Simultaneous recording of evoked potentials and T2‐weighted MR images during somatosensory stimulation of rat

Abstract

Somatosensory evoked potentials (SEP) and T* 2-weighted nuclear magnetic resonance (NMR) images were recorded simultaneously during somatosensory stimulation of rat to investigate the relationship between electrical activation of the brain tissue and the signal intensity change in functional NMR imaging. Electrical forepaw stimulation was performed in Wistar rats anesthetized with ␣-chloralose. SEPs were recorded with calomel electrodes at stimulation frequencies of 1.5, 3, 4.5, and 6 Hz. At the same time, T* 2-weighted imaging was performed, and the signal intensity increase during stimulation was correlated with the mean amplitude of the SEP. Both the stimulation-evoked signal intensity increase in T* 2-weighted images and the amplitude of SEPs were dependent on the stimulation frequency, with the largest signals at a stimulation frequency of 1.5 Hz and decreasing activations with increasing frequencies. The feasibility of simultaneous, artifact-free recordings of T* 2-weighted NMR images and of evoked potentials is proved. Furthermore, the study demonstrates-in the intact brain-the validity of functional magnetic resonance imaging for estimating the intensity of electrocortical activation. Magn Reson Med 41:469-473

Comprehensive correlation between neuronal activity and spin-echo blood oxygenation level-dependent signals in the rat somatosensory cortex evoked by short electrical stimulations at various frequencies and currents

“…Thus, fMRI is an indirect method for assessing neuronal activity. Therefore, it is essential to elucidate the relationship between BOLD signals and neuronal activity for interpreting the fMRI signals; this relationship has been investigated mainly by animal studies measuring evoked potentials as indices of the neuronal activity in the somatosensory cortex (Brinker et al, 1999;Van Camp et al, 2006;Goloshevsky et al, 2007;Huttunen et al, 2008;Sanganahalli et al, 2009). Although most of the human fMRI studies employ the event-related task design that has a short stimulus duration, animal studies generally investigate the relationship between BOLD signals and evoked potentials with a stimulus duration of several tens of seconds (Brinker et al, 1999;Van Camp et al, 2006;Goloshevsky et al, 2007;Huttunen et al, 2008;Sanganahalli et al, 2009).…”

“…Therefore, it is essential to elucidate the relationship between BOLD signals and neuronal activity for interpreting the fMRI signals; this relationship has been investigated mainly by animal studies measuring evoked potentials as indices of the neuronal activity in the somatosensory cortex (Brinker et al, 1999;Van Camp et al, 2006;Goloshevsky et al, 2007;Huttunen et al, 2008;Sanganahalli et al, 2009). Although most of the human fMRI studies employ the event-related task design that has a short stimulus duration, animal studies generally investigate the relationship between BOLD signals and evoked potentials with a stimulus duration of several tens of seconds (Brinker et al, 1999;Van Camp et al, 2006;Goloshevsky et al, 2007;Huttunen et al, 2008;Sanganahalli et al, 2009). Unlike BOLD studies, numerous optical studies performed using laser Doppler flowmetry (LDF) (Ngai et al, 1999;Matsuura and Kanno, 2001;Ureshi et al, 2004Ureshi et al, , 2005 and optical imaging (Devor et al, 2003;Martindale et al, 2003;Sheth et al, 2003Sheth et al, , 2004Jones et al, 2004;Hewson-Stoate et al, 2005;Martin et al, 2006) have investigated the hemodynamic responses such as CBF and deoxygenated hemoglobin changes to neuronal activity in the rat cortex with a short stimulus duration (2-5 s) (Matsuura and Kanno, 2001;Martindale et al, 2003;Sheth et al, 2003Sheth et al, , 2004Jones et al, 2004;Ureshi et al, 2004Ureshi et al, , 2005Hewson-Stoate et al, 2005;Martin et al, 2006).…”

“…Furthermore, most of the optical studies performed using electrical stimulation varied either in frequency (Ngai et al, 1999;Matsuura and Kanno, 2001;Martindale et al, 2003; -3 - while maintaining a fixed current strength (Brinker et al, 1999;Van Camp et al, 2006;Goloshevsky et al, 2007;Huttunen et al, 2008;Sanganahalli et al, 2009), and few fMRI studies have comprehensively investigated this relationship at different stimulus frequencies and currents.…”

“…Animal fMRI studies on somatosensory activation investigated the relationship between BOLD signals and neuronal activities by changing the stimulus frequency while maintaining a fixed current strength (Brinker et al, 1999;Van Camp et al, 2006;Goloshevsky et al, 2007;Huttunen et al, 2008;Sanganahalli et al, 2009), and few fMRI studies have comprehensively investigated this relationship at different stimulus frequencies and currents.…”

“…Unlike BOLD studies, numerous optical studies performed using laser Doppler flowmetry (LDF) (Ngai et al, 1999;Matsuura and Kanno, 2001;Ureshi et al, 2004Ureshi et al, , 2005 and optical imaging (Devor et al, 2003;Martindale et al, 2003;Sheth et al, 2003Sheth et al, , 2004Jones et al, 2004;Hewson-Stoate et al, 2005;Martin et al, 2006) have investigated the hemodynamic responses such as CBF and deoxygenated hemoglobin changes to neuronal activity in the rat cortex with a short stimulus duration (2-5 s) (Matsuura and Kanno, 2001;Martindale et al, 2003;Sheth et al, 2003Sheth et al, , 2004Jones et al, 2004;Ureshi et al, 2004Ureshi et al, , 2005Hewson-Stoate et al, 2005;Martin et al, 2006). Thus, an animal fMRI study with short stimulus duration is expected to bridge the results of the optical studies using short stimulus durations and those of the BOLD studies using long stimulus durations.Furthermore, most of the optical studies performed using electrical stimulation varied either in frequency (Ngai et al, 1999;Matsuura and Kanno, 2001;Martindale et al, 2003; -3 - while maintaining a fixed current strength (Brinker et al, 1999;Van Camp et al, 2006;Goloshevsky et al, 2007;Huttunen et al, 2008;Sanganahalli et al, 2009), and few fMRI studies have comprehensively investigated this relationship at different stimulus frequencies and currents.Thus, the present study aimed to investigate the relationship between BOLD signals and neuronal activities in the somatosensory cortex of anesthetized rats for forepaw electrical stimulation at a short duration of 4 s by changing both frequencies (1-10 Hz) and currents (0.5-2.0 mA). BOLD signals with a spin-echo echo planar imagining (EPI) sequence at 7 T and scalp-based somatosensory evoked potentials (SEP) were obtained from individual rats.…”