly decreased BOLD signal change compared with the other three sequences. The results indicated that incorporating the method of shifted echo into a standard multi-slice EPI sequence is a viable method for achieving increased sampling rate for collecting event-related BOLD data. Further, consecutively averaging every two consecutively acquired EPI volumes significantly increased the measured BOLD signal change and the subsequently calculated activation map statistics.
Highlights
It has long been known that increasing the acquisition rate of blood oxygen level dependent (BOLD) data results in more accurate measurements of the hemodynamic response function (HRF)
There has been a recent growth in the development of acquisition techniques aimed at increasing the temporal sampling of BOLD fMRI data including echo-volumar imaging (EVI; van der Zwaag et al, 2006; Rabrait et al, 2008; Witzel et al, 2008), inverse imaging (InI; Lin et al, 2006, 2010, 2012a,b), and multiplexed echo planar imaging (EPI) (Feinberg et al, 2010)
While there are a number of factors confounding the determination of an optimal TR with which to collect BOLD data including intra-subject variability, regional variability within the brain, experimental timings, and acquisition parameters, the results from these initial ultra-short TR fMRI studies all point toward the considerable benefits of collecting fMRI data with sampling rates of
Summary
It has long been known that increasing the acquisition rate of blood oxygen level dependent (BOLD) data results in more accurate measurements of the hemodynamic response function (HRF). Does theory demonstrate that MR signal is inversely proportional to the sampling rate, increased temporal sampling of the BOLD response, in particular, has been shown to result in measured signals that more closely match the predicted signal (Dilharreguy et al, 2003). Studies using both real and simulated fMRI data have shown that the accuracy with which the peak of the HRF can be determined increases with decreased repetition time (TR; Miezin et al, 2000; MacCotta et al, 2001; Dilharreguy et al, 2003). While there are a number of factors confounding the determination of an optimal TR with which to collect BOLD data including intra-subject variability, regional variability within the brain, experimental timings, and acquisition parameters, the results from these initial ultra-short TR fMRI studies all point toward the considerable benefits of collecting fMRI data with sampling rates of
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