Functional MRI of the cervical spinal cord on 1.5 T with fingertapping: to what extent is it feasible?

Thermal stimulus task fMRI in the cervical spinal cord at 7 Tesla

Although functional magnetic resonance imaging (fMRI) is widely applied in the brain, fMRI of the spinal cord is more technically demanding. Proximity to the vertebral column and lungs results in strong spatial inhomogeneity and temporal fluctuations in B0 . Increasing field strength enables higher spatial resolution and improved sensitivity to blood oxygenation level-dependent (BOLD) signal, but amplifies the effects of B0 inhomogeneity. In this work, we present the first task fMRI in the spinal cord at 7 T. Further, we compare the performance of single-shot and multi-shot 2D echo-planar imaging (EPI) protocols, which differ in sensitivity to spatial and temporal B0 inhomogeneity. The cervical spinal cords of 11 healthy volunteers were scanned at 7 T using single-shot 2D EPI at 0.75 mm in-plane resolution and multi-shot 2D EPI at 0.75 and 0.6 mm in-plane resolutions. All protocols used 3 mm slice thickness. For each protocol, the BOLD response to 13 10-s noxious thermal stimuli applied to the right thumb was acquired in a 10-min fMRI run. Image quality, temporal signal to noise ratio (SNR), and BOLD activation (percent signal change and z-stat) at both individual- and group-level were evaluated between the protocols. Temporal SNR was highest in single-shot and multi-shot 0.75 mm protocols. In group-level analyses, activation clusters appeared in all protocols in the ipsilateral dorsal quadrant at the expected C6 neurological level. In individual-level analyses, activation clusters at the expected level were detected in some, but not all subjects and protocols. Single-shot 0.75 mm generally produced the highest mean z-statistic, while multi-shot 0.60 mm produced the best-localized activation clusters and the least geometric distortion. Larger than expected within-subject segmental variation of BOLD activation along the cord was observed. Group-level sensory task fMRI of the cervical spinal cord is feasible at 7 T with single-shot or multi-shot EPI. The best choice of protocol will likely depend on the relative importance of sensitivity to activation versus spatial localization of activation for a given experiment. PRACTITIONER POINTS: First stimulus task fMRI results in the spinal cord at 7 T. Single-shot 0.75 mm 2D EPI produced the highest mean z-statistic. Multi-shot 0.60 mm 2D EPI provided the best-localized activation and least distortion.

Spatial distribution of hand-grasp motor task activity in spinal cord functional magnetic resonance imaging

Upper extremity motor paradigms during spinal cord functional magnetic resonance imaging (fMRI) can provide insight into the functional organization of the cord. Hand-grasping is an important daily function with clinical significance, but previous studies of similar squeezing movements have not reported consistent areas of activity and are limited by sample size and simplistic analysis methods. Here, we study spinal cord fMRI activation using a unimanual isometric hand-grasping task that is calibrated to participant maximum voluntary contraction (MVC). Two task modeling methods were considered: (1) a task regressor derived from an idealized block design (Ideal) and (2) a task regressor based on the recorded force trace normalized to individual MVC (%MVC). Across these two methods, group motor activity was highly lateralized to the hemicord ipsilateral to the side of the task. Activation spanned C5-C8 and was primarily localized to the C7 spinal cord segment. Specific differences in spatial distribution are also observed, such as an increase in C8 and dorsal cord activity when using the %MVC regressor. Furthermore, we explored the impact of data quantity and spatial smoothing on sensitivity to hand-grasp motor task activation. This analysis shows a large increase in number of active voxels associated with the number of fMRI runs, sample size, and spatial smoothing, demonstrating the impact of experimental design choices on motor activation.

Thermal Stimulus Task fMRI in the Cervical Spinal Cord at 7 Tesla

PURPOSE

Although functional MRI is widely applied in the brain, fMRI of the spinal cord is more technically demanding. Proximity to the vertebral column and lungs results in strong spatial inhomogeneity and temporal fluctuations in B0. Increasing field strength enables higher spatial resolution and improved sensitivity to BOLD signal, but amplifies the effects of B0 inhomogeneity. In this work, we present the first stimulus task fMRI in the spinal cord at 7 T. Further, we compare the performance of single-shot and multi-shot 2D EPI protocols, as they differ in sensitivity to spatial and temporal B0 inhomogeneity.

METHODS

The cervical spinal cords of 11 healthy volunteers were scanned at 7 T using single-shot 2D EPI at 0.75 mm in-plane resolution and multi-shot 2D EPI at 0.75 and 0.6 mm in-plane resolutions. For each protocol, the BOLD response to thirteen 10-second noxious thermal stimuli applied to the right thumb was acquired in a 10-minute fMRI run. Image quality, temporal SNR, and BOLD activation (percent signal change and z-stat) at both individual- and group-level were evaluated between the protocols.

RESULTS

Temporal SNR was highest in single-shot and multi-shot 0.75 mm protocols. In group-level analyses, activation clusters appeared in all protocols in the ipsilateral dorsal quadrant at the expected C6 neurological level. In individual-level analyses, activation clusters at the expected level were detected in some, but not all subjects and protocols. Single-shot 0.75 mm generally produced the highest mean z-statistic, while multi-shot 0.60 mm produced the best-localized activation clusters and the least geometric distortion. Larger than expected within-subject segmental variation of BOLD activation along the cord was observed.

CONCLUSION

Group-level sensory task fMRI of the cervical spinal cord is feasible at 7 T with single-shot or multi-shot EPI. The best choice of protocol will likely depend on the relative importance of sensitivity to activation versus spatial localization of activation for a given experiment.

The current state of spinal cord functional magnetic resonance imaging and its application in clinical research

Since its development, spinal cord functional magnetic resonance imaging (fMRI) has utilized various methodologies and stimulation protocols to develop a deeper understanding of a healthy human spinal cord that lays a foundation for its use in clinical research and practice. In this review, we conducted a comprehensive literature search on spinal cord fMRI studies and summarized the recent advancements and resulting scientific achievements of spinal cord fMRI in the following three aspects: the current state of spinal cord fMRI methodologies and stimulation protocols, knowledge about the healthy spinal cord's functions obtained via spinal cord fMRI, and fMRI's exemplary usage in spinal cord diseases and injuries. We conclude with a discussion that, while technical challenges exist, novel fMRI technologies for and new knowledge about the healthy human spinal cord have been established. Empowered by these developments, investigations of pathological and injury states within the spinal cord have become the next important direction of spinal cord fMRI. Recent clinical investigations into spinal cord pathologies, for example, fibromyalgia, multiple sclerosis, spinal cord injury, and cervical spondylotic myelopathy, have already provided deep insights into spinal cord impairments and the time course of impairment-caused changes. We expect that future spinal cord fMRI advancement and research development will further enhance our understanding of various spinal cord diseases and provide the foundation for evaluating existing and developing new treatment plans.

Spatial distribution of hand-grasp motor task activity in spinal cord functional magnetic resonance imaging

Upper extremity motor paradigms during spinal cord functional magnetic resonance imaging (fMRI) can provide insight into the functional organization of the cord. Hand-grasping is an important daily function with clinical significance, but previous studies of similar squeezing movements have not reported consistent areas of activity and are limited by sample size and simplistic analysis methods. Here, we study spinal cord fMRI activation using a unimanual isometric hand-grasping task that is calibrated to participant maximum voluntary contraction (MVC). Two task modeling methods were considered: (1) a task regressor derived from an idealized block design (Ideal) and (2) a task regressor based on the recorded force trace normalized to individual MVC (%MVC). Across these two methods, group motor activity was highly lateralized to the hemicord ipsilateral to the side of the task. Activation spanned C5-C8 and was primarily localized to the C7 spinal cord segment. Specific differences in spatial distribution are also observed, such as an increase in C8 and dorsal cord activity when using the %MVC regressor. Furthermore, we explored the impact of data quantity and spatial smoothing on sensitivity to hand-grasp motor task activation. This analysis shows a large increase in number of active voxels associated with the number of fMRI runs, sample size, and spatial smoothing, demonstrating the impact of experimental design choices on motor activation.

Hearing, touching, and multisensory integration during mate choice

Mate choice is a potent generator of diversity and a fundamental pillar for sexual selection and evolution. Mate choice is a multistage affair, where complex sensory information and elaborate actions are used to identify, scrutinize, and evaluate potential mating partners. While widely accepted that communication during mate assessment relies on multimodal cues, most studies investigating the mechanisms controlling this fundamental behavior have restricted their focus to the dominant sensory modality used by the species under examination, such as vision in humans and smell in rodents. However, despite their undeniable importance for the initial recognition, attraction, and approach towards a potential mate, other modalities gain relevance as the interaction progresses, amongst which are touch and audition. In this review, we will: (1) focus on recent findings of how touch and audition can contribute to the evaluation and choice of mating partners, and (2) outline our current knowledge regarding the neuronal circuits processing touch and audition (amongst others) in the context of mate choice and ask (3) how these neural circuits are connected to areas that have been studied in the light of multisensory integration.

Investigating the human spinal sensorimotor pathways through functional magnetic resonance imaging

Most of our knowledge about the human spinal ascending (sensory) and descending (motor) pathways comes from non-invasive electrophysiological investigations. However, recent methodological advances in acquisition and analyses of functional magnetic resonance imaging (fMRI) data from the spinal cord, either alone or in combination with the brain, have allowed us to gain further insights into the organization of this structure. In the current review, we conducted a systematic search to produced somatotopic maps of the spinal fMRI activity observed through different somatosensory, motor and resting-state paradigms. By cross-referencing these human neuroimaging findings with knowledge acquired through neurophysiological recordings, our review demonstrates that spinal fMRI is a powerful tool for exploring, in vivo, the human spinal cord pathways. We report strong cross-validation between task-related and resting-state fMRI in accordance with well-known hemicord, postero-anterior and rostro-caudal organization of these pathways. We also highlight the specific advantages of using spinal fMRI in clinical settings to characterize better spinal-related impairments, predict disease progression, and guide the implementation of therapeutic interventions.

A new MRI subject position to explore simultaneous BOLD oscillations of the brain and the body

BACKGROUND

Anatomically and physiologically, there is strong relationship between the brain and body. A new MRI platform covering both the brain and the limb would be beneficial for a more thorough understanding of the brain-body interactions.

NEW METHOD

A new arm-over-head (AOH) position was developed to collect MRI of the brain and one arm simultaneously. Subject's tolerability and SNR of both the brain and limb under a serial of seven different TR (250-3000 ms) were tested. Then, blocked motor imagery tasks were performed to test the possible brain-body oscillations.

RESULTS

The new MRI position provided structural images with good quality, and the AOH position had the best SNR under TR 3000 ms (p = 0.03 for the brain; p = 0.064 for the limb). Then, by using both hypothesis-free independent component analysis (ICA) and a priori seed-based functional connectivity (FC) analysis, it is demonstrated during motionless motor imagery tasks there existed possible brain-body BOLD oscillations connecting especially arm flexors to default mode, vision, and sensorimotor networks. The FC appeared at network density as low as 5%.

COMPARISON WITH EXISTING METHODS

We have developed a new MRI subject position to explore the possibilities of more extensive neuronal and physiological networks.

CONCLUSIONS

The results of this preliminary experiment indicate that functional brain networks might extend outside the brain. A bottom-up circulatory effect might explain this phenomenon. Nonetheless, considering the mechanism of neural top-down control and the nature of complex brain networks, the existence of a more extensive whole-body functional network is rational and possible.

Organization of the intrinsic functional network in the cervical spinal cord: A resting state functional MRI study

Resting state functional magnetic resonance imaging (rsfMRI) has been extensively applied to investigate the organization of functional networks in the brain. As an essential part of the central nervous system (CNS), the spinal cord has not been well explored about its intrinsic functional network. In this study, we aim to thoroughly investigate the characteristics of the intrinsic functional network in the spinal cord using rsfMRI. Functional connectivity and graph theory analysis were employed to evaluate the organization of the functional network, including its topology and network communication properties. Furthermore, the reproducibility of rsfMRI analysis on the spinal cord was also examined by intra-class correlation (ICC). Comprehensive evaluation of the intrinsic functional organization presented a non-uniform distribution of topological characteristics of the functional network, in which the upper levels (C2 and C3 vertebral levels) of the cervical spinal cord showed high levels of connectivity. The present results revealed the significance of the upper cervical cord in the intrinsic functional network of the human cervical spinal cord. In addition, this study demonstrated the efficiency of the cervical spinal cord functional network and the reproducibility of rsfMRI analysis on the spinal cord was also confirmed. As knowledge expansion of intrinsic functional network from the brain to the spinal cord, this study shed light on the organization of the spinal cord functional network in both normal development and clinical disorders.

Lateralization of cervical spinal cord activity during an isometric upper extremity motor task with functional magnetic resonance imaging

The purpose of this study was to use an isometric upper extremity motor task to detect activity induced blood oxygen level dependent signal changes in the cervical spinal cord with functional magnetic resonance imaging. Eleven healthy volunteers performed six 5minute runs of an alternating left- and right-sided isometric wrist flexion task, during which images of the cervical spinal cord were acquired with a reduced field-of-view T2*-weighted gradient-echo echo-planar-imaging sequence. Spatial normalization to a standard spinal cord template was performed, and group average activation maps were generated in a mixed-effects analysis. The task activity significantly exceeded that of the control analyses. The activity was lateralized to the hemicord ipsilateral to the task and reliable across the runs at the group and subject level. Finally, a multi-voxel pattern analysis was able to successfully decode the left and right tasks at the C6 and C7 vertebral levels.

On the impact of physiological noise in spinal cord functional MRI

Functional magnetic resonance imaging (fMRI) techniques are widely exploited for the study of brain activation. In recent years, similar approaches have been attempted for the study of spinal cord function; however, obtaining good functional images of spinal cord still represents a technical and scientific challenge. Some of the main limiting factors can be classified under the broad category of "physiological noise," and are related to 1) the cerebrospinal fluid (CSF) flux in the subarachnoid space surrounding the spinal cord; 2) the cord motion itself; and 3) the small area of the cord, which makes it critical to have a high image resolution. In addition, the different magnetic susceptibility properties of tissues surrounding the spinal cord reduce the local homogeneity of the static magnetic field, causing image distortion, reduction of the effective resolution, and signal loss, all effects that are modulated by motion. For these reasons, a number of methods have been developed for the purpose of denoising spinal cord fMRI time series. In this work, after a short introduction on the relevant features of the spinal cord anatomy, we review the main sources of physiological noise in spinal cord fMRI and discuss the main approaches useful for its mitigation.

A systematic review of spinal fMRI research: outlining the elements of experimental design

Object

Since the first published report of spinal functional MRI (fMRI) in humans in 1996, this body of literature has grown substantially. In the present article, the authors systematically review all spinal fMRI studies conducted in healthy individuals with a focus on the different motor and sensory paradigms used and the results acquired.

Methods

The authors conducted a systematic search of MEDLINE for literature published from 1990 through November 2011 reporting on stimulation paradigms used to assess spinal fMRI scans in healthy individuals.

Results

They identified 19 peer-reviewed studies from 1996 to the present in which a combination of different spinal fMRI methods were used to investigate the spinal cord in healthy individuals. Eight of the studies used a motor stimulation paradigm, 10 used a sensory stimulation paradigm, and 1 compared motor and sensory stimulation paradigms.

Conclusions

Despite differences in the results of various studies, even when similar stimulation paradigms were used, this body of literature underscores that spinal fMRI signals can be obtained from the human spinal cord. The authors intend this review to serve as an introduction to spinal fMRI research and what it may offer the field of spinal cord injury research.

Reduction of physiological noise with independent component analysis improves the detection of nociceptive responses with fMRI of the human spinal cord

The evaluation of spinal cord neuronal activity in humans with functional magnetic resonance imaging (fMRI) is technically challenging. Major difficulties arise from cardiac and respiratory movement artifacts that constitute significant sources of noise. In this paper we assessed the Correction of Structured noise using spatial Independent Component Analysis (CORSICA). FMRI data of the cervical spinal cord were acquired in 14 healthy subjects using gradient-echo EPI. Nociceptive electrical stimuli were applied to the thumb. Additional data with short TR (250 ms, to prevent aliasing) were acquired to generate a spatial map of physiological noise derived from Independent Component Analysis (ICA). Physiological noise was subsequently removed from the long-TR data after selecting independent components based on the generated noise map. Stimulus-evoked responses were analyzed using the general linear model, with and without CORSICA and with a regressor generated from the cerebrospinal fluid region. Results showed higher sensitivity to detect stimulus-related activation in the targeted dorsal segment of the cord after CORSICA. Furthermore, fewer voxels showed stimulus-related signal changes in the CSF and outside the spinal region, suggesting an increase in specificity. ICA can be used to effectively reduce physiological noise in spinal cord fMRI time series.

Non-invasive measurement of oxygen saturation in the spinal vein using SWI: quantitative evaluation under conditions of physiological and caffeine load

Susceptibility-weighted imaging (SWI) has been used for quantitative and non-invasive measurement of blood oxygen saturation in the brain. In this study, we used SWI for quantitative measurement of oxygen saturation in the spinal vein to look for physiological- or caffeine-induced changes in venous oxygenation. SWI measurements were obtained for 5 healthy volunteers using 1.5-T MR units, under 1) 3 kinds of physiological load (breath holding, Bh; hyperventilation, Hv; and inspiration of highly concentrated oxygen, Ox) and 2) caffeine load. Oxygen saturation in the anterior spinal vein (ASV) was calculated. We evaluated changes in oxygen saturation induced by physiological load. We also evaluated the time-course of oxygen saturation after caffeine intake. For the physiological load measurements, the average oxygen saturation for the 5 subjects was significantly lower in Hv (0.75) and significantly higher in Bh (0.84) when compared with control (0.80). There was no significant difference between Ox (0.81) and control. Oxygen saturation gradually decreased after caffeine intake. The average values of oxygen saturation were 0.79 (0 min), 0.76 (20 min), 0.74 (40 min), and 0.73 (60 min), respectively. We demonstrated a significant difference in oxygen saturation at 40 and 60 min after caffeine intake when compared with 0 min. In conclusion, we demonstrated the feasibility of using SWI for non-invasive measurement of oxygen saturation in the spinal vein. We showed changes in oxygen saturation under physiological as well as caffeine load and suggest that this method is a useful tool for the clinical evaluation of spinal cord oxygenation.

Characterization of the hemodynamic response in the rat lumbar spinal cord using intrinsic optical imaging and laser speckle

Quantifying spinal cord functions is crucial for understanding neurophysiological mechanisms governing the intact and the injured spinal cord. Intrinsic optical imaging (IOI) and laser speckle provides measures of deoxyhemoglobin (HbR) and oxyhemoglobin (HbO(2)) concentrations, blood volume (BV) and blood flow (BF) at high spatial and temporal resolution. In this study we used IOI and laser speckle to characterize the hemodynamic response to neuronal activation in the lumbar spinal cord of anaesthetized rats (N=9). We report consistent temporal variations of HbR, HbO(2), BV and BF located ipsilaterally at L3-L5. Responses were significantly higher when stimulation intensity was increased. Vascular changes extended several millimetres from the epicenter, supporting the venous drainage observed in functional magnetic resonance imaging studies.

Laminar optical tomography of the hemodynamic response in the lumbar spinal cord of rats

Intrinsic optical imaging (IOI) has emerged as a very powerful tool to assess neuronal function in small animals. Although it has been used extensively in the brain, its application to the spinal cord is rare. The inability of intrinsic optical techniques to resolve different depths and embedded gray matter hampers their capacity to distinguish larger vasculature contributions of hemodynamic signals originating from motoneuron and interneuron activation. Laminar optical tomography (LOT) is a recently-developed method that fills the gap left between IOI and diffuse optical imaging. With distinct source-detector separations, light that propagates deeper into tissues can be distinguished from light originating from the surface, providing depth sensitivity. In this work, LOT is investigated for the first time to image spinal cord activation with simultaneous IOI of the cortex in rats. Such proof of concept provides a powerful imaging modality to study spinal cord activation and disruption after injury.

Applying functional MRI to the spinal cord and brainstem

Functional magnetic resonance imaging of the spinal cord (spinal fMRI) has facilitated the noninvasive visualization of neural activity in the spinal cord (SC) and brainstem of both animals and humans. This technique has yet to gain the widespread usage of brain fMRI, due in part to the intrinsic technical challenges spinal fMRI presents and to the narrower scope of applications it fulfills. Nonetheless, methodological progress has been considerable and rapid. To date, spinal fMRI studies have investigated SC function during sensory or motor task paradigms in spinal cord injury (SCI), multiple sclerosis (MS) and neuropathic pain (NP) patient populations, all of which have yielded consistent and sensitive results. The most recent study in our laboratory has successfully used spinal fMRI to examine cervical SC activity in a SCI patient with a metallic fixation device spanning the C(4) to C(6) vertebrae, a critical step in realizing the clinical utility of the technique. The literature reviewed in this article suggests that spinal fMRI is poised for usage in a wide range of patient populations, as multiple groups have observed intriguing, yet consistent, results using standard, readily available MR systems and hardware. The next step is the implementation of this technique in the clinic to supplement standard qualitative behavioral assessments of SCI. Spinal fMRI may offer insight into the subtleties of function in the injured and diseased SC, and support the development of new methods for treatment and monitoring.

Detection of normal spinal veins by using susceptibility-weighted imaging

PURPOSE

To evaluate the visualization of the spinal veins using susceptibility-weighted imaging (SWI).

MATERIALS AND METHODS

A 1.5-T magnet equipped with a spine matrix coil was used. Axial SWI scans of 20 healthy volunteers were obtained with a three-dimensional fast low-angle shot (3D-FLASH) sequence. Maximum intensity projection (MIP) of the phase images were reconstructed and five MIP images (at the levels of T11, T11/12, T12, T12/L1, and L1) were selected for the evaluation. The anterior median vein (AMV), posterior median vein (PMV), anterior radiculomedullary vein (ARV), posterior radiculomedullary vein (PRV), and sulcal vein (SV) were evaluated using a 4-grade scale (0, none; 1, weak; 2, moderate; and 3, prominent).

RESULTS

The AMV was detected in all the subjects (100%). The detection rates of the other veins were lower: PMV, 65%; right ARV, 45%; left ARV, 15%; right PRV, 10%; left PRV, 30%; and SV, 0%. The average scores for AMV, PMV, right ARV, left ARV, right PRV, left PRV, and SV were 0.98, 0.24, 0.20, 0.08, 0.08, 0.14, and 0, respectively.

CONCLUSION

SWI of the spine is feasible. The extrinsic spinal veins can be visualized by SWI without using contrast materials.

Tactile sensory and pain networks in the human spinal cord and brain stem mapped by means of functional MR imaging

BACKGROUND AND PURPOSE

Touch and brush sensory stimuli elicit activity in discriminative touch pathways involving specific regions in the spinal cord and brain stem. However, no study has mapped normal sensory activity noninvasively in healthy humans. The purpose of this study is to map the neuronal activity of sensory input to understand abnormal sensory transmission.

MATERIALS AND METHODS

In the present study, spinal fMRI (by using SEEP) was used to map the activity involved with light touch (2 g and 15 g von Frey filaments) and brush stimuli in the brain stem and spinal cords of 8 healthy volunteers. The results were spatially normalized and analyzed with custom-made software. Areas of SEEP activity were identified by using general linear model analysis.

RESULTS

The 2 g von Frey filament showed predominant activity in the medulla around the ipsilateral dorsal gracile and cuneate nuclei. The 15 g filament elicited significant activity in the ipsilateral dorsal and contralateral ventral gray matter areas of the spinal cord, areas around the olivary nuclei, pontine reticular formation, periaqueductal gray, and raphe nuclei in the rostral pons and midbrain. The brush stimuli elicited more activity in the medulla around the ipsilateral cuneate and gracile nuclei.

CONCLUSIONS

The 2 g filament and brush stimuli activated areas associated with a touch response. The 15 g filament activated areas associated with a pain response. The results from this study identify specific neuronal regions in the brain stem and spinal cord involved in sensory transmission and help understand altered sensory and pain states.

Tactile-associated fMRI recruitment of the cervical cord in healthy subjects

Using spinal cord functional magnetic resonance imaging (fMRI), 12 right-handed healthy subjects were scanned during a tactile stimulation of the palm of the right hand. The task-related mean signal change was computed for all activated voxels within the cervical cord, and separately, in the four cord quadrants (right and left anterior, right and left posterior) from C5 to C8. The frequency of fMRI activity at each cord level was obtained by assigning a score of 25% at each active quadrant and by averaging the percentage of active quadrants at each level of all subjects. The difference in the occurrence of fMRI activity (a) in right versus left, and anterior versus posterior cord, and (b) among the different cord levels, was evaluated using a random effect logistic regression model, with the frequency of fMRI activity as the dependent variable and the subject as the grouping factor. The task-related mean signal change of all activated voxels of the cord was 3.2% (SD = 0.8%). During the tactile stimulation, subjects showed a higher occurrence of fMRI cord activity in the right than in the left cervical cord (odds ratio = 2.25, 95% confidence interval = 1.31-3.87, P = 0.003). A significant heterogeneity in frequency of fMRI activity between cord levels was also observed (P < 0.001), with the highest frequencies of fMRI activity detected at C6 and C7. Spinal cord fMRI enables to obtain reliable physiological information on the activity of human spinal circuits associated to tactile stimulation. This holds significant promise for a better planning and conduct of studies of people with diseased spinal cords.

Characterization of cardiac-related noise in fMRI of the cervical spinal cord

Magnetic resonance imaging (MRI) has recently been applied to study spinal cord function in humans. However, spinal functional MRI (fMRI) encounters major technical challenges with cardiac noise being considered a major source of noise. The present study relied on echo-planar imaging of the cervical cord at short TR (TR=250 ms; TE=40 ms; flip=45 degrees), combined with plethysmographic recordings to characterize the spatiotemporal properties of cardiac-induced signal changes in spinal fMRI. Frequency-based analyses examining signal change at the cardiac frequency confirmed mean fluctuations of about 10% (relative to the mean signal) in the spinal cord and surrounding cerebrospinal fluid (CSF), with maximal responses reaching up to 66% in some voxels. A spatial independent component analysis (sICA) confirmed that cardiac noise is an important source of variance in spinal fMRI with several components showing a response coherent with the cardiac frequency spectrum. The time course of the main cardiac components approximated a sinusoidal function tightly coupled to the cardiac systole with at least one component showing a comparable temporal profile across runs and subjects. Spatially, both the frequency-domain analysis and the sICA demonstrated cardiac noise distributed irregularly along the full rostrocaudal extent of the segments scanned with peaks concentrated in the ventral part of the lateral slices in all scans and subjects, consistent with the major channels of CSF flow. These results confirm that cardiac-induced changes are a significant source of noise likely to affect the detection of spinal Blood Oxygen Level Dependent (BOLD) responses. Most importantly, the complex spatiotemporal structure of cardiac noise is unlikely to be accounted for adequately by ad hoc linear methods, especially in data acquired using long TR (i.e. aliasing the cardiac frequency). However, the reliable spatiotemporal distribution of cardiac noise across scanning runs and within subjects may provide a valid means to identify and extract cardiac noise based on sICA methods.

Potential clinical applications for spinal functional MRI

Functional MRI (fMRI) of the spinal cord is a noninvasive technique for obtaining information regarding spinal cord neuronal function. This article provides a brief overview of recent developments in spinal cord fMRI and outlines potential applications, as well as the limitations that must be overcome, for using spinal fMRI in the clinic. This technique is currently used for research purposes, but significant potential exists for spinal fMRI to become an important clinical tool.

Functional MR imaging of the cervical spinal cord by use of 20Hz functional electrical stimulation to median nerve

PURPOSE

Functional MR imaging of the human cervical spinal cord was carried out on volunteers by 20Hz functional electrical stimulation to median nerve, in order to detect signal changes arising concomitant to neuronal activity.

METHODS

Functional MR imaging data were acquired in six subjects with single-shot fast spin-echo sequence (SSFSE) on a 1.5T GE Clinical System. Cervical spinal cord activation was measured both in the sagittal and transverse imaging planes. Postprocessing was performed by AFNI (Analysis of Functional Neuroimages) software system.

RESULTS

Activation correlated with the time course of stimulation was consistently detected in both sagittal and transverse imaging planes of the cervical spinal cord. Regions of the spinal cord associated with motor and pain response were observed by 20Hz functional electrical stimulation to the median nerve.

CONCLUSION

The functional MR imaging signal can be detected in the human cervical spinal cord with functional electrical stimulation. Investigating the FES response in the spinal cord using the spinal fMRI will be helpful for the further discussion on the diagnosis and functional recovery to spinal cord diseases.