Functional magnetic resonance imaging of the lumbosacral cord during a lower extremity motor task

Blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) can be used to map neuronal function in the cervical cord, yet conclusive evidence supporting its applicability in the lumbosacral cord is still lacking. This study aimed to (i) demonstrate the feasibility of BOLD fMRI for indirectly mapping neural activity in the lumbosacral cord during a unilateral lower extremity motor task and (ii) investigate the impact of echo time (TE) on the BOLD effect size. Twelve healthy volunteers underwent BOLD fMRI using four reduced field-of-view single-shot gradient-echo echo planar imaging sequences, all with the same geometry but different TE values ranging from 20 to 42 ms. Each sequence was employed to acquire a single 6-min rest run and two 10-min task runs, which included alternating 15-s blocks of rest and unilateral ankle dorsi- and plantar flexion. We detected lateralized task-related BOLD activity at neurological levels L3-S2, centered at the ipsilateral (right) ventral spinal cord but also extending into the ipsilateral dorsal spinal cord. This pattern of activation is consistent with our current understanding of spinal cord organization, wherein lower motor neurons are located in the ventral gray matter horn, while interneurons neurons of the proprioceptive pathway, activated during the movement, are located in the dorsal horns and the intermediate gray matter. At the subject level, BOLD activity showed considerable variability but was lateralized in all participants. The highest BOLD effect size within the ipsilateral ventral spinal cord, as well as the highest split-half reliability, was observed at a TE of 42 ms. Sequences with a shorter TE (20 and 28 ms) also detected activity in the medioventral part of the spinal cord, likely representing large vein effects. In summary, our results demonstrate the feasibility of detecting task-related BOLD activity in the lumbosacral cord induced by voluntary lower limb movements. BOLD fMRI in the lumbosacral cord has significant implications for assessing motor function and its alterations in disease or after spinal cord injury.