Separation of physiological very low frequency f luctuation from aliasing by switched sampling interval f MRI scans Vesa Kiviniemi * , Jyrki Ruohonen, Osmo Tervonen Department of Diagnostic Radiology, University of Oulu, OYS 90029, Finland Received 16 March 2004; accepted 22 September 2004 Abstract Anesthetized children have dominant blood-oxygen-level-dependent (BOLD) signal sources presenting high-power fluctuations at very low frequencies (VLF b 0.05 Hz). Aliasing of frequencies higher than critically sampled has been regarded as one probable origin of the VLF fluctuations. Aliased signal frequencies change when the sampling rate of the data is altered. In this study, the aliasing of VLF BOLD signal fluctuation was analysed by switching the repetition time (TR) of magnetic resonance (MR) images. Eleven anesthetized children were imaged at 1.5 T using TRs of 500 and 1200 ms. The BOLD signal sources were separated with independent component analysis (ICA). Occipital cortex signal sources had nonaliased VLF fluctuation (~ 0.03 Hz) in 9 of 11 subjects. Arterial signal sources failed to present stable power peaks at frequencies lower than 0.42 Hz presumably due to aliasing. Cerebrospinal fluid (CSF)-related signal sources showed nonaliased VLF in four subjects. In conclusion, the VLF BOLD signal fluctuation in the occipital cortex is a true physiological fluctuation, not a result of signal aliasing. D 2005 Elsevier Inc. All rights reserved. Keywords: BOLD; Fluctuation; Aliasing; Anesthesia; ICA 1. Introduction Blood-oxygen-level-dependent (BOLD) T2*-weighted magnetic resonance (MR) images reflect cortical blood flow and oxygenation alterations. The BOLD signal variability is related to either neuronal activation or spontaneous vaso- motor waves [1–5]. The spontaneous BOLD fluctuations have a nonrandom structure and they reflect functional connectivity and default mode brain activity via vasomotor- wave-like temporal BOLD signal fluctuations [3,5–9]. The very low frequency fluctuations are so strong in anesthetized child brain cortex that they can be detected as statistically independent BOLD signal sources with independent com- ponent analysis [7]. The cerebral blood flow and metabolism of children are markedly enhanced (i.e., up to two times) compared to adults [10,11]. The elevated baseline blood flow in children may enhance the conduction of the cardiorespiratory pulses inside the cranial vault. The physiological pulsations of CSF induce steady-state free precession (SSFP) disturbances in serial echo-planar imaging (EPI) scans having shorter TR than the T2-relaxation time of CSF (T2 CSF ). The SSFP disturbances can induce low-frequency signal oscillations in serial EPI image signal [12]. Physiological pulsations that have higher frequencies than critically sampled may, on the other hand, cause aliasing of fluctuation power to lower frequencies [13]. The aliasing problem is present with all conventional fMRI scans that have a long TR [13]. In children, heart pulsation may be over 2 Hz and so the image TR should be smaller than 250 ms if aliasing is to be theoretically avoided (Appendix A). Fast MRI data sam- pling rates and data filtering can be used to avoid signal aliasing, but then usually TR has to be set low and spatial coverage has to be compromised [13–15]. When using low TRs, the fluctuations related to disturbance of SSFP are increased and BOLD signal-to-noise ratio (SNR) is reduced [12,13]. K-space phase data analysis and temporal reorder- ing of images can provide data on physiological fluctuations for estimation of noise aliasing [13,16]. The temporal 0730-725X/$ – see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.mri.2004.09.005 * Corresponding author. Department of Diagnostic Radiology, Oulu University Hospital, P.O. Box 50, OYS 90029, Finland. Tel.: +358 8 3152462; fax: +358 8 3152112. E-mail address: vesa.kiviniemi@oulu.fi (V. Kiviniemi). Magnetic Resonance Imaging 23 (2005) 41 – 46